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Oxfordian to Valanginian palaeoenviron- mental evolution on the western Moesian Carbonate Platform: a case study from SW Bulgaria

Ivanova D., Kołodziej B., Koleva-Rekalova E., Roniewicz E.

Annales Societatis Geologorum Poloniae

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2008

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Vol. 78, No 2

65-90

EN

Three sections (Rebro, Lyalintsi and Velinovo) of the Upper Jurassic-Lower Cretaceous carbonate sequences from the Lyubash unit (Srednogorie, Balkanides, SW Bulgaria) have been studied for elucidation of biostratigraphy and palaeoenvironmental evolution. Palaeontological studies of foraminifera, supplemented by studies of calcareous dinoflagellate cysts and corals, enabled the determination of the Oxfordian-Valanginian age of the analysed sequences. They were deposited on the Dragoman Block (western part of the Moesian Platform), and during Mid-Late Cretaceous included to the Srednogorie. A possible Middle to Late Callovian age of the lowermost part (overlying the Bajocian-Lower Bathonian Polaten Formation) of the studied sections assumed till now has not been confirmed by the present studies. Eleven facies have been distinguished and attributed to depositional environments. Marine sedimentation on a homoclinal ramp started in the Oxfordian and till the Early Kimmeridgian - in all three sections - was dominated by fine-grained peloidal-bioclastic wackestones to grainstones. Since the Late Kimmeridgian, when a rimmed platform established, facies pattern underwent differentiation into (i) the inner platform (lagoon and tidal flat facies) - only in Velinovo, (ii) reef and peri-reef facies/bioclastic shoals - mainly in Lyalintsi, and (iii) platform slope - mainly in Rebro. Sedimentation generally displays a shallowing-upward trend. Two stages in evolution of the rimmed platform are postulated. The mobile stage lasting till the Tithonian/Berriasian boundary was followed by a more stable stage in the Berriasian to Valanginian time. Reefs are developed mainly as coral-microbial biostromes, lower coral bioherms or coral thickets, in the environment of moderate energy and sedimentation. They contain highly diversified corals (72 species). Micro- bialites contributed to the reef framework, but they never dominated. Locally, microencrusters and cement crusts formed important part of reefal framework. During the mobile stage of the platform evolution a relative sea-level rise interrupted reef development, as evidenced by intercalations of limestones with Saccocoma. During the second stage high carbonate production and/or regressive eustatic events, not balanced by subsidence, decreased accommodation space, limiting reef growth and enhancing carbonate export to distal parts of the platform.

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Middle to Upper Jurassic stromatactis mud-mounds in the Pieniny Klippen Belt, Carpathians

Aubrecht R., Krobicki M., Schlogl J.

Volumina Jurassica

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2006

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Vol. 4, no. 1

76-76

EN

Stromatactis mud-mounds are structures still enigmatic, despite of many years of research. Recently, most authors consider stromatactis mud-mounds to be fossil microbial reefs. Particularly enigmatic is the origin of stromatactis structures that appear to be the main megascopic component of the mounds. Stromatactis mud mounds occur since Neoproterozoic time and reach their maximum in Palaeozoic, especially in Carboniferous and Devonian. Mesozoic examples are rare. They were reported mostly from Jurassic; later examples are dobtfull. Our research concerns Jurassic stromatactis mud-mounds. To date, stromatactis structures were only reported from the Early Jurassic of the Upper Austroalpine of the Eastern Alps, Early Jurassic of Sicily, Oxfordian mud-mounds of southern Germany, offshore Nova Scotia, Oxfordian of Cracow Upland, Poland and Lower Kimmeridgian of southern Portugal. Recently, Middle- and Upper Jurassic stromatactis mud-mounds were found in the Czorsztyn Unit of the Pieniny Klippen Belt in Slovakia and Transcarpathian Ukraine: Slavnicke Podhorie, Atepnicka Skala, Babina, Kyjov-Puste Pole, Priborzhavskoe and Velyky Kamenets. Stratigraphic range of the mounds is Bajocian to Lower Tithonian. Geometry of the mounds could only be studied at Atepnicka Skala, Priborzhavskoe and Veliky Kamenets, where flat mound shapes are revealed. Other outcrops show only fragments of the mounds or their shape is merged with the surrounding rocks. Rocks of the mounds are mostly micritic to micropeloidal mudstones, containing fauna of pelecypods, brachiopods, ammonites and crinoids. All the occurrences are characterized by mass occurrence of stromatactis structures. In some of them (Slavnicke Podhorie, Priborzhavskoe and Veliky Kamenets), the stromatactis cavities occur also in the crinoidal limestones underlying the mud-mounds which is an atypical feature. Only three sites investigated by us involve considerable portion of sponge spicules in the mound matrix which contradicts to a theory favouring sponges as stromatactis builders. There was no discernible type of biota that might serve as being responsible for the stromatactis structures. Also biota itself is variable. The Bajocian to Callovian mounds contain more benthic biota, e.g. brachiopods, bivalves, sponges, agglutinated foraminifers etc., although biota as a whole was dominated by planktonic representatives, as shells of Bositra bivalves. The Oxfordian and younger occurrences were fully dominated by planktonic biota, e.g. planktonic foraminifers Globuligerina, planktonic crinoids Saccocoma, ammonites, shells of Bositra or coccal algae Globochaete alpina. The latter type also lacks micropeloidal matrix which is ubiquitous in the first type. The micropeloidal to clotted structures are usually attributed to be typical for microbialites.

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Age, sedimentary environment and palaeogeographical position of the Late Silurian oolitic beds in the Holy Cross Mountains (Central Poland)

Kozłowski W.

Acta Geologica Polonica

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2003

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Vol. 53, nr 4

341-357

EN

A thin sandy-oolitic formation [Jadowniki Formation (JF)] is described from the Upper Silurian of the Łysogóry Unit, Holy Cross Mountains (Central Poland). Its numerous trilobites (e.g. Homalonotusknighti, Acastella spinosa) and position in the lithological column testify for the late Ludfordian age of this formation. The sedimentary environment of the Jadowniki Formation is interpreted as extremely shallow-barrier environments with episodes of emersion and subaerial early diagenesis and erosion. However, these shallows were not connected with the nearshore belt of Baltica land and were separated from them by a belt of deep basin sedimentation - the graptholitic shales. Silurian shallow marine sediments of the Łysogóry Unit were deposited on top of a clastic wedge, which in the Ludlovian infilled the offshore - deep basin of the SW margin of the East European Craton. The wedge was probably connected with the process of dockage of new terranes onto the south-west periphery of the East European Craton. The second factor that led to the development of shallow marine environments was probably the uppermost Ludfordian regressive event.