Dolny ordowik północno-wschodniej części obniżenia podlaskiego

Bronisław Szymański




The rock complex of Lower Ordovician age found to occur in the north-eastern part of the Podlasie depression (Fig. 1), and conformably laid down on the underlying top schists of the Cambrian Skupowo Beds, is built of faunistically evidenced Lower and Upper Tremadocian, Lower and Upper Arenigian and Llanvirnian formations. The Lower Tremadocian (Pakerort) is represented by Lower Obolus Sandstones, Separating Shales, Upper Obolus Sandstones and Dictyonema Shales. The Upper Tremadocian (Ceratopyge) consists of Bryograptus Shales. The Lower Arenigian (Latorp) is represented by glauconitite. The Upper Arenigian (Volkhov) is built of dolomites and glauconite limestones and Llanvirnian that terminates here the section - consists at limestones with ferruginous oolites. Both the Lower Obolus Sandstones and the Separating Shales - which lack graptolites and which represent the lowermost Lower Tremadocian - are tentatively referred to the sub-zone Dictyonema flabelliforme desmograptoides and the overlying Upper Obolus Sandstones - to the younger sub-zone Dictyonema flabelliforme sociale. On the basis of graptolite succession, three sub-zones may be distinguished in the dark Dictyonema Shales that overlie the sandstone-cley formations with the Obolus fauna: sub-zone Dictyonema flabelliforme graptolithicum, sub-zone Dictyonema flabelliforme flabelliforme, and sub-zone Dictyonema flabelliforme bryograptopides. The upper part of the Tremadocian dark shales, i.e. bryograptus shales with Kiaerograptus kiaeri, Bryograptus ramosus and ,,Didymograptus” ex gr. primigenius corresponds in age to the Upper Tremadocian (Ceratopyge).

The Lower Ordovician profile is characterized by two distinctly dated sedimentary gaps which, in the adjacent regions of the platform, may hardly be evidenced: - the first sedimentary gap is observed to occur between the Bryograptus shales of Upper Tremadocian age and the transgressive glauconitite series which represents the Lower Arenigian; - the second sedimentary gap is distinctly visible between the dolomites and glauconite limestones of Upper Arenigian age, and the overlying limestones with ferruginous oolites of Llanvirnian age. The rock complex of the Upper Cambrian and of the lower part of the Ordovician distinguishes itself by two sedimentary macrocycles. The first macrocycle of Upper Cambrian - Tremadocian age - is of clastic nature, the second macrocycle of Arenigian - Llandeilian age - is represented by glauconite-carbonate deposits. The clastic macrocycle is subdivided into three cycles. The first, Upper Cambrian cycle I-l comprises the Skupowo Sandstones and the Top Shales; the second, Lower Tremadocian cycle I-2 - the Lower Obolus Sandstones and the Separating Shales, and the third cycle I-3, which should be related to the younger Lower Tremadocian and the Upper Tremadocian, is represented by the Upper Obolus Sandstones, and by the Dictyonema and Bryograptus Shales. The glauconite-carbonate macrocycle is subdivided into two cycles. The first, Arenigian cycle II-4, is represented by glauconitite, and by dolomites and glauconite limestones. The second, Llanvirnian-Llandeilian cycle II-5 - by limestones with ferruginous oolites and grey limestones.

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