Statistical analysis of structures commonly used to determine palaeoslopes from within mass transport deposits

Abstract

Mass transport deposits (MTDs) contain deformation structures often that are used to determine the kinematics of palaeo-mass flows and hence the orientation of the original slope or gradient upon which the mass flow developed. When integrated with stratigraphic data, the azimuth of the palaeoslope can help elucidate the depositional evolution and palaeogeography of a region. However, most studies have defined palaeoslopes from well-exposed MTDs, which raises some questions regarding the validity of establishing palaeoslope definition where MTDs are poorly exposed. We examine MTDs of the Itararé Group (Paraná Basin), in southern Brazil, that are only partially exposed, but widely distributed both vertically and horizontally. Datasets and transport direction (palaeoflow) obtained from different structures, variable methods and multiple stratigraphic levels and geographic localities were statistically evaluated to verify the robustness of the methodology. This allow the orientation of local palaeoslope to be established for different time intervals, even from disconnected outcrops. Faults and folds, that are the main structures used to define palaeoslopes, display the greatest potential to accurately determine palaeoflow in the examined MTDs. The use of other structures such as asymmetric boudins, quarter structures and injectites, helped to clarify flow kinematics. The integration of palaeoflow data with palaeocurrent data from associated deposits give additional support for determining the orientation of the palaeoslope.