Browse

81 - 90 of 3,531 items :

  • Geosciences, other x
Clear All
Llandovery microfossils and microfacies of the Hýskov section, Prague Basin

Abstract

The lower Silurian strata of the Prague Basin, as well as in many other regions, are primarily represented by black shales, with limited to no record of benthic life. In this paper, we describe an exceptional late Aeronian volcanic-carbonate succession from the Hýskov locality near Beroun, which contains a diverse and well-preserved fossil assemblage and specific microfacies features. The studied section is mostly represented by well-washed, poorly sorted skeletal grainstones, rudstones and floatstones along with volcaniclastic material. The environment is interpreted as agitated, above storm wave base, and with rapid sedimentation and burial. The collection of acid-resistant microfossils includes abundant scolecodonts, conodonts and chitinozoans, of which only the chitinozoans have previously been studied. The scolecodonts recovered represent at least 14 polychaete genera and 20 species. The fauna bears a great similarity to the early Silurian assemblages previously reported from Baltica, and some environmentally sensitive species allow constraining the assemblage to shallow-shelf settings. The recovered conodonts similarly show close affinities to Baltic faunas, and suggest a Rhuddanian – Aeronian age for the studied section. The chitinozoan assemblage, containing at least eight species, is dominated by Conochitina, and most likely belongs to the C. alargada Biozone.

Open access
Miocene quartz sands redeposited on subaqueous and alluvial fans during the Saalian: Interpretation of the depositional scenario at Ujście, western Poland

Abstract

The 14-m-thick sandy succession at Ujście in western Poland formed during the Odranian stadial of the Saalian glaciation, is exceptional in being very well sorted, almost mono-fractional (fine-grained sands) and mono-mineral (mainly quartz grains) and in lacking Scandinavian erratics. The lower sequence (5 metres in thickness) consists of three stacked packages of clinoforms (inclined cross-stratified sands) and is interpreted as having been deposited on a subaqueous fan in a shallow lake during two phases of rising water levels. The upper sand (9 metres in thickness) with (sub) horizontal stratification was redeposited on a subaerial alluvial fan. Distinctive distributary channels that occur in the uppermost part of the subaqueous fan and in the lowermost portion of the alluvial fan may indicate a change in sedimentation style from subaqueous to subaerial. Moreover, the subaerial position of the fan supports the presence of ice-wedge casts that developed under periglacial conditions in the upper part of alluvial fan. The results of granulo-metric analysis, rounding and frosting of grains and mineral analysis indicate that the sands are derived from Gorzów Formation of Early Miocene age. The only feasible explanation is that the 14-m-thick unit must have been redeposited during the Saalian glaciation.

Open access
New interpretation of the provenance of crystalline material from Oligocene flysch deposits of the Skole Nappe, Poland: evidence from heavy minerals and clasts in the Nowy Borek section

Abstract

The Futoma Member (Oligocene, Rupelian) of the Menilite Formation is present only in the northern part of the Skole Nappe. Some diatomitic layers of this member in the Nowy Borek section contain coarse-grained detrital material composed of a variety of metamorphic, volcanic and sedimentary rock fragments. The material derives from primary and secondary sources. Most abundant are debris of metamorphic rocks, mostly gneisses and mica schists. The metamorphic origin of these rocks is confirmed by the composition of heavy mineral assemblages and garnet chemistry. These rocks could have been transported from a local source located close to the margin of the Skole Basin or within that basin. The volcanic rocks reflect Paleogene volcanic activity that was widespread in the Carpathian region. Cherts, which could have been subjected to synsedimentary erosion, may have been derived from the older portions of the same formation.

Open access
A new type of slumping-induced soft-sediment deformation structure: the envelope structure

Abstract

The sediments of the Cretaceous Gyeokpori Formation in south-western South Korea accumulated in a lake in which mainly siliciclastic rocks were deposited, with some interbedded volcaniclastics. The nearby volcanic activity resulted in unstable lake margins inducing a dominance of gravity-flow deposits. The high sedimentation rate facilitated soft-sediment deformation on the sloping margin. The deposition of numerous gravity-flow deposits resulted in a vertically heterolithic stratification. The slumps are composed of different lithologies, which is expressed in different types of deformation due to the difference in cohesion between sandy and mussy layers within the slumps. Coarser-grained (cohesionless) slumps tend to show more chaotic deformation of their lamination or layering. The difference in slumping behaviour of the cohesive and non-cohesive examples is explained and modelled.

