The Miocene Climatic Optimum (MCO) represents a global warm period (approximately 17–14.7 Ma) interrupting a long-term period of Cenozoic cooling. To elucidate if bauxitization took place in ...southeastern European mid-latitude areas during the MCO, we studied a section of undated massive karst bauxite (Crveni Klanac, CK) in central Dalmatia, Croatia, hosted in Upper Cretaceous limestones and overlain by Miocene Sinj Basin lacustrine deposits. Integrated mineralogical, morphological and geochemical analyses indicate the predominant mineral phases of the homogenous bauxite matrix are authigenic, subhedral to euhedral kaolinite and gibbsite. The in-situ mineralization was a consequence of pedogenic processes, indicating the CK bauxites formed autochthonously. In situ U–Pb zircon ages of the lower, middle and upper parts of the CK bauxite are very similar, dominated by Miocene and Oligocene ages, indicating that they all share similar protolith(s). Subsequent high-precision chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) analyses indicate a maximum depositional age (MDA) for the pre-bauxitic material of 16.9576 ± 0.021 Ma (2
σ
uncertainty, incorporating decay constant uncertainty). This MDA, a maximum age of autochthonous bauxitization, coincides with the onset of the MCO. Based on currently available geochronological constraints, the maximum timeframe for CK bauxitization was less than ~ 700 ka, which matches the records of the MCO in paleo-mid-latitude Europe. The potential imprint of pre-17 Ma bauxitization and contribution of older (i.e., Upper Paleogene) bauxite deposits resedimented to the CK profile, as well as degree of potential parautochthonous origin of the CK bauxites, is yet to be investigated. More than simply aligning with regional and local reconstructions of continental climatic conditions during the onset and the early stages of the MCO, the high degree of autochthony of the CK bauxites provide a precise climatic constraint. For in-situ bauxitization to occur in the southeastern parts of mid-latitude continental Europe, paleoclimatic and paleoenvironmental conditions must have had mean annual temperature greater than 17–22 °C and mean annual precipitation of more than 1100–1200 mm.
Mts. Kalnik and Požeška gora volcaniclastic sequences hold valuable information concerning the Miocene syn-rift evolution of the North Croatian Basin, and the evolution of the Carpathian–Pannonian ...Region and the Central Paratethys. We present volcanological, high-precision geochronological, and compositional data of volcanic glass to constrain their tephrochronology, magmatic provenance, and timing of the initial Central Paratethys flooding of the North Croatian Basin. Based on CA-ID-TIMS U–Pb zircon ages (18.060 ± 0.023 Ma for Mt. Kalnik and 15.345 ± 0.020 Ma for Mt. Požeška gora) and coeval
40
Ar/
39
Ar sanidine ages (18.14 ± 0.38 Ma and 18.25 ± 0.38 Ma for Mt. Kalnik and 15.34 ± 0.32 Ma and 15.43 ± 0.32 Ma for Mt. Požeška gora), Mt. Kalnik rhyolitic massive ignimbrites and Mt. Požeška gora rhyolitic primary volcaniclastic turbidites are coeval with Carpathian–Pannonian Region Miocene post-collisional silicic volcanism, which was caused by lithospheric thinning of the Pannonian Basin. Their affiliation to Carpathian–Pannonian Region magmatic activity is supported by their subduction-related geochemical signatures. Although Mts. Kalnik and Požeška gora volcaniclastics are coeval with the Bükkalja Volcanic Field Csv-2 rhyolitic ignimbrites, North Alpine Foreland Basin, Styrian Basin, Vienna Basin, and Dinaride Lake System bentonites and volcaniclastic deposits, reliable tephrochronological interpretations based on comparison of volcanic glass geochemical composition are not possible due to a lack of data and/or methodological discrepancies. Our new high-precision geochronology data prove that the initial Middle Miocene (Badenian) marine flooding of parts of the North Croatian Basin occurred at least ~ 0.35 Ma (during the NN4 Zone) before the generally accepted ~ 15 Ma maximum flooding age at the basin scale, calibrating the timing of the onset of the widespread “mid-Langhian” Central Paratethys flooding.
Reliable reconstructions of tephrostratigraphy and emplacement mechanisms of Early to Middle Miocene volcaniclastic deposits across the Alpine-Mediterranean region may yield important clues as to the ...nature, spread, volume, magnitude and frequency of large silicic eruptions of the Carpathian-Pannonian Region. Here we report on a sequence of Middle Miocene volcaniclastic deposits intercalated with lacustrine strata from the Livno-Tomislavgrad Basin, part of the Dinaride Lake System. We applied a multi-proxy approach to elucidate their source, decipher their emplacement mechanism, and evaluate their basin-scale and regional correlativity. New high-precision zircon geochronology (~ 14.32 Ma) reveals their simultaneity with numerous volcaniclastic deposits (and their alteration products) widely spread across the Alpine-Mediterranean region. Additionally, their correlativity is confirmed at the scale of the Livno-Tomislavgrad Basin, based on similar lithostratigraphy, mineralogy and volcanic glass geochemistry. Newly obtained zircon Hf isotope data imply that Livno-Tomislavgrad Basin distal volcaniclastic deposits and ~ 14.36 Ma Harsány ignimbrite of the Carpathian-Pannonian Region had shared a parental eruptive center. However, different volcanic glass geochemistry, bolstered by the high-precision geochronology, suggests distinct eruption events, implying more frequent explosive volcanism of the Carpathian-Pannonian Region during Middle Miocene than previously recognized. We suggest that the ~ 14.32 Ma fine fallout tephra, deposited in the distal basin in the Dinarides (> 400 km from the source), was likely re-mobilized by water-driven hillside erosion from the extensive paleo-relief, and further transported via land-derived gravity flows. Upon entering the lake, the gravity flows likely transformed into subaqueous sediment density flows. These deposited ~ 1 to 7-m-thick overall graded volcaniclastic turbidites, thinning away from the presumed source of tephra re-mobilization. Although over-thickened, the ~ 14.32 Ma Livno-Tomislavgrad Basin volcaniclastic deposits can still serve as a reliable tephro- and tectono-stratigraphic markers due to their rapid mode of accumulation.
Graphical Abstract
Schematic illustration of the formation of studied ~14.32 Ma volcaniclastic successions of the SE Livno-Tomislavgrad Basin, Dinaride Lake System
Clastic sediments deposited at the bottom of the vertical, nearly 1000 m deep Njemica Cave (Biokovo Mountain, Croatia) were analysed. Owing to the vertical morphology of the cave, the occurrences of ...clastic sediments are sparse. Small, up to decimetre-thick, undisturbed sediment accumulations situated near the siphon lake revealed interesting palaeontological and mineralogical data. These data are used as a useful proxy for discussing depositional processes, the provenance of the sediments and paleo-habitats of the subterranean fauna.
The sub-recent assemblages of ostracods were discovered within the sediment, and they were shown to be correlative to the known endemic species in the wider Dinaric Karst area (Pseudocypridopsis sywulai and Phreatocandona cf. motasi). The well-preserved sediment archive indicates the allogenic origin of the sediment and defines it as an allochthonous infiltrate of the Quaternary. The detritus originates from Pleistocene–Holocene deposits in the surrounding area, whose composition was influenced by volcanic activity in the wide Adriatic region. Sedimentation occurred in a calm aquatic environment by settling from suspension. Therefore the sediments are defined as slackwater deposits.
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•Onset of Carpathian-Pannonian Region silicic volcanism is refined.•∼18.1 and ∼ 17.3 Ma eruptions made regional ignimbrites and reached distal basins.•Complex temporal variabilities ...in mantle-crust interactions are indicated.
