GEOLOGY AND PALEONTOLOGY OF THE SOUTHWEST QUARTER OF THE BIG BEND QUADRANGLE SHASTA COUNTY, CALIFORNIA By ALBERT F. SANBORN Geologist, Standard Oil Company of California Salt Lake City, Utah Special Report 63 CALIFORNIA DIVISION OF MINES FERRY BUILDING, SAN FRANCISCO, 1960 STATE OF CALIFORNIA EDMUND G. BROWN, Governor DEPARTMENT OF NATURAL RESOURCES eWITT NELSON, Director DIVISION OF MINES IAN CAMPBELL, Chief Special Report 63 Price 75$ GEOLOGY AND PALEONTOLOGY OF THE SOUTHWEST QUARTER OF THE BIG BEND QUADRANGLE SHASTA COUNTY, CALIFORNIA By Albert F. Sandorn * OUTLINE OF REPORT ABSTRACT Abstract 3 The area covered by this report is the southwest quarter of the Big Bend quadrangle in the vicinity of lntroductu.il 3 the town of Big Bendj Shasta County) California. General stratigraphy 5 This region, which has been geologically unknown, Triassic system _ 5 contains sedimentary and volcanic strata of Mesozoic Pit formation (Middle and Upper Triassic) 5 an( * Cenozoic ages. Hosselkus limestone (Upper Triassic) 7 The Mesozoic deposits are composed of pyroclastic Brock shale (Upper Triassic) 7 rocks, lava flows, tuffaceous sandstone, argillite, and Modin formation (Upper Triassic) 8 limestone. The Mesozoic formations, from the oldest Hawkins Creek member T0 the youngest, are the Pit formation of Middle and Devils Canyon member 10 I ja te Triassic age ; the Hosselkus limestone, the Brock Kosk member ll shale, and the Modin formation of Late Triassic age; . the Arvison formation of Early Jurassic age ; and the s sy em -—- -- - --- Bagley andesite and Potem formation of Early and Arvison formation (Lower Jurassic) 11 „•,,, T . ,-.., ., e ,. , „ , . .. ' . .. __ . , T 1( Middle Jurassic age. Or the seven formations mapped, Nature of the contact of the Triassic and Jurassic svstems 14 ,. , . « » .. c ■, . ,, rr six are continuations or formations found in the con- _ . ' . .. "~ i ~a7-TTi t ~~\ -. , tiguous Redding quadrangle. These formations, which Potem formation (Lower and Middle Jurassic) 14 , ° , . v • a i •* -. i ,i ,i nave been only briefly described by other authors, were Tertiary system 16 studied in detail in the field and laboratory. Montgomery Creek formation (Eocene) 16 The Modin formation is redefined and divided into Tertiary volcanic rocks (Pliocene?) 17 t h ree members, the lower or Hawkins Creek member, the Structure 18 middle or Devils Canyon member, and the upper or Kosk _.. . q member. This division is based on lithologic differences is onca geo ogy observed in the field. This formation has previously been General paleontology 10 considered Early Jurassic. However, newly discovered Systematic paleontology 20 fossils from the formation include the species Spiri- ferina suessi Winkler and Choristoceras marshi Hauer Bibliography 26 which are restricted to the uppermost Triassic of Europe. Thus, the Modin formation is Triassic rather than Ju- rassic. Marine strata of this stage of the Triassic have Illustrations no t been reported previously from California. „, .. n n , A ,.- e „„,.f ~t *u„ n;„ A new name, the Arvison formation, is proposed for Plate 1. Geologic map and sections of part of the Big ' . j?ii Bend quadrangle, Shasta County, California In pocket strata ot Early Jurassic age, which uncontormably Over- 2. Fossils 22 lie the Modin formation and conformably underlie the _ , Potem formation. This formation contains the ammonite Figure 1. "^JJ^^wJjJSS of Blg Bend area ' 4 genera Arnioceras and Aster oceras, which indicate a 2. Index map showing local geographic setting of Sinemunan (Early Jurassic) age Big Bend area, Shasta County, California ___ 4 The Cenozoic rocks consist of fluviatlle deposits of 3. Geologic section in the Big Bend area 6 Eocene age, which unconformably overlie the Mesozoic 4. Generalized columnar section of Big Bend area. _ 6 strata and are capped by lavas and pyroclastic rocks 5. Correlation table, western states 13 associated with the Pliocene Cascade lavas. Photo 1. Brock shale 8 INTRODUCTION 2. Modin formation 8 _ . _, _-. _, , , _ . . , ,„ Location. The Big Bend region is a rectangular area 3. Arvison formation conglomerate 12 „ , « c -, , , ■ ,, , . . . ,. . . . n>> of approximately 65 square miles located in north-central 4. Arvison formation volcanic breccia \2 „,, , ~ '» ,., . -, . .. , , . ,, . 14 Shasta County, California. The region is situated in the .). o em orma ion southwest quarter of the Big Bend quadrangle U. S. b. Potem formation lo ^. D . m ? , „ T .... . . . . , r . . Forest Service 15-minute planimetric map. The south- 7. Low hills underlain bv Montgomery Creek „ „ . K ,,_ ., r , ... , , formation 16 west corner of the region is at 41 north latitude and 8. Montgomery Creek formation 17 122° west longitude. o. Tertiary basalt dike 17 The region may be reached by way of a county road 10. Tertiary volcanic rocks 17 which extends from Hillcrest, on United States Highway 11. Recent displacement, Willow Springs fault 18 299, northward a distance of 17 miles to the town of Big 12. Furrow on Willow Springs fault 18 Bend. • Geologist, standard Oil Company of California, salt Lake City Field Work and Methods. The field work was under- Utah. Condensation of a dissertation presented to the btanford . . „ -„ rrt j inri a ii <• University Graduate School, in January 1952, in partial fulfill- taken during the Summers ot 1950 and 1951. A total 01 in e Geoiogy e requlrements for the degree of Doctor of Pnilos °P ny 14 weeks was spent in the region. (3) CALIFORNIA DIVISION OF MINES [Special Report 63 Figube 1. Index map showing location of the Big Bend area, Shasta County, California. Geological data were plotted on aerial photographs with a scale of approximately 1 : 20,000. The U. S. Forest Service planimetric map of the Big Bend quadrangle (1:62,500), which was constructed from the same set of photographs used for this study, proved to be very satis- factory as a base map. Geologic sections were measured and estimated by pace and compass methods and by measurements on the aerial photographs where distortion of scale was thought to be at a minimum. Topography. The greater part of the Big Bend area is drained by the Pit River and its tributaries. The Pit River, which is supplied with a large volume of water from the volcanic plateau east of the town of Burney, has cut a deep canyon into the volcanic rocks just east of Big Bend. The tributary streams, keeping pace with the rapidly downcutting Pit River, have cut deep, steep- sided canyons, giving sharp topographic relief to the region. Elevations in the area mapped range from 1,500 feet along the Pit River to about 4,800 feet along the highest ridge. Peaks immediately north of the area have elevations exceeding 6,000 feet. The area appears to be in the youthful stage of the fluvial erosion cycle. Previous Work. The area around Big Bend appears as an unmapped region on the State Geologic Map of California (1938). Early workers, however, visited the region and collected fossils near the Pit River and Kosk Creek. A survey of previous work would not be complete without a brief discussion of early work carried on in adjacent regions, especially of the papers that deal with the stratigraphic units in the Big Bend area. The first geologic account of sedimentary strata in Shasta County was by J. B. Trask (Whitney, 1865, p. 327 ) , who collected fossils from limestone at Gray Rocks near Bayha in 1855 and determined their age as Car- boniferous. A California Survey party under Whitney (1865, pp. 326-327) visited the same locality in 1863 and collected a number of fossils which were described by Meek- (Meek and Gabb, 1864) also as Carboniferous. This limestone, now known as the McCloud limestone, lies to the west of the Big Bend area. In 1883 J. S. Diller of the U. S. Geological Survey began a reconnaissance survey of northern California O MILES SCALE Figure 2. Index map showing local geographic setting of the Big Bend area, Shasta County, California. and Oregon. In a paper on the geology of the Lassen Peak district, Diller (1889) first mentioned the presence of Mesozoic sediments older than Cretaceous in the vi- cinity of Pit River and Montgomery Creek. Diller also considered these rocks as part of the "Auriferous slate series. ' ' Fairbanks (1893) wrote a general account of the geo- logy of Shasta County which was published together with the first geologic map of the county. His account included some discussion of the rocks of the Big Bend area, with a description of a fossil locality in Big Canyon (now Iron Canyon) about 6 miles southwest of Big Bend. Of these rocks and the age of the contained fossils he wrote as follows : "Slates appear in Big Canyon. They strike very regularly a little west of north ; dip almost vertical. The color is black to purplish. A few scattered fossils were found in them and in the washed boulders in the bed of the creek. . . . The most promi- nent fossil is a large-ribbed bivalve shell not found in place at this point. The character of these fossils indicates a Meso- zoic age. Some of the fossils as well as the slates resemble the Triassic of Indian Valley, Plumas County. They are certainly younger than the Carboniferous and, according to our present accepted stratigraphy, must be older than Cretaceous." 1960] SOUTHWEST QUARTER OF BIG BEND QUADRANGLE The coal-bearing shales and conglomerates of the Big Bend area he considered Cretaceous. "Below Big Bend, Pit River flows for many miles in a deep, narrow canyon. Its course has been cut at the junction of the Chico conglomerates with the older series." These beds are now considered Eocene in age. Diller (1893) mentioned fossil localities near Big Bend and near Cedar Creek in the Redding quadrangle and stated that both Jurassic and Triassic of the Taylorsville region are well represented in the Pit River Valley. Of the Big Bend fauna, he quoted Hyatt's opinion that the age of the fossils is equivalent to those of the Mormon sandstone of Taylorsville. He mentioned the Bend and Cedar formations as new formations but did not outline their stratigraphic limits or distribution, as they were to be described in the text of the U. S. Geological Survey's Lassen Peak atlas sheet, which at that time was in proof. Fairbanks (1894) published some supplementary notes on his geologic work in Shasta County. Fossils collected during the field studies had been sent to J. P. Smith for identification. Smith considered the fossils collected at Big Canyon to be contemporaneous with those of the Hardgrave sandstone of the Taylorsville area, Plumas County. The fossiliferous limestone west of the Big Can- yon area near Squaw Creek was regarded as a correlative of the Hosselkus limestone of Taylorsville and the rocks at Diller 's Cedar Creek locality, all three being of Kar- nian age. Smith (1894) summarized the results of the study of fossil material from Shasta County and briefly described the various lithologic units of the region as then under- stood. Included with these were Diller 's Upper Triassic Cedar formation and the Jurassic Bend formation, de- scriptions of which had apparently been published in 1892 in an advance copy of the Lassen Peak folio. Under the Cedar formation, Smith listed the fauna of the Swearinger slates (Brock shales), the age of which was placed as Late Norian. The first list of fauna from the Hosselkus limestone was also included, in a description of the Cedar formation. This limestone was considered to be Karnian in age, as previously reported by Fairbanks. Diller (1906) revised the stratigraphy of this area in the Redding folio, which contains an excellent geologic description of the stratigraphic units of Shasta County. For the Upper Triassic and Jurassic rocks, Diller pro- posed several new formations. The previously used name, Cedar formation, was abandoned, and replaced by Hos- selkus limestone for the strata of the lower part, and Brock shale for the overlying shale beds that previously had been referred to as the correlative of the Swearinger slates of the Taylorsville region. The Bend formation of the Lassen Peak quadrangle, which included all beds thought to be Jurassic in age, was abandoned because of prior use of the name on the Gulf Coast. The name Modin formation was proposed for the lower portion of the old Bend formation of the Lassen folio. Although many fossils were found in these beds, diagnostic forms were lacking and the formation was assigned to the Jurassic system on the basis of its stratigraphic position. The younger Jurassic beds were named the Potem forma- tion. The Potem formation was thought to be the correla- tive of the Hardgrave and Mormon sandstones of the Taylorsville region. The lacustrine deposits which uncon- formably overlie the Jurassic beds near Big Bend and along Kosk Creek were considered an extension of the lone formation and were assigned to the Miocene series on the basis of their flora. Hinds (1933) published a report on the geologic for- mations of the Redding and Weaverville quadrangles. He suggested no changes of name for the geologic forma- tions of the Mesozoic strata, but introduced the name Montgomery Creek formation for the rocks previously referred to the lone formation by Diller. Acknowledgments. Thanks are due Dr. Siemon W. Muller for guidance and advice during the field mapping and the preparation of this paper ; to Dr. R. R. Compton for suggestions concerning the study of thin-sections of the representative rocks of the area ; and to Drs. A. Myra Keen and J. J. Graham for many helpful suggestions. It is a pleasure to acknowledge the innumerable cour- tesies extended by personnel of the United States Forest Service and residents of the vicinity of Big Bend. GENERAL STRATIGRAPHY Strata exposed in the Big Bend area are Mesozoic and Tertiary in age. A maximum thickness of 12,800 feet of Upper Triassic and Lower and Middle Jurassic strata is exposed above the Pit formation. The rocks are pre- dominantly marine clastic sediments derived from the erosion of volcanic flows and associated pyroclastic mate- rial together with subordinate limestone deposits and considerable pyroclastic rocks and lavas. These rocks are well indurated. They are locally meta- morphosed by structural deformation, and some altera- tion of the pyroclastic rocks was observed. However, no obvious regional metamorphism has occurred in these sediments although the units outcropping in this area are