STATE OF CALIFORNIA EARL WARREN. Governor DEPARTMENT OF NATURAL RESOURCES WARREN T. HANNUM. Director DIVISION OF MINES FERRY BUILDING, SAN FRANCISCO 11 OLAF P. JENKINS, Chief SAN FRANCISCO SPECIAL REPORT 11 JULY 1951 GUIDE TO THE GEOLOGY OF PFEIFFER BIG SUR STATE PARK MONTEREY COUNTY, CALIFORNIA By GORDON B. OAKESHOTT Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://archive.org/details/guidetogeologyof11oake GUIDE TO THE GEOLOGY OF PFEIFFER BIG SUR STATE PARK, MONTEREY COUNTY, CALIFORNIA* By Gordon B. Oakeshott t OUTLINE OF REPORT Pa?e ABSTRACT 3 INTRODUCTION 3 GEOLOGY 5 Rock formations in the Park 7 Structural features 10 Summary of geologic history 10 Illustrations Plate 1. Geologic map and sections of Pfeiffer Rig Sur State Park _-bet. 4-5 Figure 1. Index map showing general geology of Pfeiffer Big Sur State Park 4 2. Panorama from Highway 1 across Big Sur valley 6 3. View eastward up Big Sur (Jorge 6 4. View from Redwood Pass across Big Sur valley 6 5. View from Redwood Pass 8 6. View from Highway 1 across Coyote Flats 8 7. Photo of Sur fault zone 8 8. View northwest along Sur Hill fault 8 9. Photo of Big Sur River flowing over low grade 9 10. Photo of Big Sur River flowing over low grade 9 11. Photo of slide area on Highway 1 9 12. Photo of breeciation and gouge developed in Francis- can sandstone and shale 10 13. Photo of Pfeiffer Falls 10 14. Photo of terrace gravels on white Santa Margarita sandstone 10 15. Photo of gravels being deposited by Big Sur River 11 16. Photo of Cretaceous conglomerate exposed on High- way 1 11 17. View north toward Big Sur (Jorge 11 18. Photo of mouth of Big Sur Gorge 11 19. Photo of vertically banded Sur series gneiss 12 20. Photo of dark gray-black shale of Franciscan forma- tion 12 21. Photo of massive dolomite conglomerate 12 22. Photo of black-chert breccia in Santa Margarita sand- stone 12 23. Photo of coarse black chert breccia in Santa Mar- garita sandstone 13 24. Photo of chert breccia in Santa Margarita sandstone 13 25. Photomicrograph of thin section of porphyritic vol- canic rock 13 26. Photomicrograph of thin section of porphyritic vol- canic rock 13 27. Stratigraphic column showing rock formations 14 28. Three stages in the geologic history of the Big Sur area 15 ABSTRACT Pfeiffer Big Sur State Park includes an irregularly shaped area of about 1 square mile in the lower valley of the Big Sur River which rises in the Santa Lucia Mountains and empties into the ocean a short distance south of Point Sur. The Park is crossed by High- way 1 about 27 miles south of Carmel in Monterey County. The Santa Lucia Mountains reach a maximum elevation of about 3500 feet in the vicinity of the Park. The mountains have a broad summit area of subdued mature relief, but drop off very abruptly to form steep cliffs at (he margin of the sea. The Big Sur River has cut a steep-sided narrow gorge in the higher eastern part of the Park and flows over a gentle grade in a broader valley along the Sur fault zone through the camp area. Repeated uplift in late geologic time has caused the river to leave a series of gravel-covered benches or terraces at several levels near its course. The central part of the range is made up of the very old crystal- line rocks of the Sur series which have been intruded by later Santa Lucia granite. This group of rocks has been thrust southwest ward and upward over Franciscan sandstone and shale along the Sur-Sur * Manuscript submitted for publication March 19, 1951. | Supervising Mining Geologist, California Division of Mines. Hill fault zone. The Sur fault and Sur Hill fault in the State Park are separated by a sliver of Santa Margarita sandstone a few hun- dred feet across. Movement along this major thrust fault zone prob- ably began as early as upper Miocene time and ceased by late Pleisto- cene time. Present topography is the result of repeated near-vertical uplift and erosion in late Quaternary time. INTRODUCTION Purpose of Report. The vacationing public is visit- ing our State parks in ever-increasing numbers, coming not only from California but from out-of-state by the thousands. The greatest season of influx is the summer when the Big Sur, like other parks, is filled to capacity but off-season visitors are becoming more common. Some of these visitors are occupied primarily with the purely recreational features at Pfeiffer Big Sur State Park, but many are also keenly interested in the unrivalled natural features — the plants and trees, the animal life, land forms, and