The Carpathian-Pannonian Region (CPR) hosted some of the largest silicic volcanic eruptions in Europe during the Early and Middle Miocene, contemporaneously with major lithospheric thinning of the Pannonian Basin. This was recorded as an ignimbrite flare-up event from approximately 18.1–14.4 Ma. To gain in-depth perspectives on the eruption chronology, tephrostratigraphy, and petrogenesis at the onset of CPR silicic volcanism, we applied a multi-proxy approach to Lower Miocene rhyolitic ignimbrites and pyroclastic fall deposits from the northern CPR to the Dinaride Lake System. High-precision zircon U-Pb geochronology distinguished two Lower Miocene groups of volcaniclastic rocks at ∼ 18.1 Ma and ∼ 17.3 Ma. Based on combined tephrostratigraphic signatures we propose that the ∼ 18.1 Ma Kalnik and ∼ 17.3 Ma Eger eruptions produced widespread (intermediate to) large caldera-forming massive rhyolitic ignimbrites, deposited across northern and southwestern regions of the CPR. Due to easterly winds that carried volcanic ash hundreds of kilometers to the southwest, Eger eruption products also reached distal intra-montane Dinaride lacustrine basins, recorded as pyroclastic fall deposits. Heterogeneous major and trace elemental compositions of ∼ 18.1 Ma volcanic glass shards suggest that the Kalnik eruption was sourced from complex silicic magmatic systems, with simultaneous tapping of two discrete melt bodies during the eruption. The homogeneous geochemical composition of ∼ 17.3 Ma glasses is distinct from the older glasses. Integrated zircon and bulk glass Nd-Hf isotope compositions have a positive correlation, defining a regional mantle array, and are more radiogenic in the younger phase of volcanism. The recorded systematic isotopic change, moving from older more crustal signatures to younger more juvenile compositions, imply that during the period of lithospheric thinning of the Pannonian Basin the region underwent more complex variations in the interaction between metasomatized lithospheric mantle-derived magmas and various crustal components than previously recognized.
Extensive and protracted volcanism in the Carpathian-Pannonian Region climaxed during the Early to Middle Miocene with series of major ignimbrite-forming eruptions with their products dispersed ...across the Alpine-Mediterranean Region. The spatial and temporal dispersion of these volcanic horizons make them potentially important stratigraphic markers throughout the region, allowing better understanding of temporal and spatial changes in depositional environments and paleoclimate. Several of these Middle Miocene volcaniclastic layers are preserved in stratigraphically and environmentally variable sedimentary facies on Mt. Medvednica, located in the North Croatian Basin. In order to decipher the age, depositional environment and provenance of two volcaniclastic horizons intercalated within Central Paratethys marine sediments on Mt. Medvednica, we applied an integrated approach of volcanological, geochronological, and paleontological analyses. New high-precision zircon geochronology and volcanic glass geochemistry data allow to distinguish two primary rhyolitic volcaniclastic horizons derived from distinct eruptions, “Plaz“, and the “Bidrovec“, dated at 14.937 ± 0.012 Ma and 14.835 ± 0.012 Ma. Distinguished mineralogical and geochemical data enabled the correlation of the older (“Plaz”) horizon on Mt. Medvednica with the Demjén eruption, one of the six major Early–Middle Miocene ignimbrite-forming eruptions of the Carpathian-Pannonian Region. However, a correlation of the younger (“Bidrovec“) horizon and assignment to a specific eruption could not be established due to a lack of compositional data from coeval eruption products throughout the region. The newly gathered data establishes both “Plaz” and “Bidrovec” pyroclastic deposits as valuable marker horizons for regional reconstructions, and enable a better understanding of the eruption chronology and tephrostratigraphy of the Carpathian-Pannonian Region.
•Mt. Medvednica volcaniclastic horizons were emplaced at ∼14.94 Ma and ∼ 14.84 Ma.•Primary volcaniclastic deposits were deposited in Central Paratethys environments.•∼14.94 Ma horizon can be related to Demjén eruption of the Carpathian-Pannonian Region.•∼14.84 Ma horizon is coeval with several regionally spread volcaniclastic horizons.•Better understanding of the regional widespread eruption history and tephrostratigraphy is enabled.
