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Recent studies on the Silurian of the western part of Ukraine

Łuczyński P., Skompski S., Kozłowski W.

Acta Geologica Polonica

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2016

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Vol. 66, no. 3

281--297

EN

The paper summarises the effects of recent studies carried out by a team from the Department of Historical and Regional Geology of the Faculty of Geology, University of Warsaw on the upper Silurian of Podolia (western part of Ukraine). The sedimentary history of the Silurian succession of Podolia is characterised by its cyclic pattern, with shallowing-upward cyclothems. In the traditional interpretation, the occurrence of stromatoporoid beds within each cyclothem marks the deepest (or most open-marine) sedimentary environment within the cycle. According to the results of recent studies, their occurrence is connected rather with a relatively shallow-water environment and with high energy phenomena. A substantial reinterpretation of the main sedimentary processes governing the deposition and facies distribution on the shelf is presented. Particularly, there are recognised and described high-energy sedimentary events repeatedly punctuating the generally calm sedimentation that prevailed in the lagoonal settings, some of which are interpreted as tsunami induced. Further perspectives for studies on the Silurian successions of Podolia are also discussed. The main problem is the precise correlation of particular sections that are scattered over vast distances and developed in similar facies associations.

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Stromatoporoid beds and flat-pebble conglomerates interpreted as tsunami deposits in the Upper Silurian of Podolia, Ukraine

Łuczyński P., Skompski S., Kozłowski W.

Acta Geologica Polonica

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2014

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Vol. 64, no. 3

261--280

EN

Tsunami deposits are currently a subject of intensive studies. Tsunamis must have occurred in the geological past in the same frequency as nowadays, yet their identified depositional record is surprisingly scarce. Here we describe a hitherto unrecognized example of probable palaeotsunamites. The Upper Silurian (Pridoli) carbonate succession of Podolia (southwestern Ukraine) contains variously developed event beds forming intercalations within peritidal deposits (shallow water limestones, nodular marls and dolomites). The event beds are represented by stromatoporoid and fine-grained bioclastic limestones, in some places accompanied by flat-pebble conglomerates. The interval with event beds can be traced along the Zbruch River in separate outcrops over a distance of more than 20 km along a transect oblique to the palaeoshoreline. The stromatoporoid beds have erosional bottom surfaces and are composed of overturned and often fragmented massive skeletons. The material has been transported landward from their offshore habitats and deposited in lagoonal settings. The flat-pebble conglomerates are composed of sub-angular micritic clasts that are lithologically identical to the sediments forming the underlying beds. Large-scale landward transport of the biogenic material has to be attributed to phenomena with very high energy levels, such as tropical hurricanes or tsunamis. This paper presents a tsunamigenic interpretation. Morphometric features of redeposited stromatoporoids point to a calm original growth environment at depths well below storm wave base. Tsunami waves are the most probable factor that could cause their redeposition from such a setting. The vastness of the area covered by parabiostromal stromatoporoid beds resembles the distribution of modern tsunami deposits in offshore settings. The stromatoporoid beds with unsorted stromatoporoids of various dimensions evenly distributed throughout the thickness of the beds and with clast-supported textures most probably represent deposition by traction. In some sections, the stromatoporoids are restricted to the lowermost parts of the beds, which pass upwards into bioclastic limestones. In this case, the finer material was deposited from suspension. The coexistence of stromatoporoid beds and flat-pebble conglomerates also allows presenting a tsunami interpretation of the latter. The propagating tsunami waves, led to erosion of partly lithified thin-layered mudstones, their fragmentation into flat clasts and redeposition as flat-pebble conglomerates.

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Problem tsunamitów. Dlaczego tak mało jest kopalnych osadów tsunami?

Łuczyński P.

Przegląd Geologiczny

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2012

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Vol. 60, nr 11

598–-604

EN

Tsunami is a common phenomenon and one of the most dangerous natural hazards of the modern world. Taking into account the relative high frequency of tsunamis in historical times, we can expect accordingly rich geological records of similar events from fossil formations. However, palaeotsunami deposits are surprisingly rare, mainly because of the difficulty of discerning them from sediments formed by other processes, e.g. storms. Part of the problem lies in the definition of tsunamite, which refers only to the triggering process, and in a different approach while discerning modern and fossil tsunamites. Modern tsunamites are identified by connecting the deposits to a particular historical tsunami event, even if they contain no diagnostic sedimentological features - an approach, which cannot be applied to fossil formations. This can lead to an impression that modern tsunami deposits are much more common than their fossil counterparts, which in turn suggest great intensification of tsunami events in historical times. Lately, a great number ofpapers have been published on sedimentary effects of modern tsunamis. The studies concentrated on land and near-shore areas. Many authors point out that the sedimentary record left by tsunami is commonly surprisingly mean and with a very low preservation potential. When interpreting fossil successions, the most important features that might indicate their tsunamite genesis are: mixing of material derivedfrom various facial zones, evidence of deep erosion and long transport, and association with other symptoms of seismic activity. The stromatoporoid biostromal accumulations in the Upper Silurian ofPodolia are presented here as possible palaeotsunamite deposits. Thick layers of redeposited stromatoporoids within fine-grained peritidal deposits are interpreted as an effect of onshore transport of this relatively light material during high-energy sedimentary events, such as tsunami. In many ways, the stromatoporoid beds resemble modern accumulations oftsunami-derived boulders.