Estimated times required for 25 modern river systems to form shelf-edge deltas range from 8.5 Kyrs to 116.5 Kyrs, being dependent on magnitude of fluvial sediment supply, delta width, shelf volume and shelf transport rates. These values indicate that formation of shelf-edge deltas and consequent sediment bypass into deep-marine systems may well be significant during third-order highstands of relative sea-level, dependent to a large extent on magnitude of sediment supply to the shelf. Since sediment supply is controlled largely by climate, source area tectonics and bedrock lithology, these factors must be as significant to timing of turbidite deposition as relative sea-level history. Variables such as rates of shelf transport will influence delta progradation time while details of shelf physiography may decrease the time during a highstand before shelf bypass occurs. Based on proportions of sand in modern river loads, highstand shelf bypass could lead to significant deep-marine sand deposition. Consequently interpretation of ancient sand-rich deep-marine strata as limited to times of lowstand is not always appropriate. Consideration of more palaeogeographic and palaeoclimatic data is required to determine the likelihood of highstand shelf bypass. Examples are given from Lower and Middle Jurassic turbidite strata of the Neuquen Basin, Argentina. Classically these strata are interpreted as relative sea-level lowstand deposits, but considering Jurassic palaeogeography and likely sediment budgets based on climatic and drainage basin data, many turbidite strata could equally be interpreted as trangsressive and/or highstand deposits.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah