Geology and Stratigraphy of the Seismosaurus Locality, Sandoval

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Geofogy and stratigraphy 0l the Ser'smosaurus locality, SandovalCounty, New Mexico byHilde L. Schwartz,P.0. Box156, Dixon, New Mexico, 87527 andKin Manley,4691 Ridgeway Dr., Los Alamos, New Mexico 87544

Abstract mosaurus(Gillette et al., 1985,1990; Gillette, 1986,1991; Gillette and Bechtel,1989; Schwartz et al.,1989).The fossillocalities are , The Brushy Basin Member of the Morrison Formation (Upper near the southwestflank of the southernend of the Nacimiento is Jurassic) widely exposedin westernNorth Americaand is well uplift, where it terminatesin a southward-plunginganticline. The known for its diverseand abundantdinosaur fauna. Before1985, badlandsin this region delineatethe southeasternmargin of the however, the Brushy Basinin central New Mexico had yielded few fossilsand was thought to be relatively barren of dinosaur remains. San Juan Basin. Stratigraphicunits exposedin the area include A group of new fossil sites reverses this view. They contain the the Upper TriassicChinle Formation,the furassicEntrada Sand- remains of severaldifferent dinosaur taxa, including the supergiant stone, Todilto [consideredby someauthors part of the Wanakah sauropod Seismosaurus.The new localities are in the southeaitern Formation (Condon, 1989)],and Morrison Formations,and the San Juan Basin, in an area that is both structurally and sedimen- CretaceousDakota Formation and MancosShale (Woodward and tologically complex. Mapping shows that the fossil sites do not Ruetschilling,t975). Prior to the work of Gillette and numerous occur along_asingle horizon but are distributed through at least volunteers,the only significantdinosaur remainsreported from 65 meters of section, occurring in a variety of facies. this region were a partial (20%)skeleton of a Camarasaurusand severalAllosaurus teeth (Rigby,1982). Indeed, the Morrison For- Introduction mation was thought by some to be relativelybarren throughout New Mexico (Hunt and Lucas, 1984;Lucas and Hunt, 1985). Our work was undertakento provide a geologicaland paleoen- vironmentalframework for thesenew paleontologicalsites. Local stratigraphyand the sedimentologyand petrographyof the Seis- mosaurusI site were studied by Schwartz;Manley mapped the Museum of Natural History (NMMNH). These sites have yielded regional geology and establisheda stratigraphicframework for dinosaur remains, including those of a supergiant sauropod, Sels- the network of fossil localities. r=

Alsoin this issue Upcomingmeetings p. 30 Galleryof Geology-KutzCanyon-Angel Peak p. 31 Three-dimensionalvisualization of Precambrian in New Mexico p.32 San Juan Basin E elevation southeastern E- NMGS field conferences p. 33 f 1992 conferenceinformation o c 1993 guidebook,call for papers .9 Geologichazards in New Mexice-part 1 p. 34 '6E NMGS spring meetingabstracts p.42 (U z Service/News p. 43 Staff Notes p.44

Comingsoon Geologichazards in New Mexico-part 2 New fusuliniddata and ancestralRocky Mountain deformationin the Joyita Hills,Socorro County Hikingto the top of New MexicrWheeler Peak

