Abstract

Stratigraphic contact descriptions between the Upper Cretaceous Taylor Group and the overlying Navarro Group were debated for decades due to their identical finegrained clastic lithology and surface exposure discontinuity along the Balcones Fault Zone. Initially thought to be conformable, later proposals based on a gap in the bivalve fossil record observed in outcrop determined that the contact is disconformable. The 1949 discovery of Bigfoot Oil Field in upper Taylor sands trapped under an erosional truncation in Frio County confirmed the unconformable nature of this contact in southwest Texas, which was later dated as middle Maastrichtian and named the Bigfoot Unconformity.

A detailed stratigraphic study of the Taylor-Navarro contact was undertaken using thousands of well logs adjacent to the outcrop belt to construct 28 stratigraphic cross-sections. These sections document the occurrence of the Bigfoot Unconformity continuously from the Rio Grande River to Milam County, a distance of 300 miles. Cross-sections are 15–40 miles long, oriented along structural dip, spaced 10 miles apart, contain well logs at one to four mile intervals, and collectively cover parts of 18 counties. The unconformity is visible on each section and the erosional contact is observed from the outcrop to the downdip truncation cessation where Navarro strata becomes conformable with underlying Taylor strata. The southern terminus of erosional truncation along each section represents maximum sea level regression and connecting these locations on adjacent sections defines the exact position of the Upper Cretaceous shoreline during middle Maastrichtian time. The ancient shoreline is located 12–30 miles basinward from the outcrop of the contact, has a sinuous shape, and highlights the San Marcos Arch and the Uvalde Uplift. No other method can determine the exact shoreline position at a moment in geologic time with the high degree of accuracy as the one employed in this study.