Mapping of large-scale diapir structures at the paleo-ice tongue bed in western Latvia from geophysical investigations and borehole data

This study comprises the investigation of the complex and deformed Late Pleistocene sedimentary sequence in western Latvia composed of sandy sediments that are protruded by large-scale clayey silt diapirs and covered by glaciotectonically disturbed discontinuous patches of the Late Weichselian till. We use detailed measurements by ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) combined with borehole data and sedimentological investigations at the Baltic Sea cliffs to map and characterize the large-scale deformation structures and discontinuous till patches. We distinguish the intriguing and previously not detected character-istics of the spatial distribution pattern of discontinuous till layer represented as elongated, up to 150 m long and up to 50 m wide patches, parallel to the seacoast and perpendicular to the latest ice flow direction. The upper surface of till patches is flat and not inclined in any direction that is partially explainable by the postglacial erosion of the Baltic Ice Lake. The possible locations of diapirs inland are drawn from GPR data. We relate the formation of observed large-scale deformations to subglacial diapirism induced by the ice loading of the advancing Aprik, i Ice Tongue: clayey silt sediments become loaded and overpressurized, and flowed upwards intruding into the overlying sandy sediments. The initial deposition and deformation of till layer could have been affected by the diapirism process and preferred accretion of till in inter diapir spaces. We demonstrate that the geophysical methods can greatly supplement traditional geological investigations in the field revealing charac-teristics of deformed sediment sequences that are not recognizable from the outcrop data only and thus can have significant implications on the interpretation of deformation processes.

Automated summary

This study investigates the complex and deformed Late Pleistocene sedimentary sequence in western Latvia using ground-penetrating radar (GPR), electrical resistivity tomography (ERT), and other geophysical methods. The study reveals the large-scale deformation structures and discontinuous till patches, as well as the relationship between the formation of these structures and subglacial diapirism induced by ice loading. The study also demonstrates the significance of geophysical methods in supplementing traditional geological investigations and providing insights into deformation processes.