Stratygrafia potremadockich osadów ordowiku Lubelszczyzny

Abstract

 Przedstawiono wyniki badan litostratygraficznych i biostratygraficznych potremadockich osadów ordowiku Lubelszczyzny. Dokonano korelacji 15 profilów otworów wiertniczych, w których wyróżniono i scharakteryzowano 5 formacji tworzących schemat podziału litostratygraficznego. Za pomocą metod biostratygraficznych dowiązano formacje do określonych jednostek podziału chronostratygraficznego. STRATIGRAPHY OF POST-TREMADOCIAN ORDOVICIAN ROCKS IN THE LUBLIN REGION In the Lublin region, Ordovician rocks have been encountered for the first time in the borehole column Kaplonosy IG 1 in 1962. Up to the present they have been recorded in 15 borehole columns and the gathered data markedly contributed to the knowledge of their bio- and lithostratigraphy.The paper presents lithostratigraphic subdivision of the strata into 5 formations which are the basic classification units here. The units may be easily identified on the basis of macroscopic observations and analysis of well logs. Their brief characteristics are as follows:U h e r k a  L i m e s t o n e  F o r m a t i o n (Figs. 3, 4) is a complex of carbonate rocks with a basal layer of conglomerate or breccia, passing upwards into glauconitite. Within rocks of that formation it is possible to differentiate a number of lithological members which will presumably be recognized as formal lithostratigraphic units of lower rank. Carbonate rocks are represented by (from the base upwards): marly limestones and dolomites, organodetrital limestones with ferruginous ooids and organodetrital and marly limestones. The formation rests on either Middle Cambrian or Tremadocian with erosional unconformity and it comprises strata ranging in age from the Lower Arenigian to Caradocian. Its lateral variability is connected with decrease in share of biogenic material and bioclasts from the east westwards. Thickness of the strata ranges from 20 to 40 m, decreasing in the area of the Sokołów Podlaski paleotectonic elevation.The stratotype section of this formation comes from the borehole column Busówno IG I, and the hypostratotype  - from the borehole column Terebin IG 5.W ł o d a w k a  M a r l  Fo r m a t i o n (Figs. 2, 4) is represented by gray-green and gray marls with limestone intercalations. It occurs in the Uherka Limestone Formation and the recorded rich fauna of brachiopods and trilobites made possible dating its rocks at the Caradocian - Peribaltic stages from the Idavere to Vormsi. The formation occurs in NE part of the Lublin region only, passing into the Udal Claystone Formation to SW.The interval of the borehole column Kaplonosy I G 1, in which the formation is about 25 m thick, is its stratotype.U d a l  C l a y s t o n e  F o r m a t i o n  is a packet of dark-gray and gray-green claystones (Figs. 3, 4), resting on the Uherka Limestone Formation. The rocks are dated at the Caradocian on the basis of the recorded fauna, mainly graptolites. Boundaries of this formation seem heterochroneous: lower boundary presumably passes within the Kukruse and Idavare stages, and the upper - within the Vormsi stage. The formation is known from western and southern parts of the Lublin region. Its thickness is varying form 0 to 45 m, being the greatest in the Chełm paleotectonic depression.The stratotype section of this formation comes from the borehole column Busówno IG 1, and the hypostratotype - from the borehole column Parczew IG 1.K o d e n i e c  L i m e s t o n e  F o r m a t i o n  is represented by limestones and marls varying in colour and it is characterized by the presence of brown-red intercalations (Figs. 2 4). It rests on either the Udal Claystone Formation or Włodawka Marl Formation. In the majority of sections it comprises Lower Ashgillian rocks, corresponding to the Peribaltic Pirgu stage only. Its lateral variability is connected with varying relations of gray and brown-red coloured rocks and increase in share of marly rocks in NW part of the studied area. The thickness ranges from 3 to 10 m.The stratotype section of this formation comes from the borehole column Kaplonosy IG I, and the hypostratotype - from the borehole column Busówno IG I.T y ś m i e n i c a  M a r l  F o r m a t i o n comprises a packet of gray and gray-green marls with limestone intercalations (Figs. 2–4). It rests on the Kodeniec Limestone Formation and its distribution appears limited to western and southern parts of the Lublin region. Lateral variability in its lithology is rather small whereas thickness is varying from 0 to 25 m. The rocks are dated at the Upper Ashgillian. The stratotype section comes from the borehole column Siedliska IG 1 and the hypostratotype – from the borehole column Łopiennik IG 1.Core samples from the stratotype and hypostratotype sections are housed in the Core Repository at Hołowno near Włodawa, except for those from the borehole Łopiennik IG 1, housed in the Repository at Iwiczna near Warsaw.The fauna of graptolites and trilobites is of utmost importance for biostratigraphic subdivision of the rocks (Fig. 5). Rocks of the Lower and Upper Arenigian are dated on the basis of trilobites Symphrsums anguststus (Sars et Boeck), S. dorsatus Poulsen and S. palpebrosus (Dalman) and other fossils. The faunistic record for the Llandvirnian cannot be treated as unequivocal throughout the Lublin region. That for the Llandeilian and Lower Caradocian (corresponding to the Kukruse stage) is also poor but it should be noted that the guide species Glyptograptus teretiusculus Hisinger is known from the former. The faunistic record may be regarded as good in the case of upper part of the Caradocian. In strata of unsubdivided  Idavere-Johvi-Keila stages there have been found some taxa including Panderia parvula (Holm). The richest faunistic assemblage is known from strata of the Oandu-Rakvere stage, in which there were differentiated graptolite Dicranograptus clingani zone and two corresponding trilobite zones: Chasmops macrourus and Ch. wesenbergensis. The uppermost Caradocian, corresponding to the Nabala-Vormsi stage, has somewhat poorer faunistic record. More important taxa of those found in these strata include Chasmops eichwaldi Schmidt, Illaenus roemeri Volborth and Climacograptus cf. angustus Perner. In the Lower Ashgillian, there were found guide trilobite species, including Panderia megalophthalma Linnarsson and Lonchodomas portlocki (Barrande), and the presence of the Upper Ashgillian is unequivocally shown by the record of Mucronaspis mucronata (Brongniart).