Preliminary geologic map emphasizing bedrock formations in Contra 
Costa County, California:  A digital database

Compiled by R.W Graymer, D.L. Jones, and E.E. Brabb

U.S. Geological Survey Open-file Report 94-622

Geological Explanation

This map represents an integration of the published and unpublished 
geologic maps of several previous workers (see Sources of Data in 
ccsource.txt) and more than 1000 man hours of new geologic 
mapping and field checking by us.  The data are released in a 
preliminary digital form to provide an opportunity for staff from 
governmental agencies, consultants, teachers, and other users to have 
this geologic map long before the final map is published.

Stratigraphy

Bedrock formations in Contra Costa County are divided into six 
Assemblages with a unique stratigraphic sequence bounded by 
faults.  The distinction may be either the presence of rock types not 
present in other Assemblages (e.g.. the diatomite (Tdi) in Assemblage 
III) or a different stratigraphic relationship among similar rock units 
(e.g.. The Domingene Formation (Td) is depositional on Cretaceous 
rocks in Assemblage V, but is underlain by other Tertiary rocks (Tm, 
Tmz) in Assemblage VI).  The stratigraphic differences between 
Assemblages are almost certainly due to angular unconformities and 
changes in depositional environment in one or more large 
depositional basins.  The significant differences in the stratigraphy of 
different Assemblages, originally separated but now in close 
proximity, denotes the juxtaposition of different basins or parts of 
basins by large offsets along the faults that bound them (as much as 
hundreds of kilometers, see below).
	The Great Valley Sequence is depositionally linked to the Coast 
Range Ophiolite.  Although the contact between the two is a fault 
everywhere in Contra Costa County, elsewhere in California (as close 
as Alameda County) the lowest part of the Great Valley Sequence 
(the Knoxville Formation) is clearly deposited on the ophiolite.
	The Franciscan Complex presumably underlies all of Contra 
Costa County.  It was emplaced below the Coast Range Ophiolite by 
accretionary faulting during Cretaceous time, so the contact between 
the Franciscan and Coast Range Ophiolite and the overlying Great 
Valley Sequence is everywhere faulted.  This fault is known as the 
Coast Range fault.
	Two types of Tertiary intrusive rocks occur in Contra Costa 
County, both of which intrude the strata of Assemblage V.  In the 
Concord area, the Markley Formation is intruded by plugs and dikes 
of Pliocene basalt.  East of Mount Diablo, the Great Valley Sequence 
rocks are intruded by fine grained, quartz bearing rhyolite stocks, 
dikes, and sills of late Miocene age.  The relationship of these 
hypabyssal intrusives is unknown.  Although they occur only in the 
rocks of Assemblage V, these rocks are not included in the 
Assemblage because of their intrusive nature.

Paleontology

	There are at least hundreds and perhaps thousands of fossil 
collections from Contra Costa County described in the literature 
during the past century, and even more unpublished collections 
made by geologists working for petroleum companies.  A partial list 
of references is provided by White (1990).  Preparation of a digital 
database of this information is under way by scientists at the U.S. 
Geological Survey and the University of California, Berkeley.

Radiometric Ages

	A compilation of the radiometric ages of rocks in Contra Costa 
and other counties south of latitude 38 degrees is provided by 
Lindquist and Morganthaler (1991).  Additional data are provided by 
Brabb and Hanna (1981), Wright (1974), Sarna-Wojcicki (1976, and 
written communication, 1990), Sarna-Wojcicki and others (1979), 
and Curtis (1989).

