Ananomalously large Exogyra from the Paguate Sandstone Tongue of theDakota Sandstone (Gretaceous, middle Genomanian) byBarry S. Kues,Department ol Earth & PlanetarySciences, University of New Mexico, Albuquerque, New Mexico 87131-1116 and)rin J. Anderson,New Mexico Bureau of Minesand Mineral Resources, Socono, New Mexico 87801 Abstract specimen possibly represents the sepa- the Conlinocerastarrantense through CaIy- rate, rapid evolutionary development of cocerascanitaurinum zones. Of the several A large, thick-shelled specimen of Er- a small population of E. whitneyi-llke Dakota-Mancos tongues and members in o3yra, measuring 165 mm in height, from oysters from an unknown ancestor in this sequence,the PaguateTongue of the the Paguate Sandstone Tongue, is de- west-central New Mexico, rather than a scribed and illustrated. This specimen, Dakota Sandstone contains the greatest relict population of E. whitneyior an im- diversity of marine invertebrates, with which is much larger than any oyster migratedin from mediatedescendant that more than 60 molluscan speciesreported previously reported from the middle the Tethyan province to the south. Cenomanian intertongued Dakota-lower (Cobbanand Hook, 1989).Recently, one Mancos sequence in New Mexico, is of the authors (OA) discovereda speci- Introduction morphologically similar to Exogyra whit- men of an unusually Iarge Exogyra,pre- neyi, an early Cenomanian Tethyan The intertongued upper Dakota-lower viously unknown in this sequence,in a (Texas, southern New Mexico) species. Mancos sequence is well exposed and fos- Paguate outlier in southwestern Cibola The presence of E. whitneyi or a closely siliferous at many localities in west-cen- County, New Mexico. This specimen is related species in the Paguate Tongue is tral and northwestern New Mexico, and surprising because marine connections noteworthv not onlv because of its size its faunas are diverse and well known (e.g., between southem and west-central New but also beiause its iffinities appear to be 1989). Mexico were absent from middle-early Cobban, 1977; Cobban and Hook, with a species,Exogyra whitneyi Bcise, that to middle-middle Cenomanian time and Ammonite biostratigraphy indicates that is confined to early Cenomanian strata in because no large exogyras of this type deposition of this sequence occurred dur- Texasand southern New Mexico. We de- are known from this interval anywhere ing most of middle Cenomanian and the scribe this specimen (11966,University of in the surrounding areas The Paguate beginning of late Cenomanian time, from New Mexico paleontologycollections) here and discusssome of the paleogeographic questions raised by its presencein the Pa- guate SandstoneTongue. ShatigraPhY The units of the intertongued Dakota- Mancos sequencewere initially named in the Laguna area, northeastern Cibola County, by Landis et al. (1973).Cobban (1977)summarized and illustratedthe di- verse fauna. Hook et al. (1980)extended this terminology into the southern Zuni Basin,and it has been used subsequently in mapping and coal-resourceevaluation work by McClellanet al. (1983),Anderson (1987),and Campbell (1989).Small, iso- lated outcrops of the PaguateTongue oc- cur in the areawest of AtarqueLake (Fig. U = 1), both north and south of the farolosa z Draw lobe of the North Plains basalt field N (Anderson, 1987).The Paguatehas been 3 identified in this area, in the Venadito z Camp quadrangle, on the basis of strati- graphicposition (abovethe lowestpart of the Mancos Shale and below the White- water Arroyo ShaleTongue of the Mancos Shale),as well as by its concretionaryli- thology and characteristicmolluscan fauna locally.The specimendiscussed here was collected from the middle of the Paguate ,Ec Tongue at an isolated outlier (Fig. 2) in the SE1/+SE1/+sec. 11 T5N R20W,Cibola 0 5 10mi County, about 8 mi (13 km) east of the -# New Mexico-Arizona border. Abundant 0 5 10 15km specimensof Exogyraleols Stephenson and Pycnodontecf. P. kellumi(Jones) occur in FIGURE 1-Location map and generalized geology of Atarque Lake area, Cibola County, New the upper part of the Paguateat this out- Mexico. @ marks locality where UNM-11966was collectedfrom the PaguateTongue of the Dakota crop (Fig. 3). The Paguate fauna in this Sandstone Psa, San Andres Limestone; and Triassic rocks (Entrada Sandstoneand |tr, furassic to the Acanthocerasam- Chinle Group); Kdm, intertongued upper Dakota-lower Mancos Shale; Kmha, Moreno Hill For- area is restricted mation and Atarque Sandstone;Kdmt designatesarea to the northeast with more complex stra- phibolumammonite zone, approximately tigraphy that includes the Tres Hermanos Formation; Qb, JaralosaDraw lobe of North Plains in the middle part of the middle Ceno- basalt flow Tertiary sedimentary units not shown. Basemap