Watural History Museum Of los Angeles County

Transcription

1 ixjul re^ m i L. I V U EOCENE LLAJAS FORMATION, SIMI VALLEY, SOUTHERN CALIFORNIA Watural Histry Museum Of ls Angeles Cunty Palentlgy Richard L. Squires Department f Gelgical Sciences Califrnia State Dniversity Nrthridge, CA ABSTRACT late early thrugh early middle Ecene Llajas Frmatin in Simi Valley area, surn Califrnia, represents a transitinal alluvial t marine system. Mst f frmatin is part f a transgressive sequence f strata that grades vertically upward frm 1) massive clast-supprted cnglmerate depsited in stream channels n castal alluvial fans, t 2) fssiliferus alluvial-fan like cnglmerate interbedded with we11-laminated, well srted, Qphimrpha-burrwed fssiliferus sstne depsited alng shreline, t 3) alternating laminated biturbated sstne with scattered channel-fill shelly depsits depsited in nearshre warm temperate t trpical waters, t 4) biturbated silty sstne with in situ Ecernina-Turritella- Crassatella-?Trchcyathus megafaunal assemblages that frmed where shallw-marine uter shelf t slpe envirnments intergrade, t 5) biturbated siltstne with diverse benthic framiniferal assemblages that frmed in uter shelf t slpe waters. Lcally, amalgamated turbidite sstne fills channels incised int uter shelf t slpe siltstne. uppermst part f frmatin is part f a regressive event in which uter shelf t slpe channel-fill depsits enclsing siltstne were cvered by biturbated silty sstne with shallw-marine mlluscan benthic framiniferal assemblages. regressive sequence is incmplete due t truncatin by ersin prir t depsitin f verlying, nnmarine early late Ecene thrugh Oligcene Sespe Frmatin. INTRODUCTION name "Llajas Frmatin" was first used infrmally by Schenck (1931) McMasters (1933). Cushman McMasters (1936) later frmalized it, based n brief descriptins f surface utcrps at type sectin in Oil Canyn (nw knwn as Chiv Canyn) crehle data frm Tap N. 42 il well, 2 km west f type sectin (Fig. 1). Abve an unfssiliferus basal cnglmerate, it cnsists Micene - Plicene Undifferentiated Sespe Frmatin Llajas Frmatin ( = basal cnglmerate) Santa Susana Frmatin RI9W'RI8W Gelgy by R. L. Squires Figure 1. Gelgic map shwing utcrps f Llajas Frmatin in Simi Valley area, surn Califrnia. in Squires, R.R. Filewicz, M.V., eds., 1983, Cenzic Gelgy f Simi Valley Area, Surn Califrnia, Pacific Sectin, S.E.P.M., Fall Field Trip Vlume Guidebk, p. 81-%. 81

2 82 mstly f very fine-grained sstne siltstne with scattered fssiliferus beds. Llajas Frmatin crps ut n bth sides f Simi Valley. Best expsures are n nrth side f valley at 545 m-thick type sectin (Fig. 1). Elsewhere surface utcrps are generally pr, especially alcng suth side f Simi Valley. LOWER UPPER EOCENE Sespe Fm. NONMARINE SHALLOW-MARINE (regressive) Early wrkers referred t se strata by varius names. A summatin f mst widely used early names hw y crrespnd t Llajas Frmatin is shwn in Figure 2. Llajas Frmatin OUTER SHELF t SLOPE with TURBIDITE-FILLED CHANNELS "Stewart bed' SHALLOW-MARINE (transgressive) 100m L 0 Uncnfrmity Nature f cntact nt discussed * Present nly n suth side f Simi Valley Cnfrmable cntact Figure 2. Abbreviated histrical develpment f upper Palegene stratigraphic nmenclature in Simi Valley area. discnfrmably verlies late Palecene thrugh earliest Ecene marine Santa Susana Frmatin. Except where lcal faults are present, Llajas is uncnfrmably verlain by nnmarine Sespe Frmatin, cnsidered t be early late Ecene thrugh Oligcene in age (Glz, 1976). Llajas is mstly a transgressive sequence f facies that grades vertically upward frm a castal alluvial fan, t shallw marine, t uter shelf t slpe with incised channels. uter shelf t slpe facies was subsequently cvered by regressive shallw-marine facies (Fig. 3). regressive sequence is incmplete due t er6inal truncatin by verlying Sespe Frmatin. amunt f ersin increases tward east (Squires, 1981). Llajas is late early Ecene thrugh early middle Ecene