Recognition of wave-dominated, tide-influenced shoreline systems in the rock record: Variations from a microtidal shoreline model

摘要

Existing wave-dominated fades models are based on microtidal coastlines and do not adequately address wave-dominated environments influenced by significant tidal ranges. Observations from modern environments show that such systems are abundant along tide-influenced shorelines facing wide shelves and large embayments, such as much of the northern Australia coast; yet equivalent deposits have been rarely recognized from the ancient record. Geomorphological literature shows that tidal influence on wave-dominated shorelines has the effect of shifting the shoaling, breaking, and swash wave zones up and down the beach profile; when the tidal range is appreciable, sedimentation is affected significantly. Many macrotidal, wave-dominated systems (tidal range >4 m), for example, are non-barred and are characterized by poor development of dune-scale bedforms in the subtidal zone and along the beach profile. Other systems do develop cross stratification, but this occurs in the intertidal zone rather than the subtidal zone as is implied in existing wave-dominated facies models. The association of many wave-dominated, tide-influenced environments with shallow shelves also suggests that major storms may be capable of reworking sediment significant distances from the shoreline. We present an ancient example of a wave-dominated, tide-influenced, fluvial-affected system (Wtfl from the Campanian Bearpaw to Horseshoe Canyon Formation transition near Drumheller, Alberta, Canada, which has been described in closely spaced outcrop exposures and core. Wave domination in the coarsening-upward interval is unambiguous and is represented by abundance of micro-hummocky cross stratification and other storm beds in the mudstone-dominated portions, a well-defined swaley cross stratified sandstone interval, and an up to four meter thick, horizontal planar stratified interval interpreted to have been formed by swash waves. Tide influence is suggested by common double carbonaceous and mud drapes and well-developed tidal rhythmites. The anomalously thick horizontal planar interval described above is interpreted also to be related to tides that shift the zones of wave reworking up and down the beach profile. Fluvial influence is suggested by the presence of fluid mud beds in the lower portion of the succession and the abundance of carbonaceous debris through the entire interval. The boundary between the mudstone-dominated and sandstone-dominated portions of the succession is sharp and scoured, and is interpreted as the boundary between a subaqueous delta front or mudstone belt below and a wave-dominated, tide-influenced shoreline above. Observations in this study and a survey of geomorphologic literature lead to a new proposed model of facies distributions for a wave-dominated, tide-influenced system. Additional ancient case studies are needed in order to generate robust facies models capable of dealing with the full variability of such systems.