Luka Gale

Paleogeographic significance of Upper Triassic basinal succession of the Tamar Valley, northern Julian Alps (Slovenia)

Abstract The Julian Alps (western Slovenia) structurally belong to the eastern Southern Alps. The Upper Triassic succession mostly consists of shallow water platform carbonates of the Dolomia Principale-Dachstein Limestone system and a deep water succession of the Slovenian Basin outcropping in the southern foothills of the Julian Alps. In addition to the Slovenian Basin, a few other intraplatform basins were present, but they remain poorly researched and virtually ignored in the existing paleogeographic reconstructions of the eastern Southern Alps. Herein, we describe a deepening-upward succession from the Tamar Valley (north-western Slovenia), belonging to the Upper Triassic Tarvisio Basin. The lower, Julian-Tuvalian part of the section comprises peritidal to shallow subtidal carbonates (Conzen Dolomite and Portella Dolomite), and an intermediate carbonate-siliciclastic unit, reflecting increased terrigenous input and storm-influenced deposition (Julian-lowermost Tuvalian shallow-water marlstone and marly limestone of the Tor Formation). Above the drowning unconformity at the top of the Portella Dolomite, Tuvalian well-bedded dolomite with claystone intercalations follows (Carnitza Formation). The latter gradually passes into the uppermost Tuvalian–lowermost Rhaetian bedded dolomite with chert and slump breccias, deposited on a slope and/or at the toe-of-slope (Bača Dolomite). Finally, basinal thin-bedded bituminous limestone and marlstone of Rhaetian age follow (Frauenkogel Formation). The upper part of the Frauenkogel Formation contains meter-scale platform-derived limestone blocks, which are signs of platform progradation. The Tarvisio Basin may have extended as far as the present Santo Stefano di Cadore area, representing a notable paleogeographic unit at the western Neotethys margin.

DUOSTOMINIDAE (FORAMINIFEREA, ROBERTINIDA) FROM THE UPPER TRIASSIC BEDS OF THE SLOVENIAN BASIN (SOUTHERN ALPS, SLOVENIA)

Twelve species of genera Diplotremina Kristan-Tollmann, 1960, Duostomina Kristan-Tollmann, 1960, and Variostoma Kristan-Tollmann, 1960 have been described from the Norian and Rhaetian strata of the Slovenian Basin (eastern Southern Alps). A lamellar wall structure has been confirmed for Duostomina and observed for the first time in Variostoma. The multi-layered nature of the lamellae of Diplotremina and Duostomina may be a result of a diagenetic alteration of the single-layered lamellae. The position of Variostoma in the family Duostominidae remains doubtful due to the alveolar nature of its wall.

Carnian reefal blocks in the Slovenian Basin, eastern Southern Alps

The Carnian Amphiclina beds of the eastern Southern Alps mostly consist of shale and sandstone deposited in the deep-marine Slovenian Basin, which was located near the western margin of the Meliata Ocean. In the vicinity of Cerkno (Slovenia), blocks of massive or crudely bedded limestone crop out within a succession of clastic rocks that are several hundred meters thick. Sponge-microbialite boundstone and coral pillarstone are the predominant facies within these blocks. Oncoid floatstone-rudstone and moderately sorted peloid packstone form crudely layered parts of the blocks, whereas intraclast-cortoid packstone and poorly sorted peloid packstone occur locally in cross-stratified thin beds, truncated at block margins. Detailed mapping further shows that limestone blocks form units at discrete stratigraphic levels within shale and that these units are variable in thickness. Whereas the largest blocks mostly lie concordant to the bedding, smaller blocks are poorly sorted and randomly oriented. All of this evidence suggests that the limestone blocks are olistoliths, derived from an outer platform margin and/or uppermost slope.

