Abstract

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 (χb) through the section repeatedly fluctuates between low values (average 3.51 × 10−8 ± 1.59 × 10−8 m3/kg) in parent loessite and higher values (average 5.70 × 10−8 ± 2.70 × 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 χb 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.

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