Abstract

A sequence stratigraphic framework is provided for the Lower Cretaceous (Albian) Fredericksburg Group in central and north Texas based upon a dataset of forty-two outcrop measured sections and eleven water well resistivity logs. The Fredericksburg Group is partitioned into at least 5 regionally-correlative parasequence sets. This indicates the potential for predictable reservoir compartmentalization within subsurface equivalents.

The Fredericksburg Group accumulated as a carbonate shelf comprised of six shallow marine facies. Facies are distributed within a cyclic hierarchy of parasequences and parasequence sets that stack into a lower-order composite sequence. The lower composite sequence boundary, coincident with the Trinity Group - Fredericksburg Group contact, is overlain by lowstand nonmarine to marginal marine siliciclastics of the Paluxy and Antlers Formations within the northern portion of the study area. Lowstand siliciclastics are truncated by a transgressive ravinement surface, and are overlain by retrogradational parasequence sets of the transgressive systems tract (parasequence sets 1-2: Bull Creek, Bee Cave, and Cedar Park Members of the Walnut Formation). The succeeding highstand systems tract is initiated at the composite sequence maximum flooding surface (lower portion of the Keys Valley Member of the Walnut Formation), and is comprised of progradational parasequence sets (parasequence sets 3-5: middle and upper Keys Valley Member, Comanche Peak Limestone, Edwards Limestone). The highstand systems tract has the highest proportion of potentially reservoir-prone margin shoal and rudistid buildup facies. Reservoir-prone facies are often interbedded with deeper-water, lower-quality reservoir facies across parasequence set boundaries, and are spatially offset in a basinward direction due to progradational stacking. Transgressive and highstand parasequence sets onlap north and west across the study area, and reflect accommodation loss toward the continental interior.