Pearsall (Aptian Cretaceous) Subsurface to Outcrop Sequence Stratigraphy, Central Texas: ABSTRACT

Abstract

ABSTRACT Bexar (Upper Pearsall) Shale is a widespread subsurface body of marine clay mudstone; in south-central Texas it disappears updip along a line through central Medina and Bexar Counties and the northwestern edges of Guadalupe, Caldwell, and Bastrop Counties. Bexar Shale overlies calichified Cow Creek Limestone and underlies Hensel Dolomite; in outcrop Hensel Dolomite rests on calichified Cow Creek. The Cow Creek and other mid-Pearsall limestones are local high-stand deposits; the basal Bexar is a transgressive bed and the bulk of the unit a shoaling-upward ramp deposit. Hensel silty dolomite (laminated to churned, with glauconite, collophane, beekite, and milky quartz geodes) and overlying Lower Glen Rose grainstones (containing widespread subarkose sand grains) are a high-stand lagoon/bar sequence that overlaid Bexar Shale and spread 25 km inland from the Bexar pinchout. The Hensel/basal Glen Rose shoaling sequence is the earliest of three in the Lower Glen Rose. Type Hensel Sandstone around Lake Travis was deposited (from local flash floods) after 35 m of marine Bexar shale and 35-50 m of Glen Rose carbonates farther south (an ammonite zone later). Deep red clay between calichified Cow Creek and Hensel Sandstone around Lake Travis may be remnant Bexar Shale or may be local and unrelated. Actual eustatic/tectonic/sediment-supply history does not fit a simple sequence-stratigraphic pattern.