Structure

	The structure of Contra Costa County is dominated by broadly 
distributed transpressional faults, with both strike-slip and reverse 
motions, trending roughly N30-60W.  Most of the faults have been 
active in the Quaternary and some, such as the Hayward and Concord 
faults, are creeping now (Lienkaemper, 1992, Herd, 1978a, b).  The 
faults form most of the boundaries of the Assemblages.  The 
juxtaposition of rocks with different stratigraphic histories across 
these faults lends support to the work of Prowell (1974), who 
suggested 16 to 72 km of strike slip movement along the Calaveras 
fault south of the area of this map.  Other Assemblage bounding 
faults have probably undergone a similar amount of offset, based on 
the juxtaposition of rocks originally deposited a large distance from 
one another (see above).  Dip slip movement may also be extensive, 
judging from truncation of folds and thick geologic units beneath 
dipping faults.
	Reverse and thrust faults in Contra Costa County tend to occur 
in zones of northeast or southwest vergence.  Thrust faults in the 
Berkeley Hills verge northeast, for example, wheras those on the 
west flank of Mount Diablo verge southwest.  Some thrust faults on 
the north and south flanks of Mount Diablo have an east-west trend, 
at least in part.  A few Assemblage bounding faults are thrust faults.
	The faults on this map are not intended to show hazards at any 
specific site, or to take the place of fault-rupture hazard zones 
designated by the California State Geologist (Hart, 1988).  Moreover, 
there is considerable disagreement about the location and character 
of Quaternary and Holocene faults in Contra Costa County.  See, for 
example, the different depictions of the Hayward fault by Herd 
(1978b), Radbruch (1969), and Lienkaemper (1992).  References to 
fault studies in Contra Costa County are provided by White (1990).


Description of Units

Surficial deposits (Quaternary).  Overlying bedrock of all 
Assemblages:

Qu	Surficial deposits, undivided (Pleistocene and Holocene) --  For
	more detailed reports of surficial deposits, see Helley and 
	others (1979), Atwater (1982), Herd (1978 a, b), and Borchardt 
	and Seelig (1991).

Qls	Landslide deposits (Pleistocene and Holocene) --  Landslides 
	are intentionally omitted from most of this map because they 
	are so numerous they would conceal much of the information 
	on bedrock geology.  Only a few of the large landslide areas are 
	shown.  For more comprehensive reports of landslides in the 
	area, see Nilsen and others (1979) and Ellen and Wieczorek 
	(1988). In addition, a list of pertinent literature on landslides 
	in Contra Costa County is provided by White (1990).

Tertiary Intrusive Rocks

Tb	Basaltic intrusive rocks (Pliocene) -- Hypabyssal basalt dikes 
	and sills.
Tsv	Silicic intrusive rocks (Miocene) -- Rhyolite to andesite 
	porphyry stocks, dikes and sills.

Rocks that presumably underlie the entire area.

KJf	Franciscan Complex (Jurassic to Upper Cretaceous) -- Sheared 
	sedimentary and metamorphic rock with zones and blocks
	of chert (fc), hard graywacke and shale (fss), greenstone (fg), 
	basalt and chert (fbc), melange of metamorphic rocks including 
	glaucophane and other schistose rocks in sheared sandstone 
	and shale matrix (fm), and sandstone (graywacke) (fs); also 
	includes small bodies of serpentinite, silica carbonate rocks, 
	and silicic volcanic rocks (keratophyre).    

Assemblage I - Berkeley Hills

Tbp	Bald Peak Basalt (Miocene) -- Massive basalt flows.  Ar/Ar ages 
	8.37+0.2 and 8.46+0.2 Ma (Curtis, 1989).
Tst	Siesta Formation (Miocene) -- Non-marine siltstone, claystone, 
	sandstone, and minor limestone.
Tm	Moraga Formation (Miocene) -- Basalt and andesite flows, 
	minor rhyolite tuff. Ar/Ar ages range from 9.0+0.3 to 10.2+0.5 
	Ma (Curtis, 1989).  Interflow sedimentary rocks (Tms) mapped 
	locally.
Tor	Orinda Formation (Miocene) -- Conglomerate, sandstone, 
	siltstone.  Nonmarine with abundant clasts of rocks from the 
	Franciscan Complex.
Tcc	Claremont Chert (Miocene) -- Laminated and bedded 
	diatomaceous chert, minor brown shale and white sandstone.  
	Interbedded sandstone (Tccs) mapped locally.
Tgs	Unnamed sandstone and siltstone (Eocene, Oligocene, and 
	Miocene) -- Glauconitic sandstone and brown massive siltstone. 
Tsh	Greenish gray shale (Eocene).  

