Sub-bottom and bathymetry sonar inspection of postglacial lacustrine infill of the alpine lakes (Tatra Mts., Slovakia)

An acoustic survey using sub-bottom sonar (SBS) and Autonomous Underwater Vehicle EcoMapper (AUVEM) combined with coring was used for the investigation of bottom topography, thickness of the sediments, and geodynamic factors influencing the postglacial deposition in Tatra Mts. lakes. This combined study of seven lakes shows four basic acoustic units and four subunits that are interpreted in terms of stratigraphy and lithology. However, the presence of particular acoustic units may not be encountered in all the lakes due to the effects of local and internal lake processes. Thick horizontal reflections with high amplitude reflectors characterize the bedrock or the infill with a thickness of more than 6 m. Chaotic internal reflection configuration and high amplitude reflectors are common and reflect the glacigene deposits of unknown thickness in the bottom of the lakes. Above the signals of glacigene sediments lie reflections with weak to moderate amplitude reflectors, and with a parallel and chaotic configuration. They correspond to the glaciolacustrine laminated clay and silt and postglacial gyttja that contain clastic outwash from various mass movement deposits. The transition zone between these two SBS reflections is formed by a discontinuous, parallel and wavy configuration with the acoustic bottom. This type of signal is interpreted as a mixture of cobbles, boulders, and fine-grained sediments. The exact lithological content and the presence of the acoustic units are determined by other factors that may not be visible around the lakes and can cause stratigraphic hiatuses. As a prominent and un-recognized factor in the Tatra Mts. until now is the presence of the bottom springs (pockmarks) of the non-karstic origin, which effectively disturb the fine clastic and organic matter deposition and hence can limit the use of the lake deposit for palaeoclimatic and palaeoecological studies. The mass movement processes - debris flow, rockfall - forming the mountain geo-relief are frequent and their products are visible in the marginal facies. The sonar inspection also provides evidence of mass movement deposits, which have no indication on the terrestrial environment and thereby represent intralacustrine landslides as a result of bedrock instability due to the late Pleistocene deglaciation or Holocene climatic variability.