Abstract

The medial Silurian siliciclastic succession in central Pennsylvania records two hierarchically superimposed cycles of sea-level fluctuations. The larger of these cycles controlled lithofacies architecture at the magnitude of formations and members. Five alternations of transgression and regression, with an average cycle period of about 2.5 m.y., produced wedges of coarser, shallow-water facies pointing basinward (northwest) and wedges of deeper water, basin-center mudrock facies extending marginward (southeast). Superimposed on these are smaller scale, shallowing-upward cycles, typically 1-3 m thick, that occur in three different, contemporaneous facies: basin-margin tidal flats, mid-shelf offshore bars, and below wave base in the storm-influenced transition to the basin center. Average recurrence interval for such smaller cycles was about 100 ka; their period was regular and was caused by sea-level fluctuations of about 1-3 m.

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