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Wave-Influenced Deltaic Sandstone Bodies and Offshore Deposits in the Viking Formation, Hamilton Lake area, South-Central Alberta, Canada
Subtle indications of deltaic influence can be overlooked in coarsening-upward units that resemble shoreface successions such as those found in the Cretaceous Viking Formation at Hamilton Lake (HL). Particular facies within HL strata contain physical and biogenic indicators of riverine discharge, and are interpreted as deltaic in origin. This study focuses on distinguishing between these deltaic deposits and strata reflecting normal-marine depositional conditions and relating facies within the stratigraphic framework. Based on detailed facies analysis, Viking deposits include: delta front, prodelta, upper offshore, lower offshore, shelf, slump and transgressive lag deposits.
The principal group of facies, Facies Association 1 (FA1), is interpreted as deltaic in nature, which is indicated by evidence of: high sedimentation rates, salinity fluctuations, riverine discharge, hypopycnal and hyperpycnal flows, and moderately to subtly stressed ichnological suites. Trace fossil suites indicate suppression of suspension-feeding activities despite the presence of hummocky cross-stratified units that resemble shoreface deposition. Filter-feeding organisms are typically abundant in shoreface and wave-dominated deltaic successions. In the case of HL strata, hindrance of dwellings of inferred suspension-feeding animals in FA1 is attributed to increased water turbidity and suspended sediment associated with deltaic outflow. The presence of a restricted suite of structures of inferred suspension-feeders is uncharacteristic of river-dominated deltaic units, which are primarily reworked by deposit feeders. Accordingly, sedimentological evidence in combination with the proportion of dwellings of inferred suspension-feeders signifies moderate wave mitigation of environmental stresses induced by riverine discharge indicative of a wave-influenced deltaic system.
In contrast to FA1, muddy sandstones to sandy mudstones of Facies Association 2 contain abundant and diverse suites of trace fossils interpreted to reflect deposition in the distal lower shoreface to upper offshore. Facies Association 3 is predominantly comprised of sandy mudstones with varying degrees of bioturbation indicative of lower offshore, shelfal, slump and transgressive lag deposits. The depositional history at HL includes periods of progradation alternating with transgressive flooding, subaerial exposure, stillstand progradation and wave ravinement. Four major bounding discontinuities and two major transgressive flooding surfaces separate units reflecting predominantly deltaic deposition, strictly offshore deposition, and mixed offshore and deltaic deposition. Variation in the deltaic and normal-marine signature of the deposits suggests changes in source area(s) over time, shifting shore-parallel deltaic influence, or fluctuations in riverine influx.
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