Karst bauxite deposits in the North Dalmatian piggyback basin (NDPGB) are a part of the Mediterranean bauxite belt, which is the largest European bauxite deposit zone; however, there is a general ...lack of information regarding the genesis, age, and precursor of the bauxite deposits in this region. In this study, we combined detrital zircon U–Pb geochronology with compositional, mineralogical, and morphological data from four bauxite locations in the NDPGB to provide a new palaeogeographical and palaeoenvironmental evolution model for the Lutetian–Rupelian timeframe of the NDPGB. The Eocene climatic conditions began with the Palaeocene–Eocene Thermal Maximum event (∼56 Ma), followed by the Early Eocene Climatic Optimum (∼49 Ma) and Middle Eocene Climatic Optimum (∼40 Ma), and were completed as a cooling trend culminating around the Eocene/Oligocene boundary (∼34 Ma), with a shift towards an icehouse climate. These events were coeval with the continuous drift of the African continent towards Eurasia and the subsequent closure of the western part of the former Neo-Tethys Ocean associated with massive volcanic activity. Based on the bauxite deposits of the NDPGB, Early Eocene limestones formed in the last phase of the long-lasting Adriatic Carbonate Platform. The Middle Eocene orogenic activity resulted in an elevation in this area. High average temperatures, accelerated hydrological cycles and precipitation, and intensive continental weathering with increased volcanic carbon input resulted in favourable conditions for the development of karst bauxites at this time. Further Upper Eocene tectonic deformation of the NDPGB area resulted in the development of bauxite traps and enabled redeposition of the initial bauxite material. Subsequently, the bauxite deposits were covered with clastic carbonate molasse derived from the intensive erosion of the young Dinaric orogeny. The implications of this study are as follows. First, it provides new information on the timing of bauxitisation in the area by providing the first radiometric zircon geochronology, which refined and restricted the time window for bauxite formation in this region. Additionally, our results provide a new perspective on the possibility of aeolian precursors in karst bauxite formation and provide new constraints on the first tectonic marks of the initial Dinaric orogeny.
The metamorphic conditions and evolution of the Palaeozoic-Mesozoic metamorphic complex of Medvednica Mountain (Zagorje-Mid-Transdanubian zone, Croatia) are still a matter of debate. The results of ...the investigation of five samples of metapelitic schists with the mineral association of quartz, white mica and chlorite are presented. The studied schists are part of the continental margin of Adria and were metamorphosed under upper greenschist- to amphibolite-facies conditions. The focus of this study is a sample representing the highest metamorphic grade that additionally contains chloritoid blasts. Pressure-temperature pseudosection modelling together with classical geothermobarometric calculations yielded peak metamorphic conditions of 0.94 ± 0.05 GPa and 550 ± 20 °C for chloritoid schist. Monazite in-situ U-Th-total Pb electron microprobe dating indicates two metamorphic events at 167 ± 2 Ma and 143 ± 2 Ma, which are interpreted as the time of monazite growth during two distinct metamorphic phases. The formation of the chloritoid paragenesis is related to the older event (around 167 Ma) and linked with the Middle Jurassic subduction-accretion processes of Neotethys-derived ophiolitic lithologies. The younger metamorphic event (around 143 Ma) is related to the obduction of ophiolites onto the continental margin of Adria.
Badenian (Middle Miocene) transgressive deepening-upward successions located in the NE part of Mt. Medvednica (North Croatian Basin, Pannonian Basin System) unconformably overlie Mesozoic basement. ...Triassic and Upper Cretaceous limestone pebbles, cobbles, and boulders of the Badenian basal conglomerates display abundant in situ bivalve borings of
Gastrochaenolites
and sponge borings of
Entobia
. This
Gastrochaenolites
-
Entobia
ichnoassemblage is related to the
Entobia
subichnofacies of the
Trypanites
ichnofacies, characterizing littoral rocky-shore environments (wave-cut platforms and marine transgressive surfaces with a low or null rate of sedimentation).
