Spectral analysis of a Precambrian limestone-shale sequence, lower Vindhyan, India

Abstract

The Precambrian Lower Vindhyan limestone-shale sequence which exhibits predominantly subtidal shelf facies, has been subjected to the power spectrum analysis. The limestone percentages calculated at 20 cm intervals at Pipradih I (Tuira-Pipradih) and Pipradih II (Dhabua) sections form raw data for the analysis. A significant periodicity of approximately 111 cm has been detected by the study of the power spectra. The corresponding sedimentary oscillations are also recorded relative to the position of master bedding planes in the field as well as to the variation of insoluble residue of the limestone. The relative persistence of cycle wavelength of 111 cm, which is independent of facies change throughout the stratigraphic succession, indicates an effect of a long-range rhythmic process during deposition. In view of established worldwide tectonic movements between 1600 Ma and 900 Ma (middle Proterozoic), related eustatic fluctuations of sea level have been suggested as a cause of the cyclicity in the Lower Vindhyan limestone-shale sequence. However, the difference in the pattern of power spectra of lower and upper units (Pipradih I and II, respectively) with respect to Markovian persistence and amplitude of spectral peaks, indicate a greater influence of local environmental factors in the lower unit as compared with the upper sequence.