CO-OCCURRENCE OF A BURROWED FIRMGROUND AND A NEGATIVE SHIFT IN MAGNETIC SUSCEPTIBILITY: IMPLICATIONS FOR CORRELATION AND SEQUENCE STRATIGRAPHY

The Telychian (late Llandovery) age Waco Member of the Noland Formation has been recognized in the Silurian succession of eastern Kentucky for over a century. Nevertheless, correlation and sequence interpretation of this unit have been hindered by heavy dolomitization and a relative scarcity of key biostratigraphic indicators. However, a detailed reexamination of this interval has led to the identification of sub-meter scale patterns that may be useful for correlation within Kentucky and into surrounding regions.

The Waco Member is characterized by a prominent basal dolostone interval roughly a meter in thickness overlain by as much as 4 meters of clay shale containing many thin carbonate and siltstone beds. Several distinct horizons of the basal dolostone interval are traceable across several kilometers. They are, in ascending order, a 30-50 cm carbonate bed containing tabulate corals, a 5-15 cm green clay, and a 10-15 cm carbonate bed containing abundant hypichnial burrow casts. These traces, tentatively identified here as the ichnogenus Teichichnus sp., likely represent a burrowed firmground.

Four sections of the Waco member were sampled for magnetic susceptibility. Each contained a negative spike in values coincident with the green clay horizon just below the ubiquitous occurrence of Teichichnus. If these patterns have their origin in eustatic controls, it follows that similar patterns may be observed in coeval strata in the surrounding region.

The characteristics of the lower Waco member may be explained in the context of sequence stratigraphy. The appearance of the green clay shale above the massive carbonate beds likely represents rapid flooding and subsequent shutdown of the carbonate factory (i.e., a sediment starved, drowning unconformity). The sea level rise would also result in the landward migration of shorelines and a coincident drop in detrital iron influx into the basin. The burrowed firmground is interpreted as part of a condensed transgressive succession resulting from erosional exhumation of underlying muds during a minor regression.