Albian Stratigraphy of the San Marcos Platform, Texas: Why the Person Formation Correlates with Upper Fredericksburg Group not Washita Group

Abstract

Uppermost Aptian-Albian-lower Cenomanian strata of the Texas Comanchean Series represent three long-term transgressive-regressive cycles: the Trinity, Fredericksburg, and Washita groups that are composed mainly of marine limestone and shale. Each group is composed of shorter term deepening-shallowing cycles. On the Central Texas San Marcos Platform and its downdip equivalents, subsurface strata above the Trinity Group are grouped into the Edwards and Washita groups. Correlations in the 1960s were based on wireline well logs and inferred biostratigraphy that correlated the upper Edwards Group Person Formation with the lower parts of the Washita Group. This correlation crosses a widespread subaerial unconformity that separates the Washita from the underlying Fredericksburg in the northern East Texas Basin and in the Fort Stockton Basin. New biostratigraphic and sedimento-logical data show that the Person actually correlates with the upper part of the Fredericksburg in the Gulf Coast.