ABSTRACT Cycles recognised in the gamma-ray log of the Silurian Qalibah Formation have been time-series analysed. The periodogram lengths from the analysis have ratios coincident with the ratios of the Silurian precession, obliquity and eccentricity frequencies of the Milankovitch Band orbital periodicities. Most periodogram lengths are harmonics. The periodogram lengths for P1, P2, O1, O2 and E1 are 2.41, 2.82, 4.55, 5.47 and 15.62 feet respectively, for the Sharawra Member of the Qalibah Formation. These lengths can be used to estimate sedimentation rates and time durations. The organic matter within these cyclic sediments has been quantitatively studied in an attempt to relate the orbital signal to environmental change. The primary change within the organic matter appears to be increased preservation through increased organic productivity. These changes relate to increased gamma-ray log API and therefore cyclicity.