ABSTRACT: Tectonic and Stratigraphic Evolution of the Chaco Basin of Southern Bolivia

TANKARD, A. J., Petro-Canada, Calgary, Canada, M. CIRBIAN and J. G. SALINAS, Yacimientos Petroliferos Fiscales Bolivianos, Santa Cruz, Bolivia, and K. RASKIN and H. J. WELSINK, Petro-Canada/CIDA Project, Santa Cruz, Bolivia

The Precambrian basement of the Andean foreland in southern Bolivia comprises a mosaic of terranes that have accreted to the edge of the Brazilian shield. Periodic reactivation of the terrane boundaries has controlled basin architecture and depositional style. Key elements such as the Chiquitos-Boomerang shear zone and the Izozog arch can be traced to the Infracambrian (Pan-African) event. These uplifts are linked to the Sierras Pampeanas province of Argentina. In this sense, the Chaco basin represents the transition between undeformed foreland to the north and a region of foreland basement uplifts to the south, strongly resembling the Rocky Mountain foreland.

The Chaco basin consists of a westward-thickening wedge of terrigenous clastic sediments, locally over 10 km thick along the front of the Andes. Large-scale unconformity bounded sequences record the tectonic and stratigraphic history. The succession is dominated by Silurian, Devonian, and Carboniferous sediments attributed to a period of pronounced basin subsidence and pre-Andean orogeny, and Tertiary sediments related to Andean foreland basin development. The Devonian shallow marine and deltaic sedimentation built basinward from a rising orogenic front as well as the Brazilian shield, with erosional thinning over the Izozog arch. The Carboniferous succession was deposited in glacially influenced marine and continental environments. A system of northwest-southeast-directed, deeply inc sed paleovalleys and channels was fed by mountain glaciers. The Cenozoic Andean orogeny was characterized by uplift and tectonic wedging along the Paleozoic mountain belt, without large-scale thin-skinned deformation. Transition to this phase of Tertiary foreland basin deposition is marked by a major unconformity.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)