Early Pleistocene age of fluvial sediment in the Stará Garda Cave revealed by 26Al/10Be burial dating: implications for geomorphic evolution of the Malé Karpaty Mts. (Western Carpathians)

Abstract

Assessment of vertical movements of tectonically bounded blocks is crucial for determination of geohazards in densely inhabited zones, such as the border zone of western Slovakia and eastern Austria. The morphostructure of the Malé Karpaty Mts. divides the Vienna and Danube basins in the Western Carpathian – Eastern Alpine junction, and its neotectonic activity is of high importance. This study was focused on 26Al/10Be burial dating of fluvial sediment in the Stará Garda Cave, located in the central part of the mountains. The structural research revealed predisposition of forming of horizontal passages in low angle to subhorizontal bedrock stratification together with low-grade metamorphic foliation. Fluvial origin of the passages was inferred from mezoscale erosional features on the bedrock as well as from facies character of the well preserved sedimentary profile. Cave sediment was according to petrographic analysis derived from a watershed comparable to recent one of the Stupavský Potok Stream. Three analysed dating samples provided low values of isotopic concentrations, allowing us only to calculate the minimum burial age of the deposit of 1.72 Ma. Assuming the low position of the cave above recent surface streams, resulting maximum incision rate of 26 m/Ma indicates very low uplift of the mountains horst during the Quaternary. The slow incision of the river network is in good agreement with a widespread preservation of the planation surface called "Mid-mountain level". In contrast are relatively high values of palaeodenudation rates inferred from isotopic concentrations. Generally, our results indicate that the Malé Karpaty Mts. horst underwent relatively intense but short uplift in the Early Pleistocene, followed by very moderate uplift up to the recent. Key words: Western Carpathians, Malé Karpaty Mts., fluvial cave sediment, burial dating, Early Pleistocene, neotectonics. Zgodnja pleistocenska starost fluvialnih sedimentov v jami Stará Garda, ki jo je dala 26Al/10Be datacija: uporabnost za geomorfni razvoj Nizkih Karpatov (Zahodni Karpati) Ocena vertikalnih premikov tektonsko omejenih blokov je ključna za določitev geohazardov v gosto naseljenih območjih. Morfostruktura Malih Karpatov deli Dunajski in Donavski bazen na stičišču Zahodnih Karpatov – Vzhodnih Alp in njegova neotektonska aktivnost je zelo pomembna. Študija se je posvetila 26Al/10Be dataciji fluvialnih sedimentov v jami Stará Garda v osrednjem delu gorovja. Strukturna raziskava je razkrila predispozicijo oblikovanja vodoravnih jamskih rovov pod nizkim kotom v odvisnosti od subhorozontalne stratifikacije kamnine ter tudi z nizko stopnjo metamorfne foliacije. Na fluvialno poreklo rovov smo sklepali iz srednje velikih erozijskih oblik na njihovih stenah kot tudi iz lastnosti faciesov dobro ohranjenega sedimentnega profila. Glede na petrografske analize jamski sediment izhaja iz povodja, ki je primerljiv z recentnim povodjem potoka Stupavský. Trije analizirani vzorci so nam dali nizke vrednosti koncentracij izotopov, ki so nam omogočile le izračun najnižje pokopne starosti sedimenta, ki je znašala 1,72 Ma. Če upoštevamo, da leži jama nizko nad recentnimi vodnimi tokovi, dobimo maksimalno hitrost vrezovanja dolin samo 26 m/Ma, kar kaže na zelo majhen tektonski dvig v času kvartarja. Počasno vrezovanje rečne mreže se dobro ujema s široko ohranjenim uravnanim površjem, imenovanim »Srednjegorska uravnava«. V nasprotju pa so sorazmerno visoke vrednosti hitrosti paleodenudacije, ki izhajajo iz koncentracij izotopov. Na splošno naši rezultati kažejo, da je bil horst Nizkih Karpatov podvržen relativno močnemu, toda kratkemu dvigovanju v spodnjem pleistocenu, ki mu je sledil zmeren dvig do sedanjosti. Ključne besede: Zahodni Karpati, Nizki Karpati, fluvialni jamski sedimenti, pokopna datacija, starejši pleistocen, neotektonika.

Keywords

• western carpathians
• malé karpaty mts.
• fluvial cave sediment
• burial dating
• early pleistocene
• neotectonics