International Geologiical Congress - Oslo 2008

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HPS-01 General contributions to stratigraphy

 

Relationships between magnetic susceptibility of limestones and sea level change ("direct relationship and major crises on the earth")

 

Jindrich Hladil, Institute of Geology AS CR, v.v.i. (Czech Republic)
Ladislav Slavik, Institute of Geology AS CR, v.v.i. (Czech Republic)
Leona Koptikova, Institute of Geology AS CR, v.v.i. (Czech Republic)
Petr Schnabl, Institute of Geology AS CR, v.v.i. (Czech Republic)
Frantisek Vacek, Charles University (Czech Republic)
Ondrej Babek, Palacky University (Czech Republic)
Milan Gersl, Czech Geological Survey (Czech Republic)
 

 

The development of longer and more detailed magnetic susceptibility logs through the Devonian stages provides better understanding of stratigraphic MS variation (environmentally driven MS patterns). Our experience refers to the Eifelian-to-Frasnian composite section from carbonate-platform settings (Moravian Karst; with F-F segments on slopes) and Pridoli/Lochkovian-to-Eifelian/Givetian section in upper-slope to toe-of-slope calciturbidites (Prague Synform). The first project is nearly finished and the second one is in advanced stage. The MS data are coupled with gamma-ray spectrometric measurements, detailed INAA and ICP-MS analyses of trace element compositions, thin-section and insoluble-residue petrology and other methods, which allow multi-parameter environmental synthesis. It is generally assumed that the relationship between the sea-level fluctuation (eustasy) and MS variation in (quasi)continuous carbonate stratal sequences follow the rule that "lowstands are marked by high MS values". This assumption is based on prior empirical studies and on a theoretical model for the total terrigenous flux. The model calculates that "the increase in dry-lands area must provide high amounts of weathering products". It is particularly true where "terrigenous" background sediments (eolian, riverine or coastal) are trapped in pure carbonates in ocean, somewhat resembling impurities in ice sheets. A similar role is ascribed to considerably enlarged catchment areas and prograding river mouths and deltas. The global MS correlations of the Kacak Event were the first that showed a massive inversion of typical regularities. The segment before the event datum is characterized by very low MS, where a "broad valley" has much reduced oscillations. The event datum itself is marked by an extraordinary MS peak, which is followed by high and highly oscillating values. The "comb" of high values gently fades upward. The "valley" corresponds to gradual sea level fall, whereas the elevated MS "peak-and-comb" mirrors a fast flooding episode and HST/FSST. Here we report that this inverted (mega)pattern indicative of major environmental events was also found at the Basal Chotec and Lower Kellwasser events, and, to a lesser extent also at the Lower pumilio and mid-punctata events. It may be concluded that the pre-event settings were linked to calm atmospheric conditions and reduced rainfalls while the event/post-event ones were much more dynamic than the normal climatic modes.

 

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