The upper portion of the Mary Lee coal group of the Lower Pennsylvanian (Westphalian A) Pottsville Formation in northwest Alabama is composed of the Mary Lee coal and the Newcastle coal and associated clastic facies. Sedimentological, paleontological, and geochemical investigations of fifteen sections exposing this coal group were undertaken in order to define the changes that occur among various terrestrial facies and marine facies. A relatively thin transgressive lag bed, or ravinement bed, that varies in thickness from 13.0 to 60.0 centimeters is observed throughout the area of study.
Eight distinct lithofacies are described based upon field observations and include underclay, bituminous coal, mudstone, alternating sandstone/siltstone, sandstone, mottled mudstone, "Morris Shale" (marine shale), and channel-form sandstones. Lithofacies represent coastal and marine depositional environments. Coastal environments include paleosols (underclay), peat accumulating environments (bituminous coal), clastic swamps and overbank deposits (mudstone), tidal flat (alternating sandstone/siltstone), tidal channel (sandstone and channel-form sandstones in the southern portion of the study area). Marine environments include the ravinement bed (mottled mudstone), shallow marine shelf ("Morris Shale"), and offshore shoals (channel-form sandstone).
Geochemical analyses were conducted on samples collected from seven sections along the perimeter of the study area in order to define any significant lateral changes. The analyses conducted involved inductively coupled atomic plasma spectrography (ICAP) for seven elemental oxides (aluminum, iron, silica, calcium, potassium, magnesium, and manganese). Atomic absorption was utilized to determine sodium content. Carbonate carbon was determined by weight percent difference after hydrochloric acid treatment, and organic carbon was determined by use of a carbon analyzer on a LECO thermal conductivity carbon analyzer. Sulfur content was also determined using a LECO carbon analyzer fitted with a sulfur detector. Loss-on-ignition (LOI) percentage was based upon change in weight of each sample after a period of 30 minutes in a muffle furnace at a temperature at 1000oC.
The utilization of most major elemental analyses was inconclusive in the differentiation between terrestrial and marine lithofacies. The exception was the comparison of sulfur and organic carbon ratios in mudrocks as previously demonstrated by Berner and Raiswell (1984). Sulfur and organic carbon data were plotted on a bivariate graph along with results from a regression analysis. The regression line from the Berner and Raiswell (1984) study was superimposed onto the plot. The results in the present study were similar to those obtained by Berner and Raiswell. Rocks interpreted as marine plotted within the marine zone on the graph; rocks determined to be of terrestrial origin plotted within the terrestrial zone.
The organic carbon:sulfur ratio method of Berner and Raiswell (1984) is useful in distinguishing either a marine or terrestrial origin for mudrocks. It is difficult to discern the depositional environment for coarser grained clastic rocks based only upon major elemental analyses. Sedimentological and paleontological characteristics in addition to geochemical data are most useful in the determination of either a terrestrial or marine origin for any particular lithofacies. Trace element geochemistry would probably be more conclusive in delineating between rocks of terrestrial and marine origin.