Sequence stratigraphy of Carboniferous paralic deposits in the Lublin Basin (SE Poland)

Maria I. Waksmundzka

Abstract


Based on lithofacies analysis of clastic, clay and carbonate rocks, wireline logs and sequence stratigraphy, sixteen depositional sequences have been distinguished in the paralic Carboniferous succession of the Lublin Basin from the Viséan to the base of Westphalian B. The facies evolution and depositional architecture of the deposits belonging to three types of depositional systems tracts, i.e. lowstand (LST), transgressive (TST) and highstand (HST) have been reconstructed. The sequences are bounded by type 1 basal unconformities formed during subaerial erosion following relative sea-level fall and lowstand. This erosion, that in some cases reached down to the LST deposits of the underlying sequences, mainly affected the HST deposits. Relative sea-level rise controlling the base level of the rivers during lowstand was the basic factor influencing facies development, cyclicity and thickness of fluvial deposits, as well as the vertical and lateral transformation of rivers in the paralic Carboniferous succession. Vertical transition of high- to low-energy fluvial environments can be observed within the LST. The LST deposits typically occur in incised shelf valleys and in non-incised fluvial systems. Within the LST of sequences 2 and 4 to 10 commonly occur medium and large systems of simple incised valleys that developed in a coastal-plain system. The LST of sequences 11 to 15 comprises large systems of compound incised valleys that developed in a piedmont system. Relative sealevel rise in the late lowstand, sea-level oscillations during transgression and highstand and the volume of sediment supply were the main factors influencing facies development, cyclicity, thickness and lateral distribution of the deltaic deposits, shallow-shelf shales and limestones. Within the TST commonly occur coarsening-upward and non-gradational cyclothems that correspond to parasequences and were formed in the distal parts of the inner-shelf delta lobes. In the HST, the most common types are coarsening-upward cyclothems developed during the progradation of innershelf delta lobes and small lake deltas, as well as non-gradational cyclothems formed in lakes on a delta plain without influence of delta lobes. The transgression of sequence 7 probably had the widest extent of all the sequences in the paralic Carboniferous succession of the Lublin Basin. It was presumably much wider to the N, NE and NW than the presently accepted boundaries of the basin. Based on analysis of three curves of relative sea-level changes in the paralic Carboniferous succession of the Lublin Basin and the transgressive-regressive curve for the Carboniferous of Western Europe, the sequences distinguished have been correlated with the chronostratigraphic scheme for the Carboniferous System. The diachronous commencement of sedimentation has been confirmed and evaluated. The lack of deposits of sequence 5 in the eastern part of the basin suggests the presence of a stratigraphic gap encompassing the upper Brigantian. The position and range of stratigraphic gaps present elsewhere in the basin has also been indicated. In the north-westernmost and easternmost parts of the study area the gap has the widest range, encompassing the upper Arnsbergian to lower Marsdenian strata. The mid-Carboniferous boundary between the Mississippian and Pennsylvanian has been located at the base of sequence 8.


Keywords


Carboniferous; Sequence stratigraphy; Lublin Basin; Depositional architecture; Stratigraphic gaps; Cyclothems

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