Sedimentology of the upper cretaceous red beds of Angostura Colorada formation in the western sector of the Northpatagonian Massif, Argentina

Abstract

Detailed sedimentological analysis of the Angostura Colorada Fm. (Upper Cretaceous) in the Comallo Jacobacci Gastre areas of the western sector of the North Patagonian Massif has enabled the definition of three members based on sedimentary structures, paleocurrents, grain size, compositional characteristics and lithofacies associations. These members are: lower (alluvial fan), middle (sandy braidplain) and upper (flood plain) developed within a rift sequence. Compositionally, two alternating source rocks are shown in the geological record, a continental block and recycled orogen provenance for the lower and middle members (Jacobacci area) and magmatic arc provenance for the middle (Gastre area) and upper members. This is interpreted as a progressive magmatism towards the K-T boundary during the rising of the Andean Cordillera to the west. The granitic provenance is attributed to igneous bodies of Paleozoic age to the N and SW of the study area. The smectite-zeolite rich clay mineral association of the upper member is interpreted as product of alteration of thick pyroclastic deposits. The paleogeography of this unit includes two different paleodepositional systems, to the SW an alluvial fan system running from the SW to the NE, to the NE a sandy braided river running from the NW to the SE, perpendicular to the previous one in a wide paleovalley. Later flood plain and lacustrine shaley deposits developed to the east which are equivalent to the Alamitos Fm. This distribution of paleodepositional systems is coherent with the main structural trends within the study area. A late Campanian age is assigned to this unit.