Anoxic facies, depositional processes and herpetofauna of a Tithonian-Berbiasian marine ramp in the Neuquen Basin (Ysera del Tromen), Nequen Province, Argentina

An important marine reptile-bearing locality from the Tithonian-Berriasian of the central Neuquen Basin (La Yesera del Tromen, Neuquen Province, Argen tina) is studied. This fauna is located in the lower portion of the Vaca Mu erta Formation, which is composed of thick intervals of black shares associ ated with black marls, thin packstone-wackestone beds and fine-grained pyro clastic 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 carbonate fall-out sediments, with sporadic incursions o f very distal storm-induced orbital and gravitational flows. A high nutrien t input supported a diverse biota, with exceptionally abundant phytoplankto n, and nektonic invertebrate and vertebrate fauna. Fourteen remains of off- shore top pelagic predators (ichthyosaurs, pliosaurs and metriorhynchid cro codiles), including some of the largest Tithonian-Berriasian marine reptile s, were found in an area of 2.1 km(2), and are here presented and described . The high rate of planktonic productivity, combined with physiographic iso lation, low surrounding relief and dry climatic conditions favoured the sta gnation of sea water, the generation of a thermo-pycnocline and the develop ment 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 s ubstrate composed of a thick column of water-saturated muds.