CAUFORNIA

GEOLOGY

A PUBUCATlON OF THEDEPARTIIEN'T OF CONSERVATIONDIVISION OF ...es AND GEOlOGY

s-.ofe-- PETE WILSONGo~

The "--cN~ DOUGLAS P WHEELER5(Jcr9lary for ResoutWS

~ 01 Conserva\lOll EDWARD G HEIDIG{)rrBClot

In This Issue ICOAST RANGES FIELD TRIP AND GUIDEBOOK 98GSA ANNUAL MEETING 98GEOLOGY AND MINERALOGY OF RING MOUNTAIN 99NEW EYES ON EASTERN CALIFORNIA ROCK VARNISH 1071991 Eel SYMPOSIUM 115TEACHERS PAGE ON GEOTHERMAL 116BOOK REVIEWS 117MAIL ORDER FORM 117CALIFORNIA GEOLOGY SUBSCRIPTION FORM............................ 118DMG RELEASES 119

DMG OFR 90-16 MINERAL LAND CLASSIFICATION OFHANNAH RANCH SITE 119

SP 109 GEOLOGIC EXCURSIONS IN NORTHERN CALIFORNIA 119GEOLOGIC DATA MAP NO. 7 120

Cover: View from Rmg Mounlaln Preserve across Tiburon and Belvedere towardsan Franclsco_ Boulders in the foreground are serpenllne. the olliClal slate rock ofCalilorma A sheet of serpenline IS a cap rodl at RIJ'IQ Mountain. AssOClaled withthe serpentine afe e:totlC rare rocks. Some rare plants lIVing on the serpenlme areonly lound here Because 01 liS umque rocks. plants. and archeologICal artIfacts,Ring Mountam has been set aside as a nature preserve An article aboutlhls areastarts on page 99 Photo by RICk Yorlr

Don DupliiSLena TabllIOL__

Jell TlIITQlIt

JAMES F DAVISSlate Geologlsf

CALIFORNIA GEOlOGY Slall

TectInIcal EditorAssastilnt Ed'IOiGrapha and DesIgn:PubllcabOnS Supennsor-

CALL FOR PAPERSCoast Ranges Field Trip and Guidebook

The South Coast Geological Society's fall 1991 field trip will be to thesouthern Coast Ranges from near Santa Barbara to Santa Maria. The tripwill be in late September or early October. Articles are invited for the guide­book that will accompany the field trip. Subjects need not be confined to theexact area 01 the field lrip. Deadline for submitted papers is July 15. 1991.For informaHon about article format and conlent call Lavon Lewis al 714­858·9602. Send submHted articles 10: Lavon Lewis. South Coast GeologicalSociety. P.O. Box 10244, Santa Ana. CA 92711 Y

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CAlIFOANIII GEOLOGY \ISSN~ aS55j kI pubkIheclmonll'Oy by 1lIe DepanmenI 01 eor-v.loon, 0Ma00n 01 "'oJIftllIlCl GIology The Reco<ds ono:t .. 8t 1721-2OIh S!r8tltSiItt_LO. ell 95814 Socand CIa..~ paid .,s.a.,nenro, ell f'oslmaSlllf 5e1ld addt chanon 10CAliFORNIA GEOlOGY (USPS 350 &1(1), Box 2980 SIer.­menlO, CA 95812-2910

AI9at'Ia COI'CIfI'W'Il 0rv1II0tl of 1.1...wid a.olooY~....,~W1d.- Mms.-ated 10 !he -'" _ onea.-.. 1nClI.ded on !he~_ e-Ibul«l-"Clet,pI>c*l\lf/lPhS._-'_~-.u""""-----THE CONCl.USIOHS AND OPINIOHS EXPRESSED INAATICLIES ARE SOt.E1.Y 1liOSE OF nE AUTl«JAS AHDARE NOT NECESSARl..Y ENOORSED BY TIE OEPART­WENT OF COHSERYAnQN

Cones_nee o.nould De .""•••..cl 10 Ell 10<CAl.FOAN!AG£CJ..OOY. 660 a.o.. 0.-.s.a-, CA_14·(1131

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May 1991fVolume 441Number 5

CGEOA 44 (5) 97-120 (1991)

GSAAnnual Meeting

The annual meeting of theGeological Society of Americawill be held October 21-24,1991 in San Diego. The themeof this meeting is MGIobaI Chal­lenge- and there will be accom­panying sessions about naturalhazards. global change. and thelimits of natural resources. Forfurther information contact:Vanessa George. Meetings Coor­dinator, Geological Society ofAmerica, 3300 Penrose Place.P.O. Box 9140. Boulder. CO80301. (303) 447-2020:'><"

GJ.{JI m,l( \1 SO( In)" OF \ \URI( ,\

(,S-\'I'N!

1991 ANNUAL MEETING~n !Mgo, CMifotniA. Octoblr 21-24

GlDBAL CHALLENGE• T.1l1liell Stuiorls• EJ.llibits• FIIId T""s• Short Courus

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98 CALIFORNIA GEOLOGY MAY 1991

Geology and Mineralogy of Ring Mountain- A Popular Nature Preserve

Marin County

By

SALEM RICE AND DAVID WAGNER, GeologistsDivision 01 Mines and Geology

CALIFORNIA GEOlOGY

Encroaching urbanization promptedThe Nature Conservancy to set asidethis exceptional area as a preserve. Itwas later declared a Scientific ResourceZone by the Department of Conserva­tion's Division of Mines and Geologyunder authority of the Surface Miningand Reclamation Act of 1975. By 1985The Nature Conservancy acquired titleto 377 acres and initiated a manage­ment program that provides protection.scientific research and education. andpublic use of the area (Photo 2). RingMountain is a preserve: collectingrocks or plants is prohibited.

..

GEOLOGIC SETTING

Marin County is part of the northernCoast Ranges geomorphic province andis underlain primarily by assorted rocksof the regionally extensive FranciscanComplex (Photo 3). Franciscan rocksinclude grayYJacke. sandstone. shale. al­tered basaltic rock (greenstone). chert.

<iIII Figure 1. Location map at the RingMountam Preserve. Tiburon Peninsula,Marin County, Calilornia.

VallejO

MAY 1991

24

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...Photol. View of Rmg Mountain lookingsouthwestward across Corte Madera.Photo by D.L. Wagner.

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PACIFIC

INTRODUCTION

The 602·fool~high Ring Mountain isat the northern end of the Tiburon

Peninsula (Photo 1) in south centralMarin County. about 15 miles north ofdowntown San Francisco (Figure 1). Itis named for George E. Ring. a countysupervisor who lived near the area althe lum of the century (Holing. 1988).The Ring Mountain area is protectedbecause of the distinctive mineralogy.flora. fauna. and Indian petroglyphsthat occur there. This area is the naturalhabitat of several species of rare andendangered plants and a unique speciesof spider. the blind harvestman spider.In addition to these rarities. Ring Moun­tain has a distinctive assemblage of un­usual rocks and is the type area of themineral lawsonite, an important indica­tor mineral discovered there in 1895and named for the famed University ofCalifornia geology professor. AndrewLawson (Holing, 1988).

-

Debris flow type landslides occur in ...some areas that are underlain by thickclay-rich soils which accumulate fromweathered material. Many 01 the slopesabove Paradise Drive between TrestleGlen Boulevard and the ridge spursouth of El Campo are mantled withlandslides developed in this type of soil.Wrinkled and sagged topographic fea-tures that occur on clay-rich soils arecharacteristic of active or recent land-slides. The lack of forest cover on thenorthern side of Ring Mountain alsosuggests slope instability. In contrast.lhe more stable slopes that occur to thesoutheast are heavily forested with bayand live oak trees.

Photo 8. Large landslide on the north slope of Ring Mountain near the Paradise Driveentrance to the preserve. The hummocky topography is typical 01 landslide terrains.The large rock slabs in the middle ground have moved downslope from the serpentmesheets thai cap the ridge top. Photo by S. J. Rice.

These faults are not active and thestresses exerted on the rock units in thisarea now are different from those thatcaused the Ihrust faulting. However,these faulted contacts do representzones of relative weakness that affectslope stability; sheared and shatteredrocks and abundant landslides in thisarea are evidence of this instability.

LANDSLIDES

The diverse rock types that makeup Ring Mountain differ significantlyin such characterisllcs as permeability.stability. and in the type of weatheredproducts. The weakest of these is theintensely sheared matrix materials inthe Franciscan Complex melange. II isthe source of many landslides in thisarea. particularly the larger ones.

Melange matrix material is finegrained. relatively impermeable and. inthe near-surface oxidizing zone. tendsto alter to soils that are rich in swellingclays. The melange acts as a groundwa­ter barrier beneath the more perviousserpentine sheets that cap the ridgecrests in the Ring Mountain locality.Groundwater readily accumulates inthe innumerable cracks of the coarselysheared and lractured serpentine andis the source of numerous seeps andsprings in the area.

Groundwater saturation in the po­rous melange and its swelling soils con­tribute to landslide activity below themelange-serpentine contact. Over timethis process undermines the otherwisemore stable serpentine cap rock: frag­ments. including acre-size serpentineslabs. have been added to the otherlandslide debris and carried downslope.

This type of slope failure is almostcontinuous around the periphery of the.serpentine sheets in the Ring Mountainarea. especially where the melange isthickest (Photo 8). In places where theunderlying melange is thick. compositelandslide aprons extend from near thehill crests to the San Francisco Bay.Large slabs of .serpentine and promi­nent blocks 01 resistant metamorphicrocks have moved well dO\Wl1slope oftheir original positions.

[n the past some 01 these transportedrocks have been mapped as in-place:however. eroded "windows" reveal thatthese units flowed over underlying sand­stone units. Such a window of un­sheared in-place sandstone exposed inthe creek bed southwest of MarinCounly Day School is evidence that aridge spur near the school is the lobeof an old landslide.

