The frequencies of Scottish Pendleian allocycles

Abstract

At least two orders of allocycles, termed for convenience ‘low frequency’ and ‘high frequency’, are detectable in the Pendleian sedimentary succession of central Scotland (Read and Forsyth 1989; Read 1993 a ). New evidence suggests that the high frequency cycles were orbitally forced and provides an incentive to estimate their frequency, despite the inadequate framework of radiometric dates for the earliest Namurian. The low frequency allocycles, which Read (1993 a ) provisionally equated with the basic third-order cycles of the Exxon Production Research Company’s (EPR) sequence stratigraphy (see Mitchum and Van Wagoner 1991), reflected a series of major marine transgressions. These may be traced over the greater part of the Midland Valley, into England and (in some cases) far beyond. Until recently, it had been assumed (Read 1993 a ) that only three such major transgressions could be identified within the Scottish Pendleian succession, namely the Top Hosie Limestone, the Black Metals and the Index Limestone. A further undoubtedly major marine transgression associated with the Orchard Limestone marks the base of the Arnsbergian. The Eumorphoceras pseudobilingue and Cravenoceras cowlingense ammonoid (goniatite) bands which correlate with the Index and Orchard horizons respectively (Ramsbottom 1977), have been reported from as far away as the Donetz Basin (Aisenverg et al . 1979), strongly suggesting a eustatic origin. However, palaeontological evidence suggests that there are two more Pendleian transgressions which mark low-frequency allocycles. Firstly, as N. J. Riley has informed the author, Cravenoceras cf. malhamense has been recorded as far afield as Poland (Korejwo 1969). This form characterizes the ammonoid band . . .