rock formations. Park naturalists, in conducting their nature-study programs and trips, are faced with pointed and specific questions which deserve carefully considered and scientifically sound answers. This report is the result of a study made by the State Division of Mines, at the request of the State Division of Beaches and Parks — both in the Department of Natural Resources — for the purpose of giving a background of information on the geologic features of the Park. Geologic Study of the Park Area. Few areas of the size of Big Sur State Park expose evidence of such a strik- ing geologic history. Steep and high coastal cliffs and terraces, visible from Highway 1, the high crest of the Santa Lucia Mountains with its subdued summit relief, and the spectacularly rugged gorge of the Big Sur River giving way to the quiet flat-floored, tree-covered valley in the Park area focus attention on the possible geologic interpretation of such a landscape. The geologist, who deals with land forms, rocks, and the history of their formation, recognizes the great length of time required in development of the features of the land surface as seen today. His concept of time is in terms of thousands, millions, and even hundreds of millions of years. "When this fundamental concept is recognized, little further stretch of the imagination is needed to visualize, for example, the Big Sur River deepening its channel across the Sur gneiss by the constant erosive action of running water carrying rock fragments to form the pres- ent steep and narrow gorge. Even a downward cutting of a tenth of an inch a year would form a deep channel in a half million years, if the process were uninterrupted. Actually, such processes are often interrupted, particu- larly by crustal deformation caused by little-known forces within the outer layers of the earth's crust. Three types of crustal deformation may be observed in Big Sur : broad uplift accompanied by gentle tilting of the earth's sur- face, folding of rock formations, and breaking and relative displacement of rock formations along faults. ( 3 ) Special Report 11 Fioukk 1. Index map showing general geology of the Monterey coast and location of Pfeiffer Big Sur State Park. STATE OF CALIFORNIA DEPARTMENT OF NATURAL RESOI IEPORT I EXPLANATION GEOLOGIC PERIODS AND APPROXIMATE AGES IN YEARS QUATERNARY PERIOD RECENT EPOCH (few thousand years) PLEISTOCENE EPOCH (lev/ thousand lo one million yeors) TERTIARY PERIOD MIOCENE EPOCH (10 IOI5 million yeors) GEOLOGIC FORMATIONS JURASSIC PERIOD UPPER JURASSIC EPOCH (110 to 120 million yeors) PRE-JURASSIC PERIOD (Age unknown bul probably over 200 million yeors) Grovels ond sonds recently deposi by Bio 5ur River below the Gorgi Younger terraces Grovels deposited by B 2^ is fc.E.2 V 3-g OT3 =3 M *2.2s 3^ 2' E 2~f « c i. a. mi* a a, ft>-« g^alg --a. so„ ,Q CO O m to ■ f § .s s cu c3° G E 3 ft*, J3 O t. d «j Sen 2*5 tie E« hSX j3f Park boundary the brecclated, brok .i /..ne on Highway I, about 1.6 miles southeast dees near center oi photograph are located °- —••■ ^Xtf^^&js wp&gssr ,,n the Pfeipfer Big Sur State Park Figure 9. Slump zone along Sur Hill fault, northwest side of Big Sur Gorge. Photography, Leonard B. Penhale. while first movements along the Sur fault were taking- place. The Franciscan rocks were intensely broken and fractured to form a breccia made up of angular particles of Franciscan rock, including some sandstone, but par- ticularly the hard, dense, black, resistant chert. The black- chert breccia is most strikingly exposed along the west bank of Pfeiffer Creek just north of the Sur fault, and in steep cliffs about opposite the junction of the Oak Grove Trail and the Pfeiffer Creek Trail. It is also well exposed in steep cliffs along the road at the Sawmill Flat Camp Ground, where the Sur River has for thousands of years been undercutting its right bank to form a series of steep slopes near the trace of the Sur fault. Good outcrops of the Santa Margarita sandstone may also be seen in the roadcuts at the foot of the terrace near the Old Homestead cabin and along the nearby portion of the Sur River. The formation of this breccia is evidence that movement along the Sur Hill fault began in early Santa Margarita time, that is, toward the close of the Miocene epoch, possibly ten million years ago. Terrace Gravels and Alluvium. The present Big Sur River rushes at steep grade through the hard, resistant crystalline rocks of the Sur series to form the narrow rock-bound