SERPENTINE SOILS

Because the serpentine solls in theTiburon Peninsula are low in essentialplant nutrients. but contain toxicamounts of magnesium. nickel. chro­mium. and cobalt. most plants cannotgrow in these soils. The plants that dogrow on these toxic soils have adaptedto the harsh conditions. Most rockscontain abundant aluminum. an elementthat forms the principal ingredient inclays derived from weathering proc­esses. However. because serpentine hasalmost no aluminum. clay minerals donot fonn as a weathered by-productfrom it. Serpentine also has scarceamounts of potassium. sodium. calcium.and phosphorous; all important plantnutrients. Consequently. serpentinelorms very thin gravely, nutrient-poorsoils where only specialized plants haveadapted.

Three notable plant species occurnaturally only on the harsh serpentinesoils of the Tiburon Peninsula: TiburonIndian Paintbrush (Castilleja neg/ecra).Tiburon jewel flower (Streptanthusniger) (Penalosa. 1963), and Ca/ochor'(us riburonensis) (Hill, 1973). The lastspecies. commonly called the TiburonMariposa lily. was discovered only a fewyears ago and occurs on Ring Mountainwithin an SO-acre area underlain by .ser­pentine (P. Ellman. personal communi­cation. 1976).

'" CALIFORNIA GEOlOOY MAY 1991

Following the discovery of lawsonite.other relatively rare minerals and rocktypes have been found in the vicinity ofRing Mountain. This locality continuesto be the focus of study by geologistsand mineralogists because of its intrigu­ing rock and mineral associations. Aselected list of technical literature aboutthis area includes: Ransome (l895),Holway (1906). Smith (1906), Taliaf­erro (1943). Switzer (1951), and Dudley(1969 and 1972). In addition. this local­ity is referenced in perhaps anotherhundred technical earth science-relatedpapers.

In some of the older technical litera­ture. the Ring Mountain area is referredto as the "Reed Station locality," aname derived from the railroad siding atthe old Reed Ranch headquarters in thearea. Photo 4. Weathered serpentine on the crest of Ring

Mountain. Photo by D.L. Wagner.

Photo 5. Vein of pumpellyite In a block of glaucophane schist.The vein is about one inch across. Photo by D.L. Wagner.

GEOLOGIC HISTORY

The Ring Mountain area is made up of detached rock slicesor wedges emplaced by a dynamic thrust fault system thatjuxtaposed older rock units over younger ones (Figures 2. 3).Younger sedimentary rocks occur in the lower strata of themountain while much older metamorphosed rocks are on top.

sandstone and Shale

Well stratified unmetamorphosed sedimentary units. formedfrom sand and mud. were deposited in a deep oceanic envi­ronment probably in the Late Cretaceous (about 80 millionyears ago). These sandstone and shale units are exposed inmany road cuts along Paradise Drive and Tiburon Boulevardnorthwest of where they intersect Trestle Glen Boulevard.They are also exposed in wave-cut cliffs along much of thenortheast shore of the Tiburon Peninsula. Good exposures ofthese sedimentary rock sequences reveal that they are mainlycomposed of sandstone beds. ranging from a few inches to afew feet in thickness. and are interbedded with shale. Beddingis characteristically steeply dipping and is contorted into tightfolds that are often overturned.

Serpentine and Melange

Much of the crest of Ring Mountain is capped by largesheets of serpentine rock. Serpentine is formed by igneousprocesses and is thought to originate in the Earth's mantlethat lies deep beneath the crust. 11 most certainly formed fardeeper than did the sedimentary units that now lie beneath iI.Although these serpentine sheets have not been dated, theyare perhaps the oldest rocks on the Tiburon Peninsula: proba­bly more than 150 million years old.

CALIFORNIA GEOLOGY MAY 1991 '"

FIgUre 2. GeologIC map of the Ring Mounlaln area. Tiburon Peninsula Map uMSare; sp. serpentine: 1m _ melange; gl • glaucophane sctust. chi. chlOf1le schist;Is • landslide: x rnar1ts outcropS or blocks too small to map at thIs scale. Geologyafter RICe and others. 1976.

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TIle serpentine sheets that cap Ihetwo hill cresls in the Ring Mountainarea (Agure 2) are se~raled from theunderlying sanqslone and shale units bya thick zone composed principally ofintensely sheared Franciscan Complexmelange (Photo 6). The melange repre­sents an ancient fault zone and is evi-

dence that highly altered serpentinizedmantle rock was thrust over the unmeta·morphosed sedimentary units. In placesthe melange is only a few lens of feetthick. However. under the main saddlebetween the two serpentine crests. themelange is much thicker and Is perhapsseveral hundred feet In thickness.

ExotIC MetamorphIC Rocks

Many prominent dark colored bkx:kymasses occur as outcrops principallydownslope of the serpentine sheets(Photo 7). These unusuall.Wather-resis­tant metamorphic rock bodies are in­strumental in making Ring Mountain acelebrated geologic locality. In general.each of these monolithic masses has adIfferent assemblage of minerals thanthe nearest adjoining rock mass. Mostof these bodies are coarse-grained. mas­Sive to schistose rock typeS: eclogites.hom~met amphibolites. glauco­phane-gamet schists. stilpnomelane­riebedute-quartz schists. chlorite schists.and actinolite schists. Some haw suchunusual mineral compositions that noneof the standard rock names apply. Thepetrology and mineralogy of these umtshave been investigated in considerabledetaU by DOOley (19671.

These peculiar metamorphk: rockmasses range in size from less than afoot to many tens of feet across. Theyare-or I.Wre-embedded in less-resis­tant sheared melange matrix or in ser­pentine. Some are partially exposed byerosive processes or the less resistantrock surrounding them and appear tobe In place and partially embedded.Others have eroded out and lie as looserock masses on the surface of theground. and many have been trans­ported downslope by landslides: someas far as the San Francisco Bay.

WEST

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0= Melange 01 Il'\tensety sheared rock matenal that contams dislocated blocks 01 exotICmetamorphic roc::Xs SlJd'I as edogIle. glaul::of)hane sctuslS, and garnet arnphibolites

Ftgure 3. DlagrammalJC cross sectIOn ollhe RIng Mountaln area. Tiburon P8fIU'l$Ula. Mann County, Ca~fon'lla. shoWII'IQfault slices of serpenllTl8 and melange thrust CIVet' steeply dipptng sandstone and shale Aher RICe, J981

"" CALIFORNIA GEOlOGY MAY 1991

New Eyes on Eastern CaliforniaRock Varnish

By

DAVID H. KRINSlEY, GeologistRONALD I. DORN, Geographer

Department of GeologyArizona Stale University, Tempe

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Some of the earliest observations of ferro-manganese coat·ings on rocks were made near the Salton Sea in southernCali/ornia (Black. 1855). Von Humboldt. Darwin. and other19th century natural scientists realized thaI the major problemin understanding the rock varnish phenomenon was the de­termination of how manganese imparts a distinctive dark col­oration on rocks.

INTRODUCTION

This anicle presents findings from recent Investiga1l0ns of howrock varnish lorms and describes the manner in which thIs unde,­standing can aid researchers. Rock varnish is typically a glossy­brown to black coaling thai commonly develops on rod\; surfacesin arid climates. (t may take tens of thousands of years 10 form acomplete coating over rock surlaces.... editor,

Eastern California deserts have long been an exceptionalarea for studying rock varnish. Although olten referred

to as Mdesert varnish. ~ the same phenomenon has been ob·served on glacial moraines. periglacial stone garlands.waterfalls in Yosemite. and on rocks in virtually every terres­trial weathering environment in California. Because this phe­nomenon is produced by environments other than that of adesert. the term ~desert varnish~ is renamed in this articlewith the broader term ~rock varnish~ (Oorn and Oberlander.1981b. 1982; Krumbein and Jens. 1981).

A number of hypotheses have been proposed to explainthe occurrence of rock varnish. The following explanationsoriginated during examination of rock varnishes in the MojaveDesert; (1) the role of pollen in providing manganese (White.1924). (2) the role of lichens in somehow catalyzing varnishaccretion (Laudermilk. 1931). (3) physical and chemicalchanges althe rock surface (Hooke and others. 1969). and(4) the role of bacteria in concentrating manganese (Oornand Oberlander. 1981a).

MOJAVE DESERT

Four different types of rock varnishes were first recognizedin the Mo;ave Desert: (I) black surface varnish exposed to theatmosphere. (2) a shiny ground-line band at the soil-atmos­phere-rock interface. (3) an orange bottom varnish that coversthe underside of stones in a desert pavement, and (4) crackvarnishes that formed in rock crevices and range from blackmanganese (Mn)-rich to orange Mn-poor/iron-rich (Engel andSharp. 1958: Hooke and others. 1969. Darn and Oberlander.19821.

Researchers who worked in the Mojave Desert proposedthree possible ways rock varnish may form: (1) the constitu­ents of rock varnish are derived from the underlying rock(Hooke and others. 1969). (2) rock varnish is the result ofexternal sources (Polter and Rossman. 1977. 1979a.b: Oornand Oberlander. 1982). and (3) rock varnish is the result of acombination of the underlying rock and from outside material(Engel and Sharp. \958).

The mineralogy of rock varnish was established primarilyby work on samples from the Mojave Desert (Figure 1). Potierand Rossman (1977. 1979a.b) found black varnishes to becomposed of up to 70 percent clay minerals (mixed·layer illite­montmorillonite. with some kaolinite) and up to 30 percent

Bolded terms are in Glossary on page 115.

