Block Generation, Deformation, and Interaction of Mass‐Transport Deposits With the Seafloor

Abstract

17 Mass transport processes are notorious for their ability to carry large blocks or mega 18 clasts, to deform sediments, and to interact with the seafloor through deformation and/or 19 erosion of the substrate. These processes, together with their influence on slope 20 sedimentation, are themes we address via direct field observation of three Carboniferous21 aged mass transport deposits (MTDs labelled I, II and III) from Cerro Bola, NW Argentina. 22 Internal deformation can be observed in all three MTDs, although it is best developed in MTD 23 II, a 180 m thick vertically zoned MTD with deformation evolving upwards from a simple24 shear dominated base, to a pure-shear middle zone, and finally back into a simple-shear 25 dominated top-most zone. The contact between MTDs I and II and their underlying 26 sandstone substrates are also locally deformed, with plastic deformation affecting up to ~20 27 m of substrate below the MTDs base. Conversely, the basal contact between MTD II and the 28 substrate is also in part erosional, marked by scours and grooves that truncate the bedding 29 in the top-most layers of the substrate. Additionally, the presence of large blocks composed 30 of diverse lithologies embedded within the MTDs, together with the sedimentological 31 description of the MTD ́s matrix and the aforementioned interaction with the seafloor, 32 suggest at least two processes accountable for block generation within MTDs. 33 34 Key Points 35 Vertical zonation of MTD II is based on soft-sediment deformation, block type and matrix 36 behaviour. 37 Basal erosion and deformation is recorded below the MTDs, suggesting both frictional 38 and plastic interaction between the MTD and the seafloor 39 Sandstone and siltstone blocks are present throughout the MTDs, indicating blocks may 40 be potentially generated by at least two different processes within the same flow. 41 42