Stratygrafia dewońskich utworów węglanowych wschodniej części Gór Świętokrzyskich

Abstract

Na podstawie głębokiego otworu wiertniczego Janczyce I (1251,6 m) - najpełniejszego z opisanych dotychczas profilów dewońskiej serii węglanowej w Górach Świętokrzyskich - zdefiniowano osiem nieformalnych jednostek litostratygraficznych, reprezentatywnych dla wschodniej części synklinorium kielecko-łagowskiego oraz rejonu Bud-Jurkowic. Zestawiony w ten sposób podstawowy szkielet litostratygraficzny umożliwia korelację profilów wierceń surowcowych oraz odsłonięć opisanych z tego obszaru, a także ekstrapolację wyróżnionych jednostek na sąsiednie obszary. Jednostki te nawiązują częściowo do wydzieleń dawniejszych badaczy, w wielu jednak przypadkach można było obecnie ściślej sprecyzować ich definicje i granice oraz rozszerzyć opisy litologiczne. Zrywając z dotychczasową praktyką stratygraficzną, dyskusję dotyczącą chronostratygrafii opisywanych utworów oddzielono od opisu następstwa litologicznego.STRATIGRAPHY OF THE DEVONIAN CARBONATES IN THE EASTERN PART OF THE GÓRY ŚWIĘTOKRZYSKIE MTSThe investigated Janczyce I borehole is located in the eastern part of the Góry Świętokrzyskie Mts, central Poland (Fig. 1). It represents almost a complete transgressive Devonian carbonate shelf sequence, exceeding 1100 meters in thickness, and assigned. to the Middle and Upper Devonian. This sequence is underlain by terrigeneous quartzose sandstones and shales, from 100 to 260 m thick, and generally regarded as Lower Devonian. Upwards, it passes gradually into mostly argillaceous sediments of the Early Carboniferous age. Based on the Janczyce I profile the present authors defined eight informal units that constitute a representative lithotratigraphic framework of the carbonate Devonian in the eastern·Góry Świętokrzyskie Mts. The described units are of considerable lateral persistence and occur farther to the west as well. The distinguished units are as follows (from bottom to top) 1. Skeletal and/or bioturbated dolomicrites and dolosparites. Thickness - 129.0 m (Tabl. I, Figs 4-7).2. Crypto- to fine crystalline dolostones devoids of skeletal content. Thickness - 158.2 m (Tabl. II, Figs 8-11; Tabl. III, Figs 12, 14).3. Stromatoporoid·coral limestones, irregularly but mostly in their lower part replaced with the phanerocrystalline dolomite. Thickness - 493.7 m (Tabl. Ill, Fig. 13; Tabl. IV, Figs 15-17; Tabl. V, Fig. 18).4. Detrital limestones. Thickness - 35.6 m (Tabl. V, Fig. 19; Tabl. VI, Figs 20-22).5. Marly nodular limestones. Thickness - 44.9 m (Tabl. VII, Figs. 23, 25).6. Marly laminated limestones with intraformational disturbances of slump origin. Thickness 200,9 m (Tabl. VII, Fig, 24).7. Wavy-bedded argillaceous limestones. Thickness - 17.6 m. 8. Black marly shales with some marly limestone intercalations. Thickness - more than 33.4 m (total thickness not known). It is generally accepted in the geological literature, although not always confirmed by the appropriate fauna evidence, that the Devonian deposits comparable to the units 1 and 2 belong to the Eifelian Stage, unit 3 - to the Givetian, and units 4 trough 8 represent the Upper Devonian stages. The preliminary conodont investigations would place the Frasnian/Fammennian boundary within the monotonous unit 6. The latest Devonian and Early Carboniferous deposits are not represented in the Janczyce I profile owing to erosion. The depositional environment of the unit 1 may be interpreted as a shallow marine subtidal and partly intertidal carbonate facies abundantly inhabited by various burrowing and/or skeletal organisms. The dolomitization was probably mostly a post·depositional process, opposite to the unit 2. Here, subaerial erosion horizons, cryptalgal and Stromatolitic laminations, oolites, lack of skeletal and soft-bodied fauna. and in particular relics of evaporites would point to the hyper-saline eogenetic environment of a sabkha type. The unit 3 represents slightly deeper intertidal to shallow subtidal mosaic of calcareous facies with abundant biostromal skeletal accumulations. The basal part of the unit has been affected by a mesogenetic ascending dolomitization. The detrital calcareous deposits (unit 4) can be attributed to a shallow marine erosion of large biohermal structures that in places grew over the Givetian biostromal platforms. The units 5 -8 represent a gradual deepening of a sedimentary basin and an increase in supply of a terrigeneous mud content paralleled by a degradation of a subtidal skeletal benthic assemblage. Such a facies evolution finally resulted in anaerobic pelagic sedimentation in a deeper subtidal zone. The nodular structure present in the unit 5 probably originated owing to eogenetic selective cementation phenomena, with some influence of a gravitational sedimentary boudinage.