Sedimentological and Ichnological Characters of Shallow Marine Deposits
Speaker: Lee Yen-Ching Date:01/17
Abstract
Siliciclastic marginal marine and shallow marine subenvironments of deposition are controlled manly by three factors: river, wave and tide. We present an ancient example of a wave-dominated, tide-influenced, fluvial-affected system (Wtf) from the Campanian Bearpaw to Horseshoe Canyon Formation transition near Drumheller, Alberta, Canada, which has been described in closely spaced outcrop exposures 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, 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. Fluvial influence is suggested by 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.
The key evidence of direct tidal control includes uniform sediment calibres from the upper shoreface to the offshore, and little or no mud preserved in the lower shoreface. Additionally, sands in the lower shoreface and offshore tend to be intensely bioturbated. Where primary stratification is preserved, it largely comprises currentgenerated structures. Such shoreface deposits are referred to herein as “tide-influenced shorefaces”, and are expected in settings with low storm-wave input coupled with strong tidal currents. Indirect tidal influences are manifest by the lateral translation of wave zones across the shoreface profile owing to changes in water depth during the tidal cycle. This is best developed in macrotidal to megatidal settings. Indirect tidal influences are more pronounced in the upper and lower shoreface, and are recorded through the interbedding of sedimentary structures produced by shoaling waves, breakers and surf, swash–backwash, and surface runoff. The boundaries between shoreface subenvironments are correspondingly poorly defined. Using well-established sedimentological and ichnological criteria for recognizing wave-dominated (nontidal) shorefaces — wherein sediment deposition is nearly wholly controlled by fair-weather wave and stormwave processes — a conceptual model is developed for discriminating fair-weather (non-tidal) shorefaces, storm-influenced (non-tidal) shorefaces, and tidally influenced shorefaces. Five shoreface archetypes are defined: storm-affected, storm-influenced, storm-dominated, tide-influenced, and tidally modulated.
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