Lokalne dodatnie anomalie siły ciężkości w północno-zachodniej Polsce, związane z rynnami plejstoceńskimi

Abstract

 Na podstawie map resztkowych anomalii siły ciężkości północno-zachodniej Polski wyznaczono przebieg siedmiu meandrujących dodatnich anomalii lokalnych. Omówiono ukształtowanie i budowę geologiczną powierzchni podczwartorzędowej a także zestawiono wyniki badań gęstości utworów czwartorzędowych i podczwartorzędowych północno-zachodniej Polski. Przyczyną wyróżnionych anomalii są rynny, wycięte w osadach czwartorzędowych, trzeciorzędowych lub kredowych o małej gęstości, wypełnione cięższymi utworami plejstoceńskimi.

LOCAL POSITIVE GRAVITY ANOMALIES RELATED TO PLEISTOCENE FURROWS IN NORTH-WESTERN POLAND

In north-western part of the Polish Lowlands, there were found numerous local gravity anomalies highly varying in character. The anomalies are usually related to local structures of the Zechstein- Mesozoic complex or, sometimes, Tertiary tectonic-erosional troughs. The repeated analysis of semi-detailed gravity surveys revealed existence of a third type of local positive gravity anomalies in this region. The anomalies are traceable at distance of tens of kilometers and they are characterized by meandering axes and width not greater than a few kilometers.

The available data on structure and morphology of deposits forming Quaternary basement in north-western Poland, although scarce, implicate significant role of elongated, narrow and deep depression of the valley type. At least some of these depressions represent sub- or interglacial furrows of the Pleistocene age, cut in Quaternary basement-forming Tertiary or Upper Cretaceous deposits.

Using the results of semi-detailed gravity survey in north-western Poland, compiled in the form of maps of residual anomalies calculated by the Griffin's method (R = 2.236 km, s = 1 km), and those of regional gravity surveys in nearshore Baltic area (Bouguer anomaly maps), the authors differentiated and delineated the course of seven positive gravity anomalies which are not related to structures of the Zechstein-Mesozoic complex. The anomalies are characterized by meandering axes, length several times greater than their width, amplitudes over 0.5 mGal, and large horizontal gradient.

The Grabowa, Słupia–Bytowa, and Łupawa anomalies are situated close to the Baltic coast in central Pomerania. Their course roughly coincides with that of rivers after which they are named. The Wicko anomaly is situated in the same zone, and the Kościerzyna – south-east of it. The axis of the Myśla anomaly is passing close to the Myśla river, right tributary of the Odra river, between Dębno and Trzcińsko Zdrój.

The analysis of mean density values for Quaternary, Tertiary and Upper Cretaceous rocks occurring in this region showed that Pleistocene rocks of glacial or fluvioglaciai origin are characterized by density close to that of some Holocene alluvial deposits and markedly higher than in the case of Tertiary and Upper Cretaceous from their basement.

The comparison of the course of the anomalies (especially Słupia–Bytowa and Myśla anomalies) and the present river valleys and Pleistocene furrows showed that there is no interdependence between the anomalies and the present river valleys. When this is the case, the anomalies may be only explained as related to Pleistocene furrows cut in low density Quaternary, Tertiary and Upper Cretaceous deposits and infilled with heavier rocks of glacial or fluvioglacial origin.

Besides the above mentioned, in north-western Poland there may be expected numerous other Pleistocene furrows which, however, result in less distinct local gravity anomalies. The identification and delineation of the latter requires further revision of available gravity data and more detailed gravity measurements. In north-eastern Poland, Pleistocene furrows of a different type may be expected. The furrows, similarly as those in Lithuania and Kaliningrad district, are responsible for local negative gravity anomalies.