Litofacje jury środkowej i dolnego oksfordu obszaru radomsko-lubelskiego

Abstract

THE MlDDLE JURASSIC AND LOWER OXFDIAN LITHOFACIES IN THE RADOM-LUBLIN AREASUMMARYThe thickness is here shown of the Middle Jurassic and Lower Oxfordian sediments in the Radom-Lublin area, as well as the distribution of the lithofacies differentiated on the basis of the modified classification triangle of W. C. Krumbein & L. Sloss (Kotański 1970, 1971). Attempts have also been made to reconstruct the paleogeographic conditions prevailing at that time.MIDDLE JURASSICMiddle Jurassic members lower than the Bathonian are known only from the westernmost part of the area here considered (Fig. 1). The most complete profile of sediments – from the Aalenian to the Kuiavian – occurs along the western boundary of their present distribution where they attain the maximum thickness.Along the line indicating the eastern occurrence limits of sediments younger than the Bathonian there are only those of the Kuiavian, 5 m in thickness. The remaining parts represented the source areas supplying the basin with terrigenous material. Bathonian-Callovian sediments cover practically the whole of the area here considered, the south-easternmost part excepted. This was the occurrence area of the Lublin land – a prolongation of that of the Ukraine (Niemczycka 1978 a), and of several minor paleostructures in the Central Lublin Region (Figs 2–3).Around the land areas the subsidence was rather small and the thickness of the Middle Jurassic does not exceed 10 m. An increase in thickness is distinctly observable in the south-eastern part where it attains 50 m at the boundary of their present occurrence range. Any available knowledge of the Middle Jurassic sediments older than the Bathonian has been provided mainly by the students of the Mesozoic margin of the Holy Cross Mts (References in Daniec 1970). These are sediments developed in the mudstone-sand lithofacies, abounding in a flora fairly well paleontologically documented. They have been assigned to group I of the classification triangle used here (Fig. 1, Table 1). Information concerning the Bathonian-Callovian lithofacies has been obtained from numerous bore holes drilled in our area (Figs 2–3). Lithofacial groups I, Il, III, IV and VIII have been differentiated in the Bathonian-Callovian sediments.Clastic deposits occur in the neighborhood of land areas existing at that time and are connected therewith. They are represented by mudstones and sandstones, subordinately by siltstones, while at the bottom they contain a characteristic conglomerate with elements of deposits of the Devonian and Carboniferous substratum. The clastic elements decrease with increasing distance from the land so that in the north-western part the sediments are almost exclusively carbonaceous. They are chiefly organo-detrital crinoid limestones, in the lower part slightly dolomitic, at the contact with the Lower Oxfordian bearing an ammonite fauna observable in the Nodular Bed (Dayczak-Calikowska 1969, 1873; Niemczycka 1965).LOWER OXFORDIANThe distribution range of the Lower Oxfordian sediments is slightly wider than that of the Bathonian-Callovian.The Lower Oxfordian basin widened out its range to invade the northern part of the land of Lublin, partly also some of the paleostructures (Figs 3–4). Similarly as in the Middle Jurassic, the above area was characterised by small subsidence. Its minimum occurred round the existing land areas where the thickness of deposits does not exceed 10 m, being slightly greater to the south and north-west where it attains 30 m.Eight lithofacial groups (Fig. 5) have been differentiated in the Lower Oxordian sediments, their distribution resembling that of the Bathonian-Callovian.Clastic sediments continue to occur in the vicinity of the land areas but covering a smaller area. Dolomitic deposits make their appearance as new lithofacial groups. They occur locally north and south of the land of Lublin, also in the vicinity of the Zakrzew (Z) paleostructure.A predominant part of our area is covered by limestone deposits represented by organodetrital spongy limestones. The amount of clastic material here is quite subordinate (Niemczycka 1976 a).As is reliably suggested by the distribution pattern and character of the lithofacial groups the area here considered was overflooded during the Middle Jurassic and the Lower Oxfordian by a rather shallow sea. Its islands protruded in the Central Lublin region, a peninsula which was a prolongation of the Ukrainian land into the Polish territory. At that time the sedimentation was distinctly connected with the above land areas. Their influence was, however, confined to the eastern part where terrigenous sediments were formed. Terrigenous material from the continental areas was brought into the northern part of our area only in small amounts and chemical sedimentation dominated