STRATIGRAFICKÉ DOKLADY O NÁSUNOVÉM ZLOMU VE VRTU RAŠKOVICE JA-7

Jindřich Hladil, Lubomir F. Jansa, Antonín Těžký, Kateřina Helešicová, Jiskra Hrubanová

Abstrakt

The Raškovice Ja-7 well (ESE of Frýdek-Místek) was one of the pilot wells with full cored rocks that penetrated the Carpathian nappes and continued through the entire thickness of (para)autochthonous Paleozoic to brunovistulian crystalline basement. The drilled carbonate formations (687 m) are x3 to x6 thicker than in neighboring wells [such as Morávka NP-828 (182 m), Krásná KS-9 (101 m), Ostravice NP-824 (151 m), Guty NT-5 (146 m), Kozlovice SV-4 (300 m) or Kozlovice SV-1 (236 m)]. Biostratigraphy of early 1980s indicated normal superposition of faunas in two levels, at the very base and top of limestones, in the Middle Givetian and Upper Frasnian, respectively. However, the fuzzy and alternating pattern between these two biomarker levels remained unanswered. An unlikely "Givetian-Frasnian transition" of hundreds of meters contrasted with situation in all other Moravian sections, where sedimentary condensation and hiatuses are connected to short-lived but strong sea level drop at Gi/Fr boundary. The geophysical revision of this problem required digitized well-logs, corrected for instrumentally caused differences and normalized to physical parameters of rocks. Two thick packages of the rocks - approx. 1250-1440 m and approx. 1470-1690 m - have identical geophysical sequence patterns and must be equally aged. Subsequently, the merge of three biostratigraphical databases provided an evidence for sequentially grouped biomarker patterns (such as maxima of Amphipora angusta with stepwise occurrences of Amph. pinguis and Alveolites mailleuxi) that stratigraphically repeat in the same way as shown in gamma-ray or other logs. In addition, the upper termination of the lower group of segments (at depths under and at 1450 m) is marked with Amph. moravica, which is an index fossil of Upper Frasnian. The main interface of these tectonically repeated units is the thrust fault at 1450 m. Surface of this fault involves stretched remnants of Namurian sandstones with mud-supported structure of this sediment. Shear dissolution of carbonate is coupled with this structure. The thrust kinematics is expressed also by strong folding in upper part of Ja-7 Paleozoic. Relatively slight partial bedding-parallel movements accompany the main fault. The allochthonous part of the Upper Givetian to Middle/Upper Frasnian sediments have reduced diversity and abundance of fauna, with dominance of sponges, which all largely correspond to depositional environments on the northwest. Conversely, the Upper Frasnian sediments of allochthonous part are much thicker in comparison with the (para)autochthonous part. Comparable beds are completely absent in the south and east of the Ja-7 but they were documented on the west and modelled in the north, because of latest Frasnian sea-level drop and progradding of carbonate shore. This considerable facies difference between the (para)autochthonous and allochthonous parts allows a translation up to several kilometers was from northwest to southeast.

Bibliografická citace

Hladil, J., Jansa, L. F., Těžký, A., Helešicová, K., & Hrubanová, J. (2016). STRATIGRAFICKÉ DOKLADY O NÁSUNOVÉM ZLOMU VE VRTU RAŠKOVICE JA-7. Geologické výzkumy na Moravě a ve Slezsku, 7. Získáno z https://journals.muni.cz/gvms/article/view/5108

Klíčová slova

Thrust fault; Paleozoic; Devonian; (para)autochthonous basement of Carpathians; northern Moravia and Silesia

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