Evaporite-bearing sequences in the Zechstein and Salina Basins, with a discussion on the origin of their cyclic features
Factors controlling cyclic sedimentation are discussed in a parallel study of two evaporite-bearing sequence, the Zechstein of Germany and the Silurian Salina Group of the Appalachian Basin. The Zechstein sequence was deposited in a basin that had received the debris swept in from the Variscan orogenic zone. The deposition of the evaporite-bearing sequence took place during a period of tectonic calm, preceded and succeeded by mild late Variscan movements. The sequence is divided into four major cycles by shale horizons accompanied and basinwards partially replaced by dolomites and anhydrites. Halite is the dominant sediment, it contains beds of anhydrite and potash salts, less commonly of shale, forming with the halite sedimentary cycles of diverse magnitudes. The Salina Group has been deposited in a basin that had previously received debris from the Taconic orogenic zone. The last orogenic movements had virtually ceased before the deposition of evaporites commenced. The evaporite-bearing sequence is divided into three major cycles by shale suites related to alluvial, fans of debris swept in from the previous orogenic zone. The shale beds are accompanied by dolomite beds containing stromatolitic horizons. The salt contains shale and dolomite beds of diverse thicknesses, giving rise to cycles of varied magnitudes. With increasing distance from the orogenic zone, the thinner shale interbeds in the salt grade into anhydrite. In contrast to the Zechstein sequence, in the Salina Group thicker anhydrite beds are rare and no potash zones have been found. The anhydrite deficiency is attributed by the author to bacterial reduction of the CaSO₄. The H₂S thus formed is in part retained in the sediments, in part it deposited FeS₂ or re-oxidized. The lack of potassium salts indicates a less inhibited communication with the open sea, as also witnessed by repeated incursions of marine fauna. In both sequences, most sedimentary cycles are controlled by the periodic entrance of diluted waters into the basin. Rain water enters directly as well as in the form of terrestrial run-off from the adjacent mountains, introducing mud and foreign ions, diluting and changing the ion ratios of the brines. Sea water enters the basin continuously or periodically, the concentration increases caused by the concomitant inflow of dissolved salts are mitigated by the reflux of more concentrated brines. Abrupt dilution of the brines by sea water followed by slow evaporation produces cycles of progressive solubility in the sediments resembling experimental successions. The periodic entrance of rain and sea water can be controlled by several factors. Increases in rainfall, particularly in the detritus source area, may reflect morphologically or astronomically induced climatic changes; the morphologic factors may in turn be controlled by tectonism, erosion and sediment accumulation. The ingress of sea water can be caused by intermittent subsidence in the bar area, or by a rise of sea level induced tectonically, glacio-eustatically, or simply by a change in wind direction. A few models involving parallel control of terrestrial and marine inflow are presented at the end.