Eustatic Sea Level Changes Interpreted From Seismic Stratigraphy

riK AriKi - ill Association ot i'etroleuniGcologisls Bulletin V. 70, No. 2 (February 1986), P. 131-137, 5 Figs.

Eustatic Sea Level Changes Interpreted from Seismic Stratigraphy: A Critique of the Methodology with Particular Reference to the North Sea Jurassic Record'
ANDREW D. MIALL'

ABSTRACT Seismic stratigraphic methods for interpreting eustatic sea level change are based on generalizedstratigraphic models and chronostratigraphic interpretations that have not been substantiated by publication of the necessary basic supportive data. Chronostratigraphic correlations of sea level change curves between basins are based partly on circular reasoning. Onlap and offlap seismic models may not indicate sea level change but flexural subsidence, facies shifts, or changes in sea level in theopposite direction to that proposed. Resolution of some of these problems may be achieved by publication of basic data now in confidential industry files. The widely acclaimed "Vail curves" should not be used as templates for basin correlation until such data have been provided, and such use should be supported by appropriate conventional Uthostratigraphic and biostratigraphic documentation fromeach new basin. Where such data have been made available, as with the North Sea Jurassic, serious questions of interpretation are raised. INTRODUCTION Peter Vail and his colleagues at Exxon have brought about a revolution in stratigraphic thinking during the last ten years (Vail et al, 1977a, b). Their ideas about global eustatic control of unconformity-bounded sequences are contributions offirst-rate importance to our science. This paper does not take aim at the overall model or its broader conclusions. However, I do have four concerns of lesser magnitude. 1. An independent assessment of the methodology, the data, and the results is virtually impossible because little of the primary documentation has been published. The industry confidentiality problem is a source of major difficulties.2. The global charts of coastal onlap and sea level change have become accepted as a new standard of geologic time (for example they appear on the key synthesis charts of
©Copyright 1986. The American Association of Petroleum Geoiogists. All rights reserved. ''Manuscript received, April 1,1985; accepted, Octobers, 1985. Geology Department, University of Toronto, Toronto, Ontario M5S 1A1, Canada.Thanks are due to L. F. Brown, Jr., and C. Summerhayes (journal reviewers) and to A. Hallam, for useful and encouraging comments.

Harland et al, 1982) despite the absence of a rigorous, published examination of the supporting data. This contrasts dramatically with the enormous public debate and publication effort that have been going on for many years by geologists attempting to refine andcalibrate biostratigraphic, radiometric, and magnetostratigraphic time scales. Revisions of these scales appear frequently, whereas the "Vail curves" continue to be accepted as finished products (except for the revisions of the Jurassic scale, discussed below). This leads to and is part of the third problem. 3. The existence of the widely acclaimed Vail curves seems to be encouraging a circularity inarguments about local correlation. The seismic stratigraphers explicitly state that the curves have become their standard for geologic time. For example, with reference to the Jurassic of the North Sea, Vail and Todd (1981, p. 217) stated, "several unconformities cannot be dated precisely; in these cases their ages are based on our global cycle chart, with age assignment made on the basis of abest fit with the data." And again: "Interpretations [of stratigraphic sequences] based on lithofacies and biostratigraphy could be misleading unless they are placed within a context of detailed stratal chronostratigraphic correlations" (Vail et al, 1984, p. 143). The context of the word "chronostratigraphic" in this second reference implies correlation by tracing seismic reflections. 4. The type...