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Mesozoic succession of Mt. Svilaja and signatures of the main geodynamic events

Within the Mesozoic "layer cake" succession exposed on the southwestern slope of Mt. Svilaja, Triassic, Jurassic and Cretaceous sequences were investigated. The succession reveals signatures of the main geodynamic events that took place within the Periadriatic region through the Mesozoic. The Lower Triassic carbonate-siliciclastic, sequence deposited on the Gondvana shelf comprises two units ; (1) the lower unit, that consists of thin-bedded, calcareous and micaceous, fine-grained sandstones, siltstones and silty or calcareous pelites, and (2) the upper unit, that consists of alternating skeletal grainstones, silty and/or argillaceous wackestones to packstones, and marls. The boundary between the Lower Triassic and Middle Triassic is locally marked by the disconformity, i.e. by the "Otarnik" intraformational carbonate breccias and mud-pebble conglomerate that laterally pass into the "Diplopora Limestone". Near the Anisian-Ladinian boundary, the Middle Triassic sequence contains platy pelagic limestones accompanied with vitrocrystalloclastic tuffs ("pietra verde"), cherts, sandstones and shales. The "pietra verde" interval contains bioclastic limestones characterized by an abundance of calcareous algal skeletons, but also fragments of corals, mollusks and brachiopods (JAECKS et al., 2003). It can be interpreted as the result of rift tectonics (JELASKA, 2003) and accompanying basalt magmatism (ŠĆAVNIČAR et al., 1984 ; BELAK, 2000) that indicate progressive separation and differentiation processes on the Gondwana shelf. These limestones represent the termination of the Middle Triassic sequence whose thickness here amounts up to ca. 200 metres. The topmost surface of the Middle Triassic sequence is intensely karstified during the Late Ladinian-Norian (Rhaetian?) emergence phase. On the karstified surface there are some occurrences of bauxites and terrigeneous materials, including conglomerates deposited during the Late Triassic transgression. These conglomerates, with pebbles of Middle Triassic algal limestones, are overlain by an 8 m thick interval of grain-supported limestones. This is followed by a 60 m thick sequence of dolomites with planar and corrugated stromatolitic lamination. By their general appearance, these dolomites correspond to the well-known Alpine Norian– Rhaetian Hauptdolomite. Here it represents the beginning of the shallow-water carbonate deposition on the Adriatic-Dinaridic Carbonate Platform (GUŠIĆ & JELASKA, 1993 ; JELASKA, 2003) that became more evidently separated from the Apulian Carbonate Platform during the Early Jurassic. The Triassic– Jurassic transition is continuous and characterized by ever more frequent intercalations of micritic limestone, which alternate with dolomite. The sequence of the middle Lower Jurassic is composed of thicker, muddy beds in alternation with thinner and grainier ones that clearly reveals coarsening-upward trend. In the upper part of Middle Liassic sequence, few tempestite coquina beds with large Lithiotidae bivalve shells occur. The shells are inserted in carbonate mud and oriented parallel to bedding. The Upper Jurassic sequence is exposed in total thickness of ca. 500 m. This succession can be divided into four units: (1) "Cladocoropsis" wackestones ; (2) “ Lemeš beds” with bioclastic wackestone/packstones to packstone/floatstones ; (3) “ Lemeš beds” with intercalations of chert ; and (4) bioclastic packstone/floatstones to wackestone/packstones. The composition of these units in vertical succession shows that during the Late Jurassic some inner platform parts have been sunken when intraplatform troughs were formed. These troughs were gradually filled with bioclastic material derived from the margins of the trough until the shallow-water conditions were re-established, and continued to exist during the Early Cretaceous. The Lower Cretaceous is exposed in thickness of ca. 830 metres. The Neocomian sequence is represented by mudstones and fenestral mudstones with rare, thin intercalations of Favreina packstones– grainstones. The Barremian is characterized by massive mudstones, mostly with ostracode and variable amounts of pellets and foraminifera. They are intercalated with skeletal-intraclastic packstones-grainstones with algal assemblage that in the upper part of the Barremian sequence becomes frequent. The Lower Aptian sequence is represented by Praeorbitolina/Palorbitolina wackestones and Bacinella floatstones, deposited in somewhat deeper subtidal environments, because of a relative sea level rise. On the southeastern slope of Mt. Svilaja, there are numerous rudists embedded within Lower Aptian bioclastic floatstones which are recognized as caprinid-rudist facies. Above the Lower Aptian sequence, there is a several meters thick interval of algal wackestones, followed by a series of minor or major exposure events or, in places, charophytic wackestones. Both features clearly exhibit regressive trends with fresh water influences and/or subaerial exposure features within Upper Aptian, that is a well-known fact throughout the Karst Dinarides (e.g. VELIĆ et al., 1989). The Upper Aptian - Lower Albian sequence is marked by mudstones that irregularly alternate with peloid-intraclastic-skeletal packstones– grainstones and peloid wackestones– packstones with molluscs and benthic foraminifera (Orbitolina (Mesorbitolina) texana (ROEMER)). The Upper Albian sequence is characterized by mudstones, fenestral mudstones, and rare stromatolites, alternating with peloid-skeletal wackestone– packstones and peloid-bioclastic-intraclastic packstones to grainstones with bivalves, gastropods, benthic foraminifera (“ primitive” orbitolinids and others), hydrozoans, and “ algal balls” . The Lower to Upper Cretaceous transition is marked by a sequence of laminated late-diagenetic dolomites, fenestral mudstones and intraformational breccias. This succession is overlain by Cenomanian deposits characterized by irregular alternation of micritic laminated carbonates, micritic limestones followed by fenestral mudstones and peloid-bioclastic packstone-grainstones with Broeckina (Pastrikella) balcanica CHERCHI et al. and Chrysalidina gradata d’ ORBIGNY.

Carbonate deposits; Triassic; Jurassic; Cretaceous; Mt. Svilaja; Croatia

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