49. Inclusion of Substrate Blocks Within a Mass Transport Deposit: A Case Study from Cerro Bola, Argentina

The preservation of large, relatively undeformed blocks is a characteristic feature of mass transport deposits (MTD). We examine a well-exposed succession at Cerro Bola in La Rioja Province, western Argentina, which comprises mid to late Carboniferous fluvio-deltaic sediments, turbidites and MTD’s. The main MTD, which is up to 180 m thick and crops out over 8 km, is characterized by allochthonous sandstone blocks that range in size from metres to 100s of metres in length, and are up to tens of metres in thickness. Blocks are preserved throughout the entire MTD, but are typically larger and much more abundant towards its base where they comprise up to ~30 % of the unit, and become progressively smaller and less frequent upward. Blocks were eroded from the underlying unlithified deltaic sands, and incorporated into the MTD during its transport and emplacement, resulting in local gouges and grooves in the substrate along the basal contact of the MTD. Sandstone blocks are interpreted to have undergone progressive abrasion and fragmentation as they rose through the MTD, thereby creating smaller blocks in the upper parts of the unit. We suggest that buoyancy-driven rise combined with the synchronous fragmentation of sandstone blocks that are entrained within a finer matrix, provides a mechanism for the observed distribution of blocks during overall downslope transport of the MTD.