CAMBRIAN-EARLY MISSISSIPPIAN PASSIVE MARGIN OF SOUTHERN LAURENTIA

A Cambrian to Lower Mississippian carbonate succession in northern Alabama and Mississippi represents passive-margin deposition on the Alabama promontory of Laurentia. The passive margin persisted from the time of Early Cambrian Iapetan rifting to initial Appalachian-Ouachita orogenesis in middle Mississippian. An Upper Mississippian-Lower Pennsylvanian synorogenic clastic wedge fills the Black Warrior foreland basin (BWb) in a recess within the late Paleozoic Appalachian-Ouachita thrust belt. Paleozoic rocks and structures are covered by post-orogenic Mesozoic-Cenozoic Gulf Coastal Plain strata. For the subsurface Cambrian-Lower Mississippian strata, drill holes (geophysical logs, drill cuttings, and core) provide data for stratigraphic interpretations. Stratigraphic correlations are better documented in the shallower northern BWb, where wells are more numerous; some recent wells in the southern BWb and in the subsurface Appalachian thrust belt provide new data for stratigraphic interpretation.

The Cambrian and Ordovician sequence of dolostone and limestone is difficult to subdivide. A distinct unconformity separates Lower and Middle Ordovician strata in Appalachian outcrops in Alabama but cannot be traced confidently in the subsurface. Although the base is uncertain, the Middle Ordovician clearly is a shallow-marine shelf limestone in the BWb.

The Silurian consists of limestones and mudstones in the northern BWb. These grade southeastward into partly hematitic clastic rocks along the southeastern edge of the BWb and in the exposed Appalachian thrust belt. Farther west in the subsurface in western Alabama, partly hematitic carbonates dominate the Silurian. Farther west in Mississippi, the Silurian consists of carbonates and chert.

The Devonian in the western BWb in Mississippi is a novaculitic chert that thins and pinches out northeastward in western Alabama. In the eastern BWb in Alabama, the Devonian consists of the black Chattanooga Shale, which laps onto the eastward-thinning chert and pinches out westward.

The Lower Mississippian Fort Payne Chert and Tuscumbia Limestone are chert and cherty limestones, which are thickest and coarsest in the eastern BWb. The cherty carbonates thin and grade to finer limestones to the southwest, suggesting deeper water conditions nearer the shelf edge.