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Lower Hettangian in the Holy Cross Mountains region - an example of tectonically-controlled sedimentation in the epicontinental basin of Poland

Brański P.

Volumina Jurassica

2006

Vol. 4, no. 1

80-81

Origin of the Early Jurassic depositional sequences in the epicontinental basin of Poland is mainly related to the successive super-regional pulses of sea-level with relatively lesser role of tectonic activity (Pieńkowski 2004). Development of sedimentary infill of Hettangian basin in the Holy Cross Mts. region was determined by both tectonic and eustatic factors. In earliest Jurassic times, a short-term tectonic event led to the pronounced subsidence acceleration in the southern part of the Mid-Polish Trough. It followed period of a low subsidence rate during the Rhaetian times. Geometry of the Holy Cross Mts. segment of the Mid-Polish Trough with its framing crustal fractures, rapid facies changes and shifting of depocenter indicate a strike-slip regime of tectonic reactivation, in agreement with the earlier suggestions of some authors (Hakenberg & Świdrowska 1997; Poprawa 1997). Based on the thickness changes, lithofacies and palaeoenvironment distribution, sequence correlation, as well as on the subsidence curves interpretation, two main stages of Hettangian basin evolution were identified. Accumulation of the basin-fill in Early Hettangian (Zagaje Fm.) was strongly related to the block-faulted structure of the basement and synsedimentary tectonics, controlling differential subsidence. After the transtensional (strike-slip) reactivation of the trough and local coarse-grained fluvial deposition, the extensional basin widening prevailed. It was coeval with a base-level rise (transgression), which continued through the Planorbis times (Pieńkowski 2004). Dip-slip motions of the faults became dominant and acceleration of the subsidence was very rapid. As a result of tectonic and eustatic processes, a large-scale fining-upward basin-fill succession was generated in a short time. The retrogradational, alluvial-lacustrine stacking pattern deviates from some traditional models of a progradational, coarsening-upward basin-fill in the strike-slip basins. It possibly reflects some temporal changes in magnitude of the strike-slip vs. dip-slip displacements and the rise of the base-level. After brackish-marine flooding, the nearshore and marginal-marine successions of Skłoby Fm. and Przysucha Ore-bearing Fm. were deposited in Middle-Late Hettangian time (Pieńkowski 2004). With exception of the Nowe Miasto-Iłża Fault, a synsedimentary activity of tectonic discontinuities ceased and the subsidence became much weaker. Cyclicity of sedimentation was then controlled mainly by sea-level changes. The subsidence pulse at Triassic/Jurassic boundary offers an explanation for the accumulation of anomalous thickness of the continental Lower Hettangian followed by brackish deposits of marine transgression.