Sedimentary cycles in coal and evaporite basins and the reconstruction of Paleozoic climate

Abstract

This thesis deals with large-scale processes controlling the formation of sedimentary cycles in coal and evaporite basins and their relation to large-scale fluctuations of Palaeozoic climate. Coal-clastic cycles dominate Pennsylvanian sequences in palaeo-equatorial basins from Euramerica. They formed under influence of glacio-eustatic sea-level changes driven by waxing and waning of southern-hemisphere ice sheets. It has proven difficult to unravel the sea-level signal from cyclothem records. This is attributed to the fact that multi-period, Milankovitch-driven sea-level cycles interfere with each other, which results in highly variable cyclothem thickness. It is shown based on cyclothem sequences from the Netherlands and Kentucky (USA) that is possible to reconstruct the sea-level history from a sequence only when cyclothem composition is taken into account. The sea-level signal that is present in cyclothem sequences may be overprinted by a tectonic signal. Based on an example from the Upper Silesian Basin (Poland) it is shown that pulses of thrust-controlled subsidence are represented by thick fluvial sandstone bodies whose abundance is proportional to fault-controlled differential subsidence. Evaporite basins are common throughout the Palaeozoic, but are particularly dominant during the Cambrian, Devonian and Permian. It is shown that evaporite cycles (sulphate-halite-potash sequences) have formed in response to progressive closure of ocean corridors by the progradation of basin-margin sulphate platforms. This makes evaporite deposition a self-regulatory process that automatically proceeds to halite and potash precipitation as the platform progressively obstructs basin outflow and the basin becomes more saline. Numerical modeling has shown that the process is very fast, especially in giant basins where a large surface area allows rapid closure of the ocean corridor. The typical upward thinning of successive evaporite cycles suggests that rapid basin filling triggers an isostatic subsidence reaction that generates accommodation space for the next cycle. It is further shown that evaporite basins are very extensive (~2 million sqkm) during the Cambrian, Devonian and Permian periods and much smaller (0.5 million sqkm) during the Ordovician-Silurian and Carboniferous. The transition between these two states is very abrupt and coincides with the major Palaeozoic extinction events. This is attributed to size fluctuations of the sub-tropical arid-climate zone controlled by changes in atmospheric circulation patterns according to a 120 My climate cycle.