New perspectives on deep-water sandstones: Implications

The emerging new concepts of sandy mass-transport deposits (SMTD) and bottom-current reworked sands (BCRS) have made a big impact on conventional turbidite concepts. Sediment failures near the shelf edge are the common cause of gravity-driven downslope processes. Mass-transport processes, which include slide, slump, and debris flow, exhibit elastic and plastic behaviors due to high sediment concentration (25-100% by volume). Turbidity currents which represent viscous fluid behavior with low sediment concentration (1-23% by volume) are not mass-transport processes. Four common bottom currents are thermohaline, wind-driven, deep tidal, and baroclinic types. A distinctive attribute of BCRS is their traction structures. However, depositional aspects of baroclinic currents associated with internal waves and tides are poorly understood. Seismic facies and geometries are unreliable for distinguishing types of SMTDs and BCRS in the ancient record. The only reliable method of distinguishing a specific depositional facies is by detailed bed-by-bed examination of sedimentological features in core and outcrop. Short-term events that represent only a matter of hours or days (e.g., earthquakes, tsunamis, tropical cyclones, etc.) are more important in triggering sediment failures than periods of sea-level lowstands that represent thousands of years.