Rozwój strukturalny osadów czwartorzędowej serii Czyżowa w strefie Lękińska (rów Kleszczowa)

Abstract

W osadach czwartorzędowych rowu Kleszczowa zidentyfikowano strukturę synklinalną. Ma ona charakter synkliny nadblokowej, nawiązującej bezpośrednio do podłoża mezozoicznego. W utworach wypełniających tę strukturę zaobserwowano liczne deformacje wewnątrzwarstwowe o charakterze fałdów ciągnionych, uskoków radialnych i drobnych uskoków kompresyjnych.

STRUCTURAL EVOLUTION OF QUATERNARY SEDIMENTS OF THE CZYŻÓW SERIES IN THE ŁĘKIŃSKO ZONE (KLESZCZÓW TROUGH)

The paper deals with the question of deformations of Quaternary sediments, induced by endogenic movements of the Earth crust. The studies were carried out along cross-sections through the Kleszczów Trough (Fig. 1). Figures 2 and 3 show geological structure of the trough. The structure is characterized by coplanar disturbances of Tertiary and Quaternary sediments: a wide-radius syncline and anticline. Axis of the syncline follows the course of a tectonic depression in Mesozoic basement (Figs. 4, 5) and the syncline is best developed in strata of the Czyżów series.

Two stages may be differentiated in development of the Czyżów series. The first stage was connected with its deposition in result of intermittently intense subsidence. The periods of subsidence were broken by those of tectonic quiet, evidenced by numerous erosional surfaces and rhythmically repeating cycles of fluvial and basinal sedimentation in the section (Fig. 7). The minimum value of subsidence in times of deposition of the series is estimated at 30 m. The second stage, completely postdepositional in character, was connected with origin of the syncline in result of intense downwarp of the basement. At that stage the originally flat laying strata of the Czyżów series became bent downwards for about 20 m, and faults corresponding to fractures in the Mesozoic basement originated in limbs of the syncline (Figs. 5, 6). This was accompanied by origin of numerous deformations of the intraformational type, also related to the downwarp. The deformations are especially well developed in the fossil soil horizon (Fig. 7). Folds from limb of the syncline (Figs. 8, 9) are the major structural elements traceable in this horizon. Several tens of such folds were found at distance of 200 m. The folds are characterized by northern vergence, i.e. vergence opposite to inclination of limbs of the syncline, and amplitude decreasing both upwards and downwards. This shows that they originated under sedimentary cover, i.e. are postdepositional. They cannot be interpreted as disturbances due to gravitational sliding but rather typical drag folds, with geometry matching the model of M.G. Ramsay (1967).

The drag folds are accompanied by discontinuous disturbances. Minor compressional faults are known from cores of the folds (Fig. 10). Vergence of the faults is opposite to that of folds. The faults originated possibly due to differentiated response of water-saturated sediments to deformation, depending on granulation and structure of sediments. The set of faults found in the axis of the syncline (Fig. 11) is of the tensional type. The faults are vertical and sometimes open in upper parts. Their geometry resembles that of the so-called radial hinge faults (A.M. Johnson, 1976; M.G. Ramsay, 1967).

The structural inventory of the syncline formed in the Czyżów series and location of drag folds, radial faults and other fractures (Fig. 12) indicate that we are dealing with structural assemblage typical of synclines developed above basement blocks.