STRATIGRAPHIC RECORD OF A DISTRIBUTARY FLUVIAL SYSTEM, PALEOSOLS, SOILS, AND THEIR PALEOCLIMATIC IMPLICATIONS DURING THE MIOCENE TO EARLY PLIOCENE, NORTHWESTERN ARGENTINA

Abstract

The Andean topography influences regional climates on both sides of its physical structure. Sedimentary environments and climate might have evolved along the growth of the orographic barrier caused by the topographic uplift as the result of the interaction between the Nazca and South America tectonic plates. This study present evidence of the history of the sedimentary environments evolution and the paleoclimate response and sensitivity of the uplift of the Andes during the Miocene to Early Pliocene. To determine if environmental and climatic changes occurred during the geologic record three main subjects were studied: (1) The sedimentary record of three stratigraphic columns that produced a west-east profile; (2) Paleosols that were compared with the modern soils in order to establish correlations between past and modern sedimentary environment; (3) and the study of pedogenic calcium carbonates deposits through the analyses of Carbon and Oxygen stable isotope ratios. The sedimentary record exhibits an overall upward coarsening sequence that can be sub-divided in three main sedimentary cycles. These cycles are formed by alternations between sandstones and mudstones with the sandstones presenting different geometries. The paleosols identified during the field work are present across the three stratigraphic columns. Most paleosols contain pedogenic calcium carbonate and were rich in clay, silt, amnd fine sand. The last set of analyses presented by carbon and oxygen resulted with isotopic values similar to today’s rainfall values in the area for oxygen, and a consistent δ13C values through time for carbon isotopes. The results from the sedimentary record represent a distributary fluvial system that has been progressing to the east that is correlated with tectonic events. The sub-cycles are representative of the lateral migration of the fluvial system. Paleosols present similar physical and mineralogical properties compared with modern soils. The range of values of stable isotopes suggest a climate history similar as todays. The sedimentary record presents a constant or a low variable climate of the Eastern Andean belt throughout the Miocene-Early Pliocene geologic time.