Early Silurian glaciation along the western margin of Gondwana (Peru, Bolivia and northern Argentina):: Palaeogeographic and geodynamic setting

The record of the Ordovician-Silurian transition within the southern Peru-Bolivia Palaeozoic basin (extending across Peru, Bolivia and northern Argentina) is characterized by a diamictite-bearing unit which overlies several different Ordovician units, and underlies mid Silurian shales. We review the lithostratigraphy, sedimentology and biostratigraphy of this unit, revising stratigraphic relationships and several ideas and interpretations previously proposed. Analysis of facies and lithofacies associations allow for a reinterpretation of sedimentary processes and enviromnents involved during its deposition. Turbidite and shale interbeds within the diamictites indicate a deep marine enviromnent, with interbedded mud flows, debris flows, slumps and large displaced slabs providing evidence for sediment instability and resedimentation. Chitinozoan biostratigraphy indicates a Llandovery age, in contrast with previous proposals of a Hirnantian age, and suggests the need for more detailed sedimentologic and biostratigraphic studies, and a reassessment of this diamictite unit in relation to adjacent regions in South America. The resedimented character of the deposit explains some of the Ordovician fauna previously described, which must be considered as recycled from underlying units. In situ fauna needs to be reassessed, as it may be endemic, diachronic and/or indicative of migrations within Gondwana. Glacially-faceted and striated clasts, as well as large granitoid boulders within the resedimented materials, provide evidence for glaciation of the source area, and are interpreted as recycled from former glacigenic deposits. The evidence found in the Central Andes indicates a glaciomarine origin for the diamictites and corroborates glaciation of a western source area prior to the late Llandovery (Telychian). The precise age of glaciation in this part of Gondwana cannot be confirmed do to a lack of true tillites. Tectonic deformation and the resulting relief are respectively identified as the origin for sediment instability and for local glaciation along the active margin of western Gondwana during the Late Ordovician and Early Silurian. The new data imply that the Ordovician-Silurian boundary was not preserved in some areas of the basin, and that an erosional hiatus (disconformity with variable regional significance) is present between different Ordovician units and the diamictite unit. Late Ordovician (?late Caradoc-?early Ashgill) euxinic black shales locally present in the central Altiplano and Eastern Cordillera of Bolivia may represent the youngest Ordovician unit in the region, and more research should be focused on them. Paleogeographic reconstruction of the Early Silurian Peru-Bolivia Basin depicts the extension of the diamictite unit southwards into northern Argentina and western Paraguay, connecting with the Parana basin through the Asuncion Arch. To the north it continued into northern Peru and Ecuador, connecting with the Silurian record in Colombia and Venezuela, and to the northeast into Brazil (Solimoes and Amazonas basins). The sedimentary record of glaciation in the Peru-Bolivia basin was due to the development of local ice fields to the west of the basin, related with tectonism and resulting reliefs along the active margin of Gondwana, and probably did not coincide with the development of the North African late Ashgillian ice cap. (c) 2006 Elsevier B.V. All rights reserved.