Paleoclimatic and paleobiologic implications inferred from the metabolism of Eocene tropical laterites of Uruguay and Argentina

BELLOSI, E., M. GONZÁLEZ AND J. GENISE

San Luis

Congreso; X Reunión Argentina de Sedimentología; 2004

Asociación Argentina de Sedimentología

The dynamic bio-geochemical metabolic processes occurring in tropical laterites are mainly governed by climatic changes, particularly by the rainfall regime. The bioturbated Ultisols of the Asencio Formation, would have developed in a warm-humid (1300-1700 MAP) climate (tropical savannah), with significant fluctuations in the water table (alternating rainy-dry seasons), because the high frequency and thickness of clay illuviation features, the co-occurrence of smectite (dry) and kaolinite (humid), the absence of calcite, and the abundance of spherical micropeds. Termite-reworked sediments (abandoned nests) played an important role in the formation of ancient and present laterites, which would be congruent with the Asencio example. Induration of the soils and ferricrete formation was produced during periods of intense or extended desiccation, whereas ferricrete degradation or dismantling occurred due to a high increase in rainfall by in situ geochemical evolution of the duricrust (desferruginization process). Taking into account the maturity of the paleosols and the rate of the lateritic profile development, these three climatic stages (interrupted by relatively short sedimentation periods) can involve some to several million years. The distribution of the pedofabric (abiotic soil fabric) and ichnofabric (insect and root trace fossils) recorded in different horizons of the Asencio Fm. indicates that the laterization occurred in several opportunities (at least four) on unconsolidated sediments. Consequently, the iterative sedimentation-ferricretization-dismantling events of the Asencio materials would have occurred without a significant interruption between sedimentation and laterization. For middle latitude (34oS) regions, this type of paleoclimatic conditions would be expectable at the Early Eocene Climatic Optimum.