Zmarzlinowe struktury szczelinowe w osadach interglacjału eemskiego z Wawrzyszewa

Abstract

FROST FISSURE STRUCTURES IN SEDIMENTS OF THE EEMIAN INTERGLACIAL AT WAWRZYSZEWSummaryThe present study is based on materials from a trench made in Wawrzyszew, a suburban district of Warsaw (Fig. 1). The trench, six metres deep, intersects a post-lacustrine basin filled with organic Eemian deposits. A number of fissure structures, observed on the walls of this trench, make up a polygonal net which, was probably formed in a periglacial climate.The geological section of these deposits shows the following sequence: anthropogenic banks at the top are underlain by partly aeolian and mainly fluvial sands, overlying a peat layer which rests on spongy and peat gyttias; these pass downwards into shaly gyttias with bituminous shales at the bottom. Fissure structures extend downwards from the peat layer, to a depth of two to four metres.Frost fissure No. 1 (Fig. 2) shows a characteristic curving of the intersected beds, while the sandy filling of the fissure is pushed upwards in a dome-like form; this seems to be due to secondary processes - the melting of ice situated between the layers and the compaction of sediments. The upper part of frost fissure No. 2 (Fig. 3) is filled partly with clods of peat, which are inserted vertically in the fissure. Frost fissure No. 3 (Fig. 4) is filled, in its upper part, with synclinally bent peat layers. One may assume that as a result of a fast melting of ice veins, the overlying layers collapsed and thus form part of the present filling of the fissure.The lower parts of the fissures described above are filled with fine-grained sand.The trench is intersected by the furcated frost fissure No. 4 (Fig. 5, Plates I and II). The walls of the fissure are covered with structureless loess, peat-redeposited from the walls - and gyttia. This "lining" of the walls of the fissure resulted from a seasonal, partial melting of that part of the ice vein which was in contact with the adjacent sediments, the latter flowing down inside the fissure.The central part of the fissure is filled with fine-grained sand, which was subjected to the analyses of size-grain (Fig. 8; Table 1), roundness (Fig. 9) and dullness of quartz grains and of the frequency of heavy minerals. The same analyses were made in the case of cover sands. The author' finds that the sand which fills up the fissures described here went through the following processes: water transport,aeolization, and frost disintegration of quartz grains. Frost fissure No. 5 (Fig. 6) was utilized by a compaction fault which had been caused by a nonuniform sedimentation process. Fissure No. 6 (Fig. 7) represents a fragment of the lower part of a frost fissure, forming a step-like pattern in the strongly pressed shaly gyttias.It seems that the horizontal structure of the sediments facilitated the development of the horizontal parts of fissures of that type.The fissure structures described here were formed in a postlacustrine basin, which still occurs in the form of a lowering of the area. Lacustrine sediments, underlain by an impermeable layer of clayey silts and till, were probably strongly hydrated when the fissures were being formed. Thus it seems that the fissures were first filled with ice veins and then – when the ice had melted – mineral sediments were deposited there.