in age, based n mllusks, benthnic framinifers, calcareus nannfssils. Wrkers using mllusks have reprted Llajas Frmatin as Ecene (Waring, 1917; Kew, 1919, 1924), middle Ecene (Clark, 1921, 1926; Cushman McMasters, 1936), early thrugh middle Ecene (Clark Vkes, 1936; Merriam Turner, 1937; Vkes, 1939; Merriam, 1941; Weaver et al., 1944; Squires, 1981; r late early thrugh early middle Ecene (Squires, 1983, in press; Saul, this vlume). A detailed discussin f se wrks is given in Squires (in press). late early Ecene strata are in zne f interfingering between castal w-rvy?! ASTAL ALLUVIAL FAN discnfrmity MARINE * Pacific Cast Prvincial Mlluscan "stage" ** Pacific Cast Benthnic Framiniferal Stage Figure 3. Stratigraphic clumn f Llajas Frmatin, shwing depsitinal envirnment s. alluvial-fan facies shallw-marine facies wuld crrespnd t lwer f tw faunal znes f Pacific Cast prvincial mlluscan "Capay Stage" f Clark Vkes (1936). remaining fssil-bearing part f Llajas wuld crrespnd t "Dmengine Stage" f Clark Vkes (1936). Wrkers using benthnic framinifers have reprted Llajas Frmatin as middle Ecene (Cushman McMasters, 1936) r early thrugh middle Ecene (Laiming, 1940a, 1940b, 1943; Ma]lry, 1959; Schymiczek, 1983a, this vlume). Mallry (1959) reprted that lwer Llajas crrespnds t his Pacific Cast benthnic framiniferal Penutian Stage that bulk f frmatin crrespnds t his Ulatisian Stage. An up-dated refinement f se stages, nw knwn t be time-transgressive, is given in Pre (1980). Filewicz Hill (this vlume), using calcareus nannfssils, reprt that mst f shallw-marine (transgressive) facies f Llajas Frmatin is f latest early Ecene r earliest middle Ecene age. uppermst 30 m r s f this facies rest f verlying Llajas Frmatin is f middle Ecene age. upper part f Santa Susana Frmatin is

3 83 earlist Ecene in age (Filewicz Hill, this vlume), thus unfssiliferus castal alluvial-fan facies f Llajas Frmatin may be early Ecene in age, as it is underlain by early Ecene strata verlain by latest early Ecene thrugh earliest middle Ecene strata. In last 69 years, starting with Waring (1914), re have been many investigatins dealing with megafssils f Llajas Frmatin. se previus studies have dealt with undcumented partial faunal lists, descriptins f new taxa, taxnmic refinements r miscellaneus cmments n knwn species. A listing f se previus wrkers is given in cmprehensive analysis f palentlgy bistratigraphy f se fssils by Squires (in press). Sstne beds in Llajas serve as reservirs in faulted anticline accumulatins f il in eastern prtin f "Main Area" f Simi Oil Field in vicinity f type sectin (Fig. 1). This entire field, which was discvered in 1900, had prduced nearly 3 millin barrels f il thrugh 1961 (Stipp, 1943; Califrnia Oil Gas Fields, 1961). Alternating sstne siltstne beds serve as reservirs in a faulted-anticline accumulatin f il in Unin Oil, Las Llajas Oil Field abut 5 km nrast f type sectin (Fig. 1). This field, which was discvered in 1946, had prduced 38,500 barrels f il thrugh 1963 (Tudr, 1963). Accrding t Nagle Parker (1971), Llajas is an il-prducing hrizn in r varius il fields in nshre Ventura Basin. Schymiczek (1983b) discussed ccurrence f il in Llajas Frmatin. FACIES varius facies f Llajas Frmatin as recgnized discussed by Squires (1981) are summarized belw. Castal Alluvial-Fan Facies castal alluvial-fan facies is present in lwermst part f frmatin is shwn as "basal cnglmerate" in Figure 1. It interfingers with verlying shallw-marine (transgressive) facies. castal alluvial-fan facies is present thrughut nrrn part f Simi Valley wherever basal part f frmatin is expsed. Thickness ranges frm 20 t 40 m. Accrding t Seedrf (this vlume), basal cnglmerate can be recgnized in subsurface as far nrthwest as Oak Ridge, 8 km nrthwest f type sectin. Althugh castal alluvial-fan facies is mstly cvered alng suth side f Simi Valley, it des crp ut between Runkle Oak Canyns (Fig. 1). Thickness varies frm nearly zer