MacEachern, James A.1, Janok P. Bhattacharya2
(1) Simon Fraser University, Burnaby, BC
(2) University of Texas at Dallas, Richardson, TX
ABSTRACT: Ichnology of Deltas: Organism Responses to the Dynamic Interplay of Rivers, Waves, Storms and Tides
Analysis of deltaic successions highlights recurring ichnological patterns that reflect physico-chemical stresses imposed by interaction of various delta front processes. Deltaic successions show bioturbation intensity reductions and diversity impoverishment compared to non-deltaic shorelines. This reflects increased sedimentation rates that impede infaunal colonization. River-derived stresses are profound: salinity changes, hypopycnal-induced water turbidity, distributary flood discharges with phytodetrital pulses, hyperpycnal-induced sediment gravity flows, and fluid mud deposition serve to accentuate the depauperate nature of ichnological suites. Resulting assemblages are dominated by opportunistic structures of trophic generalists. High turbidity precludes most suspension-feeding behavior, suppressing the development of the Skolithos ichnofacies.
Wave energy generally buffers these effects by dispersing fines offshore and encouraging the mixing of waters of contrasting salinity. High mud concentrations near the delta front dampen wave energy, however, limiting its effectiveness in remediating the benthic ecosystem. In wave-dominated settings, strong alongshore drift acts to extend river-derived stresses considerable distances down-drift from distributary mouths. Consequently, asymmetric deltas favor markedly different ichnological expressions on either side of river mouths. Storm energy may be effective in dispersing mud and mixing waters, but also results in erosion and episodic deposition of sediment on the bed. Concomitant precipitation induces river floods, returning river-derived stresses to the delta front. Tidal energy may trap mud plumes against the delta front, and favors marked changes in energy and salinity at the bed on a number of time scales. Resulting ichnological characteristics record the dynamic interplay and relative importance of these processes, both temporally and spatially on delta systems.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.