THE UPPER CRETACEOUS BIGFOOT EROSIONAL UNCONFORMITY ENABLES DETERMINATION OF THE EXACT POSITION OF THE MID MAASTRICHTIAN SHORELINE IN SOUTH CENTRAL TEXAS

The top of the Taylor and base of the Navarro in south central Texas have identical fine grained clastic lithologies and the nature of the contact between these Upper Cretaceous rock Groups has been long debated. Initially thought to be conformable, by the late 1920’s proposals were put forth, based on a gap in the fossil bivalve fauna, for a disconformable contact present in surface exposures in central Texas. Discovery in 1949 in Frio County of Bigfoot oilfield in upper Taylor sands trapped under an erosional truncation provided confirmation that the Taylor-Navarro contact in southwest Texas is an unconformity which was named the Bigfoot Unconformity and is dated as Mid Maastrichtian.

Correlation of thousands of well logs adjacent to the Taylor-Navarro outcrop contact enabled construction of twenty-eight cross sections which 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 stratigraphic and structural dip, spaced 10 miles apart, contain well logs at 1-4 mile intervals and collectively they cover parts of 18 counties. The unconformity is clearly visible on each of the well log cross sections and the erosional contact can be defined from the up-dip outcrop to the down-dip position where the truncation ceases and the Navarro strata above and Taylor strata below the contact become conformable. The southern terminus of erosional truncation along each cross section indicates the maximum sea level regression and connecting locations on adjacent cross sections defines the exact position of the Upper Cretaceous shoreline at a moment in time during the Mid Maastrichtian.

A map of south-central Texas was prepared that indicates the location of the Taylor-Navarro outcrop belt, the 28 cross sections and the Mid Maastrichtian shoreline. The shoreline is located from 12-30 miles basinward from the current outcrop of the contact and has a sinuous shape. The ancient shoreline highlights the San Marcos Arch and the Uvalde Uplift demonstrating that both these features were structurally positive during the upper Cretaceous. 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.