shown as metasediments elsewhere in Shasta County on the California State Geologic Map of 1938. Relatively unconsolidated fluvial shale, sandstone, and conglomerate of Eocene age unconformably overlie the Mesozoic strata. These beds in turn are unconformably capped by a thick layer of andesitic and basaltic flowr. and pyroclastic accumulations associated with the Plio- cene Cascade lavas. The formations in the Big Bend area are summarized in figure 3. TRIASSIC SYSTEM Pit Formation (Middle and Upper Triassic) Definition and Lithologic Character. The Pit forma- tion is the oldest formation (exposed) in the area studied. It was named by Fairbanks (1894), undoubt- edly for exposures along the Pit River. However, no type locality was indicated. As originally defined, the Pit formation included all sediments younger than the McCloud limestone and older than the Hosselkus lime- stone. The lower part of the formation, which consisted of shales and pyroclastic material, called the McCloud shales, was considered Carboniferous in age. The upper beds were called the Pit shales and placed as the Lower or Middle Triassic. Diller (1906) restricted the Pit for- mation. The lower beds of the original Pit formation which were considered Carboniferous in age were as- signed to the Nosoni formation. Certain extrusive and pyroclastic strata which overlie the Nosoni beds were placed in two formations, the Dekkas andesite and the CALIFORNIA DIVISION OF MINES [Special Report 63 Esti- mated maxi- Series Standard European stages Formation mum thick- ness in feet Pliocene? Volcanic flows and asso- ciated pyroclasic deposits. (Shasta Lavas Highland) 3,000 . Unconformity Eocene Middle Jurassic Bajocian Toarcian Pliensbachian Lower Jurassic .... Sinemurian MONTGOMERY CREEK FORMATION 2.600- . Unconformity POTEM FORMATION 1,000 BAGLEY ANDES1TE 700 ARVISON FORMATION 5.090 Hettangian Rhaetian Upper Triassic Norian Karnian ) Unconformity MODIN FORMATION 5,500— Middle Triassic Ladinian Anisian? BROCK SHALE HOSSELKUS LIME- STONE PIT FORMATION 400— 150— Figure 3. Geologic section in the Big Bend area. Bully Hill rhyolite. The overlying interbedded shales and tuffs, largely the Pit shales described by Fairbanks, constituted the restricted Pit formation. Field work in the Big Bend region was planned to include the investigation of rocks younger than the Pit formation. As a result, only exposures of the upper part of the Pit formation were examined. In Hawkins Creek the Pit formation consists of alternating dark gray to black argillites, light green to gray tuffs, and thin beds or lemils of gray limestone. The argillites predominate, comprising about 65 percent of the rock. Fresh outcrops are exposed only at intervals in the creek bed. Else- where, the nature of the bedrock can be judged only GENERALIZED COLUMNAR SECTION OF BIG BEND AREA Pliocene ('■') >^^V TERTIARY VOLCANICS " ' ' ! "' Lavas Pyroclastics Eocene MIDDLE JURASSIC LOWER JURASSIC * t> A &•*/> ft a ■ & t>. tf a a °^% S%-lo UPPER TRIASSIC MIDDLE TRIASSIC •o o o o°oO°0 Oo° u o°0 o°o 0°OoO OoOO i, i ,; '& ri.-'.v. :&'■. *? i.i . i--.-i-.-T ! .'. ...'...' ~r mwmF o O o q O ■ - ,-> -J I I I i: '.X}'-\7. &'■ "A,"' ^'. KA/VNaaJ MONTGOMERY CREEK FORMATION Cong I om erate Sandstone ( Arkosic) Sha le POTEM FORMATION, Argi Mite - Limestone BAGLEY ANDESITE Lavas - Pyroclastics ARVISON FORMATION Agglomerate Volcanic breccia Tuff Conglomerate Tuffaceous sandstone MODIN FORMATION Argillite Limestone Tuffaceous sandstone Kosk Lavas Agglomerate Volcanic breccia Cong lomerate Devi Is Canyon Hawkins Creek BROCK SHALE, Argillite -Tuff HOSSELKUS LIMESTONE PIT FORMATION, Argillite - Tuff Figure 4. by the fragmental material present in the regolith. These weathered materials suggest a bedrock of interbedded argillite, volcanic breccia, and coarse-grained tuff. Exposures of the Pit formation in upper Devils Can- yon are similar to those on Hawkins Creek. However, tuffaceous and agglomeratic beds predominate and lime- stone is absent. Thin-sections of samples from Devils Canyon show the tuffs to be composed mainly of subrounded to sub- angular fragments of volcanic origin and feldspar crys- tals encased in a fine-grained tuffaceous groundmass. Chlorite and leucoxene are common. The argillite is com- posed chiefly of silt to clay-sized particles of tuffaceous material together with about 20 percent black car- bonaceous material. Numerous spherical to subspherical bodies composed of chalcedony are suggestive of radio- larian remains. However, no definite organic structures could be seen, as the silica has recrystallized, and any structures that may have been present have been destroyed. 1960] SOUTHWEST QUARTER OF BIG BEND QUADRANGLE Distribution and Age. The Pit formation is confined to the northwest corner of the Big Bend area. Best ex- posures are along Hawkins Creek and in the upper part of Devils Canyon below Grizzly Peak. Elsewhere, the Pit formation is deeply weathered and forms rolling hills and gentle slopes. No fossils were found in the Pit formation in the Big Bend region. Smith (1927) reported the presence of Halobia rugosa, Trachyceras, Ceratites cf. C. humboldt- ensis and Ptychites, and placed the age of the formation as Middle and Late Triassic. Hosselkus Limestone (Upper Triassic) Definition and Lithologic Character. The Hosselkus limestone was the name given by Diller (1892) to lime- stone beds that form prominent ledges on the divide between Genessee Valley and Hosselkus Creek in the Taylorsville region, Plumas County, California. Lime- stones of similar age in the Redding quadrangle were at first included in the Cedar formation, a name proposed by Diller in the Lassen folio for those beds of the ' ' Auri- ferous slate series," limestone and shale, containing fossils of the same age as the Hosselkus limestone and associated Triassic rocks of Genessee Valley. Smith (1894) treated the Hosselkus limestone of Shasta County as a member of the Cedar formation. In the Redding folio Diller (1906) discontinued the use of the name Cedar formation and considered the Hosselkus lime- stone of the Redding area as a formation correlative with the formation of the same name in the Taylors- ville region. Smith (1894) divided the limestone into three parts. The lower division is composed of rather hard, pure limestone and consists almost entirely of fossils ; the middle division is hard, siliceous, prominently jointed limestone, which forms rugged ridges ; the upper divi- sion is siliceous, massive gray limestone. Smith (1894) later summarized his study of this lime- stone. He stated that the limestone is nearly continuous from the Cedar Creek locality northward to the Tertiary lavas but ranges in thickness from a thin edge to 400 feet on Brock Mountain where it is best developed. Smith gave a general section of the Hosselkus limestone from top to bottom as follows : Spiriferina zone (hard siliceous limestone full of brachiopods) 100 feet Coral zone ; numerous reefs of Astreidae Tropites subbullatus zone Juvavites subzone (hard limestone that carries abundant ammonites) 50 feet Trachyceras subzone (shaly limestone) 50 feet Calcareous shales full of Halobia 100 feet Distribution. The Hosselkus limestone in the Big Bend region crops out at two localities. One outcrop is on the trail from Stump Creek to Hawkins Creek, where a 20-foot bed of nearly pure, medium-gray limestone is exposed. This limestone weathers to a light blue- gray and is abundantly fractured, the resulting cracks being cemented with calcite. Although no fossils were found in this limestone, it lies below a weathered shale or argillite whose lithology and stratigraphic position are similar to that of the Brock shale at its type locality, and overlies a series of alternating, coarse tuffs and argillites typical of the Pit formation. For this reason this lenticular outcrop was mapped as Hosselkus lime- stone. The second area of outcrop is in the upper part of Devils Canyon, where the estimated thickness is about 150 feet. The limestone is thin bedded and dark gray in color, weathering to light gray. Fossils are abundant. The Hosselkus limestone in Devils Canyon weathers easily and does not form the bold outcrops that charac- terize it elsewhere. This may be due to the fact that the silicified beds are missing in the Devils Canyon area. A thin-section of the limestone shows it to be composed of about 95 percent calcite and about 5 percent black organic material. Many oval and circular areas of clear recrystallized calcite suggest the presence of microfos- sils ; however, no definite organic structures were found. Fossils and Correlation. The following fossils from the Devils Canyon locality were identified by the author : Arcestes cf. A. shastensis Smith Paratropites americanus Hyatt and Smith Tropites sp. Clionites aff. C. whitneyi Smith Halobia austriaca Mojsisovics Halobia cf. H. superba Mojsisovics Halobia sp. These forms indicate that this limestone represents the Tropites subbullatus zone of the Brock Mountain Hosselkus limestone as defined by Smith (1927) and is Karnian. Inasmuch as fossil material was gathered near the contact with the overlying Brock shale, it ap- pears that all or most of the overlying Coral and Spirife- rina zones are absent in this locality. Field observations suggest a conformable relationship of the Hosselkus with the underlying Pit formation. Folding and some distortion of the beds obscure the relationship of the Hosselkus with the overlying Brock shale. Diller (1906) said of the Redding area that ". . . there is no doubt concerning the conformability of the Brock shale and the Hosselkus limestone." If this be the case, the Hosselkus limestone is lenticular in nature, as it is not continuous either in the Redding quadrangle or in the Big Bend area. Brock Shale (Upper Triassic) Definition and Lithologic Description. The Brock shale was named by Diller (1906) in the descriptive text which accompanies the Redding folio, for the exposures on Brock Mountain, which lies between Squaw Creek and Pit River. He described the shale as being about 400 feet in thickness, of which the lower 300 feet are ". . . dark (shales) somewhat calcareous and frequently con- tain Halobia. Above these come sandy shales, gray and red- dish in color and characterized locally by Pseudomonotis sub- circularis." The Brock shale exposures in the Big Bend area con- sist mainly of beds of dark gray to black argillite with interbedded layers of medium gray tuff or tuffaceous sandstone of medium grain size. The argillite is massive in fresh exposures but shows a parting parallel to the bedding in partially weathered exposures. They are also characteristically fractured by a joint system normal to the bedding plane which breaks up these partly weath- ered layers into small blocks which are about 1 to 2 inches thick. The fine grained material weathers a brown to reddish color. Thus the blocky fragments, coupled with a reddish color of the weathered material, are char- acteristic of the Brock exposures in this area. Microscopic examination of a thin-section of Brock argillite shows that it is composed largely of silt- to CALIFORNIA DIVISION OF MINES [Special Report 63 <->'i ,* Photo 1. Brock shale outcrop in Devils Canyon. clay-sized grains that are of volcanic origin. Black mate- rial constitutes about 10 percent of the rock. The car- bonate content appears to be high, approximately 50 percent. Field examination of numerous hand specimens indicates, however, that although the argillite is usually somewhat calcareous, the carbonate content varies widely. No distinct bedding or micaceous laminae are present, but a certain rough alignment of elongate par- ticles which appear to be shards was noted. Distribution. Although the brushy terrain and deep regolith make it difficult to examine the contact through- out the area, characteristic Brock sediments were en- countered below the basal member of the Modin forma- tion and appear to be conformable with it wherever ex- posed in the Big Bend area. Thus, the Brock shale is probably continuous and varies but little in thickness. Diller reached a similar conclusion about the Brock shale in the Redding quadrangle. An excellent exposure of the Brock shale is in Devils Canyon where the area of outcrop is increased by local folding. The estimated thickness is approximately 400 feet. Age. No fossils were found in the Brock shale in the Big Bend area. Its age and correlation with the Brock shale at its type locality can only be determined on the basis of its stratigraphic position ; i.e. that it overlies the Hosselkus limestone and immediately underlies the basal member of the Modin formation. Smith (1927) con- sidered the Brock shale as part of the Upper Norian stage of the Triassic system. Modin Formation (Upper Triassic) Definition. The Modin formation derives its name from exposures near the mouth of Modin Creek at its confluence with Squaw Creek in the Redding quadrangle. The formation was defined by Diller (1906) who de- scribed it as "... an extensive succession of tuffaceous beds overlain by a greater mass of compact, fine gray shaley sandstones and shales, with a few lenses of limestone." His basal "tuffaceous beds" are composed of volcanic conglomerate and pyroclastic material consisting mainly of volcanic debris but containing limestone fragments with fossils of Hosselkus age. The writer, after examining the Modin formation along Squaw Creek near its type locality and tracing the basal beds from the Big Bend area to the northern part of the Redding quadrangle, concluded that the Modin formation is represented in the Big Bend area by an extensive accumulation of deposits reaching an esti- mated maximum thickness of about 5,500 feet. The Modin formation in the Big Bend region consists of all sedi- ments overlying the Brock formation and underlying the extensive pyroclastic breccia, conglomerate, and tuff which are to be described below as the Arvison forma- tion. Three lithologically distinct and readily mappable units are present within the formation. On this basis the author proposes a division of the Modin formation into three members : the basal pyroclastic and conglom- eratic beds as the Hawkins Creek member; the inter- mediate tuffaceous limestones and fine sandstones as the Devils Canyon member; and the upper thin-bedded argillite as the Kosk Creek member. Age. Diller (1906) originally placed the Modin for- mation at the base of the Jurassic system. Although his party had collected fossils from the Modin, the fossil material apparently did not include recognizable diag- nostic forms and the age was judged Jurassic more on the basis of stratigraphic relationship than on faunal ^ii 9D mm Photo 2. Modin formation near head of Alder Creek. Line of trees on far slope is along the contact of the Hawkins Creek mem- ber (right) and the Devils Canyon member (left). 1960] SOUTHWEST QUARTER OF BIG BEND QUADRANGLE 9 evidence. Diller quotes T. W. Stanton's opinion on the Hawkins Creek Member age of the Modin fossils: Definition and Lithologic Character. The Hawkins "Between the Pseudomonotis subcircularis horizon (Brock Creek member derives its name from exposures near the shale) and the beds yielding a well-characterized Jurassic head f Hawkins Creek in the vicinity of Little Mead- fauna (Potem formation) comparable with that found at hi h j th conglomerate is extensively ex- Taylorsville there is a broad belt in which a great thickness "> " ^ r' & . . J , of rocks is represented and from which fossils were collected posed. However, the type area is designated as that at many localities. In the field these were considered Jurassic section in Devils Canyon beginning at the contact with and I still think that most if not all of them are of that the overlying Devils Canyon member approximately 1 ^^S^^^^T^^^ZrSTfZ mile west of the confluence of Devils Canyon and Alder species and were either poorly preserved or belonged to per- Creek, extending west along Devils Canyon to the sistent types that would not aid in discriminating Jurassic contact with the underlying Brock shale. The member consists of a thick series of beds of vol- The significance of the Modin fauna will be discussed can i c material in the form of conglomerate, agglomerate, in more detail below in the description of the members volcanic breccia, and tuff. The maximum thickness is of the Modin formation. Included in the Modin fauna approximately 900 feet. The character of the beds varies are three forms considered of stratigraphic significance : markedly along the strike. In the vicinity of the head Spiriferina cf. S. suessi Winkler, Plicatula perimbricata f Hawkins Creek and Little Meadows the member is Gabb, and Choristoceras marshi Hauer. Spiriferina largely a polymictic conglomerate composed of well suessi has been reported from Upper Norian and Rhae- rounded pebbles of andesitic composition and a large tian stages of the European Triassic section. The Modin proportion of well rounded quartz and chert pebbles Spiriferina has been compared with figures and descrip- enclosed in a groundmass of coarse sandstone or grit of tions of Spiriferina suessi and appears very similar if approximately the same composition. Individual pebbles not conspecific with the European species, and also rarely exceed 1 inch in diameter, the average being about similar to a Spiriferina from the Upper Triassic Gabbs half an inch. Locally, with change in grain size, the formation of Nevada. Plicatula perimbricata appears conglomerate is interbedded with grits and coarse gray- identical to topotypes from the Gabbs formation. Cho- wacke. The presence of the quartz and chert pebbles ristoceras marshi is characteristic of the Choristoceras poses a problem as to their origin, for quartz and chert marshi zone {—Pteria contorta zone) of the Euro- are otherwise completely foreign to the sediments of this pean Rhaetian stage of the Triassic. This faunal evi- area. In fact, the entire section from the Hosselkus for- dence, plus the fact that the Modin overlies beds of mation through the Jurassic formations is remarkable Norian age and, with apparent unconformity, underlies for the paucity of silica in any form. A possible source the Arvison beds whose tuffaceous strata yield fossils could be the siliceous strata of the Hosselkus limestone, characteristic of the Sinemurian stage of the Lower Elsewhere in the member, the conglomerate occurs as Jurassic points to the conclusion that the Modin forma- tMn j and & larger proportion of vo lcanic breccia, tion is Late Triassic in age and probably a correlative agglomerate, and tuff is present. The size of the frag- with formations of the Upper Norian and Rhaetian ments of the breccia es from a fraetion of an inch stages of the European Triassic. to 6 inehes DiUer reported the p rese nce of blocks up Lithologic Character. From the top to bottom, begin- to * foot ^J™^? * n the } asa \ he $ s of the Modin for- ning at the contact with the overlying Arvison formation matlon in the R e ddin g quadrangle. In general, the frag- and ending at the contact with the Brock shale below, ments are angular; however many are slightly rounded the Modin formation consists of the following units along and a PP ear to have been subjected to erosion by water. Kosk Creek and Devils Canyon. The . matrix material is composed of medium- to coarse- J _,.. . grained tuff. Thickness Kosk member in feet In Devils Canyon the upper part of the member con- (10) Thin bedded dark gray argillites and fine sists of approximately 200 feet of volcanic flows which grained tuffaceous sandstone. Choristoceras appear to be andesite porphyrv containing feldspar marshi Hauer 1000 u a ± *. £ u i n. • (9) Andesite flows and volcanic breccia 300 phenocrysts up to a quarter of an inch in length in an (8) Thin bedded dark gray argillite 400 aphanitic greenish-gray groundmass. (7) Medium to fine grained tuff and agglomerate 285 (6) Andesite flows and volcanic breccia 165 Distribution and Age. The Hawkins Creek member (5) Coarse to medium gray tuff 250 overlies the Brock shale wherever the two units are ex- (4) Thin bedded dark gray argillite and medium d Uhi th Bi B d &rea The eontact appears to gray fine grained tuffaceous sandstone 1200 * „ . , _., a J .. . ,, , r .+ . » be conformable, lhe transition trom the deposition of Devils Canyon member fine-grained material in the Brock to coarse volcanic (3) Massive dark gray tuffaceous limestone, cal- detritus in the Modin formation is abrupt, suggestive careous tuff, fine to medium grained calcareous „ . , » , ... , , ... » ., sandstones and light gray limestone lenses. of a period of non-deposition between deposition of the Very fossiliferous locally. Spiriferina cf. 8. Brock and the Modin formations. However, the consist- suessi Winkler and Plicatula perimbricata en t thickness of the relatively thin Brock formation for Gabb 1000 great distances suggests little or no erosion of the Brock Hawkins Creek member prior to the deposition of the Modin. On the other hand, (2) Andesite porphyry 200 Hinds (1933, p. 101) suggested that a distinct erosional (l) Medium gray agglomerate, volcanic breccia, and slight angu i ar discordance exists between the Brock tuft and conglomerate <00 , ,, ,, ,. ,, , . » ., „ , t, and the Modin on the basis of evidence round on Bear Total thickness 5500 Mountain in the Redding quadrangle. Diller (1906) re- 10 CALIFORNIA DIVISION OP MINES [Special Report 63 ported fragments of limestone containing fossils of Hos- selkus age from three areas of the lower Modin forma- tion. He thought that the fragments had been dislodged from lower beds by volcanic action, but it seems possible that they could have been derived by ordinary processes of erosion. Thus there appears to be considerable evidence suggesting an unconformity between the Brock shale and the Modin formation, but as yet not enough evidence is available to confirm it. Fossils were not found in this member, and the age is determined solely on the basis of its stratigraphic posi- tion. As the upper members of the formation contain fossils of Late Norian and Rhaetian ages, and the underlying Brock shale is of Norian age, this member represents part of the Norian stage of the Upper Triassic system. The beginning of the time of Modin deposition in this area was a time of intense volcanic activity in which large deposits of volcanic material were accumulated on the fine-grained sediments of Brock shale, probably in a shallow sea. Devils Canyon Member Definition and Lithologic Character. The name Devils Canyon member is proposed for a series of mas- sive fossiliferous limestones and calcareous sandstones overlying the Hawkins Creek member. The type sections is in Devils Canyon. Its upper contact is about a quarter of a mile west of the confluence of Devils Canyon and Alder Creek. From this point the member is well ex- posed to a point about three-quarters of a mile west at the contact with the underlying Hawkins Creek member. The Devils Canyon member is composed principally of massive dark gray tuffaceous limestone, calcareous tuff, fine- to medium-grained calcareous sandstone, and nearly pure limestone lenses. The unweathered rock is a firmly indurated, tough rock that is difficult to chip with a hammer. The weathered material is tan or buff in color. As the weathering progresses, the leaching of cal- careous material results in the formation of a porous, punky rock that has the appearance of a tuff. A study of thin-sections of this rock shows that the carbonate content ranges from about 25 percent to 75 percent. Fine-grained sand- to silt-sized tuffaceous ma- terial and volcanic rock fragments which contain ande- sine compose the rest of the rock. Interbedded with this impure limestone and sandstone are a few lenses of nearly pure limestone. The lenses range from a few inches to about 10 feet in thickness and may be traced as far as one-quarter of a mile along strike. Microscopi- cally, the limestone exhibits a very fine-grained granular texture with an apparent orientation of grains to a -bed- ding plane. It appears to be a fine-grained calcarenite. Fossils were not found in these lenses but in thin- section this material is seen to contain occasional fossil fragments. Weathering of this member tends to produce rounded hiHtops and deep, steep-sided canyons. Fresh un- weathered material is encountered only in the stream beds, with the exception of the more resistant pure lime- stone '^nses which crop out in scattered localities on the hill slopes and ridges. Bedding is readily apparent in the relatively unweathered material near the bottom of stream canyons but usually is not discernible on higher slopes and ridges. Individual beds range from about 6 inches to several feet in thickness. Beds differ from one another in carbonate content and grain size of the vol- canic detrital material. Distribution. This member, which appears to be con- tinuous across the northwest part of the area, ranges in thickness from about 430 feet along the ridge extending north from Smith Flat, to an estimated 1,000 feet in Devils Canyon, where the area of outcrop is increased somewhat by local folding. The best exposures are along Devils Canyon and Alder Creek. It also crops out in Live Oak Canyon and Bull Canyon in the northeast corner of the mapped area. Good exposures of the weathered material and limestone lenses occur along the trail from Stump Creek Butte to Little Meadows. The best fossil localities are along the Stump Creek-Little Meadows trail and in Alder Creek Canyon. Fossils and Correlation. The Devils Canyon member yielded a rich, although poorly preserved, marine in- vertebrate fauna, consisting chiefly of pelecypods. Brach- iopods, of which only a few genera are present, are fairly common. Pentagonal crinoid stem plates are common, and echinoid spines are rare. The Devils Canyon assemblage includes the following : Spiriferina cf. S. suessi Winkler Rhynchonella richardsoni Smith "Terebratula" piriformis Suess Zugmayeria sp. Pecten sp. Myophoria? sp. Lima sp. Lima aff. L. costata Goldfuss Lima cf. L. terquemi Tate Plicatula perimbricata Gabb Volsella sp. Lopha sp. Ostrea sp. Philippiella sp. Pinna cf. P. blanfordi Boettger A comparison of this fauna with figured and described specimens of the Late Triassic faunas from Europe and Asia reveals that the Devils Canyon pelecypods are mainly forms that are not characteristic of a particular horizon in the Upper Triassic or Lower Jurassic systems. Apparently many new species are present in the Devils Canyon member, but the state of preservation is such that it appears impracticable to describe new species. It is hoped that further collection and study of fossils from this member will eventually provide fossil material suit- able for the illustration and description of these new forms. Spiriferina cf. S. suessi Winkler closely resembles S. suessi as described from the Rhaetian beds of the Alpine Triassic. H. Zugmayer (1882) reports that S. suessi ap- pears exclusively in the Koessener facies of the Rhaetian stage and in the Starhemberger beds. The Devils Canyon Spiriferina has been compared with a Spiriferina col- lected from the Upper Triassic Gabbs formation of Ne- vada. The two forms appear to be very similar if not identical. Specimens identified as Plicatula perimbricata Gabb have been compared with topotype material from the Volcano Peak locality in Nevada. This species is a common and characteristic fossil in the Devils Canyon beds. Gabb (1870) originally described the species as 1960] SOUTHWEST QUARTER OF BIG BEND QUADRANGLE 11 a Jurassic fossil from Volcano Peak, Nevada. Subsequent study by S. W. Muller * has shown that the Plicatula locality is actually part of the Gabbs formation, of Late Triassic age. Using the above comparisons and the stratigraphic position of the Devils Canyon member which overlies Norian beds and underlies Rhaetian (Choristoceras marshi horizon) strata, this member contains deposits that can be correlated with upper Norian and/or Rhae- tian stages of the European section and with part of the Gabbs formation of Nevada. The presence in the fauna of such forms as Ostrea, Lopha, Plicatula, Volsella, and Pinna are evidence that these fossiliferous layers were accumulated in a shallow- water, marine, upper neritic to littoral environment. Kosk Member Definition and Lithologic Character. The Kosk mem- ber derives its name from Kosk Creek. The type area is located along Kosk Creek and Devils Canyon from a point on Kosk Creek approximately 1 mile north of Arvison Flat, where the upper contact of the member is exposed, to its lower contact with the Devils Canyon member about a quarter of a mile west of the junction of Devils Canyon and Alder Creek. The dominant lithology of the member is thin-bedded, dark gray to black argillite. In fresh exposure it is firmly indurated and appears massive but has a parting along the bedding plane. Partially weathered surfaces show a dark bluish-gray color. The weathered outcrops appear as thin-bedded calcareous argillite separated by shale partings. Generally the material is very fine grained but there are many 1- to 2-inch layers of fine to medium-grained tuff. Locally, the argillite is distinctly pyritiferous. Microscopic examination of thin-sections of this mem- ber indicates that it is composed principally of clastic grains of volcanic origin, some of which appear to be water worn ; others are angular glass shards. The clastic material ranges in size from fine sand to clay ; most falls in the size-range of silt. Seen in thin-section the rock is a calcareous siltstone. Calcite constitutes about 25 to 30 per cent of the rock, carbonaceous material about 5 percent. A characteristic of this rock is the presence of subspheri- cal bodies of calcite that suggest the presence of micro- fossils. Two beds composed of andesitic flows, coarse tuffs, and agglomerates are present in the member. Both beds are approximately 300 feet in thickness. One of these volcanic beds is about a quarter of a mile south of the junction of Kosk Creek and Devils Canyon, and the other crosses Devils Canyon approximately a quarter of a mile west of Kosk Creek. Distribution. The member is best developed and of maximum exposed thickness (approximately 3600 feet) at its type locality. Outcrops continue to the northeast- ern edge of the mapped area with less thickness exposed due mainly to the overlap of the Tertiary lavas on the east. West and south of the type area the member diminishes in thickness along the strike to a point just southwest of the Stump Creek-Little Meadow trail where it disappears completely. It is apparently absent from that point to the western edge of the area. * Personal communication. Fossils and Correlation. Diligent search of the out- crops of the Kosk member revealed the presence of im- pressions of highly evolute ammonites on weathered bedding planes. At two localities, better preserved ma- terial contained one species of ammonite identified as Choristoceras marshi Hauer. It is believed that the im- pression of ammonites throughout the beds of this mem- ber are probably of the same species or closely related forms. The specimens of Choristoceras are small in size and are either all young or dwarfed specimens. Casts and impression of somewhat larger, yet smaller than normal, forms were found at other localities. These lat- ter could not be definitely identfied as Choristoceras but certainly were similar to the better-preserved material. As Choristoceras marshi is characteristic of the Rhae- tian beds of Europe this member is considered as a cor- relative of that stage. It is also correlated with the upper "member" of the Gabbs formation as defined by Muller and Ferguson (1939). These authors point out that prior to the time of their investigation, no marine beds of Rhaetian age had been reported from North America. Thus, the Kosk member of the Modin formation expands the known distribution of Rhaetian beds on this continent to northern California. As to the environment of deposition of this member, three factors are noted. First, the meagerness of the fauna which consists almost entirely of cephalopods; secondly, the possible dwarfing of Choristoceras marshi; and lastly, the presence of a sediment made up of black organic silt locally containing abundant pyrite. These factors are suggestive of a restricted environment of deposition in a basin which did not allow free circula- tion of water at depth, thus producing an environment unfavorable to benthonic life. Pelagic forms such as am- monites would be expected to be the most common fossils to accumulate in a sediment formed in such an environment. Lack of oxygen at shallower depths might inhibit but not exclude a pelagic form, with the result that a dwarf fauna might be produced. Abundant or- ganic material in the sediment and the common occur- rence of pyrite are also results of accumulation of sedi- ments in a stagnant environment. JURASSIC SYSTEM Arvison Formation (Lower Jurassic) Definition and Lithologic Character. The Arvison formation is proposed for that series of dominantly py- roclastic beds with minor andesitic flows, which are in- termediate between the Modin and Potem formations and which do not appear to be continuous with the Bagley andesite of Diller. The Bagley andesite as traced from its type area into the Big Bend region overlies and is interfingered with the basal beds of the Potem forma- tion. Because the basal strata of the Potem formation appear to lie between the Bagley andesite and the Ar- vison pyroclastic accumulations, the author deems it fitting to propose a new formation for the lower beds. The name is taken from Arvison Flat on Kosk Creek where the beds are well developed. The type area can be considered as extending from a point on Kosk Creek approximately If miles south of Arvison Flat to a point about 1 mile north of Arvison Flat at the contact with the underlying Triassic beds. Unfortunately, at its type locality part of the section is lost because of faulting, but it is even more poorly exposed elsewhere. 12 CALIFORNIA DIVISION OF MINES [Special Report 63 4* Photo 3. Arvison formation conglomerate exposed at summit of Stump Creek Butte. The bulk of the formation consists of extensive beds of agglomerate, volcanic breccia, and conglomerate. In- terbedded with these are beds of fine to coarse tuff and tuffaceous sandstone. The lava flows constitute about 10 percent of the rock ; tuff and sandstone 15 percent ; and volcanic breccia and conglomerate 75 percent. The mas- sive, coarsely jointed deposits of this breccia form promi- nent, rocky outcrops on hillsides and support only brushy vegetation ; the breccia is composed entirely of fragments of volcanic rock, probably andesite, and col- ored medium gray on fresh and weathered surfaces. Constituents range in diameter from a quarter of an inch to 3 feet. No sorting is apparent. Some fragments are vesicular. A few small lenses of stratified, tuffaceous sandstone occur at intervals within the breccia. Matrix material is mainly tuff. Conglomerate outcrops are well developed in the vi- cinity of Arvison Flat along Kosk Creek for more than half a mile. They weather to rounded slopes and support forest growth on the lower canyon slopes. The rock forms a 20-foot cliff along the stream bank of Kosk Creek. The conglomerate is composed mainly of fairly well rounded pebbles of volcanic material, with subordinate amounts of gray unfossiliferous limestone pebbles. The matrix is coarse tuffaceous sandstone. The gray calcarenite lenses of the Devils Canyon member of the Modin formation are a possible source for the limestone pebbles. Along the Stump Creek trail, between Kosk Creek and Stump Creek Butte, a lenticular bed of blue-gray lime- stone occurs interbedded between tuff and conglomerate. The limestone is compact, crystalline, and contains inter- ea tions of gray shale and impure limestone. There were fra; ments and casts of fossils but no recognizable forms wert; found. In thin-section the pure limestone layers appe; to be calcarenite. The impure limestone is com- posed luainly of rounded fragments of volcanic rocks, broken shell fragments, some phosphatic material, and calcite cement. The volcanic flows are basic igneous rocks. The flows occur intermittently throughout the formation but are not persistent along the strike. The lava is amygdaloidal in part and the more massive layers are commonly por- phyritic. Under the microscope plagioclase phenocrysts are seen in a groundmass of volcanic glass with abundant microlites. Accessory minerals are largely altered to chlorite or sepentine minerals. The phenocrysts are so badly altered that no definite determination of the com- position of the feldspar can be made, although it appears to be andesine. Tuff, and more commonly tuffaceous sandstone, of a lenticular nature is common in the formation. One such accumulation is particularly important because it yielded fossil forms sufficiently well preserved to make possible the geologic dating of the formation. The fossil- bearing outcrop occurs along the east bank of Kosk Creek, north of the junction of Kosk and Shotgun Creeks. Here the fossiliferous beds are well exposed in a bold 385-foot outcrop of tuff and tuffaceous sandstone with minor layers of fine-grained tuff and shale. Else- where in this formation tuffaceous sediments weather more freely than the associated agglomerate and volcanic breccia, but this outcrop appears to be uncommonly re- sistant, probably due to the presence of calcareous cement in the sandstone. Casts and molds of fossils are abundant in certain layers. In general, fossils are poorly preserved but several recognizable forms were collected. Fragments of carbonized wood are abundant in some of these fossiliferous layers. In the hand specimen, the rock is a greenish-gray, medium- to coarse-grained, tuffaceous sandstone. The color is the same on fresh and weathered surfaces. A thin-section of a typical fossiliferous layer shows that the sandstone is composed of fragments derived Photo 4. Arvison formation volcanic breccia exposed on Kosk Creek. 1960] SOUTHWEST QUARTER OF BIG BEND QUADRANGLE 13 almost wholly from the erosion of volcanic rocks. The grains are medium to fine in size and subangular to subrounded. The cement is partly of clay, somewhat al- tered to chlorite, and partly of calcite. Volcanic rock fragments and tuff constitute about 75 percent of the rock, plagioclase grains about 10 percent, and calcite 5 percent. Minor constituents include chlorite, quartz, and magnetite. From top to bottom the lithology of the Arvison formation as exposed at its type locality is as follows: (The upper contact of the Arvison formation is not ex- posed in Kosk Creek, and the uppermost beds shown below are at the north end of the large meadow which lies just above the mouth of Kosk Creek. The presence of this broad meadow suggests a change in lithology to a less resistant rock and may represent the area of tran- sition to the overlying Potem beds.) Feet (14) (18) (12) (11) (10) (9) (8) (7) (6) (5) (4) (3) (2) (1) Dark gray volcanic breccia and volcanic flows 250 Black hornfels, breccia zone (fault breccia?) with many small quartz veins 250 Andesite porphyry 35 Dark gray agglomerate and volcanic breccia 200 No exposures (hillside talus suggests bedrock of tuff or tuffaceous sandstone) 750 Dark gray agglomerate and volcanic breccia 200 Greenish-gray medium- to coarse-grained calcareous, tuffaceous sandstone and tuff ; very fossiliferous in certain strata. Arnioceras sp. Asteroceras sp. Pecten cf. P. acutiplicatus Meek, Entolium meeki Hyatt, Pinna cf. P. ejpansa Hyatt 385 Medium gray, coarse volcanic agglomerate 800 Fault zone — fine blue-gray fault breccia 100 Medium gray conglomerate, well rounded volcanic and limestone pebbles 750 Fine-grained medium gray tuff, weathers brown 70 Medium gray conglomerate, agglomerate and vol- canic breccia 950 Dark gray volcanic flow, vesicular and amygda- loidal structure 100 Medium gray conglomerate, agglomerate, and vol- canic breccia 250 5,090 Distribution. The Arvison formation occupies a large part of the Big Bend area. It is best exposed along Kosk Creek in its type area where an estimated thickness of 5,090 feet is exposed. East of Kosk Creek it is covered by the Tertiary lavas. West of Kosk Creek it occurs in a broad arcuate belt underlying most of the heavily for- ested, relatively flat region lying north and west of Smith Flat. With greatly diminished thickness and with a more southerly strike, its exposure is traceable into the adjacent Shoeinhorse Mountain quadrangle. Only isolated exposures of this formation are encountered to the west of the ridge west of Kosk Creek, where the streams are in weathered mantle material for the most part. The contact of the Arvison formation with the overlying Potem formation is not exposed, but can be inferred by examination of the mantle rock. This con- tact was placed at the uppermost limit of occurrence of fragments of volcanic breccia in the soil. The nature of the weathered material and talus leads to the belief that the contact is gradational. Fossils and Correlation. Although many of the tuffa- ceous and sandstone layers contain fossil fragments, rec- ognizable forms were found only in the Kosk Creek outcrop described above. A list of recognized fossils from this locality follows: Asteroceras sp. Arnioceras sp. Pecten sp. Pecten cf. P. acutiplicatus Meek Pinna cf. P. expansa Hyatt Entolium meeki Hyatt Pleurotomaria sp. Lingula sp. Stylophyllopsis sp. The ammonite genera Arnioceras and Asteroceras have been reported occurring only in the Sinemurian stage of the European Jurassic. Pecten acutiplicatus, Pinna expansa, and Entolium meeki were reported by Hyatt (Diller, 1908) from the Hardgrave sandstone of the Taylorsville region of Cali- fornia, but are not useful in precise dating of strata. The Arvison formation, on the basis of stratigraphic position as well as similar fossil content, may be cor- related with the Hardgrave formation at Taylorsville. If so, the Hardgrave sandstone occupies a position some- what lower in the stratigraphic column than that as- signed to it by Crickmay (1933) who dated it as early Middle Jurassic. Muller and Ferguson (1939) reported Pecten acutipli- catus, Stylophyllopsis and Entolium cf. E. meeki from the Sunrise formation (Lower Jurassic) of Nevada. Therefore the Arvison formation probably can be cor- related with the middle and upper portions of the Sun- rise formation, and possibly part of the Dunlap forma- tion, also of Nevada. The Arvison is tentatively cor- related with the Donovan formation of central Oregon as described by Lupher (1941) and with the Milton and Sailor Canyon formations of California. The presence of marine fossils in the tuff and tufface- ous sandstone indicate that at least part of this forma- tion was deposited in the sea. The volcanic breccia shows little or no effects of sorting or reworking by water; on the other hand the conglomerate shows the results of considerable reworking of volcanic material. The fauna and the presence of carbonized wood fragments suggest a shallow-water, near-shore environment. The Arvison formation, then, apparently accumulated in a shallow sea, in marked contrast to the deeper environment of deposition of sediments of the Kosk member of the CORRELATION TABLE (WESTERN STATES) STANOARD EUROPEAN SECTION SERIES STAGES BIG BEND TAYLORSVILLE SIERRA NEVADA CENTRAL OREGON NEVADA MIDDLE BATHONIAN ?