TIME-RESTRICTED FACIES OF THE LATE ORDOVICIAN: TECTONIC AND ENVIRONMENTAL IMPLICATIONS

Analysis of Late Ordovician Chazy, Black River, and Trenton carbonate strata of eastern North America has resulted in construction of an expanded sequence stratigraphic framework of 13 depositional sequences. These sequences have been used to correlate major outcrop successions in NY/Ontario, central PA, KY/OH and elsewhere for investigating the development of the Blountian and Vermontian tectophases of the Taconic Orogeny. Sequences were constrained using biostratigraphy, chemostratigraphic analyses, as well as K-bentonite occurrences and deformed intervals (i.e. “seismites”). Each depositional sequence was constructed using sequence bounding unconformities including erosional karstified surfaces, transgressive, high-stand and falling-stage systems tracts, hardgrounds and flooding surfaces.

These Ashbyan to Mohawkian depositional sequences contain a number of unique, laterally-extensive lithofacies that repeat in multiple sequences. These repeating intervals represent time-restricted units that occurred only intermittently during deposition. These appear similar to what were previously termed “time-restricted facies” (TRF). In the Devonian, distinct, repeating, lithologic facies were thought to be produced as a result of unique environmental conditions developed only during short time intervals. This study identifies a number of different TRF that help establish correlations and assist in calibration of time-equivalent sequences. The TRF identified in this study include:1) interbedded, nodular chert to burrow-impregnated chert-rich intervals, 2) condensed, greenish-gray, glauco-chamositic quartz-rich dolomite & shale intervals, 3) restricted marine to hypersaline carbonate facies with oolitic intervals, stromatolitic boundstones, and evaporite crystal laths, 4) intervals of polymictic intra- & extra-clastic limestone beds, 5) laminated to rhythmically-bedded calcisiltite & shale facies prone to deformation, & 6) unique chemostratigraphic events.

Recognition of these different time-restricted facies suggests that depositional environments were not only influenced by sea-level, but also by changes in ocean-circulation and rates of upwelling, increased periods of runoff impacting the supply of siliciclastic sediment, as well as periods of aridity.