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Stacked Gilbert-type deltas filling an incised palaeovalley along the cratonward margin of a foreland basin (Miocene, Western Carpathian Foredeep)

Nehyba Slavomír

Geological Quarterly

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2022

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Vol. 66, No. 3

art. no. 28

EN

The coarse-grained Brno sands are an important groundwater aquifer and represent a volumetrically significant component of the Lower Badenian/Middle Miocene sedimentary infill of the Western Carpathian Foredeep, a foreland basin formed on the eastern margin of the Bohemian Massif in front of the Western Carpathians. The integration of core and outcrop facies analysis, summary of facies features, depositional processes and architectures, and integration of provenance analysis have led to a new depositional model for the Brno sands as the product of Gilbert-type delta systems prograding into an incised palaeovalley developed on the outer foreland basin margin. The position, shape and morphology of the palaeovalley were influenced by pre-Neogene basement faults trending NW–SE. The Gilbert-type deltas recognized can be classified as foreset-dominated with other elements subordinate. The foreset beds mainly reflect low-density turbidites, with varied but important evidence of deposits of high-density turbidity currents, cohesionless debris flows and debris falls. The thick foreset deposits show, in detail, repetitive successive prograding-retrograding episodes. These are interpreted as rapid high-frequency pulses of relative sea level change i.e. as high-frequency depositional sequences. The deltaic deposition of the Brno sands was terminated by deposits of a shoal-water delta and the sands were finally drowned by a thick pile of offshore pelagic clays. The provenance of Brno sands is located into the nearby geological units. Redeposition from the older basin infill also played an important role.

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High-energy, microtidal nearshore deposits and their provenance (Lower Miocene, Burdigalian/Eggenburgian, Alpine-Carpathian Foredeep, Lower Austria)

Nehyba Slavomír, Roetzel Reinhard

Geological Quarterly

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2022

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Vol. 66, No. 4

art. no. 33

EN

During the Early Miocene (Early Burdigalian/Eggenburgian) marine transgression at the southeastern margin of the Bohemian Massif, gradual flooding occurred along a rocky coast on granitic bedrock of the Thaya Batholith under high-energy, wave-dominated, microtidal and mixed fair-weather and storm conditions. Deposits of the Burgschleinitz Formation overlie a basal unconformity above a subaerial weathered basement surface (transgressive erosional surface) and are interpreted as a transgressive systems tract. The deposits can be divided into four facies associations/depositional environments, i.e., upper-shoreface, foreshore, gravelly beach and backshore/lagoon. Two stages of transgression and successive overtopping of the basement, with different coastal physiographies, were documented. During the initial stage of transgression a barrier island system developed with relatively fine-grained deposits, reflecting the flooding of the distant parts of the Thaya Batholith with a relative flat basement morphology. The subsequent continuation of the transgression led to the flooding of the more proximal parts of the Thaya Batholith with a steeper relief and formation of a rocky shoreline with deposition of gravelly sedi- ments along palaeo-sea cliffs or wave-cut platforms. While gravel clasts of the deposits investigated originate directly from the underlying granites of the Thaya Batholith, provenance studies show that metamorphic rocks of the Moravian Superunit in the hinterland were the main source of sands. This distant source material was probably delivered mainly by small creeks and alluvial fans to the nearshore. Significant differences in heavy mineral composition of the same formation in the wider vi cinity indicate primarily local sources and rapid deposition with subordinate longshore transport, which may reflect a complex coastal palaeogeography. The Lower Miocene deposits of the Burgschleinitz Formation investigated are a rare example of ancient rocky shore deposits, which generally have low preservation potential in the geological record.