FIGURE 1-The study area in the San Juan Basin of northwestern New Mexico. Limits of the San fuan Basin after Aubrey, 1987 General stratigraphy, age, and structure positions of these two groups of sites were determined by estab- The Upper furassicMorrison Formationof the WesternInterior lishing the irrelationship to the base of the overlying fackpile is a complex, widespread sequenceof continentalfluvatile and Member. Becausethe Brushy Basin and Jackpile Sandstonemay lacustrinesedimentary rocks that is well known for its abundant be intertonguing over several meters of vertical section and be- dinosaur fauna (e.g. Ostrom and Mclntosh, 1966;Dodson et al., cause the basal contact of the JackpileSandstone is locally chan- 1980).In centralNew Mexico,the Morrison is approximately230 neled into the Brushy Basin, these measurementsallow for only m thick and consistsof four members,in ascendingorder: the an approximatecorrelation. Recapture,the WestwaterCanyon, the Brushy Basin, and the Fig. 2 illustratesthe stratigraphyof the Brushy Basinand up- JackpileSandstone Members (Flesch,1974, 7975;Owen et al., permost Westwater Canyon Members in the study area and the 1984).These units were mapped by Santos(1975) and are well stratigraphicpositions of the fossil sites.The "Seismosaurus"lo- describedin Flesch(7974, 1975) and Fleschand Wilson (7974). calities:SSI, SSII, and SSIIIare, respectively, 26,31, and 35 meters Most of the fossilsites discussed below are in the Brushy Basin below the Jackpile Sandstone. McConnell I is approximately 18 Member, the most widely exposedand fossiliferousunit of the m below the baseof the fackpile Sandstoneand McConnellII 5 Morrison Formation;one site is in the uppermostportion of the m below the base. The Martini I site is 46 m below SSI and 72 m underlying WestwaterCanyon Member. The BrushyBasin in this below the JackpileSandstone. Sites CKI and LI occurat about the part of New Mexico is exposedin broad canyonswhose slopes same stratigraphic level, approximately 32 m below the fackpile arecommonly coveredby an overburdenof sandstonerubble and Sandstone.Locality CI lies approximately 39 m below the Jackpile rotated, Iandslidblocks of well-induratedsandstone. Sandstone.The contactbetween the Brushy Basin Member and Near the Seismosaurussite, the Brushy Basin consistsof ap- underlying WestwaterCanyon Member is not sharp but vertically proximately80 m of variegatedclaystones, siltstones, sandstonei, interfingers through a stratigraphic interval of approximately 20 and rare limestones,punctuated by resistantledges of well-in- m. The lower boundary of the Brushy Basin is placed at the top durated sandstone(Fig. 2). Many of the claystonesare altered of the highest thick, continuous sandstoneunit, in accordance volcanic ash beds, as indicated by relict shard texturesin beds with Flesch (1974). that have been altered to clinoptilolite or by their bentonitic clay composition.The ledge-formingsandstones beds in the Brushy Site descriptions (SSI) Basin range in thicknessfrom severalcentimeters to severalme- SeismosaurusI ters and are dominantly coarse-grainedarkosic arenites. They are The largestexcavated site and main sourceof Seismosaurusrc- cementedfor the most part by siliceousminerals (mainly chal- mains thus far is locality SSI. It is near the top of a peninsular cedony) and minor calcite.Authigenic clays are also present in mesa that rises 40m above the surrounding valley floor and is many horizons. Individual sandstonebeds are lenticular,but some cappedby a thick, well-induratedsandstone bed. SSIhas yielded extend far enough to be useful as local marker horizons. Highly polished,subangular to subroundedpebbles and cobbles(ranging lrom 1 9m to 15cm), of exoticlithologies, also occur in the Brushy Basin.Their origin and significanceis currently being studiedby Manley (1989,7991a,b;Johnston et al., 1990).Overall, the Brushy New AAex[c@ Basindeposits in the study arearepresent fluvial channel,floodplain, pond, and playalake facies(Flesch, 7974;8e11,1986; Turner and Fishman,I99l). GEOLOGV Preliminary I(/Ar dates from the Brushy Basin Member near . Scienceand Service the Seismosauraslocality indicate a minimum ageof sedimentation {Ar/3eAr rssN 0196-944X of I54t:2.4 Ma (Woldegabrieland Hagan,in press). ages Volume 14, No. 2, May 1992 for the Morrison Formationat Montezuma Creek, Utah and Di- nosaur National Monument, Utah range from 153-145Ma (Ko- Editor:Carol A. Hiellming wallis et al.,799t), and it is unknown to what extentthe Morrison Published quarterty by Formation may be time-transgressive. New Mexico Bureau of Mines and Mineral Resources Structurally,the BrushyBasin and WestwaterCanyon Members a division of New Mexico Institute of exposedin the study area are dominated by low westerly and Mining & Tehnology southwesterlydips (<10") and are cut by severalNNW- to'ENE- BOARD OF REGENTS trending faults that are predominantly down to the southwest. Ex-Officio Bruce King, Gouernorol Nru Mexico Thesefaults are high angle and have offsetsof as much as 60 m. Alan Morgan, Superintendentof Publiclnstruction Sandstonelenses in the Brushy BasinMember containnumerous Appointed small fractureswith Lt. Gen. Leo Marquez, Pres.,1989-7995, Albuquuqw slickensidesurfaces, along which net motion Charles Zimmerly, Sec.lTreas.,1941-7997, Suorro has been negligible. Diane D. Denish, 7992-79D7,Albuquerque J. Michaet Kelly, 7992 797, Ruwell Steve Torres, 1997-1997, Albuquerque New Mexico Institute of Mining & Technology Fossil sites President. . . LaurenceH. Lattman All the fossil sites described New Mexico Bureau of Mines & Mineral Resources are in T15N, R1W and R1E. To qnd date,more than a dozendinosaur Diector StateGeologist .. . Charles E. Chapin bonesites have been discovered Sub{iptiore: Issued quarterly, February May, August, in the area;of these, nine have been documentedand studied. November; subscription price $6.00/calendaryear. The latter are designatedSeismosaurus I (SS!, SeismosaurusII Editorial matter: Articles submitted for publication (SSI!, SeismosaurusIII (SSIII), (MCI), should be in the editor's hands a minimum of five McConnellI McConnellII (5) months before date of publication (February (MCII), Martini I (MTI), Cary-Kim I (CKI), Cary I (C!, and Lat Mat August, or November) and should be no longer I (LI). Other localitieswithin a 5 km radius from the main, Seis- than 20 typewritten, double-spacedpages. All scientific papers will be reviewed by at least two mosaurusI, site have yet to be mapped and studied. people in the appropriat€ field of study. Address The nine sites discussedbelow are separatedby a maximum inquiries to Carol A. Hjellming, Editor of Nm Mexia distance Geology,New Mexico Bureau of Mines & Mineral of 2.8 km, with five of them Ioiated within a radius of Resources,Socono, NM 878014796. 1 km. The stratigraphicinterrelationships of all the sites could Publlshedas publi domin, ther4oreleryoducible without not be directly measuredbecause of erosion and faulting; one Derm ission. Sourcecred it requested. group of sites, SSI, SS[, SSI[, MTI, CKI, CI, and LI, is closelv Circulation:1,6ffi P/iflteli University of New Mexico Printing Services fpaced a1{1ot separatedby faulting. Faultsseparate this group from the MCI and MCII sites,however. The relativestratigrlphic

