Fluwioglacjalne struktury sedymentacyjne w Bełchatowie

Tomasz Zieliński


Fluvioglacial depositional structures in Bełchatów, Central Poland

In the brown coal bearing deposits in Bełchatów, Central Poland, numerous erosional washouts have been observed. These occur within a series of fine-,grained Pleistocene fluvioglacial sands (Fig. 1) and have dimensions of up to 7 m in length, 3.5 m width, and 1.8 m in depth, and are infilled with well-sorted, finegrained sands. In the upcurrent direction of the washouts inclined parallel-laminated sets are found to occur, — the so-called a -member (Fig. 4A; Pl. 1, Fig. 1; Pl. II, Fig. 2). These are derived from the avalanche-like sedimentation of material transported to the edge of the washout by migrating ripples. In the downcurrent direction, this type of lamination grades into small-scale trough cross lamination — the b -member, and climbing-ripple cross lamination, named the с -member. (Fig. 4A; Pl. 1, Fig. 1). In the с -member the following sequences have been observed; Horizontally, in the direction of flow, A—B1—В2 (Allen 1970a) and vertically, A—B1—B2—S (Fig. 2). In both cases intensity of the traction transport and flow velocity decreased. In transverse section, paleohydrodynamic conditions were different. Currents were locally directed out towards the periphery of the depression (Fig. 3), where the decreasing depth of flow caused an increase in its velocity. This is shown by the sequences occurring in the transverse sections from the center to the edges of the washout, or from the с -member to the d -member: В—A-parallel lamination (Fig. 4B; Pl. I, Fig. 2). Sediments of the a — d -membres form the basic core of the infillings and attain three-fourths of its thickness. These are dominated by aggradational structures (deltaic inclined parallel lamination — a, and climbing riple cross-lamination — c) which correspond to stream states approaching bottom equilibrium profile. Grown sets filling the depressions display small-scale though cross-lamination — member e (Fig, 4; Pl. I; Fig. 1). The existance of erosional boundaries between the sets points to an equilibrium state of the bottom, caused by the shallowing of the flow. The orientations of the described sediments in relation to other deposits in the confines of the mine, their fine-grained character, lithologic homogeneity, the wide range in dimensions of the washouts and the character of their infillings all point to the fact that the sands were deposited in a pond or depression. Erosional washouts occurred as a result of strong currents within the sedimentary basin. In the case of fluvioglacial environments, a sudden change in the mobility of the medium can be explained by the specific hydrodynamics of ablational waters, which cause rivers to swell very rapidly. The Low energy character of sediments filling the washouts clearly contrast with the wide range of erosional forms. The deposition of these sediments must be correlated with the time of low energy of fluvioglacial currents, when fine-grained material, mainly from suspension, was deposited in the sedimentary basin.

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