EXTENT AND BATHYMETRY OF NORTH AMERICAN PLATFORM SEAS IN THE EARLY SILURIAN

The North American craton is 25% covered by Lower Silurian (Llandovery) rocks based strictly upon the geographic area of surviving marine strata preserved in outcrop and the subsurface. This figure does not take into account extensive post-Silurian tectonics and erosion. Isolated structural basins in which these strata are found today were not necessarily defined as corresponding topographic basins in the Silurian. Analysis of interbasin structural arches and outliers provides important clues as to the actual extent of Early Silurian seas. Information on the age of tectonic origin for 15 prominent arches is reviewed.' The placement and significance of 11 key outliers is also discussed. Evidence of this kind indicates Early Silurian platform seas were among the most extensive of the Paleozoic. Probably more than 65% of the North American craton was then flooded. Nothing on so vast a scale serves as a recent guide to the past. The Pliocene-Holocene Bahamian Banks do provide, however, worthy models for the role of bathymetry and water circulation as related to the changeover from laterally monotonous blanket deposits to a more complex mosaic of facies. The original bathymetry of Early Silurian environments may be determined, correlated, and mapped on the basis of widespread carbonate cycles linked to sea level fluctuations. The range of fully integrated facies patterns includes (1) coral-stromatoporoid/stromatolite/ paleosol cycles (Williston Basin), (2) pentamerid brachiopod/coralstromatoporoid/stromatolite cycles (Michigan Basin), and (3) stricklandiid brachiopod/pentamerid brachiopod/coralstromatoporoid cycles ( East Iowa Basin). With shallow bathymetries of 0-30 m the Williston class tends to represent mosaic facies, while with bathymetries of 0-60 m and 10-90 m the deeper-water Michigan and Iowa classes typically involve blanket deposits. Bathymetry of other basin deposits generally fits one of these three classes. Maximum and minimum stands in sea level during Late Llandovery time are represented by bathymetric maps.