A unique soft-sediment deformation structure is recognized. This structure has not been described before, and we call it ‘envelope structure’. It consists of a conglomerate mass that has become entirely embedded in fine-grained sediment because slope failure took place and the fine-grained material slumped down with the conglomerate ‘at its back’. The cohesive laminated mudstone formed locally slump folds that embedded the non-cohesive overlying conglomerate unit, possibly partly due to the bulldozing effect of the latter. This structure presumably can develop when the density contrast with the underlying and overlying deposits is exceptionally high. The envelope structure should be regarded as a special – and rare – type of a slumping-induced deformation structure.

Open access
Oligocene turbidite fans of the Dukla Basin: New age data from the calcareous nannofossils and paleoenvironmental conditions (Cergowa beds, Polish–Slovakian borderland)

Abstract

Calcareous nannofossils found in the Cergowa beds of the Dukla and Fore-Dukla tectonic units in the Outer Carpathians indicate a time of deposition in the range of the NP23–NP24 nannoplankton zones of the Lower Oligocene. Nannoplankton assemblages reflect the paleoecological changes at the Eocene–Oligocene transition from: (i) a greenhouse to an icehouse climate; (ii) brackish to normal salinity suggesting open sea conditions, which were controlled by the Paratethys Basin closure followed by opening and connection with the Tethyan Ocean. The absence of nannofossils of NP25 zone, but their presence in the tectonic windows between 40 and 80 km to the west, shows that deposition of the Cergowa beds in the western part of the basin lasted longer than in the east. Occurrences of nannofossils indicating zones NP16 and NP21, found in the uppermost mudstone-rich parts of studied sections, may prove the remobilization and redeposition of sediments of this stratigraphic age. Potentially, eroded material could be derived from some of the following lithostratigraphic units: NP16 — the Hieroglyphic beds, Przybyszów sandstones and Upper variegated shales; NP21 — the Globigerina marls, Mszanka sandstones and sub-Chert marls and shales and/or fine-grained equivalent of these units. Reworked specimens derived from the older Mesozoic strata occur occasionally in various samples.

Open access
Recycling of Paleoproterozoic and Neoproterozoic crust recorded in Lower Paleozoic metasandstones of the Northern Gemericum (Western Carpathians, Slovakia): Evidence from detrital zircons

Abstract

U–Pb (SHRIMP) detrital zircon ages from the Early Paleozoic meta-sedimentary rocks of the Northern Gemericum Unit (the Smrečinka Formation) were used to characterize their provenance. The aim was to compare and reconcile new analyses with previously published data. The detrital zircon age spectrum demonstrates two prominent populations, the first, Late Neoproterozoic (545–640 Ma) and the second, Paleoproterozoic (1.8–2.1 Ga), with a minor Archean population (2.5–3.4 Ga). The documented zircon ages reflect derivation of the studied metasedimentary rocks from the Cadomian arc, which was located along the West African Craton. The acquired data supports close relations of the Northern Gemericum basement with the Armorican terranes during Neoproterozoic and Ordovician times and also a close palinspastic relation with the other crystalline basements of the Central Western Carpathians. In comparison, the detrital zircons from the Southern Gemericum basement and its Permian envelope indicate derivation from the Pan-African Belt–Saharan Metacraton provenance.

Open access
Review of ascocerid cephalopods from the upper Silurian of the Prague Basin (Central Bohemia) – history of research and palaeobiogeographic relationships

Abstract

The order Ascocerida Kuhn, 1949 includes rare and morphologically unique early Palaeozoic cephalopods, in which periodic shell truncation occurred during ontogeny; mature shells subsequently became inflated, with thin sigmoidal septa and phragmocone chambers situated above the living chamber. The ascocerids are at present known mainly from North America and Baltoscandic Europe. The group was first described by J. Barrande in the mid 1800’s from the upper Silurian of Bohemia. Finds of ascocerid fossils in Bohemia are generally scarce but Barrande’s collection includes tens of well-preserved specimens. These are briefly reviewed in the present paper and additional, more recently collected material is also discussed. In Bohemia (Prague Basin), ascocerids occur in limestones of Ludlow to late Přídolí age. Their maximum diversity and abundance was reached close to the Ludlow/Přídolí boundary interval. Five out of the fourteen currently recognized Bohemian species are also known from late Silurian strata in Sweden (the island of Gotland). The ascocerids thus illustrate palaeobiogeographic relationships between the Prague Basin and Baltica during the late Silurian.