FIRST EVIDENCE FOR LATE NORIAN PROGRADATION OF JULIAN PLATFORM TOWARDS SLOVENIAN BASIN, EASTERN SOUTHERN ALPS

The Late Triassic in the Southern Alps is marked by an extensive growth of carbonate platforms. Whereas Dolomia Principale dominates the western and central sections, Dachstein Limestone dominates the eastern side of the Southern Alps on the so-called Julian Platform in what is now NW Slovenia. Younger tectonic movements greatly deformed the original configuration of the Julian Platform and its margins are consequently poorly preserved. While late Tuvalian and early Norian platform progradation has been recorded on the northern and eastern side of the platform, no such information is available for the southern edge of the platform, where it borders the deeper Slovenian Basin. Three detailed sedimentological sections from the southern slopes of the Jelovica plateau span from the top of the basinal Baca dolomite to the Slatnik Formation and the prograding slope of the Dachstein Limestone. The progradation has been dated as late Norian in age, and can be correlated to a poorly-expressed coarsening event recorded in previously known sections of the Slovenian Basin. In contrast to deeper parts of the basin, shallow water conditions were established on the Jelovica plateau by the beginning of the Rhaetian.

Sedimentary record of subsidence pulse at the Triassic/Jurassic boundary interval in the Slovenian Basin (eastern Southern Alps)

Abstract In the Alpine Realm the Early Jurassic is characterized by the disintegration and partial drowning of vast platform areas. In the eastern part of the Southern Alps (present-day NW Slovenia), the Julian Carbonate Platform and the adjacent, E-W extending Slovenian Basin underwent partial disintegration, drowning and deepening from the Pliensbachian on, whereas only nominal environmental changes developed on the large Dinaric (Friuli, Adriatic) Carbonate Platform to the south (structurally part of the Dinarides). These events, however, were preceded by an earlier - and as yet undocumented extensional event - that took place near the Triassic/Jurassic boundary. This paper provides evidence of an accelerated subsidence from four selected areas within the Slovenian Basin, which show a trend of eastwardly-decreasing deformation. In the westernmost (Mrzli vrh) section - the Upper Triassic platform-margin - massive dolomite is overlain by the earliest Jurassic toe-of-slope carbonate resediments and further, by basin-plain micritic limestone. Further east (Perbla and Liščak sections) the Triassic-Jurassic transition interval is marked by an increase in resedimented carbonates. We relate this to the increasing inclination and segmentation of the slope and adjacent basin floor. The easternmost (Mt. Porezen) area shows a rather monotonous, latest Triassic-Early Jurassic basinal sedimentation. However, changes in the thickness of the Hettangian-Pliensbachian Krikov Formation point to a tilting of tectonic blocks within the basin area. Lateral facies changes at the base of the formation indicate that the tilting occurred at and/or shortly after the Triassic/Jurassic boundary

Middle Jurassic limestone megabreccia from the southern margin of the Slovenian Basin

The Slovenian Basin is a Mesozoic deep-water paleogeographic unit, located along the border between the eastern Southern Alps and the Dinarides, that records geodynamic signals from the opening of both the Piedmont-Liguria and the Neotethys oceanic domains. In the Middle Jurassic, it was bordered by the Dinaric (Adriatic) Carbonate Platform to the south and the Julian High submarine plateau to the north. The southern margin of the basin is characterized by a several tens of meters thick sedimentary sequence of Bajocian-Bathonian (Callovian?) age that is dominated by limestone megabreccia shed from the Dinaric Carbonate Platform, sedimented by debris-flows in a toe-of-slope sedimentary environment. It is accompanied by rud/grain/packstone beds sedimented via (high-density) turbidity-flows. This megabreccia unit represents the proximal equivalent of the lower resedimented limestones of the Tolmin Formation. The matrix within lithoclasts indicates resedimentation from ooidal shoals and the erosion of basinal and slope sediments. Lithoclasts are of Norian to Lower Jurassic age, and originated from (A) platform margin carbonates, i.e. Triassic marginal reef and Lower Jurassic sand-shoal limestones, (B) deep open-shelf or slope coarse bioclastic limestones, and (C) older basinal rocks. The lithoclast analysis enables the reconstruction of the platform-basin transitional zone that is not preserved (exposed) due to overthrusting. The limestone megabreccia indicates enhanced tectonic activity causing repeated collapse of the platform margin, probably connected to the initiation of intraoceanic subduction within Neotethys followed by ophiolite obduction onto the eastern distal margin of the Adria.

