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Mixed Carbonate and Siliciclastic Deepwater Systems Modified by Large-Scale Inflections in Slope Angle Below the Shelf Break


When a carbonate platform is drowned and abandoned basinward of an active shelf system, it may retain hundreds of meters of vertical relief along the relict shelf-to-basin profile and create a large-scale inflection (LSI) in the dip-angle of the slope. Additionally, these features may be spatially extensive and ‘long-lived:’ continuing for hundreds of kilometers along strike and remaining unburied along the slope profile for 10s of millions of years. While numerous LSI-modified slopes have been identified in the geologic record, very little has been done to understand how they influence the dispersal and accumulation patterns of younger sediments. This work analyzes an LSI-modified mixed carbonate and siliciclastic slope system represented in Permian outcrops of the Guadalupe Mountains and other examples identified in previous studies (n=9) in order to evaluate common stratal patterns, architectural elements, deposit types, and sequence stratigraphic settings. The studied examples of LSI-modified slope systems record: 1) sustained submarine erosion and channelization of the drowned platform margin, 2) preferential accumulation of sediment gravity flow deposits downdip of the drowned margin, and 3) temporal changes in styles of sediment dispersal in response to migration of the active shelf system. Carbonate deposits identified along and downdip of LSIs represent a combination of turbidites, breccia- and conglomerate-rich debris flows, and olistotromal blocks (mass-transport deposits) which accumulate near the slope profile of the relict platform. Conversely, siliciclastic deposits consist mainly of turbidites and hemipelagites which accumulate preferentially on the sub-horizontal basin floor. The LSI identified in the Permian outcrop is represented by a relict Leonardian platform margin that was drowned during the nexus a craton-wide, if not global, sea level rise (2nd Order). The majority of the studied examples (n=6) represent relict carbonate platforms were drowned during a concomitant 2nd order transgression and ocean anoxic event (OAE). The other examples represent former platforms that drowned during the onset of 1) an OAE without any associated long-term maximum transgressive stage or 2) the peak of a 2nd order transgression without any associated OAE.