Equatorial aridity in western Pangea: Lower Permian loessite and dolomiticpaleosols in northeastern New Mexico, USA

Lower Permian strata have been extensively cored in the subsurface of the B ravo Dome field, northeastern New Mexico. Analysis of core indicates that t hese strata consist of conglomeratic and sandy fluvial deposits and volumet rically significant eolian silt (loessite). Fluvial facies dominate the low er half of the study interval and include matrix-supported, massive conglom eratic 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 In 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 unu sual but volumetrically significant component in these paleosols. Paleogeographic reconstructions and paleomagnetic data indicate that these strata accumulated at equatorial (3-8 degrees) latitudes, but depositional and pedogenic evidence both suggest seasonally wet to markedly and conditio ns from early Wolfcampian to early Leonardian time. The loessite covers a s ubstantial area (> 6000 km(2)), making this the largest pre-Cenozoic loess accumulation yet documented. This is significant, because loess generally s uggests and to semiarid conditions. Intercalated paleosols in the loessite section record repeated cessation of silt influx coupled with landscape sta bility, which we relate to high-frequency oscillation between dry and sligh tly wetter conditions, possibly attributable to glacial-interglacial climat ic conditions that prevailed at low latitudes. At a lower frequency, the ev olution from a predominance of fluvial to primarily eolian strata, in tande m with changes in pedogenic character, reflect a long-term aridification fo r the study interval. These data corroborate independent inferences of mons oon-induced equatorial aridity in western Pangea and help constrain the tim ing of the zonal-to-monsoonal transition to earliest Permian time.