valley called the (Jorge, in the eastern part of the Park. As it emerges from the lower end of the Gorge, its velocity is slowed and it has built up a series of boulders, gravel, and sand deposits along a channel several hundred feet wide. These stream-deposited sands, gravels and boulders make up the alluvium outlined on the geologic map along the course of the Big Sur River. Older gravels, sands, and boulders formed in a similar way by deposition from the Big Sur River, occur as benches or terraces up to 300 feet above the present stream level. They represent an older, higher course of the Big Sur River and were formed prior to the most recent uplifts, which caused the river to cut deeper in its channel. The oldest of these ter- race gravels, maybe early Pleistocene in age (possibly up to one million years old ), suggests that the Sur River at one time ran westward from the present Gorge area along the Sur fault and Sur Hill fault zones. Later uplift and westward tilting have caused the Big Sur River to cut deeper in its channel and, in general, to shift toward the southwest leaving its gravel deposits behind it. This fea- ture may be particularly well observed in the terraces just north of the main bend in the Big Sur River after it emerges from the Gorge. Figure 10. Big Sur River flowing over low grade through alders in the camp area. Photography, Leonard B. Penhale. Figure 11. Slide area on Highway 1 opposite Group Organization Camp. Th.- rock i.s dark Franciscan shale with some dark gray sand- stone beds. This formation when water saturated very commonly is responsible for major slides, especially where the slopes are steepened in highway cuts. The light outcrop near left top of slide slope is a remnant of older terrace gravels left by the Big Sur River. Photog- raphy. Charles W. Chester-man. 10 Special Kkport 11 ^^^SSttJfa^SHS HiS ? a F «nc.»can sandstone of Park boundary. The prominent strftifi^ti ab i > - Ut - 15 miles sou th n&ht (northeast) indicates s tra ifirati™ f ?,! ° n dipping toward the beds, and is also approximated -the at?i£fdl he f e .l andstone and s h^le passes across the highway a few feet E ? th* th V Sur fa "'t which Structural Features toward the northeast in Lf T™ han 60 de ^ rees movement alonftK g eat £Z ^ f'l, the , ^ n ^l tural trend of the rti™T 2- Actuall y> the struc- of-north trend of tie V ° bh J™.to the general west- tl- local Surl ^^? ^V* ™* 1 " 8 ' With so that th a f„ ii. lc """ig a little bit more westward «SfB&£ KSS?*!? 8 coastli - - S certainty, but he fac that ? uT Can n0t be fixed with along the Sur I Hi fa,lt " P o reCClas were form ^ •suggests stroi y Lt 1 1 lT m e o arly S ? ta Mar ^ arita time «* earl,- as upper M&55 ftSTSfS TS? ^ pIaee oeene terrace ail ri V Le , ne tlme ,- Ih e fact that the Pleist- River tre ou. t r C r ^V** °y the old Big S ir -erodepoJi tX cr s /th tdM * ^ ^ b,,t »«^ ments along ? he S ? r M £**} " ldlcates that de- ceased bv the early p\5 L fauU an< S " r fault probably *ere haJ beer! 'a j£ SftK^i^ ** ^^ *"«■ a «wl tilting towani the west """"^ b, ° l ' k Summary of Geologic History structural ar r J , '' ;7 ,,,w 7<" k formations, the I; ""l surface, «. 1 , ,-' ,1 T* formations ' and ^e consideration of Ttttw™, ° f the re ^ ion - A Figure 13. Pfeiffer Fall* tv, ^^~^ — """ — «— — surface of Sur gneiss The falls w^ri^ ' S f owin g down a steep eroded as Pfeiffer Creek cut its course over l,T e '° ped tho "*ands of Tears ago tion approximately 20 ^^^^SS^AjISSS^ 235? tit- ■STSM'S? *"* on whi te Santa Margarita sand- W&CJ^^jgK^ cabi " « top. Pfeiffer Bk; Sur State Park 11 Figure 15. Gravels being deposited by Big Sur River about 1500 feet below the Gorge. The pebbles and boulders consist chiefly of Sur gneiss and Santa Lucia granite which have been eroded from the Gorge area and dropped by the stream as it came to lower grade. Photography, Charles W. Chestermaii. Figure 16. Cretaceous conglomerate exposed on Highway 1 near Hot Springs Creek a few miles south of the Park. This is a hard, firmly cemented conglomerate which suggests the type of solid rock that might be formed if the gravels seen in figure 16 were to be buried by later sediments and cemented over a long period of geologic time. Photography by Charles W. Chcsterman. Figure 17. View north toward Big Sur Gorge across dry abandoned channel of Big Sur River just east of South Camp Grounds. Note the most recent of the terrace deposits appearing a few feet above the channel level as a light colored, gently