Paleoclimatic interpretation from clay minerals in the early-middle Miocene of southeast Patagonia, Argentina

Abstract

The Miocene at the south coast of Santa Cruz province (Argentina) is represented by the transition between the marine sediments of Monte León Formation (late Oligocene - early Miocene) to continental deposits of Santa Cruz Formation (early - middle Miocene). During this time, global warming trends occur that peaked in the Mid- Miocene Climatic Optimum (15 -17 Ma). This period was followed by the late Miocene cooling and drying trend. The upper part of the Monte León Formation (19.3 Ma) is a sedimentary succession composed by fine sandstones, siltstones and tuffs that culminate with conspicuous coquina banks, consisting of shells of Ostrea sp. arranged in life position. This unit reflects the shallowing from a shallow marine environment with neritic and coastal conditions culminating in a paralic environment. The invertebrate fossils of Monte León Formation are related to the first influx of Antarctic waters on to the Argentinean continental shelf, but towards the upper levels an improvement in the climatic conditions is registered. The basal levels of the Santa Cruz Formation, Estancia La Costa Member (~ 16.1-16.5 Ma), is comprised of fine to coarse-size pyroclastic and volcaniclastic deposits reworked by fluvial processes. The fine materials that conforms the floodplain contain immature paleosols (rhizoliths, iron concretions) and bioturbated beds (burrows of bees and scarabeid). The coarser sediments represent sheet-floods deposits of a fluvial system. This unit bears an exceptionally rich fauna of vertebrates that was developed under relatively dry conditions with marked seasonality and open environment (grassland). Compositionally, the upper section of Monte León Formation is represented by lithic sandstones with sporadic glauconitic pellets and reworked vitric tuffs. The more frequent cements are coatings of authigenic clays and carbonate. XRD from clay fraction points to very abundant smectite with minor participation of chlorite, interstratified illite/smectite and illite. On the other hand, the Estancia La Costa Member is composed by vitric tuffs, lithic sandstones and volcaniclastic sandstones. XRD analysis show very abundant smectite with less proportion of chlorite, interstratified illite/smectite and illite. Coarse grained kaolinite is also sporadically recognized. Since the sediments have been very little affected by diagenetic processes, preliminary clay-mineral results allow interpret paleoclimatic conditions during their deposition. The interval studied is characterized by a smectite-dominated assemblage reflecting warm and seasonal conditions. However, a relative decrease in smectite couple with and increase in illite, chlorite and illite/smectite mixed layers is noticed in the transition between both units and towards the upper part of the Estancia La Costa Member. This fluctuation probably results of cool or cool and dry conditions. The kaolinite may represent local and slightly more humid conditions. The presence of illite and chlorite in the oldest rocks is related with cooling irregular stage registered during early Miocene. Nevertheless, the smectite dominance in the middle part of Estancia La Costa Member could indicate a shift to warmer and seasonal climatic conditions (Mid- Miocene Climatic Optimum). Gradual cooling and drying that followed the climatic optimum is represented by the appearance of illite and chlorite in the upper part of Estancia La Costa Member. Upon the base of results and considering the previous paleontological data, it is possible to interpret that the paleoclimate during the early- middle Miocene in southeast Patagonia was warm and seasonal with cool - ing stages related to global paleoclimatic conditions for Miocene.