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How did Pennsylvanian sea-level changes create coal cyclothems?

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Carboniferous paleogeography — Carboniferous Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Carboniferous Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

Pennsylvanian coal cyclothems were created by repetitive, oscillatory sea-level changes that caused alternating marine and nonmarine conditions across delta plains. These cycles are characterized by a sequence of sedimentary rocks that reflect the advance (transgression) and retreat (regression) of shallow seas.12

During regressive phases, delta out-building allowed terrestrial environments to develop on delta plains, where plant debris accumulated and eventually carbonized into coal seams. Marine transgressions then flooded these areas, depositing shale and limestone containing marine fossils over the coal.12

While the exact mechanisms remain a subject of study, several theories explain these oscillations:
* Climatic changes: Periodic melting and growth of polar ice caps in Gondwana likely drove global sea-level fluctuations.23
* Tectonic activity: Jerky subsidence of sedimentary basins or the growth of oceanic ridges may have triggered marine incursions.2
* Sedimentological processes: Delta out-building may have been interrupted by shifts in the river's path to the sea; the abandoned, subsiding delta-lobe was then flooded by the sea, initiating a new cycle.2