Making a homogenite: An outcrop perspective into the evolution of deformation within mass-transport deposits

Abstract Mass-transport deposits (MTDs) may show a variety of deformation aspects, from poorly-deformed layers to highly deformed and homogenized deposits, as a result of the different kinds of mass-transport processes involved in their evolution. Previous studies indicate that the downslope flow transformation of MTDs is characterized by increasing disaggregation, mixing and homogenization of a non-to poorly lithified protolith, which may ultimately generate a homogenite. We here explore the role of deformation processes in the disaggregation and mixing of sediments that tend to lead to sediment homogenization and so may also control the evolution of MTDs. Using the Itarare Group of the Parana Basin, southern Brazil as our case study, we examine several types of structures in MTDs with varying degrees of mass disaggregation and discuss the role of deformation process in the continuum homogenization of the sediment. Three deformational facies (DF-1, DF-2 and DF-3) were identified and interpreted as recording varying stages of homogenization of sediments (disaggregation and mixing) within the incipient, mature and evolved, flows respectively. These different stages show transitional aspects that indicate a continuum homogenization. The comparison between structural assemblages and the amount of matrix in the three different deformational facies indicates a close relationship between styles of deformation and the homogenization expected for mass flows. The processes that promoted disaggregation include boudinage, faulting, layer-parallel shearing and marginal deformation of intrabasinal clasts, while mixing processes that generate matrix include shearing and liquefaction/fluidization of sediments.