CALIFORNIA GEOLOGY MAY 1991 '07

TABLE 1. CHEMISTRY OF ROCK VARNISH IN THE MOJAVE DESERT, COMPARED WITH OTHER VARNISHES AROUND THE WORLD,

SITE POSITION N. MS AO ", , K C. TO Mn ", Nl C. Zn " " ,., •• ....

Sal, Springs. Mojave Dcun' U""'"~ 0-25" 4.40 25." 37.49 1.61 "' ,.~ 0.80 ." 11.77 14.SO "' "' "' "' "' "' '.25 "'T1Ili.I Fan. Death Valley Fonner Rod< FfX1U... bid'" 0.14 23.74 39.09 0.49 0.70 3.45 4.87 1.~2 10-87 13.47 0.13 0.12 0.27 ., '" '2' O~ '"M3ni~ Lal<e, Mojave Dcun :> 1m Above Soil 1.10 3M 25.77 32.3~ Ll~ 0.30 2.11 1.3~ ." 12.47 '"'' ., .n ..'" .25 0.21 0,22 '" 0.14Maunal<a TiU, Ha\O'llii WiLbSilieaSkin 0.62 1.98 21.13 29.77 0.69 0.20 3.30 4.89 .n 1l.60 2Ll3 ., "3 0.49 "' '"

., ." 0.98S;",.; Pntinsula, Esl"P' >Im Above Soil 0.28 '3' 22.94 32.81 '"

., 2.42 2.91 ... 11.97 "" ., '.25 0.42 "' 0.42 ., ." 0.27p.,,,,glyph. S. A...,ralia > I m Abov., Soil 0.17 1,21 22.81 33.34 .33 "' '.19 2.18 '" 21.70 "" ., '.AO '.AO "' '"

., ." •."Ing"';". PeN Desen A'SoilSurfaoce na·· .. 2,11 20.4~ 4~.88 .33 1.13 2.91 6.22 .~ ,." '><" ., •." 0.16 "' O.ll .,

'" 0.22Ave.. Roek, AUSlr.olia From Rod< F"""" ... "' '3' 28.77 35,69 ., ., 2.11 1.4~ 1.19 11.91 16.~7 ., '" '" ., '" ., 0.73 '"• Rcoulu.~ nonnalized '" 1~

•• Measu"""".... by PlXE (Cahill, 1986), except forLb., Sail Spring. """,pIe which we... Ollalyzed by el«:l"'" mieNp<'Ol:><'(POIler and Rouman. 1979&).••• Below Umi\ or detection•••• NOI available

manganese and iron oxides. The chem­istry of rock varnish was also deter­mined from samples taken from theMojave Desert (Engel and Sharp. 1958;Lakin and others. 1963; Hooke andothers. 1969; Potter and Rossman.1979a). Not surprisingly, the major ele­ments of rock varnish (silicon. alumi­num. manganese. and iron) reflect itsclay-oxide mineralogy.

Elements in concentration greaterthan 0.5 percent can be quite variableand typically contain magnesium. phos­phorus. potassium. calcium, titanium,and sometimes sodium. copper, zinc.barium. and lead (Table 1), Over 30other trace elements have been ob­served (Bard. 1979; Darn and others.1990).

The debate over the length of timeneeded for rock varnish formation cen­ters around field observations made inthe Mojave Desert. Engel and Sharp(I958) proposed that a complete coat­ing of varnish could form in 25 years,and this hypothesis was passed on inliterature (for example, Cooke and War­ren. 1973). Still, most Mojave Desertrock varnish researchers agree thatcomplete varnish formation takes thou·sands to tens of thousands of years(Blackwelder. 1954; Hunt. 1954; Huntand Mabey. 1966; Hooke, 1972,Darn and Oberlander. 1982; Elvidge,1982).

The disagreement over the length oftime it takes to form rock varnish arosebecause Engel and Sharp (1958) ob­seJVed varnished cobbles in a desertpavement that had been disturbed 25

years earlier in the construction of a dirtroad. The controversy was resolvedwhen it was realized that the pavementhad partially reformed with previouslyvarnished cobbles (Dorn and Oberlander.1982: E1vidge. 1982).

Rock varnish can be used as a tool tohelp interpret paleoenvironmental fluc­tuations. Potassium-argon (K-Ar) datingof volcanic flows in the Coso (Duffieldand others. 1981) and Cima (furrin andothers. 1985) volcanic fields providedage constraints that were used in testinginnovative rock varnish dating andpaleoenvironmental research methods.These methods include cation-ratio dat­ing (Dorn. 1983. 1989: Darn and oth­ers. 1990). radiocarbon dating (Darnand others. 1989), and micromorphol·ogical and microchemical changes thatare indicative of paleoenvironmentalfluctuations (Dorn, 1986, 1988, 1990).

The use of rock varnish as a geo­chemical prospecting tool has also beenproposed based on research in theMojave Desert (Lakin and others, 1963;Dorn and Oberlander. 1981b).

BACKSCATTER ELECTRONMICROSCOPY

Backscatter electron microscopy(BSE) uses electrons rather than light toview minerals in thin section. [t imagesmineral textural (grain to grain) parame­ters such as shape, size, orientation andgrain boundaries. SSE also exhibitschemical contrasts between minerals.and these contrasts show up on micro­graphs as various shades of gray. de·pending on the atomic number of the

elements composing the imaged miner­als; higher atomic numbers are brighter.

BSE and secondary electron (SE) mi·crographs of cross sections of easternCalifornia desert varnish samples wereused in this investigation to address on­going discussions concerning (1) theinternal versus external origin of varnishconstituents, (2) biological versus abioticorigin. and (3) the use of rock varnish insurface exposure dating. SSE is alsoused to identify previously unknowntextures. Although a variety of varnishtypes are present in the Mojave Desert,this investigation concerned only themost studied and most noticeable: man­ganese-rich, black-subaerial varnishformed on rock outcrops exposed onlyto airborne fallout.

INTERNAL VERSUSEXTERNAL ORIGIN

One of the longest lasting controver­sies in the study of rock varnish iswhether the constituents are derivedfrom the underlying rock or from exter­nal material. Following Walther (189l).the notion of high desert temperatures"sweating" solutions from the interior ofthe rock has remained an appealinghypothesis (Glennie, 1970: Garner,1974; Shlemon. 1978; Smith andWhalley. 1988). However. most reosearchers find conclusive micromorphol­ogical and microchemical evidence thatvarnish is an external accretion (Potterand Rossman. 1977. 1979a.b; Allen.1978: Perry and Adams, 1978; El­vidge, 1979: Dorn and Oberlander.1982).

'" CALIFORNIA GEOLOGY MAY 1991

PETROGLYPHS

In the 1970s the face of one of the large blocks of meta­morphic rocks in the Ring Mountain area was found to haveseveral ancient Indian petroglyphs (Photo 9) carved on it(Hotz and Clewlow, 1974). These were the first petroglyphsreported in the San Francisco Bay area by archaeologists.The carvings are in chlorite schist. a very soft rock, but onewith a greater resistance to weathering than many other kindsof rock types. Since their discovery, other petroglyphs havebeen found in the Ring Mountain area and all are carved insimilar chlorite schist. Some may be 2,000 years old (Holing,19881.

ACKNOWLEDGMENTS

We greatfully acknowledge the assistance of Gail Newton.DMG plant ecologist. and the manuscript reviews by DonDupras, Cynthia Pridmore, and Mary Woods. DMGgeologists.

Photo 9. Indian pelroglyphs carved into chlorite schist.Photo by S. J. Rice.

REFERENCES

Dudley, P.P., 1969, Glaucophane schists and associated rocks ofthe Tiburon Peninsula. Marin County, California: Ph. D. thesis,University of California. Berkeley, 116 p.

Dudley, P.P., 1972, Comments on the diSlribution and age of thehigh'grade blueschists and associated eclogites, and amphi·bolites from Tiburon Peninsula, California: Geological Societyof America Bulletin, v. 83, no. 11, p. 3497-3500.

Hill, A.J.. 1973, A dislinctive new Calochortusfrom Marin CountyCalifornia: Madrono, v. 22, no. 2, p. 100,104.

Holing, Dwight, 1988, Calilornia wild lands, a guide to the NatureConservancy preserves: Chronicle Books, p. 53·60.

Holway, A.J., 1906, Eclogites in California: Journal of Geology,v. 12, no. 4, p. 344-358,

Hotz. V" and Clewlow, CW.. Jr.. 1974, First report of the petro·glyphs: Master Key of Los Angeles County Museum, v. 4,p. 148.

Murdoch, Joseph, and Webb, R.W.. 1966, Minerals of California,Centennial Volume (1866-1966): California Division of Minesand Geology Bulletin 186, 560 p.

Penalosa, J., 1963, A flora of the Tiburon Peninsula, MarinCounty, California: The Wasmann Journal of Biology, v, 21,no. 1, p. 1·74.

Ransome, F.L., 1895, On lawsonite, a new rock·forming mineralfrom Tiburon Peninsula, Marin County, California: University ofCalifornia Publicaltons, Bulletin 01 the Depanment of Geology,'0',1, no. 10, p. 301·312.

Rice, S.J., 1964, A trip to the lawsonite type locality: California Di·vision of Mines and Geology Mineral Information Service, v. 17,no. 6. p. 96·98.

Aice, S.J., Smith, T.C., and Strand. A.G., 1976, Geology lor plan·ning, central and southeastern Marin County, California: Cali­fornia Division of Mines and Geology Open-file Aepon OFR76-2·SF.

Rice, S.J" 1981, Field trip guide, Stops 1, 2, and 3 (Marin Head·lands, Ring Mountain, Pillowed Greenstone) in Kleist, John A.,editor, 1981, The Franciscan Complel\: and the San Andreasfault from the Golden Gale to Point Aeyes, California: Ameri­can Association of Petroleum Geologists Pacilic Section, p, 13.

