Geologic Structure of a Part of the Barinas Mountain Front, Venezuelan Andes

摘要

The following lithologic units are exposed in the Barinitas-Santo Domingo region, southeastern Venezuelan Andes: (1) the Precambrian (?) Iglesias Group, forming the metamorphosed basement; (2) the Lower Paleozoic (?) Cerro Azul Formation, a phyllite sequence; (3) the Upper Paleozoic Palmarito Formation, mostly fossiliferous limestones and shales; (4) the Triassic-Jurassic La Quinta Formation, a continental sequence of red beds; (5) the rocks of the Cretaceous transgression and regression, consisting of the Río Negro, Apón, Aguardiente, Maraca, La Luna, and Colón-Mito Juan Formations; and (6) the Upper Eocene Gobernador and Pagüey Formations, disconformably overlying the Cretaceous sequence. Three granitic intrusions crop out, two of which, the La Soledad Quartz Monzonite and the hornblende granodiorite of Cerro Azul, intrude the Cerro Azul Formation. They were dated by the potassium-argon method as Middle Pennsylvanian and Early Devonian, respectively. The principal structural units present in the map area are enormous blocks of basement rocks (Iglesias Group) which are separated by steeply northwestward-dipping, or nearly vertical reverse faults (upthrusts), along which the blocks have been uplifted or depressed. Along the east flank of the mountain range toward the Barinas Basin the sedimentary cover has been folded into a monocline which is itself broken into blocks by steep faults. In the higher parts of the region, toward the central part of the mountain range, is a zone of extension which produced grabens. horsts, and stepped faults, represented by the Boconó fault zone. The present uplift of the Venezuelan Andes took place in post-Eocene time. Upper Eocene sediments of the Pagüey Formation, poorly consolidated during the initial stages of the uplift, were deformed by slumping, into cascade folds (overturned to the southeast), and also slid down-slope as chaotically oriented blocks which cover the Upper Eocene sediments in situ. Vertical uplift, probably combined with arching or tilting of the region, produced the steep thrust faults along the margin of the range, and also the zone of extension represented by the Boconó fault zone. Strike-slip movement of great magnitude probably has not taken place along this fault.