engleski

Preliminary results on incision rates in the area of Bilogora (NE Croatia) as implied by post-IR IRSL dating

Bilogora is a low hilly terrain in the SW Pannonian Basin along the SW margin of the Drava Depression in Croatia. Genetically linked to the evolution of the Drava Depression and the palaeo-Drava River System, Bilogora initial uplift set on during the Pliocene within the NW-striking Drava Depression Boundary Fault Zone. Bilogora was finally uplifted during Middle to Late Pleistocene and Holocene, within a range of 300 and 550 m. Exposed to recent tectonic activity the area of Bilogora is characterized by near-surface strata deformation and numerous historical and recent instrumental records of moderate seismicity. In the area of Bilogora Quaternary sediments unconformably overlie Pontian-Lower Pliocene Rhomboidea beds and are associated to the Drava River drainage system and its sedimentation. The Quaternary is represented by four aggradational Drava river terraces composed of thick series of gravel beds intercalated by sand, silt and clay. The stratigraphically oldest terraces exposed in the investigated area are unconformably overlain by Pleistocene loess and loess-like deposits, lacustrine-marsh silts and clays. In order to establish a chronological framework of the Quaternary deposits, luminescence dating was applied on loess and loess-like deposits as well as sand lenses intercalated within the terrace gravel deposits identified at several locations in the investigated area. The dating was performed on separated feldspar coarse- grains from the sand and on polymineral fine- grained material from loess, using the elevated temperature post-IR IRSL dating protocol (pIRIR) with the IR-stimulation temperature of 290°C. The advantage of the pIRIR protocol is its applicability for dating of older deposits (up to the Middle Pleistocene) as well as that it circumvents fading, which causes age underestimation in feldspar due to athermal signal loss. The luminescence signals of the sand samples are in saturation and therefore very likely imply to a Middle Pleistocene age of the river terrace, whereas the covering loess can be correlated to the Last Glacial, as seen from the pIRIR dating results. The obtained results could indicate the timing of the abandonment of the investigated river terrace and the incision rates calculated from the obtained ages could imply to the relative uplift rates of the area.

Drava river terraces; loess; Bilogora; post-IR IRSL dating

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