Exploration Implications of the Geometry and Orientation of Paralic and Shelf Sands, Wilcox Group (Paleogene), South Texas

J. A. Breyer, J. B. Bellamy, and P. Phornprapha
Department of Geology, Texas Christian University, Fort Worth

The geometry and orientation of sixteen stacked sands in a 100 mthick interval in theWilcox Group in the subsurface beneath La Salle and McMullen Counties in South Texas is remarkably consistent. Two recurring sand-body geometries are present: broad, sheetlike, shoreline-attached sands with length-width ratios near one; and narrow, elongate, shoreline-detached sands with length-width ratios greater than two. The shoreline-attached sands pass landward into a continuous sand section without shale breaks and thin basinward before pinching out into shales on the shelf. The shoreline-detached sands pinch out into shales on both their landward and basinward margins. Because the regional dip is at right angles to depositional strike, the updip and landward limits of the shoreline-detached sands coincide forming attractive stratigraphic targets.

Depositional strike averages N42E for the seven shorelineattached sands and N39E for the nine shoreline-detached sands. The zone of maximum thickness of each of the shoreline-detached sands is parallel to the orientation of the sand-body on paleoslope. The zones of maximum thickness of the shoreline-attached sands in most cases are nearly perpendicular to the zero edge of the sand-body. The geometry and orientation of the shoreline-attached “Wales sand” at the base of the sequence and the shoreline-detached “Gasper sand” near the top of the sequence extend another 50-75 km along depositional strike to the northeast beyond La Salle and McMullen Counties. The pattern of sand deposition in the middle part of theWilcox Group persists over distances of at least 100-200 km along depositional strike in this area.

AAPG Search and Discovery Article #90901©2001 GCAGS, Annual Meeting, Shreveport, Louisiana