New insights from the Neocomian basins in the West of Chubut and Santa Cruz provinces, Argentina

This paper presents a regional overview of the Neocomian basins developed in the provinces of Chubut and Santa Cruz, in Argentina, on the light of an extensive compilation of 2D and 3D seismic and borehole data. Neocomian deposits lie, mainly, on the western subsurface sector of both provinces. They are correlated with the Tres Lagunas Formation, Katterfeld and Apeleg formations, or Coyhaique Group, which crop out in Argentina and Chile. The Neocomian Cycle sensu lato was deposited during the Berriasian up to Barremian in a NW trending rift system. The base of the Neocomian fill lies either on top of the Sedimentary Volcanic Complex, (which is correlated with the Lago La Plata Formation), the Early-Middle Jurassic shales and sandstones deposits or Paleozoic metasedimentary deposits. The synrift sequence, regarded as Anticlinal Aguada Bandera Subcycle, is composed of matrix-supported conglomerates, interbedded with volcanic ashes at the base and covered by dark brown shales. The sag or postrift sequence, known as Cerro Guadal Subcycle, filled up the depocenters with sandstones and shales. This paper describes the main morphotectonic features of the area and the stratigraphic architecture of the Cycle. A marine basin is identified in the NW sector of the studied area, where an extended NE paleo-high is recognized. Toward the SE, long narrow NW-trending extensional depressions have developed. The rift infill of these depressions is lacustrine in the Southeast, changing to marine-deltaic toward the North and West. The rift infill is unconformably covered by the Chubut Group volcaniclastic and clastic sediments indicating the early inversion of these depocenters.