Demise of the Wetterstein Carbonate Platform and onset of the Dachstein Carbonate Platform recorded in deep-water successions of the East Bosnian–Durmitor megaunit (Pliješevina, northern Montenegro, Dinarides)
Abstract: During the latest Ladinian to earliest Carnian, the Wetterstein Carbonate Platform evolution led to the formation of the first rimmed platforms in the Western Tethys Realm after the Permian/Triassic mass extinction. The overall demise of the Wetterstein Carbonate Platform is related to the Lunz event (Reingraben event, Mid-Carnian Pluvial Episode or Mid-Carnian Wet Intermezzo – Julian 2). However, several questions remain open when comparing the platform demise in the Eastern Alps, the Southern Alps, and the Western Carpathians (ALCAPA) with its demise in the Dinarides/Albanides/Hellenides, where these siliciclastics are practically unknown, except the Outer Dinarides in Croatia or Montenegro (High Karst). Prior to the drowning of the Wetterstein Carbonate Platform in the ALCAPA region with siliciclastics, the platform emerged due to a sea-level drop around the Julian 1/2 boundary (Carnica event). The underfilled accommodation space between the platforms is characterized by restricted deep basinal areas with deposition of organic-rich siliceous limestones, followed by the deposition of fine-grained siliciclastics (Reingraben claystones). In contrast to the ALCAPA region a long-lasting stratigraphic gap is common in the Carnian in the Dinarides/Albanides/Hellenides. Carbonate deposition resumed during the Late Carnian after the demise and uplift of the Wetterstein Carbonate Platform. In northern Montenegro, near the village Pliješevina, the final demise of the Wetterstein Carbonate Platform around the Julian 1/2 boundary can be dated by conodont faunas (Carnica conodont zone) in a newly detected Carnian basinal sequence. Above the fine-grained resediments of the Wetterstein Carbonate Platform (Zlošnica Formation), less then 3 m of grey “filament”- and radiolarian-rich biomicrites were deposited, followed upsection by a ~6 m-thick sequence of siliceous claystones, black cherts and silicified volcanic ashes (Džegeruša Formation). The carbonate-free intercalated metabentonites of Julian 2 to Tuvalian age are composed of (biogenic) quartz, and clay minerals of the mica-group (mainly illite), montmorillonite, the smectite group, and mixed layer clay minerals. A controversially discussed environmental change around the Julian 1/2 boundary resulted in the demise of the Wetterstein Carbonate Platform and carbonate deposition was replaced by deposition of siliciclastics. Carbonate production reflecting the onset of a precursor of the Dachstein Carbonate Platform evolution started again during the latest Carnian as dated by conodonts.
Detrital zircon age constraints on low-grade sedimentary successions of the eastern Circum-Rhodope Belt, Bulgaria
Abstract: We focused on the eastern Circum-Rhodope belt (CRB) low-grade sedimentary sequences in Bulgaria, in which clastic rocks are presented. U–Pb detrital zircon geochronology indicates the latest Late Jurassic maximum depositional ages of two samples from two distinct locations. Prominent Jurassic zircon cluster in the first sample is consistent with provenance from CRB-related Evros arc, whereas the Triassic zircons come from the high-grade basement. In the second sample, the main Permian and Carboniferous zircon populations, minor Triassic clusters and two Jurassic zircons reflect a provenance mainly from the high-grade basement and to a lesser extent from the Evros arc. These new results indicate latest Late Jurasic sedimentation proximal to the Evros arc (CRB) and along the continental margin of Eurasia (Rhodope), respectively for the studied first and second sample. The results further support the presence of Mesozoic (Jurassic) oceanic lithosphere mantle remnants within the metamorphic basement of the eastern Rhodope Massif. The results obtained shed new light and could open a discussion on the Late Jurassic clastic sedimentation along the transect from the continental margin of Eurasia (Rhodope) towards the Evros arc system of the eastern CRB.
Surviving from transgression to regression of Lake Pannon: Fan deltas of the Nemčiňany Fm. persisted across the rifting until the post-rift stage of the Danube Basin, western Slovakia
Abstract: Successions deposited under rifting and post-rift settings of an isolated epicontinental basin often exhibit contrasting characteristics. Facies linked to transgression during basin rifting are typically locally sourced fan deltas transporting coarse-grained sediment, whereas the post-rift setup generally involves a normal regression marked by more extensive catchments, sediment sourced from greater distances, and an overall decrease in average grain size compared to the transgressive facies. In this study, we present a specific scenario based on a sedimentological, stratigraphic, and authigenic 10Be/9Be analysis of the Nemčiňany Formation, an Upper Miocene fan deltaic succession in the eastern Danube Basin, Slovakia. Deposition of the Nemčiňany Fm. was initiated shortly after ~11.6 Ma by the fourth rifting phase of the Pannonian Basin System, triggering the transgression of Lake Pannon. The depositional system persisted until ~9.6 Ma, likely balancing the increased accommodation rate with sediment supply in the shallow lake area situated on a basement high. Subsequently, a relative decrease in the accommodation rate prompted the progradation of shelf-slope scale clinoforms, originating from the Nemčiňany depositional system, toward the center of the Komjatice depression at ~9.4 Ma, resulting in a normal regression of Lake Pannon. The persistence of the Nemčiňany fan delta system, unlike common stratigraphic patterns, was associated to the high sediment supply yielded by the paleo-Hron river, which entered the basin in the study area. These observations underscore the need for caution in predicting provenance shifts during changes in geodynamic stages of a basin.
Unravelling the tectonic phases: The impact of the South Caspian Block on Late Cenozoic deformation in the Central Alborz, Iran
Abstract: Paleostress reconstruction through polyphase fault-slip data of a multi-deformed region suffered collisional tectonics can lead to detecting the stress phases. Based on earthquakes’ focal mechanisms and morphotectonic features, multiple deformation phases model has been proposed for the Alborz Mountains located in the collision zone between the Arabian (Central Iran) and Eurasian (South Caspian block) plates during the Late Cenozoic. In this study, paleostress analysis has been carried out in an area bounded by two (Kandovan and Taleghan) regional faults in the Central Alborz Mountains using fault-slip data. This analysis resulted in the identification of three main tectonic phases. The first, compressional phase is proposed to cause the inversion of the Alborz Mountains’ major initial normal faults to reverse faults during the convergence of the Arabian and Eurasian plates. The second, transpressional phase is offered as a factor for the reactivation of the hidden Alborz basement faults to form the NE-striking left-lateral strike-slip faults on the sedimentary cover. The third, transtensional phase is suggested to be responsible for the development of the NNE–SSW left-lateral transtensional faults. It is proposed that the second and third paleostress phases are affected mainly by the indentation of the South Caspian Block into the Alborz Mountains during the Late Cenozoic.