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

David L. Amsbury

Bexar (Upper Pearsall) Shale is a widespread subsurface body of marine clay mudstone; in south-central Texas it disappears 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 (with widespread subarkose sand grains) are a high-stand lagoon/bar sequence that spread 25 km inland from the Bexar pinchout. The shoaling sequence is the earliest of three in the Lower Glen Rose.

The 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.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas