Abstract

Rotliegend sequence stratigraphy in the Southern Permian Basin suggests climatically forced fluctuations in the level of an expanding playa lake system. The associated facies changes are abrupt and define essentially isochronous sequence boundaries. Spectral analyses of gamma ray logs have indicated that individual sequences display internal cyclicity consistent with orbital (Milankovitch) forcing. Confirmatory results have been reported from the spectral analysis of field logs from the continental Permian Brodick Beds of the Isle of Arran. These studies claim that, since the cycles are orbitally forced, they have chronostratigraphic significance. However, they are based on dubious assumptions concerning the unvarying rate and continuity of deposition of the sequences in question. In the case of the Southern Permian Basin, the assumption is applied to c. 50 m units, but the result is an implausible chronostratigraphic model. For the Brodick Beds, the assumption is applied to a 675 m succession and there is a crucial ambiguity in the relationship between the lithostratigraphic cycles and the Permian orbital cycles. The studies do not provide persuasive evidence of the consistent encoding of Milankovitch orbital cycles in Permian desert deposits.