STA TE OF CA L IFOR N IA

R L W R R N G ov e rn or EA A E . DEPAR TMENT OF NATUR AL R ESOURCES

W R R N T HA N Di re c tor A E . NUM .

DIVISION OF MINES F R R Y B N G SA N FR N C l l E UILDI . A ISCO L F P EN KIN S Chi O A . I . e f

SA N FR A NCISCO BULLETIN 1 67 JUNE 1 953

GEOLOGY OF THE

OR TIGA LITA PEA K QUA DR A NGLE

CA LIFOR NIA

By LOUIS I. BR IGGS . IR .

LIBR AR Y UNIVER SITY OF CA L IFOR NIA DA VIS

L E T TE R OF TR A N SM I T TA L

T o His E x c e llen c y The HONORABLE E ARL WARREN Go v ernor of th e S ta t e of Ca liforni a

D S IR : h on or n 1 67 Ge olo EAR I have the to t ra smit herewith Bulletin , g y o t h e Or ti a li t a P e a k u a dr a n le Ca liforn i a r e a r ed dir e c f g Q g , , p p under the la n O f . D D tion of P Jenki s , Chief of the ivision of Mines , epartment of N a u ra l t Resources . The report includes colored geologic and econom ic mine ral maps , geologic sections , and many other illustrations . The ar ea mapped lies on the west side of the San Joaquin Valley , lar gely within

Merced County, although parts of it ar e in San Benito and F resno Coun it a ties . Since desc ribes lar ge number of important oil field formations , it r is of particula interest to oil geologists . E conomic minerals pr esent in the s area a re magnesite , quick ilver , diatomite , gypsum , lime , sand , gravel , and bentonite . r t h . . e a The author , Louis I B riggs , J , p r epar ed map and report as doc t ora t e r r thesis equi ed by the University of California . The publication of the results of this pr oj ect r epresents one of the cooperative undertakings D of the ivision of Mines with the University .

Respectfully submitted , W - T H M D ARREN . ANNU , i rector Department of Natural Resources 2 6 1 9 53 Februa ry , .

CON TE N TS Page A BSTR A CT

IN TR ODUCTI ON Geography

STR A TI GR A PHY Jurassic system Franciscan group Cretaceous system Wisenor formation Panoche formation Moreno formation Pet r ography of the Upper C r etaceous sediments

Tertiary - Quaternary system Laguna Seca formation Tesla formation K r eye nha gen form ation San Pablo formation Oro Loma formation Tulare formation Stream terraces Recent alluvium Landslides GE OL OGI C S TR U CTUR E Folding Faulting Structural history GE OL OGI C HI STOR Y

E CON OMI C R E SOUR CE S Quicks ilver Magnesite Diatomite Gypsum Lime Bentonite Sand and gravel Petroleum BI BL I OGR A PHY I L L U ST R AT I O N S

m a t h e rt i a l it a 1 . G O Plate eologic p of g Peak quadrangle , California

2 E Ort i a l i t a . conomic map of the g Peak quadrangle , California G Ort i a l i t a 3 . eologic structure sections across g Pe ak quadrangle , California In pocket 4 G T L S H . eologic map of ertiary formations in the aguna eca ills ,

Merced County , California In pocket

Index map showing location of Or t ig a l it a Peak quadrangle

Columnar section of rock in Ort iga li t a Peak quadrangle “ Photomicrograph of typical F ranciscan graywacke from Ort iga lit a Peak area Photomicrograph of F ranciscan ph yll on i t e from Or t ig a lit a thrust zone Photomicrograph of F ranciscan pillow basalt from Miner Creek Photomicrograph of F ranciscan m e t a g re e ns t on e from east slope of Ort ig a lit a Peak Photomicrograph of pegmatoid vein in F ranciscan quartz gabbro sill south of Or t ig a l i t a Peak _

Photomicrogra ph of e x - solution patterns of magnetite - ilmenite altered to leucoxene and quartz Photo showing quartz gabbro sill on north bank of Miner Creek Photo showing slump bedding in silt strata Photo showing gnarly bedding of U pper Cretaceous sandstone Photo showing concretions in U pper Cretaceous rocks Photo showing conglomerate series i n north branch of L os Banos Creek Photo showing sandstone penetrating overlying conglomerate Photomicrograph of U pper Cretaceous s ubg ra ywa c ke _ Histogram of rock types in coarse conglomerate at Or t iga li t a Creek and L os Banos Creek Photo showing concretion in Panoche formation Photo showing diatomaceous shale in Moreno formation Photo showing formations in Rattlesnake Canyon Photo showing outcrop of K reye n h a g e n shale on Oro L o rna Creek Photomicrograph of jarosite and glauconite Photomicrograph of jarosite crystals and clusters Photo showing disconformity separating shale and lower member of K re y e nh a ge n formation

K re en h a e n Sa n L Photo showing y g shale , Pablo beds , and Oro oma formation

Photo showing view north across Dog L eg Creek _“ Photomicrograph of sandstone from base of S a n Pablo formation Photo showing T ulare formation underlain by Moreno shale in cliff on tributary to Ort iga l it a Creek Photo showing Piedra Azul fault i Ort i a l t a . 1 8 T . 1 2 S R . 1 0 E . Photo showing view down g Creek in sec , Photo showing view north along crest of L aguna Seca Hills Photo showing badl a nd topography in T ulare beds

Photo showing Mercy quicksilver mine , southern workings Photo showing remains of Scott furnace of Merc y quicksilver mine GE OL OGY OF THE OR TI GA L I TA PEA K E AL I F R % QUA DR AN GL , C O NIA

“ L OU I S I . B S J R . By RIGG .

A BST R A CT

Or t iga li t a Peak quadrangle is in the Coast R anges bordering the Sa n Joaquin

l x 5 L os B . T h e D Va ley , appro imately miles south of anos small part of iablo Range in the southwestern portion of the mapped area is underlain by F ranciscan ( U ppe r i s e r e n Jurassic ) graywacke , chert , and greenstone ntruded by quartz gabbro and p h llon i t e s Or t i a li t a tine , and locally converted to glaucophane schists and p y along the g F thrust , the major structural element of the area . ranciscan rocks have been thrust U northeastward over pper Cretaceous conglomerate , sandstone , and shale , and locally Wisenor shale of L o wer Cretaceous ( Horsetown ) age is exposed beneath the fault T h e U surface . thickest known section of pper Cretaceous rocks in California ( some feet ) underlies the foothill belt to the east of the thrust ; it consists of the

Panoche formation ( sandy shale , massive concretionary sandstone , and a coarse con glomerate ) overlain by the Moreno fo rmation ( purplish organic shale and sandstone ) .

- Rel atively thin bedded T ertiary s edimen ts fringe the eastern edge of the foothills . T n L S r s h hey i clude aguna eca ( Paleocene ) conc etionary sandstone , siltstone , and ale , n a x i t ic K r e e nh a e n T esla ( a u sand and shale , y g diatomite and glauconitic sandstone

E S a n o - e of ocene age , Pabl ( Miocene Pliocene ) bentonitic sandston , shale , and con ' L F - 1 1 P l io glomerate , and Oro oma ( Pliocene ) gravels , sands , and silts . lat lying % 1 are ( Pleistocene ) reddish gravel s and white - weathering silt and marl overlap the old e r L i formations and obscure much of the tilted bedrock series , especially near ttle Panoch e

Creek . T h e major uplift of D iablo Range occurred during the upper Pliocene orogeny a long the Or t ig a li t a thrust which forms the eastern structural element of D iablo Range 1 3 0 . D for some miles islocation in the thrust zone near the end of the Jurassic period , D ia bl a n i probably during the orogeny , is ndicated by coeval developmen t of glaucophane h ll on it e s i schists and p y in apparent equilibrium , and was accompanied by intrus on

. S t h e n of ultrabasic igneous masses ubsequent to Jurassic deformation , deformi g forces have squeezed the serpentinous ultrabasic rocks into faults a n d fractures of the dis r u t e d F p ranciscan rocks in the thrust zone during the upper Pliocene orogeny . E i conomic resources nclude magnesite , quicksilver , diatomite , gypsum , lime , sand , gravel , and bentonite ; however , sand , gravel , and gypsum are the products success fully exploited from this area to date . At least 7 holes have been drilled for petroleum but all have been abandoned .

IN T R ODU CT IO N

Ort i a lit a a n g Pe k quadrangle, delimited by meridia s W l est and by paralle s north , lies largely within Merced

County, although it extends into parts of San Benito and Fresno Coun ties . There are no settlements within the quadrangle , but Los Banos is 5 un D os only miles beyond the northern bo dary, and Palos , Firebaugh, 20 m i a is and Mendota are within les of the eastern boundary . The are approximately 5 0 miles west of Fresno and 1 00 miles southeast of San

Francisco . l Numerous roads enter the region ; one is oiled , and the others are wel f e r in s graded and passable except during and immediately a t r a . The r i li most convenient approach to the northern sector is by O t ga t a Road, past the oil - pumping station in the north - central map area ; but Mercy

' Springs Road (2 miles f a rt h e r e a st ) is a better access to the Laguna Seca area . Little Panoche Valley is best approached from Mendota .

0 Bas ed upon a dissertation submitted in partial satisfaction of requirements for th e D f G G D U degree of octor o Philosophy in eology , in the raduate ivision of the niver s it f b 1 9 5 0. i a y o California , Berkeley , California , Manuscript submitted for pu l c J 1 9 1 tion anuary 5 . D f G f U . Instructor, epartment o eology , niversity o M ichigan 8 OR T IGA'L I TA PEAK QUADRAN GLE [Bull 1 67

“ f f f Ort i a l ita 1 . g FIGURE Index map o part o central California showing location o the - Peak quadrangle and of other quadrangles in the vicinity published as geolog i c maps by t h e D ivision of Mines .

Mapping of O r t iga lit a Peak quadrangle was begun in the spring of 1 948 r i m m 1 948 1 949 , continued du ng the su ers of and , and completed r during the Spring of 1 9 50. Field data were plotted on aerial photog aphs (scale appr oximately and later transferr ed in the field to r Ort i a li t a the U . S . Geological Su vey topographic map of the g Peak quadrangle (scale 1 i kn l d m e n tS I r . . A c ow e XJ . am pa ticularly indebted to N L Tal aferro , Ve rh oo en i C . M . Gilbert , F . J . Turner , and J . g , of the Un versity of I r m e a f ossils f or . California , their help in this work nverteb ate g collected D during the course of the investigation were identified by J . W . urham l K i l a . and M . V . rk , a so of the University of Californi I am gratefu also for the hospitality and cour tesies shown me during the field seasons by

Mr . . . n A r u ru a . . Mr . Fra k b , Mr . and Mrs S Marsi, and P Lindbloom The r Board of Research , University of Califo nia , Berkeley , furnished mate rial aid to defr a y the major part of expenses incurred in the field . P r v i o L i t t u re n e l r a e us era . O of the ear iest and most comp ehensive p pers covering geology of the Or t ig a lit a Peak ar ea is the report on the geology and oil resources of the eastern foothill belt of Diablo Range by Anderson and Pack Although their work was of a recon r naissance nature , considerable attention was given to desc iption and l i 1 93 1 distribution of ithologic units . 0 . P . Jenk ns ) in a detailed de scription of the stratigraphic and economic significance of the K r ey e n K r e e nh a en hagen shale , mentions the occurrence of the y g in the 1 953 ] INTRODU CTION 9

i lit s Ort ga a area , and include a comprehensive bibliography up to the

1 9 0 . . year 3 . A P Bennison mapped in detail the stratigraphy of late Upper Cretaceous strata both to the north and to the south of Pacheco

Pas s ; although the results of his work were never published , they were 1 presented before the L e Con t e Club in April 940 . The Moreno shale

in Panoche Hills was subdivided into lithologic units by M . B . Payne (1 9 5 1 ) who descr ibed the continuation of the units as far north as i Ort ga lit a Creek . l 1 941 O N . L . Ta iaferro ( ) published a structure section across rtiga 1 941 1 944 lita Peak quadrangle in , and in a later paper ( ) discussed i i D Lower C retaceous sediments to the east of Ort ga l t a thrust . etailed stratigraphy of late Upper Cretaceous and Eocene sediments of Laguna 1 942 Seca Hills was described by C . F . Green ( ) in an unpublished thesis , and several stratigraphic correlation sections from the same area by P o e n oe 1 944 Stewart , p , and Snavely ( ) were published as a preliminary

Oil . chart of the U . S . Geological Survey and Gas Investigations Radio

K . laria from the r eye nh a gen shale were described by B L . Clark and

A . S . Campbell Da v ie ss 1 946 S . N . ( ) attempted to establish a mineralogical basis for

correlation of Upper Cretaceous , Paleocene , and E ocene sandstones from Or o a study of outcr op samples from Laguna Seca , Loma , and Rattle snake Creeks and from well cores taken from the floor of the adj acent

valley . Quien Sabe quadrangle to the west was mapped and the geology 1 949 described by C . J . Leith ( ) the San Benito quadrangle to the south

west was mapped by I . F . Wilson

Ge o g r a ph y

li n r h r i li a s R e ef d T o o a . r a a p g p y The a ea of O t g t a Peak quadrangle , a D part of the iablo Range , the easternmost of the Coast Ranges , is typ ical in many ways of the foothills at the western border of the San

Joaquin Valley . Flat hills of low relief rise abruptly from the valley floor with s1 0 pes approaching the eastward dip of underlying Tertiary r ll and C etaceous beds . The maximum elevation in Lagu na Seca Hi s is 800 only feet , whereas the average elevation is feet . Westward , the fl altitude slowly increases , except in the broad terraced ats of Garrisa

litos , Salt Creek, and Little Panoche Valley . Rugged and steep Fran c isc a n terrane of Diablo Range contrasts markedly with rolling foot of r r hills softe Cretaceous and Tertia y rocks , culminating at feet i a lit a in an unnamed peak . Lower but better known Ort g Peak rises t abruptly some fee from the floor of Little Panoche Valley . The gently rolling summit areas of the Diablo Range are a striking topo

“ graphic feature , contrasting with precipitous slopes of lower altitudes .

Clim a t e a n d Ve t ti n g e a o . The western slopes of Diablo Range are h o characterized climatically by t , dry summe rs , mild winters , and little s rainfall . Average daily maximum temperatures during July and Augu t 9 9 1 01 r m i are to deg ees Fahrenheit , whereas average daily nimum tem e ra t u re s D p during ecember and January are about 30 to 35 degrees .

Summer rains ar e rare . A northwesterly breeze , at times reaching gale proportions , blows throughout much the year . The predominant vegetation of the foothi lls 1 5 range grass, principally a n foxtail and wild oat , but even these are bsent over exte sive areas . S oils derived from shale and silt are largely barr en ; northern slopes and s andy 1 0 OR T I GA L I T A PEA K QUADRANGLE [Bu lL 1 67

soils support a good grass cover ; and cottonwood , sycamore , oak , and w ul illow trees mark the course of maj or str eams , in partic ar Los Banos , Ort i a lit a a g , Little Panoche , and Piedra Azul Creeks . In contr st , higher Franciscan terrane of Diablo Range supports a sparse but widespread

flora including oak , juniper , cottonwood , sycamore , and pine , as well as a short brush cover largely of sage .

