Tufa deposits sheltered by inland notches as indicators of Quaternary denudation rates

Vabljeni na predavanje z naslovom Tufa deposits sheltered by inland notches as indicators of Quaternary denudation rates, ki bo v četrtek, 31 maja 2018 ob 17.00 uri v dvorani Zemljepisnega muzeja GIAM ZRC SAZU, Gosposka 16, Ljubljana.

Predavala bo dr. Nurit Shtober-Zisu z Univerze v Haifi, Izrael.

Predavanje bo v angleškem jeziku (povzetek):
Inland notches are elongated concave-shape indentations that develop on the carbonate rocky cliffs of mountainous zones, down to the desert fringe. These unique features formed as a result of the interaction between specific lithological and climatic controls, emphasizing the importance of environment upon rock decay. The vast majority (71%) of inland notches in Israel are formed in hard, dense, and crystalline limestone deposited throughout the Turonian age. Another 27% are cut into the dolomitic sequence of the upper Albian and lower Cenomanian. The rest (2%) are dispersed in the various formations of the Cenomanian and Eocene eras. Inland notches form because of slight differences (1%–15%) in the porosity of the visor and cavity bed: the cavity bed is more porous, so more likely to erode by exfoliation and dissolution. Thus, the cavity bed retreats at a faster rate compared to the slower subaerial dissolution of the visor bed, until a critical point is reached where the visor collapses. In Israel, inland notches inhabit the same lithostratigraphic units as do large caves.

Notches are most common in semi-arid and in Mediterranean climates but mainly in areas with annual rainfall of between 400 mm and 850 mm. In more humid areas (> 900 mm/y) notches are negligible or completely absent, due to the rapid rate of chemical dissolution of carbonate rocks. In the desert fringe (200–300 mm/y), mechanical decay is accelerated and notches exhibit disintegration processes, visor collapse, and rock falls. In the desert area (< 200 mm/y), salt decay replaces the chemical decay, encouraging tafoni formation. In the Mediterranean zone, tufa stalactites and stalagmites occasionally grow within the cavity of the notch. In extremely rare cases they merge into columns, detached from the back wall. The tufa fill, both along the back wall of the notch and in detached stalagmites, is more typical of notches located on north-aspect slopes, which are characterized by a more hydric environment – that is colder, more humid and with a more stable microclimate. The Carmel tufa deposits that grew under the notches visors and on the cavity back-wall were dated by U-Th chronology at the Geological Survey of Israel using ion exchange column chemistry and MC-ICP-MS techniques. The ages were corrected for initial 230Th by using crustal average 232Th/238U atomic ratio of 4.2. In each notch the oldest tufa layer was dated giving the minimum age of the surface formation. Six layers from four tufa samples were dated giving ages spanning from 13,636 ± 834 ky to 37,562 ± 2,397 ky, implying that these notches were formed during the last glacial period, or during the last deglaciation.

Predavanje organizira Geomorfološko društvo Slovenije v sodelovanju z Geografskim inštitutom Antona Melika ZRC SAZU.