BIOSTRATIGRAPHIC STUDIES IN THE COMANCHE (CRETACEOUS) SERIES OF NORTHERN MEXICO AND TEXAS

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Abstract
The biostratigraphic relationships of the Aurora limestone in the Sierra de Tlahualilo of southwestern Coahuila to the type Aurora near Ojinaga, Chihuahua and to the type Lower Cretaceous (Comanche) series of northern Texas are discussed. Stratigraphic sections of the Fredericksburg and Washita groups in the Fort Worth-Weatherford area of north Texas are redescribed. The following stratigraphic nomenclatural revisions are proposed: (1) the division of the Goodland formation (Fredericksburg group) into two members of which the lower is designated the Marys Creek marl member (new name) and the upper the Benbrook limestone member (new name); and (2) the use of the name “Denison formation” to include the units previously referred to in the Fort Worth-Weatherford area as the Denton, Weno, Pawpaw, Main Street, and Grayson formations and here considered members. Based on the author’s collections, faunal lists of the larger invertebrate fossils for each formation and member are given and the useful zonal guide fossils are indicated. Twelve new species of pelecypods from the area are described. A fauna of 45 species of larger invertebrates from the upper member of the Aurora limestone of the Sierra de Tlahualilo is described. Of this fauna: 27 species are identified and 5 are compared with species known from the Texas Comanchean; 2 are identified with species previously known from the Mexican Cretaceous; 6 are described as new species; and 6 are given generic assignments only. On the basis of the stratigraphic ranges of the Texan and Mexican species with which the Aurora forms are identified and the ranges of the Texan and Mexican species allied to the new species from the Aurora, it is concluded that the upper member of the Aurora limestone in the Sierra de Tlahualilo should be correlated with the Washita group of Texas. The lower part of the member is correlated with the Fort Worth limestone, and the upper part of the member is correlated with the lowermost part of the Grayson marl. The base of the Aurora limestone is tentatively correlated with the Walnut marl. This correlation indicates a late Albian to possibly an early Cenomanian age for the upper member of the Aurora limestone in the area studied. Elsewhere in northern Mexico the formation has been determined as ranging from early to middle Albian in age and only at the type area has the upper part of the limestone been considered as young as latest Albian or possibly early Cenomanian. On the basis of these correlations it appears that the environment responsible for the origin of the Aurora limestone did not exist everywhere in northern Mexico at the same time nor did it persist for an equally long period at every place in the region. The Aurora limestone is interpreted as a formation of epineritic origin deposited far from any source of terrigenous clastics as indicated by the fauna and by the almost complete absence of terrigenous material within this unit. This origin differs from that of the Fredericksburg and Washita groups in north Texas where the strata of these groups are principally epineritic and infraneritic in origin as shown by the faunas and the abundance of fine-grained terrigenous clastic rocks throughout the section.

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