Wapienie z oolitami żelazistymi środkowego ordowiku Białowieży i Mielnika

Bronisław Szymański




In the areas of Białowieża and Mielnik, limestones with ferruginous oolites, revealing a sedimentary gap and washout traces, rest discordantly on an uneven, eroded and wavy top surface of glauconite dolomites of Arenig age (Tab. I, Figs. 3 and 4).
At the bottom part, the limestones are marly in character, with glauconite, numerous pebbles and small fragments: of limestones chaotically scattered on the uneven contact surface. A differentiated relief of the top surface of the dolomites is stressed here by a some centimetres thick layer, saturated with brown iron hydroxides. Lows, corroded hollows and funnels in dolomite are filled in with  brown-grey marly material that shows a great amount of scattered oolites and ferruginous ooids. Limestones with oolites are zoogenic, frequently organodetrital, nodular, or of aggregate character.
The limestones are crowded with scattered -oolites, arranged in nests, as well as with ooids and detritus of fauna impregnated with iron compounds. The oolites make here approximately 10–30% of rock volume. The limestone series exhibits thin intercalations and partings of claystones and calcareous siltstones, cherry brown and greenish in colour. Fairly numerous and irregular inter-growths give, in the entire section, a characteristic nodular appearance. The limestones are dark or light grey, and yellow, with brown tint in the parts rich  in oolites. Cherry-brown and cherry-grey colours, with lighter spots, frequently prevail in the lower portion of the section.
Towards the top, the limestones gradually lose their oolites and pass into the upper member of carbonate Ordovician deposits, i.e. into grey limestones of Llandeilo age. The limestones with ferruginous oolites belong to the Llanvirnian and, in the top part, may correspond to the lowermost Llandeilo (J. Znosko, 1964, W. Bednarczyk, 1966).
Throughout the entire Bialowieża and Mielnik areas they form a stable, slightly lithologically differentiated series referred to the Pomeranian beds. Thickness of the limestones with oolites ranges from 2,4 to 5,3 m.
The Llanrvirnian limestones sedimented in an open and shallow basin characterized by a vide epicontinental extent, where no terrigenous material was being supplied. Proximity of wave base frequently caused intraformational washouts, as well as transportation and redeposition of sediments. Oolites, ooids and detritus of fauna impregnated with iron compounds, were formed in separate portions of the basin, where, either due to the situation, or owing to the morphology of the bottom and the existing conditions, a greater concentration of ferruginous substance may have originated. The existing physic-chemical and dynamical regime enabled here precipitation and formation of oolites. Bath bathymetric conditions and dynamical regime were responsible for responsible and common occurrence of oolites.
In the Białowieża and Mielnik areas a sedimentary gap and change in facies existed at the Arenig-Llanvirnian boundary. In all probability, the time span of the sedimentary gap was not great, as proved by the stratigraphical continuity of the section (W. Bednarczyk, 1966). However, this gap was of regional character and embraced only the north-eastern areas of Poland. At the top parts of the limestones, the oolites gradually disappear and a continuous sedimentary transition into the overlying grey limestones of Llandeilo age can be observed.
The limestones with ferruginous oolites of palaeontologically evidenced Llanvirnian, occurring in the Białowieża – Mielnik section, allow to make a precise correlation with their stratigraphical equivalents of the same age encountered recently in the bore holes made within the north-eastern areas of Poland.

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