Dra in a e n d t r l D g a Wa e S upp y . rainage is towar d the east except in D a small part of iablo Range near the southwestern margin of the area .

Most of the stream valleys are dry except during rainstorms , but water flows sluggishly throughout much of the year through shallow ponds in the upper reaches of Los Banos Creek and Little Panoche Creek, seeping into alluvium before reaching the valley . T rellis drainage is well developed in Laguna Seca Hills and less per f e c t l y in the remainder of the foothill belt , whereas a dendritic pattern D is carved into more homogeneous Franciscan rocks of iablo Range .

Extensive ter race mantle indicates that Los Banos , Little Panoche , and

Sa lt Plio- r t i a li a Creeks follow Pleistocene drainage lines ; however , O g t

Creek , Laguna Seca C reek , and others appear to have developed courses , in a large part dictated by relative resistance of underlying strata , in late Pleistocene or Recent time .

Sever al minor ter race levels ar e evident even in the smallest streams , and although most of the valleys are stepped by a single terrace , Molino , Ort i a lit a Piedr a Azul , Los Banos , g , and Little Panoche Creeks have well 1 949 W 1 942 d preserved terraces at three levels . Leith ( ) and ilson ( ) note similar occur rences in adjacent regions . a e Paucity of water suitable for irrig tion of cr ps , for watering of stock , and for human consumption is perhaps the major obstacle hinder ing economic development of the fertile soil covering much of the area . In the San Joaquin Valley the pr oblem h a s been partly solved by t h e drilling of deep wells into the valley alluvium and by a vast network of irrigation canals which bring water fr om the Sa n Joaq uin River drain

of D - age system . The vast system the elta Mendota Canal now brings r wate from the Sacramento River drainage to the Mendota area , aiding agricultural development of the semi - arid regions of the San Joaquin

Valley .

The remainder of the area has less available water . Shallow wells , bored into Tular e terrace mantle or in thin alluvium of larger streams , i p roduce a meager supply of water . A cons derable numbe r of large steel water tanks are spotted throughout the central portion of the area to water sheep during the shor t spr ing grazing period . A few of these tanks are supplied from local sources , but the maj ority must be replenished with wate r trucked from the valley . Natural springs in the Diablo Range supply sufficient water for the few hundred head of cattle that remain there throughout most of the

' 2 1 3 1 0 S . . . NW z 9 . year . The sp ring in the l sec . , T , R E flows several hun dred gallons an hour of clear water , but at p resent it is not utilized ; neithe r is the sulfurous Sp ring about half a mile to the south . However , 3 Mercey Hot Springs , in Little Panoche Valley miles beyond the

r . southern quad angle limits , is a popular summer spa 3 a The structure sections (pl . ) show the inadvisability of bor ing w ter ' eds wells into Cr e t a c e ou s a n d Tertiary bedrock of the foothills , as the b dip steeply towar d the valley, even at a depth of feet below sea 1 953 ] STRATIGRAPHY 1 1

level . Most units of the Upper Cretaceous are lenticular , and probably do not carry much water ; furthermore , those waters that are present are charged with calcium sulfate , sodium chloride , and other soluble a s salts , evinced by several horizons of salt water struck by borings for oil into the adj acent valley . 8 o f Only some inches of rain falls during the year , and much this l Of i r runs off into the alluvium of the va ley . the l ttle wate that seeps into the tilted bedrock , much must return to the atmosphere by capillary action and evaporation ; therefore , the supply of water from the foothill belt is meager . The Tulare terrace mantle forms the only reservoir rock but it is very thin and patchy in distribution ; yet shallow wells obtain

‘ water from this formation in Little Panoche Valley, the north slopes Ga st onbide a of Wisenor Hills , Carrisalito Flat , the Ranch rea , and the r b e r Laguna Seca Ranch area . Elsewhere wate may obtained f om shallow l wells in the al uvium of the major streams .

S T R AT IGR A P HY

The oldest rocks of the region are Franciscan graywacke , siltstone , u and shale , interbedded with chert and gr eenstone , and intruded by q artz gabbro and serpentine . Lower Cretaceous shale is sandwiched between Franciscan r ocks and Upper Cretaceous conglomerate along Ort iga lit a Ort i a lit a thrust about a mile east of g Peak , and the maj or part of the d r a foothill belt to the east is un e lain by Upper Cretaceous s ndstone , shale , and conglomerate . Comparatively thin Tertiary and Quaternary beds bor der t h e San Joaqui n Va He y and include Paleocene Laguna Seca dl 2 sandstone and shale , lower or mid e E ocene Tesla ( 1 ) white quartzose K r e en h a en di sand , and upper Eocene y g atomaceous shale . Miocene sh a l e Pliocene San Pablo bentonitic sandstone , conglomerate , and over a n silt ' a nd l in by Pliocene u consolidated sand, , gravel complete the bed r rock sequence . Pleistocene Tulare ter ace deposits mantle much of the foothill belt .

J u ra s s i c S y s t e m F ra n c is c a n G ro u p

Dis t ribut ion n d T hi kn a c ess . Franciscan rocks underlie the southwest l portion of the map area , and form a small part of the almost continuous y exposed Franciscan core of D iablo Range covering an area of about I rt i a li square miles . n the O g t a Peak quadrangle the Franciscan is exposed in the eastern limb of an asymmetrical anticline , overturned Ort i a lit a di f adjacent to g thrust . Total thickness is ficult to estimate as ffi faults are di cult to trace in the homogeneous graywacke sediments , but erratic attitudes and some d evelopment of phyll on it es indicate a zone of dislocation southwest of Sugarloaf . The layered sequence above this zone is to feet in thickness , and if repetition is absent ’ - r i a li a or minor , the thickness (measured on section E E thr ough O t g t

Peak) is at least feet . Neither the bottom nor the top of the Fran c isc a n group is represented .

L ith olo gy . Graywacke sands tone associated with black slaty shale and siltstone is the predominant rock type o f . the diversified Franciscan a n 80 group and comprises estimated percent of the total volume . The

is - b graywacke characteristically massive , slate lue to greenish gray when fresh and tan to buff on weathered surfaces . In the upper reaches of 1 2 OR T I GAL I T A PEAK QUADRANGLE [Bull 1 67

2 f n e Or t i a l it a . . i th FIGURE Columnar section o rocks g Peak quadrangle , California

1 4 OR T IGA L ITA PEAK QUADRANGLE [Bull 1 67

4 h l l on it e Or t i a l it a FIGURE . Photomicrograph of Franciscan p y from the g F D thrust zone north of W isenor lat . eformation lamellae in quartz grains ( resembling polysynthetic t winning of plagioclase ) are aligned with pelitic

. G . fi 2 0x streaks and lawsonite prisms laucophane is common Magni cation , crossed nicols .

grains show strain shadows , fritted edges , and inclusions of minute i a n needles of a colorless prismatic amphibole ) , d of the feldspar grains , plagioclase , invariably calcic oligoclase or sodic andesine , is much more abundant than orthoclase and perthite . Present also are chlorite , mus c lin oz oisit e a covite , sericite , epidote , , sphene , m gnetite , and carbonate a 5 minerals . Lithic fr gments constitute about percent of the grains , including volcanic rock, chert , and pelitic schist . A greater proportion 70 0 r of matrix ( to 8 percent ) , smalle average grain size , and paucity of composite grains distinguish graywacke siltstone from sandstone . The striking similarity in mineral composition of sandstone , siltstone , and shale of the Franciscan graywacke sediments is their characteristic microscopic property . Crystalline chert comprises the remainder of the sedimentary rock types and is characteristic of the late volcanic sequence . The chert is typically white or pale yellowish , less commonly red or green , and bedded in lenticular layers a f e w feet to several tens of feet thick that form discontinuous outcrops one eighth to one half mile in length . Continuous outcrops of 2 or 3 miles occur locall y as along the spur south of Ort iga lit a

Peak . Countless thin lenticles , a fraction of an inch to several inches in ' - t u fia c e ou s thickness , separated by papery thin argillaceous or material , compose a single chert bed , which is characteristically contorted , frac t u re d - a ll , and intricately quartz veined . Rhythmic banding typifies varieties except the strongly sheared and completely recrystalli zed massive chert . Quartz, both cryptocrystalline and coarsely granulose, c lin oz oisit e stilpnomelane , and small acicular prisms of ( make up the 1 953 ] STRATIGRAPHY 1 5

90 minerals of these rocks , over percent of which is quartz . Colored patches of red chert contain acicular c lin oz oisit e ( and cryptocrystalline quartz, the latter exhibiting strong preferred orientation , in contrast to the coarse granulose patches which show undulatory extinction of doubly

polarized light . d s Partially or completely altered pillow basalt , an esite, intrusive ba alt t uif a nd d a s and diabase , , ag glomerate are conveniently groupe green us stone , which is most prof e in the upper part of the sequence adjacent Ort i a li a i s to g t thrust . Northwesterly cont nuation of these rock in a 1 949 1 5 similar stratigraphic position is recorded by Leith ( , p . ) who correlates them with E 1 P uerto volcanic rocks of the eastern portion of D iablo Range .

5 f f m FIGURE . Photomicrograph o Franciscan pillow basalt ro Miner Creek showing variolitic texture of flamboyant sheaths of feldspar micro

s a u s s u ri t i z e d . E liths ( ) uhedral grains are quartz and calcite , pseudo f f fi 9 0x morphs a ter olivine and eldspar and amygdules . Magni cation , plane polarized light.

Thickness of greenstone strata ranges from a few inches to se veral rt i a lit a hundred feet . A broad zone of schistose igneous rock east of O g Peak attains an apparent thickn ess of at least feet (measured on e s i structure sections ) , although xtreme hearing and fault ng prevent an

exact measurement . Some of the rock may be intrus ive , indeed abundant coarse augite grain s in a spe c flen from this area may be evidence of

fin - - . e intrusion However , prevalence of grained meta igneous roc k makes it doubtful that all greenstone in the thrus t zone east of Ort iga lit a Peak is s is intru ive ; moreover, complete disruption of the entire sequence in th area nullifies any attempt to establish the intrusive or extrusive character s ora di of the greater part of these rocks . Thin sheets of greenstone occur p 1 6 OR T IGAL I T A PEAK QUADRANGLE [Bull 1 67

6 m e t a re e n s t on e FIGURE . Photomicrograph of Franciscan g from the eastern f O rt i a l it a P fl b slope o g eak with relict augite ( lower right ) , am oyant glauco f r phane ( eathery tinted grains ) , and acicular lawsonite ( small prismatic g ains r i n g ou n d m a s s ) . fi 2 5 x a . / Magni cation , pl ne polarized light

cally in the lower graywacke beds . Many of the thinner layers , some only 2 4 n s t u if s f ew to inches in thick e s , are certainly altered , whereas a layers n nl several feet in thick ess are certai y volcanic flows . Pillow structure is in clearly shown by a flow Miner Creek a mile east of Sugarloaf, com

‘ posed largely of altered volcanic glass containing m ic rosc Opic bundles u ss ri iz s of radiating flamboyant sa u t e d feldspar microliths . Mo t of the

fin e - c hl orit iz e d greenstone is very grained, thoroughly and oxidized , i making fresh fracture surfaces a rarity . Much greenstone has an oph tic or diabasic texture ; some carries clear colorless phenocrysts of augite rimmed by colorless or green uralite , and minor amounts of leucoxene and granular sphene . b Greenstone , quartz diabase ga bro , and serpentine intrude the Fran i s ciseau sediments . A small body of greenstone located miles west of Ort i a lit a g Peak, and a half mile northwest an elliptical basaltic plug with steeply dipping curved joint planes forms a craggy prominence s visible from Wisenor Flat and adjacent ridge to the north . ul Serpentinous trabasic rock, highly sheared and locally altered to

- i l rt a it a . silica carbonate rock, occurs only along the O g thrust zone There is no thermal contact zone along the serpentine , rather the contacts are marked by shearing in both the intrusive serpentine and the invaded rock . Small irregular patches of glaucophane schist are common along the contact zones: Thin stringers and sheets of serpentin e in the disrupted 20 50 in rock of the thrust zone and a single thin sheet , only to feet i l thickness, marking the sinuous trace of Ort ga it a thr us t east of Ort i 1 953 ] STRATIGRAPHY 1 7

galita Peak , are evidence of cold intrusion during a period of deforma tion later than that accompanying the original intrusion of ultrabasic rock .

Hornblende - quartz gabbro form s the sill passing through Sugarloaf and Ort iga lit a Peak . The sheet swells and pinches within short distances s along the strike , attaining a maximum thicknes of approximately e a feet in Miner Creek, and dips st eply to the e st at angles greater than 70 degrees except immediately south of Ort iga lit a Peak where the in cli fl 45 nation attens to about degrees . Although the intrusive body is not Ort i a lit a continuously exposed between g Peak and Sugarloaf, the rela tive stratigraphic position of the exposures suggests continuity of the

’ s sill at depth . Specimen from a section across the sill in Miner Creek : e — contain the following minerals augite , hornblend including colorless , s— sa u ssu rit iz ed green , brown , and blue varietie chlorite , feldspar , quartz , e: magnetite , ilmenite , leucoxene , chromite , chalcopyrite , and calcit

Epidote and sphene are sparse in s ome of the specimens . Apatite com e mouly forms euhedral grains in quartz, and in one specimen larg euhedral grains up to 4 millimeters in length compose several percent of the rock . Augite ranges in composition from diopsidic at the top of the sill to sub c a l c ic near the base where the grains have an optic angle of about 40 D degrees . iallage parting is common and most of the grains are rimmed with secondary hornblende or partially replaced by a felted mass of

FIGURE 7 . Photomicrograph of pegm atoid vein in Franciscan quartz gabbro s 1 1 1 f Or t i a l i t a P south o g eak , showing intergrowth pattern of quartz ( white ) s a u s s u ri t i z e d fi 1 8x and feldspar ( dark ) . Magni cation , plane polarized light. 1 8 OR T IGA L I T A PEAK QUADRANGLE [Bull 1 67

chlorite and colorless amphibole . Hornblende is believed to be secondary although that which is darker colored , pleochroic , and rimmed by slender prisms of colorless amphibole might well be primary . Acicular needles of glaucophane line cleavage traces of some of the hornblende and occur in irregular patches in others . Quartz abounds in specimens taken near a n s the top of the sill , is rare in those from the base , d average about 5 percent of the bulk of the minerals . It is commonly intergranular , and micrographic intergrowths with sa u ssu rit iz e d feldspar constitute 25 percent or more of pegmatoid rocks near the top of the sill . Veins of coarse granular quartz are profuse thr oughout the intrusive rock and in the adjacent contact gr aywacke wh ereas minute granulose qua rtz s a u u r i iz — n occurs within ss t e d feldspar laths . Chlorit e both pale gree — a nd and yellow varieties makes up the greater part of most specimens , appears to have formed from augite and perhaps from amphibole . Abundance of chlor ite and actinolite may be ascribed to dynamic meta morphism in the shear zone bordering Ort ig a lit a thr ust ; incipient glau c oph a n e may be attributed to metasomatic solutions permeating the frac tures of this zone . Differentiation of the sill is indicated by decrease in optic angle of augite from top to bottom of the intrusion and by concentration of quartz at the top . The texture is diabasic near the top and clotty or gabbroic near the base although a sub oph it ic texture is common throughout the sill . Intrusion at high temperature is indicated by a h ornf elsic zone border

- r c ing the top and the bottom of the sill . No high temperatu e metamorphi minerals were detected in the contact sediments ; the matrix is coarsely crystalline chlorite and sericite with lawsonite ( and epidote , and the rock is traversed by a myriad of quartz veins . The hornfels zone border ing the top of the sill in Miner Creek is about 400 feet in thickness but r l elsewher e it rarely exceeds 50 feet . Greater b eadth of both sil and con tact zone along Miner C r eek indicates that the sill originated in this vicinity and spread southward between the bedded graywacke .

