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Stop 1. Skrzydlna quarry – olistostrome-bearing succession of the Menilite Beds (Eocene-Oligocene)

Wendorff Marek, Starzec Krzysztof, Siemińska Aneta

Geotourism / Geoturystyka

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2023

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nr 3-4 (74-75)

18--28

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Pseudo-shallow marine features in deep marine gravity-flow successions: lessons from the Menilite Beds at Skrzydlna (Oligocene; Western Outer Carpathians)

Wendorff Marek, Siemińska Aneta

Geotourism / Geoturystyka

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2023

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nr 1-2 (72-73)

79--80

EN

Quite common in publications devoted to the marine and lacustrine environments are differences of opinion on bathymetry of the basin receiving detrital sediment, especially when sedimentary structures are interpreted as indicators of specific depth-defined environments (following classic textbooks). However modern studies of deep-water environments, experimental work, modelling and numerous outcrop studies of ancient successions mitigate against such an approach. In this respect, the flysch succession of the Menilite Beds strata at Skrzydlna, which contains a variety of features that can confuse a cursory observer seems to serve as a universally applicable example. The succession at Skrzydlna records deposition in the western part of the synorogenic Dukla Basin during the Oligocene. The Menilite Beds are considered by most authors as deep marine deposits (the bathyal zone). The exposed section, almost 200-metre thick, is divided into three lithological intervals, each of which represents a radically different type of sedimentation. These are: i) a fine-grained association of terrigeonous and hemipelagic sediments; ii) infill of a canyon incised by about 40–50 m into the underlying strata, wider than outcrop and dominated by an olistostromal succession of debris flows with pebbles, boulders, slide and slump sheets; iii) succession of turbidites forming three fining-upwards sequences and ranging from thick, massive, amalgamated sandstones deposited by high-density flows in laterally migrating outcrop-scale channels, through ‘normal’ turbidites forming complete Bouma sequences (Ta-e), containing dunes and fining to Tce in the uppermost associations of thin-bedded sandstones and shales. These features suggest rapid uplift of the source area resulting in canyon incision and sudden onset of the olistostrome deposition that evolved upwards into proximal turbidite-fan sequence, which subsequently retrograded due to decreasing intensity of supply. The oldest interval (i) consists of predominantly finegrained facies, most characteristic of the Menilite Beds at their regional development. These are dark mudstones deposited in anoxic to dysoxic conditions and containing thin layers of fine-grained sandstones — turbidites with Bouma Tab; Tbc; Tabc; Tabe intervals, a 2–3 m thick intercalation of massive amalgamated sandstone, dark cherts, and locally silicified marls and limestones. The latter contain isolated lenses of medium-grained sandstone current ripple marks indicating three palaeocurrent directions. Two sets represent bipolar distribution of palaeocurrents, typical of shallow sea/ shelf sediments reworked by tidal currents. However, these are interpreted here as the products of tidal currents reworking bottom sediments of the bathyal zone, the case known from contemporary environments. In this context, the third direction, perpendicular to the bipolar flows does not represent reworking by littoral current on shelf but deep marine contour current. The main channel, or canyon (ii) incised into the slope sediments fed the depositional system with olistostrome deposits supplied from the rapidly uplifted source zone. Above there is a thinning upwards, turbidite sequence of four sub-complexes (A-D): A – conglomerate and sandstone fill three laterally migrating narrow, outcrop-scale erosional channels with a maximum depth of 15 m; B – two shallow (up to 2 m deep) distributary channels filled with very thick, massive or normally graded sandstones; C – turbidites Tb, Tc, Tbc with single occurrences of hummocky-like cross stratification and sandstone beds forming dunes at the mouth of distributary channels; D – less ordered interval of thick-, medium- and thin-bedded sandstones interbedded with mudstones, forming various incomplete sequences of Bouma intervals. Interbeds of hummocky-like cross stratification, commonly found on the shelf, are interpreted in the deep-sea environment as the effect of Kelvin-Helmholtz instability or other complex flow processes, e.g. reflections of turbidity currents. A few occurrences of ripplemarks symmetrical in outer shape show unidirectional cross-lamination in cross section. These were modified by erosion that could have resulted from occasional extremely violent storms or flow reflections off channel margins. In spite of the external shape reminiscent of symmetrical ripplemarks these features do not possess the internal structure of composite cross laminae characteristic for oscillatory reworking of sand by prolonged, rhythmic action of waves. Solitary current ripplemarks showing flow directions opposite to the main transport direction are antidunes or deposits of currents reflected/deflected by channel sides. In summary, in spite of geometrical and structural similarity to the features traditionally considered as formed on shelf, the structures described here, assessed in association with facies and evidence referred to in the introductory paragraphs, fall into the category of deposits known also from below the ‘normal’ wave base and below the shelf edge, i.e. in the slope region. Hence from deep-sea environment for which the occurrence of bipolar currents, dunes, hummocky cross-stratification and symmetrical ripplemarks are neither typical nor diagnostic, but do exist.

