SEQUENCE ARCHITECTURE AND SEA-LEVEL DYNAMICS OF UPPER PERMIAN SHELFALFACIES, GUADALUPE MOUNTAINS, SOUTHERN NEW-MEXICO

A 3.2 km profile of mixed carbonates and clastics is superbly exposed in Slaughter Canyon in the Guadalupe Mountains of southern Kew Mexico and provides a seismic-scale panorama of stratal architecture and cycl ic facies relationships across the Upper Permian shelf margin of the D elaware 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 fac ies tracts, Retrogradational, aggradational, and progradational stacki ng patterns of meter-scale cycles in the Yates define four complete hi gh-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 me ter-scale cycles, on the basis of comparable internal arrangements of Lithofacies and their seaward-thickening geometry. The evolution of th e Yates-Capitan shelf margin from Y1 through Y4 is expressed by system atic trends in downdip thickness changes, lateral extent of facies tra cts relative to the Capitan reef margin, aspect ratios of facies tract s, progradation:aggradation ratios and derived offlap angles, and prog radation rates, These trends reflect the position of individual high-f requency sequences in the larger scale Yates-Tansill composite sequenc e, Long-term changes in aspect ratios record the progressive seaward m igration and lateral expansion of the shelf-crest facies tract from Y1 through Y4 and the reciprocal seaward-stepping architecture and later al contraction of the outer-shelf facies tract, The four Yates high fr equency sequences in Slaughter Canyon are characterized by an average offlap angle of 3.6 degrees, whereas the corresponding average growth angle for the time-equivalent Capitan reef is 5.2 degrees, The higher reef growth angle reflects the greater amount of accommodation availab le near the outer shelf to-reef transition as well as limitations to s eaward growth imposed by the steepness of the reef front, The Yates sh elf prograded at an average rate of 2.1 m/k.y., whereas the time-equiv alent Capitan reef prograded at a rate of 1.7 m/k.y. The strongly prog radational architecture of the Yates shelf resulted in a progressive d ecrease in the depth of the Capitan reef through time from a maximum o f similar to 65 m during early Yates time to near sea level during the latest stages of the Yates platform. The abrupt seaward expansion wit hin each Yates high-frequency sequence occurs directly above the termi nal reef margin of the preceding high-frequency sequence, The underlyi ng reef likely acted as a foundation to localize the basinward shift i n deposition associated with sea-level fall along high-frequency seque nce boundaries, contributing to the seaward thickening and progradatio nal ''step-out'' of the outer shelf, Architectural changes controlled by the interaction between relative sea level and antecedent depositio nal topography may be a fundamental characteristic of many progradatio nal shelf margins.