May 1992 Nm Mexico Ceology the articulated remains of a single Seismosaurusindividual, in- sibility, SSI has been studied in greater detail than the other seven cluding vertebrae, ribs, chevrons, parts of the inominate bone, sites in the area. and gastroliths (Gillette, 1986, l99l). The bones trend into a hill- The bones at SSI are enclosed in several different horizons of side and excavation is continuing in search of more. Three of the moderately indurated, buff to grey, arkosic arenite. The sandstone caudal vertebrae have been prepared and are on display at the is coarse grained and moderately sorted; its constituent frame- NMMNH. Because of its paleontological significance and acces- work grains consist mainly of quattz (80-85%) and feldspar (pla-

40M JACKPILESANDSTONE OM BRUSHYBASIN MEMBER

10M

WESTWATERCANYON MEMBER 20M

=-fr-= Iffi l:lll :-: =- < MTI ------1

30M .--J LATI

E variegatedsiltstone and claystone -so F-----llwe t t rtedsan d sto n e lHil moderately-sortedsandstone ffi limestone FIGURE 2-Stratigraphy of Sandstone, .the I:ckpile Bryshy Basin, and Westwater Canyon Members of the Morrison Formation in the study area. Arrows on right side of section show the stratigraphic locaiions of the fossil localities diicussed. Meter measurements indicate distance of fosiil sites below the fackpile Sandstone-Brushy Basin contait.