Smith, J.p", 1906, The paragenesis of minerals in the glauco­phane bearing rocks of California: American PhilosophicalSOCiety Proceedings. v. 45, p, 183·242.

Switzer, G.. 1951. Mineralogy of the California glaucophaneschists: California Division of Mines Bulletin 61, p. 61-70,

Talialerro, N. L., 1943, Franciscan·Knol\:vilie problem: AmericanAssociation of Petroleum Geologists Bulletin, v, 27, no. 2,p,109-219

Glossaryamphibollle: A dark-colored metamorphic rock composed01 minerals of the amphibole group,

eclogUe: A high.grade metamorphic rock chiefly composedof garnet and pyrol\:ene.

harz:burglte: A dark plutonic rock chiefly composed 01 otivineand enstatite.

ophiolite suite: A group of rocks including serpentine. gabbro,basalt and chert that commonly occur together.

rodingite: A massive dense rock composed mainly 01 garnetassociated with serpentine; formed by chemical alteration.

schist: A rock composed of aligned platy minerals: the majorconstituent mineral is used as a modifier {e.g. chlorite SChiSt),

CALIFORNIA GEOLOOY MAY 1991 ,os

The rare Tiburon mariposa lily (Ca/ochor/us liburonensis) grows naturally only inserpentine soils al Rmg Mountain. Firsl recognized in 1973. this perennial bloomsIn May and June and has lan, Cinnamon, and yellow flowers. Courtesy of theDepartment of Fish and Game Endangered Plant Program; photo by Rick York.

Ring Mountain~ildlife Preserve

Larger individuals can produce asmany as eight flowers although theaverage is two or three. The TiburonMariposa lily flourishes among boul­ders and serpentine bedrock out­crops. possibly as a result of the nu·merous associated seeps and springsand the inaccessibility to grazing.X'

The Tiburon Mariposa lily is a bul~

bous perennial characterized by a dis­tinctive cinnamon-and-yellow-coloredflower. It can be about two feet high atfull maturity and the flowers commonlyare in full bloom by the end of May.

Although this lily occurs nowhere elsein Ihe world, it is abundant at RingMountain and provides food for a hostof native insects, birds. and small ani­mals.

Some of the unusual plants atthe preserve include Tiburon Indianpaintbrush, with a yellow flower. anOakland star tulip. with a pale pinkflower. and the rare Tiburon Mari­posa lily. This plant. classified asColochorfUS tiburonensis in thebotanical literature. only grows natu­rally in one area on Earth: in theserpentine soils of Ring Mountain.

Ring Mountain can be reachedfrom San Francisco by taking High­way 101 north to the Paradise DriveExit in Corte Madera. Follow Para­dise Drive southeast for 1 3/4 miles(do not go into the town of Tiburon).Just past Westward Drive is a fireroad with a gate and a ~Ring Moun~

tain Preserve" sign. Park off thepavement on the shoulder of Para­dise Drive and walk to the hikingtrail. For additional information con­tact: The Nature Conservancy RingMountain Preserve. 3152 ParadiseDrive, 11101. Tiburon. CA 94920;(4151 435-6465.

T he Nature Conservancy offersfree gUided natural history walks

in the Ring Mountain Preserve dur~

ing the spring wildflower displayfrom April through June. The 377­acre preserve is open daily year­round during daylight hours. A 1­1!2-mile moderately steep hikingtrail loop in the preserve providesvisitors with scenic vistas of SanFrancisco Bay and the surroundingtowns of Sausalito and Tiburon.Admission is free.

"" CALlFORNIA GEOLOGY MAY 1991

Figure 4. Continual deposition withoul ero­sion is one o! the charactenstics necessaryto successfully date lock varnish. Figure 4dillustrates a sample that will yield a reliabledale. The other figures illustrate problemsthat are identified using BSE (4a-4c) andsecondary electron microscopy (4e-41). Forscale 4a. 4d. and 4e ., 130 micrometers.4b • 15 mICrometers. 4c • 60 micrometers.4f • 35 mICrometers.

4a. Hollows eroded into varnish at lhe baseand near the top. probably by microcolonialfungi or other acid·secreting organisms.Sample collected from the Marble Moun­tains. San Bernardino County.

4b. Redeposition of manganese and ironoxide in fractures in the varnish (brightstringers). Irom Hanaupah Canyon alluvialfan. Death Valley.

4C. Erosional hollows and redeposition ofoXides. Irom lathrop Wells cinder cone,southern Nevada.

4d. layered varnish Irom Marie Byrd Land.Antarctic.

4e. Time·transgressive behavior 01 varnishdepositIon, llIustrated from granodiorile gla·ciallil! at Pine Creek. Owens Valley. Var­nish is growing on grain boundary (upperlelt). but is having trouble colonizing thesurface of a smooth quartz grain (lowerright).

4f. Unusual situallon where varnish is notcompletely abraded by aeolian abrasion inlhe Cronese BaSin. There are possibly SIXevenlS recorded: (1) a lower layer is depos·ited; (2) varnish below the double arrow ISeroded. perhaps by microcoloniallungi: (3)varnish IiIls in this depreSSIOn: (4) aeolianabrasion truncates varnish above thedouble arrow; (5) a new layer of varnishdePOSitS above the double arrow: and possi­bly (6) some aeolian abraSion may haveoccurred on top of the newest varnish layer.

Mn and Fe can be concentrated byslight Eh-pH fluctuations, accordingto the abiotic model presented by Hookeand others (1969), Elvidge (1979), andSmith and Whalley (1988). Change toslightly more acidic conditions would beexpected to mobilize more manganesethan iron from source material. Subse­quent drying or a change to a more alka­line condition would be expected to re­$l.llt in the precipitation 01 manganese.Although this mechanism is theoreticallypossible. no data have yet been pre­sented 10 support this hypothesis. Also.the only experimental data on the physi·cal and chemical hypothesis indicated bythis mechanism do not concentrate Mn(Jones. 19911.

Fe-oxidizing organisms: (3) the geo­graphic distribution of varnish; (4) meas­urements of varnish alkalinity (pH) thatare unsuitable for the physical andchemical oxidation of manganese: (5)the lack of varnish at micro-sites that areconducive to the physical and chemicaloxidation and reduction of manganese:(6) findings that Mn-Fe accretions insoils. caves. springs. lakes. ore deposits.and oceans are due to microorganisms(Khak-mun. 1973; BoIOlina. 1976;Dean and Ghosh, 1980: Ghiorse. 1984:Peck, 1986; Cowen and others. 1986:Chandramohan and others, 1987; Heinand Koski. 1987). and (7) the experi­ments of Jones (1991).

CALIFORNIA GEOLOGY MAY 1991

Backscaller imagery of Mojave Des­ert rock varnish provkles new indirectsupport for the biological hypothesis.Bacteria that concentrate manganeseoccur as rods. cocci. and filaments (PIQ­ure 3b. 3d). Micron-scale deposits ofmanganese-rich material could be fossil­ized bacteria casts (FIgure 3e). Krinsleyand others. (1990) and Jones (1991)observed stromatolite-like features thatthey allributed to a biological growthprocess. The authors also find thesestructures in eastern California (FIgures2a and 2b) mimicing, at the micronlevel. megascopic structures that havebeen described as stromatolites.

,,,

..Figure 5. Newly discovered textures Db·served in Mojave Desert varnish with aSE.Scale from top 10 bOttom of the micro­graphs; 5a K 15 micrometers, 5b .. 60 mi·crometers. 5c • 5 micrometers. 5d • 130micrometers.

Sa. Loose. poorly organized varnish grow­ing on lOP of better organized varnish fromthe Marble Mountains.

Sb. Slightly beller organized loose debris.still rich in Mn and Fe, but not layered. alsotrom the Marble Mountains, San BernardinoMountains. line indicates varnish/rockboundary.

5c. Close-up of 5b. discussed in greaterdetail in the text.

5d and Sa. Cracks through the varnish withiumerole-like mounds" at the lOp. Rede­position of Mn-Fe oxides has occurred onthe sides of cracks. Samples from DeathValley (Sd) and the Coso volcanic !ield (Se).

USE OF VARNISH IN DATING

Rock varnish forms on desert land­forms. surface artifacts. and petroglyphsthat have been difficult to date by con­ventional dating methods. Many differ­ent approaches to dating rock varnishhave been proposed. Nevertheless.prior to dating there are two basicstrategies for analyzing rock varnish.One approach is to remove the varnishfrom the rock (Dorn, 1989: Dorn andothers 1989; Liu and Zhang, 1990).The other approach is to analyze thevarnish still attached to the rock byscanning electron microscopy (SEM)(Harrington and Whitney, 1987) andparticle induced X-ray emission orPIXE (Pineda and others. 1988).

Accelerator-radiocarbon dating (Darnand others. 1989) and cation-ratio dat­ing (Dam. 1983. 1989: Harrington andWhitney. 1987; Pineda and others,1988: Liu and Zhang. 1990) are poten­tially the best methods to date rock var­nish. Regardless of which varnish datingtechnique is used. however. there are

specimens of varnish that will yield reli­able dates. as well as specimens thatwill yield unreliable dates. For example.Mojave Desert rock varnishes illustratehow BSE can be used to discriminatewhat sample types will yield inaccuratedates. as opposed to samples that willyield credible dates. Figure 4 presentssome of the newly recognized featuresassociated with rock varnish that yieldunreliable dates (Figures 4a. 4b, 4c, 4e,40 and samples that yield reliable dates(Figure 3e. 4d) for dating (Krinsley andothers. 1990).

