Based on new field mapping and wildcat drilling, the Cretaceous sequence is presented in a new classification which shows some important modifications. Only five different formations are recognized now, instead of the original, cumbersome thirteen formation-subdivision which was very impractical and unrealistic, being based partly on erroneous tectonic interpretation. Total Cretaceous thickness, which formerly was believed to amount to 11,000-13,000 meters, has been found to be not more than 6,500-7,500 meters.

The thick, Jurassic effusive rocks of mainly quartz-porphyric composition are, with a transitional contact, overlain by the marine argillites of the Zapata Formation, of Tithonian to Lower Cretaceous age. In Late Cretaceous time there were vast accumulations of flysch-type deposits, with the Punta Barrosa graywackes at their base, followed upward by the generally thin-bedded, rhythmically alternating pelites and fine-grained psammites of the Cerro Toro Formation; locally, lenticular bodies of thick conglomerates are intercalated. Toward the end of the Cretaceous these flysch-sediments were followed by a typical molasse sequence of sandstones, shales, and conglomerates, which near the top include some coal beds and terrestrial members. In many places a hiatus seems to separate the Maes richtian from subsequent Tertiary transgressions. In this area, a gradual filling of the basin is thus indicated in late Cretaceous time.