Hornblende - quartz gabbro of the Franciscan group has been described

1 948 20- 21 only from the Tesla area by Huey ( , pp . ) thus the outcrops in — — Diablo Range may hold an important though presently unknown sig n ific a n c e n in the history of the Francisca series . Glaucophane schist is restr icted largely to the Ort iga lit a thrust zone which r eaches a width of 1 % miles east of Or t iga lit a Peak and maintains a width of about 1 mile elsewhere to the northwest - The Ort iga lit a Peak Sugarloaf sill apparently formed a bulwark against which the overlying rock was sheared , as the metamorphic zone is wider adj acent to the sill and the rocks forming the uppermost part of the intrusive are m e t a m or h ose d . p The zone of metamorphism, marked by sheared graywacke at f a r the southern border of the map area , doubtless extends at least as as 5 Glaucophane Ridge , miles beyond the southern border of the quad r angle . h ll oni t e Some of the rocks , for example the shear ed graywacke or p y , owe their metamorphic recrystallization largely to stress and dislocation , W l hereas in others , such as the glaucophane schists of Piedra Azu Spring, metasomatic solutions appear to have played the dominating role . For most rocks it is difficult or impossible to conclude whether shearing stress i or metasomatism has been the primary factor produc ng metamorphism , but both have had an influence . All of the previously described Fr an 1 95 3 ] STRATIGRAPHY 1 9

c isc a n rock varieties have their metamorphic derivatives in the thrust t h e zone , and in most of them metamorphism has not progressed to degr ee that the original rock type cannot be recognized as ther e is a complete g raduation from practically unaltered sedimentary rock and greenstone

to that which is completely recrystallized . For reasons of cartographic continuity those slightly altered rocks whose parentage was certain were

mapped with unaltered rocks of the same group . Glaucophane schist occurs in a small area about Piedra Azul Spring

- surrounded by serpentine and silica carbonate rock . Glaucophane , mus

. a c ovite , actinolite , and quartz are the prominent minerals The schist p r t pears to be embedded in the se pentine , suggesting tha the metamorphic n r rock was carried up from depth withi the se pentine body , and the min e ra logy and distinctive appearance leave little doubt that metasomatism

has played a dominant role in metamorphism . Phyllon it e s of the thrust zone southeast of Piedra Azul Spr ing owe their recrystallization to shearing stress although the glaucophane a p r pears to be in stable equilibrium . The microscope reveals a mixtu e of very coarse schistose and el a st ic texture with elongate streaks of pelitic i r . a mate ial , incipient glaucophane , and rotation of grains Qu rtz show ng

d eformation lamellae , andesine plagioclase , chert , and basic igneous lithic l i fragments occur as relict e a st c grains whereas glaucophane , lawsonite ,

chlorite , sericite , stilpnomelane , and actinolite are metamorphic minerals , c lin oz oisit e and granular spene , , magnetite , and carbonate minerals may

- be eithe r relict or metamorphic . Fabric and structure of these meta gray

2 e . wackes may be cor related with the Subzone Chlorite describ d by F J .

1 9 4 r % . 8 . 3 8 Turner ( , p ) for the graywackes of southe n New ealand In this and other schists o f the meta morphic zone along Ort iga lit a thrust lawsonite takes the place of albite gener ally pr esent in rocks of corre 1 9 37 n in . s po d g composition in the green schist facies [ See G . A Joplin ( ,

424- 430 pp . ) for similar occur rences in New Caledonia]

Most m e t a g r e e n st on e is very fin e - gr ained and contains abundant glau h a n or c op e ( c rossite ) and lawsonite . It is widesp read in the shear zone east of Ort ig a lit a Peak where the eastern s1 0 pe of the peak is composed

largely of alternating greenstone and glaucophane schist , fractured into

r r - small splintery fragments . Some rar e occu ences of quartz sericite schist

appear to have developed from chert by metamorphism . Sericite in very

thin partings between massive layers of coarse - grained quartz suggests

thin argillaceous partings of rhythmically banded chert .

Dia l n ro n rt i a lit a b a 0 g e y . Franciscan rocks in the O g Peak area are in fault contact with Lower C retaceous and Upper Cretaceous sediments , thus precluding any possibility that an unconformity may have been pr eserved between F ranciscan and Lower Cretaceous rocks ; consequently

evidence of a Ju r a ssic - Cretaceous orogeny must be sought in the lithology

and fabric of the Franciscan r ocks . Common field association of glaucophane schist with serpentine has led Taliaferr o ( 1 943 ) to the opinion that glaucophane schist of the Cali

' f orn ia Coast R a n g e s is a product of pneumatolytic or metasomatic

emanations from ultrabasic intrusives . There are numerous localities where deVIOpm en t of glaucophane schist can be explained in no other n r plausible man er ; howeve , there ar e other occurrences where miner

alogy and fabric of the schist are clearly the result of dislocation , and metasomatism— although it may be instrumental in development of glau 20 OR TI GA L I T A PEAK QUADRANGLE [Bull 1 67

- n - FIGURE 8 . Photomicrog raph of ex solution patterns of mag etite ilmenite b fi altered to leucoxene and quartz in Franciscan quartz gab ro sill . Magni cation 1 8 x , plane polarized light . — c oph a n e c ould not have formed the dominant metamorphic fabric and

- Ort i a lit a s mineralogy of the schist . In meta gr aywacke of the g thru t t zone , gr anulation and rotation of grains , deformation lamellae in quar z , and large discoidal pelitic streaks ar e the pr oduct of shear ing stress during dislocation coeval with crystallization of lawsonite , glaucophane , l ch orite , sericite , and stilpnomelane . Lawsonite and glaucophane occur — in schists der ived from various F ranciscan r ock types gr aywacke , sand m et a r e en st on e—a n d stone and shale , but most abundantly in g if em a n a t ion s from ultrabasic magma ar e necessary for development of these r minerals in the thrust zone , then shea ing and fracturing , intrusion of ultrabasic magma , and metasomatism must have taken place concurrently .

- K Since serpentine and schist are restricted to Franciscan noxville rocks , it follows that dislocation in the Ort iga lit a thrust zone occurred durin g D ia bl a n the period of metamorphism , that is , during the orogeny near s the close of the Jura sic per iod .

C r e t a c eou s S y st e m W ise n o r F o r m a t ion

N a m e a n d c c u r r e n c rt i a li a O e . E ast of O g t Peak Lower C retaceous shale and thin sandstone beds extend for 2 miles northward from Little Panoche Valley between the Or t iga lit a thrust and the basal conglomerate

of the Panoche formation . They were originally included in the Panoche formation by Anderson and Pack ( 1 9 1 5 ) but Taliafer ro ( 1 943a ) distin gu ish e d them and placed them in the Shasta gr oup on the basis of lith

ology and stratigraphic position . Later (Taliafer ro, oral communication ,

R T I GA L I T A 1 67 22 O PEAK QUADRANGLE [Bull .

a 1 950) fossils indicating the Horsetown st ge as defined by F . M . Ander 1 s on ( 936 ) were collected from these beds , indicating that they are to be correlated with similar strata along the east side of Diablo R ange in

Hospital Creek and Corral Hollow . Because of their exposure in the a Wisenor Hills , the name Wisenor formation is suggested for these strat of Horsetown age . That an angular discordance exists between the Wisenor formation and the Panoche formation is seen in several localities where the strikes ifie r of the two formations d . But everywhere the angle of dip of the two formations is steep and approximately the same, so that the observed unconformity might be explained by gentle warping of the Wisenor w un c on strata before the Panoche a s laid down . If this is correct the formity may be only local and elsewhere the sa me formations might a p of rt i a lit a pear conformable , as they do north O g Creek where the lowest conglomerate beds occur about feet stratigraphically above the lowest exposed Cretaceous strata . The shale and thin sandstone beds at the base of this section , however , are lithologically similar to those inter bedded with Panoche conglomerate , and the presence of very coarse Pan oche conglomerate at the base of the section in Ort iga lit a Creek indicates that if Lower Cretaceous beds are to be found farther north their ex osu re p must be very limited .

A single thin section of the fin e - grained fla ggy sandstone typical of the Wisenor formation has approximately the following composition

M in e ra l con s t i t u e n t Quartz F eldspar Matrix C a lcite L ithic fragments C a rbonaceous material

Chlorite , biotite , epidote

1 00 Carbonate cement consists of coarse to fine crystalline interstitial cal m seric it iz e d cite and veins . Much of the feldspar is twinned and so e is or n l r kaolinized ; both quartz and feldspar grains are fractured and a gii a . Matrix material consists of recognizable crystallized clay ( granular a quartz , chlorite , and sericite ; the chlorite listed sep rately appears to be

' u a rt z it e c h e rt altered biotite . Lithic fragments include q , , schist , and 1 949 other rock types . The sandstone is a graywacke (Tallman , ; Petti 1 95 0 f j ohn , ) although the texture and proportion o argillaceous matrix are intermediate between Franciscan graywacke and Upper Cretaceous sub ra wa c k e g y . Sandstone of the Wisenor formation resembles micro sc o ic a ll p y sandstone of the Shasta group from Lake County, California

(Brice ,

ri in O g . There is some evidence that Lower Cretaceous sediments were deposited in a continuous geosyncline extending throughout the central and northern Coast Ranges within the approximate limits of the previous a n d Franciscan basin ( Taliaferro , Preponderance of black shale abundant carbonaceous fragments suggest that deposition was in quiet waters under anaerobic conditions . Interbeds of sandstone are indicative of occasional currents competent to transport sediment of sand size . 1 953 ] STRATIGRAPHY 23 R T I GAL I T A 1 67 24 O PEAK QUADRANGLE [Bull .

P a n oc h e F o r m a t io n Anderson and Pack (1 9 1 5 ) subdivided Upper Cretaceous sediments (previously called the Chico formation) on the east side of Diablo Range north of Coalinga into two formations , namely the Panoche formation l r 943 a . . below and the Mo eno shale above . Taliaferr o ( ) and F M Ander son ( 1 941 ) have proposed divisions of the Panoche formation wherein ’ Anderson s Pioneer gr oup is appr oximately the equivalent of Talia ’ ’ ferro s Pacheco gr oup and Anderson s Panoche and Moreno groups are ’ T li f r o r r vi inclusive of a a e r s Asuncion g oup . Taliafer o made his di sions in t h e Santa Lucia Range wher e Upper Cr e t a e c ou s strata are separated by a strong unconformity that rep resents the Santa Lucian orogeny . He cor related conglomerate strata bearing reworked Turonian 1“ fauna along D ’ the eastern slopes of iablo Range with the disturbance . Anderson s sub ou division is based the same fossilifer ous conglomerate beds , thus the r close c o respondence in their Upper Cretaceous grouping . That lithologic units in the Upper Cretaceous can be separated within limited areas was recognized by Anderson and Pack who subdivided the

Panoche and Moreno formations along the 1 30- mile strip north of Coa linga ; but because they also r ealized that the units could not be extended r with any degree of ce tainty beyond local areas , they did not attempt to subdivide Upper Cretaceous sediments along Diablo Range into units more refined than the Panoche and Moreno formations . Because Cre t a c e ou s rocks are not continuously exposed along the east side of Diablo

Range , owing to consider able overlapping of Quaternary terrace deposits and alluvium and to extreme l e n t ic u l a r it y of the Upper Cretaceous lith ol o ic r g units , exact correlation between isolated a eas is not possible . Stratigraphic terminology of the Upper C retaceous of D iablo Range r r is not satisfactory . Fossils are elatively sparse th oughout great thick nesses of sedimen tary strata and where preserved they are frequently

- long r ange forms .

Di t r i u ti on n n s b a d R e la tio s . The Panoche formation is the most wide spread formation unit of the map area ; it underlies approximately 1 00 r t i a l it a d square miles of the area cover ed by O g Peak qua rangle . The outcrops in this region comprise but a small part of the very extens ive belt of early Upper Cretaceous rocks along the west side of the San

Joaquin Valley that includes the type section in Panoche Hills , which r extend into the southeastern pa t of the mapped area .

The greatest thickness accurately measured on cr oss - sections where both the base and top of the formation are exposed is shown on structure sec ’ - C 3 r tion C (pl . ) where the e are feet of beds mapped as Panoche E f S uropean names applied to stages o the Cretaceous ystem . S eries S tages D anian

M a es t ri c h t ia n U ppe r Cretaceous Senonian Turonian Cenomanian A lbian L ower Cretaceous A ptian N eocomian

26 OR T IGA L I T A PEAK QUADRANGLE [Bull 1 67

1 2 S - r r f U FIGURE . pheroidal reddish brown limy concretions cha acte istic o pper Cre t a c e o u s P r S 1 1 . 1 3 S . R . 1 0 E . r rocks . anoche sandstone nea alt Canyon , sec . , T , Hamme

is 1 8 inches long.

' E - E and a total of feet of Upper C retaceous sediments . Section through Ort iga lit a Peak indicates a thinning of the Upper C retaceous rocks as only feet are here exposed of which about feet e W comprise the Panoche formation . Exposur of Lower C retaceous isenor strata along this section may indicate a local ridge in the floor of the basin ove r which Upper C retaceous sediments were deposited . T h e angular discordance separating the Wisenor and the Panoche for Ort i a lit a mations east of g Peak has been discussed . Elsewhere , Upper C retaceous sediments are in fault contact wi th the Franciscan formation o r the contact is obscur ed by Quaternary deposits . The line of separation between the Panoche and Moreno formations is grada tional .

