Biostratigraphy of Albian-Cenomanian Boundary in Texas and Implications for Middle Cretaceous Stratigraphy Elsewhere in North America: ABSTRACT

Abstract

On the basis of comparison of ammonite zonations in Texas and western Europe, the Albian-Cenomanian boundary in Texas is placed in the lower part of the Grayson Formation of the Washita Group. This position contrasts with the Comanchean-Gulfian Series boundary, normally placed at the top of the Washita Group. Comparison of polynomorphs from the Grayson and associated strata with those of other North American middle Cretaceous units reveals inconsistencies in the designations of Albian ages in other areas. Spore-pollen assemblages from the subsurface Dantzler Formation in Mississippi and Louisiana (previously considered Albian) include age-diagnostic taxa characteristic of the Maness and lower Woodbine formations (Cenomanian) in Texas. The Albian-Cenomanian boundary therefore occurs at a major unconformity between the Dantzler and underlying Fredericksburg Group, rather than at the unconformable top of the Dantzler, below Tuscaloosa Group sandstones. Palynology also indicates correlation of the type Dakota Formation of Nebraska and Iowa to the Cenomanian Maness-Woodbine in Texas, overlying the major middle Cretaceous unconformity. In northern Arizona, the Dakota Sandstone contains a Cenomanian assemblage similar to that of the type Dakota, but in Wyoming and Colorado, units traditionally referred to the Dakota Group (e.g., Muddy formation) are clearly Albian in age, and lie belowmore » the middle Cretaceous unconformity. These correlations shed new light on the sedimentary and stratigraphic relationships of unconformities and lithologic units in the middle Cretaceous of central and western North America.« less