t 10 m in that area. facies cnsists f cnglmerate (80%) sstne (20%) (Fig. 4). cnglmerate cnsists f channel-fill depsits that average 3 m thick. A few well-expsed beds can be traced fr abut 100 m. Well runded clasts cnsist predminantly f cbble-size light-gray, tan, r purple quartzite. Lcally, granite, green esite, /r gneiss make up as much as 30 percent f clasts. Mst f cnglmerate is prly srted, clast-supprted, unstratified, ungraded (Figs. 5A, B). Sheared clasts half-rund clasts are fairly cmmn. Quartzite bulders with percussin marks are present but are uncmmn. sstne beds are lenticular are interbedded with cnglmerate. y average 1 m thick. M6t f lwer cntacts f sstne beds are gradatinal, upper cntacts are ersin surfaces. Mst f sstne is silt t granule subarkse, very prly srted, subangular t subrunded, usually massive. In sme places, it is crss-bedded r laminated (Fig. 5C). Active il seeps frm basal cnglmerate are present at type sectin nearby Chiv Canyn. seeps are related t small faults that cut cnglmerate. At type sectin, sme f sstne is il-stained sme f laminated sstne cntains gilsnite veinlets. Where gilsnite is present as 6mall streaks parallel t laminae, it resembles carbnized plant fragments. Discussin massive, prly srted, interbedded channelized sequences f cnglmerate sstne are similar t fld-dminated, stream-channel depsits n alluvial fans. Nilsen (1982, fig. 43C) als interpreted m t be streamflw depsits. interfingering f upper part f facies with shallw-marine depsits indicates that this alluvial-fan system prgraded int marine waters. Alluvial fans that prgrade int a sting bdy f water frm an adjacent highl have been defined as fan-deltas (Hlmes, 1965; McGwen, 1970). castal alluvial-fan facies culd be called a fan-delta facies, but fr sake f cnsistency with earlier designatins (Squires, 1981, in press), name "castal alluvial fan" will be used. It is envisined that castal alluvial-fan depsitinal setting f Llajas resembles thse f mdern high-relief areas alng west cast f Baja Califrnia, Mexic. Accrding t Hwell Link (1979), hydrlgy hydraulics f cbble transprt fr such areas are fld-dminated. steep tpgraphy prduces narrw cnfined channels that rapidly give way t castal plains. Where braided rivers carry carse sediment int cean, y prduce steep fan deltas. Similar cnditins existed in Ecene depsitinal systems f San Dieg area (Hwell Link, 1979; Link, et al., 1979), as well as in Late Cretaceus depsitinal systems f Santa Ana Muntains (Cper, et al., 1982). clasts f cnglmerate beds f Llajas castal alluvial-fan facies are well runded. T prduce well-runded quartzite requires a great deal f transprt r abrasin, unless y were derived frm nearby expsures f lder cnglmerate that cnsist f well-runded clasts. nearby Palecene Simi Cnglmerate prtins f Santa Susana Frmatin cnsist f well-runded clasts very similar in cmpsitin (Sage, 1973; Janes, 1976; Parker, this vlume) t thse f Llajas basal cnglmerate culd have been surce rcks. prly srted nature f clasts als supprts lcal-surce interpretatin. Shallw-Marine Facies shallw-marine facies is divisible int tw parts. transgressive phase f shallw-marine facies is present in middle part f frmatin whereas regressive phase is in uppermst part (Fig. 3). Shallw-Marine (Transgressive)

4 Ttfzr, mrs -rr Tnrr mnr ZZZ Trsr' c "a Lieu CL _ en xz =J O 0.5 Km EXPLANATION Cnglmerate 133 Sstne ^ Biturbated siltstne mnr = ' -yjnr "JW ' ~mr Tnnr. mnr w. 5 ^ x: Pseudphrgming } beds lr?nr mnr ~8TT&-. Stewart bed" 540 m- = Laminae m Crss Biturbatin Vertical burrws Channel-fill depsits Cncretins Megafssils * Wd WVWSA Sespe. Fm. I, uncnfrmity 300- Castal-Alluvial q Fan Facies ft Shallw-Marine 280- S> Facies > c 3 O s^ c k.» c 480- Santa Susana Fm. discnfrmity cvered V w w v v Figure 4. Stratigraphic sectin f Llajas Frmatin measured at type sectin alng west side f Chiv Canyn, SE1/4 suth central prtin f sectin 30, T3N, R17W, Santa Susana quadrangle, Califrnia.