Neu Mexico Geology May 1992 gioclase, microcline, and orthoclase, L5-20Vo).The quartz grains (approximately l5-55V"). The largest Pore sPacesin the bone are are subrounded to subangular and contain abundant inclusions. the haversian canals, which are tubes that lie parallel to the long Quartz overgrowths and clay rims are common. Feldspar grains axis of a bone. In living bone, the canals are filled with blood are generally subangular and range in preservation from nearly vessels,connective tissue, and nerve filaments, but these organic pristine to completely altered. Both fresh and altered grains com- materials decay and wash out of bone with time and burial. Hav- monly exhibit twinning, zoning, or perthitic texture. ersian canals in the interior, spongy parts of Seismosaurusbone Minor detrital grains in the SSI sandstoneinclude heavy min- are cemented exclusively by chalcedony.Variable sorting of the erals (predominantly zircon) and rock fragments. The rock frag- sandstoneaccounts for discrete,concretionary lenses (pore sPaces ments are mainly volcanic materials, consisting of feldspar laths largely filled with chalcedony)in the quarry area, some of which and quartz phenocrysts in a dark, fine-grained matrix. Light- are associatedwith fossil bone. A later generation calcitecement colored mottling in the matrix of some of the fragments suggests is also present locally in the sandstoneand bone; its distribution relict shard textures. Rare sedimentary rock fragments (chert and is related to the location of large (to several centimeters thick) silicified siltstone) and igneous rock fragmen[s (polycrystalline root and caliche-filled cracksthat criss-crossthe sandstoneat the quartz with muscovite) also occur in the sandstone. Bone frag- SSIquarry (Fig. 3). ments make up a significant percentageof the detrital grains in A microstratigraphic section of the quarry wall at SSI (Fig. 3) the SSI sandstone near the contactsbetween dinosaur bone and shows the internal structure oI the Seismosaurus-bearingsand- matrix as well as in sediment-filled cracks in larger bones. stone. Planar laminations, trough cross-stratification,scour and Diagenetic alterations in bone and matrix at SSI were recog- fill, and repeated fining-upwards sequencesPredominate, sug- nized using petrographic and scanningelectron microscopes.Ce- gesting that the deposit representsa fluvial channel sandstone mentation is prevalent, occurring as grain coatings, void fillings, bed (Fisk, 1944; Folk and Ward, 1957; McGowen and Garner, and overgrowths. Clay (smectite) rims are common around de- 1970).Crossbedding at the quarry site shows that current direc- trital grains, pore spaces,and voids in both sandstoneand bone, tions were generally to the east and northeast. Scroll bars, rep- and fill in approximately 45-85% of the sandstone porosity. Lo- resentingparts of the point barsof largerivers (Fisk,19t14; Sundborg, cally, a secondgeneration of chalcedonycement and quartz over- t956), are selectively cemented and weathering out across the growths is volumetrically important in pore space reduction upper surface of the Seismosaurusmesa, several meters strati- E

@l pebblelao tr<

I I I I I I I I I OM 1M 2M 3M 4M 5M 6M 7M 8M

FIGURE 3-Stratigraphy of the SSI quarry: top, lithology and bedding in the northern quarry wall; bottom, position of Seismosaurusremains exposed in the south end of the quarry. Bones are colored solid black; bone-bearing, plastered blocks are indicated by diagonal lines. The plastered blocks have not yet been fully prepared and thus the precise positions and shapes of the bones in them is not known. Both bones and blocks are shown superimposed upon a vertical surface. Vertical positions of the bones and blocks were measured by sighting from an established benchmark.

