The controlling mechanism of Carboniferous cyclic sedimentation

Abstract

The Carboniferous cycles of sedimentation of Europe and North America are particularly well developed examples of a phenomenon which occurs throughout the geological column. The Carboniferous cycles are shown to be caused by sedimentation following an abrupt rise in sea level relative to the plane of deposition. Changes in sea level relative to the plane of deposition may take place either from eustatic sea level changes or from vertical crustal movements of tectonic origin. Corresponding to these possibilities, two hypotheses have been proposed to explain the controlling mechanism of sedimentary cycles: (i) the eustatic hypothesis —cyclo-thems are formed in a subsiding region as a result of eustatic changes in sea level; (ii) the tectonic hypothesis- —in a subsiding region cyclo-thems are formed by subsidence of the crust taking place in rapid steps separated by longer periods of relative quiescence. Examination of sedimentary cycles at the margins of Carboniferous basins shows that they do not conform to the pattern which would be expected if the sea-level changes were eustatic. This is because eustatic rise in sea level must equally affect the margins and central part of the area of deposition. Deposits of this type are rare in the geological column outside the Quaternary where eustatic lacustrine and sea level changes are known. They are unknown in the Carboniferous basins of deposition. The tectonic hypothesis can be readily understood, however, in the setting of the mantle flow theory for the origin of sedimentary basins ( Bott 1964 ). This theory links isostatic uplift of a denuding mountain range with contemporary sinking of adjoining basins and accounts for the presence of deep sedimentary basins filled with uniformly shallow water deposits. Modern ideas on the rheology of the crust and the upper mantle suggest that subsidence to form sedimentary basins may normally take place in sudden steps along fault lines. It is suggested that such repeated crustal subsidence provides an explanation of Carboniferous cyclic sedimentation.