LARGE-SCALE INFLECTIONS IN SLOPE ANGLE BELOW THE SHELF BREAK: A FIRST ORDER CONTROL ON THE STRATIGRAPHIC ARCHITECTURE OF CARBONATE SLOPES: CUTOFF FORMATION, GUADALUPE MOUNTAINS NATIONAL PARK, WEST TEXAS, USA

摘要

A large-scale inflection (LSI) in the slope profile may develop as a carbonate platform is drowned and abandoned basinward of the active shelf break. The impact of LSIs on the sediment dispersal patterns and stratigraphic architecture of carbonate slopes remains understudied. Outcrops of the Lower-middle Permian Cutoff Formation in the Guadalupe Mountains National Park provide a good view of a mixed carbonate and siliciclastic slope system modified by an LSI during the accumulation of a 2-4 My composite sequence (Permian composite sequence 9, PCS9). Processes of channelization, bypass, and slope failure associated with the LSI contribute to the development of basin-restricted carbonate, shale, and sandstone strata which demonstrate apparent onlap to the relict platform. Correlation of high-frequency sequences between the slope and shelf-equivalent strata demonstrates that channels incising the relict platform LSI served as conduits for confined gravity flows and depocenters for organic-rich shale throughout the transgression and much of the ensuing highstand of PCS9. In the latest highstand regression, an increase in the production of carbonate mud by the active shelf system contributed to the complete filling of channels across the LSI and accumulation of carbonate mud-dominated mass-transport deposits basinward of the LSI. A critical review of previous studies reveals that LSIs are common in the geologic record, across multiple basins and time periods. LSIs that developed due to drowning of carbonate platforms demonstrate dimensions and stratal patterns comparable to the relict Leonardian platform. LSI development in the identified examples occurred in association with 1) the transgressive stages of 10-100 My (second order) sequences, 2) ocean anoxic events, or 3) times of concomitant sea-level rise and ocean anoxic events.