MAJOR AND TRACE ELEMENTAL ANALYSIS OF LATE PENNSYLVANIAN MIDLAND BASIN SHALE MEMBERS FROM TEXAS

The Pennsylvanian of the North American mid-continent is characterized by cyclical deposition of sandstone, coal, black shale, limestone, and grey shale, in the repeated successions known as cyclothems. The dark shale lithofacies within these cyclothems often contain a well preserved and abundant benthic fauna. Here we compare paleoenironmental conditions, as reconstructed using trace element geochemistry, from eight shale members, with the goal of understanding whether similar lithofacies within different cyclothems represent similar environmental conditions, or whether apparently similar deposits can reflect a range of benthic environments. Our ultimate aim is to gain insight into how habitat conditions control benthic paleocommunity structure.

We collected bulk sediment samples from the Late Pennsylvanian of North Central Texas, specifically the shale lithofacies of the Graham, Colony Creek, Wolf Mountain, and Mineral Wells Formations from the Midland Basin. Samples were cleaned, powdered, and combusted at 550°C.

Energy dispersive x-ray fluorescence spectroscopy (ED-XRF) was used to quantify the major and trace element composition of shale members. Enrichment factors (EF) were calculated based on average shale compilations, and redox sensitive transition metals were used as paleoenivronmental proxies. There is little compositional and EF variation between the various shale members, and the coefficient of variation is <1. This suggests that, over this stratigraphic interval, environmental conditions of shale deposition were similar during each depositional cycle. Shales show no appreciable enrichment in the redox proxies Zn, Cu, Ni and only minor enrichment of V, Cr, and Mo. These data suggest deposition under dysoxic environments. Moderate enrichment of Ba suggests moderate paleoproductivity. These chemical signatures suggest a shallow shelf environment experiencing oxic to dysoxic, moderately productive environments.