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Paleoclimatic inferences from paleopedology and magnetism of the Permian Maroon Formation loessite, Colorado, USA

Kristy L. Tramp^,1, G.S. (Lynn) Soreghan^,2 and R. Douglas Elmore^,2

¹ Department of Earth Science, Rice University, Houston, Texas 77005, USA
² School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma 73019, USA

The Maroon Formation in the eastern Eagle basin (Colorado) consists of >700 m of lithified loess with >200 paleosols interpreted as Protosols and Argillisols on the basis of field, petrographic and geochemical data. Additionally, magnetic susceptibility aids assessment of the intensity of pedogenesis. Bulk magnetic susceptibility (_b) through the section repeatedly fluctuates between low values (average 3.51 x 10^–8 ± 1.59 x 10^–8 m³/kg) in parent loessite and higher values (average 5.70 x 10^–8 ± 2.70 x 10^–8 m³/kg) in paleosols. Moreover, magnetic susceptibility positively correlates with abundance of clay-sized material as well as Al₂O₃ and K₂O and effectively distinguishes Protosols and Argillisols. Low- temperature demagnetization indicates the presence of ultra-fine-grained magnetite. The integration of geochemical, petrographic, and rock magnetic data suggest that changes in magnetic susceptibility reflect pedogenesis and relate primarily to climate- and time-dependent pedogenic production and concentration of ultra-fine- grained magnetite. The Maroon Formation loess and associated soils accumulated in an overall arid system, documented in part by formation of incipiently formed paleosols developed into Argillisols by eolian clay and carbonate additions rather than by in situ clay formation. However, the paleosols showing bulk magnetic susceptibility values of greater than 200 _b document a high- frequency (10⁴–10⁵ yr) fluctuation between arid times of loess accumulation and slightly wetter times of reduced silt influx and resultant pedogenesis. This fluctuation likely reflects glacial-interglacial climate shifts that operated in low-latitude Pangea during icehouse conditions. These results suggest that climate-related magnetic susceptibility variations within loess successions can be preserved and useful in very ancient (pre– Pliocene–Pleistocene) sequences.

Key Words: magnetic susceptibility • loess • paleosols • Eagle basin • Ancestral Rocky Mountains

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