Megabreccias and Associated Carbonates of Rader Member, Capitan Reef Complex (Permian), Guadalupe Mountains, West Texas

Ellen C. Lawson

The Rader Member contains coarse allochthonous carbonate debris deposited basinward of a ^approx 500-m high angle of repose foreslope. Sheet-like megabreccias with internal channel fills up to 35 m thick are traceable 6.5 km along the reef escarpment (depositional strike) and extend 13 km southeast from the toe of slope into the Delaware basin. A prominent erosion surface at the base of the megabreccias may represent a major basin-sloping unconformity.

At maximum thickness (65 m) of exposed Rader, three lithologic units are recognized in upward succession: (1) fine-grained, bioturbated limestones interbedded with fine-grained, laminated sandstones; (2) matrix- and clast-supported megabreccias comprised of chaotically oriented limestone blocks (up to 10 m across) with a fine-grained sandstone matrix; and (3) coarse to fine-grained carbonate turbidites interbedded with sandstones. The megabreccias truncate 30 m of section within 2.5 km of the shelf edge, completely eliminating unit 1. Basinward fining and thinning result in a depositional extent of 13 km for the megabreccias. Several turbidites (< 1 m thick) of unit 3 persisted for more than 17 km across a gently sloping basin floor.

Megabreccias (unit 2) are complexly channelized, have an erosive base, and contain multiple flows. Well-cemented limestone clasts within the flows were probably derived from high on the shelf margin. The megabreccias and overlying turbidites (unit 3) are time-distinct units, as evidenced by intervening organic-rich siltstones. The erosion surface at the base of the megabreccia may have a time-correlative surface (sea level fluctuations?) on the shelf.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.