Paleozoik obniżenia podlaskiego

Borys Areń

Abstract


PALAEOZOIC OF THE PODLASIE DEPRESSION

Summary

The present paper deals with the previous research works made in the Podlasie depression by means of deep drillings. Summarized sections of Palaeozoic series are presented, and Cambrian, Ordavician, Silurian, Carboniferous and Permian deposits occuring in the area of the depression are shown, too. The Podlasie depression represents a structul1al unit similar to a tectonic graben of an east-westerly direction. The depth to the crystalline basement in its axial western part exceeds 5000 metres, whereas in its eastern portion, it ranges from 1000 to 1600 metres. Cross section of the depression is asymmetrical: the southern slope is abrupt and the northern one dips gently. The depression is filled in with Palaeozoic deposits. However, the overlying Mesozoic formations do not reflect the presence of this depression, except for the Lower Buntsandstein deposits that, consequently with the Palaeozoic formations, are of greater thickness along the axis of the depression. The remaining Mesozoic deposits are found to rest regularly within the area of the depression and of the adjacent elevations, and form a mantle gradually increasing its thickness towards the west.
Within the Palaeozoic deposits a mobility of the bottom can be observed not only within the entire depression, but also in its individual portions.
A stratigraphical comparison, made with respect to the area of the individual Paleozoic formations, distinctly demonstrates that the uplifting and the lowering of the bottom took place irregularly, as proved by various character of the horizons and stages of the deposits occurring in the depression. For example, the Silurian – Ludlovian deposits at Tłuszcz are 176 m. in thickness, at Okuniew being already 1175 m. The thickness of the Venlockian deposits at Tłuszcz is 85 m., and at Okuniew only 70 metres. These are values illustrating various nature of the conditions that prevailed in the same basin and at the same time interval.
This fact should be explained by tectonic movements of the basement. For Palaeozoic this is the Pre-Cambrian crystalline basement dislocated by deep fractures, which are a basis for zones of tectonic displacements situated along the margins of the depression, and in other places. Directions of the fractures should also be different, mainly along the marginal zone of the platform and perpendicularly to it. Complications of these deep fractures must have led to the formation of a rich dislocation net characterized by various directions. This in turn caused that the, individual sectors (blocks) could have mutually been displaced, elevated and sunk. The Podlasie depression makes, together with the adjacent elevations and depressions, a system of key elements characterized by high mobility.
The first oil traces were reported from the area of Żebrak, where they were found in Cambrian deposits at a depth from 2399,8 to 2435,0 m. Gas seepages were observed at Tłuszcz in Cambrian deposits, at a depth of 2270–2278 m. Recently, oil and gas traces were noted to ·appear also in Cambrian formations at an interval from 3646 to 3656 m. In all the cases, the oil- and gas-bearing Cambrian formations rest under the Silurian cover. A sealing nature of the Silurian deposits is highly favorable for preservation of oil and gas here. Under these conditions the search for hydrocarbons should be carried on in the areas where Silurian and Cambrian deposits are of considerable thickness.


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