Orbital-climatic forcing of Namurian cyclic sedimentation from spectral analysis of the Limestone Coal Formation, Central Scotland
Authors: Weedon G. P. , Read W. A. 
Published: April 2008

The Limestone Coal Formation of the Glasgow-Stirling area of the Midland Valley of Scotland was deposited in a composite basin divisible into the fluvially-influenced or proximal Kincardine Basin in the east and the distal, delta-dominated Kilsyth Trough. Three main factors influenced deposition and determined the geometry of the various lithologies: tectonism, glacioeustatic changes and localized irregular crevassing/avulsion processes. If the sea-level changes were induced by orbital-climatic control of global ice volumes, then the resulting sedimentary cyclicity might be expected to be regular (near-constant wavelength). However, the irregular processes would produce superimposed irregular sedimentary cyclicity. In this study spectral analysis of time series was used to test for regularity and to distinguish the various forms of sedimentological control acting within the basin.

Six localities spanning proximal to distal settings, and exactly the same stratigraphic intervals in the upper part of the formation, were used. Digitization of rock types using different numeric codes for coal, claystone, siltstone, fine-, medium- and coarse-grained sandstone was used for time series generation. Blackman-Tukey Fourier power spectra revealed that in the lower half of the sections four localities contained evidence for regular cyclicity significant at the 90 0.000000e+00vel. In the upper half of the data only one section revealed regular cyclicity. However, following decompaction the same regular cyclicity was detected in the lower part of the data, but with a considerable concentration of variance at particular frequencies, the significance had risen to 95%. In the upper data decompaction revealed two regular cycles. The decompaction procedure appears to have substantially removed compactional distortions related to rock type.

Very crude estimates of the periodicity of the regular cycles indicate that a high-frequency, possibly obliquity or short eccentricity cycle is involved. All the regular cycles detected in the decompacted series occur in dominantly distal fluvio-deltaic sections. This suggests that the main signal relates to successive progradation of deltas in distal settings under regular glacio-eustatic control. Recategorizing the data into simple non-sandstone/sandstone and coal/non-coal categories revealed no major changes in the spectral results. This supports the model of the cyclicity as arising from a simple grain-size oscillation related to deltaic progradation. The lack of regular cyclicity in the more proximal areas and in the upper half of the data probably reflects fluvial sedimentation producing more irregular grain-size variations related to channel avulsion, crevassing and meander migration plus, perhaps, increased hiatuses and the influx of coarse siliciclastic material forming thick wedges of multi-storey sandstones. The study illustrates the importance of applying decompaction and of using multiple sections for the study of regular cyclicity in cyclothem series.