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Department of Geology and Geophysics, University of Wisconsin, Madison, 53706 WI, USA
Variability in lithofacies and depositional environments of the Upper Ordovician mixed carbonate-siliciclastic Glenwood and Platteville Formations in Wisconsin are studied and analysed within a sequence framework. Two depositional sequences are identified in these strata. Sequence boundaries correspond to identified and interpreted exposure surfaces with or without angular discordance. These surfaces mark an abrupt upward increase in shale, and correspond to widespread discontinuity surfaces interpreted to represent time-stratigraphic breaks. Sequences show a base with either sandstones or shales grading upward into carbonate lithologies devoid of terrigenous clastic deposits. Cycles are the smallest stratigraphic packages of regional significance (tens to hundreds of kilometres) and correspond to interpreted upward-shallowing successions. Cycle attributes change from sequence to sequence.
Depositional environments are interpreted similarly to many ancient carbonate ramps. Sequence S1 contains diverse facies indicating deposition in wide palaeowater depth ranges from tidal-flat to deep subtidal outer ramp environments. Rapid lateral facies change with irregular spatial distribution observed in sequence S1 is interpreted to be related to the high ramp gradient and irregular antecedent topography during deposition of sequence S1. Facies succession in sequence S2 shows upward changes from storm-dominated mid-ramp to low-energy inner ramp environments without intervening high-energy shallow-water shoal belts. This differs from most storm-dominated carbonate ramp facies models in the geological record, in which storm bed rich mid-ramp deposits show transition to inner ramp successions that contain shoreface barrier sediments such as ooid-skeletal grainstone and boundstone. This model is interpreted as a consequence of infrequency of significant wave activity, in which incoming oceanic waves lost their energy very gradually over a wide area. Without inherited topography, the broad epeiric sea modified circulation and the overall result is a decrease in energy landward. A low wave-energy, microtidal, storm-dominated ramp model is postulated for such successions.
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