1 3 FIGURE . Conglomerate series in north bank of L o s Banos Creek in northwest corner 2 1 1 - . b . 1 S . R . 9 E . F of sec , T , , containing angular oulder of ranciscan type greenstone - b b ( under hamme r ) , and reddish rown concretionary sandstone oulder ( right ) charac t r i e s t i c of U pper Cretaceous strata . 1 953 ] STRATIGRAPHY 27.

1 4 S FIGURE . andstone penetrating overlying conglomerate as a result of compaction “ % b and slumping of conglomerate . Flow lines accentuated y weathering continue into b penetrating area and are crossed y compaction shear surfaces at nearly right angles . D n b b E epositional curre ts were pro a ly from left ( west ) to right ( east ) . xposure in north L R . I b o s 2 1 1 1 . S . 9 E ank of Banos Creek , in northwest corner of sec . , T . , nch markings on hammer handle .

i h fla L t ology . Siltstone and shale intimately bedded With thin ggy

- sub ra wa c k e n 1 949 lime cemented g y sandsto e ( Tallman , ) ar e typical r ock types of the Panoche formation , particularly the lower part . Massive s sandstone is al o pr ominent , especially the huge lens extending across the area to feet above the base of the formation , and several a other concretion ry sandstones ar e of mappable size , namely, one form ing the low ridge of Laguna Seca Hills (Joaquin Ridge sandstone ) , and seve ral along Los Banos C r eek . Concretionary sandstone is p rominent als o along the lower r eaches of Salt Canyon and elsewhere throughout the Panoche formation as thin beds less than 1 00 feet in thickness .

- r Coarse conglomerate lenses containing well ounded pebbles , cobbles , 2 1 2 a r and boulders to inches cross , occur spo adically throughout the lower feet of the formation . A single lens of conglomerate is generally no mor e than 30 feet in thickness and many are only several 1 2 feet acr oss , but in Los Banos Creek as many as individual lenses merge at the canyon walls to form a continuous exposur e of c on gl om f 3 00 40 D crate in sheer cli fs rising to 0 feet above the valley floor . ense

‘ tan limestone occurs a s t h in lenses within massive concretionary sand stone beds although limestone makes up but a minor part of the sedi mentary sequence . Coarse polym ic t i c conglomerate indistinguishable lithologically from that higher in the section occurs at the base of the Pan och e formation a n d is absent for a short distance only along the F ranciscan contact near W isenor Flat . In detail , however , the conglomerate is lenticular and discontinuous as few beds extend along the strike for more than a mile 5 0 a r or reach a thickness greater than feet . Conglomerate lenses e cross bedded on a lar ge scale , grading laterally into coarse biotite sandstone w D ithin several feet . iscontinuous lenticles of coarse to fine sandstone and thin - bedded siltstone and shale are interbedded with the c on gl om

r t i - e a c . strata Thin alternating beds of soft dark gray shale , siltstone , 28 OR T I GA L I T A PEAK QUADRANGLE [Bull 1 67

FIGURE 1 5 . Photomicrograph of U pper Cretaceous calcareous s u b g r a yw a c k e b - b a cemented y coarse grain e d car onate minerals . Calcite gr ins ( high relief ) are fi 2 x 5 . clastic . Magni cation , plane polarized light a n d hard fla ggy biotitic feldspathic sandstone occur with the c on gl om crate and comprise the major part of the lower beds of the Panoche for n t h e mation . In places thick massive sa dstone is pr ominent . as near head i of Or t a lit a bu t . waters g Creek , such occur rences ar e relatively rare Shale of the lower Panoche beds is lighter in color and more silty and the interbedded fla ggy sandstone contains much less carbonaceous debris than that of the underlying Wisenor formation . A giant sandstone lens extends more than 1 2 miles across the mapped area to feet above the base of the Panoche formation , r each ing a maximum thickness of feet near Piedr a Azul C reek . In the Wisenor Hills this member can be separated into a lower massive sand

st a n ds on e - stone and an upper fissile t . Lime cemented concretionary coar se gray biotitic su b gr a ywa c k e of the lower unit weathers to buff W cavernous exposur es , and in isenor Hills str eam courses ar e broken by nume rous falls and small amphitheaters eroded into the vertical s n trata which ar e favorite haunts for deer a d smaller game . Thin pebble conglomerate is commonly interbedded and north of Carrisalito Flat there are several lenses of coarse conglomerate lithologically similar to those higher and lower in the section . A thin pebble conglomerate bed composed of r ounded black and gr een limestone and black shale frag ments crops out along the south bank of Piedr o Azul C reek and through W f r out isenor Hills , a fo ding an excellent marker zone .

30 OR T I GA L I T A 1 67 PEAK QUADRANGLE [Bull .

1 P f FIGURE 7 . S pheroidally weathered limy concretion in anoche ormation north of 1 1 1 E S . . 1 3 S . R . 0 . alt Canyon , sec , T , Ma%or fractures paralleling hammerhead follow

original bedding planes .

' northward the s e q u e n c e t ot a ls but 1 00 feet in thickness . Northwar d the Ort i a lita beds can be traced intermittently to Salt C reek . South of g C r eek four conglomerate strata disappear beneath Tular e gravels of the

Laguna Seca - Ranch area to reappear as a single stratum which continues 2 a roxi southward for about miles , giving a lateral distribution of pp 1 0 r mately miles for the entir e series . The conglomerate units th ough which Los Banos Cr eek has cut a steep - walled gorge are made u p of thin beds that reach their maximum thickness and extent in the near - vertical 1 1 1 9 . 9 . walls of the canyon in sec . , T . S , R . E Thickness of the con glomerate series within the Ort iga lit a Peak area is feet (measured on structure section A Although the beds - have not be en traced northwestward to their termination , conglomerate in approximately the same position has been observed as far north as Highway 1 52 on the if area covered by the Pacheco Pass quadrangle , which , continuous with the conglomerate of Los Banos C reek, denotes a lateral extent of at least 1 0 miles . Excellent exposures of conglomerate at L os Banos Creek contain 1 9 1 5 reworked early Upper Cretaceous fossils . Anderson and Pack ( ,

4 - 44 pp . 3 ) noting the presence of the fossilifer ous boulders remarked One o f the most significant features of the conglomerate i s the fact that many beds contain boulders of sandstone and conglomerate in which typical Chico ( U pper S a n d Cretaceous ) fossils occur . uch boulders were found at many places at no less f than three di ferent horizons , the lowest being in the basal conglomerate overlying the F ranciscan formation northeast of Or t ig a l it a Peak T h e explanation of these fossiliferous inclusions that appears most reasonable is that at various times oscilla tions of the shore line took place which brought littoral areas of the sediments above le v e l h sea and allowed banks to be cut in t em , from which lumps of sand and gravel , somewhat consolidated by the lime of the contained shells were washed down into the T sediments that were continually being deposited nearby . hese beds are therefore similar to the intraformational conglomerates in the Cambrian and Ordov ician of N e w Y ork and Pennsylvania described by Walcott . The composition of about 200 pebbles and boulders at both Los Banos Ort i a lit a Creek and g Creek , determined megascopically , indicate the On e relative pr oportions of rock types included in the conglomerates . 1 953 ] STR A TIGR APHY 31

of the commonest components is porphyry which is not indigenous to r the bedrock se ie s of either the Coast Ranges or the Sierra Nevada , suggesting a western source for the bulk of the Upper Cretaceous sedi o ment . Further supp rt is lent by a northeasterly decrease in grain size

of the coarser constituents of the conglomerate in Los Banos Creek , and an unconformity within the conglomerate sequence along Los Banos 1 1 1 . . 9 . n r 2 . S C eek in the northwest corner of sec . , T , R E tru cating uptilted strata of sandstone and siltstone which could be deformed only by later al movement of the overlying conglomerate strata from west to

east . Other boulders of the conglomerate are granite , diorite , aplite , l pegmatite , gneiss , schist , and quartzite which are simi ar to rocks com e ll prising the Sur series of the w stern Coast Ranges , and sma amounts

of greenstone , graywacke , and red chert are found that could have been

derived from a Franciscan provenance . Thus there is considerable evi dence supporting a western source for the conglomeratic strata . Units lying over the highest conglomerate member in Or t iga lit a C r eek and below the massive sandstone of Laguna Seca Hills are composed of

diversified lithologic types . Lenses of sandstone , siltstone , and shale

interdigitate along the strike of the beds . The strata are p r edominantly e r ode d ' a soft and easily , factor responsible for the low depression ex tending northwestward from Little Panoche Creek to beyond the north — e rn boundary of the mapped area . Thin bedded to massive concretionary su b r a wa c k e l g y forms an important part of the beds , especial y to the

south of Laguna Seca Ranch . A prominent shale stratum , attaining 800 r i a li about feet in thickness , occurs about feet above the O t g t a d Cr eek conglomerate , and shale is abun ant at Los Banos Creek as well .

Tulare gravels and residual soil obscure much of this member . Bennison ( 1 941 ) established the Mustang Creek formation which he Ort i a lit a extended as far southward as g Creek, beyond which it loses its r i identity . It is best exposed in the O t ga lit a Peak quadrangle at Los Banos Creek where the lower unit is massive concretionary sandstone forming a low topographic ridge from Laguna Seca Ranch northwest of ward beyond the limits the quadrangle . The low summ it area of Laguna Seca Hills is underlain by concretion ary biotitic subgra ywa c k e and thin beds of soft fin e - grained sand and silt which pass northward into thin - bedded sandstone and siltstone near Or t i a lit a g Creek . At Los Banos Creek contemporary units were grouped into the Mustang Creek formation and the Oa t Gulch shale by Bennison (1 941 ) who termed the massive sandstone of Laguna Seca Hills the n Joaquin Ridge sandstone . The Quinto member described by Ben ison includes sandy shale and two beds of massive biotitic sandstone at Los

Banos C reek and at Salt Creek, but the units at Salt C reek are poorly a exposed , being partly cover ed by slides of terrace mantle . The R gged ' s Valley hale and the B rown Mountain sandstone , poorly defined strati r of g aphic units the late Upper Cretaceous , are included in the uppermost

%

part of the Panoche formation .

A e a n d orre la ti on - g C . Fossil bearing strata are widespread through I out the Panoche formation but few are abundantly fossiliferous . n the course of this investigation diagnostic ammonite and rudistid fauna we re as u l oc a lit ie s found in only fo r .

fi l e P U n i The fossils and their descriptions are on with the Museum of aleontology , L —o n versity of California , Berkeley . ocalities are marked on aerial photographs

fil e . r S r D G with Mr Theo C ook , enio Museum Curator , epartment of eological

S U l . ciences , niversity of California , Berke ey 32 OR T I GA L IT A PEAK QU ADR A N GL E [Bull 1 67

A fossil locality in basal conglomera te east of Ort iga lit a Peak ha s yielded a diagnostic rudistid species interbedded in the matrix of the conglomerate pebbles, and the same species has been identified from fossiliferous boulders in a conglomerate lens that crosses the upper 34 1 1 see S . . 9 E f reaches of Salt Creek in . , T . , R . O those found in the K w n a . . 1 950 b sal conglomerate , M V irk ( ritte communication , ) states

A - 6 1 2 D u ra n i a ? c a li orni c a e . T L ocality 6 c ontains f Anderson ms . in plac his fossil

D a n i a % . c li or o G is apparently the same as f c fr m Peterson Ranch , lenn County , Cal i forn ia s P er v in uri a i n a t e - where it occurs a sociated with q fl , a world wide U pper Albian S S marker . At Peterson Ranch these fossils occur in the highest part of the hasta eries , ” f - just below the uncon ormity which marks the Mid Cretaceous disturbance .

If the reworked Albian fossils at the southern extremity of the c on glom e ra t e series that at Los B a nos Creek contains an unconformity mark the boundary between the Pacheco and Asuncion groups in the Ort iga lit a n Peak quadrangle , they indicate that the unco formity at Los Banos

Creek is a product of the Santa Lucian orogeny . 32 1 2 1 0 see S . R . E . A fossil locality in . , T . , yielded a rich ammonite i Ba cu lit es fauna representing at least three species of wh ch one , c hi c oensis indicates an age not older than Turonian and probably a n i n Th e Coniacian or S t on a . fossils are found in a limestone interbedded with sandy shale approximately three - fourths of a mile southeast of Langan Ranch within a zone stratigraphically above the Los Banos conglomerate series . The sandstone bed at the mouth of Los Banos Creek yielded a poorly P a ra a c h disc us s preserved ammonite tentatively identified as p y p . and Gl c one ris v ea t c hii abundant pelecypods including y y .

‘ Fossils in the Panoche formation indicate an age from Cenomanian to — possible Ma est ric ht ia n a range that includes almost the entire Upper i . A sun c on rou s Cretaceous epoch Both the Pacheco and g p of Taliaferro , or the Pioneer and Panoche groups of Ande rson a re represented though they have not been satisfactorily separated in this area .

Mo re n o F o r m a t io n

Dis t ri bu tion a n d R e la tions . The Moreno formation includes the youngest Upper Cretaceous beds exposed between Los Banos Creek and Wildcat Canyon and in a small area of Panoche Hills in the southeast Or t i a lit a corner of the map area . Tulare ter race cover near Salt Creek , g W Creek, ildcat Canyon , and Little Panoche Creek , makes correlation f di ficult between isolated exposures , and causes doubt as to the exact

- stratigraphic position of the Panoche Mor eno contact at Los Banos C reek,

Salt Creek , and the Panoche Hills . Thickness of the Mor eno formation is p ractically constant in the n Laguna Seca Hills , the only area within the quadra gle where both top Ort i a lit a and bottom of the formation are exposed , being feet at g

Creek and feet at Hamburg C reek and Wildcat Canyon .