	Great Valley sequence (Late Jurassic and Cretaceous) -- 
	Consists of:
Kph		Pinehurst Shale (Late Cretaceous) -- Siliceous shale with 
		interbedded sandstone and siltstone. Includes maroon, 
		concretionary shale at base.  This formation was 	
		originally considered to be Paleocene, but it contains 
		foraminifers and radiolarians of Late Cretaceous age in its 
		type area and throughout its outcrop extent.
Kr		Redwood Canyon Formation (Late Cretaceous) -- Massive, 
		distinctly bedded sandstone with minor interbedded 
		siltstone
Ksc		Shephard Creek Formation (Late Cretaceous) -- Mainly 
		shale with minor sandstone.
Ko		Oakland Conglomerate (Late Cretaceous) -- Conglomerate 
		containing mainly silicic volcanic clasts and massive 
		biotite-rich quartz sandstone.
Kjm		Joaquin Miller Formation (Late Cretaceous) -- Shale 
		with minor sandstone.
JKk		Knoxville Formation (Late Jurassic to Early Cretaceous) -- 
		Mainly dark, greenish - gray shale with sandstone 	
		interbeds; also, locally includes conglomerate and 	
		sandstone in its lower part.  Locally contains abundant  
		specimens of bivalve fossil Buchia.

	Coast Range Ophiolite (Jurassic) -- Consists of:
Jsv		Keratophyre.  Altered silicic volcanic rocks including 
		keratophyre and quartz keratophyre.
sp		Serpentinite.  Mainly sheared serpentinite, but also 
		includes massive harzburgite.

Assemblage II - Hercules Area

Qmz	Montezuma Formation (Pleistocene) -- Poorly consolidated 
	sand, with minor gravel, silt, and clay.

	Mullholland Formation of Ham (1952) (Pliocene) -- Consists of:
Tmlu		Upper member; conglomerate, sandstone, and mudstone.
Tmll		Lower member; sandstone and mudstone

Tus	Unnamed sedimentary and volcanic rocks (Miocene and 
	Pliocene) -- Includes marine and nonmarine conglomerate, 
	sandstone, and siltstone, as well as basalt (Tub) and limestone 
	(Tul) mapped locally.
Tpt	Pinole Tuff (Miocene) -- Tuffaceous sandstone containing 
	pumice fragments.  Tuff dated by K/Ar about 5.2 Ma (Sarna-
	Wojcicki, 1976).
Tlt	Lafayette Tuff (Miocene) -- K/Ar age of 8.2 + 2.0 Ma (Sarna-
	Wojcicki, 1976).
Tn	Neroly Sandstone (Miocene) -- Blue, volcanic - rich, shallow 
	marine sandstone, with minor shale, siltstone, tuff, and 
	andesitic conglomerate. 
Tc	Cierbo Sandstone (Miocene) -- Blue, brown, gray, and white 
	marine sandstone, minor conglomerate, tuff, and shale.

Tbr	Briones Formation (Miocene) -- Sandstone, siltstone, 
	conglomerate and shell breccia.  The Briones Formation in this 
	assemblage contains a tuffaceous layer with a K/Ar age of 
	14.5+0.4 Ma (Lindquist and Morganthaler, 1991).  

	In the southern part of the assemblage, divided locally into:
Tbg		G member of Wagner (1978) -- Massive sandstone, 
		pebble conglomerate, and shell breccia.  Locally 	
		subdivided into:
Tbf		F member of Wagner (1978) -- Fine -grained feldspathic 
		sandstone and locally prominent brown shale.
Tbe		E member of Wagner (1978) -- Medium - grained 	
		sandstone with abundant shell breccia beds; lithologically 
		similar to unit Tbg.
Tbd		D member of Wagner (1978) -- Massive, medium - 
		grained sandstone with local conglomerate layers.

	In the northern part of the assemblage, divided locally into
Tbu		Upper sandstone and shale member.
Tbh		Hercules Shale Member; gray shale and siltstone.
Tbl		Lower sandstone and siltstone member.

Tro	Rodeo Shale, Hambre Sandstone, Tice Shale, and Oursan 
	Sandstone, undivided (Miocene)
Tr	Rodeo Shale (Miocene) -- Brown siliceous shale with yellow 
	carbonate concretions.
Th	Hambre Sandstone (Miocene) -- Massive, medium - grained 
	sandstone, weathers brown.
Tt	Tice Shale (Miocene) -- Brown siliceous shale
To	Oursan Sandstone (Miocene) -- Greenish gray, medium - 	
	grained sandstone with calcareous concretions.
Tcs	Claremont Shale (Miocene) -- Brown siliceous shale with yellow 
	carbonate concretions and minor interbedded chert.
Ts	Sobrante Sandstone (Miocene) -- Massive white, medium - 
	grained calcareous sandstone.
Tts	Tuffaceous sandstone (Oligocene or Miocene?) -- Light - gray 
	tuffaceous sandstone, minor conglomerate and siltstone, 
	marine.  May be equivalent to Kirker Tuff in Assemblage VI.
Tsr	San Ramon Sandstone (Oligocene and/or Miocene) -- Massive, 
	medium to coarse - grained, fossiliferous, marine sandstone.
Tshc	Shale and claystone (Eocene) -- Also contains minor sandstone.

