In contrast to prevailing concepts, Pennsylvanian (Pottsville) sedimentary rocks of Tennessee, except for coals and adjacent strata, are considered herein to have been deposited in marine water of considerable depth.
Blanket sandstones, restricted to the lower third of the column, were deposited by southwestward-moving currents in a shallow (i.e. tens of feet deep) arm of the sea, which was greatly restricted by Appalachia to the southeast and the emergent Cincinnati arch to the northwest. Digitate sandstones occur in the upper beds and extend westward partly across the coal field. These digitate sandstones were deposited on the east flank of the Interior Sea where sediment-laden water moved down the slope into deeper water; each tongue may have matched a stream mouth which had its headwaters in Appalachia. At this time the Cincinnati arch was buried. Shale blankets were deposited in deep, quiet water. Widespread coal zones mark periods of abrupt withdrawal of the sea.
On the assumption that the bulk of the strata are marine, the writers have applied the concept of four-phase cyclothem deposition and divided the lower third of the column into five cyclothems. Phase 1 is commonly represented by coal beneath blanket sandstones. Phase 2 is represented by a widespread marine sandstone blanket deposited in shallow water; local coals within these sandstones indicate local filling up of the shallow sea by rapid sedimentation adjacent to Appalachia. Phase 3 is represented by the widespread coal zone in normal position above the blanket sandstones. Phase 4 is represented by a widespread shale blanket deposited in a deep sea. Stratigraphic cross sections and paleogeographic block diagrams illustrate each phase.