Cenozoic Deposits, Upper Magdalena Valley, Colombia

ABSTRACT As much as 12,000 m of nonmarine Cenozoic deposit is preserved in the Upper Magdalena Valley between the Central and Eastern Cordilleras in southwestern Colombia. Near the end of the Cretaceous Period early Andean deformation uplifted the Central Cordillera. Concomitantly the sea began to retreat from the East Andean geosyncline whose axis lay along the site of the Eastern Cordillera. After withdrawal of the sea the latest Cretaceous and Paleocene Guaduas Formation, having an average thickness of 1,000 m, accumulated on a poorly drained lowland stretching eastward to the Llanos. It consists of drab, mottled, and reddish-brown mudstone, and dark-gray sandstone with abundant dark grains of Cretaceous chert and mudstone. Carbonaceous deposits are more common than in any other Cenozoic formation. During the Cenozoic Era a succession of disturbances generated four major alternations of coarse- and fine-grained deposits. The Central Cordilleran provenance persisted, but detritus also was derived increasingly from local sources within the basin and on the site of the Eastern Cordillera, and the Cretaceous sedimentary mantle progressively was stripped from basement blocks. Eocene and early Oligocene Lower and Middle Gualanday deposits, as much as 2,500 m thick, and the Oligocene(?) and early Miocene(?) Upper Gualanday and La Cira formations, as much as 2,000 m thick, constitute two depositional cycles of thick chert-pebble conglomerate overlain by drab, mottled, and reddish-brown mudstone. The third cycle comprises the late Miocene Honda Formation, as much as 3,000 m thick, and composed largely of arkosic and volcanic detritus in local alternations of conglomeratic channel sandstone overlain by floodplain mudstone. A volcanic-rich redbed facies in the upper part of the formation reflects waning accumulation on well-drained alluvial plains. The fourth cycle is the Pliocene(?) Mesa Formation, about 1,000-1,500 m thick, which consists of a lower and upper polygenetic conglomerate and a middle unit of volcanic debris derived from great cones on the Central Cordillera. The upper conglomerate shed largely from the Garzón massif at the southern end of the Eastern Cordillera records the last important aggradation of the region. During and after deposition of the Mesa Formation, the major Andean orogeny, accompanied by continued volcanism on the west, deformed the region and differentially uplifted the entire Eastern Cordillera. With onset of regional uplift, deep erosion produced the present relief.