The Triassic in Southwest Bulgaria. Upper Triassic red beds (Moesian Group) in the Golo-bârdo unit

Abstract

The Upper Triassic red beds (Moesian Group) in the Golo-bârdo unit are subdivided into three formations. The basal Slavey Formation is situated over the Upper Triassic dolomitic Rusinovdel Formation, and consists of variegated marls, dolomites, and oolithic, nodular or biodetritic limestones often containing foraminifers, ostracods, algae and coprolites. The foraminiferal assemblage includes Angulodiscus friedli, Nodosaria ordinata and Semiinvoluta clari, and indicates a Norian age. The following Komštica Formation (red shales, siltstones and sandstones) and Vetruška Formation (variegated conglomerates built up of carbonate pebbles) are situated in superposition and with lateral interfingering. The section of the Moesian Group in the Golo-bârdo unit is covered after wash-out and with a slight unconformity by the Middle Jurassic Polaten Formation. The pebbles in the Vetruška Formation come mostly from the Upper Triassic Rusinovdel and Trân Formations, and the Anisian-Ladinian radomir Formation. The pebbles from the Radomir Formation yielded conodonts which prove the conodont zones Pridaella cornuta R. Z., Pridella excentrica I. Z. and Pridaella bakalovi R. Z. The pebble content of the Vetruška Formation hints to the Ljubaš unit (situated at northwest of the Golo-bârdo unit) as the possible source area of the pebbles as far as the Lower Triassic and Lower Anisian formations in this unit are covered directly by Lower and Middle Jurassic formations thus indicating a deep Late Triassic erosion and denudation. This evidence disproves the idea about a complete alochthoneity of the Golo-bârdo unit.

Stratigraphic problems of the Moesian Group (Upper Triassic, peri-Tethyan type), Bulgaria

“…Borima Subgroup (Чаталов, 1984; revised by Тронков in Тенчов, ad., 1993) Vetroushka Formation (Budurov et al, 1993) Chelyustnitsa Formation (this study)…”

“…The southernmost outcrops of psephytic and psammo-psephytic formation of the Moesian Group are outlined in the Golo Burdo unit and introduced as a Vetroushka Formation (Budurov et al, 1993). These outcrops are clearly delimited within a comparatively small palaeo-graben (Fig.…”

“…The formal introduction came two years later (×åìáåðñêè et al, 1974). Later on to the group have been added (×åìáåðñêè et al, 1974;×åìáåðñêè, Âàïöàðîâà, 1975Vaptsarova, Chemberski, 1984;×àòàëîâ, 1984;Budurov et al, 1993; the Borima Subgroup (Strouga and Boulata Members), Ambaritsa, Goren Dubnik, Kobile, Komshtitsa, Kozlodouy, Slavey, Touchenitsa, Veslets and Vetroushka Formation as well as the Glavatsi and Beli Izvor Formation (Òðîíêîâ, Ìîíîâ, 1971), whose status has been questioned (s. Òåí÷îâ, ed., 1993). According to the last revised definition (×åìáåðñêè in Òåí÷îâ, ed., 1993, p. 217), "The group unites formations in whose compositions terrigenouscarbonate rocks are predominant, and they are very typical with their variegated coloration.…”

Introduction to the geology of SW Bulgaria

“…The Komshtitsa Formation (Tronkov, 1968) is built up mostly of red siltstone and shale (rarely marl) that are interbedded with whitish oligomictic quartz sandstone (locally with conglomerate), red carbonate conglomerate to breccia (pebbles from Triassic limestone and dolomite), and rarely in the upper parts, with thin beds of whitish or brownish limestones. Thick masses of carbonate conglomerate to breccia in the Goloburdo unit have been individualised as a Vetroushka Formation (Budurov et al, 1993). The position of these psephytic rocks indicates a differentiation of the marine bottom, with fast subsiding grabens next to island horsts.…”

“…The Golo-burdo unit represented a parautochthonous to allochthonous structure with a probable low-angle decollement at the base. The unit is built up almost entirely (Moes, 1971(Moes, , 1978TpoHKOB, 1975TpoHKOB, , 1981TpoHKOB, , 19833aroplfes et at., 1991, 1994TeulfoB, ed., 1993;Budurov et at., 1993Budurov et at., , 19953aroplfeB, 1994) ofTriassic and Jurassic sedimentary rocks. The principal unconformity has a parallel character, and is observed at the base of the Middle Jurassic Polaten Formation.…”