Middle Pleistocene debris flows in the low mountains: Geological and georadar surveys at the Mosty site (Holy Cross Mountains, Poland)

摘要

We present the palaeorelief model and depositional environment of debris deposits in the western part of the Holy Cross Mountains (SE Poland). During the Middle Pleistocene the studied area was in a close contact with the front of the Scandinavian ice sheet. Ground penetrating radar surveys, interpretation of geological drillings, outcrops and petrographic and quartz grain analyses have been used in palaeogeographical reconstructions. Several packets (up to 12 m thick) of fine limestone debris with an admixture of sand within a 30-m-thick sand series were documented under the surface of fluvial plain. The total volume of debris is estimated at 5,350,000 cubic meters. Based on the lithology and morphometric features of the deposits and their geological setting, the origin of these packets was related to the debris flows from the slopes of a low mountain range (330 m a.s.l.). We found that the large spread of the debris material is caused by i) the frost weathering of poorly resistant limestone hills under periglacial conditions, ii) the occurrence of deep ravines which were the material transport routes, and iii) the presence of a wide river valley at the foot of the slope. The triggering mechanism for debris flow were both snow and permafrost melting. It is possible that debris accumulation was also influenced by faults activity. The periods of increased activity of slope processes were correlated with the stages of surface outflow formation, following the retreat of the Odranian ice sheet (Saalian, MIS6) from its maximum extent line. The site described is an exceptional example of relic slope processes, and therefore is an important point on the palaeogeographic map of Europe. We believe that the presented model of slope sediment deposition can be applied in other reconstructions of periglacial phenomena in the low mountains in the close foreland of the Scandinavian ice sheet.