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Detection of near-surface geological heterogeneity at Starunia palaeontological site and vicinity (Carpathian region, Ukraine) based on microgravity survey

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Porzucek S. , Madej J.

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tom Vol. 79, No 3

365-374

EN

Results of a microgravity survey performed in the abandoned Starunia ozokerite mine (Carpathian region, Ukraine), where in the early 20th century well preserved remains of large, extinct mammals were found, are discussed in the paper. A number of gravity anomalies indicating the geological heterogeneity at the sub-Quaternary strata have been observed. The assumed measurement observations also enabled the authors to interpret the results in view of density changes in the Quaternary strata. Most of the registered microanomalies coincide with the high-halite and ozokeritic Miocene salt-bearing Vorotyshcha beds. The distribution of the microanomalies reveals neither their correlation with the thickness of overburdens, nor any gravity impact of numerous abandoned wells. Gravity anomalies were mostly connected with the lithology of sediments and rocks obtained from geological boreholes drilled in the years 2007 and 2008, on the basis of which the anomalies' origin could be determined. A concentric, relatively negative gravity microanomaly of ca. 25 m in diameter was registered in the place, where very well preserved relics of the woolly rhinoceros had been found. They may be indicative of the existence of Pleistocene lakes (and/or palaeoswamps), into which the woolly rhinoceroses had sunk. Hence, an assumption can be made that the further search for successive zoological relics can be made in areas with similar anomalies under the low-density sub-Quaternary subcrop.

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Distribution and origin of gaseous hydrocarbons and carbon dioxide in the Quaternary sediments at Starunia palaeontological site and vicinity (Carpathian region, Ukraine)

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Kotarba M. J. , Sechman H. , Dzieniewicz M.

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tom Vol. 79, No 3

403-419

EN

Near-surface gases were investigated in Starunia, where remains of mammoth and woolly rhinoceroses were discovered at the beginning of the 20th century. Samples were collected at 30 measurement sites, from 0.8, 1.6, 2.4, 3.2 and 4.0 m depths and analysed for their molecular and isotopic compositions. Most of the methane and all higher gaseous hydrocarbons accumulated within the near-surface zone are of thermogenic origin. The distribution of thermogenic methane and higher gaseous hydrocarbons in Quaternary sediments is variable and, generally, increases with depth. Microbial methane or a mixture of microbial and thermogenic methane also occurs at the near-surface zone. The presence of hydrogen and alkenes in the near-surface gases also suggests a contribution from recent, microbial processes. Helium of crustal origin migrated through the Rinne Fault and other tectonic zones located in the study area, and was subsequently dispersed during migration through the lithologically diversified Pleistocene and Holocene sediments. The high variability of the carbon dioxide concentration and stable carbon isotope composition at various depths in the near-surface zone is caused by its polygenetic origin (thermogenic, microbial, and oxidation of hydrocarbons and Quaternary organic matter), and also by its dissolution in water and oil during migration through lithologically diversified Pleistocene and Holocene sediments. Zones of thermogenic methane occurrence within Pleistocene sediments, dominated by muds saturated with bitumen and brines, provide the most favourable environment for preservation of large extinct mammals. It is likely that during the Pleistocene winters, when thick ice and snow covered the tundra lake and swamp, zones of outflow of brines, oils, helium, and thermogenic gaseous hydrocarbons had a higher temperature, which could have resulted in melting and cracking of the ice cover and drowning of large mammals. The most favourable conditions for preservation of large, extinct mammals probably still exists in the vicinity of sampling probe sites Nos 21, 22, 23 and 36. Zones of dominance of microbial methane concentrations are less favourable sites for preservation of extinct mammals, because this methane was generated within recent swamps and also within recent descending meteoric waters which infiltrated into Pleistocene and Holocene sediments. Older Pleistocene microbial gases have already escaped to the atmosphere.

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Molecular and isotopic compositions of gases adsorbed to near surface sediments at Starunia palaeontological site and vicinity (Carpathian region, Ukraine)

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Dzieniewicz M. , Sechman H. , Kotarba M. J.

