| Popis: |
The Miocene sedimentary succession of Northern Croatia belongs to the SW margin of the Central Paratethys (Pannonian Basin System). In this study, beside the Lower Badenian sediments of the Plaz locality near village Cucerje (SE Medvednica Mt.) described previously (e.g. Coric et al. 2009), we also investigate underlying, approximately 150 m thick, succession of upper Karpatian to Lower Badenian that extends along the Bokanjscica stream. Upper Karpatian (Bokanjscica stream): The lowermost part of marine sedimentary succession (F--G intervals) consists of fossiliferous calcareous mudrocks (marls), sporadically with more silt-grade particles. These sediments were most probably deposited on the deeper shelf and in the offshore transition zone, below the fair-weather wave base. According to calcareous nannoplankton, Helicolith horizon (NN4 Zone) have been determined. The last appearance datum (LAD) of Helicosphaera ampliamperta is noticed in the sample G where Sphenolithus heteromorphus Paracme ends, that represents a very useful correlation tool defining the Subzone MNN4b. This zone is correlated with the NN4 Zone in the Karpatian successions of the Paratethys (Svabenicka 2002). It is characterised by diverse Helicolith species known as Helicolith horizon with Helicosphaera ampliamperta accompanied with rare (5-10%) small placoliths of genus Reticulofenestra, which is in agreement with our new results (6-10%) and almost completely lacks specimens of other genera such as Discoaster, Umbilicosphaera and Sphenolithus detected as well during the late Karpatian to Karpatian/Badenian boundary. These sediments contain well-preserved foraminifera, whereas ostracods are very rare. Planktonic foraminiferal assemblage consists of numerous specimens of the cool surface water indicators such as Tenuitella munda, Tenuitella angustiumbilicata and Globigerinita glutinata. The frequent occurrence of calcareous benthic foraminiferal infauna I genera Bolivina, Bulimina and Melonis indicate a low oxygen but organic carbon reach sea bottom. Lower Badenian (Bokanjscica stream): The middle part of marine sedimentary succession (intervals AE), 70 m thick, consists also of fossiliferous calcareous mudrocks (mostly marls, sporadically siltstones), deposited on the deeper shelf environment. On the basis of calcareous nannoplankton Lower Badenian Helicosphaera waltrans horizon (NN5 Zone) is determined. The regular occurrence of Sphenolithus heteromorphus observed from samples E to A, together with close records of Helicosphaera waltrans upwards, corresponds to the Mediterranean Subzone MNN5a (Sphenolithus heteromorphusHelicosphaera waltrans). In the Paratethys, it is detected in the lower part of the Helicosphaera waltrans horizont of Early Badenian (Svabenicka 2002). In addition, according to Coric and Rogl (2004), the prevalence of small Reticulofenestra species over Coccolithus pelagicus, which were detected in sample E, characterise transgressive base of the Badenian. The Lower Badenian sediments (samples A-E) contain reach and highly diversified calcareous benthic foraminiferal assemblage dominated by infauna! organic carbon-preferring genera such as Bulimina, and low-oxygen adapted taxa like Bolivina, Uvigerina and Pappina indicating middle shelf environment. Genus Bolivina reaches the peak in abundance in the small size fraction. The first occurrence of the shallow and warm water preferring species like Elphidium, Glabratella and Pararotalia have also been detected. Planktonic foraminiferal association is characterized by cool water taxa (Turborotalita quinque/oba, 10%) and abundant presence of warm surface-water preferring taxa (Cassigerinella chipolensis, 50%). The Lower Badenian ostracod fauna (samples E and D) is dominated by deep marine species: Cytherella compressa, Cytherella aff. vandenboldi, Henryhowella asperrima, Parakrithe dactylomorpha, Krithe sp., Krithe cf. monosteracensis, Cardobairdia sp., Argilloecia acuminata, Paijenborchella iocosa and Macrocypris sp. A few valves of the shallow- water species Costa edwardsi, Loxocorniculum cf. hastatum, Loxoconcha sp., Grinioneis haidingeri, Xestoleberis sp., Tenedocythere sp. and Aurila sp. are also present. The entire ostracod assemblage is well known from the Badenian deposits of the Central Paratethys. Discovery of very rare lndo-Pacific species Paijenborchella iocosa indicates the establishment of connections between the Paratethys Sea, Mediterranean realm and Inda- Pacific Ocean. Palynomorph analyses of this interval show moderately well represented organic matter. The amorphous matter prevails with about 60% share. Lignohumine clasts of kerogen (remnants of higher plants) make about 30% of the palynofacies. Liptinite component which comprises dinoflagellate cysts, different kinds of pollen grains, some spores and many foraminiferal test linings, denotes 10% of the organic residuum. Dinocyst association is relatively abundant and the most frequent species are Systematophora placacantha, Lingulodinium machaerophorum, Melitasphaeridium choano-phorum, Spiniferites sp., Lejeunacysta sp. All these facts indicate sedimentation in marine somewhat deeper and distal environment. Lower Badenian (Plaz section): The lower part of the Plaz section predominantly consists of calcareous mudrocks which are intercalated with layers of sandstones (mostly marls, silty or sandy, greywackes and calcareous sandstones to calcarenites). The upper part of the profile is predominantly composed of sandstones (calcareous sandstones to calcarenites), and to a lesser extent of calcareous mudrocks (marls and siltstones), with volcaniclastic sediments (tufts) on the top. It is a coarsening-upward succession indicating shallowing of the marine environment. Sediments of the Plaz section contain shallow and warm water preferring benthic foraminifera like Elphidium, Asterigerinata, Ammonia and Amphistegina, whereas planktonic foraminifera are absent or very rare. In the sample Plaz 4 shallow-water ostracod species with ornamented valves are present. These include species Cnestocythere lamellicosta, Cytheridea acuminatq, Olimfalunia spinulosa, Loxocorniculum hastatum, Aurila punctata, Aurila sp. and Xestoleberis g/abrescens. Echinoderms, bryozoans and molluscan carbonate shell fragments are also present. Planktonic foraminifera are absent or very rare. Macerates of samples of the Plaz section (samples 0-10) are rich in organic matter. Larger, mostly biostructured lignoghumine clasts dominate with about 60% share, while amorphous matter makes about 30% of the palynofacies. Liptinite kerogen component makes about 10% of the organic content and it is composed of diverse association of spores, pollen grains, foraminiferal test linings, dinocysts and fragments of colonial green alga Botryococcus braunii. Following dinocysts are most frequent in palynoassociation: Opercu/odinium centrocarpum, Opercu/odinium sp., Hystrichosphaeropsis obscura, Hystrichokolpoma rigaudiae, Melitasphaeridium choanophorum Polysphaeridium zoharyi, Lingulodinium machaerophorum, Spiniferites sp. and Selenopemphix sp. Samples are Badenian in age and sedimentation took place in marine but somewhat shallower and more proximal environment in reach of terrestrial input. The upper Karpathian sediments of the investigated section correspond to the Mi2 cooling event that induced climate instability and larger temperature oscillations. Almost all warm water species has scattered occurrences throughout that time. During Early Badenian warmer climate and the Middle Miocene Climatic Optimum (MMCO) was re-established, but with more fluctuating environment. |
