Paleontology of the jurassic and cretaceous of west central Argentina / Палеонтология юрского и мелового периодов западно-центральной Аргентины

Early in the Mesozoic the western portion of the South American continent was subjected to differential but gentle crustal deformation which resulted in a series of crustal downwarps situated at intervals from Colombia on the north to Southern Patagonia on the south. These downwarps were in the nature of small geosynclinal troughs separated one from the other by peninsula-like land areas of probable low topographic relief. During a part of the Triassic the western portions of these downwarps in Chile were to some extent invaded by oceanic waters forming irregular shaped embayments of deposition. Farther to the east local lacustrine basins were in existence, and were receiving the sediments derived from the erosion of the extensive land areas to the east. During this epoch, wide-spread volcanic activity was prevalent, and the lavas and ash were accumulating not only upon the land but also within the lacustrine and marine basins.
One of these downwarps is termed in this study the West Central Argentine Mesozoic embay-ment, and included a large part of the province of Mendoza and the territories of Neuquen, Las Pampas and Rio Negro together with a considerable portion of Central Chile lying immediately to the west. This downfold came into existence during the Triassic and continued to oscillate differentially upwards and downwards during the Jurassic and Cretaceous, but with a predominant downward movement. Vulcanism continued somewhat intermittently during the Lias or lower Jurassic, but with gradually diminishing importance. The products vary from an acidic to an intermediate composition. During the Lias the downwarp was occupied by oceanic waters across Central Chile and for an unknown but considerable distance into Mendoza and Neuquen. The products of marine deposition consist mainly of sandstone, conglomerate and stratified volcanic tuffs. The marine invertebrate faunas are closely allied to those of the Lias of Central and Southern Europe, and since the majority of the forms are not free swimming types probably they migrated to this region along the north shores of the African-Brazilian or Gondwanaland continent. <...>