_?_? POTEM ^^^ <-"BAGLEY < ANOESITE FORMATlON^^ ? -?-? ARVISON FM ... ? ? _ ? _ _ H_,LL_FM _MO_0NJHlN.E_F.tf _ MORMON FM MONTE DE ORO FORMATION JURASSIC 6AJOCIAN THOMPSON FM __£A_ !__:_. HARDGRAVE SANDSTONE MILTON FM (SAILOR CANYON FM) __i_E_E_GR COLPITS GR MOWICH GR TOARCIAN DUNLAP FM LOWER PLIENSBACHIAN DONOVAN FM SINEMURIAN LILAC FM TRAIL FM (OF DILLER1 SUNRISE FM HETTANGIAN RHAETIAN MOOIN FM. UPPER NORIAN GABBS FM BROCK SHALE SWEARtNGER SL HOSSELKUS LS HOSSELKUS LS KARNIAN LUNING FM PIT FM MIDDLE TRIASSIC LADINIAN EXCELSIOR FM Figure 5. 14 CALIFORNIA DIVISION OF MINES [Special Report 63 underlying 1 Modin formation. In the Taylorsville region the comparable beds, transitional between Triassie and Jurassic (Trail formation of Diller [1908, p. 36]), con- tain fresh or brackish water crustacean and plant re- mains. Nature of the Contact of the Triassie and Jurassic Systems The contact between the uppermost Triassie beds and the basal Jurassic strata appears to be unconformable. The contact is an abrupt division between fine-grained, relatively deep-water sediments and thick accumulations of coarse pyroclastic debris containing marine beds with a fauna characteristic of a shallow-water, near-shore en- vironment. Areal distribution of the Modin formation and the overlying Arvison formation also suggests an uncon- formable relationship. Muller and Ferguson (1939) re- port continuous sedimentation between Triassie and Jurassic beds in Nevada about 250 miles southeast of the Big Bend area. From this evidence and from Lupher's (1941) report of an angular unconformity between the Triassie and Jurassic systems in central Oregon, the shoreline of this transitional Triassic-Jurassic sea ap- pears to have been to the southeast of the Big Bend area. Bagley Andesite Definition and Lithologic Character. Although the Bagley andesite overlies and is intercalated with part of the lower beds of the Potem formation in the Big Bend area, it is older than most of the Potem, which is here described as the next younger formation. The Bagley andesite was originally described from the Redding area by Diller (1906) as follows: "The Bagley andesite includes the lavas and pyroclastics of a succession of volcanic eruptions of similar general char- acter. It is commonly filled with an abundance of small phenocrysts of plagioclase, and rarely, also with dark grains in a greenish groundmass." The type area is Bagley Mountain, located in the northeast corner of the Redding quadrangle about 1 mile from the southwest corner of the Big Bend area. The Bagley andesite, in the Big Bend area, is best developed on Oak Mountain, where it reaches an esti- mated thickness of 700 feet. Here it forms a rugged mountain whose flanks are covered by a sparse, brushy vegetation, and whose summit area supports oak tree growth, whereas surrounding areas support fir and pine forests. Tuffaceous layers comprise about 30 percent of the beds, and agglomerate and volcanic breccia about 50 percent. Volcanic flows of andesite porphyry which occur near the ridge between Pit River and Iron Canyon, make up the remaining 20 percent of the forma- tion. A thin-section study of three of the lava flows shows the plagioclase to be andesine. The feldspars appear fresher, in general, than those of the Arvison formation. The phenocrysts are imbedded either in a groundmass of i ack devitrified volcanic glass containing feldspar mi rolites or in an almost noncrystalline groundmass of feh iar laths. On -rence. The Oak Mountain area comprises the largest occurrence of Bagley andesite in the mapped area. The western ridge of the west tributary of Iron Canyon is, omposed entirely of Bagley andesite lavas. Porphyritic extrusive flows, 300 to 400 feet in thick- ness, mapped with the Bagley andesite, are interbedded with Potem strata that contain Middle Jurassic fossils. One such flow crosses the Pit River at Little Joe Flat where it causes a constriction in the river canyon and a sharp bend in the river. An igneous rock that appears to be an intrusive dike or volcanic neck, crops out on both sides of the mouth of Kosk Creek and is shown as Bagley andesite on the map. This dike or neck is located stratigraphically and geo- graphically in such a position that it could have con- tributed lava and debris to the Bagley accumulations. However, field relationships, and laboratory determina- tions showing that the intrusive rock contains a some- what more calcic plagioclase than was observed in Bag- ley rocks, cast doubt on the correlation of this rock mass with the Bagley formation. Age and Correlation. Since the Bagley andesite appears to be a volcanic facies of the Potem formation, its age and correlation is considered to be the same as that of the Potem formation in this area. Potem Formation (Lower and Middle Jurassic) Definition and Lithologic Character. The Potem for- mation in the Redding quadrangle was described by Diller (1906) from the Jurassic beds which overlie the Modin and Bagley formations. The type area is along Potem Creek. The Potem Creek exposures are essentially along the strike near the base of the formation ; the Potem section is better exposed along the Pit River in the 15-minute Montgomery Creek quadrangle of the larger Redding sheet. In the Big Bend region the Potem formation consists of argillites and fine-grained tuffaceous sandstone with small amounts of conglomerate, tuff, medium to coarse Photo 5. Potem formation. Exposure of argillite near head of Iron Canyon. 1960] SOUTHWEST QUARTER OF BIG BEND QUADRANGLE 15 tuffaceous sandstone, and a few beds of limestone and " A "***"a W" 'flffKtVf ^MP^H IF BIC^P'gA, eoarse pyroclastic material. These strata weather deeply *& % £l "^JgfcfciSs ^^'^E^V^^^ B^Jtt^ 1 ^ and form rounded topography. They are exposed only in i^ ! ~ U ^^*f?*^ ''^KfiSi^^^^^^^ fc^^T^- the deep canyons of streams and along the Pit River. r'.r - 'j^te^*- -^^^Sfej |«y --£" .^^i'**^?^ The argillite is massive with minor parting along bed- ^!f^~ 'sP**^ - -I;,,^- ^'V ^**^^^ : '* v ding j)lanes. The partly weathered exposures have a ^-gfefa 'v^* 1 ^* '>*)$$&&y-\*,\ . + shaly appearance. The fresh rock is dark gray to black ^5L*# * . ■"'*' ' * ' '*" j^'*"*^ \-?- v ^ x 4, 4^^. -».- but weathers to a light gray or buff and occasionally % , ^^ftr-^^^*^ -*^ SSL reddish to purplish color. Much of the rock is calcare- ■-- «*» ' ,-"- ■<- ^^ ^jL j^.-. '>"S fc * ll * t '**^^^P ? ^£j^- mis. [n outcrop this rock is similar in appearance to the jff*v -bfc^ jfcal J^^^^m* 1 *^^; ■* *>£'j?fa * '-&_ W^ Kosk member of the Modin formation. The argillite f ^^£3K^ -* \.^^+ -^ ^^''^'J? -~ :*i**\,C *2% / JS!z*% grades locally into dark gray, fine-grained tuffaceous *•**, ■^fr*' >: '' '- -•■*$■ -" *^7 ■'^•it'^W» sandstone which predominates in the lower part of the "^■^'■'^f , '«M--{l; ' ■ t'^*»*' L>' +1. ■ -■* • *"'• *^^i/^8 Potem formation. A few beds of tuff and volcanic breccia -£->*** 4£-H>' '' * ' '* -**■• ■ frr* Vvw"'"'' * ^^' * ^^* as much as 50 feet in thickness crop out along the eastern *!|fel|£tii^^ slope of the west branch of Iron Canyon. -"■m*- *£ T -„,^' .*.vO^ -^"^ - fc *«?P^' ^3l *S In the Pit River Canyon north of Little Joe Flat the . .." ' -W^^S^ft^S^^^^fc ^%^ following beds crop out: - •» • >-'v^ *, " * '' »•- Feet J- .*».■' (5) Interbedded dark gray clastic limestones and medium- ... to fine-grained sandstones, contain I'osidonia alpina, "'%'■'% Trigonia, Rhynchonella 210 - ■ ' ' , (4) Dark gray, massive, fine-grained sandstone weathers - ,. £ ^ ; lavender, contains Lingula sp. and I'osidonia alpina- 150 (3) Gray, coarse- to medium-grained, pebbly sandstone; weathers reddish 200 (2) Medium gray conglomerate, pebbles of volcanic origin, ,, „ _ . , .. „.,.,., i to 2 inches in thickness. Matrix coarse-grained an- l H ° T0 6 " Potem Ration. PostAonta c ifpina-bearing argillite. gular sandstone. Contains Trigonia v-costata, T. un- Exposure in road cut near Pit R.ver Power House No. o. dulata 15 (1) Dark gray argillite, fine-grained tuffaceous sandstone , Q , 1T - T .. ~ „,.„,, and tuffs 410 years ago by S. W. Muller (L.S.J. U. fossil locality 2971). Fossils from these localities are listed below: The clastic limestone, and medium- to fine-grained Lj ma sp. sandstone beds listed as (5) above lie immediately below Pecten sp. the thick andesite porphyry flow mapped as Bagley an- Weyla aff. w. alata (Buch) desite, which crosses the Pit River at Little Joe Flat. £l£!E , !^ 1,abile Hya " The limestone is highly fossiliferous locally, but its mas- Parallelodon sp. sive character makes it difficult to obtain good specimens Myacites depressus Meek from the unweathered material. Volseila sp. Microscopic examination of this limestone showed it This assemblage lacks diagnostic forms but is listed to be a calcarenite. Fine-grained particles of caleite, fos- separately because it comes from strata that appear to sil fragments, microfossils, and caleite cement constitute be stratigraphically lower than those collected from the about 75 percent of the rock ; tuffaceous fragments 10 Pit River. Specimens identified here as Entolium equa- percent, quartz 5 percent, and black organic material 5 bile were compared with the illustrated and described percent. Subspherical, recrystallized caleite bodies sug- form credited to Hyatt by Crickmay (1933), who listed gest the presence of microfossils. it from the Mormon sandstone in the Taylorsville region. _.,„,. ~ , . „ , , tip, Weyla alata is a common Liassic * form and is recorded Distribution. Only a portion of the lower beds of the from near the t of the Sunrise formation of Nevada Potem formation are within the mapped area of the Big by MuUer and Fe rguson who correlated that part of the .Bena region. Sunrise formation approximately with the Lilac forma- Two general areas of Potem exposures occur in the tion described by Crickmay (1933, p. 396) and the Big Bend region. The first and larger area of exposure Hardgrave sandstone in the region of Taylorsville. Inas- is that of Iron Canyon and its tributaries. Excellent ex- much as these strata overlie the Arvison beds of Early posures occur on the east slope of the west branch of Jurassic age and underlie deposits containing a fauna Iron Canyon where the strata are relatively undisturbed considered as Middle Jurassic, it seems probable that the and have a regular northerly strike and easterly dip. In lower Potem beds represent deposits of Early Jurassic Iron Canyon argillite predominates. The second area of age. Potem exposure lies along the Pit River and is connected j n the Pit River section fossils were found in many with the first by exposures around the north end of Oak localities but at approximately the same horizon. This Mountain. In the Pit River section the strike is east and assemblage includes the following forms : the dips are south, some dips are as low as 10°. „ . . , . ,~ . ^ ' l Posidonia alpina (Gras) Fossils and Correlation. Poorly preserved fossils Trigonia v-costata Lycett „ ., , ,., . . ,y i, , „ .. Trigonia undulata t romherz were found at many localities in the rotem formation. Trigonia cf. T. denticulata Agassiz The fauna found in the Iron Canyon area is thought to Tri >nia cf. T. spinulosa Young and Bird be older than that along the Pit River. Included in this Trigonia sp. Study is an assemblage collected in Iron Canyon some • Lias is the oldest division of the European Jurassic system. 16 CALIFORNIA DIVISION OF MINES [Special Report 63 Entolium sp. Astarte sp. Gryphaea sp. Pinna sp. Ostrea sp. Phylloceras sp. Rhynchonella ef. R. varians Schlotheim Lingula sp. Orbiculoidea sp. Astrocoenia sp. Posidonia alpina is abundant in the fine sandstone and argillite in the Pit River section. Guillaume (1927) cited the European range of this fossil as Upper Toarcian to Lower Callovian. Trigonia v-costata, T. undulata, T. spinulosa and T. denticulate/, are all reported from the Jurassic beds of Great Britain and are restricted to the Inferior Oolite (Bajocian stage) of the Middle Jurassic epoch. Rhynchonella varians and related species are com- mon in Lower and Middle Jurassic beds of Europe. In view of the above relationships it appears reasonable to regard those Posidonia and Trigonia bearing beds of the Potem formations as Middle Jurassic in age. The Potem formation yielded little fossil evidence that would suggest correlation with beds in the Taylors- ville region. However, on the basis of stratigraphic posi- tion and age assignment the formation can tentatively be correlated with the Thompson and Mormon forma- tions and associated non-fossiliferous strata. Crickmay (1933) placed the Hardgrave as early Middle Jurassic, equal in age to part of the Potem formation. However, the author is of the opinion that the Hardgrave is better correlated with the Arvision formation, which appears to carry some fossils reported in the Hardgrave sand- stone, whereas the younger Potem formation in the Big Bend area does not contain fossils characteristic of the Hardgrave. Muller and Ferguson (1939) approximately correlated the upper part of the Sunrise formation (lower Pliensbachian stage) with the Hardgrave sand- stone, and also suggested an earlier age for the Hard- grave sandstone than that assigned by Crickmay. The Potem formation is tentatively correlated with the Dunlap formation of Nevada, and the Mowich, Col- pitts, and Izee groups of central Oregon as defined by Lupher (1941). TERTIARY SYSTEM Montgomery Creek Formation (Eocene) Definition and Lithologic Character. The Mont- gomery Creek formation was first mentioned in the liter- ature by Williams (1932, p. 215) and later by Hinds (1933, p. 114) Both authors credited the name to an unpublished manuscript by R. Dana Russell. The forma- tion is extensively exposed along Montgomery Creek in the vicinity of the town of Montgomery Creek in .the Burney quadrangle. Hinds discussed the formation as follows : "Along the southeast slopes of the Klamath Mountains in Kosk Creek and Pit River Canyon, farther to the south on Montgomery Creek and at a few other localities is a series of dominantly fluviatile brownish, arkosic sandstone, sandy ! iales, and conglomerates which Diller, in the Redding Folio, led the 'lone' formation and dated Miocene on the basis of '■mination of fossil leaves made by Knowlton. These strata been recently examined by R. Dana Russell . . . ind will be described by him as the Montgomery Creek formal ion in a report now in preparation." In the Big Bend area this formation is represented by non-marine conglomerate, arkosic sandstone, and shale. Its maximum thickness is approximately 2,600 feet. The conglomerate, which appears to be in lens-like deposits, crops out along the road to Power House No. 5 on the Pit River. It is poorly consolidated, but forms a high, rounded ridge. It has been quarried for use as gravel in nearby dams on the Pit River. This conglom- erate is composed of well rounded pebbles that range from 1 to 8 inches in diameter, the majority being about 2 inches in diameter. About 70 percent of the pebbles are derived from fine-grained igneous rocks, about 20 percent from hornfels, gneiss, and quartzite, and the remaining Photo 7. Low hills in foreground underlain by Montgomery Creek sandstone. Oak Mountain in background composed of Bagley andesite. 