Revisited Play Concept for Distally-Steepened Carbonate Ramps: The Relevance of Sediment Density Flows in the Stratigraphic Record

Since the pioneering facies model proposed in the late 60's by W. F. Bishop for the extensive Jurassic Smackover Formation, crossing the southern United States, the definition and classification of carbonate ramps has been the topic of a vast and rich scientific debate. Detailed outcrop research coupled recently with analysis of seismic data led to models based on depositional profile (shelf vs. ramp), facies distribution, and to genetic approaches based on responses to interacting sea-level changes, changes in hydrodynamics and ecology, etc. Recent detailed work on Lower Pleistocene, distally-steepened heterozoan carbonate-ramp deposits on Favignana Island (southern Italy) indicates the occurrence of large volumes (e.g. up to 60 m-thick deposits over ca. 10 km2) of coarse-grained skeletal remains transported down-ramp by bimodal energy flows, representing either water flows generating sub-critical subaqueous dune deposits or high-energy, upper-flow-regime density flows generating thick strata consisting of antidune, chute-and-pool and cyclic-step sedimentary structures. The latter, forming individual beds up to 6 m in thickness, are made of carbonate grains with highly irregular shape and high intragranular porosity, the latter related to frequent interlocking of skeletal grains. The mostly bioclastic debris consists of fragmented coralline red algae, bryozoans, molluscs, echinoderms and benthic foraminifera re-sedimented on ramp slope and toe position in an intrashelf basin. The occurrence of similar facies associations lacking distinct seismic signatures has been identified at several locations in the Mediterranean region and may be recognized in other comparable, distally steepened heterozoan or rhodalgal carbonate ramps. Possible examples include the Upper Oligocene-Lower Miocene carbonates of Perla (Venezuela) and similar successions in the Caribbean, Iran, Philippines, and China. The addition of this newly identified facies associations to play concepts for distally steepened carbonate ramps in intrashelf basins is relevant from an exploration and development perspective in order to assess fully the potential of reservoirs extent and predict fluid-flow in carbonate reservoirs