ABSTRACT: A High-Resolution Local Sea level Curve From Detailed Mapping of the Wales Sand Interval (Wilcox Group, Paleogene), LaSalle and McMullen Counties, South Texas

PHORNPRAPHA, PRASERTCHAI, JOHN A. BREYER, and ARTHUR B. BUSBEY, Department of Geology, Texas Christian University, Fort Worth, TX

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 ft).

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 mi 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 n rtheast.

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.

AAPG Search and Discovery Article #91014©1992 AAPG GCAGS and GC-SEPM Meeting, Jackson, Mississippi, October 21-23, 1992 (2009)