Rising water flow as a factor of organic material importation into caves

Abstract

The Carlucio Cave is located on the left bank of the Peruacu River canyon, in a National Park located in the semiarid zone of Minas Gerais, Southeastern Brazil. Although the Carlucio Cave is in an unfavorable relief position in terms of material importation into its interior, it harbors an important fossil deposit. This paper sought to investigate such fossil assemblage and describe material importation and deposition events. For that purpose, a survey of the existing fossils was conducted, including a description of the deposition sites and taphonomic signatures. A topographic survey was carried out of representative samples and the deposit chronology was established by means of carbon-14 and uranium-series dating. The fossil assemblage comprises mainly terrestrial epigeal mollusk shells, totaling 1,399 individuals, 70% of which are Drymaeus sp. (Bulimuloidea, Bulimulidae). In addition, 20 bones, 57 vegetal deposition sites, four charcoals, one palm fruit and one corncob were recorded. The cave location and morphology and the deposit’s taphonomic signature indicate that the material was carried upward by the rise in water level (WL) of the Peruacu River due to episodic barriers to the natural river flow, followed by floating and retention of the material once the WL went gradually down. At least four flood events that reached the Carlucio Cave were identified in the Peruacu River canyon during the Holocene. This study provides evidence of a mechanism of material transport and accumulation in the cave caused by the rising water flow in connection with flood events in the river canyon. Key words: Cave deposits, Holocene chronology, fluvial geomorphology, Peruacu River canyon. Narascanje nivoja vode kot faktor vnosa organskega materiala v jame Jama Carlucio je na levem bregu kanjona reke Peruacu v nacionalnem parku na polpuscavskem obmocju province Minas Gerais v jugovzhodni Braziliji. Ceprav je jama v neugodnem reliefnem položaju glede vnosa materiala, v njej ležijo pomembne fosilne najdbe. Ta clanek je poskusal raziskati fosile in opisati dogodke vnosa in odlaganja materiala. Zato smo pregledali obstojece fosile ter med drugim opisali odlagalisca in tafonomske podpise. Izvedli smo topografsko raziskavo reprezentativnih vzorcev ter z dolocanjem starosti z ogljikom C14 in uranovo serijo vzpostavili depozitno kronologijo. Fosilni ostanki obsegajo predvsem kopenske epigealne mehkužce s 1399 posamezniki, od tega 70 % Drymaeus sp. (Bulimuloidea, Bulimulidae). Poleg tega smo opazili 20 kosti, 57 rastlinskih talisc, stiri oglja, eno palmovo sadje in eno koruzo. Lokacija jame, njena morfologija in tafonomske raziskave sedimentov nakazujejo, da je material odneslo navzgor s porastom nivoja vode reke Peruacu zaradi epizodnih ovir pri naravnem toku reke. Temu sledi lebdenje in zadrževanje snovi, ko je nivo postopoma upadel. V holocenu smo v kanjonu reke Peruacu identificirali vsaj stiri poplavne dogodke, ki so segli v jamo Carlucio. Ta studija dokazuje mehanizem materialnega transporta in akumulacije v jami, ki jo povzroca narascajoci nivo vode v povezavi s poplavnimi dogodki v recnem kanjonu. Kljucne besede: jamski sedimenti, holocenska kronologija, recna geomorfologija, kanjon reke Perucu.