Open access
Rhynchonelliform brachiopods and trilobites of the ‘upper dark interval’ in the koněprusy area (Devonian, Eifelian, Kačák Event; the Czech Republic)

Abstract

The dark pelbiodetrital limestone beds of the ‘upper dark interval’ in the Koněprusy area, Central Bohemia, the Czech Republic, the assumed manifestation of the Kačák Event in this area, yielded a moderately diverse faunule of rhynchonelliform brachiopods and trilobites. In total, 15 species have been recognized (13 brachiopods and 2 trilobites), but the majority of them are only tentatively determined to generic level due to fragmentation, rarity or poor preservation. Brachiopod genera Leptaenopyxis, Protodouvillina, Douvillinella, Holynetes, Poloniproductus, Iridistrophia, Mystrophora, Pentamerella, Quasidavidsonia, Carinatina, Mimatrypa, Leptathyris, Eoreticularia and a single trilobite Astycoryphe were determined. The mode of preservation indicates transport of skeletal bioclasts from shallower parts of the basin and their fragmentation in debris flows together with fragments of terrestrial plants, crinoid detritus and numerous pelagic fossils, especially the dacryoconarid Nowakia ex gr. otomari. Associated conodonts (as well as trilobites) indicate a late Eifelian age for the fauna (Polygnathus ensensis Zone).

Open access
Secular and catastrophic processes reflected in sediments of the Suchedniów water reservoir, Holy Cross Mountains (Poland)

Abstract

The Suchedniów water reservoir is located in the central section of the River Kamionka in the northern part of the Holy Cross Mountains of central Poland. This area once belonged to the Old Polish Industrial District that, during the Middle Ages, was very intensively developed by iron metallurgy. Many forges and mills along the rivers used water power, which led to the construction of an anthropogenic, small-scale water retention system. At the beginning of the twentieth century many of these reservoirs were drained after the collapse of metallurgical activities. The present-day reservoir was built in 1974 and drained in 2017. Research into the drained basin has documented various forms and sediments, some of which record present-day depositional processes (fire proof clay layer, inland fan delta), while others represent the historical period (lacustrine sediments of older reservoirs). Traces of catastrophic events have been preserved as well; an assemblage of megaripples marks the sudden drainage caused by a dam break in 1974.

Open access
The siliciclastics/carbonates shift in the Jurassic of the Western Caucasus (central northern Neo-Tethys): reconsidering research over the last 50 years

Abstract

A chain of carbonate platforms evolved in the northern Neo-Tethys during the Late Jurassic, but current knowledge remains incomplete as long as data from several larger regions, such as the Western Caucasus, are not included. In order to fill this gap, it is here suggested to reconsider the information accumulated chiefly during Soviet times. Although these data are too general, they still matter with regard to some regional characteristics and tentative interpretations. Available data on the spatio-temporal distribution of Bajocian-Callovian sedimentary rocks are summarised in a novel way which permits documentation of depositional trends at six representative localities in the Western Caucasus. The extent of the carbonate platform increased at two localities since the Late Callovian and at a third since the Middle Oxfordian. Three additional sites were characterised either by non-deposition or deep-marine sedimentation. The onset of carbonate platform development marked a remarkable shift from chiefly siliciclastic to carbonate deposition, although this event was not sudden everywhere. The Bathonian pulse of tectonic activity, coupled with the eustatic sea level rise, allowed shelves to expand during the Callovian-Oxfordian, with a reduction in siliciclastic input from islands and sea-water that became well oxygenated and warmer. These conditions were conducive to biogenic carbonate production, allowing the carbonate platform to expand subsequently.

Open access