10 percent from quartz. The matrix material is a coarse- grained arkosic sandstone cemented by clay and iron oxide. The outcrops are pale gray on fresh surfaces but the weathered material is reddish brown and the pebbles are coated with a red iron oxide stain. The buff arkosic sandstone is the predominant litho- logic type in the formation. This sandstone forms low, gently rolling hills, and the weathered surface charac- teristically contains large, calcareous, dark brown sand- stone concretions 1 to 6 feet in diameter. The rock is fairly soft and friable. The grains are subangular to subrounded in shape and poorly sorted in size. The ap- proximate composition is 40 percent feldspar, 50 percent quartz, 5 percent biotite, with clay cement. The shale beds are brown on fresh exposures, and weather to a light brown or buff color. These beds are soft and incompetent, and form a typical hummocky land-slip topography. Plant remains are common in this shale and a few thin beds of carbonaceous shale and lig- nitic coal are present. The largest coal seam observed was 20 inches in thickness. The coal is too impure and soft for commercial use. 1960] SOUTHWEST QUARTER OP BIG BEND QUADRANGLE 17 3» _/v f VE^P- Photo 8. Montgomery Creek formation. Shale with beds of lignitic coal. Old coal prospect near head of Kosk Creek. Distribution and Age. The Montgomery Creek for- mation has been exposed by the erosion of the Pit River and its tributaries, which have stripped off the covering Tertiary lava cap. South of the town of Big Bend the exposures are confined to an area east and south of the Pit River. Near the Pit River, in the south part of the mapped area, the Montgomery Creek strata have been eroded away, exposing the older rocks below. Exposures occur north of the Pit River along the east side of Kosk Creek south of the junction of Kosk and Shotgun Creeks, They are delimited on the west by Kosk Creek and on the east by the overlying Teritary volcanics. As regards the age of the Montgomery Creek forma- tion, Hinds (1933, p. 115) wrote: ". . . determination by R. W. Chaney of a flora discovered by Russell indicates a later middle Eocene age for the formation and Russell's field and laboratory studies prove that the for- mation is not the lone formation as Allen has recently re- defined it along the eastern margin of the Great Valley." The Montgomery Creek formation overlies the older formations with distinct angular unconformity. Farther to the south in the Redding quadrangle the Upper Cre- taceous "Chico" formation unconformably overlaps the older Mesozoic strata. If the Cretaceous sea extended over the Big Bend area, the sediments were completely eroded away before the Eocene sediments were deposited, as no trace of Cretaceous deposits was found in the area. Tertiary Volcanic Rocks (Pliocene?) A thick and extensive cover of basalt and andesite flows and pyroclastic deposits covers the eastern part of the Big Bend area. Peacock (1931) included the Big Bend area in the Shasta Lavas Highland and considered the age of the flows to be Pliocene (?) on the basis of their general similarity to the lavas regarded as Pliocene in the Cascade province. It appears that Kosk Creek and the Pit River marked the approximate western limit of this lava field. Only at one place in the Big Bend area do Tertiary lavas occur west of this line. This flow is on the ridge between Iron Canyon and its west branch. The lava is basalt, approximately 150 feet thick, and has an areal extent of about half a square mile. It was con- sidered part of the Tertiary lavas because of its relative freshness, composition, and attitude. - ■■ T^ m 4 Photo 9. Tertiary basalt dike cutting Eocene Montgomery Creek conglomerate. ■ r ■ .- Photo 10. Tertiary volcanic rocks in Pit River Canyon at Hagen Flat. Elevation of river 1,800 feet ; that of the peak in background, 5,700 feet. 20 California division of mines [Special Report 63 bratula" piriformis Suess, Lima costata Goldfuss, and L. The Potem formation contains two distinct fossil hori- terqueni Tate. Of the above only Spiriferina suessi is zons. The lower fauna yielded pelecypod forms that are useful in the correlation of these beds with the Upper not useful in dating the beds. However, Entolium equa- Triassic of Europe. Plicatula perimbricata Gabb is bile Hyatt, which was described from the Mormon sand- common to this member and the Upper Triassic Gabbs stone of the Taylorsville area, is common in the lower formation of Nevada. A diligent search of available Potem beds. literature on the Upper Triassic and Lower Jurassic A younger horizon of the Potem formation yielded an series of other parts of the world failed to reveal other abundant fauna which contains pelecypod species useful species similar to those found in the middle member of i n the correlation of these beds with the European the Modin formation. The correlation and age of the Jurassic. Posidonia alpina (Cras) occurs abundantly in member was made on the basis of the above similarities the Potem formation along the Pit River. The European and on its stratigraphic position above beds of known range f this species is Upper Toarcian to Lower Callo- Norian age and below the Kosk member which contains v i an . The genus Trigonia is well represented in local Choristoceras mar sin, a species restricted to strata of beds in the Potem formation. Forms similar to Trigonia Rhaetian age in Europe. The absence of ammonites in v-costata Lycett, T. undulata Fromhertz, T. spinulosa this fauna can be explained in two ways. Either this Young and Bird, and T. denticulata Agassiz are recog- fauna accumulated in an isolated sea or ocean currents nized. These species are characteristic of the Bajocian prevented the accumulation of the buoyant ammonite s tage of the European Middle Jurassic, shells. The presence of the large percentage of forms not These pelecvpods and the presence of the gene ra reported elsewhere suggests a rather isolated environ- 0strea ^ Gryp f laea and Pinna in indicate shallow ment However, it is probable that the fauna wasincom- water eonditions for the fauna of this hori zon. The pletely preserved, as Muller and Ferguson (1939) re- preS ence of Lingula and Orbiculoidea suggest brackish ported that beds of approximately the same age in water collditions during a part of this w Small coral Nevada contain an abundant fauna, of which many reefs composed of Astrocoenia indicate a warm water species are identical with those of Europe. The genera enviroment Lima, Plicatula, Volsella, Lopha, Ostrea, Phillippiella, ' „ _,, . and Pinna indicate an upper neritic or littoral environ- Froni a s ™: e - y of these faunas * appears that environ- ment. Lopha is characteristic of warm water in the mental conditions were remarkably similar in this area present seas. Thus, the Devils Canyon fauna probably from late Tnass ! c thro ^ h the Middle Jurassic. Shallow, lived in warm, shallow seas. Marine strata of the Upper ™ seas predominated and intercommunicating sea Norian and Rhaetian stages (European Upper Triassic) lam f J* 110 * 1 connected with the European area were have not previously been reported from California. probably continuous throughout this entire time. The movement which elevated the Shasta County Triassic The Kosk member of the Modm formation yielded one rocks above the sea at the end of the Rha etian stage species, Choristoceras marshi Hauer. This species is con- apparently did not materially affect the distribution of fined to the Rhaetian stage of the European Triassic. these warm Mesozoic seas so as to create a barrier be- The lack of other fossils and the character of the enclos- tween western North America and the European areas, ing sediments suggest a deep-water environment. Inter- communicating sea lanes with Europe apparently were systematic paleontology present during the Rhaetian age to allow this species to Phylum BRACHIOPODA exist both in Europe and on the west coast of North Class articulata America Order TELOTREMATA mv e c 4.1. a • f *■■ u 4-1, + •+ Family SUESSIIDAE Waagen The fauna of the Arvison formation shows that its en- Genus spiriferina D'Orbigny, 1850 vironment is very similar to that of the Devils Canyon Spiriferina cf. 8. suessi Winkler,' 1882 fauna. The Arvison fauna is composed mainly of pele- Plate 2, Figures 7 and 8 cypods, many of which indicate upper neritic or littoral Description. Shell inequivalved. Ventral valve pyramidal with environments. The presence of carbonized wood frag- median sinus extending from the beak to the anterior margin. mentS also suggests close proximitV to land. Specimens Dorsal valve somewhat arcuate with median fold extending from of the cephalopod genera Asteroceras and Arnioceras *« L^f 1 ff *? ^ e an * eri . or ™ argin - C t t rdi K ^Ti^' ^ » -i • V • » • mi we 'l developed delthvnum beginning near the beak of the ventral were found in this formation. These genera are re- valve widen ing and 'terminating at the hinge line. Valves radially Stricted to the Sinemurian Stage of the Lower Jurassic striate. Fine growth lines more or less regularly spaced on both in Europe and make it possible to correlate the fossil- valves. Shell material coarsely punctate, bearing horizon of the Arvison formation with that Dimensions. stage. The presence of these genera, which are restricted Width 24 mm. to a narrow stratigraphic range indicates that the early Height 36 mm Jurassic seas, like those of the late Triassic, were con- Height cardinal area ~~~~~"~~~~~~~ 23 mm! nected to the European region by east-west sea lanes. ,„ TT , T .... „„_„ , „ Q ^^ T . .. p. „ r d ^ Locality. L.S.J.U. Localities 2!)<0 and 29 j- 7- i j 7JT™,* 7- e v *^„„i„; the Modin beds is too poor to permit observations of the internal ville ion. P. acuttphcatus and Entolium cf. E. meeki partSj i(lentification mu ' st depen ^ on a c ,, mi , iU . is()I1 of the external are i rted from the Sunrise formation (Lower Juras- features. Zugmayer states that Spiriferina suessi appears exeiu- sic) Of N: .ada. sively in the Koessener strata of the Rhaetian stage and in the 1960] SOUTHWEST QUARTER OP BIG BEND QUADRANGLE 21 Starhemberger beds. Trechman (1918) has named a variety from Xew Zealand, S. suessi var. attstralis, which occurs in the HaJobia beds of Karnian age. Family RHYNCHONELLIDAE Grav Genus RHYNCHONELLA Fisher, 1809 "Rhynchonella" richardsoni Smith, 1927 Plate 2, Figures 5 and 6 Description. Shell small ; quadrangular in shape, biconvex ; beak small, pointed and not prominent. Ornamentation of 8 suhangular costae continuous from umbo to anterior margin. Dorsal valve with high ridge containing three plicae ; ventral valve with deep sinus containing two plicae. Dimensions. Length 12 mm. Width 13 mm. Height 9 mm. Locality. L.S.J.U. 2970. Devils Canyon member of the Modin formation. Geologic age. Late Norian or Rhaetian age, Late Triassic epoch. Discussion. As the internal characters of this specimen could not be observed, definite identification of this form was not possible as to genus or species. However, this specimen appears to be iden- tical in external features with topotype specimens of R. richardsoni. Rhynchonella cf. R. varians Schlotheim, 1820 Plate 2, Figures 3 and 4 Description. Shell inequivalved ; triangular to quadrangular in shape ; biconvex ; beak small, recurved toward umbo. Ornamenta- tion consists of 18 to 26 subangular costae continuous from umbo to the anterior margin on both valves. The ventral valve with a prominent sinus containing three to six costae ; seven to eight costae on each lateral slope. The dorsal valve with a strong medial fold containing two to five costae. Internal structures not preserved. Dimensions. Length 14-21 mm. Width 16-22 mm. Height 11-14 mm. Locality. L.S.J.U. 2982. Potem formation. Geologic age. Bajocian age, Middle Jurassic epoch. Discussion. Definite specific identification was not possible be- cause internal features were not preserved. This form closely resembles several Lias.sic and Middle Jurassic species including Rhynchonella schopeni Di Stefano, R. plicatissima Quenstedt, and R. angulata Sowerby. Thirty specimens were found at Locality 2982. The variability of characters is such that no two specimens are alike. There is considerable variation as to shape, number of costae on the valves and on the sinus and fold, and in convexity of the valves, although the latter may be due in part to compression. It appears that this form of Rhynchonella is extremely variable and individual specimens might be taken for different species. How- ever, the intimate association of these forms in the rock, which in places is largely composed of these brachiopods, suggests that the various forms belong to one species. Family TEREBRATULIDAE Gray Genus "TEREBRATULA" Miiller, 1776 "Terebratula" sp. The Devils Canyon member of the Modin formation contains numerous T 'erebrat it la -like forms. The external features are clearly preserved. However, the preservation of the internal structures is so poor that sectioning failed to give sufficient information for generic as well as specific information. On the basis of external features alone, at