Palaeoclimatic significance of open-marine cyclic sequences
The offshore facies of the basal Lias of S.Britain was studied as a typical example of an open-marine cyclic sequence. The sedimentology, geochemistry and power-spectral analysis were investigated in order to understand the cause of the interbedded rock types. Three sediment types were deposited on the sea-floor: light marl, dark marl and laminated carbonate-rich shale. Calcite microspar, the dominant carbonate component, appears to have been formed from the neomorphic aggradation of coccoliths supplied in zooplankton faecal pellets. During sulphate reduction, the most carbonate-rich horizons in the light marl and laminated shale beds were cemented by carbonate, producing early diagenetic limestone and laminated limestone beds and nodules. Walsh power-spectral analysis of several measured sections in the basal Lias indicate that two regular sedimentary cycles, with periods of tens of thousands of years, are present. The regularity, stability and periods of the cycles invokes the Milankovitch Theory of orbital forcing of sedimentation; the cycles thus probably represent periods of 41,000 and 21,000 years. The sedimentation appears to have been linked to climatic variation by the levels of runoff and the formation and destruction of wedges of brackish water. During dry periods relatively little runoff and low clay input allowed turbulent, oxygenated bottom-waters and the deposition of burrowed, organic-poor marl. During wet periods, brackish wedges caused widespread density stratification, bottom-water anoxia and high clay inputs that resulted in laminated shale beds. Walsh power spectra were generated for one Silurian, five Upper Lower Jurassic, one Kimmeridgian and one Oligocene formation. Unexpectedly the Early Jurassic appears to have been dominated by the Milankovitch cycles related to obliquity and precession rather than eccentricity. Of the thirteen spectra produced, including five from the basal Lias, ten (or about 80%) contain evidence for regular sedimentary cycles consistent with orbital forcing of sedimentation. Therefore the Milankovitch Theory should be considered whilst investigating open-marine 'cyclic' sequences.