Sedimentology and Ichnology in Mixed-Energy Coastal Systems
Open-coast tidal flats (synonymous with ‘mixed-energy coasts’, ‘intertidal shoreface’, and ‘wave-dominated tidal flats’) are marginal marine environments, where both tides and waves play an important role in nearshore sedimentation. Despite their widespread occurrence in the world coasts, most research has focused on the simplified end-member settings, where sedimentation is controlled by a single major process. On-going research in the western Korean coast and eastern Vancouver Island, Canada, has shown that despite the large tidal range the open-coast tidal flats are wave-dominated due to lack of a significant tidal prism. As a result, the tidal flats consist of wave-generated sedimentary structures with a minor component of tidal facies. Stratigraphic reconstruction from vibracores also shows that the succession coarsens upward, from bioturbated mud to hummocky cross-stratified sand, resulting in strong similarities with wave-dominated shorefaces.
To aid in the recognition of ancient open-coast tidal flats, we have conducted integrated sedimentologic and ichnologic study, because organisms have limited tolerance for physiologically stressful environments. The study results have shown that new criteria for the identification of open-coast tidal flats are (1) wave-generated tidal rhythmites that record the shallowing and deepening of wave base due to fluctuating water depth; (2) composite storm deposits that reflect tidally modulated storm sedimentation; (3) small wavelength of hummocky cross-stratification (HCS) due to limited surface wave size; (4) overall a higher abundance of animal traces compared to bathymetrically 'equivalent' upper shoreface and foreshore deposits; (5) the distributions of ichnofacies are attributable to seasonal processes rather than water depth; (6) ichnological trends are strongly influenced by local factors, such as the locations of swash bars and channels; and, (7) like other tidal flat settings, deposit-feeding is as common as suspension feeding, but the distributions and abundances of traces are sporadic and variable. This study provides the tools to better interpret ancient mixed-energy coastal deposits. Such deposits are represented in many hydrocarbon reservoirs. Future research needs to focus on such transitional environments in order to provide a more diverse suite of models for use in the interpretation of ancient deposits.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009