O przedtortońskiej dolinie erozyjnej wypełnionej osadami trzeciorzędowymi w okolicach Chrzanowa

Stanisław Panek, Marek Szuwarzyński


Pre-Tortonian Valley with Tertiary fill near Chrzanów (Cracow Upland, Poland)

This paper has to do with topography and paleo-drainage pattern at the Lower Tertiary planation surface in the Cracow Upland. The geologic events which gave rise to this surface, are noted below: After a prolongued period of erosion, following late Mesozoic and early Tertiary tectonic movements, the relief of the Cracow area was reduced and a peneplain was developed upon Paleozoic and Mesozoic rocks Fault movements, both prior to and during the Tortonian transgression, which terminated the cycle of peneplanation, broke up the ancient surface into east-west trending horsts and grabens. In consequence, much of the Cracow Upland became inundated and was covered by a mantle of Tortonian marine sediments (chiefly claystones). One of the controversial issues concerning the erosion surface, related to the ancient drainage pattern. Dżułyński (1953) following earlier workers, regarded the present valleys, transecting the horst as exhumed parte of an older pre-Tortonian river system. He postulated that the surface in question was drained to the south and that the valleys predate the Tortonian faulting. The hypothesis set forth by Dżułyński was based upon false premises and has not been confirmed by later investigations (e. g. Gradziński, 1962). In consequence, Dżułyński has revised his earlier ideas and recently furnished evidence in favour of a post-Tortonian age for the present valley system (Dżułyński et al., 1966). Although Dżułyński’s original contention proved untenable the concept of ancient, pre-Tortonian drainage to the south deserves further attention. The investigations carried out by the present writers in the Matylda mine, near Chrzanów, throw new light on this question. In the mine workings a prominent valley was discovered underneath the Tortonian sediments. The valley is incised into Triassic carbonates (Figs. 1, 2, 3), and is 30 to 70 m deep and 50 to 100 m wide (Figs. 4, 5, 6, 7, 8). The floor of the buried pre-Tortonian valley as well as the adjacent slopes and the erosional surface itself are riddled by numerous karst cavities and sinkholes (Figs. 10, 11, 14) similar to those known from other parts of the Cracow-Silesia area (see Gradziński, 1962). The lower part of the incised river valley is filled with pre-Tortonian continental sediments (Fig. 9). These sediments are comprised of argillaceous residues derived from the Triassic, Jurassic and presumably also Cretaceous carbonate rocks, dolomitic sands derived from granular desintergration of the ore-bearing dolomite, fragments of chert, fine grained quartzose sands and calcareous tufa. Of particular interest here is a layer of crystalline calcite (Fig. 13), interpreted as the infilling of a sag fracture, developed in horizontal strata overlying a sinkhole. The upper part of the entrenched valley is filled with marine Tortonian clay (Figs. 9, 15). The valley under consideration is a part of an ancient, pre-Tortonian river valley draining southwards. Formation of the valley predates the Tortonian faulting. The upstream portion of this valley exposed in the uplifted zone is manifested at the present-day surface as a shallow depression, developed in soft Miocene sediments filling the valley. The ancient, lower Tertiary erosion surface of the Cracow upland was drained to the south and immediately prior to the Tortonian transgression was uplifted, so that meandering streams became incised into pre-Tortonian rocks.

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