The C retaceous - Paleocene boundary is gra da t ion a l within the Orti

- - of galita Peak area . Purplish b rown shale at the top the Moreno along most of Laguna Seca Hills thins north of Rattlesnake Canyon and is Ort i a lit a Or t i a lit a absent at g C reek . Absence of the shale member at g Creek c a n be ascribed to overlap of the Paleocene Laguna Seca beds or to lateral gradation of shale to sand . The shale member underlying the “ ” uppermost sandstone bed ( Glyc yineri s reef ) at the mouth of Los

Banos Creek is placed near the top of the Panoche formation . 1 953 ] STR ATIGR APHY 33

- 1 8 . W f FIGURE hite weathering diatomaceous shale in the Moreno ormation . View

W s e e 5 . S 1 1 E L f 1 3 R . . north across ildcat Canyon in . , T evel summit sur ace is capped with Tulare gravel . If the map designation of the base of the Moreno formation at Los e u a v a le n t Banos Creek is correct , the Moreno is in part q to the Garzas fo rmation of Bennison ( 1 941 ) suggesting northward gradation of the Moreno formation from typical shale of Panoche Hills and Laguna Seca n Hills into sandstone of the Garzas formation . Paucity of sandsto e in the Moreno formation of Panoche Hills and a gradual increase in the sandstone to shale ratio in Laguna Seca Hills from 29 percent at Wildcat Canyon on the south to 60 percent at Ort iga lit a Creek on the north lends r further support , as does paleontologic and st atigraphic evidence voiced P o en oe 1 944 by Stewart , p , and Snavely ( )

T h e fossiliferous upper part of the Moreno ( near Garzas Creek is F . M . Ander ’

G . U son s arzas member It contains a fairly distinct pper Cretaceous fauna , including

- f r T u r r i t e lla c h a n e i Cu c u lla e a F i s u s . U n o y , a new species of and a c like gastropod t u na t e l y this fauna has not been recognized in the L aguna Seca area and t h e strata

cannot be traced directly across the entra nce to Pacheco Pass . It is not likely that strata containing T u r r i t e ll a c h a n e yi have been eroded from the top of the Moreno in

L a . the guna Seca area where no unconformity has been recognized It is likely , how U G s ever , that pper Cretaceous strata have been removed from the arza Creek section just before or during deposition of the conglomeratic sandstones ( placed at the base 1 2 of the Paleocene in Garzas Creek ) shown in column . Possibly the fauna containing T u r ri t e lla c h a n c y i is the equivalent of that found in the L aguna Seca area about ” to feet below the top of the Moreno ( column but it may be older .

- The Cretaceous Paleocene boundary is difficult to establish in the field . Good exposures occur along Rattlesnake Canyon where the La guna Seca

strata consist largely of argillaceous sandstone and thin limestone beds ,

including several fossil reefs . Near the base of the Laguna Seca forma ' 1 - tion , fossils occur within 00 feet of the purplish brown shale of the

Moreno formation ; the intermediate beds , however , grade imperceptibly

through fine sandstone , argillaceous white sandstone , brown silt , to the

purplish shale . The contact is placed at the top of this shale as was done a n d previously by Anderson and Pack C . F . Green P o en oe Stewart , p , and Snavely

L ith olo gy. The Moreno formation is characterized by organic clay

W - shale . here well exposed it is purplish brown , soft , bedded in paper thin 34 OR T I GA L I TA PEA K QU A DR A N GL E [Bull 1 67

r - laminae which commonly weathe to small pencil shaped fragments , h W and intricately veined wit gypsum . At ildcat Canyon the Moreno

includes massive white - weathering diatomaceous shale that is lit h ol ogi K r cally indi stinct from the younger ey enh a ge n shale of E ocene age . Anderson and Pack ( 1 9 1 5 ) reported that diatoms are as abundant in r this member of the Moreno fo mation as in the Tertiary diatomite . Massive concretionary sandstone identical to that of the Panoche for mation comp r ises a considerable part of the Moreno formation of the

Laguna Seca area , and like that of the Panoche , the sandstone is lentic r ular and grades late ally into shale . Sandstone occurs in beds a few inches

to several feet in thickness alternating with siltstone and shale , and in

massive lenses as much as 5 00 feet or more in thickness . Most of the sandstone is calcareous and concretionary and contains

- abundant biotite and feldspar , but some is light colored and friable , and

contains few visible fer romagnesian minerals . Glauconite is widespr ead

but never abundant . Conglomeratic sandstone is limited to several zones

all of which contain fossils . Sandstone dikes within the Mor eno formation are not as common in the Or t ig a lit a Peak quadrangle as they are else o D where al ng the eastern front of iablo Range , but several small dikes a few feet in thickness a re well exposed in the southern tr ibutary to r t i a lit 2 1 1 1 0 O a SW 8 . S . . g Creek in the i sec . , T , R . E

C . F . Green lists the following foraminifera from the Moreno shale

D 2 2 . o SW . 1 . 1 S taken from a single locality in g Leg C reek ( i sec , T , 1 0 R . E . )

Gl obo t ru n c a n a a r c a M a r i n u li n a m de s t a a g c f . o R uess of Cushman and P rker S i h o e n e ri n oi d ol a r k i p g e s c f . Cushman and Campbell S ip h og e n e r in oide s wh it e i Church B u li m i n t r i a n u l a r h a of . g i s Cus man and Parker B u li m i n a p r o lix a Cush man and Parker i i d C b c i e s 3 p . n E p o ide s 8 p . G lo big e r i n a c r e t a c e a ' Globig e ri n e ll a v ol u t a White G u e n be li n a g lo b ulos a r idi n Gy o a 3 p . L a g e n a a c u t ic os t a R uess N odo a r n u da s i a c f . R uess of Cushman and Church R t a l o ia 3 p . Ve n t i la br e lla or n a t iss i m a Cushm an and Ch urch

’ This assemblage p robably falls into Gou dk ofi s (1 945 ) D - l zone a l though the re ar e several forms including Siph og en e ri noides whit c i B u li m i n t r i n l ri u b in a c . a u a s G e m e l a Church , f g Cushman and Parker , lobu losa Ve n tila b r e ll rn a i g , and a o t ssim a Cushman and Church that do n i . h i not belong t s zone .

P e t r o g r a ph y of t h e U pp e r Cr e t a c eou s S e d i m e n t s Because there is little to distinguish rocks of the Moreno formation

from those of the Panoche , except the content of organic shale , the two

units will be described together .

M e a sc o i c F e t u r e g p a s . Perhaps the most striking featur e of the Upper

Cretaceous is the great thickness of strata involved . Several measure ments along st ructure sections have been given which show a maximu m e thickness of about feet . The r lative proportions of sa ndstone and shale have been computed from the sections with the assumption that n 1 0 sandy shale u its contain percent sand . 1 953 ] STR A TI GR A PHY 3 0

’ Section ( north to south ) E - E E ntire U pper Cretaceous

T otal thickness , feet

Sandstone , feet S andstone , percent

Shale , feet

Shale , percent

Conglomerate , feet o Congl merate , percent L imestone , percent The average composition of the Upper Cretaceous sediments in the ar ea covered by the Ort iga lit a Peak quadrangle includes 3 9 per cent sand 5 9 1 2 stone , percent shale , to percent conglomerate , and a trace of lime stone . Throughout the Upper C retaceous section the rocks a re lithologically similar and a hand specimen of Moreno sandstone cannot be distin ish gu e d from one of Panoche sandstone . Exceptions are coarse biotitic ol m ic t ic sandstone and p y conglomerate of the lower Panoche strata , and r organic purplish shale of the Mor eno fo mation . Indeed , a lithologic dis

- tinction can be made only by grouping of rock units .

'

Sedimentary structures include such features as limestone concretions , “

- and exposures are dotted with these dark reddish brown . spherical can

- 1 0 On non balls which attain a diameter of feet or more . fr esh exposur es , r howeve , the concretions ar e the same color as the surrounding sandstone r “ f rr and can be distinguished only by th eir concentr ic str uctu e . Small e u in ou s r g concretions , many having irregula and grotesque shapes , occur in shale , but few reach more than a few inches in breadth . In sandy shale units a thin stratum of argillaceous limestone commonly separates thin e r bedded platy sandston from overlying shale , and at seve al localities in the Panoche formation the upper surface of the sands tone adj acent to the limestone is contorted into small undulations with an amplitude of less than an inch .

- Slump bedding, another type of non deformational folding , occurs in the Panoche formation along Salt Canyon about a mile above the junc w 1 1 1 1 9 . 3 . . tion ith Little Panoche Creek in sec . , T S . , R E The names “ ” “ ” ” glide bedding , curly bedding , and hassock structur e have been

l u 6 - m used as wel for this str cture . Folding in a inch silt stratu inter bedded with papery - thin argillaceous shale is outlined by darker bands 1 942 203 234 W . within the white silt stratum . ilson ( , pp , ) cites a similar example in the Call sandstone member of the Panoche formation along the west bank of Moody Canyon in the southeast corner of the Panoche “ Valley quadrangle and suggests that the contortion is p r obably due to sliding or slumping while the siltstone still contained water and could ” act as a plastic mass . 1 9 50 1 45 Pettij ohn ( , p . ) wr ites that such deformation is due to sub aqueous slump or gliding (and is ) most characteristic of thick silt

% shale sequences of gr aded beds which mark delta - like accumulations of geosynclines . The common occurrence of thick shale beds interrupted by thin silt strata indicates that intraformational slumping I S mor e pr obable in this I 1 s depositional environment . f sediment deposited on a gentle submar ine c slope , the weight of the a cumulated overburden of mud and silt would expel the entrapped water from the mud into the more permeable silts , v making them more fiu ida l and r educing their v isc os it . Even on a very 3 6 OR T I GA L I T A PEA K QU ADR A N GL E [Bull 1 67

gentle slope , weight of the sedimentary overburden could lead to sub marine slumping ; such movement p robably would occur during or shortly after deposition when the greatest compaction of the muds a n d expulsion of the entrapped water takes place .

Feldspar (including orthoclase , micr ocline , perthite , and sodic plagio clase ) is the major constituent of the sandstones e qualing or exceeding quartz in all specimens . Orthoclase is the p redominant feldspar ; plagio clase is especially conspicuous in the lowe r Panoche sandstones where it r o k is commonly associated with volcanic c fragments . Biotite character 3 5 izes the Upper Cretaceous sediments , generally making up to percent of the mineral components , but it too becomes increasingly abundant in the lower part of the section and r eaches a maximum in the upper mem n h ber of the giant sandstone lens near the base of the P a oe e formation . r Abundant heavy minerals are epidote , sphene , tourmaline , g een horn blende , and black opaque minerals , wher eas zircon , garnet , rutile , and staur olite are widespread but in minor quantities . Staurolite is a promi nent heavy mineral only in the Moreno sediments . Lithic fragments (com osit e 1 5 p grains ) are present in all samples ( to pe rcent ) . Carbonate cementation is typical of perhaps 30 to 5 0 percent of the Uppe r C retaceous sandstones but an exact proportion is difficult to de f termine in so thick and extensive a formation . It is likewise di ficult to as ce rtain whether the cement is of primary deposition o r of secondary n r r l p recipitation from i t a st a t a solutions . Those sandstones containing widely disperse d elastic grains in a medium of 30 to 5 0 percent carbo nate matrix are most likely the r esult of accumulation of e l a st ic min eral and carbonate grains with subsequent recrystallization of the carbonate minerals . Cementation of sandstone areas closely associated with fossil molds and c a st s is clearly the result of solution of carbonate fossils and r edeposition of the lime in interstices of the . enclosing sandstone . Although marin e life was common in the Uppe r C r etaceous seas in central California , as shown by abundant fossiliferous boulders in

- Panoche conglomerate , well p reserved fossils ar e r elatively rare in the

Upper Cretaceous sediments , indicating that most of the carbonate cement was derived from solution of organic r emains .

T e rt i a ry - Q u a t e r n a ry S y st e m

The thick monotonous sequence of Upper Cretaceous sediments e on t r a st s strikingly with thinly bedded Te rtiary sediments . Concretionary su b r a wa c k e g y , thin argillaceous limestone , dark siltstone , and shale of r Paleocene age closely r esemble those of the Cretaceous ; however , qua t z ose a n a u x it ic sandstone and shale , glauconitic greensand , siliceous dia t om a c e ou s e n on ic and radiolarian shale , and b t sandstone , shale , and conglomerate stand in sharp distinction to all but Moreno diatomaceous shale . Diagnostic fossils have been found in only the lowermost Paleocene strata , often refer red to the Martinez formation (Anderson and Pack , 1 9 1 5 5 8 P o e n oe ly 1 944 , p . ; Stewart , p , and Snave , ) but named Laguna

Seca formation by Payne Radiolaria and diatoms , abundant in the white siliceous shale , have been described in detail by Clark and e Campbell but pr cise cor relation is not possible ; however , the K r e e n h term y a g e n shale is sanctioned by p revious usage in this area . The bentonitic beds are believed correlative with the Neroly formation Ort i a lit a of the San Pablo group . The formation in the g Peak quadrangle

38 OR T I GA L I T A P E A K QU A DR A N GL E [Bull 1 67

1 942 C . F . G reen ( ) erected four units within the Paleocene and E ocene

K r e y e n h a en - 1 —4 strata underlying the g shale and labeled them Te to Te ,

- l n n inclusive . The Te unit is equivalent to the Tej on of A derson a d Pack

i - ? r 2 - and the Tesla ( ) of this eport . Units Te through Te 4 are lithologic subdivisions of the Laguna Seca formation .

” Dist ri bu ti on a n R e la t ions d . The Laguna Seca formation is intermit V t en t ly exposed between Or t ig a lit a C reek and Y ildc a t Canyon in the Ort i a lit a 1 0 g area , a distance of about miles . A somewhat constant thick ness of feet is maintained thr oughout the exposur es ; however , the top of the formation is not exposed at either Or t ig a lit a C r eek or Wildcat n Ca yon . The line of separation between the Laguna Seca and adjacent formations is gradational and commonly poorly exposed .

L i th olo gy . The Laguna Seca formation is lithologically similar to the Upper C retaceous sediments , excepting organic shale and coarse n grai ed conglomerate . Although glauconite is abundant thr oughout the W 5 0 formation and locally , as in ildcat Canyon , compr ises about percent

W - of an individual bed , greensand is not typical . hite , tan , and dark gray y i n siltstone and cla shale pr edom nate , although abundant thi biotitic

y a r e r . sandstone beds , man containing calcareous concretions , inte bedded s w Mas ive , friable , hite sandstone streaked with red and or ange occurs Ort i a lit a locally , as in g C reek , laminated with thin shale and calcareous

n - r biotitic sa dstone beds . C ross bedding and ipple marks were observed in several localities and conglomeratic sandstone is well exposed along

W b u fi - ildcat Canyon . Thin , hard , colored argillaceous limestone beds , commonly associated with fossiliferous sandstone containing nodular “ ”

- a n d n . can on ball concretions , occur throughout the formation Petrographically the arenaceous sediments resemble those of the Upper C retaceous— sorting is fair as to size of the grains but not as to the r n minerals comp rising the grains . The mineral g ains are suba gular to r f su b ou n ded . O , and moderately close packed but not interlocking the 0 3 . total bulk , feldspar averages about percent Clay is sparse in most of the samples , forming the matrix of most of the sandstones and lending

e - s an unsort d appearance to the hand specimen . Heavy mineral include i hornblende , tourmaline , sphene , epidote , garnet , zircon , apatite , metall c ores , jarosite , and limonite . F i d A oss ls a n g e . Fossils are most abundant near the base of the forma

1 1 . 1 . S . 0 34 . tion along Rattlesnake Canyon (sec . , T , R E ) and in the n same zone alo g the low ridge for sever al miles to the southeast . Petrified logs are profuse along this r idge and also a quarter of a mile southeast it ll a h o ns is . a r e a c e e e of Laguna Seca Creek The Paleocene marker , p , occurs in the middle of the formation on a small spur about a quarter of a mile north of Laguna Seca C reek .

Con d it ion o D e iti n a s f pos o . The grad tional contact with the Moreno formation i n dicates that deposition did not cease in the geosynclinal basin e P h ola s at the close of the M sozoic era ; however , bo rings in sandstone and numerous petrified logs near the base of the Laguna Seca formation

e v a - suggest local sh l ing at the beginning of Terti ry time . Cross bedding , abundant molluscan and coralline fossils , floods of petrified logs and u smaller wood fragments , and ubiq itous glauconite are indicative of continued shallow marine environment throughout the Paleocene and 1 953 ] STR ATIGR A PHY 39

m possibly into the lowe r E ocene . Similar ity of Laguna Seca sedi ents to those of the Uppe r Cretaceous is suggestive of continued degradation of the Upper C retaceous landmass to supply the detritus . It is possible , however , that the sediment may have been derived from C retaceous beds exposed nearby .

T e s la F o r m a t io n

r - N a m e a nd Ch a ra c t e . Overlying the Paleocene Lowe r E ocene ( La guna Seca formation with apparent conformity are thin beds of quartzose sands and kaolinitic clay, brown shale , and local pebble conglomerate pre v iou sly mapped as Tej on by Anderson and Pack The only fossils found in these beds are poorly p reserved carbonaceous wood fragments and fragmental leaf impr essions , but str atigraphic position of the strata between fossilifer ous Paleocene beds and diatomaceous upper E ocene K r e en h a en beds of the y g formation restricts their age to late Paleocene ,

early or middle E ocene . Strata of similar lithology have been described from the Tesla area by

1 948 33 - 3 8 Huey ( , pp . ) and from other areas of the Coast Ranges and the 1 9 48 P o e n oe Sierra Nevada by Allen Huey ( ) and Stewart , p , and ' Snavely ( 1 944) noted the close similar ity in fauna and lith ol ogy b et we en the Tesla formation in Corral Hollow and beds along the east side of Diablo Range in the Or e st im b a area and Huey suggested use of the name f r u Tesla formation o the latter beds . The uniq e occurrence of quartz anauxite sandstones in Eocene strata of central California (probably cor r elative to the Ione formation of the western foothill belt of the Sierr a Nevada) leaves little doubt that the Tesla ( formation designated in Or t i a lit a the g area is correlative , at least in part , to similar beds in the Ore st im b a area and the type Tesla formation in Corral Hollow .

Dis t ribu ti on a n d R e la tion s . The Tesla ( formation has been traced for app r oximately 6 miles from Rattlesnake Canyon southeastward t o 2 W within miles of ildcat Canyon , wher e it is covered by ter race gravels and alluvium . A mile north of Rattlesnake Canyon a small exposure of pebble conglomerate is questionably assigned to the Tesla ( as is a thin glauconitic sand along the steep north bank of Rattlesnake Canyon man t le d K r n h a by soil derived fr om the overlying eye ge n shale . The contact with the underlying Lagu na Seca formation is gradational n and is easily discer ed where well exposed in steep stream cuts . E lse where soft basal sands of the Tesla ( formation r eflect a sharp topo graphic break with the harder concretionary sandstone of the Laguna

Seca ; consequently , throughout much of the area the contact is placed

at the top of the uppermost ridge - forming sandstone of the Laguna Seca

formation . An erosional surface separating the Tesla ( and overlying K r ey en hagen formations is clearly exposed along the north fork of Laguna Seca Creek where l arge blocks of white shale and sand of the Tesla ( i? ) are included in the base of the lower greensand of the K r eye n h a g en forma su b r ou n d e d tion , and elsewhere along the contact angular or fragments of white shale are abundant within basal K re ye n h a gen pebbly glauconitic

beds .

L ith ol r ogy . The sediments esemble those of the Ione formation de

scribed by Allen Thin - bedded white a n a u xit i c clay shale lami n a t e d with white quartzose sands that are particularly diagnostic form R T I GA L I T A P E A K A DR A N GL 1 67 40 O QU E [Bull .

2 K re e n h a e n b L FIGURE 0. Bold outcrop of y g shale along the east ank of Oro oma Creek 1 E 1 1 . 0 f 2 S R . . a in the southwest corner of sec . , T . , Tesl ormation poorly exposed in foreground . the top of the formation northward from Laguna Seca C reek to Or o Loma

3 0 5 0 . C reek . Maximum thickness of the beds does not exceed to feet The unde rlying strata include soft b rown claystone containing delicate len t i c u l a r partings of white to yellow a n a ux it ic sands and many carbona c e ou s Or o plant remains , and along the norther n branch of Loma Creek near the base of the formation is a conside rable thickn ess of white quartz a n a u xit ic ose sandstone that is red streaked , and clayey, minutely cross n bedded, and locally co glomeratic . The conglomerate pebbles , composed v ol c a m c largely of white to gray chert , quartzite , and acid rocks are ex c e t ion a ll 1 ou n d e a 1 l n p y well d and aver ge about inch diameter , occur ring in t thin lenticles only locally exceeding a foot in hickness , widely dis e rs e d p thr oughout the lower beds . 1 3 k A thin tuffaceous sandstone , foot to feet in thic ness , is interbedded with the lower conglomeratic sandstone in the southern part of the La guna Seca area , and although the outcrops are discontinuous , it is believed that this ej ected volcanic material is restr icted to a single zone . Pastel red to lilac colors p revalent in the high er beds mav be evidence of volcanic a sh accumulation . Petrographic study of thr ee r ock specimens (white quartzose sand stones having a variation in clay content ) reveals quartz to be the pre 1 0 dominant mineral , and clouded feldspar rar e , comprising less than e p rcent of the bulk . Anauxite or kaolinite , present in all samples , is pre n po derant in a specimen of laminated sand and shale . Biotite is common k 1 0 in the lower beds , muscovite and glauconite ma e up as much as per of cent the minerals in some of the sandstone beds . Heavy minerals are extremely rare , but they include the diagnostic mineral andalusite , pre su m a bl y derived from metamorphic r ocks of the Sierra Nevada , as well

s n . as metallic mineral , zircon , garnet , epidote , sphene , and tourmali e The clay mineral abundantly p resent is a member of the kaolinite gr oup p robably the species an auxite described by Allen ( 1 941 ) fr om the l on e formation of California ; however , the Optical properties do not exclude other members of the kaolinite group . 1 95 3 ] STR A TIGR A PHY 41

ri in r O g . The p esence of anauxite and andalusite suggests a Sierran

p r ovenance , but an increase of feldspar in the lower beds suggests some

other source area as well . Glauconite signifies a marine environment of

- shallow clear waters , evinced by cross bedding , ripple marks , and local 1 941 77 a n a u xit ic conglomerate beds . Allen ( , p . ) suggested that the E ocene beds of California were formed in shallow marine basins filled by sediments derived partly from the embryo Coast Ranges and partly from

the lateritic soils blanketing the Sier ra Nevada . A Paleocene or early Me a n os y E ocene time is indicated , either the g or Capa stage , and the p resence of wood fragments associated with kaolinitic clay and quartz i sand suggests a warm moist climate leading to deep chem cal weathering .

K re y e n h a g e n F o r m a t io n

The K r e ye n h a ge n formation of this report includes the K r e ye nh a gen shale as described by Anderson and Pack ( 1 9 1 5 ) and laminated sandstone

and shale , radiolarian shale , and pebbly greensand which are termed the K r e n h a n lower member of the ye g e formation . The basal beds have been mapped separately , but as no diagnostic fossils have been found in the r K r e e nh a en lower membe , the two units ar e gr ouped into the y g forma tion . In 1 945 Clark and Campbell published a monogr aph on radiolaria from K re e n h a en the y g formation near Los Banos , California , from samples ’ r o Be n n ison s collected by Allen Bennison near O Loma Cr eek . strati graphic section includes 45 feet of laminated sandstone and shale at the K r e e n h a e n 50 base of the y g shale , underlain by approximately feet of

- silty shale , glauconitic sand , gray sand , and coarse grained conglomerate “ Dom e n in e D o assigned to the g formation . The pebbly and glauconitic m e n gin e strata have bee n traced southward from Rattlesnake Canyon f or 6 about miles in which interval the thickness varies from 2 to 5 0 feet .

Dis t ri bu tion n R la ti n K r e e nh a en a d e o s . The br illiant white of the y g shale forms a conspicuous feature in the foothill belt bor dering the San Joaquin Valley from Rattlesnake Canyon southward almost to Wildcat o t Canyon , visible for miles both to the east and to the west . The zone of u c r 0 6 p in the Laguna Seca area extends approximately miles , and a thick ness of about 700 feet is maintained although an erosional surface sep K re e n h a en arates the y g shale from the overlying San Pablo beds .

L i th ol o gy . The basal pebbly glauconitic sand is a consistent marker zone of the lower sand and shale member and is no mor e than a foot or

- two in thickness . F r agments of well r ounded chert , quartzite , and various s 5 r porphyr itic igneous rock half an inch to inches in diamete , are the pebbles . Included i n this bed and the overlying greensand are fragments a n a u xit ic z it of quart sand and shale of the Tesla ( ) formation , overlain at Oro Loma C reek and North Fork of Laguna Seca C reek by 3 to 4 8 ds 40 feet of white to gray sandstone , feet of green san tone , and feet ‘ r a diol a i ia n l a of brown arenaceous c y shale which weathers a light gray . The 45 foot gritty white laminated sandstone and shale unit descr ibed by Benn ison reaches its maximum thickness along the northeastern bank Or o of Loma Creek and has a total later al extent of about 1 mile . The member consists of white , yellow, lavender , and gray , coarseg rained, cross bedded , thinly banded sandstone containing lenses of diatomaceous shale . Locally it carries well rounded black chert granules . It grades ver tically through sandy shale to b rown and buff diatomaceous shale 42 OR T IGA L I T A PE A K QU AD R A N GL E [Bu lL 1 67

2 FIGURE 1 . Photomicrograph of %arosite ( minute prisms and dark clusters ) and glau conite ( larger light to dark gray grains ) from lower member of K r e y e n h a g e n formation

R . fi 9 0x . one mile south of attlesnake Canyon Magni cation , plane polarized light

r of ' of the K eyen h a ge n formation . Near the base this member ripple marks and slightly curved tubules about a foot in length and half an inch in diameter (which may be molluscan burrows ) are indicative of very shallow water deposition . — The remainder of the K re ye nh a g e n formation the K reyenh a gen — - shale is composed of white weathering diatomite and radiolarite , with dense ocherous limestone lentils and thin layers of volcanic ash inter wn bedded near the base . F resh exposures are predominantly light bro to pinkish brown , massive , blocky, and thinly bedded however , weathered

- surfaces are of pure white , paper thin , flexible laminae . Locally, as along Oro r ff Loma C reek, bold pictu esque cli s are eroded into the soft white shale near the base of the unit . Gypsum is everywhere p resent in thin

- veins , both cross cutting and interbedded with the shale , whereas on ‘ flat summits e fiiorc sc e n t gypsum forms extensive deposits several feet in thickness . Ja rosit e comprises 30 to 40 percent of the Dom en gin e basal e on 1 glomerate mile southeast of Rattlesnake Canyon , occurring throughout the rock in drusy aggregates and individual crystals up to millimeter m 1 52 0 in diameter . Glauconite akes up percent ; quartz, acid feldspar , lithic fragments and clay compose the remainder . Replacement of glau conite by jarosite is clearly ' shown by a complete transition from clear ovoid grains of glauconite (some coarsely c rystalline , others crystalline aggregates ) to patches of jarosite incorporating scattered fragments of glauconite . Small crystals of j arosite are aligned along the basal cleavage planes of glauconite , and locally along the cleavage traces of alkaline 1 953 ] STR ATIGR APHY 43

a rosit e feldspar . J was observed in rocks of the lower member of the K r eye nh a gen formation - between Rattlesnake Canyon and Oro Loma 2 Creek , a distance of about miles , in the laminated sands at the base of r h n ro the K e yen a ge shale at O Loma Creek , and in a thin section of

Laguna Seca siltstone from Wildcat Canyon . Its presence was not dis covered until mapping of the area was p ractically complete , thus the i distribution may be more widespread than ind cated above .

r l t o A g e a nd Cor e a i n . The lower member is possibly correlative with n in the type D om e g e formation (middle E ocene ) south of Panoche Hills , but lack of diagnostic fossils prevents exact correlation at this time . The nh K r eye a gen shale is considered upper Eocene by most authorities ,

Oligocene by others , and for a complete review of the stratigraphic 1 9 30 problems and a copious bibliography up to and including the year , the reader is referred to O. P . Jenkins

FIGURE 2 2 Photomicrograph of %arosite crystals and clusters showing facets of indi i b K r e e n h a e n vidual gra ns . From glauconitic basal conglomerate of lower mem er of y g f f t l s n a k fi x i R a t e . 3 7 5 ormation , one mile south o Canyon Mag ni cation , plane polar zed ?i h g t .

Con dit ion D e o ition - di s o s . f p Abundant glauconite , cross bed ng and local molluscan ( t? ) bur r ows indi cate shallow mar ine deposition of the s i lower bed of the formation . A special zed envir onment was necessary ‘ — for p rodigious growth of siliceous r a diol a r ia n s a n d diatoms perhaps silica from volcanic tuffs now in the lower part of the formation attracted o these planktonic marine forms . There is c nsiderable controversy con cerning the depth of water accompanyi ng radiolarian and diatomaceous r deposition ; howeve , it is reasonably certain that the radiolarite of the

lower member was formed in relatively shallow water , but such cannot 44 OR T I GAL I T A PE A K QU A DR A N GL E [Bull 1 67

FIGURE 2 3 . D isconformity separating white a na u x i t ic Tesla ( shale and darker glau c on it i c pebbly sand s of the lower member of the K r e y e n h a g e n formation along the L f e s ee 1 . 1 2 . R . 1 1 E th S . 8 S . eastern bank o north fork of aguna eca Creek in , T , Tilted blocks of white shale and angular inclusions of sh a le in t h e base of th e glauconitic

b t h e . member are elow hammerhead Inch markings are on hammer handle .

A ila . o . c s be said f the upper shale Fish scales , shark teeth , and molds of p constitute the only fossil evidence besides siliceo us microorganisms in 5 00 n the top feet of this shale , but regularity and thin ess of bedding are suggestive of quiet deposition , perhaps in somewhat deeper water .

