Monograph: Late Cenomainian and Turonian Ammonite Faunas from North-East and Central Texas

The late Cenomanian Sciponoceras gracile Zone ammonite fauna of the Britton Formation of the Eagle Ford Group in north-central Texas comprises twenty-seven species referred to seventeen genera, of which one genus and five species are new. The fauna consists of: Borissiakoceras orbiculatum Stephenson, 1955, B. auriculatum sp. nov., B. desmoceratoides sp. nov., Desmoceras (Moremanoceras) scotti (Moreman, 1942), Placenticeras cumminsi Cragin, 1893, Metengonoceras acutum Hyatt, 1903, Calycoceras (Calycoceras) naviculare (Man tell, 1822), Eucalycoceras sp., Pseudocalycoceras angolaense (Spath, 1931), Tarrantoceras (Sumitomoceras) conlini Wright and Kennedy, 1981, T. (S.) crassum sp. nov., T. (S.) proteus sp. nov., T. (S.) bentonianum (Cragin, 1893), Euomphaloceras septemseriatum (Cragin, 1893), Metoicoceras geslinianum (d'Orbigny, 1850), Nannometoicoceras gen. novo acceleratum Hyatt, 1903, Metaptychoceras reesidei (Cobban and Scott, 1973), Puebloites corrugatus (Stanton, 1894), Anisoceras sp. novo aff. plicatile (J. Sowerby, 1819), Allocrioceras dentonense (Moreman, 1942), A. larvatum (Conrad, 1855), A. annulatum (Shumard, 1860), A. conlini sp. nov., Allocrioceras sp., Sciponoceras gracile (Shumard, 1860), Worthoceras vermiculus (Shumard, 1860), and Yezoites delicatulus (Warren, 1930). Dimorphism is recognized in Placenticeras, Metoicoceras, Nannometoicoceras gen. nov., Allocrioceras, Sciponoceras, Worthoceras, and Yezoites. Nannometoicoceras gen. novo is interpreted as a dwarf offshoot of Metoicoceras.

The Turonian ammonite faunas of the upper part ofthe Eagle Ford Group from the Texas/Oklahoma border 400 km (250 miles) south to Austin comprises twenty-eight species referred to eighteen genera of which one genus and seven species are new. The fauna consists of: Puzosia (Puzosia) serratocarinata Kennedy and Cobban, 1988a, Parapuzosia (Austiniceras) seali Clark, 1960, Tragodesmoceras sp., Watinoceras reesidei Warren, 1930, W. coloradoense (Henderson, 1908), Romaniceras (Romaniceras) mexicanum Jones, 1938, incertae sedis, Spathites (Spathites) puercoensis (Herrick and Johnson, 1900a), Mammites sp., Collignoniceras woollgari (Man tell, 1822) regulare (Haas, 1946), Prionocyclus hyatti (Stanton, 1894), P. macombi Meek, 1876b, P. bosquensis sp. · nov., P. wyomingensis Meek, 1876a, Prionocyclites mite gen. et sp. nov., Hoplitoides sandovalensis Cobban and Hook, 1980a, Coilopoceras springeri Hyatt, 1903, C. inflatum Cob ban and Hook, 1980a, Metaptychoceras crassum sp. nov., M. annulatum sp. nov., Metaptychoceras sp. A., Metaptychoceras sp. B., Baculites yokoyamai Tokunaga and Shimizu, 1926, Scaphites (Scaphites) carlilensis Morrow, 1935, S. (S.) larvaeformis Meek and Hayden, 1859, S. (Pteroscaphites) inaffectus Crick, 1979, Worthoceras minor sp. nov. , and Y. cf. delicatulus (Warren, 1920).

The faunas can be placed in the standard zonal scheme for the southern US Western Interior recognized by Cobban (1 984a). In the Dallas area there is a marked unconformity just below the base of the Arcadia Park Formation, with Upper Cenomanian Sciponoceras gracile Zone Britton Formation overlain by a thin 2.5 m sequence of Pseudaspidocerasflexuosum and Vascoceras (Greenhornoceras) birchbyi Zone age that also belongs to the Britton. There is no evidence for the lower Collignoniceras woollgari woollgari Subzone of the C. woollgari Zone, but the basal Kamp Ranch Limestone Member of the Arcadia Park and, at some localities at least, the Britton shales below belong to the regulare Subzone. Above, the bulk of the Arcadia Park Formation is Prionocyclus hyatti Zone, the top just extending into the P. macombi Zone and overlain unconformably by Coniacian Austin Chalk. To the north of Dallas, the top of the Eagle Ford extends to the P. wyomingensis Zone and perhaps higher. In the Waco area, the upper part of the Lake Waco Formation is P. hyatti Zone, the succeeding South Bosque Formation spanning the upper P. hyatti Zone to the Scaphites whitfieldi Zone, overlain unconformably by Coniacian Austin Chalk. In the Austin area, the Eagle Ford Condensed Zone yields a P. hyatti Zone fauna and rests unconformably on C. woollgari Zone sediments referred to as South Bosque Formation in 'recent publications.