Sequence architecture and sea-level dynamics of Upper Permian shelfal facies, Guadalupe Mountains, southern New Mexico

ABSTRACT A 3.2 km profile of mixed carbonates and clastics is superbly exposed in Slaughter Canyon in the Guadalupe Mountains of southern New Mexico and provides a seismic-scale panorama of stratal architecture and cyclic facies relationships across the Upper Permian shelf margin of the Delaware basin. The upper Seven Rivers and Yates Formations exposed in the mapped profile behind the Capitan reef margin are characterized by extreme seaward thickening and strongly progradational architecture. Meter-scale cycles exhibit systematic lithofacies changes in a seaward direction, spanning pisolitic shelf-crest to skeletal outer-shelf facies tracts. Retrogradational, aggradational, and progradational stacking patterns of meter-scale cycles in the Yates define four complete high-frequency sequences (Y1-Y4) and the lower half of a fifth (Y5) that continues into the overlying Tansill Formation. Individual Yates high-frequency sequences are fundamentally macroscale versions of Yates meter-scale cycles, on the basis of comparable internal arrangements of lithofacies and their seaward-thickening geometry. The evolution of the Yates-Capitan shelf margin from Y1 through Y4 is expressed by systematic trends in downdip thickness changes, lateral extent of facies tracts relative to the Capitan reef margin, aspect ratios of facies tracts, progradation:aggradation ratios and derived offlap angles, and progradation rates. These trends reflect the position of individual high-frequency sequences in the larger scale Yates-Tansill composite sequence. Long-term changes in aspect ratios record the progressive seaward migration and lateral expansion of the shelf-crest facies tract from Y1 through Y4 and the reciprocal seaward-stepping architecture and lateral contraction of the outer-shelf facies tract. The four Yates high-frequency sequences in Slaughter Canyon are characterized by an average offlap angle of 3.6°, whereas the corresponding average growth angle for the time-equivalent Capitan reef is 5.2°. The higher reef growth angle reflects the greater amount of accommodation available near the outer shelf-to-reef transition as well as limitations to seaward growth imposed by the steepness of the reef front. The Yates shelf prograded at an average rate of 2.1 m/k.y., whereas the time-equivalent Capitan reef prograded at a rate of 1.7 m/k.y. The strongly progradational architecture of the Yates shelf resulted in a progressive decrease in the depth of the Capitan reef through time from a maximum of ~ 65 m during early Yates time to near sea level during the latest stages of the Yates platform. The abrupt seaward expansion within each Yates high-frequency sequence occurs directly above the terminal reef margin of the preceding high-frequency sequence. The underlying reef likely acted as a foundation to localize the basinward shift in deposition associated with sea-level fall along high-frequency sequence boundaries, contributing to the seaward thickening and progradational "step-out" of the outer shelf. Architectural changes controlled by the interaction between relative sea level and antecedent depositional topography may be a fundamental characteristic of many progradational shelf margins.