5 85 Figure 5. Castal alluvial-fan facies, type sectin: A) Cnglmerate with interbedded lenticular sstne; scale is 1.5 m. B) Unstratified ungraded cnglmerate; scale is 1.5 m. C ) Laminated sstne interbed; pen is 14 cm lng. (Frm Squires, 1981). shallw-marine (transgressive) facies cnsists f fssiliferus cnglmerate sstne where re is interfingering with cnglmerate beds f castal alluvial-fan facies (Figs. 4, 6A). fssiliferus cnglmerate beds are similar in gemetry, fabric, lithlgy t unfssiliferus cnglmerate beds in castal alluvial-fan facies. fssiliferus cnglmerate beds grade upward int channelized laminated cquinas cnsisting mstly f streid unidentifiable gastrpd pelecypd remains (Fig. 6B). In places, re are thick-shelled Venericardia (Pacificr) aragnia iaquinensis fragments. Grwth series f thick-shelled Turritella megansensis prtumescens can be fund lcally. This taxn (Fig. 6C) is cnfined t se beds characterizes where castal alluvial-fan facies interfingers with shallw-marine facies. Mst f sstne in this interval f interfingering facies is very fine-grained, well srted subarkse. sstne is usually laminated (Fig. 6D) althugh in places lw-angle crssbedding, ripple bedding, channels are present. Vertical burrws, sme f which are Ophimrpha (Fig. 6E), are mre cmmn in this part f Llajas Frmatin than anywhere else. Lcally, re are pieces f Teredsbred petrified wd transprted cncretins which have brehles made by phladid bivalves. Overlying interval f interfingering castal alluvial fan shallw-marine facies, shallw-marine (transgressive) facies cnsists f alternating laminated biturbated sstne (Figs. 7A, B) with scattered fssiliferus units. laminated beds can be up t 2 m thick. Many f lwer cntacts are ersin surfaces. sstne is very fine-t fine-grained subarkse mderately well srted, subangular, usually cntaining sme brken small calcareus framinifers. Cmmnly, re are penecntempraneus, very lw-angle ersin surfaces within laminated beds (Fig. 7C). Lcally, sme beds have undulatry bedding, seemingly related t scuring. Well-preserved Ophimrpha burrws are cmmn. Biturbated sstne is in beds that can be up t 3 m thick. Usually, lwer cntacts are gradatinal with laminated sstne. biturbated sstne is very fine t fine subarkse, mderately well srted subangular. sstne is usually thrughly mttled by biturbatin (Fig. 7D). Ophimrpha is recgnizable in places. scattered fssiliferus units in shallw-marine (transgressive) facies are mstly channel-fill shelly depsits (Figs. 8A,B) that can be up t 50 cm in thickness 30 m in width. depsits are calcareus subarkse with variable amunts f mllusks calcareus benthic framinifers. Mst gastrpds framinifers are cmplete shw little r n signs f abrasin r runding. In many beds, apices f Turritella shells shw a strng bimdal rientatin pint mstly tward nrthwest r suast. Mst