Blocky Mass Transport Deposit, Argentina
Presented by Mr. Matheus Silveira SOBIESIAK on 23 Sep 2013 from 17:15 to 19:15
Type: Poster presentation
Session: Poster session
Track: Poster presentations
Downslope mass movement involves changes in material properties and rheology induced by compactional dewatering and shear strains. The structures and kinematics indicative of these strains are very complex; they have been described in seismic data (e.g. Bull et al., 2009), but equivalent structures may be very difficult to identify in outcrop studies, largely due to the large scale difference. The study area is the Cerro Bola Anticline, which exposes part of a Carboniferous pericratonic basin located in the western part of Argentina, that was filled with glacially-derived sediment during and immediately after the mid to late Carboniferous glaciations of Gondwana. The mass transport deposit consist of a re-sedimented subaqueous rain-out till. It ranges in thickness between 100-140m and displays considerable variation in texture and structures; it can be subdivided into 3 main zones (lower, middle and upper) (Dykstra et al., 2011; Milana et al., 2010; Fairweather & Kneller, in prep.). The lower zone is characterized by sandstone blocks, interpreted as being derived from the underlying fluvio-deltaic sandstone. The middle zone comprises structureless greenish coloured siltstone with glacial dropstones inherited from the protolith, and rafts of relatively undeformed parent material. The upper zone consists of folded and sheared siltstone. The presence of blocks of the underlying unit within the lower zone of the MTD bears witness to the degree of coupling between the mass flow and the substrate, with deformation extending many metres into the autochthonous material below the obvious lithological contact. Drag across this interface resulted in blocks being plucked from the substrate and incorporated into the mass flow. Observations were made in the lower zone of MTD where deformation zones around sandstone blocks are visible as sheared sand streaks, folds and faults, which act as strain markers. We compare these deformation zones to quadrant structure that develops around rigid objects in mylonites zones (e.g. Fossen, H. 2010; Hanmer & Passchier 1991). Since the allochthonous sand blocks are more rigid than the main MTD matrix, deformation zones tend to develop all around the blocks, due the differences in rheology, and to the resulting distribution of shear strain around the blocks; an asymmetric distribution of structures is expected over the four quadrants around the blocks. Deformation zones are complex and vary from block to block, depending on their orientation, degree of translation and/or stratigraphic level.