LATE ORDOVICIAN SEA-LEVEL FLUCTUATIONS IN THE CENTRAL APPALACHIAN BASIN

One of John Dennison’s quests was to find stratigraphic time-lines, and one of his trademarks was his ability to recognize widespread correlatable sea-level events in the stratigraphic record. He was the first to recognize the relationship between the Queenston Delta and the Late Ordovician glaciation, attributing the redbeds to both eustatic and tectonic causes.

Detailed measurement and analyses of bedding patterns in the Oswego and Juniata Formations have yielded several orders of stratigraphic cyclicity within the Queenston redbed sequence. The Juniata is characteristically cyclical, containing meter-scale ‘parasequences’ of tidal to fluvial origin which are readily observable in outcrop. Fischer Plot-type analyses of smaller sandstone-shale couplets indicate several scales of cyclicity, some of which are higher order, and some lower order than the ‘parasequences’. Similar analyses of strata penetrated by deep wells suggest that these patterns are recognizable throughout the Ordovician. In all, at least five orders of cyclicity are recognized at the Milankovitch scale and lower.

The recognition of glacio-eustatic patterns in the Paleozoic is handicapped by the lack of a continuous deep marine record. In cratonic stratigraphic successions like the Queenston, deposition is discontinuous, and it is a challenge to delineate cyclicity that is truly eustatic. However, in the deeper parts of the Appalachian Basin, deposition is probably more continuous, and therefore has the potential of providing a more complete record. The simple stratigraphic techniques of correlation and facies analysis may provide a useful tool for interpreting eustasy.