Gastrochaenolites torpedo
,
Gastrochaenolites lapidicus
, and
Entobia
recorded in Badenian basal conglomerates (compared with modern Northern Adriatic rocky-shore environments), enabled more precise palaeoenvironmental interpretations. The occurrence of
G. torpedo
(produced by lithophaginid bivalves) on all sides of individual limestone lithoclasts in the Gornje Orešje basal conglomerate, coupled with truncation of the formation (possibly indicating multiphase colonization), reflect gravel transport, roll-over, overturning and erosion by wave action in high-energy rocky-shore settings. Gornje Psarjevo-2 basal conglomerate boulders were probably not subjected to significant movement and abrasion, as suggested by good preservation of both
G
.
lapidicus
(potentially produced by gastrochaenid bivalves), associated
G
.
torpedo
, as well as abundant shallow
Entobia
borings. The Badenian
Gastrochaenolites
-
Entobia
ichnoassemblage also could be the result of a composite development. However, direct cross-cutting relationships between
G
.
torpedo
and
G
.
lapidicus
and/or
Entobia
were rarely observed. In addition, Badenian boring tracemakers might have coexisted at the same water depth. Northeast Mt. Medvednica Badenian successions probably formed during different Central Paratethys transgressive pulses (NN5 and NN6 Zones). However, exact timing of Badenian transgressions, stratigraphic correlations and tectono-eustatic implications are unresolved, due to sparsely integrated biostratigraphic and high-precision geochronological studies of Early–Middle Miocene North Croatian Basin deposits and due to the absence of a uniform biostratigraphic zonation and regional chronostratigraphic division of Central Paratethys.
Rocky shorelines in the geological record, which represent major transgressive surfaces and provide us with crucial information about paleoshorelines and ancient sea levels, often involve a basal ...conglomerate associated with an unconformity. The unconformity between the Mesozoic basement and the overlying Middle Miocene (Badenian) deposits (which belong paleo-geographically to the southwestern margins of the Central Paratethys, and geotectonically to the Pannonian Basin System) in the Gornje Orešje section (northeast Mt. Medvednica, northern Croatia) is marked by basal Badenian conglomerates. The Upper Cretaceous limestone lithoclasts occurring in basal conglomerates show abundant truncated
Gastrochaenolites
and
Entobia
borings (represented by an in situ rocky substrate community of bivalves and sponges, respectively), with
Gastrochaenolites
being the dominant ichnogenus.
Gastrochaenolites
-
Entobia
ichnofossil assemblage related to the
Entobia
subichnofacies and in turn assignable to the
Trypanites
Ichnofacies, is very typical of Neogene rocky shores. This association characterizes littoral rockground environments, indicating wave-cut platforms and marine flooding surfaces (transgressive surfaces) with a low or null rate of sedimentation. Erosion of a pre-existing Mesozoic basement rocky shore during a marine transgressive phase in these high-energy littoral conditions, which formed basal conglomerates analyzed here, is also evidenced by truncation and the occurrence of
Gastrochaenolites
borings on all sides of limestone clasts. Calcareous red algae encrusting bored lithoclasts and often-forming rhodoliths also imply that the basal conglomerates occur at the start of a deepening-upwards succession of a transgressive systems tract deposits. The biostratigraphic correlation of the transgressive deepening-upward sequence from the Gornje Orešje section (ascribed to a single Badenian transgressive event that affected the Mesozoic basement) is based on the presence of the orbulinas in the uppermost outer shelf marl interval. The co-occurrence of
Praeorbulina glomerosa circularis
and
Orbulina suturalis
points to the short interval between FO (first occurrence) of
Orbulina suturalis
and LO (last occurrence) of
Praeorbulina glomerosa circularis
. These samples belong to the plankton zone M6 (
Orbulina suturalis
Zone) or the base of NN5 zone, marking the third-order sequence TB 2.4, which represents the second, main Badenian transgression in the Central Paratethys.
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