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Sedimentary environment and tectonic controls of the Snochowice Beds (Lower Jurassic, western margin of the Holy Cross Mountains, Poland)

Kozłowska M.

Geological Quarterly

2012

Vol. 56, No 2

299-314

The Snochowice Beds represent the lowermost part of the Lower Jurassic succession in the western part of the Mesozoic margin of the Holy Cross Mountains (Central Poland). This thick, gravelly has a different lithological character to other Lower Jurassic sandy and fine-grained deposits. Eight lithofacies have been distinguished, mainly gravelly and sandy, grouped into four genetic associations: M – massive gravels, interpreted as clast-rich debris flow deposits, G – horizontally to cross-stratified gravels, interpreted as gravelly bars and bedforms, S – cross-stratified and massive sandstones, interpreted as sandy bars and bedforms and F – massive mudstones and claystones, generally fine-grained deposits, interpreted as overbank deposits. Analysis of depositional geometry and lithofacies successions allowed reconstruction of the sedimentary environment: landforms of alluvial fan type, formed mainly by braided river processes and rarely by sediment gravity flows. A decrease in the thickness and content of gravelly lithofacies towards the north and north-east is documented. Analysis of transport directions has revealed that the clastic material was transported toward the north and north-east, perpendicular to the main faults fringing the basin. Depositional geometry facies and palaeocurrent patterns of the Snochowice Beds indicate that sedimentation was controlled by tectonic activity.

Architecture, evolution and hydrocarbon potential of the Late Jurassic - Early Cretaceous carbonate platform in SE Poland and W Ukraine

Gutowski J., Popadyuk V., Urbaniec A., Złonkiewicz Z., Gliniak P., Krzywiec P., Maksym A., Wybraniec S.

Volumina Jurassica

2006

Vol. 4, no. 1

46-48

Well, outcrop, seismic and gravity data were used to construct a series of palaeogeographic maps revealing architecture and evolution of the carbonate platform developed within the northern shelf of Tethys in SE Poland and W Ukraine during Oxfordian through Barremian times. The platform developed in a transition zone between the epicontinental Mid-Polish Trough and the Outer-Carpathian basins. A variety of depositional systems included open shelf spongemicrobial bioherms, coral reefs and oolitic-bioclastic grainstones (see Gutowski et al. 2005, Gliniak et al. 2005) which form hydrocarbon reservoirs in connection with overlying Cenomanian sandstones. Palaeogeographic distribution of the depositional systems evolved in time and was controlled by syn-depositional basement normal/transtensional and strike-slip faulting. A series of analogue models (Gutowski & Koyi 2006a, b) helped to understand a role of strike-slip movements along deep fault zones directed obliquely to the axis of the Mid Polish Through and related to modifications of the extension direction (Gutowski & Wybraniec 2006). These movements controlled fault geometry and the shift of depocenters. Pelagic, black and grey, often bituminous shales (Karolina Formation in Western Ukraine and Cieszyn Shales in Poland), deposited in front of the bioherm-reef belt on the shelf margin, form excellent source rocks. They were deposited since the Late Kimmeridgian due to breaking-up of the peri-Tethyan carbonate platform (Âtramberk type carobonates) and opening of the Silesian Basin (Fig. 1). Consequently, promising traps should mainly be located close to the Late Kimmeridgian - Early Cretaceous shelf margin (Fig. 1), the location of which was inferred using gravity data (Gutowski & Wybraniec 2006). Additional source rocks are of Palaeozoic or Middle Jurassic age. Quality of the reservoirs was often enhanced by diagenesis (e.g. dolomitization), fracturing and pre-Cenomanian karstification. The reservoirs are sealed by Miocene evaporates and/or clays and Upper Cretaceous marls. The Mid-Polish Trough was inverted during the Late Cretaceous and Palaeogene. As a result of Carpathian thrusting, the basin in its southernmost part was covered by the Miocene sediments of the Carpathian foredeep and/or by the Outer Carpathian nappes. Although these tectonic processes modified the evolution of the hydrocarbon system, the Late Jurassic - Early Cretaceous facies development and evolution of the carbonate platform were decisive for the primary distribution of the source rocks and potential reservoirs. Therefore, they should play a key role in hydrocarbon exploration strategy.