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S. George Pemberton, University of Alberta (Canada)
James A. MacEachern, Simon Fraser University (Canada)
Kerrie, L. Bann, Ichnofacies Analysis Inc. (Canada)
Murray K. Gingras, University of Alberta (Canada)
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Shallow-water, marine environments comprise a multitude of sedimentological regimes, most of which are subject to marked fluctuations in physical and chemical parameters. Detailed investigations of numerous coastal marine zones have illustrated the value of using biogenic sedimentary structures to delineate such ecological parameters as oxygenation, salinity, temperature, depositional rates, and energy levels. Coastal morphodynamics are subject to a wide range of variables depending upon whether the coast is prograding or under transgression. Barrier islands, for example, are generally a function of transgression whereas river deltas are more apt to develop along prograding coastlines. Such shallow-marine environments constitute major hydrocarbon reservoirs.
Shorefaces consist of seaward-sloping sediment wedges extending from the low- tide mark, generally to fairweather (minimum) wave base, commonly corresponding to approximately 10-25 m of water depth. The shoreface setting is dominated by wave energy and, as a result of decreasing wave interaction with the substrate in a seaward direction, shows a pronounced basinward fining. Although the large-scale context may vary, the specific subenvironments of the shoreface are not significantly different whether they occur as part of a strandplain, barrier island or wave/storm-dominated delta.
The shoreface and adjacent environments can easily be grouped into three depositional complexes; the offshore, the lower-middle shoreface, and the upper shoreface-foreshore. Each depositional complex corresponds to discrete environmental parameters, reflected by the prevailing physical processes and biogenic activity. The presence of zones of Macaronichnus segregatis is highly significant to the interpretation of sandstone successions as upper shoreface and foreshore deposits. In the archetypal Seilacherian ichnofacies scheme the nearshore zone should be dominated by the dwelling burrows of suspension feeders, which characterize the Skolithos Ichnofacies. The Macaronichnus assemblage characterizes higher energy shoreface environments where domiciles cannot be established because of shifting substrates and where filter feeding may be precluded. These shorefaces are instead dominated by horizontal feeding burrows that are formed interstratally.
Ichnological analyses of numerous deltaic successions likewise highlight recurring patterns that record physico-chemical stresses imposed upon infaunal organisms through the dynamic interplay of river discharge, tidal flux, wave energy, and storm events. Ancient examples persistently show reductions in bioturbation intensity and impoverishment in ichnological diversity compared to their non-deltaic counterparts. Deltaic ichnological suites locally display size reductions of ichnogenera and a paucity of suspension-feeding ethologies due to heightened water turbidity. Resulting ichnological suites are overwhelmingly dominated by deposit-feeding behaviours, even in sandy facies.
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