Sucessões neoproterozoicas do Supergrupo Damara no noroeste da Namíbia: estratigrafia, paleoambientes e proveniência

Abstract

The Damara Supergroup is a Neoproterozoic succession (> 1000 m thick) that was deformed and metamorphosed during the Pan-African Orogeny, which gave rise to the Damara and Kaoko belts, in southwestern Africa. In northwestern Namibia, this supergroup consists of carbonate and siliciclastic successions that form from the base to the top, the groups Nosib, Otavi/Swakop and Mulden. The groups Nosib and Otavi/Swakop record rift to proto-ocean depositional phases during the Rodinia supercontinent break up. The Nosib Group includes alluvial fan deposits and felsic and mafic rocks that record syn-rift bimodal magmatism. The Otavi/Swakop groups can be subdivided in two tectonic sequences. The lower sequence includes slope and proximal fan, the upper sequence consists of slope debrites and medium to distal fan deposits. Both tectonic sequences include pelagic/hemipelagic deposits. The Chuos Formation in the lower sequence, and Ghaub Formation in the upper are interpreted as related to the Sturtian and Marinoan glaciations. Carbonate olistoliths and isolated blocks (pebble to boulder size) interpreted as sieved from gravitational flows occur in several formations of the two tectonic sequences. In this context, a non-glacial origin for the Chuos and Ghaub formations is considered as a possible alternative. Moreover, 569 concordant ages (U-Pb LA-ICPMS in detrital zircon grains) allowed to infer the deposition intervals and the provenance of the successions. Between ca. 870-760 Ma were deposited the Nabis Formation and the base of the Chuos Formation (base of the lower sequence); between ca. 760-635 Ma the middle part of the lower sequence to the base of the upper sequence. This interval also includes a gap represented by the unconformity between the two tectonic sequences. In the interval between 663-590 Ma were deposited the other successions of the upper sequence. It was also possible to distinguish that the main source area for the Nosib and Otavi/Swakop groups is the Angola-Congo Craton. Ages between 750 Ma and 630 Ma are interpreted as derived from syn-rift igneous rocks and/or the Coastal Terrane of the Kaoko Belt. Ages between 1000 Ma are considered originated from a basement structural high that prevented influx of sediments from source areas linked to the Kalahari proto-craton. The data show that this structural high was part of this proto-craton, so the Matchless amphibolites record a narrow ocean between the structural high and the proto-craton. As there is no evidence of oceanic rocks in the Outjo Basin, it is suggested that the Angola-Congo and Kalahari proto-cratons were part of the same paleocontinent during Rodinia times. In angular unconformity on the two tectonic sequences and basement occur foreland basin deposits of the Mulden Group. Neoproterozoic detrital zircon ages between 700-650 Ma predominate in these deposits, and are probably related to erosion of the Coastal Terrane of the Kaoko Belt