7-32 Geochemical Analysis of Pennsylvanian Carbonates from the Bird Spring Basin, Southern Nevada

Session: Undergraduate Research, Part I (Posters)

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Authors:

Abstract:

The Mississippian-Permian Bird Spring Formation of Nevada contains km of coarsening-upward carbonate packages. Most of the strata were deposited on a carbonate ramp, though portions of the southern part of the basin experienced further shallowing, represented by facies changes and upsection increase in evaporites and dolostones. In this study, we combined pre-existing δ13C analyses with new X-ray fluorescence (XRF) spectrometry data to evaluate this drying signal at a section in the southern part of the Bird Spring basin.

In total, 183 samples were collected from the 330-meters of Pennsylvanian strata at Mountain Springs Pass near Las Vegas, NV. All samples were analyzed using X-ray fluorescence (XRF) spectrometry (Bruker Tracer 5i) to measure both major elements (Ca, Mg, Si, Fe) and trace elements (Ni, Zn, Sr, Zr, Mn, Ti). Repeat analyses were conducted on every fifth sample, and the average of the two measurements was used for those samples.

The XRF analysis indicates that the samples are mainly dominated by calcium, with concentrations ranging from 8.3 to 40.1 wt% (mean = 31.7 wt%). Calcium values remain stable throughout the stratigraphic column. Magnesium concentrations increase upwards from 0.8 wt% to 1.2 wt%, with Mg/Ca ratios nearly doubling from 0.026 to 0.050. Silicon also generally increases upsection. Iron concentrations, Sr/Ca ratios, and carbon isotope values (mean 𝛿13C = 2.29‰ VPDB) remain relatively constant throughout the section. Increasing Si may be due in part to increasing presence of diagenetic chert and from silica dust blowing into the basin. The increase in Mg/Ca ratio indicates an increase in high-Mg calcite and/or dolomite, consistent with previous drying interpretations.

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