ABSTRACT

Eight sands, B₁ through B₈, from oldest to youngest, comprise the Wales sand interval in LaSalle and McMullen Counties, South Texas. B₁ and B₂, the lower and upper Wales of industry are the most widespread sands and make up much of the approximately 150 feet of the Wales interval. B₈, which caps the sequence and is probably equivalent to the Bartosch sand along strike to the northeast, has the most limited distribution and is the thinnest of the eight sands (<10).

The lower Wales (B₁) strandplain extends basinward across the shelf almost to the growth-fault zone. The upper Wales (B₂) also extends down paleoslope almost to the shelf edge. Following deposition of the upper and lower Wales, the shoreline stepped back some 15 to 20 miles to the northwest, and shelf muds accumulated across McMullen County. Stacked shoreline sands (B₃-B₅) in LaSalle County are higher on paleoslope than B₁ and B₂ sands. Sand carried from the northeast by shelf currents was deposited in McMullen County while the B₅ shoreline stood in LaSalle County. The shoreline continued to retreat to the northwest and deposition of shelf muds took place over most of the two county area except in northern McMullen County where sands (B₆-B₈) preserve the advance of shelf sands from the northeast.

Changes in local relative sea level in the Wales sand interval can be determined by mapping the individual sands, but not from the stacking patterns seen on individual well logs. The sea level curve shows an overall rise in relative sea level, with successively younger parasequence sets deposited farther landward in a retrogradational pattern, punctuated by periods of relative stillstand during which the parasequences within each set stack in an aggradational pattern.