129-25 Sandstones and Paleosols of the Mingus Shale (Desmoinesian, Pennsylvanian), Palo Pinto County, Texas

Abstract:

The Mingus Shale (Desmoinesian, Pennsylvanian) is a widespread shale below the Brazos River Sandstone on the Eastern Shelf of the Permian Basin. At two localities in the outcrop belt a prominent, purple, sandy paleosol was observed. At the Ward Mountain (WMTN) locality, the paleosol is broken into two units; a lower white sandstone with well-developed vertical root traces and an upper mottled purple sandstone. At the Lake Palo Pinto Creek Spillway (MSPP), the paleosol consists of a thin, lower white sandstone with abundant horizontal roots and an upper thinner grayish purple sandstone. Paleosols at both localities were characterized by geochemical and petrological analysis. Thin sections were prepared for petrographic and EDS analysis. Powders were prepared for X-Ray Fluorescence and X-Ray Diffraction analysis. Total organic carbon analysis (TOC) was completed with powdered samples using the Loss on Ignition method. The distinctive purple color of the upper paleosol was caused by increased hematite concentration. The reddish hematite mixed with gray clays drive the unit color to purple. WMTN hematite concentrations were higher, thus displayed a more vibrant purple coloration. MSPP featured less hematite but had an increased iron content expressed as pyrite. The switch to pyrite and introduction of manganese in birnessite resulted in a duller, grayish purple color. Overall, the paleosol at WMTN is more quartz enriched and more birnessite depleted. The MSPP paleosol has greater mineral diversity, as displayed by lower quartz and increased birnessite, along with calcite and microcline. WMTN had an order of magnitude lower TOC than MSPP in the purple paleosol. Overall, the Mingus Shale in these localities reflects a coastal setting without substantial deltaic input as observed with a lack of deltaic sedimentary structures and finer sands. WMTN was drier, featuring well-developed vertical root structures in thicker sand bodies and overall thicker soil development. Lower TOC and more hematite indicate more oxidation. MSPP was swampier, presenting horizontal roots in thinner sand bodies as well as more carbonate material. Indication of less oxidation and organics was supported by increased TOC and more pyrite.

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