Annual Sedimentation Cycles in Rhythmites of Carboniferous Tidal Channels
Stephen F. Greb, Allen W. Archer, 1998. "Annual Sedimentation Cycles in Rhythmites of Carboniferous Tidal Channels", Tidalites: Processes & Products, Clark R. Alexander, Richard A. Davis, Vernon J. Henry
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Three occurrences of rhythmites in Carboniferous tidal channels from the Appalachian and Illinois Basins were compared in order to assess the controls on the rapid accumulation of tidal sedimentation in paleoenvironments in which channelized flow and daily reworking normally preclude such preservation. In all three channels, rhythmites consist of stacked composite bedsets. Daily tidal sedimentation in these rhythmites is marked by submillimeter- to millimeter-scale laminae couplets. Bundles of 12 or fewer couplets represent fortnightly neap-spring deposition, alternating thick and thin neap-spring bundles represent perigeen and apogeen cycles, and the association of 24 or fewer neap-spring bundles in each bedset represents annual sedimentation cycles. The most complete annual bedsets are often divided into sand- and shale-dominated halves, which are inferred to represent seasonal differences in current energy during aggradation.
Interpreting the various orders of tidal cyclicity is complicated in many bedsets because of the thinness of daily laminae couplets and typical incompleteness of continuous daily sedimentation records in neap-spring bundles of the three channels studied. In many cases, bundles of laminae or ripples with foreset drapes are superficially very similar to laminae couplets. In such examples, fortnightly cycles can be misinterpreted as daily deposits. Furthermore, annual cycles can be misinterpreted as monthly deposits because 24 weekly cycles look like 24 daily deposits.
In all three channel-fill deposits, annual bedsets thin upward, through (1) a vertical decrease in the number of daily couplets per neap-spring bundle, (2) loss of neap bundles per monthly pair, and (3) loss of monthly bundle pairs in the shalier, low-energy, seasonal parts of each bedset. These vertical changes reflect a strong accommodation control, especially considering the lack of rhythmic sedimentation in lateral deposits. In each of the channels, the most complete tidal records were recorded toward the base and axes of the fills, and daily, neap-spring, monthly, seasonal, and annual cycles amalgamated upward and laterally in response to shallowing. This is significant because accommodation-controlled thinning and amalgamation causes substantial changes in the rhythmicity of the fill and results in subtle hiatal surfaces, which could lead to incorrect inferences of temporal duration spatially across the channel, even within the same bedset.