Subaqueous Sediment Flows And Their Deposits: Current Events

Donald R. Lowe

Sedimentologists distinguish four main end-member sediment-flow types based on sediment support mechanisms: turbidity currents, liquefied flows, grain flows, and cohesive debris flows. Only turbidity currents and cohesive debris flows deposit large volumes of sediment in the deep sea. Classically, the practical discrimination between coarse turbidites and debris flow deposits has been relatively straightforward: turbidites are grain-supported, low-detrital-mud-matrix deposits that show a Bouma sequence of current-produced structures (low-density turbidity currents) or the S or R divisions of high-density-flow deposits. Debris-flow deposits are mud-rich, commonly matrix-supported, unsorted or poorly sorted deposits. Three developments necessitate reconsideration of these s mple criteria. (1) Many thick-bedded sandstones previously interpreted as high-density turbidity-current deposits have been reinterpreted as sandy debris-flow deposits. (2) Slurry flows have been described having properties transitional between those of turbidity currents and debris flows and forming deposits with distinctive structures and textures. (3) Many thin, current-deposited sands in deep-water sequences have been reinterpreted as contourites instead of turbidites. These developments provide a stimulus to reassess and refine our understanding of sediment gravity flows and their deposits. Presently, sandy debris flows are about as well understood as fluxoturbidity currents were in the 1960s, and the concept of mud-poor, laminar, cohesive flows that can transport large volumes of s nd over low subaqueous slopes is based largely on a misinterpretation of previous studies rather than sound mechanical principles. Slurry-flow deposits are major components of many mud-rich deep-water systems and need to be studied separately from turbidites and debris-flow deposits. Contourites have been greatly overrated.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California