Palaeoclimatic inferences from upper Palaeozoic siltstone of the Earp Formation and equivalents, Arizona-New Mexico (USA)

Abstract

The Late Palaeozoic configuration of Pangaea contributed to a palaeoclimatic extreme that was characterized by both icehouse and monsoonal conditions. This study uses sedimentological, geochemical, and provenance data from silty facies of the Earp and equivalent Supai Formations (Arizona, New Mexico) to shed light on atmospheric circulation and glacial-interglacial climate change in westernmost equatorial Pangaea. Five silt-rich facies comprise both loessite and marine and fluvially reworked loessite. An initial aeolian origin for the silt is indicated by the remarkably invariant grain size and the laterally continuous, sheet-like geometry of beds. The silt-rich facies occur in repetitive facies associations (1-20 m scale) that form mixed continental-marine (loess, marine-reworked loess), shallow-marine, and continental (loess, palaeosol) 'sequences'. Facies repetitions of both mixed continental-marine and shallow-marine sequences reflect a linked glacioeustatic-glacioclimatic control, whereas the continental (loess-palaeosol) couplets reflect a primary glacial-interglacial climatic cyclicity linked to glacioeustasy. Stratigraphic interpretations suggest that aeolian silt flux maximized during glacial to incipient interglacial stages (lowstand to early transgression), and decreased significantly or ceased during interglacials (highstand to early falling stage). Detrital-zircon geochronological data indicate a transition from dominantly north-easterly winds during the Middle Pennsylvanian to north-westerly and south-easterly winds by the Early Permian, which trend is inferred to reflect the onset of monsoonal circulation in western Pangaea. Relative grain-size data support the detrital-zircon data, and exhibit a significant decrease from the Sedona arch/Central Arizona shelf (north) to the Pedregosa basin (south) sections. Whole-rock geochemical data suggest a relatively unweathered source for the silt in the north, and detrital-zircon data indicate significant silt was derived from the local basement. These large piles of silt(stone) preserve valuable information for reconstructing both long-term evolution in atmospheric circulation and short-term fluctuations in glacial-interglacial climate. Many such indicators for long have been applied to 'recent' (Plio-Pleistocene) loess, but are equally applicable to 'deep-time' strata.