Postępy erozji młodotrzeciorzędowej w okolicy Leśnej na Pogórzu Izerskim

Józef Oberc, Stanisław Dyjor



Gneisses outcropping in the vicinities of Leśna, lzerskie Pogórze, are covered successively by deposits of highland and valley group. The highland group comprises (from base to top): clay-sandy beds, occassionally containing some coal, lower basalt cover, thin mantle of sedimentary rocks, and upper basalt cover. Sedimentary rooks underlying basalts are of the late Early Miocene age, whereas the eruptive series is of the Middle Miocene age. The highland group underwent dissection by river pattern in post-Middle Miocene times. In the resulting river valleys deposition of the valley group took place during the Late Miocene and Early Pliocene. The group comprises clay-sandy deposits of the Poznań series and gravels and loams of the Gozdnica series. Late Miocene and Quaternary erosional cycles resulted in partial uncovering the valley pattern filled up with deposits of the valley group. The authors suppose that erosion active on the turn of the Middle and Late Miocene, and preceding deposition of the Poznań series, reached to the depth of about 240 m. In turn, Late Pliocene and Quaternary erosion reached to the depth of about 40 m in the vicinities of Leśna. The authors distinguish the following sequence of geological events which took place mainly in the Late Tertiary times on the area ion question: 1. Formation of pre-basalt sedimentary series 2. Basalt eruptions } the highland group, 3. Dissection of the highland group 4. Limnic sedimentation of Poznań series 5. Fluvial sedimentation of Gozdnica series, accompanied by partial erosion of Poznań series 6. Late Miocene and Quaternary erosion the valley group

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