ABSTRACT

The geometry and orientation of sixteen stacked sands in a 300-350 ft-thick interval (100 m) in the Wilcox 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 to form attractive stratigraphic targets.

Depositional strike averages N42°E for the seven shoreline-attached sands and N39°E 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 interval and the shoreline-detached "Gasper sand" near the top extend another 30-50 miles (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 the Wilcox Group persists over distances of at least 60-125 miles (100-200 km) along depositional strike in this area.