S a n P a b lo F o r m a t io n

n l n P 1 1 9 - k 5 . 5 9 N m e a d R a tio s . a c a e Anderson and ( 9 , pp 7 ) defined the San Pablo formation of the eastern part of the Diablo Range as follows T f a In the vicinity of esla , resting upon the undi ferenti ted Miocene and southward L T a o as far as ittle Panoche Creek , overlapping the older ertiary or Cret ce us strata , fin e is a formation composed mainly of bluish sandstone , and coarse grained con gl om e ra t i c sand T hey may be traced more or less continuously from the T esla region northward al ong the edge of the S a n Joaquin Valley to the north side of Mount D iablo where they have been more thoroughly studied and definitely correlated with the Sa n Pablo formation of S a n F rancisco S outhward from L one T ree Creek beds which represent a portion of the S a n Pablo as exposed in the area are traceable f or 3 0 h i about miles , nearly to Garzas Creek where t ey d sappear beneath the alluvium S a B l n . of the Joaquin Valley eds of somewhat simi ar character , occupy ing a similar Or t i a li t a a n d stratigraphic position , appear at the edge of the low foothills between g L ” ittle Panoche creeks .

The line separating the San Pablo formation from the K reyenh a gen shale marks the most p rofound erosional disconformity to which the sediments of this depositional basin were subj ect since the beginning l d ffi of Uppe r C retaceous . Angu ar discordance , if p resent , is i cult to show as the underlying siliceous shale is subj ect to local crumpling and slumping and the overlying conglomeratic beds are strongly cr oss - bedded and extr emely lenticular . The surface separating these formations is somewhat ir regular and its trend in the field cannot be pr edicted with l 600 certainty . Loca ly, as far as feet northwest and feet southeast of r ilic ifi d K r e enh a en Hamburg Creek , large blocks of ha d s e y g shale , several hundred feet in diameter , lie in the base of the San Pablo sandstone , and

46 OR T I GA L I T A PEA K QU A DR A N GL E [Bull 1 67

2 D L 5 . o e . 7 . 1 2 S . R . 1 1 E . FIGURE View north across g g Creek ( sec . T , ) showing white - weathering S a n Pablo bentonitic sandstone overlain by reddish unconsolidated L gravel of the Oro oma formation . Beds forming the subdued topography on left and in

- r K re e n h a e n 3 5 40 S a n J . fo eground are y g shale . Beds dip degrees beneath oaquin Valley

strengthened by volcanic detr itus in the rocks of Garzas Creek , the Ort i a lit a I n g area , and the highest beds of the San Pablo group of the D Mount iablo region . Fragments of horse teeth found in the N eroly formation along the west side of the San Joaquin Valley in the Carbona area have been assigned to the lower Pliocene by Stirton and flora of the Neroly is con s idere d a n 1 943 - l by Chaney, Conduit , d Axelrod ( ) to be Mio P iocene ; r 1 943 b t howeve , B . L . Clark ( ) and most inverte ra e paleontologists agree that the San Pablo invertebrat e fauna is upper Miocene . The imperfectly preserved pelecypod casts in the lower sands tone of r i li D t a t a W . O . the g area cannot be positively identified J . urham ( oral “ h r e communication , w o examined the casts , stated that they i u l semble a Sp s a , which would indicate brackish water or marine con ” dit ions .

Oro L o m a F o r m a t i o n Anderson and Pack (1 9 1 5 ) included in their Tulare ( formation beds overlying the San Pa blo for mation along the front of Laguna Seca 40 Hills which dip beneath valley alluvium at angles up to degrees , and fla t t ish beds bordering Little Panoche Creek which are structurally and l Or o lithologica ly distinct . Here the name Loma is given to the strata of Oro Laguna Seca Hills . The type section and name are taken from Loma

Creek along which there are excellent exposures . ro 6 The O Loma formation , continuously exposed for about miles south v of Rattlesnake Canyon , is o erlapped both north and south by Tulare 300 terrace mantle . The maximum thickness of approximately feet occurs r in the vicinity of O o Loma Creek .

L i th olo gy . The beds are an assortment of loose sand , silt , and gravel , of deep reddish color . The gravel , locally cemented to conglomerate , is a prominent lithologic type throughout the formation , however , near the

so - fin e r top of the sequence a ft , light gray, g ained , argillaceous easily 1 95 3 ] STR ATIGR APHY 47

eroded sand , can be followed for several miles south of Rattlesnake

Canyon . A section along Rattlesnake Canyon (measur ed by pacing) shows the following sequence :

1 x - . Argillaceous gray silt , soft , poorly e posed , weathers to clay like

2 W - fin e - i . hitish gray grained sand or s lt , uncemented , contains small well rounded chert granules about 2 millimeters in diameter

- h 3 . Orange red gravel , weathers a deep ric brown and contains subangular s u b rou n de d t to pebbles of red , green , black , and white chert , vein quar z , F G u graywacke , granitic rocks , ranciscan schist and greenstone . rades p ward through concretionary pebbly sandstone to overlying gray silt F ine grained argillaceou s red sand 3 Gravel , like unit Red argillaceous silt F ine red a rgil laceous gravel B entonitic conglomerate Arenaceous gravelly clay 1 R e d c 0 . silty lay 1 1 . Gravel

- 1 2 . Arenaceous silt , red colored G 1 3 . ravel

4 - 1 . t ite weathering arenaceous clay 1 5 G . ravel 1 6 R ed . clay and some gravel 1 G 7 . ravel 1 8 . Red clay 1 9 S . ilty gravel

20 F - . ine grained silty sand Sa n Pablo formation 1 T u fia c e ous 2 . conglomerate

- r th FIGURE 2 6 . Photomicrograph of clay cemented qua tz sandstone from e base t h e S a n P L r of ablo formation in Oro o na Creek . Halo of clay shows around many

n fi 2 5 x d . of the grains . Mag i cation , plane polarize light 48 OR T I GA e A PEAK QU ADR A N GL E [Bull 1 67

r ti on A e a n d Co r e la . Or o g As no fossils were found in the Loma beds , the age of the beds must be inferred from stratigraphic position and r structu al details . Underlying San Pablo beds (either upper Miocene r or lower Pliocene ) grade imperceptibly into the O o Loma formation . Date of the first pulse of the Coast Ranges orogeny which folded and faulted the Tertiary and older strata has been determin ed in adj acent 1 943 a e areas by Taliaferro ( ) to be upper Plioc ne , thus the beds in question may be lower or middle Pliocene in age , possibly both . Strata of similar lithology and stratigraphic position occur along the west side of the San Joaquin Valley northward from Pacheco Pass and include many beds mapped by Anderson and Pack as Tulare (

T u l a re F o r m a t i o n

i t ri ti n n hi c n s D s bu o a d T k ess . The most extensive occurrence of the Tulare formation in the area covered by the Ort ig a lit a Peak quadrangle are at Little Panoche C reek and between Los Banos and Ort iga lit a

Creeks ; however , there is considerable extent of the formation elsewhere 3 50 throughout the region . A thickness of at least feet is attained near s Little Panoche Creek , but the lowermost bed only are exposed to the south along the north - tr ending part of Little Panoche Valley ; thus as much as 5 00 feet of sediment may have been deposited in this area . Else where the beds ar e 1 00 feet or less in thickness and generally measure a few tens of feet .

L ith olo r o gy . Lithologic similarity of the Tular e to the O Loma led r Anderson and Pack to consider the formations a single unit ; howeve , Oro separation can be made on lithology and structural character istics . Loma beds are folded with the b e droc k ser ies and the Tulare strata ex

t en siv el a ll . y overlap of these units , including the Franciscan formation Thin argillaceous limestone of the Tulare formation which has no counter part in the Or o Loma formation is most extensive south and east of Little

Panoche Creek, especially in the bordering Panoche Valley quadrangle .

R - i f a FIGU E 2 7 . Flat ly ng dark reddish gravel and white marly silt o the Tulare form tion underlain by dark - weathering Moreno shale dipping 3 5 degrees into the cli ff along b b Or t i a l it a f the east ank of the southern tri utary to g Creek , in the southwest corner o

2 1 1 0 1 00 . . 8 . 1 S . R . E . sec , T , The cliff is approximately feet in height 1 95 3 ] STR ATIGR A PHY 49

Significant amounts of marl or marly silt occur north and east of A rbu ru a 1 2 r i l S . R . 9 s a it 1 . O a 7 1 2 t . . . Ranch (T , ea t of g Creek (sec , T S , R . 1 0 22 1 2 1 . . 0 and east of Laguna Seca Ranch (sec . , T S , R . A nearly complete section of the Tulare formation occur s in Little

6 1 . . 1 0 . 3 . 3 S 1 Panoche Valley (sec . , T , R E ) where approximately 50 to 200 feet of strata are exposed in the westward facing , deeply colored ff W rt i a li a cli s . hite silt beds are prominent between O g t Creek and Rattle snake Canyon and in the broad expanse of terrace between Ort iga lit a

Creek and Los Banos Creek . Gravel composed characteristically of sub r s angula pebbles , cobble , and boulders dispersed through an incoherent

red silt matrix is the dominant phase of the Tulare formation , which

may be appropriately termed terrace gravel . Franciscan debris including

- h che rt , diabase gabbro , glaucophane sc ist , greenstone , and graywacke is r the most prominent constituent . Relative f equency of the Franciscan s rock types in the Tulare varies with geographic location ; green tone , a r gabbro , and graywacke predominate ne Piedra Azul Creek and in the

Laguna Seca Ranch area ; chert , glaucophane schist , and graywacke p r edominate in northwestern Little Panoche Valley ; and sheared gr ay

wacke predomi nates in southe r n L ittle Pan oche Valley . That the compo sit ion of the gr avel in th e Tulare formation directly r eflects the lithology of the adjacent Franciscan terrane is evidence of local origin of the

Tulare beds . Locally, as along Los Banos Creek , C retaceous debris especially pebbles and boulder s from Panoche conglomerate ) is promi S is nent in the Tulare beds . Concretionary Panoche andstone a minor but widespread constituent and other Cretaceous sediments m a y have

contributed to the silty matrix .

t ru t r l R ti n S c u a e la o s . The Tulare formation was deposited upon a weathered and e roded surface of low r elief with a gentle easterly Slope r c ov toward the San Joaquin Valley . Initial dips recorded f om the beds 2 3 ering Little Panoche Valley reach a maximum of to degrees , but local iz e d uplift of Wisenor Hills tilted the Tulare beds lapping ar ound the northern rim of this poor ly defined dome and subsequent erosion has di largely removed the beds from the summit areas . The steepest p 1 recorded is 6 degrees .

A a nd rr la ti n r e Plio g e Co e o . The Tulare fo mation postdates the upp r

r i - cene orogeny and p edates the m d Pleistocene orogeny, thus limiting

- Con se the time of deposition to Plic Pleistocene and early Pleistocene . o r quently, age and degree of def rmation ag ee closely with the typical

Tulare formation south of Coalinga .

S t re a m T e r r a c e s

Remnants of older stream levels occur along most of the larger str eams r r and locally, as in Pied a Azul and Molino C eeks , remnants of two or t three ter races have been preserved . The highes and oldest level is approx im a t el 5 0 75 r n y to feet above the p resent st eam beds , whereas the you gest 2 1 0 and most extensive terrace stands from to feet above stream bottoms . In Piedr a Az ul trough materials composing stream terraces closely r e semble those of the Tulare formation ; therefore the line of separation is ‘ w r arbitrarily dra n . Good sections of local st eam terrace are well exposed

in the lower reaches of Laguna Seca and Hamburg Creeks . 50 OR T I GA L I T A PEA K QU A DR A N GL E [Bull 1 67

R e c e nt A llu v i u m

Sediment der ived from local areas by sheet flood _and gulley erosion is r periodically deposited in the g aded portions of streams , valleys , and depressions during the short seasonal rainfall . The San Joaquin Valley is the largest area of aggradation and the ultimate site of deposition for Di the sediment derived from the adj acent foothills and ablo Range . E ro sive action of wind continually removes , t ransports , and sorts loose soil and sediment but the over - all bulk is small compared to th at of running r r wate ; howeve , infrequent winds of strong velocity remove considerable t volumes of fine dus , p rincipally from the border areas of the San Joaquin

Valley .

L a n d s fide s Local slides of considerable quantities of material occur only in Fran ' c is c a n r k E lsewh e r e serpentine between Piedra Azul and Molino C ee s . small slides of shale in the Panoche and Moreno formations are present along steep stream banks . Large cones of conglomerate talus in Los Banos Creek were not considered landslides as their formation was piecemeal and not by movement of large masses of material .

G E O L O GIC S T R U CT U R E The general picture of Diablo Range has been termed by Anderson “ 1 9 1 5 1 08 a s and Pack ( , p . ) a broad anticline which has been subj ected to long continued erosion . This erosion has stripped the cover of younger beds from the central part and exposed a core of folded and contorted Franciscan rocks flanked by steeply inclined Cretaceous and younger ’ b e ds f

In the Ort ig a lit a Peak area this anticlinal structure has been disrupted on its eastern flank by the thrusting of Franciscan rocks northeastward over younger sediments . The dismember ed flank i ncludes bedded Creta ' c e ou s and Tertiary sediments dipping eastward in a homoclinal sequence overturned adjacent to the thrust . The crest portion of the anticlinal structure is difficult to locate and escaped detection by Leith ( 1 949 ) in the adjacent Quien Sabe area . The p rincipal folds in the Franciscan rocks appear to be westward - trending anticlines and synclines as indicated 1 949 W 1 942 t by Leith ( ) and by ilson ( ) in the San Beni o area , and a similar structure is suggested by a local decrease in inclination of the Ort i a lit a F ranciscan strata south of g Peak . However , the main anticlinal axis must trend southeastwar d across the southwest corner of the area a s dips as low as 25 degrees northeast a r e recorded within 2 miles of the thrust , but it is just as probable that in regions several miles west of

n - the fault the dominating structure t re ds east west , a possible result of

Dia bla n - r the and the mid C etaceous orogenies .

F oldi n g

Di bl A n ti i D a o c l n e . uring the upper Pliocene o rogeny some feet of preponderantly sedimentary strata were flexed into a single anti clinal fold with the intensity of u pb owin g decreasing gradually toward the east and terminating abruptly at the border of the San Joaquin 1 7 se e s . Valley, as is indicated in well cores in the adjacent valley in and 28 1 2 1 r S . 1 , T . , R . E . Moreover, numerous st eam terraces indicate that 1 95 3 ] GEOL OGIC STR U CTU R E 5 1

2 8 . a A A . S FIGURE Piedr zul trough , near Piedra zul spring mall dark knolls are silica - carbonate rock ; hill in right background is serpe ntine cappe d in r O rt i a l i t a f r pa t with Tulare gravel . g thrust orms easte n border of hill and trends %ust right of knoll and Franciscan rocks in foreground . the region is still tectonically active and that future deformation may be expected to transgress eastward and to fold the bordering sediments of the San Joaquin Valley . U pb owin g of Diablo Range probably started in lower Miocene and is reflected in the unconformity separating the K r eye n h a gen and the San r Pablo formations ; howeve , some feet of Upper Cretaceous sedi ments must have been removed p revious to exposure of the Franciscan r rocks . P robably the greater pa t of the upper Miocene and Pliocene formations are composed of redeposited Cretaceous sediment , and the feet of Upper C retaceous beds in the area covered by Ort iga lit a Peak quadrangle may have been re presented by less than feet

along the axis of the p resent Diablo Range . Thus the total elevation of D iablo Range since the Miocene lies between and feet , de pending upon the thickness of Upper Cretaceous and Lower Cretaceous strata originally present ther e and the thickness of Franciscan rocks v e subsequently remo ed by rosion . The major part of this uplift probably Or t i a lit a took place along g thrust in upper Pliocene time .

i n r l flexu r e of W W s e o F o d . The small in the northern portion isenor

- Hills was probably caused by the mid Pleistocene orogeny .

