Lorraine Coates1, James A. MacEachern1
(1) Simon Fraser University, Burnaby, BC
ABSTRACT: Ichnological and Sedimentological Criteria for Differentiating Between River-Dominated Deltas, Wave-Dominated Deltas, and Shorefaces: Examples from the Cretaceous of Western Canada
Analysis of several Cretaceous intervals has led to ichnological and sedimentological criteria that aid in identifying and differentiating subenvironments of river-dominated delta, wave-dominated delta, and shoreface successions. The principal differences occur in prodelta and lower delta front deposits, and the analogous offshore and shoreface deposits of strandlines.
Prodelta mudstones of river-dominated deltas are largely unburrowed and contain low numbers of deposit-feeding and grazing structures. Massive silty mudstone beds and soft-sediment deformed intervals are common. Prodelta mudstones of wave-dominated deltas yield a diverse, though low abundance Cruziana ichnofacies. Thin, sharp-based tempestites are abundant, and display burrowed tops or are mantled by post-storm mudstones. Offshore deposits of shoreface successions contain both diverse and abundant distal and archetypal Cruziana suites, displaying a close affinity with the prodelta deposits of wave-dominated deltas. Discrimination between them is difficult in storm-dominated successions. Reduced numbers of burrows in prodeltaic mudstones may reflect high sedimentation rates.
Lower delta fronts of river-dominated successions contain structureless sandstones interbedded with synsedimentary deformed sandstones. Ichnological suites show low numbers of deposit-feeding and grazing structures, with very rare suspension-feeding structures. Wave-dominated lower delta fronts consist of stacked, erosionally amalgamated tempestites, with a low abundance but high diversity of trace fossils. Trace fossil suites are dominated by deposit-feeding structures with few elements of the Skolithos ichnofacies, defining a proximal Cruziana ichnofacies. This contrasts with shoreface deposits, which typically possess abundant deposit-feeding and suspension-feeding structures, defining a mixed Skolithos-Cruziana or archetypal Skolithos ichnofacies. Delta front deposits probably lack suspension-feeding structures due to higher water turbidity.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado