Abstract

The Cutoff Formation (middle Permian) of the western escarpment of the Guadalupe Mountains preserves a record of alternating erosion and deposition over a drowned shelf margin. The underlying carbonate bank (Victorio Peak Formation-Bone Spring Limestone) was abruptly terminated by erosion of the shelf edge, which formed a two-mile-wide scarp with a relief of 900-1000 ft. The fine-grained Cutoff strata mantled this unconformity. Along this scarp, post-Cutoff erosion locally removed Cutoff strata so that it occurs today in three discontinuous areas: shelf, shelf margin, and basin. Within this section, five lithostratigraphic units, consisting of alternating intervals of lime mudstone and shale, can be traced from the shelf edge to the basin. Recognition of these five units appears to resolve the uncertainty of correlating the Cutoff Formation into the basin area. Within these units, variations in the sedimentary structures, color, and fauna of the lime mudstones record anoxic basin conditions.

The upper and lower contacts of the Cutoff Formation are unconformities, and an intraformational unconformity separates upper and lower units of the Cutoff Formation. For all three unconformity surfaces, maximum truncation of underlying beds is along the shelf margin. Steep, spoon-shaped “half-channels” are enigmatic features along the unconformities that truncate at least 300 ft of strata along the shelf margin. Oriented parallel to the shelf edge, much of the margin relief is formed by these surfaces. Within the Cutoff Formation are numerous, less persistent surfaces ranging from broad (180-300 ft wide), flat-sided scours to narrow (less than 100 ft wide), steep-sided channels. The primary channel fills are intraclastic rudstone, megabreccia, and lutites. Clast lithologies indicate the source was both underlying units, and lithified Cutoff strata. The depositional geometries of the lutite beds, and the interrelationship of channel fillings and erosion surfaces, indicate that a model of a density stratified basin with both bottom currents and interflows can be applied to deposition of Cutoff strata.

Previous faunal collections from the Cutoff Formation are largely allochthonous, reworked material. Fusulinids collected during this study from the upper Cutoff Formation are of Guadalupian age. The intraformational unconformity within the formation appears to be the first easily recognized, chronostratigraphic horizon below the reported Guadalupian fusulinids.