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Factors controlling a depositional architecture in synorogenic Outer Carpathian basins – an example of Oligocene-age successions from the Fore-Magura Unit, Poland

Siwek Piotr, Waśkowska Anna, Wendorff Marek

Geotourism / Geoturystyka

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2023

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nr 1-2 (72-73)

62--63

EN

A Fore-Magura Unit is strongly tectonically-engaged tectonic unit of the Polish Outer Carpathians, sandwiched between Magura and Silesian nappes. Due to poor and sparse exposure of the Fore-Magura Unit, which is covered by the Magura Nappe, there has been no comprehensive interpretation of depositional systems of the Fore-Magura Basin (Eocene–Oligocene), a part of the Paratethys realm. Therefore, in order to broaden our knowledge about depositional conditions in this part of the Outer Carpathian basins, two turbidite sequences (Szczawa and Klęczany) were subjected to detailed lithofacies and sedimentological analysis. The 100 m thick Szczawa section is predominantly composed of thin and medium thick turbidite sandstones associated with co-genetic turbidite mudstones, which thickness greatly exceeds that of underlying sandstone. The latter ones show another peculiar features, like opposite palaeocurrent directions between base and top of a bed, mud-rich banded and heterolithic structures, and combined-flow bedforms, including small-scale hummocky-type structures. All those sedimentary features reflect deposition from mud-rich low-density turbidity currents enclosed within small confined basin, which prevent each flow from further down-current propagation, and eventually resulted in trapping (ponding) of the whole flow within confinement, a process associated with flow reflections and internal Kelvin-Helmholtz waves propagation (Siwek et al., 2023). This mini-basin can be situated on the southern flank of the Fore-Magura Basin, i.e., on the slope of the Fore-Magura Ridge (Siwek et al., 2023). The 170 m thick succession at Klęczany is composed of thick-bedded amalgamated sandstones, grading into sandstone-mudstone turbidite sequences. The former reflect deposition from high-density turbidity currents and hybrid flows, and are stacked into a few to over ten metres thick tabular lobes, and can be interpreted as lobe axis or distributary channel deposits. These lobes are often topped by socalled ‘bypass’ facies indicating the moment a lobe attained a critical thickness which prevented the accommodation of new deposit, thus heralding a feeder channel avulsion. The recurring process of lobe building and feeder channel avulsion resulted in compensational stacking of subsequent lobes (Piazza & Tinterri, 2020). The upper part of the Klęczany section reflects deposition from low-density turbidity currents and aggradation of turbidite beds into upward-thickening sequences resulting from lateral compensation and/ or forward progradation of subsequent lobes. Considered as a whole, the Klęczany succession is fining upward, and shows decrease of sand net-to-gross, accompanied by increase of more distal facies. Therefore, that depositional system can be situated within single submarine base-of-slope fan featured by retrogradational stacking pattern. Ponded turbidite beds, together with their whole inventory of sedimentary structures, are an evidence of the crucial influence of structural confinement on unrestricted flow propagation on the seafloor. The presence of structural confinement on the basin slope may have been associated with regional compression and tectonic activity of the Outer Carpathian basins. In the case of the Klęczany section, shortterm autocyclicity is manifested in compensational lobe stacking pattern and cyclic feeder channel avulsions. A longterm variability, probably covering the whole Fore-Magura realm, can be identified with one sequence stratigraphy cycle – from forced regression resulting from sea-level falling stage to sea-level lowstand, reflected in the transition from amalgamated massive sandstones to sandstone-mudstone turbidite sequences (Catuneanu, 2006). Alternatively, the uplift-denudation cycle due to tectonic activation of source area (Mutti et al., 2003) can be considered as an explanation of retrogradational stacking pattern of the Klęczany Fan, with eustatic sea-level fall involved (Pszonka et al., 2023). To conclude, the regional and local changes of depositional conditions in deep-water basins can be related to tectonics, as well as to eustatic short- or long-term sea-level changes, or combination of both, and can give the readable rock record in sedimentary successions accumulated especially in synorogenic marginal basins (Pszonka et al., 2023). These include foreland-type Outer Carpathians basins during Oligocene times, which were located in the Central Paratethys isolated from the Tethys Ocean during Eocene-Oligocene geotectonic reconstruction of the Circum-Carpathian realm.

