Correlation of Pecan Gap Chalk in Texas

Abstract

This paper presents the surface mapping of the Pecan Gap chalk from White Cliffs, Arkansas, across northeast Texas to the type locality of the Pecan Gap chalk in Delta County, then southward through Farmersville in Collin County, Rockwall in Rockwall County, Marlin and Lott in Falls County, Rogers in Bell County, Taylor in Williamson County, and into the Anacacho limestone as it occurs west of San Antonio. The Pecan Gap chalk is divided into three paleontological zones, Flabellammina compressa zone (basal), Diploschiza cretacea zone (middle zone), and Bolivina incrassata zone (upper) by means of the microfauna. These faunal zones reveal the fact that the variation in the width of outcrop of the chalk in northeast Texas is due to an unconformity at the base of the chalk ac oss Hunt, Delta, and western Red River counties and to a progressive overlap from Arkansas to Hunt County, where the upper Taylor clays rest unconformably on progressively older beds toward the west. Due to this overlap cutting out the upper chalk zone and the unconformity of the lower zone, the width of outcrop is very narrow in Delta County and western Red River County. The chalk exposed at the type locality at Pecan Gap, Delta County, is equivalent to the lower 50 feet of the chalk at White Cliffs, Arkansas. A subsurface section is given in which the Pecan Gap chalk is shown as a continuous chalk deposited east of the area where on the surface it is apparently missing due to some possible structural features, probably faulting.