FOUR UNCONFORMITIES IN THE SOUTHERN APPALACHIAN ORDOVICIAN-MISSISSIPPIAN SUCCESSION

The Ordovician-Mississippian succession in the southern Appalachian thrust belt includes four regional unconformities: below Middle Ordovician, below Lower Silurian, below Lower Devonian, and below Upper Devonian-Lower Mississippian. Unconformity-bounded stratigraphic units pinch out southeastward across the thrust belt, and on the southeast, Lower Mississippian strata locally rest on Lower Ordovician passive-margin carbonate.

The unconformity below Middle Ordovician corresponds to the craton-wide unconformity between the Sauk and Tippecanoe Sloss sequences. Above the unconformity, synorogenic (Taconic foreland basin) Middle Ordovician black shale grades northwestward to limestone in the Taconic foreland.

The unconformity below Lower Silurian and a Silurian hematitic clastic facies with interbedded carbonates represent the distal Taconic foreland. The clastic facies pinches out southeastward between two bounding unconformities and grades northwestward into carbonate facies.

The unconformity below Lower Devonian truncates the Silurian clastic facies and oversteps the southeastward pinch-out of the Silurian strata onto Middle-Upper Ordovician rocks. A Lower-Middle Devonian sandstone and chert succession is irregularly distributed between two bounding unconformities and includes internal unconformities. The unconformity at the top corresponds approximately to the top of the Tippecanoe Sloss sequence. The thin, shallow-marine succession is coeval with the time between the Taconic and Acadian orogenies.

The unconformity below Upper Devonian-Lower Mississippian oversteps the older unconformities onto Lower Devonian, Silurian, Middle-Upper Ordovician, and Lower Ordovician strata. Thin Upper Devonian-Lower Mississippian shale (condensed section) defines the distal Acadian foreland, and overlying Lower Mississippian carbonate records post-Acadian transgression. In a broad onlap relationship, the Lower Mississippian carbonate oversteps the pinch-outs of the shale units and locally rests unconformably on Lower Ordovician carbonate. Alleghanian and Ouachita Mississippian-Pennsylvanian synorogenic clastic wedges prograde over the Mississippian carbonate and span the time of the craton-wide unconformity between the Tippecanoe and Kaskaskia Sloss sequences.