Sedimentologic-magnetic record of western Pangean climate in upper Paleozoic loessite (lower Cutler beds, Utah)

Abstract

Sedimentologic, pedologic, and magnetic data within the upper Paleozoic lower Cutler beds of the southwestern Paradox basin (Utah) record high- and low-frequency climate changes that operated at equatorial latitudes of western Pangea. The lower Cutler beds consist of ∼250 m of lithified eolian silt (loessite) and marine-reworked and fluvially reworked loessite, with abundant intercalated paleosols comprising Protosols, Argillisols, and Calcisols. The evolution from loessite and marine-reworked loessite with abundant Calcisols in the lower section to loessite and fluvially reworked loessite with abundant Argillisols in the upper section records a long-term transition from semiarid conditions in western equatorial Pangea in latest Pennsylvanian time to seasonally wet conditions in earliest Permian time. This shift could record intensification of the Pangean megamonsoon and associated seasonal incursions of moisture-laden westerlies. Paleosols record relatively high-frequency fluctuations between drier, dustier glacials and wetter interglacials of the late Paleozoic. Bulk magnetic-susceptibility values in paleosols exhibit variations that track paleosol type and are significantly elevated relative to parent loessite, attributable to the occurrence of both ultrafine-grained (superparamagnetic) and coarser-grained (remanence-carrying) magnetite. This signature reflects in situ pedogenic production of ferrimagnetic phases and a subordinate component of allochthonous, magnetic dust influx during pedogenesis, analogous to processes inferred for the magnetic signature in the Pliocene–Pleistocene loess-paleosol sequences of, e.g., the Chinese Loess Plateau. Integration of sedimentologic, geochemical, and magnetic data further suggests that enhancement of magnetic susceptibility in loessitic paleosols of this section relates primarily to climatic conditions and secondarily to durations of pedogenesis. Whereas peak susceptibility values in mature paleosols (Argillisols and Calcisols) do not vary significantly through the study section, peak values for Protosols track facies evidence for wetter conditions through time. Accordingly, relative changes in paleosol susceptibility values can provide paleoclimatic information, but should be integrated with other data to fully assess the origin of the signature. Overall, our data document the applicability of analytical approaches used on recent loess to very ancient loessite; this result is significant, because loess commonly records high- resolution evidence of terrestrial climate and climate change.