Lithified talus slopes, Quaternary, Northern Calcareous Alps

The development of talus slopes is controlled by geological, morphological, and climatical factors. Slope dip, segmentation, and facies of talus are controlled by processes each of transport, (final) deposition, and deposit overprint. Because these processes relate to climate, to talus "maturity" (rockwall heightalus height) and rockwall morphology, fossil talus successions record past climates and previous cliff configurations. Inquiry of the long- term development of talus, however, is often impeded by lack of outcrop and methodical difficulties. For this purpose, lithified talus successions are ideal, since they provide insight into a long time interval of talus development. In the Northern Calcareous Alps (NCA), lithified Quaternary carbonate talus slopes up to more than 100 m thick triggered speculations on their paleoclimatical and geomorphological significance, yet all hypotheses suffered from lack of absolute age-data. In addition, to date the stratigraphy, sedimentology and diagenesis of these successions are nearly unexplored. Preliminary results suggest that talus stratigraphy can be formulated into a new scheme of depositional sequence development. Sedimentary facies indicate that the considered talus successions developed in a humid climate (precipitation>evaporation). Many talus breccias onlap the base of present cliffs, or lap out into rectilinear rocky slope segments with a dip of 35-40. Many lithified talus slopes show a close relation to the present cliff configuration, but a few successions are totally "isolated" from their present geomorphic surrounding. Many breccias contain phenocrystalline calcite cements that may allow for determination of a (minimum) age by Th/U spectrometry. Moreover, stable isotope values of oxygen and carbon of the calcite cement may provide data on paleotemperature and type of precipitation (e.g. water, snow). The lithified talus slopes of the NCA may provide a valuable, yet unexplored source of data for a contribution to the topics of long-term cliff retreat, Quaternary landscape evolution and climate of the Alps, and may provide a concept for long-term development of coarse-clastic hillslopes in mountains.

Rock cliffs and talus slopes are characteristic of high-mountainous regions. The development of a talus slope is controlled by cliff height, rock type and deformation, and by climate. For the Northern Calcareous Alps (NCA), common relicts of lithified talus slopes had repeatedly triggered hypotheses on landscape history and palaeoclimate, yet all speculations were undermined by the unknown age of these deposits. Many lithified talus slopes are cemented by newly precipitated crystals of calcite that contains traces of 234 U and 230 Th. The radioactive decay 234 U 230 Th represents a physical clock that can be used to age-date the calcite crystals and, hence, to determine a "minimum depositional age" of a lithified talus relict. The U/Th method turned out to be well-suited for age determination of calcite cements of lithified talus relicts. Age-dating of lithified talus slopes of well-established Late-Glacial to Holocene age, and geological considerations indicate that calcite precipitation commonly proceeds fairly soon after deposition of a talus to be lithified, and within an interval of time no longer than the statistical standard error of age-dating (i. e. within a few decades to, in a few cases, perhaps a few millenia). Both deposition and lithification of the talus relicts were not confined to a specific interval of time. For Eastern Alpine lithified talus relicts, previously, a common assumption was that at least most of these talus slopes accumulated and lithified during the Last Interglacial (about 130.000-115.000 years before present). All together, however, the cementation ages of the talus successions investigated by us plot over an age range between 480.000 years to a few thousands of years. In addition, no cumulation of U/Th cementation ages during the Last Interglacial could be recognized. These results render the "traditional" tentative age assignment of the lithified talus relicts of the NCA as of the Last Interglacial obsolete and provide, for the first time, numerical minimum ages to such deposits. Under cold-humid ("periglacial") climatic conditions, talus slopes do accumulate geologically rapidly. Because of Pleistocene climatic oscillations, thus, conditions propelling effective talus formation shifted, repeatedly, over a wide range of altitude. Lithification of talus slopes by precipitation of calcite cement is a fairly common process, and is not unique to a specific interval of time during the Pleistocene. No simple relation is recognized between the position of a talus relict within the present landscape versus the age of that relict. Reconstructions by previous authors of the geometrical relation of talus relicts to former rock cliffs rest on assumptions that are erroneous or prone to very large errors. In high mountain ranges, aside of talus slopes, catastrophic rockslides or sturzstroms are common, too. Sturzstrom deposits may also be lithified, locally, by newly precipitated crystals of calcium carbonate. For the Fern Pass sturzstrom (third largest sturzstrom of the Eastern Alps), U/Th dating of calcium carbonate precipitated closely after the event yielded an age of 4190 100 years; this is the most precise proxy of depositional age of this sturzstrom at present. The approach to minimum age-date sturzstrom events by U/Th dating of cements is all of new. For the largest sturzstrom of the Alps, that of Flims in Switzerland, calcite cements shall be dated to potentially further precise the age of this very well-investigated deposit.