ABSTRACT

The Eocene Yegua Formation, one of a series of Tertiary clastic wedges, prograded across the Houston Diapir province of southeastern Texas and thickens significantly basinward of the platform margin. Sandstones of the expanded Yegua represent neritic, distal mouth-bar, and shelf environments downdip from the strandline. Structural movements (growth faulting and salt diapirism) were contemporaneous with deposition and exerted a strong influence on patterns of sand dispersal, whereas sedimentation mobilized the underlying salts and triggered faulting over the unstable shelf edge.

Major depocenters became established along strike-parallel growth faults, and the source for these sediments was dip-oriented feeder systems from the northwestern shelf. The faults, which trend northeastward through Liberty and Hardin Counties, served to localize thicker sand sections on their downthrown side. Salt diapirism created a major depocenter over a large withdrawal syncline in south-central Liberty County, assumed to be the source for most piercement features in the study area. Thick sand sections also accumulated in other low areas due to salt withdrawal or sediment loading and to damming behind growing domes. In contrast, salt domes and ridges were generally positive topographic highs that channeled sand through interdomal conduits. Diapirism has thus compartmentalized the depocenters.

Multiple Yegua sandstones now occur in several favorable structural positions (especially along growth faults and over the salt-withdrawal syncline), and these areas of thick sand accumulation constitute primary targets for petroleum exploration.