Luki i kondensacje na pograniczu jury środkowej i górnej
Abstract
ON NATURE OF BREAKS IN SEDIMENTATION AND CONDENSATIONS AT THE MIDDLE AND UPPER JURASSIC BOUNDARY
Summary
There is growing evidence for omnipresence of stratigrapic gaps and/or condensations in the Middle-Upper Jurassic junction beds in the Mediterranean and adjoining regions as well as zones predestined for intense and continuous sedimentation (e.g. Viking and Central Grabens, 26, and Mid-Polish Trough) in Europe and other continents. These phenomena may be easily explained in terms of overlap of crises in clastic and carbonate sedimentation. The former appears due to advanced transgression and resulting flood of landmasses and decrease in supply of clastic material. The crisis in carbonate sedimentation was recently explained in terms of CCD rise due to the above mentioned transgression (25, 19) or the transgression and accompanying intensification of subaqueous volcanic activity at mid-oceanic ridges (17). The paper presents results of tests of the above and other hypotheses with reference to the results of studies on classic Callovian-Oxfordian sections in the Cracow-Częstochowa Upland, where condensations and gaps are fairly well evidenced (20, 10, 15, 14, 9, 16).
Our studies showed the presence of high concentrations of cosmic dust - products of ablation of meteorites, which indicate either drastically low rate of sedimentation or, which is also not excluded, a catastrophic event connected with asteroid impact (as assumed in the case of the Cretaceous-Tertiary boundary - 8). The omnipresence of subaqueous corrosion has been found in both the Callovian (20) and lower parts of Oxfordian. Phenomena of redeposition of clasts up to 20 cm in size or even larger appear more common than hitherto inferred ( 14, 9 and others). A part of Oxfordian limestone layers display large intraclasts with specific, mineralized fractures which do not continue in surrounding rock. These and other features suggest that this may be a material of autoclastic breccia (formed due to seismic shocks - 21 ), redeposited by storms. The redeposition also affected ammonites, resulting in significant difficulties in establishing stratigraphic position of the strata, including those of lower part of the Middle Oxfordian. Moreover, geochemical studies showed marked enrichment of the strata in Fe, Mn and several other elements including Ni, Co, REE, V and PGE. The enrichment seems to give support to the hypothesis linking the CCD crisis with intensification of subaqueous volcanic processes and exhalations.
Figure 4 shows an attempt to reconstruct phenomena affecting history of sedimentation of the Callovian-Oxfordian junction beds in the vicinities of Kłobuck and Częstochowa (NW part of the Cracow-Częstochowa Upland). In result of these processes, thickness of 9 zones of the Callovian and Oxfordian is here reduced to 5 m or less and the strata are enriched in the above mentioned metals. We assume to be dealing with some kind of feedback: subaqueous volcanic activity led to marked decrease or even breaks in sedimentation of carbonates and the lower is the rate of sedimentation the higher is concentration of products of the volcanism and exhalations in the resulting sediment. The studies indicate that it would be purposeful to continue the surveys of condensed sequences of this type as in some of them there could have originated economic concentrations of metals.
Summary
There is growing evidence for omnipresence of stratigrapic gaps and/or condensations in the Middle-Upper Jurassic junction beds in the Mediterranean and adjoining regions as well as zones predestined for intense and continuous sedimentation (e.g. Viking and Central Grabens, 26, and Mid-Polish Trough) in Europe and other continents. These phenomena may be easily explained in terms of overlap of crises in clastic and carbonate sedimentation. The former appears due to advanced transgression and resulting flood of landmasses and decrease in supply of clastic material. The crisis in carbonate sedimentation was recently explained in terms of CCD rise due to the above mentioned transgression (25, 19) or the transgression and accompanying intensification of subaqueous volcanic activity at mid-oceanic ridges (17). The paper presents results of tests of the above and other hypotheses with reference to the results of studies on classic Callovian-Oxfordian sections in the Cracow-Częstochowa Upland, where condensations and gaps are fairly well evidenced (20, 10, 15, 14, 9, 16).
Our studies showed the presence of high concentrations of cosmic dust - products of ablation of meteorites, which indicate either drastically low rate of sedimentation or, which is also not excluded, a catastrophic event connected with asteroid impact (as assumed in the case of the Cretaceous-Tertiary boundary - 8). The omnipresence of subaqueous corrosion has been found in both the Callovian (20) and lower parts of Oxfordian. Phenomena of redeposition of clasts up to 20 cm in size or even larger appear more common than hitherto inferred ( 14, 9 and others). A part of Oxfordian limestone layers display large intraclasts with specific, mineralized fractures which do not continue in surrounding rock. These and other features suggest that this may be a material of autoclastic breccia (formed due to seismic shocks - 21 ), redeposited by storms. The redeposition also affected ammonites, resulting in significant difficulties in establishing stratigraphic position of the strata, including those of lower part of the Middle Oxfordian. Moreover, geochemical studies showed marked enrichment of the strata in Fe, Mn and several other elements including Ni, Co, REE, V and PGE. The enrichment seems to give support to the hypothesis linking the CCD crisis with intensification of subaqueous volcanic processes and exhalations.
Figure 4 shows an attempt to reconstruct phenomena affecting history of sedimentation of the Callovian-Oxfordian junction beds in the vicinities of Kłobuck and Częstochowa (NW part of the Cracow-Częstochowa Upland). In result of these processes, thickness of 9 zones of the Callovian and Oxfordian is here reduced to 5 m or less and the strata are enriched in the above mentioned metals. We assume to be dealing with some kind of feedback: subaqueous volcanic activity led to marked decrease or even breaks in sedimentation of carbonates and the lower is the rate of sedimentation the higher is concentration of products of the volcanism and exhalations in the resulting sediment. The studies indicate that it would be purposeful to continue the surveys of condensed sequences of this type as in some of them there could have originated economic concentrations of metals.