Eustatic and autocyclic influences on deposition of the lower Pennsylvanian Mary Lee coal zone, Warrior Basin, Alabama

Abstract

Deposition of marine strata associated with the Lower Pennsylvanian Mary Lee coal zone (Pottsville Formation) was controlled by the allocyclic mechanism of eustatic sea-level change. The vertical stacking of marine, marginal marine, and terrestrial facies indicates that this interval was deposited during a single transgression-regression-transgression cycle. Ravinement surfaces, and associated autochthonous macro-invertebrate and trace-fossil assemblages record the landward retreat of the shoreline and abrupt deepening due to rapid rise in sea-level. These horizons are overlain by terrigenous clastic facies, either tidal sand bodies or shelf mudstones, which indicate shoaling due to progradation during the subsequent filling of the generated accommodation space. Although marine influence, including the effects of tidal currents, continued during the more terrestrial phases of deposition, autocyclic factors, including the control of paleotopography by differential compaction of buried peat bodies, were also important in determining facies distribution.