DEPTH-DEPENDENT SEDIMENTATION AND THE FLEXURAL EDGE EFFECT IN EPEIRIC SEAS - MEASURING WATER DEPTH RELATIVE TO THE LITHOSPHERES FLEXURAL WAVELENGTH

In sediments deposited within about 300 km of shore, downlap is an expected consequence of the lithospheric flexure associated with the edge of sediment and water loads at an epeiric sea''s shoreline. For loose carbonate sediments in a typical, slowly transgressing, epeiric sea, the dependence of sediment accumulation rate dS/dt on water depth H tends to magnify the otherwise subtle downlap, making it more practical to determine the dimensions of the water load from the dimensions of the preserved sediment load. Synthetic stratigraphies for a steadily transgressing sea in which dS/dt.varies.H (the form of depth-dependence previously demonstrated for the eastern North American Middle Ordovician sea) reveal downlap of a certain form and scale as the signature of near parity between sediment and water loads in the nearshore zone. The K-bentonite-outlined internal geometry of Middle Ordovician strata that lap onto the Wisconsin Arch shows this signature. Comparison of the paleontologically [fossil assemblage] quantified relative bathymetric histories of two stratigraphic sections in this sequence with synthetic histories further indicates that this geometry was the product of an ongoing flexural process. The asymptotic water depth offshore of the Wisconsin Arch is calculated at very roughly 2 m, consistent with stratigraphic and sedimentologic evidence for a depth near wave base in this and other epeiric seas.