Fourier and Autocorrelation Analysis of Estuarine Tidal Rhythmites, Lower Breathitt Formation (Pennsylvanian), Eastern Kentucky, USA

Abstract

ABSTRACT Outcrops of the Pennsylvanian Breathitt Formation in eastern Kentucky reveal a rhythmic pattern of siliciclastic sedimentation in a marginal marine coastal setting. A 15-23 m thick stratigraphic interval of thinly interbedded, fine sandstone and shale displays tidally generated features such as flaser and wavy current ripple bedding, bipolar paleocurrents, and cyclic thickening and thinning of mud-draped sandstone layers. Bioturbation is common and increases toward the top of the section. Ichnogenera include Asterosoma, Arenicolites, Olivellites, Conostichus, Scalarituba, Planolites, Zoophycos, Teichichnus, Curvolithus, Chondrites, Neonereites, and Rosselia. A statistical analysis of sand layer thickness was carried out using shale partings as bounding surfaces for the individual sand units. Fourier and autocorrelation analyses were performed on two vertical sequences containing a total of over 2100 layers. The results reveal the presence of four cycles of thickness variation. First-order cycles consist of alternating thick-thin sand layers. These daily couplets may reflect unequal flood and ebb currents during a single tidal cycle or dominant and subordinate tidal deposits in an ebb or flood dominated semidiurnal or mixed system. Second-order cycles typically consist of 11-14 sand layers and reflect spring-neap variations in tidal range and current velocities. Third-order cycles are usually composed of 24-35 layers and are formed in resp nse to monthly variations in tidal range resulting from the ellipticity of the moon's orbit. Fourth-order cycles generally contain about 150 layers (range, 100-166) and were caused by seasonal maxima in tidal range associated with the solstice (winter, summer) and seasonal minima associated with the equinox (spring, fall).