The tectonic factor in sea level change: A countervailing view

Abstract

It is now demonstrable that many unconformity‐bounded assemblages of strata, the sequences and groupings of sequences of seismic stratigraphy, can be correlated subglobally. The facies tracts that are identifiable within sequences clearly represent depositional environments that bear a systematic relationship to the strandline and to water depths, and thus to sea levels. Lacking a more suitable explanation for the intercontinental correlation of sequences it has become widely accepted that eustatic changes of sea level are responsible for the interruptions in sedimentation that separate sequences and for the shifts of coastal onlap and of facies tracts implicit in sequence analysis. Students of cratonic basins are convinced that continental interiors are subject to a tectonic evolution of positive and negative vertical movements and that these control relative sea level and the distribution of sedimentary environments. Proof of this control is buried hi the sedimentary fills of cratonic basins, but it has been difficult to produce such proof in a form digestible by Neo‐Neptunist doubters. Digitization of isopach maps of successive stratal units produces thickness data amenable to numerical study. For example, the logarithms of thicknesses of a stratal unit commonly exhibit a uniform areal distribution, the hallmark of a bivariate‐normal distribution. This revelation makes available a number of Gaussian statistical measures as descriptors of time‐variable basin geometry. Extraction and analysis of geometric parameters lead to the realization that basin evolution, necessarily including relative sea levels and the transgression and progradation of strandlines and sedimentary environments, has a strong tectonic component that gives evidence of synchrony of widely separated basins on separate cratons. Secular variation in radial heat flow, leading to episodic inflation and deflation of continents is suggested as a possible globally effective mechanism.