engleski

Preservation of microscale erosive glacial features in carbonate rocks of Croatia

Mountain glaciers have a high climate-sensitivity and are ideal records of changes in climate during past glaciations (Owen et al., 2009). The mountain paleoglaciers in Dinarides attracted the attention of early researchers (e.g., Hranilović, 1901 ; Cvijić, 1917). However, the poor chronological control of this features limited its paleoclimate application in comparison with other records. Thus, the study of past glaciations became less relevant for decades. However, the improvements on dating techniques during the last decade restored the interest for the study of paleoglaciers, including the Mediterranean region (Hughes et al., 2006 ; Hughes & Woodward, 2008). In Croatia, despite the early pioneer studies and the punctual subsequent contributions (e.g. Nikler, 1973 ; Belij, 1985 ; Marijanac & Marjanac, 2004 ; Bognar & Faivre, 2006 ; Velić et al., 2011 ; Marjanac, 2012), there is no complete detailed catalogue/cartography of glaciated areas. Therefore, further basic geomorphology and Quaternary geology on this topic is to be developed in the region. Microscale glacial erosion features (e.g., polished surfaces, striations, small grooves) are geomorphic indicators to support the evidence of glaciation (Bennett & Glasser, 2009). However, preservation of carbonate surficial features depends on weathering. Morphologically similar features can result from agents different than a glacier (e.g., tectonic, anthropogenic). Therefore, considering the preservation criteria of these features since deglaciation could help to constrain its genesis. Weathering of carbonate rocks depends on many parameters including climate (Dreybrodt, 1988 ; Appelo & Postma, 1993), rock properties, and local context (Wainwright, 2009). In Croatia several weathering rate studies have been conducted in different locations (Pahernik, 1998 ; Perica 1998 ; Perica & Orešić, 1999 ; Krklec, 2011). Despite the variability in weathering rate, all results at surface conditions are within the same order of magnitude (i.e., 4 to 7 mm/ka). Subsurface weathering rates are frequently higher (e.g., >10 mm/ka) than those in the surface (e.g. Pahernik, 1998, Perica & Orešić, 1999). In this context, the dissolution is the principal weathering process, but physical and biological erosion may also be important. It should be noted that for the dissolution, the surface of the features have to be in contact with unsaturated water in an open system (i.e., water flow). Thus in buried microscale glacial erosion features, the weathering is expected to be higher unless the cementation/cohesion of sediments prevents water circulation along rock/clast surface. The figure 1 represents a theoretical approach showing the time needed for a microscale glacial erosion feature on the surface of a rock (or clast) to be lost. In general terms, glacial polish surfaces disappear within 1 or 2 decades, and all striations could disappear in less than 200 years. Thus, only grooves could be identified after longer time periods. This is a theoretical approach, but gives the range of magnitude for the preservation of these delicate geomorphic elements. In Croatia glaciers disappeared from the mountains thousands of years ago (e.g., Belij, 1985). Thus, the preservation of glacial erosion microscale features in Croatia is expected to be null under surface conditions. Only two scenarios could allow their preservation in other settings: 1) a burial context under impermeable sediments due to their cementation and/or cohesive properties, or 2) a recently exposed rock (or clast) that was under the previous scenario. Any other subsurface scenario should result in weathering of these fine morphological elements. We have studied glacial sediments from Paklenica National Park, in the Malo Rujno moraines. Although microscale glacial erosion features were not expected, one section provided gravels with striations. The gravels are pebbles in size and show striations <1 cm in length (normally some millimetres) with different orientations, typical for gravels rotating during glacial transport (Benn & Evans, 2007). The depth of the striations reaches several hundreds of micrometres, showing a good preservation. They were found in loose sediments with almost no carbonate cements. However, the cohesion of the deposit is high due to the presence of micrite (silt/clay size) in the matrix that minimizes porosity. The clast supported sediment has massive structure and shows occasional orientation of elongated pebbles, suggesting some deformation of the sediment by the glacier push within or shortly after sedimentation. This study demonstrates that the preservation of delicate erosion morphologic elements after deglaciation in Croatia is possible. However, this requires a burial history of the rocks (or clasts) under very specific conditions where dissolution would be prevented

microscale erosive glacial features ; carbonate corrosion ; weathering

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