--> Abstract: Opposed Passive- and Active-Margin Slope and Basin-Floor Depositional Systems, Ozona Sandstone, Val Verde Basin, Southwest Texas, by H. S. Hamlin; #90937 (1998)

Datapages, Inc.Print this page

Abstract: Opposed Passive- and Active-Margin Slope and Basin-Floor Depositional Systems, Ozona Sandstone, Val Verde Basin, Southwest Texas

HAMLIN, H. SCOTT, Bureau of Economic Geology, The University of Texas at Austin

Contemporaneous Ozona Sandstone slope and basin systems display a range of depositional styles and facies, which can be understood as responses to tectonic setting and sediment supply In the Early Permian Val Verde Basin, active and passive continental margins faced each other across a basin floor that was less than 40 km wide. The northward-converging Ouachita orogenic belt formed the southern basin margin, whereas a precollision cratonic margin persisted in the north. Sandstones derived from both margins are intimately interbedded along the narrow basin floor, forming a major low-permeability gas-producing trend. Well logs, cores, and regional 2-D seismic formed the data base for this study. Although the stratigraphy is complex, dense well control allowed high-resolution mapping of genetic stratigraphic units a few tens of meters thick. Depositional elements were traced across broad areas and related to regional stratal architecture.

The Ozona Sandstone interval is 300 m thick and displays three distinct depositional styles: (1) A line source slope apron onlaps the northern passive margin. Thick-bedded turbidites and debrites fill a network of bifurcating channels, which diminish in size and disappear into mud-dominated sheet lobes. (2) Poorly channelized lobes composed of thick- to thin-bedded turbidites offlap the southern active margin. (3) Axial channel systems on the basin floor parallel the opposing margins. Tributary channels feed into the axial channels from both sides.

Ouachita convergence resulted in northward migration of three important structural zones: thrust-related uplift, foredeep subsidence, and forebulge uplift. Thrust-related uplift in the south supplied large volumes of sediment for offlapping sand-rich turbidite lobes. The migrating foredeep progressively depressed the no them passive margin, creating space for sustained onlap. Forebulge uplift on the northern craton created a constant though distant sediment source for the onlapping strata. The axial channel system is a product of asymmetric convergence and basin-floor tilting. Contemporaneous Ozona slope and basin systems experienced the same eustatic sea-level history but exhibit a variety of depositional styles and facies. Hemipelagic drapes bound depositional units but are rarely regionally extensive. Shifting depositional loci resulted in sand deposition that was continuous in time but not in space Depositional patterns persisted through time but responded in space to tectonic setting and sediment supply.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah