Cyclostratigraphic reasoning and orbital time calibration

Cyclostratigraphy postulates that the stratigraphic record cyclically encodes the periodic orbital forcing of terrestrial insolation, thus providing a time calibration. Regular cycles are sought using spectral analysis of lithological data series, but there are inherent ambiguities in this method. It may detect more cyclicities than conventional orbital forcing allows, but only those with the closest correspondence to estimated orbital frequencies are used for time calibration. Irregular cycles are subjectively defined in terms of non-rhythmic repetitions of facies. The calibrations assume that they record the spatially distorted sedimentary effects of orbitally forced periodicity in insolation. The null hypothesis that such non-rhythmic repetitions are autogenic, rather than orbitally forced, cannot, however, be rejected. Both types of cyclicity conform to an 'expected universe' where orbital forcing is reliably and recognisably encoded in the stratigraphic record. Neither form of cyclicity rules out the presence of hiatuses; thus, even if orbital in origin, neither can provide dependably refined, orbitally scaled, time calibration.