Podstawowe zmiany facjalne i miąższościowe ordowiku platformowego północno-wschodniej Polski

Zdzisław Modliński




The discussion of the principal thickness and lithofacies variations of the platform Ordovician has been based on studies of three maps constructed by quantitative method for the Arenigian-Ashgillian depositional cycle. The thickness of this cycle is 25 to about 100 m. Abundant clay and carbonate sediments are accompanied by minor marls, siltstones, bentonites, glauconitites, and conglomerates. The only potential reservoir rocks for bitumens are carbonates. Therefore to establlish the prospects of bitumen occurrence in the Ordovician complex it was desirable to define the total thickness of the Ordovician, the relationship between the carbonates and the remaining sediments (the carbonate coefficient - Fig. 2) and the total thickness of the carbonates (the carbonate isolith map - Fig. 4). A number of thickness distribution zones subparallel to the margin of the Precambrian platform edge have been noted. Assuming that in most cases the subsidence of the platform Ordovician was compensated by deposition, the individual zones would correspond to palaeotectonic uplift and submergence zones (Fig. 1). The only exception is a belt of small thicknesses extending along the Smołdzino IG-l, Kościerzyna IG-l, and Szczawno IG-l boreholes called the "elevated" edge of the platform. Probably during the Ordovician the area was not elevated and may be regarded as a case of non-compensated subsidence. Towards the axis of this element the thickness decreases and simultaneously the deposits bear evidence of increasingly deeper marine environment. This is confirmed by the increasing share of clay sediments in the sequence (Fig. 3). The comparison of the Ordovician isopachyte map (Fig. 1) with the carbonate coefficient map (Fig. 2) shows that the carbonate share increases on elevations with the exception of the "elevated" platform edge. Three areas of increased thickness values can be observed on the Ordovician carbonates isolith map: Elbląg (VII), Middle Lithuanian (I), and Brest (II). Despite the small porosity of the carbonates the most prospective area for the bitumen occurrence seems to be a part of the Elbląg depression where the carbonates attain considerable thickness, but only if these sediments prove heavily fissured.

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