Assemblage III - Pinole Area

Tcgl	Conglomerate, sandstone, siltstone (Miocene and Pliocene) -- 
	Contains abundant clasts of Claremont chert.  Includes rhyolite 
	tuff and tuff breccia (Tcglt), correlated with the 5.7 to 6.1 Ma 
	Roblar tuff of Sonoma County (Sarna-Wojcicki, written 
	communication,1990).
Tut	Tuff (Miocene) -- Tuffaceous sandstone containing pumice 
	fragments.
Tdi	Diatomite (Miocene) -- Light gray to white with minor brown 
	shale.
Tsa	Sandstone (Miocene) -- Massive, light gray, fine to medium - 
	grained.
Tmu	Mudstone, shale, and siltstone (Miocene).

Assemblage IV - Martinez Area

Tbr	Briones Sandstone (Miocene) -- Sandstone, siltstone, 
	conglomerate and shell breccia.
Ts	Sobrante Sandstone (Miocene) -- Gray to brown, fine - to 
	medium - grained sandstone and minor conglomerate.
Tsr	San Ramon Sandstone (Oligocene and/or Miocene) -- Bluish - 
	gray to brown, medium - grained sandstone with conglomerate 
	locally present in basal part.

Tes	Escobar Sandstone of Weaver (1953) (Eocene) -- Massive, 
	medium - to coarse - grained, brown sandstone with shale in 
	basal part. Locally, divided into:
Tehs		Sandstone and shale member.
Teh		Basal shale member.

Tmr	Muir Sandstone of Weaver (1953) (Eocene) -- Massive, yellow - 
	weathering arkosic sandstone.  Locally, divided into:
Tmru	Upper member.  Sandstone, lithologically similar to 
		unit Tmr
Tmrl		Lower member.  Claystone with thin sandstone in basal 
		part.

Tlj	Las Juntas Shale of Weaver (1953) (Paleocene and Eocene) -- 
	Gray shale with minor siltstone.  Locally divided into:
Tlju		Upper member.  Shale.
Tljl		Lower member.  Sandstone.

Tvh	Vine Hill Sandstone of Weaver (1953) (Paleocene) -- 
	Glauconitic sandstone.  Locally, divided into:
Tvhu	Upper member.  Sandstone and shale.
Tvhl		Lower member.  Glauconitic sandstone.

Ku	Great Valley sequence (Cretaceous) -- Sandstone, siltstone, 
	shale, and minor conglomerate.  Locally, divided into:
Kcs		Gray, massive quartz arenite.
Ksh		Siltstone and shale.
Kus		Sandstone, siltstone, and shale.
Kuh		Massive sandstone, minor siltstone.

Assemblage V - Diablo Valley Area

Tgvt	Green Valley and Tassajara Formations of Conduit (1938), 
	undivided (Miocene and Pliocene) -- Non - marine sandstone, 
	siltstone, and conglomerate.  Locally includes a 5 meter thick 
	tuff marker bed (Tgvtt).  A tuff in this unit has a K/Ar age of 
	4.0+1.0 Ma, while tuff layers lower in the unit has been 
	correlated with the Roblar tuff in Sonoma County which has 
	K/Ar ages of 5.7+0.5 Ma and 6.1+0.1 Ma and the Pinole Tuff of 
	Assemblage II which has a K/Ar age of 5.2+0.1 Ma (Sarna-
	Wojcicki, 1976).
Tn	Neroly Sandstone (Miocene) -- Brown, massive, marine 
	sandstone with abundant volcanic clasts.
Tc	Cierbo Sandstone (Miocene) -- Light - gray, massive sandstone 
	with marine fossils.  Contains sandstone and conglomerate 
	near the base (Tcsc).

