Title:

Magnetic Mineralogy of Lacustrine Carbonates from the Sheep Pass Formation, Nevada

Ancient lacustrine carbonates offer a means of reconstructing important paleoclimate and paleoenvironmental information. However, developing a reliable chronology from them directly is difficult. Magnetic reversal stratigraphy can help guide age determination, but this method assumes magnetic remanence was acquired soon after deposition, which is not always true for carbonates. The late Cretaceous – Eocene Sheep Pass Formation of eastern Nevada preserves microbial lacustrine carbonates that are being developed as a paleoclimate record for high altitude environments during a greenhouse interval and preliminary demagnetization shows that these carbonates retain a stable remanent magnetization that can be useful for magnetostratigraphy. We applied isothermal remanent magnetization (IRM) experiments to a range of observed lithologies from the Sheep Pass Formation as a means of determining the magnetic mineralogy and identifying potential post-depositional magnetic mineral alteration. IRM was applied along a single axis in a stepwise manner up to a maximum field of 1.1 T, followed by a backfield IRM of 0.3 T to calculate an S-ratio. Next, magnetic fields of 1.1 T, 0.4 T and 0.12 T were applied along the X, Y and Z axes to remagnetize the hard, intermediate, and soft coercivity fractions, respectively. This remanence was then removed using stepwise thermal demagnetization. All samples were dominated by low coercivity minerals, as seen by their high S-ratios (> 0.9), saturation under low applied magnetic fields, and the proportion of minerals remagnetized along the weakest direction (0.12 T), suggesting remanence is held primarily by minerals such as magnetite or maghemite. These magnetic mineralogy results will guide future magnetostratigraphic and paleoclimatic interpretations from the Sheep Pass Formation and contribute to a better understanding of magnetic remanence acquisition in microbial carbonates.

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