O strukturach deformacyjnych i sedymentacji piasków sarmatu w rejonie Chełma

Abstract

Deformational structures and sedimentation of Miocene (Sarmatian) sands in the area of Chełm (Eastern Poland) Deformations of sedimentary structures associated with reversed density stratification were observed in Upper Miocene (Lower Sarmatian, Vohlynian zone) sands in the area of Chełm, east of Lublin. The area of sity stratification were observed in Upper Miocene (Lower Sarmatian, is presented in Fig. 1. Two lithological complexes are present in the Sarmatian of the area of Chełm. The lower one consists of loose sands, the upper one of sands with irregular layers and lenses of sandstones with siliceous cement. Both complexes contain a rich Sarmatian fauna (K. Kowalewski, 1958), and siliceous oolites are present in the sandstones (M. Turnau-Morawska, 1950). The grain-size distribution of the sandstones is presented in Fig. 3 and Fig. 6, with the use of parameters of D. L. Inmann (1952). The deformational structures occur mostly in the upper part of the lower complex (Fig. 2, Fig. 5, Plate XXXI and Plate XXXII), in medium and fine-grained sands, often cross-stratified. The directions of cross-straticifation indicate palaeocurrents from the west to the east. The consistency ratio, calculated after J. R. Curray (1956) has a high value of 0,87. However, on palaeogeographical grounds is it assumed that the source of the detrital material could not be situated in the west, and was situated east of the investigated area, in the Ukrainien massif. Synsedimentary deformational structures of load deformation type, associated with reversed density stratification (S. Dżułyński, 1966), have various forms. Most frequently individual cross-laminae or sets of laminae are folded and deformed, while deformations of cosets are relatively rare. Diapire-like structures, sometimes filled with fine-grained structureless sand were also observed (Fig. 2, Fig. 5 B). Some diapires piercing the overlying laminae reached the depositional surface (Fig. 5 A). Drop-like structures of coarse-grained sand embedded in fine-grained sand were also observed (Fig. 5 D). The intensity of deformations is variable. Most frequently in sets of cross-laminae the top laminae are bent upwards or overturned (Fig. 2 C, Plate XXXI, Fig. 1). Such structures were described as intraformational recumbent folds or as overturned cross-strata (P. E. Potter and H. P. Glass, 1958, R. Gradziński, 1970). In other cases the laminae of one set are strongly deformed, while the underlying and overlying sets remain undeformed (Plate XXXI, Fig. 2). In the strongest deformations folding affects several sets of cross-laminae (Plate XXXII, Fig. 2). The deformational structures and especially the diapire-like and the drop structures are associated with alternating layers differing in grain size. Such deformations are nearly entirely absent in sands with uniform grain size. Evidently the sorting and bulk density of sand layers was depending upon grain size (Fig. 6). The layers with coarser grains having a greater bulk density squeezed up the underlying fine-grained sand upon liquefaction, producing thus the diapire-like structures. The mass occurrence of deformational structures in the Sarmatian sands is probably associated with the high degree of rounding of the large sand grains, which assisted relative movements of grains upon liquefaction. The discussed structures are analogous to those described by J. Butrym et al. (1963), J. M. Anketell et al. (1970) and J. Cegła and S. Dżułyński (1970), It should be stressed upon that the synsedimentary deformational structures formed in the area of Chełm in Sarmatian times, in a typically warm climate, are analogous to structures regarded by some authors as periglacial (e. g. A. Jahn, 1951, J. Dylik, 1952, J. E. Mojski, 1958).