from Dane and Bachman, 1965. manian stage(Cobban and Hook, 19891 Nm Mexico Geology August 7994 FIGURE2-View to northwestof outcropof PaguateTongue (the light-coloredunit). Spec- imen of E. cf. E. whitneyiwas collectedfrom transitionalunit at baseof blocky,spheroidal weatheringunit in upperpart of exposure.Ta- lus and colluviumare derivedfrom overlying Quaternarybasalt flow (JaralosaDraw lobeof North Plainsbasalt flow). Description of specimen UNM-11966(Figs. 4A, C, D) is an ar- ticulated specimendisplaying most of the left (larger) valve, with the right valve present but almost completely covered with matrix. The surface of the left valve is severely worn and extensivelyperfo- rated with holes and canals of boring sponges,thus obscuring surficial details of the valve. The specimen is approxi- mately 155 mm high; restoration of miss- ing portions of the ventral margin suggests an original height of 175* mm. Estimated original length is 115 mm and width is approximately 70 mm. The left valve is thick and massive. The beak is inflated, f Locallyan additional6-ft.-thick fossiliferous gently coiled against the posterodorsal I sandstonesimilar to unit below. valve surface,and is stronglyinclined, such -.' Heavilybioturbated and hematite-stained that it approximately parallelsand slightly ' uppersurface suggests subaerial exposure. overhangs the hingeline. The outer sur- face of the beak is depressedrather than T Lowerfine-grained, grayish-orange (10 YR Z4) convex, suggestinga 40-mmJongattach- I quartzosesandstone. Low-angle crossbeds in ment scar.In lateralprofile, the valve sur- smalltrough sets may be tidallyinfluenced; faceis moderatelyand evenly convexfrom spheroidalweathering is common. I Oblate beak to ventral margin, with the maxi- 6' calcareoussandstone concretions uo to 3 ft in diameterare commonand generally mum valve width slightly above mid- fossiliferous.Fauna includes Exogyra levis, height. In dorsal(beak up) view (Fig.4D), 5- Pycnodontect. P. kellumi, and Pinna sp. the profile from anterior to posterior mar- Burrowingis common, gin is strongly convex, flattening slightly + Exogyract. E. whitneyi across the umbonal midline. Valve orna- Uppervery fine to lowerfine-grained, mentation is poorly preserved, but ex- yellowish-gray(5Y 712)to grayish-orange(10 YR amination of the less-worn valve surfaces 714)quat?.ose sandstone with micaceouszones, yielded no indication of significant radial thin,flat bedding;brown-weathering calcareous ribbing and slight suggestion of lamellate sandstoneconcretions up to 3 ft. in diameter; growth lines. 10- sparselyfossiliferous; calcareous cement; slight upward-coarseningtrend. Specimenof Exogyra Taxonomic discussion ct. E. whitneyi found at top of interval. The fauna of the Paguate and related Uppervery fine to lowerfine-grained, tongues of the upper Dakota-lower Man- yellowish-gray(5Y 712)quartzose sandstone cos sequencein west-centralNew Mexico with micaceouszones and small,coalified peat has been well summarized and illustrated b fragments;upper part has thin sets (<0.1ft,) of by Cobban (1977)and Cobban and Hook I low-anglecrossbeds and smallmud clasts; (1989).No exogyrasof this sizeand form 15- I scatteredfossils include small fragments of have been documented from these units, I gastropodsand bivalves(Exogyra? ); nor from coeval strata to the southeastin calcareouscement. Lincoln County (Cobban,1986). The larg- _]- est Paguateexogyras, E. trigeri (Coquand) Section below is covered. and E. cf. E. oryntas(Coquand), only rarely 4 4i attain a height of 100mm. Typical speci- a Qt mens of E. trigeri possessa left valve of low convexity, with prominent lamellate growth lines (e.g., Cobban, 1977,pI. L7, fi Burrowed(trace fossils) figs. 7-10), that differs considerably from A Calc.concretions the much larger, more convex specimen dG Fossiliferous FEI crossbeddedsandstone FIGURE 3-Measured section of Paguate Tongue, SEI/ISEIL sec. 11 T5N R2OW Cibola lTl rin"-grainedsandsrone County, New Mefco (measuredby O. Ander- son, Sept. 1993). August 1994 Nan Metico Geology taa1:! FIGURE 4-Exogyra cf. E. whitneyi (UNM 1f966; PaguateSandstone Tongue, Cibola County), and E whitneyi (UNM 11971;D9l !o clay, Cerro de Cristo Rey); all t 0.5. A, C, D, E. cf. E. whitneyi,right (A), left valve exterior (C) and dorsal (D) views. B, E, E whitneyi,right (B) and dorsal (E) discussedhere. E. cf. E. oxyntashas prom- Two species of Exogyrafrom the Cen- ratio of 0.41 and is more gently conuex, inent, closely spacedradial costae.Cob- omanian of the WestemInterior-Texas area with maximum width at about mid-height. ban (1977,p. 20)did note specimenswith attain the unusually largesize, thick valves, E. forresterialso has a more acutely convex "very massivethick shellsthat are much and general valve proportions