DATING OF THE UDIN BORST CONGLOMERATE TERRACE AND IMPLICATION FOR TECTONIC UPLIFT IN THE NORTWESTHERN PART OF THE LJUBLJANA BASIN (SLOVENIA)/DATACIJA KONGLOMERATNE TERASE UDIN BORST IN NJENA UPORABA ZA DOLOCITEV TEKTONSKEGA DVIGOVANJA V SEVEROZAHODNEM DELU LJUBLJANSKE KOTLINE (SLOVENIJA)

The northwestern part of the Ljubljana Basin is filled mostly with fluvioglacial sediments deposited by rivers coming from Alpine mountain groups. The Trziska Bistrica River, flowing from the Karavanke Mountains, has deposited a large alluvial fan consisting predominantly of carbonate pebbles with sub-ordinate amounts of siliciclastic pebbles. The oldest infill, cemented into a conglomerate terrace named Udin Borst, overlies an erosional surface on Oligocene mudstone. The thickness of the conglomerate terrace is up to 50 m. The conglomerate terrace is well karstified; the surface is dissected by numerous dolines and covered with a thick soil sequence. There are several caves. The most important are spring caves formed on the contact with the underlying impermeable basement. Samples of quartz pebbles were taken from the walls and ceiling in the 815 m long spring Arneseva Luknja Cave for cosmogenic nuclides burial age dating. The calculated burial age yielded an age of 1.86 ± 0.19 Ma that gives (i) the age of the oldest known infill in the Ljubljana Basin and (ii) indicates the time of change of the sedimentary system in the Basin from erosion to deposition. The age of the Udin Borst karst and caves is significantly younger. The age dates provide grounds for a first relatively firm estimate of the long-term tectonic uplift of the Udin Borst terrace to be between 0.06 and 0.04 mm/yr. This tectonic uplift rate may be related to the activity of the regional Sava Fault.