| dc.description.abstract | Interest in applications of x-ray fluorescence (XRF) is greatly increasing, but the diversity
of applications of XRF data has yet to be fully exploited. This dissertation presents three
projects showcasing different applications of XRF. The first one uses molybdenum and
uranium ratios (from XRF) and in conjunction with data from sedimentary structures, lack
of benthic fauna, and biomarkers suggests that the depositional environment of Eagle Ford
Group can be characterized as storm-dominated shallow-water, photic zone, episodically
euxinic. Also the paleotopography seems consistent with that of a drowned carbonate platform.
The second project extends the stratigraphy of Eagle Ford Group into Big Bend National
Park, at a publicly accessible outcrop, by identifying the sequences boundaries through
chemostratigraphy of XRF data and stable isotopes, supported by nannofossil biostratigraphy.
This provides a downdip analogue of subsurface Eagle Ford section with intervals
not described in outcrops in central and north Texas.
The third project generated a regional stratigraphic correlation of Eagle Ford Group throughout
west Texas in the Trans Pecos area. Both a gamma-ray based correlation and carbon
isotope chemostratigraphic correlation are provided. These correlations were constrained
by nannofossil, biostratigraphy, and unpublished U-Pb ash bed dates. Also euxinia
(proxied by covariation molybdenum and uranium ratios) is interpreted to have been
diachronous. It was present earlier in the western part of Trans-Pecos during the early-mid
Cenomanian, but later was absent during the late Cenomanian. However, it was persistent
and was the dominant redox state in the eastern part of the Trans-Pecos as seen in the data from Antonio Creek and Lozier Canyon. Also the use of iron-sulfur-organic carbon (TOC)
data from Antonio Creek revealed that in the Lower Eagle Ford that while the Fe/S ratio
parallels the pyrite Fe/S ratio, it has more sulfur than stoichiometry predicts if all sulfur
was in pyrite. That the low Fe/S is associated with high TOC seems to support the idea
that low Fe/S is because that the TOC is sulfurized, suggesting that in certain mudrock and
source rock units, the Fe/S ratio can reveal the presence of sulfurized TOC. | en |
