Upper Albian and Lower Cenomanian ammonites from the Main Street Limestone, Grayson Marl and Del Rio Clay in northeast Texas

The ammonite faunas of the Main Street Limestone, Grayson Marl and Del Rio Clay form the basis for a revised upper Upper Albian to lower Lower Cenomanian zonal sequence in northeast Texas. A Mortoniceras (Subschloenbachia) rostratum Zone, previously known to be well-represented in the Denton Clay, Weno Limestone, and Pawpaw Shale, extends into the lower Main Street Limestone. A M. (S.) perinflatum Zone is also recognised in the Main Street on the basis of the occurrence of the index species, although precise stratigraphic relations of the scattered records of the two index species are not known. The succeeding Mariella (Wintonia) brazoensis Zone is an Acme Zone, yielding a monospecific assemblage of the zonal index that is abundant in the Main Street Limestone, although the index species ranges both lower and higher in the sequence. The base of the Cenomanian, in ammonite terms, is drawn at the base of the succeeding Graysonites adkinsi Zone, on the basis of the first occurrence of Mantelliceratinae with inner and outer ventrolateral tubercles. The index species of the succeeding G. wacoensis Zone is the senior synonym of Graysonites lozoi Young, 1958, and G. wooldridgei Young, 1958. The Lower Cenomanian sequence is generally truncated by the erosive base of the terrigenous-clastic Woodbine Formation in the study area, but relics of Buda Limestone, and of Grayson Marl with ammonites of the upper Lower Cenomanian Budaiceras hyatti Zone are present locally in Denton and Grayson Counties. A total of 30 ammonite species are described, of which Moremanoceras flexuosum, Stoliczkaia (Stoliczkaia) conlini, and Neophlycticeras (Neophlycticeras) fascicostatum are new. © 2005 Elsevier Ltd. All rights reserved