Analiza sedymentologiczna drobnoziarnistych osadów górnoordowicko-dolnosylurskich basenu podlasko-lubelskiego

Abstract

Upper Ordovician-Lower Silurian fine-grained sediments in the Podlasie-Lublin Basin.A b s t r a c t. In western and central parts of the Podlasie-Lublin Basin, Upper Ordovician–Lower Silurian mixed siliciclastic-carbonate section is dominated by fine-grained sediments (claystones, mudstones), which are passing into more carbonaceous ones towards the east. In this region, subsidence was controlled by flexural bending of the East European Craton margin. Sedimentological analysis of drill cores revealed that deposition of those fine-grained sediments originated from suspension and traction currents. Amount of oxygen plays important role in the rock record (Fig. 1) whereas occurrence of pyrite is significant for determinations of sedimentary conditionsion, also in terms of sea level changes (Fig. 2). The rocks display abundant sedimentological structures such as current ripples, wave ripples, hummocky cross-stratification (HCS), graded beds, water escape structures, soft sediment deformation and scours (Fig. 3). The rocks are also characterized by predominance of wave ripples, common scours, graded beds and soft sediment deformations whereas current ripples, water escape structures and HCS are rather rare. The most frequent of the ichnofacies elements include unlined, rarely branched Planolites, which represent Scolithos, Cruziana and Zoophycus ichnofacies. The size of elements of Planolites ichnofacies appears dependent on environmental conditions and ranges from 0.5 mm to 15 mm. Chondrites, Anconichnus, Helminthopsis and Teichichnus, very important environment indicators, are present but not that common (Fig. 4). Textural analyses along with the records of diagenetic features made it possible to distinguish a succession ofstratigraphic parasequences (Fig. 5). Early carbonate cementation appeared important, especially in the case of suble or invisible changes in grain size. On the basis of the obtained results, depositional environment of fined-grained sediments in the Podlasie-Lublin Basin was interpreted as a wave-dominated shelf with turbidite episodes. Detailed sedimentological observations are crucial for definiting facies types and prediction of their distribution, but also for tracing the most organic-rich units in the sedimentological profile, as the basic in terms of gas shale prospects. Apparently monotonous fine-grained sediments (claystones and mudstones) are shown to be in fact miscellaneous.