Abstract

The Deseado and San Julian late Paleozoic-Mesozoic rift basins occupy sizable parts of central Patagonia and offshore southern Argentina, respectively. The basins record a similar (linked) tectonostratigraphic evolution since their inception in the late Paleozoic. Both evolved as continental intracratonic rifts developed above a basement comprising Devonian low-grade metasediments overlying older continental crust, in response to regional tectonic extension that preceded late disaggregation of Gondwana. This Late Permian rifting was followed by continuous filling of the basin during Triassic thermal subsidence. Jurassic extension produced a framework of grabens and half grabens filled with mafic and felsic proximal volcanics and volcaniclastics. The east-west orientation of these Jurassic rift features, in sharp contrast to the north-south trend of the Permian-Triassic rift systems, suggests that the Jurassic depocenters shifted in response to oblique extension: development of a transtensional pull-apart basin. In the upper Valanginian-Aptian, a strong compressive event, marked by a prominent angular unconformity, affected the Deseado and San Julian basins. Wrenching associated with this compressive event produced transpressive structures in both basins. A period of relative tectonic stability was observed during the Upper Cretaceous and Tertiary, the only interruption being a mild transpressive reactivation along major preexisting faults during the Miocene Andean orogeny. The first marine incursion occurred in the Paleocene as the seas transgressed over an extensive peneplained terrain.

The Deseado and San Julian basins remain virtually unexplored—only three wells having been drilled. Oil seeps in Neocomian strata in the Deseado basin indicate that this frontier basin has petroleum potential. Source quality intervals were encountered in the Neocomian Bajo Grande Formation of the San Julian Basin; distal lacustrine facies of the Neocomian La Matilde Formation are expected to be potential source rocks in the Deseado basin. The Permian-Triassic section is also thought to contain well-developed source rocks; however, the section has yet to be penetrated. Reservoir-quality rocks occur in clastics of the Upper Cretaceous, Neocomian, Triassic, and Permian formations. Possible play types include Paleozoic reservoirs in positive transpressive features, lateral updip pinch-outs of Neocomian sandstones, and fractured Jurassic volcanics adjacent to fault zones.

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