RECENTLY DISCOVEREDVARNISH TEXTURES

Backscatter electron micrographs(SSE) of newly discovered textures werefirst observed on Mojave Desert var­nishes. One such texture shows loosedebris that is mostly of clay size (lessthan 2 micrometers) and appears to reston well-formed layered rock varnish(Figure Sa). Note that it is composed ofa variety of minerals. indicated by vari­ous shapes and chemical contrasts.

'" CALIFORNIA GEOLOGY MAY 1991

Figures 2a-2g. Backscancr electron micros­copy mICrographs. IlIuSlraling aspects 01 thevarnish/rock boundary and varnish weather­Ing. scales are the distance top to bollom01 the micrographs: about 130 micrometerslor 2a,2b.2e.2g: and 60 micrometers lor2c.2d.2f.

2a and 2b. Varnish Irom Death Valley Can­yon alluvial fan. Death Valley. The stroma­tolite·llke growth IS lighter than the underly­Ing rock because it is rich in manganeseand Iron. Note how the varnish has tormedacross mineral boundanes in the underlyingrocks and the manner In which distmct mor­phological change occurs trom the varnishto the rock.

2c. With time. rock under the varnishweathers and the varnish collapses into thegrowing VOid. resulting in a mIl( of varnishand rock. shown by this close-up view ofthe bottom 01 varnish. collected Irom Ha'naupah Canyon alluvial Ian. Death Valley.

2d. An erosional unconformIty within var·nish formed on the Bishop TuH. These mi·crodepressions may have been eroded byacids secreted by mlfoco!onlal lungi grow­ing in the past, and later relilled by mineralsof quanl and feldspar (dark) and bariumsulfate (bright) mll(ed with newly accre\lf1gvarnIsh (lighter materiat).

2e. Nicely layered varnish on HanaupahCanyon alluvial Ian. Death Valley, wherethe rock has weathered under the varnish,btlngmg about the collapse 01 varnish intothe underlying VOids.

2f and 2g. Areas of 10wer·atomlC numberminerals (darker) where the manganeseand iron have been leached away by waterf10wmg through the varmsh. This allows thedetrital grains rich in tltamum. baflum. andiron to stand out as bright spots. Sampleswere collected from ancestral shorelines ofLake Maml( (Meek. 1989). Line along thebottom of 2g indicates the varnish/rockboundary.

Well-defined breaks between the var­nish and different rock minerals--clearlyindicating an external origin for rockvarnish-are shown in Figures 2a and2b. Figures 2c-2e also illustrate an ex­ternal origin for most varnish constitu­ents as well as indicating ways in whichpieces of rock can become incorporatedinto varnish: rock particles may fall intodepressions created by varnish growthand organisms that erode the varnish(Hgure 2d). Rock material may also be­come mixed into the developing varnishduring sequences of rock weatheringand varnish collapse Into underlyingvoids (Figures 2c. 2e). Such evidence

supports field·based suggestions thatrock varnish may form a coating thatcan preselVe the original rock fromeroding (Butler and Mount. 1986).

Given the complexity of the varnish­rock interface displayed in Figure 2. it iseasy to see how an original rock sourcefor lhe varnish continues to be postu­lated (Smith and Whalley. 1988). Somerock detritus can become incorporatedinto the varnish. as the process of var­nish accretion and rock weathering con­tinues. However. it is well documentedthat the clay. oxide. and trace elementconstituents thaI make up varnish are

derived from sources external to therock (Palter and Rossman. 1977.1979a.b: Allen. 1978: Perry andAdams. 1978: Elvidge. 1979: Darnand Oberlander. 1982).

BIOlOGtCAL VERSUSABIOTIC ORIGIN

There is considerable ongoing con­troversy over how iron-and especiallyhow manganese-are enriched in rockvarnish. The manganese concentrationsin black varnish are typically 50 timeshigher than in the underlying rock ordesert dust (Oorn and Oberlander.

CALIFORNIA GEOLOGY MAY 1991 '"

Figure 3. Bacterial origin of rock varnish In

the Moiave Desert. SCales Irom top to bot·10m 01 micrographs are 5 micrometers !orb. 110 micrometers for c. and 15 microme·lers for d and e. Images 3b--3d are Iromsecondary electron (SE) microscopy: 3e islrom backscaner electron microscopy.

Ja and 3b. Figure 3a illustrates !WO en·ergy-dispersive X'ray fluorescence analy­ses: AI • aluminum. Si • silica. K • potas·Slum, Ca • calcium, n • titanium, Mn •manganese, Fe • Iron. Figure 3b showsrod·shaped bacteria (genus unidentilied)growing on rock varnish on granite fromIndian Cove at Joshua Tree NationalMonument. These bacteria were activelyconcentrating manganese. as illustrated inthe larger Mn peak in the upper analysis ofFigure Ja. The lower Mn peak in the loweranalysis illustrates the overall concentra·tion of manganese in the surrounding var·nish.

3c. Microcoloniallungi, shown here grow,ing on varnish from Warm Spl"ings allUVialfan, Death Valley. While these fungi donot coocenlfale manganese, they can 01·ten erode microdepressions inlo the var·nlsh, probably by the secretion of organicadds.

3d. Budding bacteria growing on varnishon a talus cone in southern Death Valley.This type at bacteria has been identified asMeta/Iogenium (Dorn and Oberlander,1982) or Arthrobacter (Palmer and others,1985). Like Ftgure 3a, manganese wasbeing actively enhanced.

3e. Fossil bacteria have been very dillicullto identify in cross section. However, it ispossible that the approximately 1 micron·wide bright spots in this ngure may be bac,teria thaI are encapsulated in manganeseoxides. Sample Irom Hanaupah Canyonalluvial fan, Death VaHey.

1982; Jones. 1991). Bulk analyses ofmanganese to iron ratios (Mn:Fe) ofvarnish scraped from rocks in theMojave Desert are typically in the rangeof 2: 1 to 1:4. but can vary on the mi·cron'scale from 1: 100 to over 100: 1(Dom 1990).

Analyses of bulk varnish samples col·lected from lhe Mojave Desert showmanganese and iron concentrationstypically in the range of 10·15 percentfor both, but again. these values canvary greatly over distances of a micronor so. Twe types of mechanisms ofmanganese enhancement have beenproposed, a biotic model and an abioticmodel.

Biotic Model

The biotlc model suggests that Mnand Fe are oxidized and concentratedby bacteria (Krumbein, 1969: Krumbeinand Jens. 1981: Dorn and Oberlander.1981a, 1982: Palmer and olhers.1985: Dorn and Dragovich. 1990:

Jones. 1991) and possibly by otherrock-surface organisms (Krumbein andJens. 1981: Taylor·George and others,1983). Support for this model. mostlyfrom the above articles. is based on: 11)in situ obseRVations of bacteria thaI con­centrate Mn and Fe (such as in FiguresJa. 3b. 3d): (2) cultures of Mn- and

'" CALIFORNIA GEOlOGY MAY 1991

Glossary

atomic number: An element is defined by the number of protons in lis nucleus. The number ofprotons is called the atomic number. For example, the atomic number of hydrogen is 1, the atomicnumber of the major elements in varnish are aluminum 13. silica 14. manganese 25. and iron 26.

Eh-pH nuctuaUons: For chemical reactions in which electrons are transferred from one ion to an­other. the oxidation potential of an aqueous solution is called Eh and is measured in volts. Oxidationpotential is directly measured by a meter. A positive value indicates that the solution is oxidizing; anegative value indicates that it Is chemically reducing. When the pH and Eh of a solution aTe known.the stability of the minerals In contact with the water can then be detennined.

thin .section: Nearly all rocks are transparent when ground sufficiently thin (the standard thickness is0.03 mm). When placed under a polarizing petrographic microscope with a rotating mounting stage,the optical properties and textures of the constituent minerals and clasts reveal how the rock formed.A thin section viewed under crossed polarized light resembles stained glass in color.

periglacial stone garlands: Tongue-shaped mass of fine sediments on the downslope mass of stonyembankments found in cold climates. such as arctic and alpine regions. Note: often found atop west­ern Ranges not high enough to be glaciated~

• • • • • • • • • Announcement. • • • • • • • •

1991 EEZ Symposium

The 1991 Exclusive Economic Zone(EEl) Symposium will be held Novem­ber 5-7. 1991 in Portland. Oregon.The theme of the meeting is ~Working

together in the Pacific EEl" and willfocus on results of ongoing seafloormapping and research in the westernUnited States. The meeting is cospon­sored by the U.S. Geological SuJVeY(USGS). National Oceanic and Atmos­pheric Administration (NOAA) JointOffice for Mapping and Research(JOMAR). the American Associationof State Geologists, and the OregonDepartment of Geological and MineralIndustries.

This is the fifth in a series of biennialsymposia held since the issuance of theExclusive Economic Zone (EEZl Procla­mation in 1983. Previous symposiahave been held in the Washington. D.C.area and have focused on EEl mappingand research from a national perspec­tive. The intention of the 1991 Sympo­sium is to refine the specific interests

and activities within each of the EEZsubregions (east coast, Gulf of Mexico.west coast. Alaska and islands) and toidentify ongoing mapping and researchprograms in these geographical areas.

The Symposium will include approxi­mately 30 individual presentations andposter displays from scientists and or'ganizations active in the EEZ of thewestern United States (California. Ore­gon. Washington. Alaska. and Hawaii).Subject matter will focus on the seafloorand will include an overview of ongoingfederal and state activities. results ofseafloor research projects, uses and us­ers of the EEZ. applications of data andinformation. and technological ad­vances. Symposium sessions will discussthe relationship among federal. state.academic, and private sector activities inmapping. research. and determining theresources of and providing informationin the EEZ. technological advances. andcomputer modeling of seafloor proc­esses.

