Piaszczysto-żwirowe osady mioceńskie z Łęk Górnych

Marek Doktor


Sandy Graves of Miocene age from Łęki Górne

South-east of Tarnów, near the village Łęki Górne sandy gravels are found cropping out in a small area adjoining edge of the Carpathian Flysch overthrust. These are exposed in various parts of the „Podraża Cliff”, a rooky escarpment near the southern end of the village and in the bottom of the Dulcza stream valley {Fig. 2). They were first described by J. Wdowiarz (1951). Their Sarmation age may by inferred from their occurrence over the argillaceous deposits of the Upper Badenian age, their tectonic tilting (6—10°) to the north, the heavy minerals assemblage and indirectly from other premisses. The dominant lithologies are sands, sands with gravel and gravels, locally cemented into conglomerates (Figs. 3, 12). Clay balls of Badennian clays are scattered within (Fig. 14). Horizontal bedding may be discerned due to the lithological variation and the presence of the erosional surfaces of various extent; however individual beds are not strongly differentiated. Many beds are lenticular; thinning out of beds may be often observed. Grading is common in the gravels as well as in the sands. It is better expressed in coarser deposits. The grain-size of the deposits is highly variable within the beds. In sandy deposits horizontal-arid cross laminations are present (Figs. 14, 15). The gravel-grade sediments are imbricated (PI. I., Fig. 2 and Pl. II., Fig. 1). Imbrication measurements are shown in Fig., 16. Most gravels were deposited by an unidirectional flow from SW to NE, as the diagram indicates. The grain-size of the deposits falls within a wide range of size-classes (Fig. 5); the coarse material dominates over the finer one which oc curs in small amounts only (Fig. 4). The maximum diameter of the pebbles is 350 mm. The sorting is poor, both in the gravels (Pl. II., Fig. 2) and in the sands (Fig. 7 and Pl. IV). The pebbles of 32—64 mm size prevail among the gravels (Fig. 6). The petrographical composition of pebbles of every size grade is shown in Table 3. The shape analysis in relation to the petrographical composition revealed the predominance of the discoidal pebbles (47,7%), (Fig. 8 and Table 5). The pebbles are well rounded (Fig. 9); with some differences between the petrographical classes (Fig. 10). Flatness measurements lack any significant differentiation (Fig. 11). Results of the petrographical analysis of gravels are shown in Table 1. The gravels are polymictic and the pebbles could be grouped into 7 petrographical classes. The provenance of the material is from the exotics- bearing beds occurring within the Carpathian Flysch and from the Miocene deposits transgressively overlying the Flysch. The high content of redeposited exotic pebbles is marked by the great quantities of the limestone pebbles. The mineral composition of the cemented sands is presented in Table 2. Petrographical composition and directional features of the investigated deposits indicate their provenance from the south from the Carpathians. The high content of the exotic pebbles suggests a small source area and a short transport route. It is confirmed by the rounding of the gravels as a whole as well as by the differences in rounding coefficients between the exotic and the Lithothamnium limestones. Basing on the works by Kuenen (1956, 1959), Unrug (1957) and Nawara (1964) it may be inferred that such a degree of rounding would occur in pebbles of the given petrographical composition after a transport only few kilometres long. This is in accordance with the supposed localization of the source area of the gravels from Łęki Górne. The sedimentary features of the deposits are indicative of the hydrodinamic conditions of deposition oscillating near the boundary between the lower and upper flow regimes. The described deposits show many similarities in their sedimentary structures and textural to the deposits of the alluvial fans (cf. Blissenbach 1954, Bluck 1964, Bull 1972) and of the braided rivers depositing gravels and sands (cf. Williams and Rust, 1969; Smith, 1970; Rust , 1972). The features indicative of deposition in a beach environment (cf. Gradziński et al. 1976, p. 469) are absent in the described deposits. It may be thus concluded, that the gravels and sands from Łęki Górne were deposited (in the Sarmatian time) in form of a small alluvial fan of a short stream rising in the Carpathians. It seems that the fan was being deposited in the continental conditions, close, to the mountainous region, during the Sarmatian.

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