Abstract
The Main Glauconite Bed (MGB) is a pelleted greensand located at Stone City Bluff on the south bank of the Brazos River in Burleson County, Texas. It was deposited during the Middle Eocene regional transgression on the Texas Gulf Coastal Plain. Stratigraphically it lies in the upper Stone City Member, Crockett Formation, Claiborne Group. Its mineralogy and geochemistry were examined in detail, and verdine facies minerals, predominantly odinite, were identified. Few glauconitic minerals were found in the green pelleted sediments of the MGB. Without detailed mineralogical work, glaucony facies minerals and verdine facies minerals are easily mistaken for one another. Their distinction has value in assessing paleoenvironments. In this study, several analytical techniques were employed to assess the mineralogy. X-ray diffraction of oriented and un-oriented clay samples indicated a clay mixture dominated by 7 and 14Å diffraction peaks. Unit cell calculations from XRD data for MGB pellets match the odinite-1M data base. Electron microprobe analyses (EMPA) from the average of 31 data points from clay pellets accompanied with Mössbauer analyses were used to calculate the structural formula which is that of odinite: Fe3+ 0.89 Mg0.45 Al0.67 Fe2+ 0.30 Ti0.01 Mn0.01) Σ = 2.33 (Si1.77 Al0.23) O5.00 (OH)4.00. QEMSCAN (Quantitative Evaluation of Minerals by Scanning Electron Microscopy) data provided mineral maps of quantitative proportions of the constituent clays. The verdine facies is a clay mineral facies associated with shallow marine shelf and lagoonal environments at tropical latitudes with iron influx from nearby runoff. Its depositional environment is well documented in modern nearshore locations. Recognition of verdine facies clays as the dominant constituent of the MGB clay pellets, rather than glaucony facies clays, allows for a more precise assessment of paleoenvironmental conditions.
Highlights
Green clay-rich sediment is recognized throughout the geologic record, and the paleoenvironmental implications of green clay facies are not always understood clearly
This study reports the clay mineralogy and geochemistry of the Main Glauconite Bed (MGB), a shelly and sandy mudstone that has an olive-gray hue and is bioturbated, pelleted, and trace fossiliferous, with diverse body fossils
As determined by QEMSCAN, reveals sediment composed of clay pellets, as much as 50%, in a clay matrix
Summary
Green clay-rich sediment is recognized throughout the geologic record, and the paleoenvironmental implications of green clay facies are not always understood clearly. This study reports the clay mineralogy and geochemistry of the Main Glauconite Bed (MGB), a shelly and sandy mudstone that has an olive-gray hue and is bioturbated, pelleted, and trace fossiliferous, with diverse body fossils. These characteristics, especially the abundant ovoidal, small clay pellets, probably led to its being named the Main Glauconite Bed. very few glauconitic minerals are present in the MGB. Published research on green minerals in sedimentary rocks reveals a lack of consistency in use of mineralogic terms. In the case of the MGB, the few glauconitic minerals detected were usually confined to sparse, isolated grains
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