least two distinct forms are recognizable. The larger of the two is oval in shape, has a shallow sinus and fold confined to the anterior portion of the shell, contains fine but well defined growth lines. Both valves are strongly arched. The beak is prominent and curved slightly forward. This form closely resembles "Terebratula" pyriformis Suess, common in the Norian stage. The second form is nearly circular in outline. Both valves are arched and show only slight plication or none along the margin. The juncture of the valves is slightly curved or straight. Growth lines are faint. This form resembles the genus Zugmeyeria Waagen, a Rhaetian form. Phylum MOLLUSCA Class CEPHALOPODA Order AMMONOIDEA Family AECOCERATIDAE Neumayr Genus ARNIOCERAS Agassiz-Hyatt, 1867 Arnioceras sp. Plate 2, Figures 1 and 2 Description. Shell discoidal, evolute ; cross section of whorl sub- quadrate. Whorls increase in size gradually. Shell smooth in the initial three and one-half whorls. Outer whorls ornamented with radial ribs which number about thirty in the outer whorl at the diameter of 18 mm. Ribs flat and broad in earlier portion of shell hut narrow and acute in the outer whorl. Ribs extending from the umbilical shoulder to the ventral shoulder, where they are abruptly terminated by turning orally to form well developed lateral keels. Venter broad, with well defined keel and shallow lateral furrows. Ventral shoulders sharply rounded ; flanks nearly flat ; umbilical shoulder sharply rounded overhanging the umbilical seam. Sutures not observed. Dimensions. Diameter of shell (DS) 18.0 mm. Diameter of umbilicus (DU) 8.6 mm. Height of outer whorl (HOW) 6.2 mm. Width of outer whorl (WOW) 6.0 min. Height of preceding whorl 3.2 mm. Width of preceding whorl 3.0 mm. Ratios. HOW = 1 DS = 2 HOW = l ds 3 TTu T wow T Locality. L.S.J.U. 2968. Arvison formation. Geologic age. Sinemurian age, Early Jurassic epoch. Discussion. Only a single specimen was found of this form, which closely resembles the figure and descriptions of Arnioceras hartmunni (Oppel) and Arnioceras bodleyi (Buckman) (Hyatt 1899). Identification as to species was not made because the suture could not be seen and, as the specimen was small, it was thought that this might be a juvenile form or the inner whorls of a broken specimen. The genus itself, however, is of such short geologic range that it serves as an excellent index fossil. Genus ASTEROCERAS Hyatt, 1867 Asteroceras sp. Plate 2, Figures 18 and 19 Description. Shell discoidal, somewhat involute. Last whorl covers about two-thirds of the preceding whorl. Whorls increasing in size rapidly. Flanks slightly convex, sloping toward the venter. Venter narrow with depressed keel and well developed lateral keels. Ventral shoulder rounded at lateral keel. Umbilical shoulders acutely rounded and overhanging the umbilical seam. Flanks nearly smooth in the body whorl but having rounded ribs in the chambered part of the shell. Suture not well defined. Dimensions. Diameter of the shell (DS) Approx 43 mm. Diameter of the umbilicus (DU) Approx 12 mm. Height of the outer whorl (HOW) Approx 22 mm. Width of the outer whorl (WOW) Approx 18 mm. Ratios. HOW = 1 DS = 3^7 HOW = 11 DS 2 DU ~T~ WOW 9 Locality. L.S.J.U. 2968. Arvison formation. Geologic age. Sinemurian age, Early Jurassic epoch. Discussion. This specimen is a broken fragment. It has the general form, degree of involution, suture, and simplification of ornamentation of the outer whorls which are often characteristic of Asteroceras. It is very similar to Asteroceras impendens (Young and Bird), but the keel is more depressed in the Arvison specimen. (Wright 1881, plate XXIIA, Fig. 1-3.) It is also similar to A. brooki (Sowerby) and A. turneri (Sowerby) but differs in its smaller size, greater degree of involution, and early development of gerontic simplification of ribs in the body whorl. DIVISION OF MINES SPECIAL REPORT 63, PL. 2 FOSSILS 1960] SOUTHWEST QUARTER OF BIG BEND QUADRANGLE 23 EXPLANATION OF PLATE 2 Figures 1 and 2. Arnioceraa sp. L.S.J.U. Loc. No. 2068. Sinemurian Age, Early Jurassic. Arvison formation. Figure 1 Side view X3 Figure 2 Venter X3 Figures 3 and 4. Rhynchonella cf. R. varians Schlotheim L.S.J.U. Loc. No. 2982. Bajocian Age, Middle Jurassic. Potem formation. Figure 3, Dorsal valve. Figure 4, Anterior. Figures 5 and 6. "Rhynchonella" richardsoni Smith L.S.J.U. Loc. No. 2970. Late Norian or Rhaetian Age, Late Triassic. Devils Canyon member, Modin formation. Figure 5, Dorsal valve. Figure 6, Anterior. Figures 7 and 8. Spiriferina cf. 8. suessi Winkler L.S.J.U. Loc. No. 2970. Late Norian or Rhaetian Age, Late Triassic. Devils Canyon member, Modin formation. Figure 7, Ventral valve. Figure 8, Posterior. Figure 9. Pinna cf. P. expansa "Hyatt" Crickmay L.S.J.U. Loc. No. 2972. Late Early Jurassic or Early Middle Jurassic. Potem formation. Side view, both valves. Figure 10. Pinna sp. L.S.J.U. Loc. 2970 Late Norian or Rhaetian Age, Late Triassic. Devils Canyon member, Modin formation. Side view. Figures 11 and 12. Volsella sp. L.S.J.U. Loc. No. 2970. Late Norian or Rhaetian Age, Late Triassic. Devils Canyon member, Modin formation. Left valves. Figure 13. Entolium meeki "Hyatt" Crickmay L.S.J.U. Loc. No. 2968. Sinemurian Age, Early Jurassic. Arvison formation. A single valve. Figure 14. Entolium equable "Hyatt" Crickmay. L.S.J.U. Loc. No. 2568. Late Early Jurassic or Early Middle Jurassic. Potem formation. A single valve. Figures 16 and 17. Choristoceras marshi Hauer L.S.J.U. Loc. No. 2981. Rhaetian Age, Late Triassic. Kosk member, Modin formation. Figure 17, Venter X4 Figure 16, side view X4 Figures 18 and 19. Asteroceras sp. L.S.J.U. Loc. No. 2968. Sinemurian Age, Early Jurassic. Arvison formation. Figure 19, Venter. Figure 18, Side view. Figures 20 and 15. Trigonia cf. T. v-costata Lvcett L.S.J.U. Loc. No. 2966. Bajocian Age, Middle Jurassic. Potem formation. Figure 20, left valve. Figure 15, right valve, young specimen. Plaster cast. Figure 21. Pecten cf. P. acutiplicatus Meek. L.S.J.U. Loc. No. 2968. Sinemurian Age, Early Jurassic. Arvison formation. A single valve. Figure 22. Lopha sp. L.S.J.U. Loc. No. 2970. Late Norian or Rhaetian Age, Late Triassic. Devils Canyon member, Modin formation. A single valve. Figure 23. Trigonia cf. T. spinulosa Young and Bird. L.S.J.U. Loc. No. 2982. Bajocian Age, Middle Jurassic. Potem formation. Right valve. Figure 24. Posidonia alpina (Gras). L.S.J.U. Loc. No. 2963. Bajocian Age, Middle Jurassic. Potem formation. A single valve. X4 Figure 25. Trigonia cf. T. denticulata Agassiz. L.S.J.U. Loc. No. 2982. Bajocian Age, Middle Jurassic. Potem formation. Both valves. Figures 26 and 27. Plicatula perimbricata Gabb. L.S.J.U. Loc. No. 2970 and 2975. Late Norian or Rhaetian Age, Late Triassic. Devils Canyon member, Modin formation. Figure 27, clay cast. Figure 28. Trigonia undulala Fromherz. L.S.J.U. Loc. No. 2966. Bajocian Age, Middle Jurassic. Potem formation. Left valve. 24 CALIFORNIA DIVISION OF MINES [Special Report 63 Family CERATITIDAE Mojsisovics Genus CHORISTOCERAS Hauer, 1865 Choristoceras marshi Hauer, 1865 Plate 2, Figures 16 and 17 Description. Shell evolute, showing a tendency to uncoil in later whorls. Cross section of outer whorl subcircular. Whorls increasing in size gradually. Shell ornamented with coarse rounded ribs, about forty in the outer whorl at the diameter of 14.6 mm. Ribs charac- terized by the presence of two lateral nodes on the venter. Nodes forming two parallel lateral rows on the venter. Ribs nearly obso- lete across venter between nodes. Suture not observed. Dimensions. Diameter of the shell (DS) 14.6 mm. Diameter of the umbilicus (DU) 5.1 mm. Height of the outer whorl (HOW) 4.8 mm. Width of the outer whorl (WOW) 4.9 mm. Ratios. HOW = 1 DS = 3 WOW = 1 DS 3 DU 1 HOW 1 Locality. L.S.J. U. localities 2981, 2969 and 2976. Kosk member, Modin formation. Geologic age. Rhaetian age, Late Triassic epoch. Discussion. This fossil was compared with descriptions and illustrations of Mojsisovics (1893) and, although the suture could not be observed, the external characters of the Kosk Creek speci- men appear to be identical with those illustrated by Mojsisovics. Several specimens were obtained with ornamentation moderately well preserved. The amount of uncoiling is slight in these speci- mens. However, it is possible that the extended parts of the un- coiled whorl may have been very fragile and easily lost before preservation. The genus Choristoceras is restricted to the upper Norian and Rhaetian stages of the European Triassic. Choristoceras marshi is a Rhaetian form reported from the clays and marly limestone of the Koessener beds in the northern Alps. Class PELECYPODA Order PRIONODESMACEA Family OSTREIDAE Lamarck Genus LOPHA Bolten, 1798 Lopha sp. Plate 2, Figure 22 Description. Shell subquadrate in form ; anterior margin con- vex ; posterior margin concave. A long medial ridge extending from the umbonal area to the ventral margin. Supplementary ribs ex- tending from the central ridge to anterior and posterior margins, the juncture with opposite valve being strongly plicate. Ribs smaller and more numerous on the posterior margin. Dimensions. Length 48 mm. Width 26 mm. Convexity of the right valve 21 mm. Locality. L.S.J.U. 2975. Devils Canyon member of the Modin formation. Geologic age. Late Norian or Rhaetian ages of the Late Triassic epoch. Discussion. The various species of this genus are difficult to distinguish even with well preserved material. Therefore, the strati- graphic value of poorly preserved specimens is questionable. How- ever, the form is important as an indicator of environment. In the present seas it is characteristic of warm water and shallow depth and we may assume that the same relationship obtained in the past. Family PECTINIDAE Lamarck Genus ENTOLIUM Meek, 1864 Entolium meeki "Hyatf'-Criekmay, 1933 Plate 2, Figure 13 -iption. Shell nearly circular with peripheral margin con- it the beak at an angle of 109°. The area near the umbo with -al concentric ridges and the vicinity of the border marked by tv. (acentric lines. Remaining portion of the shell smooth. Ears tic ;!■ equal, their dorsal margins meeting at an angle of approximately 180°. Dimensions. Length 34 mm. Width 35 mm. Convexity of one valve 2 mm. Locality. L.S.J.U. 2968. Arvison formation. Geologic age. These specimens are external molds in tuffaceous sandstone. The concentric ribbing near the umbo is nearly lost but can be recognized. This form has been reported from the Hardgrave sandstone of the Taylorsville area of Plumas County, by A. Hyatt (1892) and by Crickmay (1933). Although Entolium meeki was listed by Hyatt in 1892, it remained a nomen nudum until 1933 when it was described and illustrated by Crickmay from Hyatt's original collection. Crickmay credited the name to Hyatt. Entolium equabile "Hyatf-Crickmay, 1933 Plate 2, Figure 14 Description. Shell nearly circular in outline on the ventral two- thirds of the valve, above which point both margins converge in nearly straight lines to the umbo, forming an obtuse angle. Valves slightly inflated. Ears small, nearly symmetrical, extending above beak. Shell surface nearly smooth, marked only by very faint and closely spaced concentric striae. Dimensions. Length , 25 mm. AVidth 21 mm. Convexity one valve 4 mm. Apical angle 97° Locality. L.S.J.U. Localities 2563 and 2971. Potem formation. Geologic age. Late Early Jurassic or Early Middle Jurassic. Discussion. This form was reported by Hyatt (1892) and Crickmay (1933) from the Mormon sandstone of the Taylorsville area in Plumas County, California. Entolium equabile remained a nomen nudum from the time of its first mention by Hyatt until Crickmay included it among his descriptions of Hyatt's material. Crickmay (1933) credited the name to Hyatt. Family MYTILIDAE Fleming Genus VOLSELLA Scopoli, 1777 Volsella sp. Plate 2, Figures 11 and 12 Description. Shell elongate, solenoid, inflated, becoming com- pressed posteriorly. Oblique ridge on both valves extending from the beak to posterior ventral border. Ventral border nearly straight, dorsal border arcuate, curving downward posteriorly to the junc- tion with the ventral border. The junction of the valves forming an acute angle. Beak subterminal. Growth lines on the ventral part parallel to the ventral margin as far as the oblique ridge, thence curving back sharply to the dorsal border. The dorsal portion of the shell from the oblique ridge to the dorsal border covered with fine transverse striae which slightly overlap the ridge posteriorly but not extending to the ventral border. Striae immediately dorsal to the ridge, forming a slight chevron-shaped angle, with apex directed beakward. Dimensions. Length 72 mm. Width 23 mm. Convexity of one valve 15 mm. Locality. L.S.J.U. 2970 and 2975. Devils Canyon member of the Modin formation. Geologic age. Late Norian or Rhaetian age of the Late Triassic epoch. Discussion. This is an abundant and characteristic form from the middle of the Modin formation. Its presence suggests a shallow water, near-shore environment for the associated fauna. This fossil is similar to Volsella sowerbyana (D'Obigny) but differs in the fact that the concentric growth lines are less prominent and do not appear to bifurcate on passing the ridge. It also differs in the possession of transverse striae which gives the shell a cancellate ornamentation. This form also is similar to Lithophaga verbeeki Boettger. 