Abstract

The Latest Leonardian-Early Guadalupian global transgression that marks the main second-order eustatic rise within the Lower San Andres Formation pushed siliciclastic sediments well landward of the Permian Basin margin, marking a prolonged phase of pure carbonate shelf-to-basin deposition. This carbonate phase is followed by a punctuated basinward migration of inner shelf siliciclastics through upper San Andres, Grayburg, Queen, Seven Rivers units, culminating in the depo sition of the 6 mixed clastic-carbonate Yates Formation high-frequency sequences defined by Tinker (1998). Though much discussion has centered around the mechanisms for transportation and deposition of the Yates siliciclastics, the model that has dominated thinking over the last four decades has been that the sheet-sand stratal architecture, absence of channel geometries, and lack of obvious bedforms, is indirect evidence of a reciprocal LST eolian /TST marine ravinement/HST stabilization-carbonate deposition model. Borer and Harris further added to this model that the Yates shelf was “born-to-bypass” and that a series of high-frequency-cycle-scale beats was the main mode of siliciclastic bypass into the Delaware Basin rather than a longer-wave-length sequence-scale pulsated delivery of clastics.

This paper adds two pieces of information to the collective knowledge of Yates stratigraphy. First, a distinct increase in exposure occurs in the Yates 1-6 sequences. The top-Yates 5 HFS, or Hairpin member of the shelf, in particular illustrates a major basinward shift in abnormal subaerial exposure at least onto the reef flat, most likely forcing a large volume of siliciclastics basinward to form the main productive Delaware Mt. Group sand, the Ramsey sandstone. The Yates 6 HFS or Triplet, records continued lowering of baselevel. Detailed mapping of the Triplet shows it to be a shelf-margin wedge deposit with a coarse basal conglomerate and rapidly basinward-expanding geometry, perched on the precursor Hairpin margin. A minimum estimate for the eustatic fall at the end of Hairpin deposition is 30 m.

Support for the major eustatic fall marked by this late Guadalupian Yates 5-6 phase of deposition is provided by 3D seismic data on the NW shelf. A dramatic incised valley system with a basal incision of approximately 200 ft. cuts into a classic fluvial meanderbelt system. There seems little doubt that along at least portions of the Delaware Basin, fluvial point-source bypass of the shelf was alive and well. Close re-examination of both outcrop and subsurface data will likely lead to renewed appreciation for the depositional and erosional processes associated with Yates clastics and their Bell Canyon counterparts.