G. Shanmugam¹

(1) The University of Texas at Arlington, Arlington, TX

ABSTRACT: Deep-Marine Tidal Bottom Currents and Their Reworked Sands in Submarine Canyons: Implications for Sand Distribution

Up and down tidal currents have been documented using current-meter records in modern submarine canyons worldwide, ranging in depths from 46 to 4,200 m. Currents in canyons commonly attain velocities of 25 cm/sec. Recent satellite altimeter data show significant dissipation of tidal energy to oceanic depths. Core data from modern deep-water fjords show cyclic silt-mud rhythmites caused by tidal processes. Core and outcrop features indicative of reworking by tidal currents in ancient deep-water deposits are sand-mud rhythmites, double mud layers in sand, climbing ripples, mud-draped ripples, alternation of parallel and cross lamination, sigmoidal cross bedding, lenticular bedding, flaser bedding, internal erosional surfaces, and clean sand. Many of these tidal features have been recognized in hydrocarbon-bearing deep-water sandstone reservoirs worldwide. Unlike shallow-water estuarine deposits, deep-water tidal deposits are closely interbedded with deposits of sediment-gravity flows and pelagic-hemipelagic mud.

From petroleum exploration point of view, tidal bottom currents are capable of forming "clean" sands in submarine canyons and channels where tidal energy is more focused than in open basinal areas. Misinterpretation of deep-marine tidal deposits as "turbidites" has important implications for predicting sand distribution. For example, interpretation of climbing ripples as levee turbidites implies deposition outside channels, whereas interpretation of the same climbing ripples as deep-marine tidal deposits implies deposition within canyons or channels. Furthermore, deep-marine tidal deposits are likely to develop elongate bars aligned parallel to channel axis, whereas turbidites are more likely to develop lobes at channel mouths aligned perpendicular to channel axis.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado