Abstract
Rock magnetic results from paleosols in two North American Paleozoic loessite sequences, the Maroon Formation (Colorado) and the lower Cutler beds (Utah), indicate that bulk magnetic susceptibility (χb) variations can be complex. In Maroon Formation profiles, χb increases with increased pedogenic intensity and the major contributor to the augmentation of the χb signal is the ferrimagnetic susceptibility (χf). These trends are consistent with previous studies in which χb has been linked to climatically controlled pedogenesis. Thermal demagnetization of low-temperature saturation isothermal remanent magnetizations (SIRMs) indicates this increase correlates to a greater abundance of superparamagnetic (SP) grains, most likely magnetite, within the paleosols. The presence of coarser magnetite is indicated by the Verwey transition. In contrast, paleosols in the lower Cutler beds show a less predictable pattern in the χb signal. The paramagnetic χ (χp) and χf contribute to varying degrees to the χb within the loessite-paleosol couplets. Thermal demagnetization of low-temperature SIRMs indicates that both SP and coarser magnetite is present. There are no consistent patterns and often no significant variations in the amount of SP material within loessite-paleosol couplets. The thermal demagnetization curves for samples of the lower Cutler beds display a gradual decay of remanence and a weakly developed Verwey transition, both of which probably relate to maghemitization. Most paleosol and loessite samples differ in that there is more remanence-carrying material, predominantly hematite and some magnetite and maghemite, in the loessite. Rock-magnetic differences among loessite-paleosol couplets of the lower Cutler beds may relate to the heterogeneous distribution of pedogenic carbonate and/or to the presence of abundant hematite. This study shows that combining rock-magnetic techniques with sedimentologic evidence is important for deciphering the origin and preservation of χb patterns in ancient strata.
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