MIDDLE MISSISSIPPIAN (LATE OSAGEAN) GLAUCONY HORIZONS IN EAST-CENTRAL U.S.A.: A FORELAND-BASIN SEDIMENTARY RESPONSE TO LITHOPSHERIC RELAXATION DURING NEOACADIAN TECTONISM

In east-central U.S., up to five glaucony horizons are widespread in upper parts of the Lower-Middle Mississippian Borden-Grainger delta sequence. The thickest and most widespread of these horizons is the Floyds Knob Bed. The glaucony is pelletal and occurs with bioturbation and phosphorite; horizons occur in upper parts of the sequence at the transition from coarser clastics to carbonate-rich shales and mudstones.

The glaucony horizons are part of a Mississippian foreland-basin sequence, reflecting a complete flexural cycle in the final tectophase of the Acadian/Neoacadian orogeny. In early parts of the sequence, Lower Mississippian black shales reflect major loading-related subsidence. Once this loading ceased, the lithosphere responded to the static load by proximal subsidence and eastward bulge migration. All the while, erosion of the highland load filled the Appalachian Basin with the Price-Pocono-Borden-Grainger deltaic complex. The Borden and Grainger represent subaqueous, prodelta and delta-front parts of the complex in east-central U.S., and their westward progradation continued until late Osagean time, when major clastic sedimentation ceased, glaucony precipitation briefly ensued, and carbonate-rich muds terminated the sequence.

We suggest that the late Osagean events reflect eastward migration and emplacement of the relaxational bulge such that westward deltaic sedimentation was cut off or diverted, leaving the broad delta platform in east-central U.S. in a sediment-starved state. Lacking major clastic influx, organismal activity and stillstand conditions in near-shore waters allowed for brief intervals of glaucony precipitation before delta-destruction facies (carbonate-rich shales) overwhelmed the platform. Although the Floyds Knob Bed probably reflects final emplacement of the bulge, lesser glaucony horizons may represent stages of bulge development or reactivation of local structures. These glaucony horizons developed in an overall shallowing-upward sequence and do not seem to reflect deepening events. Because full development of glaucony horizons may take 25,000 years or more, the presence of sediment-starved, stillstand conditions may be most critical, and flexural/tectonic mechanisms can be one way of developing such conditions across widespread areas.