Speleogenesis and depositional history of paleokarst phreatic caves/cavities; Podgrad, SW Slovenia

Abstract

UDC 551.44:551.35(497.434) Bojan Otoničar: Speleogenesis and depositional history of paleokarst phreatic caves/cavities; Podgrad, SW Slovenia The studied paleokarst corresponds to an uplifted peripheral foreland bulge when Upper Cretaceous diagenetically immature eogenetic carbonates were subaerially exposed, karstified and subsequently overlain by upper Paleocene/lower Eocene palustrine limestone. Among the subsurface paleokarstic features, both vadose and phreatic forms occur. The phreatic caves/cavities include features characteristic of the mixing zone speleogenesis at the interface between freshwater (brackish water) lenses and the underlying seawater. They were found in various positions with respect to the paleokarstic surface, the deepest being about 75 m below the surface. Three indistinct horizons of cavities/caves and intermediate vugs were recognized. Subsequently, all cavities were completely filled with detrital sediments and speleothems in the phreatic and vadose zones. In general, the phreatic cavities of the lower two horizons are geopetally filled with mudstone derived from incomplete dissolution of the host rock and overlain by coarse-grained, blocky calcite. Shallower below the paleokarst surface, a large phreatic cave of the third horizon is filled with flowstone overlain by reddish micritic carbonate sediment with intercalated calcite rafts. In the upper part of the cave, sediments derived from the paleokarst surface are gradually becoming more abundant. Vadose channels, which may also intersect the cave sediments, are mainly filled with "pedogenic" material derived from the paleokarst surface. Immediately prior to marine transgression over the paleokarst surface, some cavities were filled with marine-derived microturbidites. In general, the diversity of cave fills and the amount of surface material decrease with distance from the paleokarst surface. Below the Izvleček UDK 551.44:551.35(497.434) Bojan Otoničar: Speleogeneza in sedimentacijska zgodovina paleokraških jam/votlin prežete cone; Podgrad, JZ Slovenija Raziskovani paleokras se je oblikoval na dvignjeni periferni izboklini, ko so bili diagenetsko nezreli zgornje kredni karbonati dvignjeni nad morsko gladino in zakraseli. Kasneje je bil ta zakraseli del periferne izbokline ponovno potopljen pod morsko gladino, paleokraško površje pa prekrito z zgornje paleocenskimi do spodnje eocenskimi palustrinimi apnenci. Na raziskovanem območju se pojavljajo podpovršinske paleokraške oblike značilne tako za prežeto kot neprežeto hidrogeološko cono. Kraške jame in manjše votlinice prežete cone kažejo značilnosti speleogeneze v območju mešanja meteorne in morske vode. Pojavljajo se v različnih nivojih glede na paleokraško površje, najgloblje približno 75 m pod njim. Jame in votlinice so razporejene v tri neizrazite horizonte, vendar se tudi med njimi pojavljajo manjše pore in kanali, ki so večinoma nastali ali so bili vsaj deloma preoblikovani z raztapljanjem. Vse omenjene votline različnih dimenzij so bile kasneje v prežeti in neprežeti hidrogeološki coni popolnoma zapolnjene z večinoma drobnozrnatimi sedimenti in sigami. Na splošno so bile votline prežete cone spodnjih dveh horizontov geopetalno zapolnjene s karbonatnim blatom, ki je nastal kot posledica nepopolnega raztapljanja prikamnine votlin, preostanki nad drobnozrnatimi sedimenti pa z grobozrnatim mozaičnim kalcitom. V tretjem horizontu zapolnjenih votlin, plitveje pod paleokraškim površjem, se nahaja tudi velika jama, ki je tudi nastala v prežeti coni. Ponekod v spodnjem delu vsebuje sigo, sicer pa je večinoma zapolnjena z rdečkastim karbonatnim mikritom v katerem so pogosta zrna, ploščice in svežnji plavajočega kalcita. V zgornjem delu velike jame postajajo sedimenti čedalje bogatejši z materialom, ki je bil v 1 Karst Research Institute ZRC SAZU, Titov trg 2, SI-6230 Postojna, Slovenia; e-mail: otonicar@zrc-sazu.si * Corresponding author Received/Prejeto: 12.04.2021 DOI: 10.3986/ac.vi.9945 ACTA CARSOLOGICA 50/1, 67-96, POSTOJNA 2021 COBISS: 1.01