Case studies will be presenteddescribing the results of a cooperative

effort to assess the geology and seafloorprocesses around the Farrallon Islandsoff central California; the Pacific Map­ping Program at the University of Ha­waii: the development of a comprehen­sive digital surficial sediment andbathymetric data base for the westcoaSt. Hawaii. and Alaska. and the on­going work of individual western statesto map and assess the resources of theircoastal waters.

For more information contact,

Millington LockwoodUSGS-NOAA Joint Office for

Mapping and Research915 National CenterReston. VA 2209217031 648~6225

0'

Gregory McMurrayOregon Department of Geology

and Mineral Industries910 State Office Building1400 SW Fifth AvenuePortland, OR 97201(5031229~5580~

CALIFORNIA GEOLOGY MAY 1991 '"

A Page For Teachers

Geothermal Energy(Adapted in part from ~Steam Press.- published by lhe Geothennal Education Office. v Ln. 1. 1988.)

"Steam Press. Geothermal EducationOffice. 664 Hilary Dnve. Tiburon. CA94920. (SOO) 8664GEO ThIs journal is

-Technical Map of GeothermalResources of California {GeologICData Map No 51.- compiled bo; H HMaJmundar 1983 Department ofConservatIOn. Division of Mines andGeology. This Willi :;ize map shows tnerelatlOllship of tectonic features. such asfaults and volcanoes. to the occurrenceof geothermal resources."'"

publtshed annually to further educateyouth and other interested readers aboutgeo(hermal energy and its Vital role Inhelping to ~ain a healthy and deanI.lIOTk:I Class sets of "5l:eam Pres.s~

are available for a nominal charge.

.About Geotherma[ Energy. ~ Geotl1ermal EducatiOn Office. 664 Hilary Dove.Tiburon. CA 94920. (SOO) 866·4GEOThis booklet is in a cartoon format andprovides an introduction to geothermalenergy.

"Geothermal Energy in California.~ textby S,F Hodgson, Illustrations by JSpnggs 1988, California Department ofConsel\lation. Division of Oil and Gas.1416. Nll1th Slreet. Room 1310. Sacra­mento. CA 95814 22 p. This bookletprovides an IntroductiOn to geothermalenergy In CalifotTiia It Is designed lorstudents in fourth through ninth gr<ldesand readers .....ho u.'ant a revIeW of thesub,ect IndMduaJ and classroom seesare available free-of-<harge.

TIle follou.,ng are availab£e al the re­gIOnal DMG Publkalions and Informa­tIOn offICeS: Los Angeles (213) 62Q.3560. P1eas.ant HiD ('lIS) 646·5921.

and Sacramento (916) 445-5716

"Geothermal Re$OUrces in Cahfor-" nia. - by S Bezore 1984 CAU-'> FORNlA GEOLOGY. v. 37.

." n 6. p. 115-118 This--- ", article explains the origin

"'\ and characteristics ofgeothermal resources

in CalifOrnia and

?highlights commu-

""-5rlI __ nities in California

..- r000l7"- that utilize this• resource

od.:i... , ~ -Geothermal--- ~ Resources 01 Cali-. ,-

.:;--~ _ ::::- lomla (GeologicData Map No 4).compiled bo; C T

HlQ9lns. 1980. Department of C0nser­vatiOn. OMsion of Mines and GeoiogyThis Willi SIZe map is a compilallon of aIthe thermal spnngs and u.oeIIs on recordin California

--......

•~­

G2

•

[n contrastto otherrenewableenergyresources.gcothennalis a veryreliablesource ofelec1ricity.It is avail-

.bIo "'"lime. dayand mght

0.::-',>-"

•Natural steam is one of the least expen­sive ways of aD the fuel) to generate eIec·trielty. In fad. the cost of dnnmg a naturalsteam u.'CU pays lor It.5"11 in a few years.which makes the fuel free alter that.

Solar energy is only available when thestnl IS shining and UoiInd energy only u.henthere is a UoiInd bkJwIng

poIIutm because nothing IS burnedTIle steam IS produced COllrt~ ofMother Nature,

Like solar and wind. geothermal Is a re­newable energy source. which means that....'C won't run oot of It Geothermal. aswen as these other renev.table energysources. has no fuels that must be trans·ported. so there are no chances of acci·dents like oil spills.

GEOTHERMALRESOURCEAREAS

Sources of Information:

1be sleam is used to tum the blades 01a turbine and generate electricity.

The hot water is used to heat anotherliquid that has a lower boiling pointThe liquid turns into a gas and is usedto tum .a turbtne.

To release the steam and water from thereservoir a deep hole is drilled and a pipeis inserted.

Visible forms of geo­thermal energy include:

Reservoirs of hot Willer and steam aretrapped in fractured rock or sediment Inthe Earth's CnJ5t.

HoI spnngs. geysers. or natural~e.am that occurs ....flen Wilt('\'"comes In contad UoiIth hot rocksUoiIthln the crust and rises to thesurface of the Earth

Volcanoes......hich OCCUl'" .....henmagma Itself surfaces as \ava

How is geothermal energyused to produce electricity?

The angLlli'l1 hot wat('\'" Is returned 10the Earth

Why is geothermal energyimportant?

Geothermal Is a clean. reneu.oable. rehable. and economical energy source thatproduces electricity ....'thout burmng

TraditJOl\i'lI pou.-er plants bum fossllfuelsto make steam that turns the turl:lIl1eS tomake electricity. The burning of fossilfuels produces large amounts of carbondioxide and other pollutants. In contrast.at a geothermal power plant there Is hllle

II \s heal energy that comesIrom the interior 01 the Earthand is transferred to the crustGeothennal energy becomeshydrothermal energy when itconverts water to hot waterand/or steam.

What is geothermal energy?

Geothermal resources are an Impol'1antsource of energy for both the United Slatesand the 1OoOrid. Right now in the UmtedStates there is enough electricity generatedfrom geochermal to &JPpIy ~r three mU­lion households with electricity; thatsenough electricity to supply the needsof Oregon and Washington combined,

'" CALIFORNIA GEOLOGY MAY t991

This texture suggests that the materialwas Mdumped" in place. and cementedwith Mn and Fe oxides.

Figure 5b may display the next stagein reorientation of loose debris. Thegrains are somewhat smaller and thepore spaces are smaller. The subsurfacedebris in Figure Sb is surrounded by­and grades into-layered. more typicalvarnish. The sequence appears to beleast organized at the top and most or­ganized at the boHom. It is likely thatFigures Sa and 5b represent one waythat aeolian detritus is incorporated intovarnish.

To provide some idea of the greatvariability of debris that presumablyoriginated from airborne fallout. highmagnification images were examined(Figure 5c). At high magnification. en­ergy-dispersive X-ray analysis, X-raydiffraction. and backscattered electrontextural analysis indicate that loose de"bris consists of iron oxide, manganeseminerals. clay minerals, feldspars, andquartz, Some of the very small materialmay be amorphous. as suggested byhigh resolution transmission electronmicrographs.

Another previously unknO\.VTl varnishtexture (Figures 2f and 2g) is indicativeof abundant water flow. The waterleached much of the manganese andiron. making the varnish darker withelements of a lower atomic number.The water also speckled the varnishwith minerals that are rich in elementswith higher atomic numbers (such astitanium and barium). Erosion of man­ganese and iron from the varnish makesit more porous and easier to see thedetrital particles included.

Another textural feature Indicative ofmanganese and iron mobility is "infiltra­tionM(Figure 4b). The white stringersthat run across the grain in Figure 4bare high in manganese. iron. and alumi·num and silica (indicative of clay miner­als). The mobility of Mn and Fe wouldindicate a redistribution 01 varnish con­stituents. Precipitation of manganeseand iron can widen existing rock frac­tures. suggesting a new type of weath­ering mechanism (Krinsley and others.1990).

MFumerole-like" mounds are fre­quently found at the top of the rockvarnish (Figures ScI and 5el. Thesemounds sil atop fractures that fre­quently penetrate an entire varnish sec­tion. Fractures that end in mounds areusually hned in places with zones con­taining high concentrations in Mn andFe. It is likely that the Mn and Fe aremobilized and transported easily intofractures where they precipitate. Fluidsthat reach the surface precipitate andbuild the surface mound. This is yet an­other mechanism of redistributing var­nish malerial.

CONCLUSION

Eastern California deserts are widelyrecognized by scientists for the excellentexamples of rock varnish that occurthere. The occurrence of extensive var­nish development. vegetatlon-environ­mentaltransilions from arid to humid.the presence of K-Ar dated Cenozoicvolcanic rocks. and its proximity to ma­jor universities. makes this region an ex­ceptional area for Investigating rockvarnish.

Recent findings using backscatterelectron microscopy are giving re­searchers additional insights into thisphenomenon. This technology permitsresearchers to view rock varnish chem­istry and texture simultaneously andpermits sources of varnish constituents.origin of manganese enhancement Invarnish. reliable rock varnish dating.and new microscopic textures to bestudied in great detail. lt is now appar­ent that a number of varnish accretionprocesses occur other than depositionin even layers (Figures 3e. 4d). Theserecently discovered varnish processesmay be used by researchers for datingand for reconstructing past environ­mental changes.

ACKNOWLEDGMENTS

This research was supported bygrants to D. Krinsley from Mifflin &Associates. Incorporated. and to R.I.Darn from the National Science Foun"dation and the National GeographicSociety. We thank SW. Anderson. T.A.Cahill, D. Darn, T.E. Gill. and P. Trustyfor discussions and field and laboratoryassistance _

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Blackwelder. E.. 1954. Geomorphic proc·esses m the desert: Calilornla State Divi­SIon of Mmes Bullelin, v. 170, p. 11-20.

Black. W P" 1855. Geological report. Explo­rations and surveys lor a railroad routefrom the MISSIssippi River to the PaCifiCOcean: Special Executive Document 78,33rd Congress. second session. pt. III.v. 5, P 263.