1960] SOUTHWEST QUARTER OF BIO BEND QUADRANGLE 25 Family PINNIDAE Meek Genus PINNA Linnaeus, 1758 Pinna sp. Plate 2, Figure 10 Description. Shell probably quadrate in cross section (most specimens deformed by compression). Compressed, the angle be- tween the two margins approximately 90° near the beak and 45° near the posterior margin. Hinge line and ventral margin long and straight. Anterior extremity and the umbones not preserved. Ornamentation of radial ribs equally prominent on both valves and much fainter, irregularly spaced, concentric growth lines. Specimens characterized by rugose, irregular folding which is roughly normal to the radiating ornamentation, apparently a secondary feature possibly the result of compaction forces which squeezed and compressed the thin shell material while it still remained in its normal upright position in the sea-bottom muds. Dimensions. Length (reconstructed) 79 mm. Width varies from about 5 mm. at umbones to approxi- mately 37 mm. at extremity. Convexity varies from about 5 mm. at umbones to ap- proximately 25 mm. at extremity. Locality. L.S.J.U. Localities 2970 and 2975. Devils Canyon member of the Modin formation. Geologic age. Late Norian or Rhaetian age, of the late Triassic epoch. Discussion. This form is probably a new species but the poor and incomplete preservation of the specimens prevented its recog- nition as such. The genus has a long stratigraphic range and may be regarded as a persistent one. Several descriptions and figures of this type of Pinna, ranging in age from Triassic to lower Ter- tiary, were studied. Few if any significant morphologic differences could be observed which Would serve to distinguish these Modin specimens from others of earlier or later ages. This form compares very favorably with the Pinna cf. P. blanfordi Boettger described from the Rhaetian beds of Upper Burma ( Healey, 1908). Pinna cf. P. expansa "Hyatt" Crickmay, 1933 Plate 2, Figure 9 Description. Shell probably quadrate in cross section, com- pressed, angle between two margins approximately 120°. Fine line and ventral margin long and straight. Anterior extremity and umbones not preserved. Ornamentation consists of 14-16 radial ribs equally prominent on both valves. Dimensions. Length (reconstructed) 85 mm. Width varies from about 5 mm. at umbones to approxi- mately 35 mm. at extremity. Convexity varies from about 5 mm. at umbones to ap- proximately 40 mm. at extremity. Locality. L.S.J.U. locality 2072. Potem formation. Geologic age. Late Early Jurassic or Early Middle Jurassic. Discussion. This form was reported by Hyatt (1892t and Crickmay (1933) from the Hardgrave sandstone of the Taylors- ville area of Plumas County, California. Pinna expansa remained a nomen nudum from the time of its first mention by Hyatt until Crickmay included it among his descriptions of Hyatt's material. Family SPONDYLIDAE Fleming Genus PLICATULA Lamarck, 1819 Plicatula perimbricata Gabb, 1870 Plate 2, Figures 26 and 27 Description. Shell inequivalved ; larger valve convex and con- siderably inflated in the central portion of the shell ; smaller valve flat or slightly concave or convex. Margins rounded but converging rapidly toward the beak in the dorsal half of the shell. Valves ornamented by well defined imbricating concentric ridges and somewhat less prominent radial plications. Beaks indistinct. Hinge with two strong teeth with resilifer pit between them. No attach- ment scar apparent. Dimensions. Length 36 mm. Width 25 mm. Convexity of larger valve 20 mm. Locality. L.S.J.U 1 . localities 2970 and 2975. Devils Canyon member of the Modin formation. Geologic age. Late Norian or Rhaetian age of the Late Triassic epoch. Discussion. This species is quite common in the middle of the Modin formation where a large number of specimens were col- lected. They are quite variable in size and shape but are constant in the characteristic imbrication of the concentric growth lines. No scar of attachment was observed on any of the specimens but this may be a local characteristic, as the original substratum was a soft calcareous mud. Plicatula perimbricata as originally de- scribed by Gabb (1870) was found at the "Jurassic" locality of Volcano, Nevada. Subsequent study by S. W. Muller (personal communication) has revealed that the Plicatula perimbricata- bearing strata at this locality are actually Upper Triassic. Com- parison of the Modin specimens with a number of P. perimbricata collected by Muller at the type locality of the Triassic Gabbs formation show the California forms to be conspecific with the Nevada specimens. Family PTERIJDAE Meek Genus POSIDONIA Bronn, 1828 Posidonia alpina (Gras), 1852 Plate 2, Figure 24 Description. Shell thin, inequilateral, oval, compressed; periph- ery rounded to subquadrate ; hinge line short, straight. Beak generally below the hinge line, rarely extending slightly above it, and anterior of the median line. Ornamentation of strong concen- tric costae which vary from coarse to fine. Costae rounded and wider than the interspaces, converging near the hinge line and generally coarser in the earlier portion of the shell, becoming progressively finer toward the periphery. Well preserved specimens with a posterior furrow from the umbilical region to the ventro- posterior border. Dimensions. Length 6-13 mm. Width 4- 9 mm. Locality. L.S.J.U. localities 2963, 2964, 2977 and 2982. Potem formation. Geologic age. Bajocian age of the Middle Jurassic epoch. Discussion. This form is similar in size and shape to Posidonia bronni Voltz but differs in the anterior rather than the median lo- cation of the umbo. It is also very similar to P. ornati Quenstedt, P. buchi Roemer, P. parkinsoni Quenstedt, P. opalina Quenstedt and P. suessi Oppel. According to a study of Posidonia made by L. Guillaume (1927), the group named above are all conspecific with P. alpina; the described differences all fall well within the variation of the species P. alpina and the manner of preservation of the fossil material. Guillaume gives the geologic range of P. alpina as from Upper Toarcian to Lower Callovian stages. Family TRIGONIIDAE Lamarck Genus TRIGONIA Bruguiere, 1789 Trigonia cf. T. spinulosa Young and Bird, 1828 Plate 2, Figure 23 Description. Shell ovately triangular, convex, somewhat de- pressed. Area relatively broad, representing approximately one-half the area of the anterior portion, ornamented with fine, more or less regular striae which meet the carina at a right angle and give rise to small tubercles on the carina. Carina very slightly curved, rather prominent, tuberculated. The anterior portion orna- mented with approximately fifteen tubercated costae of which ten appear on this broken specimen. Costae arranged concentrically, arising at the carina at an acute angle, and curving to the anter- ior border. Size of the costae and tubercles increasing to a maximum at a point about three-eighths of an inch from the carina and diminishing toward the anterior border. Details of the area and escutcheon-not-preserved. Dimensions. Length approximately 48 mm. Width 53 mm. Convexity one valve 16 mm. Locality. L.S.J.U. 2982. Potem formation. Geologic age. Bajocian age of the Middle Jurassic epoch. Discussion. This form is represented by a broken, incomplete portion of one valve. This fossil also shows similarities to T. forwosa Lycett and T. striata Miller but differs from the former by the presence of fewer ribs and from the latter by the presence of a prominent carina. All three forms are confined to the In- ferior Oolite (Bajocian stage-Middle Jurassic) of the British Isles. 26 CALIFORNIA DIVISION OK At IN lis [Special Report 63 Triffonia cf. '/'. denticuluta Agassi/, 1840 I'hite 2, Figure 25 Description. Shell ovately trigonal, convex. Umbones moder- ately pointed, incurved. Area ornamented by several rows of diverging denticulated costae extending from the umhones to the posterior border, at least one being much more prominent than the rest. Area flat near the umbones becoming somewhat convex near the border. Marginal carina curved, prominent, denticulate. Ribs about twenty in number, prominent, uniform, meeting the marginal carina at an acute angle directed toward the umbo and curving to the anterior border with a gentle undulation. Dimensions. Length 41 mm. Width 37 mm. Convexity one valve 17 mm. Locality. L.S.J.U. 2982. Potem formation. Geologic aye. Bajocian age of the Middle Jurassic epoch. Discussion. This member of the Costatae group resembles in part the following species; T. costata Sowerby, 7'. tenuicosta Lycett, and T. bella Lycett. All of the above, including 7'. denti- culata, are reported from the Inferior Oolite (Bajocian stage- Middle Jurassic) of the British Isles. Triyonia cf. T. v-costata Lycett, 1850 Plate 2, Figure 20 Description. Shell ovate, trigonal, moderately convex, anterior border curved, hinge border slightly concave. Area narrow, con- cave beneath the apices but flattened posteriorly. Anterior portion with 14-16 costae which are moderate in size, subtubercated near the point of greatest curvature. Costae commencing at the anterior border and curving obliquely downward, the six costate below the umbo curving upward to the marginal carina, those below less curved and more oblique, the anterior portions forming an acute angle with the posterior continuations of the costae. Dimensions. Length 45 mm. Width 25 mm. Convexity one valve 1(5 mm. Locality. L.S.J.U. 2966. Potem formation. Geologic aye. Bajocian age of the Middle Jurassic epoch. Discussion. This species differs from T. v-costata in the pos- session of fewer costae. but as the full description of T. v-costata allows for considerable variation of the species, this form could well fall into the species. T. v-costata is- confined to the Inferior Oolite (Bajocian stage-Middle Jurassic) of the British Isles. The genus Vaugonia set up by Crickmay (1930) contains two species, V. veronica and V. mariajosephinae, both of which would have fallen into Lycett's concept of Triyonia r-costata. This new genus includes the undulated trigoniae that are characterized by a break or interruption of the costae at the apex of the ''V" on those forms that show Y-shaped costae. The age assigned to the rocks which contain the genus Vauyonia is Middle Jurassic. The form described here differs from Vaugonia in its larger size, un- broken costae and number of curved costae below the beak that do not form a "V. Triyonia undulata Fromherz, 1840 Plate 2, Figure 28 Description. Shell ovate-trigonal, convex. Anterier border curved sharply near the umbo, decreasing downward. Hinge border straight. Area narrow, costellae not preserved, marginal carina weak. Anterior portion with 12-14 costae of moderate size curving downward near the middle of the anterior portion, then upward to the marginal carina and joining the latter with an acute angle. Dimensions. Length 30 mm. Width 20 mm. Convexity one valve 8 mm. Locality. L.S.J.U. 2966. Potem formation. ologic age. Bajocian age of the Middle Jurassic epoch. ssion. This species is a common and characteristic form of iddle Jurassic of Europe. BIBLIOGRAPHY Crickmay, C. H.. 1930, The Jurassic rocks of Ashcroft, British Columbia : Univ. California Dept. Geol. Sci., Bull. 19, no. 2, pp. 23-74. Crickmay, C. H., 1933. Mount Jura investigation : (Jeol. Soc. America Bull., vol. 44, pp. 895-926. Diller, J.S.. 1889, Geology of the Lassen Peak district: U.S. Geol. Survey Ann. Rept. X. pp. 401-432. Diller, J. S., 1892. Geology of the Taylorsville region: Geol. Soc. America Bull., vol. 3, pp. 369-394. Diller, J.S., 1893. Cretaceous and early Tertiary of northern California and Oregon : (Jeol. Soc. America Bull., vol. 4, pp. 205- 224. Diller, J.S.. 1906, Redding folio. California: U.S. Geol. Survey Geol. Atlas of the U.S., folio 138. Diller. J.S.. 1908, Geology of the Taylorsville region of Cali- fornia : U.S. Geol. Survey Bull. 353. Fairbanks, H.W., 1893, Geology and mineralogy of Shasta County: California Min. Bur. Rept. 11, pp. 24-53. Fairbanks, H.AV., 1894, Notes on some localities of Mesozoic and Paleozoic in Shasta. California : Am. Geologist, vol. 14, pp. 25-31. Gabb, W.M., 1870, Descriptions of some secondary fossils from the Pacific states: Am. Jour. Conchology, vol. 5. pp. 5-18. Guillaume. L, 1927, Revision des Posidonomyes Jurassiques : Soc. (Jeol. France, ser. 4, vol. 27, pp. 217-234. Healey, M., 1908, The fauna of the Napeng beds of the Rhaetic beds of Upper Burma : (Jeol. Survey of India, Palaeontologia Indica, vol. 2, n.s. Mem 4. Hinds, X.E.A., 1933. Geologic formations of the Redding, Weav- erville districts, northern California : California Jour. Mines and Geology, vol. 29. no. 1, pp. 76-122. Hinds. N.E.A., 1935, Mesozoic and Cenozoie eruptive rocks of the southern Klamath Mountains, California : Univ. California Dept. (Jeol. Sci. Bull., vol. 23. no. 11, pp. 313-380. Hyatt, A., 1889, Genesis of the Arietidae: Smithsonian Inst. Contr. to Knowledge, vol. 26. Hyatt. A., 1892, Jura and Trias at Taylorsville, California: (Jeol. Soc. America Bull., vol. 3, pp. 395-412. Lupher, R.L., 1941, Jurassic stratigraphy of central Orgon : Geol. Soc. America Bull., vol. 42, pp. 219-270. Lycett, J., 1874, A monograph of the British fossil Triyoniae : Palaeontographical Soc, vol. 28, pp. 54-92. Lycett, J., 1877, A monograph of the British fossil Trigoniae : Palaeontographical Soc, vol. 31. pp. 149-204. Meek, F.B., and Gabb, W.M., 1864, Paleontology of California : California Geol. Survey, Paleontology, vol. 1. Mojsisovics, E., 1893, Die Cephalopoden der Hallstsetter Kalke : Abhand. der K.K. Geol. Reich., Wien, pp. 56-563. Muller, S.W., and Ferguson, H.G., 1939, Mesozoic stratigraphy of the Hawthorne and Tonopah quadrangles, Nevada : Geol. Soc. America Bull., vol. 5, pp. 1573-1624. Peacock, M.A., 1931. The Modoc lava field, northern California : Geog. Review, vol. 21, no. 2, pp. 259-275. Smith, J. P., 1894, Metamorphic series of Shasta County : Jour. Geology, vol. 2, pp. 588-612. Smith, J. P., 1927, L'pper Triassic marine invertebrate faunas of North America: U.S. Geol. Survey Prof. Paper 141. Trechman, C.T., 1918, The Trias of New Zealand ; Qr. Jour. Geol. Soc, vol. 73, pp. 165-244. Whitney, J.D., 1865, Geology of California : California Geol. Survey, Geology, vol. 1. Williams, H., 1932, Geology of the Lassen Volcanic National Park, California : Univ. California Dept. Geol. Sci. Bull., vol. 21, no. S, pp. 195-385. Wright, T., 1881, Monograph of the Lias Ammonites of the British Islands : Palaeontographical Soc, vol. 35, pp. 265-328. Zugmayer, H., 1882, Untersuchen ueber Rhiitische Brachiopo- den : Breitraege zur Paliiontologie Osterreich-Ungarns und des Orients, Wien. A251 :!,r,oo pl-IHteil IH CALIFORNIA STATE PK1NTINC OM-1CL DIVISION OF MINES IAN CAMPBELL, CHIEF STATE OF CALIFORNIA DEPARTMENT OF NATURAL RESOURCES SPECIAL REPORT 63 PLATE I EXPLANATION m Montgomery Creek fm mm Polem formolion and Boqley ondesite Arvison lormotion •fem-k •^m-d "fern-h Mo din (orm a Block shole ^ Hosselkus hmeslone P.I formation Approximate contacl Conceded contacl 122*00' T. 36 H.- R Map base Irom U.S. Forest Service w Jp Jb ~~~~l V T " \. \ IJVrmc l"00" + SI Strike and dip of beds rike and dip ot undulating bed • 2972 Fossil locolity T 36 U . - H. 1 E OVr SPRINGS GEOLOGIC MAP AND SECTIONS OF PART OF THE BIG BEND QUADRANGLE SHASTA COUNTY, CALIFORNIA SCALE IN MILES