PERMIAN SHELF-EDGE DEPOSITION IN THE MARFA BASIN AT SHAFTER, TEXAS

The Marfa Basin at Shafter, Texas represents the southwestern portion of the Permian Basin. The Middle Permian limestones of the Ross Mine and Mina Grande formations in the Shafter area are contemporaneous with the slope-to-basin strata of the Delaware basin exposed in the Guadalupe, Apache, and Glass mountains. The Capitan reef is not present at the surface or in the subsurface in the region. Examination of selected cores from several hundred recovered by Silver Standard Mining Company indicates that the debris flows and turbidites of the Ross Mine and Mina Grande formations prograded from northeast to southwest. The shelf edge beds are mostly redeposited fossiliferous limestones containing shallow-facies faunas of bryozoans, brachiopods, fusulinids, calcareous algae and echinoderms. Individual beds of the Mina Grande thin to the southwest, where they overlie and grade into black shales (Bell Canyon equivalent).

Limestones from the cores have yielded the conodonts Jinogondolella nankingensis (Roadian) and Mesogondolella idahoensis (Kungurian) which suggests a basal Guadalupian age assignment. The overlying Mina Grande Formation contains the Capitanian fusulind Polydiexodina. Late Permian, Castile-equivalent rocks do not occur in the Shafter area, although Tessey-equivalent collapse breccias appear to be present. The subsurface samples are currently being studied to determine the precise relation of the Shafter rocks to Marfa Basin stratigraphy and a possible oceanic outlet of the Marfa Basin to the southwest during the Middle and Late Permian.