Retyk i lias na wschodnim Mazowszu

Ryszard Dadlez




In the area situated east of Warsaw (Fig. 1) both Rhaetic and Lias series have been examined in 15 deep bore holes. The deposits of these ages have been subdivided, according to the area of North-East Poland (R. Dadlez, 1968), into the following lithostratigraphical members: Nidzica Beds (Lower? - Middle? Rhaetic), Bartoszyce Beds (Upper Rhaetic); Liwiec Beds (Lower Lias - ?pars?), Olsztyn Beds (Middle Lias - ?Damerian?), Ciechocinek Beds (Lower Toarcian), Borucice Beds (Upper Toarcian - ?+ Middle Jurassic pars?). The development of these members has been illustrated in Figs. 3-6, and their thicknesses given in Table 1. The stratigraphic position of the Nidzica Beds has been accepted among others on the basis of ostracods Darwinula globosa (Duff.). The other complexes reveal characteristic assemblages of megaspores: Bartoszyce Beds - an assemblage with Trileites pinguis, Liwiec Beds - an assemblage with Nathorstisporites hopliticus, Ciechocinek Beds - an assemblage with Thomsonia phyllicus and Erlansonisporites sparassis. At that time, almost the entirely limnic sedimentation was subject to general rules that governed the marginal zone of the pre-Cambrian platform. A general tendency to increasing subsidence and to completion of profile from east to west was in certain periods complicated by vertical movements of basement blocks along dislocation zones running in a direction, perpendicular to the platform margin. The most considerable part was played at that time by the ENE-WSW trending zone that restricted the Young-Palaeozoic Padlasie depression from the south. It was rejuvenated still before the Rhaetic time, and this was responsible for a violent erosion and redeposition of only slightly altered older formations, mainly of Triassic age, in the southern part of the area; these are the Nidzica Beds of Lower Rhaetic age. In the Upper Rheatic and Lias the dislocation zone mentioned above was inactive. The south-eastern margin of the basin was flat, and predominantly clay sediments were laid in its vicinity. The profile was strongly reduced, showing numerous sedimentary and erosional gaps. The completion of the profile and an increase in thickness took place towards north-west and west (Fig. 7); coarser clastic material was brought from north-west. The most uniform facies, and probably the easternmost and southernmost extent are characteristic of the deposits of Lower Toarcian age (the Ciechocinek Beds). In the lower part of the Middle Jurassic, the areas in the east and in the south rose again and in consequence of this were subject to denudation. Just at that time the present-day boundary of the Rhaetic and Liassic formations became stabilized (Fig. 1). At Upper Bathonian time the levelled plane was invaded by the sea.

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