The sedimentology, paleoenvironment, and diagenesis of the Asencio Formation of Western Uruguay

The uppermost sandstones of the Asencio Formation of southwestern Uruguay are characterized by a striking iron-rich geochemical overprint, which has been interpreted as paleopedogenic in origin. Among the notable features of the Asencio is the extension of the overprint into tubes that extend several meters into the underlying Mercedes Formation. Field observations and geochemical analyses of the Asencio Formation, and its parent lithologies from four localities around Uruguay allow the identification of the Asencio overprint as a ferricrete, displaying a number of the features typically associated with ferricrete formation: iron and aluminum enrichment; conversion of labile minerals to kaolinite; vermiform, pseudoconglomeratic and mottled textures; and loss of bulk density in comparison to the unmodified parent. The nature and uniformity of the geochemical profiles through the Asencio Formation suggests that this paleopedogenic modification occurred during a single phase of intense tropical weathering on a stable land surface, possibly over a geologically short time period. Paleoenvironmental proxies suggest this weathering could have occurred at temperatures around 18 °C with about 1500 mm of precipitation, although these proxies are not developed for paleosols lacking a strong Bt horizon, like the Asencio. No existing hypotheses can satisfactorily explain the nature of the tubes extending from the base of the Asencio Formation, nor do any of the observed features aid in the dating of the Asencio.