Bolotina. I.N" 1976, Influences of enVlfon­mental conditIOns on the development ofmanganese-depositing soil mlcroorgan·Isms: Soviet SOlt SCience. v, 8 (1). p. 29­34.

Butler. P.R. and Moont. J.F.• 1986. Cor·roded cobbles in southern Death Valley:then relallonshlp to honeycombed weath­ering and lake shorelines: Earth SurfaceProcess and Landforms, v. 11. p 377­387.

Cahill. T.A" 1986. Particle-Induced X-rayemission in Whan. R.E. coordinator. Met·als handbook. 9th edition, Matenals char·acterlzation: American Society fOf Metals.V. 10, p. 102-108.

Chandramohan. 0 .. Lokabharathi. P.A..Nair, S. and Matondkar, S.G.P.. 1987,Bacteriology of terromanganese nodulesfrom the Indian Ocean~ GeomicrobiologyJournal. v. 5, p. t7-31.

Cooke. R.U. and Warren A.. 1973. Geomor·phology in deserts: University 01 CalifOf­

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Cowen, J.p.. Massoth. G.J. and Baker. E.T..1986, Bacterial scavenging of Mn and Fein a mid-to far-lield hydrothermal particleplume: Nature. v. 322. p. 169-171.

Dean. W.E, and Ghosh. S.K.. 1980. Geo­chemistry 01 freshwater ferromanganesedeposits in North America In Varentsov.t.M. and Grasselly. G.Y. editors. Geologyand geochemistry of manganese: manga'nese on the bottom of recent basms: E.Schwelzerbarfscf!e Vorlagbuchhandlung,Stullgart. Germany. v. III. p. 255·277.

Dorn, R.I" 1983. Catlon·ratlO dating: a newrock varnish age determinatIOn tech·nlque: Quaternary Research. V. 20.p.49-73.

Dorn. R.I.. 1986. Rock varnish as an indica­lor of aeolian environmental change inNlckling. W.G.. edItor. Aeolian geomor­phology: Allen & Unwin. London. p. 291­307.

CALIFORNIA GEOLOGY MAY 1991 '"

Dorn. A.I .. 1988. A rock varnish interpreta­tion of alluvial·lan development In DeathVaHey, California: National GeographicResearch. v. 4. p. 56·73.

Dorn. A. I.. 1989, Cation·ratio dating ofrock varnish: A geographical perspec·live: Progress In Physical Geography. v.13. p. 559-596.

Dorn, A.I., 1990. Ouaternary alkalinity fluc­tuations recorded in rock varnish ml­crolaminatlons on western U.S.A. vol·canics: Palaeogeography, Palaeoclima­tology, Palaeoecology. v. 76. p. 291­310.

Dom, A.I., CahiM, T.A.. Eldred, A.A.. Gill.T.E.. Kusko, B.H., Bach, A.J. and Ellion­Fisk, D.L., 1990. Dating rock varnishesby the cation ratio methOd with PIXE.ICP. and the electron microprobe: Inter·national Journal of PIXE. v. t, p. 157·195.

Dorn, A.I. and Dragovich, 0 .. 1990. Inter­pretallon of rock varnish in Australia:Case studies from the arid zone: Austra­lian Geographer, v. 21 p. 18·32.

Dorn. A.t.. Jull. A.J.T., Donahue, D.J.. lin­ICk, T.W. and Toolin, L.J., 1989, Accel·erator mass spectrometry radiocarbondating of rock varnish: Geological Soci­ety AmerICa Bulletin. v. 101, p. 1363­1372.

Dern. A.I. and Oberlander, T,M.. 1981a.Microbial origin of desen varnish: Sci­ence. v. 213, p. 1245-1247.

Dorn. A.I. and Oberlander, T.M., 1981b,Rock varnish origin. characteristics andusage: Za;lschrifl fur Geomorphologle.v. 25. p. 420-436.

Dorn. R.I. and Oberlander. T.M.. 1982,Rock varnish: Progress in Physical Ge­ography, v, 6. p, 317-367.

Duffield W.A. and Bacon, C.A., 1981. Geo·logiC map of the Coso volcaniC field andadjacent areas. Inyo County, California:U.S. GeologIcal Survey MiscellaneousInvestigations $erles. v. 1-1200.

Elvldge. C.D.. 1979. Distribution and forma·tlon 01 desert varnish in Arizona: ArizonaState University. M.S. theSIS, 109 p.

Elvidge. C.D.. 1982. Aeeltamination of therate of desert varnish formatIon reportedsouth of Barstow. California: Earth Sci·ence Surface Processes and Land­forms. v. 7, p. 345-348.

Engel. C.G. and Sharp. A.S.. 1958. Chemi­cal data on desert varnish: GeologicalSociety of America Bulletin. v. 69,p.487-518.

Garner, H.F., 1974, The origin of land­scapes. a synthesis of geomorphology:Odord University Press. 734 p.

Ghiofse. W.C.. 1984, Bacterialtranslorma­tions 01 manganese in weiland environ'ments m Klug. M.J., and Reddy, CA,

editors. Current perspectives in microbialecology: Amencan Society for Microbiol­ogy. p. 615-622.

Glennll~. K.W.. 1970, Desert sedimentaryenVIronments: ElseVier. 222 p.

Harrington. C.O. and Whitney, J.W, 1987,Scannmg electron microscope methodfor rock varnish dating; Geology. v. 15.p. 967-970.

Hein. J.A.. and Koski. R.A.. 1987, Bacteri­ally mediated diagenetic origin for chert·hosted manganese depoSits in the Fran­CIscan Complelt. Call1ornia CoastRanges: Geology, v. 15, p. 722-726.

Hooke. A.L.. 1972, GeomorphIC eVidencefor late Wisconsin and Holocene tectonICdeformation. Death Valley. California:Geological Society of America Bulletin,v. 83. p. 2073·2098.

Hooke. A.L., Yang, H. and Weiblen. P.W.,1969. Desert varnish: an electron probestudy: Journal Geology. v. 77. p.275­288.

Hunt. C.B.. 1954. Desert varnish: Science.v. 120. p. 183-184.

Hunt, C.B. and Mabey. D.A., 1966, Strati·graphy and structure. Death Valley. Cali·fornia: U.S. GeologICal Survey Proles­sional Paper. v. 494A. p. 90-92.

Jones, C.E.. 1991. CharactenstlCS and ori­glfl 01 rock varnish from the hyperafldcoastal deserts of nonhern Peru: Oua'ternary Research, v. 35, p. 116·129.

Khak·mun. T.. 1973. PartiCipation of micro­organisms In the formation of manga­nese-iron concretions In soils of thebrown forest zone 01 the far east: SovietSoil Science. v. 5, p. 96-99.

Krlnsley, D., Dorn. A.I. and Anderson, S.,1990, Factors that may interfere With thedating of rock varnish: Physical Geogra·phy,v. tl.p.97-119.

KrumbelO, W.E" t969. Uberden Emliussder Mlkroflora auf die Exogene Dynamik(Veswrrterung und Krustenbi/dung): Ge­oJoglsche Rundschau, v. 58, p. 333·363.

KrumbelO, W.E. and Jens, K.. 1981. BIO­geniC rock varnishes of the Negev Des·ert (Israel): An ecological study 01 Ironand manganese translormatlon by cya­nobactena and lungi: Oecologla. v. SO.p.25-38.

Lakin. H.W.. Hunt, C,B., DaVidson, D.F,and Odea. U.. 1963. Vanallen 10 minor·element content of desert varnish: U.S.Geological Survey ProfeSSional Paper.v. 475-B, p. B28-B31.

Laudermllk. J.C.. 1931. On the orlQlO ofdesert varnish: Amencan Journal of Sci·ence. v. 21. p. 5t-66

Liu. T. and Zhang. Y., t99O. Establishment01 a cation· leaching curve of varnish forthe Dunhuang region. western China:Seismology and Geology, in press.

Meek, N.. 1989. Geomorphic and hydro·logic Implications of the raped Incision ofAfton Canyon, Malave Desert. Calilor­nia; Geology. v. 17. p. 7-10.

Mustoe. G.E.. 1981, Bacterial oXldallon 01manganese and iron in a modern coldspring: Geological Society 01 AmericaBulletin, v. 92. p. 147·153.

Palmer. F.E .. Staley. J.T.. Murray, A.G.E..Counsell, T. and Adams. J.B.. 1985,Identification of manganese-oltldizlngbacteria from desen varnish: GeomlCro­biology Journal, v. 4. p. 343-360.

Peck. S.B., 1986. Bacterial depoSition ofiron and manganese oltides in NorthAmerican caves: National Soil ScienceBulletm, v. 48, p. 26·30.

Perry, A.S. and Adam. J., 1978, Desert var­nish: evtdence of cyclic deposition 01manganese: Nature, v. 276. p. 489-491,

Pineda. C.A.. Jacobson, l. and Pelsach.M., 1988. Ion beam analySIS for the de·termination 01 cation-ratIOs as a meansof daMg southern Alrican rock var­nishes: Nuclear Instruments and Meth·ods in PhySICS Research. v. B35. p. 463·466.

Polter. R.M. and Aossman. G.A., 1977,Desert varnish; The Importance of dayminerals: Science, v. 196, p. 1446-1448.

Palter, R.M. and Rossman. G.A., 1979,The manganese-and iron-oltide mineral­ogyof desert varnish: Chemical Geol­ogy, v. 25, p. 79-94.

Paller. A.M. and Rossman. G.R.. 1979.Mineralogy of manganese dendntes andcoatings: AmerICan MlOeraloglst, v. 64.p.1219·1226.

