Mineralogy and Geochemistry of the Main Glauconite Bed in the Middle Eocene of Texas: Paleoenvironmental Implications for the Verdine Facies

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.

High‐frequency cycles of brachiopod shell beds on subaqueous delta‐scale clinoforms (early Pliocene, south‐east Spain)

“…They can be interpreted as mixed assemblages, in part within-habitat time-averaged, and in part census death assemblages (Kidwell, 1998). The occurrence of glauconite in the terebratulid pavements points to conditions of very low terrigenous input (Odin & Fullagar, 1988;Harding et al, 2014). Preservation of glauconite grains (often as well-preserved casts of foraminifera) points to their autochthonous or parautochthonous origin (Amorosi, 1997(Amorosi, , 2012.…”

Depositional setting and limiting factors of early Late Cretaceous glaucony formation: implications from Cenomanian glauconitic strata (Elbtal Group, Germany)

“…Allegedly, glauconitisation, i.e., the formation of 10-Å glauconitic smectite and smectitic glauconite characterizing the glaucony facies, only occurs in fairly deep marine settings under cooler temperatures, low accumulation rates, and on long timescales (outer shelf to upper slope; Odin and Fullagar 1988) while shallow marine warm-water settings are characterized by the verdine facies with a different suite of rather rapidly forming green 7-Å clay minerals such as odinite and ferric chlorite (Odin and Sen Gupta 1988;Odin 1990). However, the geological record of the verdine facies is scarce with only a few occurrences in pre-Quaternary strata (e.g., Harding et al 2014) and several new findings show a mismatch between the recent mode of glaucony formation and ancient examples (e.g., Chafetz and Reid 2000;El Albani et al 2005;Chafetz 2007;Banerjee et al 2012Banerjee et al , 2016Huggett et al 2017;Bansal et al 2018Bansal et al , 2020.…”

Mineralogical and Crystal-Chemical Constraints on the Glauconite-Forming Process in Neogene Sediments of the Lower Guadalquivir Basin (SW Spain)

“…Glauconitic minerals are highly stable and resistant to dissolution in the marine environment [46]; however, they tend to destabilize in weathering profiles. In fact, glauconite grains showed some evidence of post-depositional alteration as a result of chemical weathering processes (Figure 11).…”