Doctor of Philosophy

dissertationThe ichnology and mineralogy of selected occurrences of trace fossils and green minerals in verdine and glaucony facies were investigated, and the nature and significance of their association were evaluated. Trace fossils and green marine clays commonly occur together and are genetically related. At each site fecal pellets in marine shelf sediments were determined to be the most likely precursor of the green minerals. Fecal pellets, which are common on shelf sea floor, are organic rich, and they provide microenvironments of reduction for green mineral authigenesis, which promotes the formation of glauconitic pellets or odinite-rich pellets depending upon the paleoenvironmental conditions. Fecal pellets in intensely burrowed sediment represent a direct link between ichnology and green mineral authigenesis. Five Phanerozoic sites representing terrigenous marine shelf deposits were examined: the Middle Eocene Crockett Formation in eastern Texas; two Cambrian sites including the Reno Member, Lone Rock Formation in southern Wisconsin and the Lion Mountain Member, Upper Riley Formation in central Texas; and two Mesozoic sites including the Jurassic Curtis Formation in central Utah and the Cretaceous Shannon Sandstone in central Wyoming. All sites represent times during the geologic past when global temperature was abnormally high. The verdine facies, dominated by odinite-rich pellets, was identified in the "Main Glauconite Bed" (MGB) of the Crocket Formation. Odinite in the modern sea floor is confined to tropical latitudes, and its association with trace fossils made by shallow marine animals suggests a shallower and more tropical paleoenvironment during the Middle Eocene than was previously recognized. Glauconitic pellets were found at all the other sites, where sedimentologic characteristics, stratigraphic occurrence, ichnofabric, and trace fossil assemblages indicate a multistage, reworked depositional history. The story of sea level dynamics unfolds where glauconitic minerals indicate elevated sea level during a marine transgression. Subsequently, quartz grains and shallow-marine burrows indicate an interval of regression. Finally, shallow shifting-sand substrate occupied by burrowing organisms indicates adaptation for life in water too shallow for glauconitic mineral authigenesis. These findings document the paleoenvironment of the marine shelf during times of fluctuating sea level and warm paleoclimate

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

<div><p>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 Mg_0.45 Al_0.67 Fe²⁺_0.30 Ti_0.01 Mn_0.01)_{Σ = 2.33} (Si_1.77 Al_0.23) O_5.00 (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.</p></div

Plot of isomer shift vs. quadrupole splitting (in mm/s).

<p>A) Clays from the MGB (this study). B) MGB clays compared to published analyses of glauconite.</p

Top of MGB QEMSCAN images with specific mineral area %, sample TX-17.

<p>Top of MGB QEMSCAN images with specific mineral area %, sample TX-17.</p

Diagrammatic verdine facies model depicting the idealized paleoenvironment at a tropical river mouth (modified after Odin [16]).

<p>Diagrammatic verdine facies model depicting the idealized paleoenvironment at a tropical river mouth (modified after Odin <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0087656#pone.0087656-Odin1" target="_blank">[16]</a>).</p

Photomicrographs from central MGB, A) TX-9, B) TX-8, cp = clay pellet, hp = heterogeneous pellet, sh = shell, Q = quartz.

<p>Photomicrographs from central MGB, A) TX-9, B) TX-8, cp = clay pellet, hp = heterogeneous pellet, sh = shell, Q = quartz.</p

Unit cell calculation from XRD data (odinite-1M (monoclinic)).

<p>Eleven peaks were indexed. MGB pellets compared with ICDD. All units are in angstroms (Å).</p

Mössbauer parameters of 7Å clay mixture.

<p>IS: Isomer shift (mm/s).</p><p>QS: Quadrupole splitting (mm/s).</p><p>Width: FWHM (mm/s).</p><p>Spectrum temperature 295 K.</p>*<p>parameter fixed.</p

Geologic Atlas of Texas, Austin Sheet, Burleson and Brazos Counties [60]. Stone City Bluff and Stone #1core located. Wilcox Group (Ewi); Crockett Fm., also known as the Cook Mountain Fm. (Ecm); and Caddell Fm. of the Jackson Group (Eca).

<p>Geologic Atlas of Texas, Austin Sheet, Burleson and Brazos Counties <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0087656#pone.0087656-Barnes1" target="_blank">[60]</a>. Stone City Bluff and Stone #1core located. Wilcox Group (Ewi); Crockett Fm., also known as the Cook Mountain Fm. (Ecm); and Caddell Fm. of the Jackson Group (Eca).</p