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tom Vol. 79, No 3

421-437

EN

The near-surface geochemical survey of gases desorbed from sediment samples was carried out in the area of an abandoned ozokerite mine in Starunia, where remnants of mammoth and three woolly rhinoceroses and one almost completely preserved rhinoceros carcass were discovered in 1907 and 1929. Numerous hydrocarbon seeps (gas and oil "eyes") occur on the surface of the study area. Analyses of molecular and stable carbon isotope compositions of adsorbed gases were carried out in two variants. The first included sampling of cuttings from 30 auger boreholes at depths of 4.8, 5.6 and 6.4 m. In total, 88 samples were collected. In the second variant core samples were collected from 17 selected boreholes. In total, 78 samples were taken from various depths to 12 m. The results of molecular composition analyses of desorbed gases indicated high saturation of near-surface sediments with the oil. The highest concentrations of alkanes were detected in Miocene strata. Hydrocarbon migration from deep accumulations to the surface was relatively fast and proceeded along the faults, fractures and cracks. In the near-surface zone hydrocarbons were subjected to oxidation and dehydrogenation, which resulted in generation of unsaturated hydrocarbons and hydrogen. These processes were most intensive in the Pleistocene sediments and in the mine dumps. Increased concentrations of hydrogen may also originate from water radiolysis in the presence of hydrocarbons. Concentrations of carbon dioxide in the adsorbed gases show the higher values in comparison with the analysed gaseous compounds. However, a slight increase in CO2 concentration was detected in the mine dump, which may indicate conditions more favourable for hydrocarbon oxidation. Carbon dioxide from the analysed adsorbed gases is of thermogenic origin. Occasionally, insignificant influence of secondary hydrocarbon oxidation and/or Quaternary organic matter can be observed. Concentrations of alkanes, alkenes and carbon dioxide in the gas derived from desorption of rock samples are lower and the concentration of hydrogen is higher than those measured in free gases. This indicates that additional effects from recent (e.g. microbial) processes are absent.

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Młode procesy tektoniczne w strefie uskoku Dien Bien Phu (NW Wietnam) w świetle wstępnych wyników badań osadów czwartorzędowych

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Zuchiewicz W. , Quoc Cuong N. , Bluszcz A. , Michalik M.

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tom Vol. 50, nr 10/1

861-870

PL

Strefa współcześnie lewoprzesuwczego uskoku Dien Bien Phu (DBP), o orientacji NNE-SSW do N-S, jest jest jednym z najbardziej aktywnych uskoków sejsmotektonicznych w Indochinach. W NW Wietnamie uskok ten, 160 km długości i 6-10 km szerokości, przecina skały osadowe i metamorficzne wieku późnoproterozoicznego, paleozoicznego i mezozoicznego, a także paleozoiczne i górnotriasowe granitoidy. Wzdłuż DBP występuje 5 niewielkich basenów typu pull-apart (od północy: Pa Tan, Chan Nua, Lai Chau, Muong Pon oraz Dien Bien Phu), spośród których przeanalizowano szczegółowo dwa największe, tj. Lai Chau i Dien Bien Phu. Oba baseny są obramowane przez uskoki lewoprzesuwcze i lewoprzesuwczo-normalne, wywołujące ugięcie i przemieszczenie sieci drenażu, obecność licznych grzbietów zagradzających (shutter ridges), jak również zuskokowanie tarasów i stożków napływowych. Na obecność normalnej składowej ruchu uskokowego wskazują: trójkątne lica progów uskokowych, wysoko wzniesione tarasy skalno-osadowe w przełomowych odcinkach dolin, zawieszone doliny oraz wysokie i niewyrównane spadki koryt rzecznych. Minimalne rozmiary przemieszczeń lewoprzesuwczych wynoszą od 6-8 m do 20-50 m dla dolin holoceńskich oraz 2-2,5 km dla dolin środkowo-późnoplejstoceńskich w różnych segmentach uskoku. Miąższość osadów czwartorzędowych zmienia się od kilku--, kilkunastu metrów w Lai Chau do blisko 130 m w basenie Dien Bien Phu. W basenie Lai Chau osady tarasu średniego (23 m) rzeki Nam Na (stanowisko most Muong Te) zostały wydatowane techniką OSL-SAR na 26-45 ka. Osady te zostały przecięte uskokiem normalnym o zrzucie ok. 12 m w ciągu ostatnich 13 tys. lat i przykryte przez gliny stokowe, 8-12 m grubości, z kilkoma poziomami klinów koluwialnych z poziomami ostrokrawędzistego gruzu, a utworzonymi przypuszczalnie dzięki epizodom palesejsmicznym po dacie 13 ka. Kontakt między utworami stokowymi a piaskowcami i mułowcami triasu również ma charakter uskoku normalnego. W basenie Dien Bien Phu aluwia tarasów wczesno-środkowoholoceńskich zostały wydatowane OSL-SAR na 6,5-7 ka, podczas gdy najmłodsze pokrywy aluwialne zostały zdeponowane 1,8-2,7 ka oraz niemal współcześnie. Aluwia górnoplejstoceńskich stożków napływowych na wschodnim obramowaniu basenu są pocięte serią spękań oraz drobnych uskoków o orientacji zbieżnej z orientacją strefy DBP (N20oE). Przemieszczone tarasy i stożki napływowe pozwalają sądzić, iż minimalne tempo ruchu lewoprzesuwczego wynosiło 0,6-2 mm/rok w holocenie oraz 2-4 mm/rok w środkowym i późnym plejstocenie. Natomiast tempo holoceńskich ruchów wypiętrzających zmieniało się od 1 mm rocznie w rejonie Lai Chau do 0,4-0,6 mm/rok na zachód od Dien Bien Phu. Długoterminowe, średnie prędkości czwartorzędowych ruchów podnoszących są trudne do oszacowania; przekraczały one jednak wartość 0,05 mm/rok.