May 7992 New Mexico Geology graphically above SSI. They also have an overall east-west ori- Lat I (LD entation and are part of a resistantsandstone that is continuous Site LI, about 0.3 km south-southeastof SSI, contains many acrossseveral kilometers. Trough cross-bedding,scroll bars, and Iarge fragments of dinosaur bone. The fragments are along one the sheetlike geometry of the sandstonesat SSI all suggest that horizon, along 25 m of hillside, but are not in situ; their depo- the Seismosaurusdied on or near the banks of a large meandering sitional setting is thus indeterminate. Although one large bone river. fragment was found in a landslid sandstoneblock, the rest of the bones lie on a reddish claystone. The hillslope at LI is capped by (SSID SeismosaurusII a well-indurated buff sandstone;it lies between 5 and 8 m above SSII is 0.2 km southeastof, on the same mesa as, and 5 m the level at which most of the bones are found. stratigraphically below SSI. It consists of several piles of bone fragments. The fragments are parts of large bones that eroded Conclusions (apparently in place) from a sandy shelf composed of poorly indurated, buff sandstone.The fossiliferoussandstone was depos- The Brushy Basin Member of the Morrison Formation in the ited directly above a well-indurated, dark sandstone bed (with southeastern San Juan Basin is much richer in dinosaur remains (at least abundant 1-cm-sizedcalcite concretions), and is interpreted to be than was previously believed. Both sauropods Seismo- a paleochanneldeposit. saurus and Camarasaurus,and possibly others) and a theropod have been identified. Nine documented fossil localities within the SeismosaurusIII (SSIII) study area (approximately 2.24 km2) occur over 55 m of vertical section. They are found in both fine-grained and sandy units, An isolated limb bone was excavated from the SSIII locality. representing a range of paleoenvironmentsfrom distal floodplain The bone was found in situ in an olive-green claystone, approx- and pond to fluvial channel and channel top; the fossiliferous imately 4 m stratigraphically below SSII. The claystone lies be- sediments may have accumulated near the southern margin of a tween two channelsandstones and is itself an overbankdeposit. large alkaline, saline lake (LakeT'oo'dichi'), under semiarid to arid SSIIIis locatedapproximately 0.2 km northeastof SSI,on a iedge climatic conditions (Turner and Fishman, L99l). on the north side of the samepeninsular mesa. For the most part, the dinosaur remains in the study area rep- isolated partial (articulated and disarticulated) (MCI) resent skeletons McConnell I and isolatedbones (both completeand broken). In the Morrison MCI consistsof an accumulationof disarticulatedskeletal el- Formation to the north of New Mexico, isolated partial dinosaur ements,including ribs, teeth,vertebrae, one tooth, and one gas- skeletons and bones occur mainly in floodplain and overbank trolith in situ. The bones represent at Ieast two individuals, a environments, while mass skeletal accumulations are found mainly sauropod and a carnosaurliillette, 1991).They are at the base in paleochanneldeposits (Dodson, et al., 1980).The lack of mass of a short, steepslope in a silty olive claystonethat cropsout 2.8 accumulations of dinosaur bone in Morrison Formation paleo- km southeastof SSI.The MCI stratumis part of a thick floodplain- channels in the southeastern San fuan Basin suggests that pa- overbank sequencethat is near the top of the Brushy Basin Mem- leoenvironmental conditions varied widely across the Morrison ber. depositionalbasin (andLake T'oo'dichi') during the Late|urassic. The local abundance of dinosaur bone in this area shows that McConnell II (MCII) the Morrison Formation in central New Mexico has great potential Only 0.1 km south-southeastof MCI, the MCII site is strati- for further vertebrate paleontological studies, particularly with graphically 13 m higher than its neighbor. One large bone and respect to taphonomy and paleoecology,and deserves further numerous bone flakes and fragments (unidentified to date) are investigation. exposed here. They are eroding from a dark reddish-brown silt- Ac

