EVIDENCE OF DEPOSITION FROM INDIVIDUAL TIDES AND OF TIDAL CYCLES FROM THE FRANCIS CREEK SHALE (HOST ROCK TO THE MAZON CREEK BIOTA), WESTPHALIAN D (PENNSYLVANIAN), NORTHEASTERN ILLINOIS

A silt-clayrock sequence in the Francis Creek Shale (Pennsylvanian, Westphalian D) of northeastern Illinois is characterized by (1) the presence of numerous clayrock-paired bands; and (2) cyclic variations in thickness of silt between and within these clayrock pairs. The sequence is interpreted to result from tidal influence on inner estuarine sediments; the clayrocks representing deposition from suspension at times of tidal slack, the interbedded silts and fine sands representing deposition from bottom traction during ebb and flood tidal flow. The cyclic variations in thickness is interpreted to represent lunar neap-spring forcing of tidal range in a dominantly diurnal tidal system. The association of tidal characteristics provides a means whereby the rate of sedimentation can be quantified at about 1.0 m/yr of compacted sediment during times of active deposition. Such a high rate of sedimentation would account for the rapid burial and preservation of soft-bodied organisms as indicated by the associated Mazon Creek Biota. Similar rates in similar settings (i.e., tidal estuaries) have recently been reported. Although the depositional setting of many Westphalian sequences was favorable to the preservation of tidal features, few examples have been recorded. It is suggested that evidence of Westphalian tides may have been misinterpreted or overlooked in field studies elsewhere. © 1990.