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Coastal sandy spit deposits (Lower Burdigalian/Eggenburgian) in the Alpine-Carpathian Foredeep of Lower Austria

Nehyba Slavomír, Roetzel Reinhard

Geological Quarterly

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2021

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Vol. 65, No. 4

art. no. 50

EN

In the type-area of the Eggenburgian regional stage (Lower Burdigalian) sands with large-scale clinoforms were studied north-west of Eggenburg (Lower Austria). Stratigraphic and facies architecture, palaeocurrent pattern and inferred palaeogeographic setting show that these sands are deposits of W-E trending and SW to SSE prograding coastal spit systems, attached to crystalline shoals or islets in the shallow marine Eggenburg Bay. The spits were dominantly formed by shoal parallel accretion above fair-weather wave base due to longshore transport. The 4-5 m thick clinoforms with bottomset, foreset and topset structures contain up to 3.6 m thick, steeply inclined foresets, dominated by sediment gravity-flow deposition. Relatively stable depositional conditions, characterized by strong unidirectional currents, high sand supply and sufficient accommodation space are assumed for their formation. However, internal reactivation surfaces indicate variations in current activity, orientation and velocity. The deposits of spit systems are interpreted as part of a transgressive systems tract. The provenance analysis reveals the local Moravian and Moldanubian crystalline rocks as principal source. The spit sands of the Burgschleinitz Formation show the highest mineralogical maturity within the studied Lower Miocene succession. Intense reworking and redeposition of material from older deposits is evident. Due to the ongoing transgression deposits of the following Gauderndorf Formation and Zogelsdorf Formation exhibit a larger catchment area with input of high amounts of fresh weathered material.

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High (ultrahigh) pressure metamorphic terrane rocks as the source of the detrital garnets from the Middle Jurassic sands and sandstones of the Cracow Region (Cracow-Wieluń Upland, Poland)

Meres S., Aubrecht R., Gradziński M., Sykora M.

Acta Geologica Polonica

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2012

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Vol. 62, no. 2

231-245

EN

The Middle Jurassic (Upper Bathonian/Lower Callovian) sands and sandstones of the Cracow.Wieluń Upland contain detrital garnets with high contents of the pyrope molecule (30.73 mol %). The predominance of detrital pyrope garnets, and inclusions represented mainly by omphacite and kyanite, show that the garnets were derived from high (ultrahigh) pressure (H/UHP) metamorphic terrane rocks (garnet peridotites, eclogites and granulites). Their source is unknown. The Moldanubian Zone of the Bohemian Massif is closely comparable. However, the terranes between this zone and the Cracow-Wieluń Upland are dominated by almandine garnets. The relatively low proportion of almandine garnets in the examined samples indicates that transport of the detrital material could not have been from a far distant source as the garnet assemblage would otherwise be strongly dominated by almandine. A less distant possible source could have been the Góry Sowie Mts., which incorporate UHP/HP metamorphic rocks, but the exposed areal extent of these rocks is too small. It is possible that larger portions of these metamorphic rocks are buried beneath the Cenozoic cover and might have earlier represented a larger source area. Reworking of the entire heavy mineral spectra from older clastics is improbable because of the low maturity of the heavy mineral assemblages (higher proportion of less stable minerals). The source area therefore remains unknown. Most probably it was formed by primary crystalline complexes of lower crust to mantle origin, outcrops of which were not far distant from the area of deposition. Similar detrital garnet compositions were also recorded in the Outer Western Carpathians (Flysch Zone, Pieniny Klippen Belt), i.e. the crustal segments which formed the Silesian and Magura cordilleras; the Czorsztyn Swell was also formed by similar rocks.

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Mysterious high-pyrope detritic garnets in the Middle Jurassic clastics of the Cracow region

Aubrecht R., Meres S., Gradziński M., Sykora M.

Geologia / Akademia Górniczo-Hutnicza im. Stanisława Staszica w Krakowie

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2009

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T. 35, z. 3/1

9-15

PL

W celu rozpoznania składu minerałów ciężkich środkowojurajskich piasków i piaskowców, analizie poddano utwory tego wieku z kilku odsłonięć okolic Krakowa (Dębnik, Dębnik-Czatkowice, Racławice, Paczółtowice). Wykazano w ich obrębie wysoki udział minerałów z grupy granatu, wskazując na duże podobieństwo do minerałów tego typu znanych z zewnętrznych Karpat Zachodnich. Tym niemniej bezpośrednie wskazanie obszaru źródłowego tych minerałów nie jest możliwe, a stwierdzić można jedynie, że ten enigmatyczny region obfitował w granulity, eklogity i perydotyty.