	Domingene Formation (Eocene) -- Consists of:
Tdu		Upper member.  Massive, pebbly, white sandstone.
Tdl		Lower member.  Gray shale and minor sandstone.  	
		Locally, includes:
Tdls			Sandstone.  Fine grained, white, quartz sandstone 
			marker bed.


Ku	Great Valley sequence (Cretaceous) -- Undivided sandstone
	siltstone, and shale.  Locally divided into:
Kslt		Siltstone with minor shale, claystone, and sandstone.
		(Late Cretaceous)
Kus		Sandstone and shale (Early Cretaceous).
Ksh		Shale with minor sandstone (Early Cretaceous).

Assemblage VI - Eastern Area

Ttu	Tulare Formation (Pliocene) -- Poorly consolidated, non-marine, 
	gray to maroon siltstone, sandstone, and conglomerate.  This 
	formation contains tuff correlated with the Putah Tuff which 
	has a K/Ar age of 3.3+0.1 Ma (Sarna-Wojcicki, 1976).
Tl	Lawlor Tuff (Pliocene) -- Non-marine, pumiceous, andesitic 
	tuff.  In this area the Lawlor tuff has a K/Ar age of 4.0+0.2 Ma. 
	(Sarna-Wojcicki, 1976).
Tn	Neroly Sandstone (Miocene) -- Blue, volcanic - rich, cross-
	bedded sandstone and conglomerate; mainly nonmarine.  The 
	Neroly Sandstone in this assemblage contains tuffs with K/Ar 
	ages of about 9.7 Ma (Black Diamond Park Tuff ) and 10-12 Ma 
	(Alves Tuff) (Sarna-Wojcicki, written communication, 1990). 
	Includes minor siltstone, locally mapped (Tns).
Tc 	Cierbo Sandstone (Miocene) -- Blue, volcanic - rich sandstone 
	with abundant marine fossils.
Tkt	Kirker Tuff (Oligocene) -- Pumiceous, white tuff.  Includes 
	minor tuffaceous sandstone, conglomerate and siltstone, locally 
	mapped (Tks).

Tmk	Markley Formation (Eocene) -- Sandstone and shale.  Locally, 
	divided into:
Tmku	Upper member.  Thin - bedded sandstone, siltstone, and 
		claystone.

		Sidney Flat Shale Member.  Consists of:
Tsu			Upper part.  Black shale with minor siltstone and 
			sandstone.
Tsl			Lower part.  Interbedded shale and sandstone.

Tmkl	Lower member.  Thin - bedded to massive sandstone 
		with minor siltstone and mudstone.  Locally, includes:
Tlu			Upper siltstone bed.
Tll			Lower siltstone bed.

Tnv	Nortonville Shale (Eocene) -- Brown to grayish - green 
	mudstone and claystone with minor siltstone and sandstone; 
	marine rocks.

Td	Domingene Formation (Eocene) -- Locally, divided into:
Tdu		Upper member.  Brown sandstone with minor mudstone 
		and conglomerate.
Tdl		Lower member.  Siltstone and claystone with minor 
		sandstone and basal conglomerate.

Tm	Meganos Formation (Paleocene) -- Sandstone, shale, and 
	conglomerate.  Locally, divided into:
Tme		Upper member.  Greenish - gray to light gray, biotite-rich 
		siltstone and silty mudstone, with abundant plant debris 
		in places.  Division E of Meganos Formation of Clark 
		(1921)
Tmd		Sandstone member.  Medium grained, light gray to 
		bluish-gray sandstone with carbonaceous laminations; 
		pebble conglomerate present locally at base.  Division D of 
		Meganos Formation of Clark (1921).
Tmc		Shale member.  Bluish - gray shale with sandstone 
		interbeds.  Division C of Meganos Formation of Clark 
		(1921).  Locally includes:
Tmcss		Sandstone interbeds
Tma		Lower member.  Sandstone with basal conglomerate.  
		Medium to coarse grained, clean, white, distinctly cross 
		bedded, biotite bearing arkosic sandstone.  Equivalent to 
		Divisions A and B of Meganos Formation of Clark (1921).

Tmz	Unnamed sandstone and shale (Paleocene) -- Locally, divided 
	into:
Tmzu	Upper member.  Siltstone and shale.
Tmzl		Lower member.  Glauconitic sandstone.

