RECONSTRUCTING THE PALEOENVIRONMENT OF THE UPPERMOST ASENCIO FORMATION, WESTERN URUGUAY

The top of the Asencio Formation of western Uruguay is a 4+ meter thick interval of iron-stained sandstone that lies immediately above, and sometimes overprints the Late Cretaceous, dinosaur-bearing sandstones of the Mercedes Formation. The exact stratigraphic relationship of these units has been the subject of much debate, with some authors classifying both as separate formations and others grouping them together as the Mercedes Formation. Here we examine a previously undescribed section through this interval to better characterize the nature of the Asencio Formation and cast light on both its age and relationship to the underlying sediments.

Using a combination of field description, XRD and XRF analyses, the sediments of this part of the Asencio Formation were identified as vermiform laterites that are lithologically distinct from the Mercedes Formation, albeit overprinting those sediments, irregularly in places. Our data indicate that these Asencio Formation sediments formed through a lengthy (possibly >2 Myr) period of intense weathering, likely on an extensive, low-topography paleosurface. Mean annual precipitation estimates calculated from these analyses (1100-1400 mm yr^-1) were in agreement with those expected for laterite formation, but mean annual temperature estimates were ~5-6ºC lower than typically expected. These estimates suggest that the Asencio Formation was generated in a temperate seasonal environment, a conclusion that is supported by the observed trace fossil assemblage. While some workers have suggested that the formation of laterites is possible in low temperature environments (particularly on basalts), this apparent contradiction requires further investigation. The age of laterite formation cannot be better constrained by these analyses.