EN

The Dien Bien Phu (DBP) fault zone, oriented NNE to N, is one of the most seismically active fault zones in Indochina. In NW Vietnam, this zone is 160 km long and 6-10 km wide, cutting through sedimentary and metamorphic rocks of the Late Proterozoic, Palaeozoic and Mesozoic age, as well as Palaeozoic and Upper Triassic granitoids. Along the DBP fault zone there occur relatively small, narrow pull-apart basins (from the north: Pa Tan, Chan-Nua, Lai Chau, Muong Pon, and Dien Bien Phu), the two largest of which (Lai Chau and Dien Bien Phu) have been studied in detail. Both are bounded by sinistral and sinistral-normal faults, responsible for offset and deflected drainage, presence of numerous shutter ridges and displaced terraces, and alluvial fans. The normal component of motion is testified to by well-preserved triangular facets on fault scarps, highly elevated straths in river watergaps, overhanging tributary valleys, as well as high and uneven river-bed gradients. Our observations indicate a minimum recent sinistral offset ranging from 6-8 m to 20-50 m for Holocene valleys to 2-2.5 km for middle-late Pleistocene valleys in different fault segments. The thickness of Quaternary sediments varies from a few to a dozen or so metres in Lai Chau to a few tens or even 130 m in DBP basin. In the Lai Chau basin, the middle terrace (23 m) alluvia of Nam Na River at Muong Te bridge, shortly north of the confluence with Song Da, have been OSL-SAR (optically-stimulated luminescence; single aliquot regenerative dose technique) dated to 26-45 ka. These sediments were normal-faulted by some 12 m after 13 ka, and mantled by vari-coloured slope loams, 8-12 m thick, containing colluvial wedges composed of angular debris. These wedges were probably formed due to at least two palafoseismic events postdating 14-13 ka. The contact between slope sediments and Triassic bedrock is along a normal fault (N10°W, 78E), accompanied by 0.3-0.35-m-wide fault gouge. In the Dien Bien Phu basin, in turn, alluvium of the upper Holocene terrace has been OSL-SAR dated to 6.5-7 ka, whereas the younger terrace sediments give ages of 1.8-2.7 ka. Displaced terraces and alluvial fans allow us to suppose that the sinistral and sinistral-normal faults bounding narrow pull-apart basins in the southern portion of the DBP fault reveal minimum rates of left-lateral strike-slip ranging from 0.6 to 2 mm/yr in Holocene and 2-4 mm/yr in middle-late Pleistocene times, whereas rates of Holocene uplift tend to attain 1 mm/yr north of Lai Chau and 0.4-0.6 mm/yr west of Dien Bien Phu. Long-term, average Quaternary uplift rates are difficult to estimate, the minimum values certainly exceeding 0.05 mm/yr.