sloping surface seen through the trees. Photography, Charles W. Chesterman. Figure 18. Mouth of Big Sur Gorge. The valley here is narrow with extremely precipitous side slopes. The huge fragments of Sur gneiss which lie in river bed have fallen from the steep slopes and have been moved a very short distance downstream during flood periods of the river. Photography, Charles W. Chesterman. history, from what we see today in the rock formations structures and land forms of the Big Sur area. More than 200,000,000 years ago in pre-Mesozoic time, a great thickness of sedimentary rocks was accumu- lated, probably largely below sea level, later to become the Sur series gneiss. This early Sur series was intruded by molten rock, which crystallized to form the Santa Lucia granite far below the surface of the earth. After a gap in the known record of something like 100, 000, 000 years, the next major event in the history of the region was the formation of a long trough in which the marine sedimentary rocks of the Franciscan forma- tion were deposited. This trough was not local in the 12 Special Report 11 HB^BI ■«Bt Si',r7wlo Vertically-banded Sur series gneiss near mouth of Big Sur Oorge. This rock was probably deposited as a sediment in a. nearly flat attitude in early geologic time and has since b£n turned on edge and intensely altered by heat and pressure (metamorphosed) in mounta.n building. Photography, Charles W .Chest erman ' Dosed RE on°' w^fn sray-black shale of the Franciscan formation ex- posed on Fire Road southeast of Redwood Pass. Photography Charles W. Chesterman. ' {' ■ 4 '■*'■ ' --* ^ . V,*>' '•-"-' :,■/■} ;^ %- / ■J Figure 21. Massive Franciscan conglomerate exnnsprl ™ Vi- o ,, southeast of Redwood Pass. This conglomerate lies ust w th shale beds noted in figure 20. PhotogrZp^Charte? W "chef™^?. mMwr^M roadcut It Sa^mni^fJj M e * Ccia i n Santa Mar e a "ta sandstone" in roadcut at Sawmill Flat. Notice the extreme irregularity in the dis- tribution of the black-chert fragments distributed through the coarse n£h St^K sands , tone - This outcrop is just a few feet north of the probable trace of the Sur fault zone, which here follows the right bank of the river. Photography, Charles W. Chesterman. Pfkiffer Big Sur State Park 13 Figure 23. Coarse black-chert breccia in Santa Margarita sand- stone at Sur fault on Pfeiffer Redwood Creek. Photography , Charles W. Chesterman. Big Sur area, but extended in general through the entire Coast Range of California. Sediments formed by erosion of mountainous areas to the east, and possibly some west- ern lands also, were deposited in a sinking trough to form the sands, muds and cherts of the Franciscan formation. Intermittent volcanic activity took place, some of it prob- ably below sea level. Marine formations of Cretaceous age are evidence that the area was also a seaway in part of Cretaceous time. In the early part of the Tertiary period, that is, in the Eocene-OHgocene epochs, the Big Sur area was prob- ably above sea level as no sedimentary rocks of these ages are known. The seas advanced in early and middle Miocene time, and then in upper Miocene time (Santa Margarita) the area was locally elevated. Part of the uplift of the land mass formed may have been along the Sur fault zone, which probably came into being during upper Miocene time. As the land mass was elevated, rapid erosion formed the sandy deposits of the Santa Margarita formation in the margin of the sea to the west. The upper Miocene seas, and probably also Pliocene seas, must have extended from the Pacific area several miles east of the present Figure 25. Photomicrograph of thin sec- tion of porphyritic volcanic rock which in- trudes Franciscan sandstone on Highway 1 near Chief Ranger's residence. X-nicols. 70x. Long slim phenocrysts of albite feld- spar in groundmass of albite and intersti- tial altered biotite and pyroxene. Figure 24. Chert breccia in Santa Margarita sandstone north of Sur fault on Pfeiffer Redwood Creek. Photography, CharlesW. Chesterman. coastline, with a shoreline possibly east of the present Sur Hill fault in the Park area. Thrust faulting continued and was sharply renewed toward the close of the Pliocene epoch and into early Ple- istocene time. Following thrust faulting a series of more or less vertical uplifts of the whole Santa Lucia block began, which brought the main part of the range upward by as much as 3000 to 5000 feet during Pleistocene and Recent times. Westward tilting occurred along with the regional uplift. Evidence of a succession of late Pleist- ocene to Recent uplifts is found in the numerous marine terraces along the present coast, in river terraces like those in the Park area left by the ancient Big Sur River, and by the deep narrow valleys cut by west-flowing streams. Figure 26. Photomicrograph of thin sec- tion of porphyritic volcanic rock which in- trudes Franciscan sandstone on Highway 1 near Chief Ranger's residence. X-nicols. 