L Ba n F l os os o d . A small imperfectly formed dome is evident from the ab rupt westerly swing in the trend of the Upper Cretaceous rocks in L os the northwest corner of the map area . Panoche sediments south of 3 5 W . Banos Creek strike N . and dip eastward whereas north of the creek the beds strike westwa rd and dip northward 1 0- 20 e e flxu re d gr es . The center of this e lies in the neighboring Quien Sabe area and is shown by Anderson and Pack affect the attitude 2 of beds in a circular area about miles across . Alignment of the dome 52 OR T IGA L IT A PEA K QU ADR A N GL E [BulL 1 67 with two small areas of late Pleistocene volcanic flows to the northwes t in the area covered by the Pacheco Pass quadrangle led to the suggestion t hat the doming is caused by a local igneous intrusion , not exposed by

er os 1 0 n .

F a u lt i n g

r Ortiga lit o Th ru s t . The fault fo ming the contact between Franciscan I and younger rocks is one of the maj or structural features of this area . t was first shown by Anderson and Pack (1 9 1 5 ) in the Ort iga lit a area but 4 6 2 1 9 3 . 1 Taliaferro ( , p ) noted that the extent in this area is but a small part of the major thrust that bounds Diablo Range along its eastern edge D from the New Idria area northward to Mount iablo , a distance of about 1 20 r rt i a lit a r miles . The no theast continuation of O g th ust in Quien Sabe area was described by Leith (1 949 ) who mapped a parallel fault in Fran i a c sc n terrane northwest of Carrisalito Flat . Undoubtedly the continua z tion of this structure forms the western boundary of Piedra A ul trough , and perhaps the identical fracture is responsible for the escarpment rt i a lit a O facing Little Panoche Valley south of O g Peak . In essence rti galita thrust forms the most easterly fracture of a broad zone of sheared

‘ 2 rt i a lit a Franciscan rock , a mile to miles in width near O g Peak, narrow ing slightly north of Wisenor Flat . The southern continuation of the thrust beneath Little Panoche Valley must pass near Mer cey Hot Springs as it is known that F ranciscan rocks and Cretaceous sediments are ex n r posed i adj acent outc ops through Tulare terrace mantle in this vicinity .

(Taliaferro , oral communication . ) Rapid later al and vertical variation in the inclination of the fault sur face is a striking characteristic of the Ort ig a lit a thrust which dips 20 to 3 0 degrees southwestward near Or t iga lit a Peak and gradually steepens W r northwest of isenor Flat , becoming essentially ve tical north of Piedra

Azul Spring . Convexity of the fault surface , evident where Franciscan W 1 943 a rocks are thrust over isenor shale , has been noted by Taliaferro ( ) “ who states that in the vicinity of Ort iga lit a Creek (section VII ) the dip varies from less than 3 0 to 45 degrees in a vertical distance of ’ ’ nearly feet . . Preliminary fractures in this zone appeared near the close of the Upper Jurassi c during the Dia bl a n orogeny but extens ive e thrusting along this zon occurr ed during the upper Pliocene orogeny .

The fault may have broken again during the mid - Pleistocene or ogeny ;

Ort i a lit a - Pl eist o however , g thrust p r oper was not active during the mid cene as the Tulare (Plio- Pleistocene and early Pleistocene) formation Or t i a lit a laps over the g thrust in Carrisalito Flat , Piedra Azul trough , and Little Panoche Valley without dislocation .

Min or L on itu din a l a n T r a n e r F a lt d sv se u s . g Faults , most of minor displacement , are abundant within the Franciscan rocks but only a few of these were mapped . A strong offset along the northern border of Wisenor Flat is p robably a fault having a horizontal displacement of

feet or more ; the same fault may account f or a - foot easterly i displacement of the serpentine contact a mile to the east . If it d splaces Ort i a lit a r the g thrust , it is clearly of Pleistocene o Recent age , and may be associated with the doming of Wisenor Hills . Faults displacing con glomerate beds in Wiseno r Hills are believed to be tension fractures formed at the same time .

5 4 OR T I GA L I T A PEA K QU ADR A N GL E [Bull 1 67

1 8 1 2 . E . 29 w l i f . . Ort i a ta . S R . 1 0 FIGURE View do n g Creek in southeast corner o sec , T , Massive outcrops of left and right middle ground are coarse conglomerate str ata whi ch 4 dip downstr eam about 5 deg rees .

l : . accumu ated sandstone , silt , shale , chert , limestone , and conglomerate i s Submarine volcan sm, deposition of chert , intrusion of ba ic and ultra di basic igneous rock into the wet geosynclinal se ments , and local develop ment of glaucophane schists heralded the close of the Upper Jurassic . E arly Lower C r etaceous sediments are not recorded from the Orti i gal ta area, but gently folded late Lower Cretaceous shale and sandstone , angularly discordant with Upper C retaceous sediments , indicate con t in u e d - if geosynclinal deposition and mid Cretaceous folding, upl t , and regression of the sea which exposed the tilted Lower Cretaceous sediments to erosion .

3 r L a a S a C . FIGURE 0 . View north along c est of L aguna S eca Hills across gun ec reek S ubsequent draina g e pattern well developed in the alternating hard and soft strata of the massive concretionary sandstone unit that underlies greater part of the summit area. 1 95 3 ] GEOL OGIC HISTOR Y 5 5

3 - f FIGURE 1 . Badland topography eroded into Tulare beds in westward facing blu fs in

- s e e . 3 6 . 1 3 S R . 1 0 E . W , T The upper light colored beds are hite , red , and bluish cross d b d e e r u n c on s ol i bed ed gravel , sand , and silt underlai y p bluish g een and deep red ni ated siit.

Subsequent sinkin g allowed ingress of the broad shallow Upper Cre t a c eou s se a that lapped against a deeply eroded Sierra Nevada of low relief near the eastern border Of the present Great Valley . A vast thick ness (over feet in the Ort iga lit a Peak region ) of shallow - water marine sediment was deposited in this basin during Upper Cretaceous time . The sea persisted until middle E ocene when local uplift allowed a sh llow scoring of the previously deposited sediments . In middle Eocene the long - lived western landmass contributed little or no detritus to the basin ; deeply weathered crystalline rock of the Sierra Nevada supplied e quartz sands and refractory kaolinitic clay . The climat was tropical , warm and moist , and chemical weathering deeply altered exposed rock

s . w ma ses Near the close of the Eocene epoch the sea returned , teeming ith plankton , and siliceous tests accumulated on the sea floor to a thickness of K r en h a en several hundred feet to form the ey g shale . Silica derived from volcanic ej ecta showered into the sea from distant volcanos possibly a t i tracted these organ sms . R enewed epeirogenic uplift drained the nar row upper Eocene seaway and laid bare to erosion the freshly formed siliceous shale in lower D Miocene time . enudation was followed by ingress of a shallow sea and deposition of a thin stratum of q u a r t gose sand close ashor e o r in b rackish r wate . Local uplift at the close of the Miocene epoch flooded the area with coarse detritus coeval with deposition of great thicknesses of wind ff blown tu aceous silt and sand . Volcanism ceased in lower Pliocene but deposition of detritus derived largely from nearby F ranciscan rocks continued . The material sp read as broad alluvial fans from the embryo D iablo Range onto vast flood plains to the east . Deposition was inter rupted in upper Pliocene by u pb owin g of the sediments into the broad anticlinal Diablo Range while other ranges of the Coast Ranges were folded and thr ust into much their present topo r i li di graphic form . Rupture of the rocks along the O t ga t a thrust splaced Franciscan over Cretaceous and di sjointed the eastern limb of the anti i clinal structure . The bor dering foothills were eroded and mperfectly l 1 67 56 OR T I GAL I T A PEA K QU A DR A N GL E [Bul

v . FIGURE 3 2 . Mercy quicksil er mine , southern workings 1 95 3 ] GEOL OGIC HISTOR Y 5 7

3 3 R f f m . FIGURE . emains o S cott furnace o Mercy quicksilver ine 5 8 OR T IGA L I T A PEA K QU ADR A N GL E [Bull 1 67 planed in early Pleistocene and a thin mantle —the Tulare formation

- was spr ead over the surface . The mid Pleistocene orogeny locally folded and fractured the rocks , and the invigorated streams cut rapidly into soft sediments to form the present drainage pattern . Several more recent elevations temporarily renewed the activity of streams which cut into previously graded channels to form local terraces along their valley s . The geologic history recor ded in the Ort ig a lit a Peak area is much the D same as that elsewhere along the eastern bor der of iablo Range . This area was a central basin for accumulation of Upper Cretaceous sediment , W and perhaps for Franciscan sediment as ell ; however , the Tertiary rec ord throughout the Coast Ranges varies greatly from area to area , even where they are adj acent , as the Tertiary seas were narrow and much of

- the rocks were land laid .

E CO N O MIC R E S O U R C E S

Q u i c k s i lv e r

There is no recorded production of quicksilver from the Ort iga lit a

Peak area ; however , the Mer cy mine lying only a quarter mile beyond oim a r the southern b d y of the area is a lar ge producer . According to “ Y 1 945 25 r ates and Hilpert ( , p . ) It is the leading p oducer in the dis triet with a total recor de d pr oduction of flasks of quicksilver . r 1 860 Mexicans started mining cinnaba about , but it is reported that the I 1 9 1 1 property had been explored earlier fo r silver . n the Pacific Quick silver Company took over the mine , enlarged the operations , and installed

- 1 1 4 24 r 9 . a ton Scott furnace , which was Ope ated until about the end of ’ Since then operation has been intermittent .

2 33 1 3 S . . se es . 3 . The mining claims extend northward into and , T , R 1 0 r t i a lit a E . in the area covered by the O g Peak quadrangle and ore of economic value may exist along the kaolinized zone extending north Of D Mercy mine . eeply colored green clay of this zone is similar to the deeply colored beds at the base of the Tulare form ation in the adjacent wa s area to the east , indicative that a vast amount of quicksilver ore w stripped from this zone during lo er Pleistocene time .

M a g n e s it e Associated with F ranciscan serpentine are small thin veins of mag n e sit e W that have been worked on a small scale , as that north of isenor

Flat . However , no production has been recorded . Fracturing and shear ing of the serpentine bodies and the ir regularity and small size of the veins makes it doubtful that these deposits are economically valuable .

Di a t o m it e

The K r e ye n h a ge n shale is an almost in exh a u st a ble supply of pure t h e r t i a lit a diatomite in O g Peak area . Argillaceous diatomite , almost as pure , is in the Moreno formation of Wildcat Canyon . Neither of these sources has been exploited .

Gy ps u m

Vein gypsum , most too dispe rse to be of economic value , occurs abun a n l r E fflor e sc en t d t y in the K e y enh a g en and Moreno shales . surface de posits are in the same formations and are especially abundant on the 1 953 ] ECON OMIC R ESOU R CES 59

fla t t ish - K r e enh a e n r summits of the ridge forming y g shale . North of O o Loma C reek such material was being cleared of top soil in the spring of 1 9 5 0 fo r marketing by the Agriculture Minerals and Fe rtilizer Com pa n D a n d of Los Banos , California . Mr . A . . Sousa of this company p rospected prepar ed the deposits , which contain highly concentrated gypsite that demands little processing , and is of easy access for the local users of agricultural gypsum .

L i m e Lime is locally mined and processed for agricultural use from Tulare marl at the summit of Panoche Hills in the adjacent Panoche Valley 2 1 0 1 9 1 5 . area . Anderson and Pack ( , p ) listed partial analyses of two 1 4 1 1 N . 30 . . . S samples taken in 1} sec , T , , R E

P a r t i a l a n a l s e s o a l a r e u s be ds i n t h e t e rr a e a in t h e P a n h il y f c c o c c pp g a c e H l s i n s e e . T 1 3 0, . 4 R . 1 1 E . A n a l t e r e t i r ( y s , G o g S e g e ) 2

Sample 1 was considered a fair average of the marl ; sample 2 was taken from a thin hard limestone intercalated with a cl y .

K re enh a e n o The white , y g shale locally contains a notable proporti n

of lime and much of the shale will eff ervesce freely with acid . Several T 2 N 2 . 1 . 1 0 W . S . . samples taken from }; sec , T , R E were analyzed by means of the standar d alkalimeter tests following sedimentary laboratory pr o

c e du re giving a range of Ca C0 3 for white diatomite of 1 00 percent

(four tests ) and for brown diatomite of percent (two tests ) .

Be n t o n it e

Bentonitic shale , sandstone , and conglomerate Occur abundantly throughout the San Pablo formation along the eastern front of the im Laguna Seca Hills . It is doubtful that the deposits are of economic portance at the present time as the bentonite is commonly intermixed ri with silt , sand and gravel a d in most localities it forms the cementing el a st ic medium for these sediments . Somewhat pure bentonite of limited extent does occur within the San Pablo sequence ofie rin g a possible Of m future supply this aterial for local consumption .

S a n d a n d G ra v e l 2 sees . 3 Sand, and gravel from Los Banos Creek is being worked in 3 3 1 . 1 0 S . 0 . and , T . , R E . for aggregate and road metal Fresno County se operates a small g ravel pit at the mouth of Little Panoche Creek in e . 1 5 1 3 S 1 1 T . . , R . E . which is worked intermittently for use on local

county roads . There is almost a n unlimited supply of such materials .

P e t ro le u m

At least 7 holes have been drilled within the area covered by Or t iga lit a h Peak quadrangle in sear ch of oil and gas ; owever, all have been aban 60 OR T I GA L IT A PEAK QU A DR A N GL E [Bull 1 67

n d K r e enh a en n do e . The highly organic shales of the y g and More o formations have been long considered the source beds for much of the petroleum found in the San Joaquin Valley and bordering areas , and the presence of the same formations in the Ort iga lit a area suggests Oil i n r r e the possibility of economic accumulations of the bo de ing vall y. r However , the attitude of outc opping strata in the foothills Offers no clue to a favorable structure in the San Joaquin Valley .

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9 . F : D i v . S . resno and Merced Counties , California California Mines pecial Rept N e w Y 1 9 5 0 F 1 9 49 S : H B . Pettijohn , . J . ( ) edimentary rocks arper and ros , ork ( )

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0 9 6 I 1 p “5 6 611 9 ‘ 1 li q “ 30 10 3 9

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T HI S BOOK I s our: ON T HE L A ST AM P E D BE L OW

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