Anomalia irydowa na granicy jury środkowej i górnej

W. Brochwicz-Lewiński, A. Gąsiewicz, W.E. Krumbein, G. Melendez, L. Sequeiros, S. Suffczyński, K. Szatkowski, R. Tarkowski

Abstrakt


IRIDIUM ANOMALY AT THE MIDDLE/UPPER JURASSIC BOUNDARY

Summary
The hypothesis put forward by D.M. Raup and J. Sepkoski (15), linking major mass extinctions with cyclic cosmic catastrophes, focused attention on several geological boundaries. This is also the case of the Middle/ /Upper Jurassic boundary, shown as the most doubtful of 12 probable events (15, Fig. 1). Therefore, some major laboratories appeared interested in analysis of material from our sections of the boundary. Analyses made by G.M. Kolesov and L.D. Barsukova (Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow) revealed the presence of Ir anomaly in 4 sections from southern Poland and Spain (Fig. 2). Further analyses (made by M. Zolensky and A.V. Murali, NASA, Houston, and F. Asaro, Berkeley Laboratory, University of California) confirmed the above results, showing significant concentrations of Ir and Os (in preparation to press).
There remains to be solved the question whether the Ir anomaly and very high enrichment of rocks in products of ablation of one or more cosmic bodies (18, see also 16) are explainable as due to unusual drop in rate of sedimentation or a catastrophic event. The recorded geochemical anomaly appears difficult to explain as due to selective binding of metals by organisms building bacterial-fungal Fe-stromatolites. The anomaly is marked also below and above the stromatolites and concentrations of metals in stromatolite matrix are so high that biological forms fail to appear in scanning maps of distribution of individual metals. Therefore, it seems more plausible that these specific stromatolites have originated in result of some significant changes in chemistry of marine water.
Stratigraphic analyses showed that rocks displaying the anomaly most probably originated in the Lamberti zone. The available data (especially those indicating catastrophic nature of "nodular layer" - 2-5) speak against a drop of rates of sedimentation to a level lower than that of oceanic clays. An increased frequency of fall of small cosmic bodies (meteoritic shower) also does not seem to be a solution here. It may explain the recorded high concentration of cosmic material and, eventually, the Ir anomaly, but fails to explain their coincidence with a phase of block movements and volcanic and hydrothermal activity. Similarly as in the case of the Frasnian-Famennian boundary (11) the events seems most easy to explain in terms of "tectonic phase", most probably triggered by a fall of one or more large cosmic bodies and propagation of stresses along reactivated and newly formed weakness zones.
Our further works are concentrated on a more unequivocal proving a lateral transition from "nodular layer" facies (with or without Fe-stromatolites) to phosphate- nodules-bearing glauconitic facies in southern Poland and. other regions and, what is also not excluded, to that of black shales.

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