	Great Valley sequence (Jurassic and Cretaceous) -- Consists of:

Kdv		Deer Valley Sandstone of Colburn (1964) (Late 
		Cretaceous) -- Fine - to medium - grained, gray, distinctly 
		bedded to massive, biotite-bearing arkosic sandstone and 
		minor conglomerate.

		Unit E - Siltstone and mudstone (Late Cretaceous) -- 
		Consists of:
Keu			Upper member.  Light gray siltstone, interbedded 
			with medium to coarse grained, clean, white and 
			orange, lithic sandstone with many large (as much 
			as 50 cm diameter) iron concretions.  Weathers to 
			light orange.
Kel			Lower member.  Light gray to gray-brown, 	
			foraminifer bearing siltstone and mudstone.  	
			Reddish-brown weathering and iron concretions 
			conspicuous.  Locally, includes (mapped in part):
Kels				Sandstone interbeds.  Coarse grained, clean 
				white, fossiliferous, lithic sandstone.  Grains 
				include quartz, feldspar, and black lithic 
				grains.  Mica is rare.  Iron concretions 	
				common.

Kd		Unit D - Sandstone (Late Cretaceous) -- Medium to coarse 
		grained, light gray, clean sandstone.  Grains include 
		quartz, feldspar, and biotite.  Spherical weathering 	
		common.  In places interbedded with fine to medium 
		grained, biotite and muscovite bearing wacke with 	
		mudstone rip-up clasts.  Sandstone beds form packages 
		up to 10 meters thick with 1 to 2 meters of interbedded 
		siltstone and mudstone.  Locally, divided into:
Kds			Shale member.  Brown to gray, micaceous mudstone 
			and brown micaceous siltstone.  One layer is dark 
			gray-brown to dark gray, massive, foraminifer rich, 
			siliceous mudstone.

		Unit C - Sandstone and shale (Late Cretaceous) -- Consists 
		of:
Kcu			Upper member.  Shale and siltstone.  Mapped 
			locally, includes:
Kcus	 			Upper member sandstone.  Sandstone 	
				interbeds.
Kcm			Middle member.  Medium grained, brown to gray, 
			biotite rich wacke with some mudstone rip-up 
			clasts.  Contains interbeds of siltstone, shale, and 
			conglomerate.
Kcl			Lower member.  Shale and siltstone with minor 
			sandstone.  Mapped locally includes:
Kcls				Lower member sandstone.  Sandstone 	
				interbeds.

Kb		Unit B - Sandstone and shale (Early Cretaceous) -- Base 
		marked by 1 m thick 	bed of cobble conglomerate 	
		overlain by 10 meters of conglomeratic sandstone. 	
		Includes at least one layer, 10 meters thick, of white 
		rhyolite tuff.  Locally, includes (mapped in part):
Kbsh			Shale member.  Olive-gray mudstone and micaceous 
			siltstone.  Forms reddish soil.  Contains sandstone 
			interbeds.
Kbs			Sandstone member.  Medium grained, olive-brown 
			wacke with large biotite grains and many small (3 
			mm long) mudstone rip-up clasts.  Interbedded 
			with thin beds of siltstone and mudstone.

Ka		Unit A - Shale, with minor sandstone (Early Cretaceous) 
		-- Olive-brown to dark gray siltstone and mudstone.  
		Spherical concretions common.    Locally, includes 	
		(mapped in part):
Kas			Sandstone member.  Fine to medium grained, 
			brown to olive, biotite-rich wacke with micaceous 
			siltstone interbeds.  In places grades into coarse 
			grained, bluish-gray wacke with large (up to 3 cm 
			long) mudstone rip-up clasts and phosphate 	
			nodules.

JKk		Knoxville Formation (Late Jurassic to Early Cretaceous).  
		Mainly dark, greenish - gray shale with sandstone 	
		interbeds; also, locally includes conglomeratic sandstone 
		in its lower part.  Locally contains abundant specimens of 
		bivalve fossil Buchia.

	Coast Range Ophiolite (Jurassic).  Consists of:
Jpb		Pillow basalt, basalt breccia, and minor diabase.
Jdb		Diabase.  Mainly sills and dikes; also includes screens of 
		pillow basalt.
sp		Serpentinite.  Mainly sheared serpentinite, but also 
		includes massive harzburgite.
sc		Silica carbonate rocks.  Altered serpentinite.