Equatorial Aridity in Western Pangea: Lower Permian Loessite and Dolomitic Paleosols in Northeastern New Mexico, U.S.A.

Abstract

ABSTRACT Lower Permian strata have been extensively cored in the subsurface of the Bravo Dome field, northeastern New Mexico. Analysis of core indicates that these strata consist of conglomeratic and sandy fluvial deposits and volumetrically significant eolian silt (loessite). Fluvial facies dominate the lower half of the study interval and include matrix-supported, massive conglomeratic debris-flow units and laminated arkosic sandstone, whereas loessite dominates the upper half of the study section and consists of massive, well-sorted quartzose siltstone that locally reaches thicknesses as much as 120 m in the greater study region. Paleosols are present throughout the study interval and consist of protosols and dolosols, commonly exhibiting vertic features. Dolomite that is interpreted to be of pedogenic origin is an unusual but volumetrically significant component in these paleosols. Paleogeographic reconstructions and paleomagnetic data indicate that these strata accumulated at equatorial (3-8°) latitudes, but depositional and pedogenic evidence both suggest seasonally wet to markedly arid conditions from early Wolfcampian to early Leonardian time. The loessite covers a substantial area (> 6000 km2), making this the largest pre-Cenozoic loess accumulation yet documented. This is significant, because loess generally suggests arid to semiarid conditions. Intercalated paleosols in the loessite section record repeated cessation of silt influx coupled with landscape stability, which we relate to high-frequency oscillation between dry and slightly wetter conditions, possibly attributable to glacial-interglacial climatic conditions that prevailed at low latitudes. At a lower frequency, the evolution from a predominance of fluvial to primarily eolian strata, in tandem with changes in pedogenic character, reflect a long-term aridification for the study interval. These data corroborate independent inferences of monsoon-induced equatorial aridity in western Pangea and help constrain the timing of the zonal-to-monsoonal transition to earliest Permian time.