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,flood- plain, 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