TOWARD A LATE ORDOVICIAN (CINCINNATIAN) SEA-LEVEL CURVE

Sequence stratigraphy provides a range of geological approaches to identifying changes in relative sea level. One powerful approach is to use detailed facies analyses to establish lateral and vertical environmental changes. These regional studies can be linked with biostratigraphic and other chronostratigraphic data to identify potential eustatic changes. We have used this methodology in studying Upper Ordovician (Cincinnatian, late Caradocian-Ashgillian) carbonate units along the eastern (Appalachian Basin), southern (Oklahoma-Texas-New Mexico), and western (Great Basin) margins of Laurentia. These widely separated areas reveal a similar pattern of relative sea-level change. Cincinnatian strata consist of seven complete stratigraphic sequences although some sequences are missing in some areas. For example, the lowest sequence is either missing or contained within the underlying Eureka Quartzite in the Great Basin, and uppermost sequence (approximately equivalent to the Hirnantian glacial interval) is absent, or highly condensed in most areas or Laurentia. The lowstand to early transgressive deposits of an eighth sequence that continues into the lowest Silurian (Llandoverian) occur in the uppermost N. persculptus biozone. This pattern is similar to sequence interpretations of equivalent sections around the Cincinnati Arch and in Baltica. We attribute the similarities in sequence patterns to eustatic sea-level changes. Despite the similarity in timing of these sequences, there are some important differences. The deepest water facies occur at different times: in the Edenian (Sequence 1) in the Appalachians, in the Maysvillian (Sequence 3) in west Texas, and at the base of sequence 6 (Richmondian) in the Great Basin. We attribute the difference in timing of the apparent maximum transgression (maximum flooding surface) to continental-scale tectonic effects.