Late Tertiary Northwestward-Vergent Thrusting in Valle del Cauca, Colombian Andes

C. A. Alfonso, P. E. Sacks, D. T. Secor, Jr., F. Cordoba

The Valle del Cauca is a topographic basin situated between the Cordillera Central and the Cordillera Occidental in the Colombian Andes. The basement is Mesozoic mafic igneous rock of the Volcanic and Amaime Formations and clastic sediments and chert of the Espinal and Cisneros Formations. The basement was intruded by middle Cretaceous granodiorites (including the Batolito de Buga) and was deformed and metamorphosed to greenschist facies. The Mesozoic rocks originated in an oceanic setting and were accreted to northwestern South America during the Cretaceous or early Tertiary.

Unconformably overlying the Mesozoic basement are the Eocene and Oligocene Vijes (marine limestone) and Guachinte and Cinta de Piedra (fluvial and deltaic sandstone and mudstone). In the Cordillera Central, the Cinta de Piedra is unconformably overlain by fanglomerate of the Miocene La Paila Formation. These clastics coarsen and thicken eastward.

Geologic mapping and structural analyses show that the Mesozoic basement and its Tertiary cover are faulted and folded. Folds are asymmetric and overturned westward. Faults dip at shallow to moderate angles to the east and carry older sedimentary or basement rocks westward over younger rocks. Thrust faults carry the Amaime westward over the Cinta de Piedra and La Paila in the Cordillera Central, and the Cauca fault carries the Volcanic Formation over the Vijes and Guachinte in the Cordillera Occidental. Faults that carry basement over cover propagate in the cover as fault-propagation folds and imbricate fans. The middle Tertiary and older rocks are overlain with angular unconformity by the late Tertiary and early Quaternary Zarzal and Jamundi Formations, which are undeformed. Thrustin began before deposition of the La Paila and ceased by the end of the Tertiary. The Cauca basin formed as a consequence of differential subsidence related to crustal loading by thrust faulting during the middle or late Tertiary orogenic activity.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.