Facies Patterns And Cyclicity In The Tide-Dominated Silurian Clinton Formation At Schuylkill Gap, Pennsylvania

Abstract

The Lower Silurian Clinton Formation in eastern Pennsylvania represents a tectonically stable, tide-dominated, marginal-marine environment following the regional transgression associated with the ending of the Taconic Orogeny. It consists of repeated fining-upward cycles (facies sequences) which are highly variable in thickness, overall coarseness, channel variability and channel scouring ability. The cycles contain two major facies, the cross-bedded sandstone (subtidal channel) facies and the overlying interbedded/bioturbated (tidal-flat) facies. The latter facies encompasses such a wide range of lithologic conditions that it is subdivided into three subfacies: the nonburrowed interbedded (restricted-flat) subfacies, the vertically burrowed sandstone (intertidal channel) subfacies and the bioturbated red/green (levee) subfacies. The lateral migration of tidal channels across the tidal-flat environment in conjunction with regional subsidence causes the vertical accumulation of adjacent facies deposits. Such lateral migration results from the meandering of tidal channels and is very similar in process to the lateral migration of river channels. The sea which existed during Clinton time was shallow, restricted, and slightly brackish. Fossils, except for trances of burrowing organisms, are scarce. The Taconic Mountains supplied sediment to the Clinton environments, generally through a succession of river systems, longshore drift and tidal currents. Sedimentation rates and subsidence rates are approximately equal. A tidal environmental model explains the cyclicity in the Clinton Formation and applies as well to all similar clastic, semi-restricted, tidal-flat environments having a moderate tidal range and associated with a stable shoreline following a regional transgression.