Role of Deltas in the Formation of Some British Lower Carboniferous Cyclothems

Abstract

Well-developed cyclothems are characteristic of the Yoredale Series of the Lower Carboniferous in northern England and Scotland. Each cyclothem consists of up to eight members, all impersistent, some especially so. The presence of the top three members is conditional upon the occurrence of sandstone. A maximum of eight major cyclothems is present in the English outcrops; some of the major cyclothems contain minor cyclothems of much more restricted extent, whose appearance is again conditional upon the occurrence of sandstone in the major cyclothem. Individual cyclothems have not yet been traced into the Scottish outcrops. Each major cyclothem is the result of interplay of two environments: a shallow epicontinental sea, whose normal sediment was limestone, and the delta of a large river draining the continent north of the sea. Comparison of the ancient delta with that of the Mississippi River suggests that the ancient delta advanced laterally down the coast of the continent, pushing out southwestward from the mouth of the alluvial valley. Regional subsidence, slower than delta deposition but continuing after the delta built up above sea level, gradually drew the bypassed parts of the delta plain below the sea. Finally, when the active head of the delta lay far to the southwest of the alluvial valley, major crevassing of the trunk river caused abandonment of the old delta and the initiation of the new. In the meantime the old delta sank below sea level, and limestone was deposited upon its surface by the encroaching sea. The mechanism of cyclic advance of the delta is sufficient to account for all the observed features of the Yoredale Series cyclothems without recourse to more fanciful ideas.