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Lithostratigraphic classification of the Tsodilo Hills Group: a Palaeo- to Mesoproterozoic metasedimentary succession in NW Botswana

Wendorff Marek, Świąder Andrzej

Geology, Geophysics and Environment

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2019

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Vol. 45, no. 4

305--318

EN

The Tsodilo Hills Group strata exposed in the Tsodilo Hills are an association of meta-arenites, metaconglomerates, quartz-mica schists, sandstone, red siltstone and sedimentary breccia deposited on an open siliciclastic marine shelf between the Late Palaeoproterozic and Late Mesoproterozoic, and outcropping in NW Botswana. The succession is dominated by three micaceous quartzite units interlayered with subordinate lenses and wedges of other rock types. Facies gradients from S to N are expressed by: decreasing content of muscovite at all levels of metasediment organisation from thin wedge-shaped units to thick quartzite complexes, as well as a decrease in pebble content and increase in arenaceous matrix in some metaconglomerate beds, matching regional palaeotransport direction. Well-rounded pebbles of extrabasinal rocks are flat, suggesting redeposition from a beach environment. Lenticular conglomeratic bodies with erosional lower boundaries represent infills of local incisions in the sandy bottom sediments. The abundance of laterally discontinuous lithological units reflects shelf palaeotopography controlled and modified by deposition and migration of large bed forms, ranging from megaripple marks (or submarine dunes) to sand waves. Deposition was influenced by tides and two regressive events. The older regression resulted in a marker unit of tidal mudflat-related red-bed facies: mudstone, siltstone, channel-fill sandstone and sedimentary breccia. The second regression is indicated by a tabular conglomerate marker reflecting increased input of coarse terrigenous material.

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Facies architecture of the Cambrian succession at the western margin of Baltica in the Podlasie region (E Poland)

Wendorff Marek

Annales Societatis Geologorum Poloniae

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2019

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Vol. 89, No. 4

453--469

EN

Sedimentary features of the Cambrian-age succession transected in seven borehole cores sited in the Podlasie region document vertical and lateral variations of shallow-marine sedimentary facies, deposited at the rifted western margin of the Baltica Palaeocontinent. The boreholes are distributed along two lines of cross-section (E–W and NE–SW) running roughly perpendicular to the margin of the palaeocontinent. The easternmost borehole represents a proximal setting located on a relatively stable, shallow basement in the east; the remaining boreholes document conditions of deposition in the subsiding shallow-marine basin, extending towards the SW. Fourteen sedimentary facies defined on the basis of their lithological and sedimentary features are interpreted in terms of the sedimentary environments they represent. Strata deposited upon the stable craton in the east document a stratigraphically condensed succession of proximal facies, 240 m thick, whereas a sequence three times thicker is positioned distally, 170 km to the west. Facies associations in the proximal section represent the lower to upper shoreface in the lower part of the section and evolve upwards to the intermediate shoreface. Facies complexes in the remaining, intermediate and distal areas form a symmetrical megasequence, composed of a positive (i.e., fining-upwards – FU) transgressive sequence, overlain by a negative (coarsening-upwards – CU) regressive sequence. The vertical arrangement of the sedimentary subenvironments during the transgression indicates a tidally influenced shoreline followed by oscillations between the swash zone, the upper, intermediate and lower shoreface, and the offshore. The symmetry of the megasequences and the rhythmic pattern of the component facies complexes indicate that the intensity of supply in the terrigenous material and the efficiency of its reworking and redistribution within the basin were similar during the transgression and the regression. The facies types and variations within the basal part of the succession reflect syndepositional movements of tectonic blocks parallel to the rifted basin margin. Differences in total thickness and facies associations between the two lines of cross-section approximately perpendicular to the basin margin indicate that sedimentation was also influenced by a synsedimentary hinge fault, extending in a WSW–ENE direction.