70x. Longest white phenocryst is pyroxene which has been completely replaced by calcite (white) and chlorite (dark patches) ; the smaller phenocryst is albite feldspar. The groundmass is principally albite, with interstitial altered biotite and pyroxene. 14 Special Report 11 AGE Quaternary: Recent (Age: a few thousand years) Pleistocene (Age: a few thousand to I million years) Tertiary: Miocene (Age: 10 to 15 million years) Cretaceous (?) (Age: 60 to 110 million years) Jurassic: Upper (Age: 110 to 120 million years) Pre-Mesozoic (Age: unknown, but probably over 200 million years) FORMATION Alluvium Younger terrace ROCK TYPE DESCRIPTION Older terrace oooQocaooo *> o o o o o o o . o O O o .o o - o. " o ' O ' . o • o~ o O. O .o ■ • O . *. o . O . o o o /c?'o *d • o'.o Gravels and sands recently deposited by Big Sur River. Bench and terrace gravels deposited by ancient Big Sur River. Santa Margarita Franciscan Santa Lucia granite Sur Series gneiss Oldest (highest) bench or terrace gravels left in earliest stages of river deposition. Fine- to coarse-grained buff to gray sandstone, probably deposited below sea level; very coarse black chert breccia near Sur Fault zone. Black slate of uncertain age _"^-l_-^J exposed as slivers in Sur Hill fault zone. Dark gray to black sandstone, conglomerate, and interbedded shale; thin lenses of black and red chert. Intruded by small irregular bodies of porphyritic "volcanic" rock. Coarse-banded gneiss, quartzite, and gray Gabilan limestone beds or lenses; derived from sedimentary rocks by intense heat, pressure, and chemical changes brought about by mountain building. Intruded by Santa Lucia granite. Figure 27. Stratigraphic column sh owing rock formations in Pfeiffer Big Sur State Park. Pfeiffer Big Sur State Park THREE STAGES IN THE GEOLOGIC HISTORY OF THE BIG SUR AREA 15 Big Sur Accumulating Santa Margarita sands and chert br eccia^ FR4 ^ZZlZZZVZZtt ZO» E TO E POS,T CH ERT «.«*« .NO S.NOS F S*NT> MARGARITA FORMATION Sea level Big Sur U EARLY PLEISTOCENE (About I Million Years Ago) SURFACE OF LOW ELEVAT.ON AND LOW RELIEF, AFJEJL SUR THRUSTING BUT J3EFORE. 3,000-5,000 FOOT UPLIFT AND WESTWARD TILTING. Santa L ucia Mou ntains Faulting later Sea level HIGH fLEV^^NTrRANGE OF LOW RELIEF-, MOUNTAIN BLOCK UPLIFTED, TILTED WESTWARD, AND ERODED TO FORM STEEP AND RUGGED COASTAL MARGIN. Figure 28. Three stages in the geologic history of the Big Sur area. 16 Special Report 11 Present features of the land surface, as seen in the Park today, arc therefore the result of geologically late processes of uplift and erosion by running water. The Big Sur River must have started as a westward-flowing stream down the tilted rising slope of the Santa Lucia .Mountains, possibly in early Pleistocene time. As further uplift and tilting took place, and as the Sur fault and Sur Hill fault movements occurred, the Sur River incised itself in the crystalline rocks of the Sur series to form the deep rugged Gorge and cut its way along the more easily eroded brecciated rock in the thrust fault zones toward the northwest. With continued Recent uplift and west- ward tilting, the stream has had a tendency to slip off toward the southwest, forming, for example, the major loop in the Park area just below the Gorge. The present stream is exerting most of its energy in rapid down-cut- ting over its steeper gradient in the resistant ervstalline rocks of the Sur series and in side-cutting and under- mining its banks on the outside of its loops through the relatively flat valley of the Park area. In the next few thousand years, unless some unforeseen geologic inter- ruptions occur, it can be expected that the Big Sur River will lower its grade and widen its channel slightly in the Gorge area, and in the valley below will widen that valley at about its present level, by undercutting its banks to the south in the area of the South Camp Grounds, to the north in the area of Sawmill Flats, and to the northwest ij the area northwest of the Lodge. 1 5M printed in California state printinc offic