Shlemon. A.J.. 1978. Quaternary soil-geo­morphic relationships, southeasternMojave Desen. Calilornla and Anzona.in Mahaney, W.C., editor: OuaternarySoils: UniverSity of East Anglia. England,p.187-207.

Smith, B.J. and Whalley. W.B.. 1988. Anote on the characteristics and possibleorigins 01 desert varnishes from south­east Morocco: Earth Surface Processesand landforms. v. 13. p. 251-258.

Taylor-George. S., Palmer. F.E.. Staley.J.T., Borns. D.J.. Curtiss, B., andAdams, J.B.. 1983, Fungi and bactenaInvolved in desert varfllsh formation:Microbial Ecology. v. 9. p. 227·245.

Turrin. B.D.. Dohrenwend. J.C.. Drake, A.E.and Cunlss, G.H., 1985. K·Ar ages fromthe Clma volcanIC lield. eastern MalaveDesen, Callforflla: Isochron West. v. 44,p.9-16.

Walther, J.. 1891, Ole Denudation inderWusle: Akademl der Wissenschaflen:Malhemafischk-Physicalisdle, Abhand­lungen, v. 16. p. 435-461,

White, C.H.. t 924, Desen varnish: Ameri·can Journal 01 Sctence. v. 9. p. 413·420.

continued.

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California produces nearly half of theoil it consumes every year. In 1989.this state produced 337 million barrelsof oil while consuming 706 million bar­rels. Oil and gas exploration in Califor­nia is continuing. Since its inception inthe 1930s. the oil exploration serviceknown as ~mud logging~ has expandedto encompass several geoscience. pe­troleum, and engineering disciplines.Although there has been a plethora ofnew high-tech oil and gas exploratorytechniques in recent years, the only wayto find out exactly what lies under­ground is to drill a well. It remains atried and true way to accurately under­stand the subsurface geology. As it

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Special Publication 109 contains guidesand explanatory text for II field trips con­ducted in associatiOn with the 87th AnnualMeeting of the Cordilleran Section of IheGeological Society of America (GSA) and theSeismological Society of America. held inSan Francisco. March 25-27. 1991

This volume was planned by the TechOlcalCommlUee 01 the 87th Annual Meeling andorganized by a subc:ommillee consisting ofM.C. Blake. Doris Sloan. and D.L. Wagner.Twenty-eight authors. representing govern­ment (12). universities {lZ}. and industry (4)conTributed to the publication. There areabundant tables. photos. and illustrationsthroughout tne volume,

In the past. meetings oflhe GeologicalSociety of America have provided impetus forthe Division of Mines and Geology to releasepublications describing the geology of Califor­nia Bulletin 190. Geology of Northern Cali­fornia. was published in conjuncllon with the1966 GSA meeting held in San Francisco.II contained guides for seven field trips in lheBay area. the northem Coast Ranges. theSacramento Valley. and the Sierra NevadaNow. 25 years later. the gUides in SpecialPublication 109 cover much of the sameground. but the geology is interpreted froma profoundly different perspective.

The advent of plate tectonics in the late196& h<'ld an enormous impact on the inter-

availability thai is used for land-use decisionsby county and cily ollicials. and planners. ItIs important Ihal land-use and pl3nning dei::i­sions be made wilh the knov.iIedge 01 under­lying mir'ICral deposits.

Open-File Report 90·16 presents the re­sults of the Division of Mines and Geology'sIOMG) mineral land classification study of the775 acre Hannah Ranch site in TulareCounty. Classification was based on the pres­ence or absence of malerial suitable for useas porlland cement concrete ~regate. Thisevaluation is based on data prOVided by thepetitioner. Kaweah River Rock Company.The data were confirmed by a ooe-day fieldexamination by DMG stall of tile petitioned

prctallon of the geology and tectonic hisloryof California The concepts of seafloorspreading. subducTion. forearc basins. arc­related magmatism. and transfonn faultingwere applied to the fonnerty enigmatic andcontradictory relationships of the FranciscanComplex. the Great Valley sequence. theSalinlan block. and the Sierra Nevada batho­lith. Application 01 plate-Tectonic models tononhem California made it the type ex­ample of an active continental margin.

The field guides in Speclal Publication109 provide all up·to-date application ofthese current models and concepts to Cali­fornia geology. Trips I. 3. 4. 5. and 8 em·phasize the Quatemary tectonics of the SanAndreas transform system and its role inrecent sedimentation. Trips 6 and 10 em­phasize Neogene and Quatemary volcanism.lhought by some to be related to passage ofthe triple junction rather than to subduction

[n the Santa Cruz Mountains near lornaPrieta rrrip 5). and in Marin County [Trip2}. paleontologic and paleomagnetic data in­dicate thaI Franciscan terranes east of theSan Andreas faull have been translatedIlOrlhward from near-equatorial latiludes.reqwring that large-scale transform faultingtook place prior to the development of theSan Andreas system. Trip 2 also presentseW:lence that the Sahnian block has beenoffset about 90 mi (ISO km)ln the last lO­IS million years (my) by the San Gregoriofaull.

Trip 7. in the Berkeley HiUs of the cen·tral Coast Ranges. suggests lhat during (he

properly in June. 1990. High-qualily ag­gregate resources occur on the propertywhich are suitable for a variety of aggre­gate commodities. The aggregate re­sources meet the criteria established by theState Mining and Geology Board lor port­land cement concrete aggregale

Reference copies of Open-file Report90-16 are available in the Los Angeles.Sacramento. and Pleasant Hill offices ofthe Division of Mines and Geology. Acopy of the reporl may be purchased over­the·counter from the Geologic Informationand Publications Office In SacramenlO. IImay be ordered by prepaid mail orderfrom, Division of Mines and Geology. P 0Box 2980. Sacramento. CA 9581Z-2980.

laSI 5 my. there have been both large·scalestrike slip and thrust faulting along the Hay­ward. Calaveri'lS. and other aclive faults Inthe region.

To the southeast. in tne Diablo Range[Trip 9). evidence is presented that indicatesthat the Coast Range ophiolite was the sileof a volcanic arc during the latest Juri'lSSicand that the well-exposed low-angle faultthat fonns the present boundary betv.leenlhe ophiolite and the underlying Franciscanmetagri'lywacke. is probably a normal fault.formed during uplift of the Franciscan sub­dllCliOn complex.

Fanher to the easl. in the Sierra Nevadarrrip II). detailed structural i'lnd isotopicstudies indicate that the development ofcleavage and metamorphism along the Foot­hills suture zones was taking place at thesame Ume as deposition of the older pan ofthe Great Valley sequence and early meta­morphism of the Franciscan Complex

Thus. nearly every manifestation of platetectonics can be seen in nOrlhem California.and Ihe geologic features described in (hisguidebook provide an excellent introducTion10 the geologic and tectonic history of theregion.

Special Publication 109 is available forreference or purchase at the Division ofMines and Geology olflces in Los Angeles.Pleasant HIll. or Sacri'lmento. It may be or­dered by mail from, Division of Mines andGeology. P.O. Box 2980. Sacramento. CA95812-2980.

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GEOLOGIC DATA MAP NO.7

ISOSTATIC RESIDUAL GRAVITY MAP OF CAUFOR­NIA AND OFFSHORE SOUTHERN CAUFORNIA. By Car­ter W. Roberts, Robert C. Jachens, and Howard W. Oliver.scale 1:750.000, $12.00.

TIle isostatic residual gravity map of California effectivelyseparates gravity anomalies caused by geological variations inthe crust from large &uguer gravity anomalies that resultfrom isostatic compensation of the topography. By removingthe isostatic effects of mountain roots, the smaller geologicanomalies are better defined. This map should be useful foroil and mineral explorationists as well as for anyone inter­ested in the interpretation and extrapolation of subsurfacegeology.

This regional-residual separation reveals important featuresthat are not easily recognized on the Bouguer gravity mapand converts others Ihal are difficult to interpret into anoma­lies that can be readily analyzed. Major residual anomaliesthat are now more easily recognized include (l) a gravityanomaly caused by the Gorda plate sulxlucted beneath north­ern California. and (2) a pattern of linear gravity highs alongthe western margins and gravity IOVJS in lhe eastern parts ofboth the Sierra Nevada and Peninsular Ranges batholiths(Jachens and Griscom, 1985).

Prominent anomalies that now are more amenable toquantitative analysis include (l) gravity highs defining a major

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detached thrust sheet within the western Klamath Mountains,and (2) a gravity low caused by Iow-density sedimentary rocksin the Ventura basin modified by the associated mantle up­warp accompanying isostatic compensation 01 the basin lill(Jachens and Griscom. 1985).

The gravity map is printed on the wall-size colored geo­logic base map of California (scale 1:750.000). Approxi­mately 65.000 land gravity stations were used in the compila­tion; an increase of several thousand stations since theBouguer compilation of Oliver and others (1980).

Copies of Geologic Data Map No. 7 are available for refer­ence at Division of Mines and Geology offices in Sacramento.Pleasant Hill. and Los Angeles. Copies may be purchasedfrom lhese offices for $12.00.

REFERENCES

Jachens. R.C" and Griscom. A.. 1985. An isostatic reSidual gra'lIlymap or Calirornia: a residua! map for interpretation 01 anomaliesIrom intracrustal sources. in Hinze. W.J.. editor. The ulility 01 re­gional gravily and magnetic anomaly maps: Society of Explora­tion Geophysicists, Tulsa. Oklahoma. p. 347-360.

Oliver. H.W., Chapman. R.H.. Biehler. S., Robbins. S.L. Hanna.W.F.. Griscom. A., Beyer. LA., and Silver. EA. 1980. GravityMap or California and its continental margin: California Divisionof Mines and Geology. Geologic Dala Map No.3, scale1:750.000.""

,,. CALIFORNIA GEOLOGY MAY 1991