Geophysical records of dispersed weathering products on the Frasnian carbonate platform and early Famennian ramps in Moravia, Czech Republic: proxies for eustasy and palaeoclimate
- 作者：Hladil ; Jindrich
- 关键词：carbonate sequences ; γ ; -ray spectrometry ; magnetic susceptibility ; eustasy ; Late Devonian ; Czech Republic
- 刊名：Palaeogeography ; Palaeoclimatology ; Palaeoecology
- 出版年：2002
- 期刊代码：68_00310182
- 类别：ear
- 出版时间：June 20, 2002
- 卷：181
- 期：1-3
- 页码：213-250
- 文件大小：835 K
摘要
An exceptionally high sea level in Frasnian times (early Late Devonian) changed many peneplained, cratonised and tectonically undisturbed continental margins of tropical belts into carbonate platforms which became covered by thick banks of limestone. The stages marked by very pure carbonates developed during partial sea-level highstands since, when the flooding reached its maximum, the shoreline was distant and conditions for reef growth were good. An increased extension of emerged, flat, coastal areas was coupled with an opposite situation, when oscillating sea level fell to its momentary minimum and the accommodation space available for accumulation of carbonate material was low. Subaerially exposed surfaces were subject to weathering and the products were dispersed by aeolian or aquatic means to form layers when these weathering materials were trapped in carbonates. This overall scenario relating to the trapping of weathering products in pure carbonates was tested at geographically isolated platform with a stable and only slowly subsiding basement (Eastern Moravia, Czech Republic). The amounts of weathering products dispersed in limestone were documented using the combined methods of γ-ray logging, γ-ray spectrometry and magnetic susceptibility measuring, i.e. according to their radioactivity (K, Th and U) and magnetism (Fe). Six third-order Frasnian eustatic cycles have been inferred from the data, with an estimated period of 1 Ma. The Late Frasnian and especially the Frasnian/Famennian (F/F) transition brought the total destruction or at least a strong overlay on this hypothetical background setting. The record of strong sea-level fluctuations in the F/F interval is shown in strong oscillations and offsets related to pronounced hiatuses and flooding surfaces. Such rapid sea-level fluctuation is typical for glacioeustatic control, although the present estimates of the maximum amplitudes are larger than realisable by glaciation events and may be explained in combination with tectonoeustatic models. Comparison of sections between the centre and periphery of the platform substantiates the concept that oscillating Late Frasnian shorelines were rapidly prograding. Very low sea-level stages of early Famennian times changed the depositionary systems from extensive platforms to narrow carbonate ramps which were positioned out and down on the extinct reef bank slopes. The central parts of the Eastern Moravian and also other Frasnian platforms in the world were not normally covered by F/F sediments, the transgressive onlaps above any crystalline highs were not observed and therefore the drowned Frasnian reefs on their periphery, although emphasised by many geologists, may reflect sliding or tectonic subsidence on the margins of platform blocks rather than eustatic rise of sea level.