Continental sediments filling major extensional block-faulted basins in the rock record traditionally are ascribed to plate margin rifting. The Triassic Cuyo basin of central Argentina is such a continental sedimentary basin, which contains over 3,600 m of interbedded fluvial, lacustrine, and volcanic deposits and which produces about 98,000 BOPD. It, however, formed within a back-arc tectonic setting as a reentrant into the Pampean craton prior to the onset of compressional deformation. The southern half of this basin (about 9,000 km²) was studied using logs and cutting descriptions from 101 exploratory wells.

The basin began as a series of subparallel fault blocks and accumulated at least 2,750 m of fining-upward detritus in its first cycle of sedimentation. Initial deposits were oxidized lithic conglomeratic units confined by elongated grabens. As faulting waned, high-gradient fluvial sandstones became prevalent (41%), derived primarily from reworking of interbedded felsic tuffs (29%). The end of tensional faulting is marked by widespread bituminous lacustrine shales that form the source beds for oil production. Broad basin subsidence, probably associated with lithospheric cooling, initiated a second episode of basin sedimentation in which tuffs and sediments containing fluvial sandstones (48%) markedly overlap the original basin boundaries. Sandy lag conglomerates of an intermontane brai ed-stream system cap the interval. These deposits were temporarily interrupted by valley basalt flows (Upper Jurassic-Lower Cretaceous), which are similar in composition to diabase dikes of the underlying strata.

End_of_Article - Last_Page 1449------------