Neu Mexico Geology May 1992 Flesch, G A , and Wilson, M D , 1974, Petrographv of Monison Formation (Ju- Manley,K , 1991a,Gastrolith identification and sauropoddinosaur migration (abs ): rassic) sandstone of the Ofito Spring quadrangle, Sandoval County, New Mexico: GeologicalSociety of America, Abstracts with Programs, v 23, no 4, p 45 New Mexico Ceologrcal Society, Guidebook to 25th Field Conference, pp 1.97- Manley, K , 1991b,Two techniquesfor measuring surfacepolish as applied to gas- 210 froliths:lchnos, v 1, pp 313-316 Folk, R L , and Ward, W , 1957, Brazos River bar: A study in the signrficance of McGowan,J H , and Garner,L E ,1970, Physiographicfeatures and stratigraphic grain-size parameters: journal of Sedimentary Pehology, v. 27, pp. 3-26. types of coarse-grainedpoint bars: Modern and ancient environments: Sedimen- Cillette, D. D.,1986, A new giant sauropod from the Monison Formation (Upper tology, v 1.4,pp 77-1.1"1 Jurassic) of New Mexico (abs.): 4th North American Paleontological Convention, Ostrom, J H , and Mclntosh,I S ,7966, Marsh'sDinosaur-the collectionsfrom Boulder, Colorado, August 12-15, 1986, Proc., v. 4, p. A18. Como Bluff: Yale University Press,New Haven, Connecticut, 388 pp Grllette, D. D., 1997, Seismosurus halli, gen. et sp. nov., a new sauropod dinosaur Owen, D E, Walters,L L lr., and Beck, R G, 1984,The JackpileSandstone from the Monison Formation (Upper Jurassic/Lower Cretaceous) of New Mexico, Member of the Morrison Formation in west-centralNew Mexico-a formal defi- USA: Journal of Vertebrate Paleontology, v. 11, no. 4, pp. 417 433. nition: New Mexico Geology, v 5, no 3, pp 45-52 Gillette, D. D., and Bechtel, J. W., 1989, Pelvic and caudal anatomy of the siant Rigby,I K., Jr ,7982, Carnarasauruscf supremus from the MorrisonFormation near sauropod dinosaur "seismosaurus" (Morrison Formation, New Mexicol 1jbs.1: San Ysidro, New Mexico-the San Ysidro dinosaur: New Mexico Geological So- Journal of Vertebrate Paleontology, Abstracts of Papers, 49th annual meeting, v ciety,Guidebook to the 33rd Field Conference,pp 277)72 9, no. ), p.22A. Santos,E S ,7975, Lithology and uranium potentialof Jurassicformations in the Cillette, D. D., Bechtel, J. W., and Bechtel, P., 1990, Gastroliths of a sauropod SanYsidreCuba and MajorsRanch areas, northwestern New Mexico:U S. Ge- dinosaur from New Mexico (abs.): Journal of Vertebrate Paleontology, Abstricts ological Survey, Bulletin 1329,22 pp. of Papers, 50th annual meeting, v. 10, no. 3, p.24A. Schwartz, H L , Gillette, D D , and Manley, K , 1989,Taphonomy and paleoecology Cillette, D. D., Cillette, J. L., and Thomas, D. A., 1985, A diplodocine dinosaur in of the Upper Jurassic(?) "Seismosaurus"locality, San Ysidro, New Mexico:Journal the Morrison Formation of New Mexico (abs.); Annual Symposium of Southwest of VertebratePaleontology, Abstracts of Papers,49th annual meeting, v. 9, no 3, Geology and Paleontology, Museum of Northern Arizona, Flagstaff, p. 4. o 37A Hunt, A. P., and Lucas, S. G., 1984, Rariiy of dinosaurs in the Upper jurassic Sundborg, A , 1956,The river Klaralven: A study of fluvial processes:Geografica Morrison Formation, New Mexico (abs.): Geological Society of America, Abstracts Annales, v 38, pp 1.25-316 with Programs, v 16, no 4, p 225 Turner,C E, andFishman,N S,1991,JurassicLakeT'oo'dichi': Alargealkaline, Johnston, R. G., Manley, D., and Lemanski, C. L., 1990, Characterizing gastrolith saline lake, Morrison Formation, easternColorado Plateau:Geological Society of surface roughness with light scattering: Optics Communications, v.74, pp. 279- America,Bulletin, v 103,no 4, pp 538 558 283. Woldegabriel,G , and Hagan,R, in press,Diagenetic processes and burial history Kowallis, B. J , Christia nsen, E. H., and Deino, A. L. , 1997,Age of the Brushy Basin of the Morrison formationin the southernSan Juan Basin, New Mexico;in D D Member of the Monison Formation, Colorado Plateau, western USA: Cretaceous Cillette (ed ), Chemistry of preseruationof dinosaur bone: Springer-Verlag,New Research, v 12, pp 483-493 York Lucas, S. G., and Hunt, A. P, 1985, Dinosaurs from the UpperJurassic Morrison Woodward, L A , and Ruetschilling,R L , 1976,Geology of SanYsidro quadrangle, Fomation in New Mexico: New Mexico Journal of Science, v 25, no. 1, pp. 1- New Mexico: New Mexico Bureauof Mines and Mineral Resources,Geologic Map 37, scale7:24,O00 n V""Ly, X , 1989, Technique for distinguishing bona fide and suspected gastroliths (abs.): Annual Symposium of Southwestern Geology and Paleontology, Museum of Northern Arizona, Flagstaff, p. 18.

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30 May'1992 Nm MexicoCeology