Published 1995 as part of Memoir 62
Copyright © 1995 The American Association of Petroleum Geologists.   All Rights Reserved.

Andean deformation in the Tertiary resulted in a foreland fold and thrust belt of which the Boomerang Hills structures in the northern Chaco basin form the frontal subsurface culminations. Hydrocarbons are trapped in anticlinal structures that mark the arcuate trend of the Boomerang Hills. Within this hinge zone, the Silurian-Devonian sedimentary wedge thins dramatically over basement that was structured by Silurian extensional faults. The basal detachment of the Andean deformation lies within the Silurian wedge. It mimics the basement geometry and thus controls the shape and orientation of the Andean folds. Geometric models confirm that it was movement along this stepped detachment surface that formed the type of folding observed on seismic data. The wedge-shape geometry of the Paleozoic succession enhances closure of the Cretaceous and Tertiary megasequences. The east-west orientation of the hinge zone coincides with the east-west trend of the Silurian sequence, which resulted in a configuration of the Boomerang Hills that is oblique to the Andean tectonic transport direction. 

Along the Chiquitanas trend and beyond the influence of Andean contractional deformation, subtle anticlinal structures are associated with early Paleozoic half-grabens. These extensional forced folds formed when the listric half-graben faults were reactivated. These faults dip toward the Guapore shield and appear to merge with low-angle detachment surfaces which we interpret as preexisting thrust faults. Seismic data demonstrate that extensional reactivation of some of these faults was Silurian (half-grabens), Late Devonian-Early Carboniferous (extensional forced folds), Cretaceous, and Tertiary (Andean) in age.