Perm dolny obniżenia podlaskiego

Jędrzej Pokorski




A thin complex of clastic rocks, referred to the Rotliegendes occurs at the bottom of the Zechstein formation within the central part of the Podlasie depression (Fig. 1). The boundary of their present erosional range resembles the original one (Fig. 2). The Rotliegendes formations represent the main types of clastic deposits, i.e. conglomerates, sandstones, siltstones and claystones. Apart from the distinct difference in fraction, all these rocks reveal a considerable analogy in mineral composition, and are arkoses, or disclose mineral composition of these latter. Conglomerates showing variously grained textures, bad selection and sorting degree are red, pink or spotty in colour. Fragments of granitoid rocks and quartz-microcline and quartz rocks are their main components. Fragments of sedimentary rocks are Cambrian, Ordovician and Silurian in age. Sandstones and conglomeratic sandstones are characterized by psammitic texture, their structures being chaotic or parallel and finely laminated. Colours are from grey to red. The main clastic components are represented by quartz-microcline and quartz granitoid rocks, and by sedimentary rocks. Clastic mineral material consists of quartz, feldspars, biotite and muscovite. Siltstones reveal aleurite texture, their structures being parallel and finely laminated, characterized by a directional orientation of lamellar minerals. Heavy minerals are highly important components of the rocks here considered (Figs. 5 and 6). To the assamblage of heavy minerals that come from metamorphic rocks belong: epidotes, garnets (dark pink and salmon-coloured), disthene, staurolite, hypersthenes, hornblende, spinels and andalusite. Hypersthene occurs as unweathered, sharp-edged fine grains. Both staurolite and epidote were somewhat longer transported, but no doubt they are in situ. Abundant zircon, pale-pink garnet, tourmaline and rutile are for the most part redeposited from the older sedimentary rocks. The sedimentation of the formatiom here considered was preceded by a period of erosion and denudation. This period, begun already at the Stephanian time, continued still in the Lower Rotliegendes. Structural reconstruction that took place at the boundary of the Lower and Upper Rotliegendes (Saal phase movements) led to the formation of the Podlasie depression characterized by diversified morphology (Fig. 3). The formations of Rotliegendes age were laid down under large-lake and desert conditions. Mineralogical examinations paint to warm and dry climate. Desert conditions governed due to a lack of vegetation cover. Sedimentation took place in a water environment, in large lakes or in rivers. The sedimentary basin of the Podlasie depression was distinctly isolated from the Kujawy-Pomerania one. A connection was possible through the Magnuszew bay only. Lithofacial analysis (Fig. 4) and heavy mineral analysis (Fig. 6) demonstrate that transportation was short, and the main alimentation area was within the Mazury-Suwałki elevation. So, the Mazury-Suwałki elevation considerably affected the nature of the Rotliegendes deposits, being at that time the main area of clastic material supply.


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