Abstract

The San Angelo Formation (lower Guadalupian) represents infilling of the Midland Basin during the generally regressive phase of the Permian seas. It is subdivided into the upper Flowerpot Member and the lower Duncan Member, each representing a different environment of deposition.

The Duncan Member was deposited by meandering fluvial processes that flowed east into the Midland Basin. This member is characterized by channel lag conglomerates, fining-upward, point bar sequences, floodplain mudstones, and abandoned channel sands and silts.

A marine transgressive limestone marks the boundary between the fluvial Duncan Member and the deltaic Flowerpot Member. The delta system was a high-constructive, fluvially dominated, lobate delta, characterized by numerous distributary channels, well developed delta front and delta plain deposits, and pronouced coastal progradation.

The subsurface displays sheet-like sand deposits that are gradational from the distal portions of the lobate delta. The San Angelo subsurface deposits correlate with the upper Flowerpot Member at the surface and result from progradation of the delta system into the slowly retreating seas of the Midland Basin.