MORPHOMETRIC APPROACH TO DETERMINATION OF LOWER JURASSIC SIPHOVALVULINID FORAMINIFERA

Siphovalvulina is among the first foraminifera that appear on carbonate platforms of the Lower Jurassic, forming a conspicuous element of low-diversity assemblages prior to recovery after the end-Triassic biotic crisis. The high morphologic variability of species of this genus is usually not documented, which makes the determination of species difficult and subjective. We demonstrate the variability in five morphotypes of Siphovalvulina in Sinemurian and Pliensbachian carbonate rocks from the Dinarides and the Southern Apennines. Due to the different interpretation of its architecture, an emendation of the genus Siphovalvulina is proposed. One morphotype has been left in open nomenclature and could belong to either S. variabilis Septfontaine or to S. beydouni BouDagher-Fadel & Noujaim Clark. Three morphotypes, differing in apical angle and/or size belong to S . ex gr. gibraltarensis BouDagher-Fadel, Rose, Bosence & Lord. We also describe a new genus and species, Radoicicina ciarapicae gen. n., n. sp. from the lower Sinemurian of the Southern Apennines. We suggest a close phylogenetic relationship between the two genera and introduce a new family, Siphovalvulinidae fam. n. of the superfamily Eggerelloidea.

Biotic composition and microfacies distribution of Upper Triassic build-ups: new insights from the Lower Carnian limestone of Lesno Brdo, central Slovenia

The architecture and composition of Middle to lower Upper Triassic platforms is often obscured by dolomitization. Hence, comparatively little is known about their architectures compared to their size and geographic extent. An active quarry near Lesno Brdo (central Slovenia) offers an excellent exposure of Lower Carnian (Julian) massive limestone, which is diagenetically little altered. A detailed microfacies analysis along a 15.5-m log revealed the presence of three facies types: fine-grained limestone as a groundmass, blocks and globular masses of sponge-microbialite boundstone, and lens-like aggregations of polychaete (terebellid) tubes. Sponge-microbialite boundstone contains a rather small number of cosmopolitan sponge taxa, solenoporacean red algae, microproblematica, bryozoans, and a small proportion of dwelling fauna. Instead, stromatolites represent the main constituent. While some blocks appear to have truncated margins, others show mammillary-like protrusions of microbialites into the surrounding sediment, suggesting active growth of microbialite-producing organisms. Aggregations of terebellid worm tubes show a highly irregular relief, with tubes placed sub-parallel to the ancient sea floor. The presence of fibrous rim cement, crystal silt, and in some cases fragmentation of the tubes, suggest at least moderately energetic waters. Aggregations are thus interpreted as preserved in situ, but not in toto. The entire complex was probably deposited at the margin or upper slope of a carbonate platform. Although the presence of a large number of terebellids associated with microbialites boundstone may indicate some sort of environmental stress, such a stress remains to be identified.

Characterization of silicifid fossil assemblage from upper Carnian “Amphiclina beds” at Crngrob (central Slovenia)

The village of Crngrob (central Slovenia) is known to geologists for the silicified fossils found in the thick soil covering the “Amphiclina beds”, a Carnian lithostratigraphic unit of the Mesozoic Slovenian Basin. Due to silicification, the assemblage may provide an important insight into Carnian marine life; however, the nature of the fossil assemblage, as well as the timing and mode of silicification are unknown. A detailed sedimentological section, which covers the uppermost part of the “Amphiclina beds” and their transition to the “Bača Dolomite”, was recorded at one of the fossil-bearing localities. The section consists of limestone (including bioturbated filament and radiolaria-filament wackestones, laminated filament packstone, bioclast wackestone, peloid packstone, floatstone with intraclasts and bioclasts, and intraclast floatstone and rudstone), marlstone and dolomite. Chert is locally present in nodules or as dispersed silicified patches, giving a speckled appearance to the host rock. According to our interpretation, the deposition of the limestone occurred via hemipelagic settling, turbidite currents and debris flows in a slope-to-basin or outer ramp setting. The composition of the grains (which include green algae fragments, thick-shelled bivalves, corals and solenoporacean algae) clearly points to the allochthonous nature of the material, which was largely derived from shallow-water environments. Fossils, as well as the intraclasts and sometimes the matrix, are locally replaced by chalcedony and granular megaquartz. This type of silicification does not appear to be selective to particular microfacies. As the silicification occurred after the down-slope transport of shells and the deposition of the sediment, the Crngrob fossil assemblage represents a thanatocoenosis or even a taphocoenosis, and it is not representative of autochthonous Carnian associations.