Study of an upper Eocene mixed carbonate–siliciclastic succession from N Romania (Rodnei Mountains): microfacies, mineral associations and depositional conditions

Abstract The microfacies analysis and mineralogical investigations (X-ray diffraction, scanning and transmission electron microscopy, and energy-dispersive X-ray spectroscopy) of a four-meter sedimentary succession in the Cave from Izvorul Tăușoarelor (Rodnei Mountains) provided new insights into paleontological and paleoenvironmental features of a ramp-type depositional system during the late Eocene. The studied succession consists of rhythmic alternations of limestone, carbonate micro-breccia, sandstone, and a thin clay level. The microfacies analyses of the limestone beds highlighted their compositional and textural features and permitted the establishment of two main facies types: bio-extraclastic grainstone/packstone (MFT1) and bioclastic wackestone with Chapmanina (MFT2). The present micropaleontological association (benthic foraminifera, red algae, echinoids, bivalve fragments, bryozoans) is characteristic of a shallow-water depositional environment with frequent hydrodynamic oscillations from the middle-innermost parts of a carbonate ramp system. The presence of stylolites and numerous fissure sets in these facies highlights ongoing post-depositional processes, while the abundant extraclats (quartz grains) may confirm a proximal clastic source. Mineralogically, the main constituents of the non-carbonate units from the sedimentary succesion include quartz, calcite, muscovite, pyrite and chlorite, with occasional plagioclase feldspars and kaolinite. The clay fraction is dominated by muscovite and chlorite, with palygorskite and rectorite occurring in several samples. Secondary minerals formed through the exposure of the deposits to the cave environment are aragonite, iron hydroxides, gypsum and natrojarosite. In this context the presence of pyrite suggests reducing depositional conditions, while rectorite may indicate late diagenesis of the deposits. The observed microfacies variations between the lower and upper limestone beds, supported by the compositional features of the non-carbonate units, reveal a shallowing-up tendency for the whole sedimentary succession in the cave. Based on their paleontological and sedimentological features, the limestone beds from the cave are comparable to some of the Priabonian carbonates of the Transylvanian Basin and, more regionally, to many upper Eocene shallow-water ramp-type carbonates of the Neotethys.