Facies anóxicas, procesos deposicionales y herpetofauna de la rampa marina titoniano-berriasiana en la Cuenca Neuquina (Yesera del Tromen), Neuquén, Argentina

Abstract

An important marine reptile-bearing locaLity from the Tithonian-Berriasian of the central Neuquén Basin (La Yesera deL Tromen, Neuquén Province, Argentina) is studied. This fauna is located in the lower portion of the Vaca Muerta Formation, which is composed of thick intervaLs of black shales associated with black marLs, thin packstone-wackestone beds and fine-grained pyroclastic fall deposits. The lower Vaca Muerta Formation is the deposit of a basinal to outer ramp environment, characterised by a cyclic arrangement of siliciclastic and carbonato fall-out sediments, with sporadic incursions of very distal storm-induced orbital and gravitational flows. A high nutrient input supported a diverse biota, with exceptionally abundant phytoplankton, and nektonic invertebrate and vertebrate fauna. Fourteen remains of off-shore top pelagic predators (ichthyosaurs, pliosaurs and metriorhynchid crocodiles), including some of the largest Tithonian-Berriasian marine reptiles, were found in an area of 2.1 km² , and are here presentes and describes. The high rate of planktonic productivity, combined with physiographic isolation, low surrounding relief and dry climatic conditions favoured the stagnation of sea water, the generation of a thermo-pycnocline and the development of anoxic bottom conditions in the basinal and outer ramp portions of the basin. Excellent preservation of reptile skeletons is attributed to the lack of both bottom predators and bottom currents, as well as to a soupy substrate composed of a thick column of water-saturated muds.