International Geologiical Congress - Oslo 2008

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HPF-01 General contributions to paleontology and historical geology - Part 1

 

The gross environmental phenomenon of the classical Pragian stage (hot lowstand)

 

Jindrich Hladil, Institute of Geology AS CR, v.v.i. (Czech Republic)
Ladislav Slavik, Institute of Geology AS CR, v.v.i. (Czech Republic)
Petr Schnabl, Institute of Geology AS CR, v.v.i. (Czech Republic)
Leona Koptikova, Institute of Geology AS CR, v.v.i. (Czech Republic)
Jaroslav Frana, Nuclear Physics Institute AS CR, v.v.i. (Czech Republic)
Frantisek Vacek, Charles University (Czech Republic)
Ondrej Babek, Palacky University (Czech Republic)
 

 

The Lochkovian and Pragian stages were defined in the Prague Synform, in upper-slope to toe-of-slope calciturbidites. The tectonic settings in distant N Gondwana peripheries correspond to late "oceanization" stages, thinning of Perunica transitional crust and reshaping of adjacent Rheic abyssal plains; paleomagnetic position 20 deg S. The Lochkovian is dominated by blackish-grey rhythmites with silica, phosphate and organic matter. On the other hand, the Pragian limestones are variegated (ammonitico-rosso type), with frequent hiatuses and occurrences of "white" reefs. Fine-bedded (amalgamated) yellowish-grey, pinkish and purple deposits contain recycled and submarine-weathered material.
Cements are much reduced with increasing depth and distance from islands; the typical highly polydisperse particle/grain-size distributions are tri- to tetramodal. Abundances of graptolites, marine plankton, siliceous sponges and conodonts dropped down (opposite to cephalopods and tentaculitoids). The partial restoration of "fresh-bioclast/lithoclast-fed" turbidite systems occurred in Zlichovian (dark grey, shale band rhythmites, with channelized breccia flows above the base, silica, organic matter and phosphate). The remarkable difference of the classical Pragian from the underlying and overlying sequences was confirmed by results of high-resolution outcrop logging: high Th/U ratios (4.0-GRS, 2.5-INAA) (x10, x5, compared to Lochkovian) and high magnetic susceptibility values (x4). K, Al, and almost all lithofile element concentrations are elevated by 200-300%. Hematite and iron oxyhydroxides occur instead of pyrrhotite-pyrite. Concentrations of LREE are apparently more increased than those of HREE (remineralization effects).
These overall characteristics of the Pragian are far more important - Similar hiatuses occur in the Appalachian basin or E Australia, and analogous facies features occur worldwide (e.g. Carnic Alps, Asturia). The environmental synthesis strongly suggests that the Pragian was a period of extremely low sea level and quite effective mixing/oxygenation of ocean water. Considering the intense chemical weathering, the terrestrial climates should be interpreted as "hot and humid", at least in comparison with Lochkovian and Early Emsian conditions. Such a "hot climate" concept may also fit the slow deposition of "red dacryoconarid (pteropod-like) oozes", rapid increase of faunal diversity in shallow subtidal habitats and boom of bioeroders. The long-term sea-level low (emerged land) is consistent with increased terrigenous flux.

 

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