Abstract
Raponzolo is a paleo-phreatic cave explored in 2011 in the Brenta Dolomites (Trentino, Italy), at the remarkable altitude of 2,560 m a.s.l. Differently to all other caves of the area, it hosts well-cemented fine to medium sands of granitic-metamorphic composition. The composition suggests a sediment source from the Adamello and Tonale Unit, separated from the Brenta by one of the most important tectonic lineaments of the Alps (Giudicarie Line). The fine-sand sediment was sampled to determine burial time and thus a minimum age of the cave. Cosmogenic isotopes (26Al and 10Be) in quartz grains allowed to estimate a minimum burial age of 5.25 Ma based on the mean sediment transport time at the surface and infer original altitude of the catchment area. Detrital apatite fission-track (AFT) and U-Pb dating on zircons provide information on the source, both from a regional and altitude (exhumation) perspective. Two populations of detrital AFT ages center at 17 (−2.3 + 2.6) Ma and 23 (−3.3 + 3.9) Ma, whereas the main detrital zircon U-Pb age populations are younger than 40 Ma. These correspond to intrusive and metamorphic sources nowadays outcropping exclusively above 2,200–2,300 m a.s.l. in Northern Adamello and Tonale. The results point to a late Miocene erosion and infilling of the cave by allochtonous sediments, with important implications on the timing of cave speleogenesis, as well as the paleogeographical connection, tectonic evolution and uplift of different structural units of the Alps. The roundness and the well sorted size of the quartz grains suggest a fluvial or aeolian origin, possibly recycled by glacial activity related to cold events reported in high latitude areas of the world at 5.75 and 5.51 Ma. These glacial phases have never been documented before in the Alps. This information confirms that the valleys dividing these geological units were not yet deeply entrenched during the onset of the Messinian Salinity Crisis (5.6–5.5 Ma), allowing an efficient transport of sediments across major tectonic lineaments of the Alps. This study shows the potential of cave sediments to provide information not only on the age of speleogenesis but also on the paleogeography of a wide area of the Alps during the late Miocene.
Highlights
Understanding how the topography and physiography of the Alps evolved from the Eocene to the Miocene has been one of the main challenges in Alpine geology over the last decades (Schmid et al, 2004; Garzanti and Malusà, 2008; Schlunegger and Mosar, 2011; Campani et al, 2012; Winterberg and Willett, 2019)
While the evolution of the Southern Alps has been studied through structural mapping and tectonic reconstruction (Doglioni and Bosellini, 1987; Castellarin and Cantelli, 2000), scarce information is available on the absolute timing of the events, uplift, erosion rates and especially paleogeographic arrangements in pre-Quaternary times (Stefani et al, 2007; Malusà et al, 2009; Potter and Szatmari 2009; Fox et al, 2016)
A set of conditions has to be satisfied to consider a cave as a potential site for sediment burial studies and to meaningfully link cave development to tectonic evolution: 1) The presence of quartz-rich terrains in the vicinity of the karst system, procuring the sediment source; 2) geological and environmental conditions able to transport these allogenic sediments from the source area to the karstified units; 3) the injection of the sediment into the phreatic or epiphreatic conduits, often developed close to the local water table
Summary
Understanding how the topography and physiography of the Alps evolved from the Eocene to the Miocene has been one of the main challenges in Alpine geology over the last decades (Schmid et al, 2004; Garzanti and Malusà, 2008; Schlunegger and Mosar, 2011; Campani et al, 2012; Winterberg and Willett, 2019). While calcite speleothems can grow at any (usually air-filled) stage once the void is formed, and are mainly useful for paleoclimatic and paleo-environmental reconstructions (Fairchild et al, 2006), the introduction of allochthonous sediments within cave systems is often synchronous or occurs shortly after the genesis of cave passages (Häuselmann, 2007; Calvet et al, 2015), i.e., when the base level is still more or less at the same altitude of the caves themselves (Columbu et al, 2018) In this case, burial time corresponds to the approximate age of the phreatic karst network at specific water table levels (Granger et al, 2001; Häuselmann and Granger, 2005). If this later transport would happen the age obtained with cosmogenic dating could not be associated to the correct speleogenetic phase (Häuselmann et al, 2020)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
