The lower part of the Mary Lee coal zone records deposition in a tidally-influenced coastal zone during the Early Pennsylvanian (Westphalian A). This sequence consists of: (1) a quartzose sandstone body; (2) coal seams and paleosols; and 3) thinly interlaminated sandstone and mudstone units.
The quartzose sandstone body exhibits sedimentary structures indicative of deposition within a tidally-influenced, nearshore environment. Trough cross-bedding, with mudstone and plant nannodetritus drapes, reactivation surfaces, and rippled surfaces indicates fluctuating flow velocities and directions.
The interlaminated sandstone and mudstone shows striking rhythmicity in lamination thicknesses with bundles of thicker sandstone laminae corresponding to spring tides and thinner laminae to neap tides. These laminations preserve daily (diurnal and semi-diurnal), fortnightly, and yearly cyclicity in deposition. Trace fossils indicate a diverse, active fauna on the tidal flats.
Coal seams and paleosols in the sequence preserve evidence of deposition in stacked paludal environments. Autochthonous and hypoautochthonous fossil plants associated with these units indicate that vegetation in these forested swamps was lycophyte-pteridosperm dominated. Coal and inter-coal thickness was controlled by the paleotopography of the underlying quartzose sandstone body.
The interpreted depositional environments in the lower part of the Mary Lee coal zone represent a regressive sequence. This genetic stratigraphic package is bounded by transgressive surfaces below the nearshore sandstone and above the coal zone. The point of maximum regression is located at the aerially extensive Blue Creek coal. The points of maximum transgression are located at condensed sections within shelf mudstones below and above the coal zone. The recognition of marginal marine deposits within the coal zone is essential to correct delineation of the paleogeography, and evaluation of eustatic, tectonic and progradational controls on facies distribution.
See: Bryant, Degges, and Liu for related M.Sc. Theses