Early Guadalupian (Permian) Bank Margin Erosion Surfaces, Guadalupe Mountains, Texas: ABSTRACT

Abstract

Two basin-sloping erosion surfaces occur in the early Guadalupian carbonate rocks on the western Guadalupe Mountains escarpment. Their longitudinal profile resembles a slump scar. Each truncates (with 20 to 40° dips) 70 to 100 m of flat-lying bank-top strata, and then flattens basinward. The younger surface, previously unreported and virtually inaccessible because of sheer cliffs, sharply truncates about 70 m of upper Grayburg shelf strata at its headwall. It flattens basinward and appears to more gradually truncate the uppermost part of the Getaway (also Grayburg) bank. Initiation of the characteristic high-angle foreslope deposits of the Goat Seep and Capitan reef began at this 70-m-high headwall. Early Goat Seep foresets, contrasting with later Goat Seep and Cap tan, have little rock equivalent in a gentler-dipping toe-of-slope section. The older erosion surface is the regional unconformity at the base of the Brushy Canyon Sandstone. At its headwall (the Brushy Canyon pinch-out) about 100 m of Cutoff and Victorio Peak Formations are truncated abruptly. The two Guadalupian erosion surfaces somewhat resemble the closely associated late Leonardian basin-sloping (5 to 10°) surface that truncated 200 + m of Victorio Peak bank in pre-Cutoff time. We believe all formed in a submarine environment. Conceivably they are half-channels with their south or southwestern side eroded or kilometers away. We believe they were formed by shelfward retreat of the depositional bank margin. The erosion agent and mechanism are enigmatic. We believe the Victorio Peak and probably the Grayburg were rock when eroded, but we are uncertain regarding pre-erosion lithification of the Cutoff Formation. End_of_Article - Last_Page 768------------