WEIMER, ROBERT J., Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO

ABSTRACT: Sequence Stratigraphy: A Historical Perspective

Sequence stratigraphy was originally defined by Sloss as the study of genetically related strata that are bounded by unconformities. A sequence was regarded as a lithostratigraphic unit. The definition has been expanded to "bounded by unconformities or their correlative conformities" (Mitchum et al.) and a sequence was changed to a chronostratigraphic unit.

Using this broadened definition, a new hierarchy of chronostratigraphic terms has been introduced to subdivide a depositional sequence (Exxon model). Contrary to this proposed usage, I believe that the new terms are still lithostratigraphic in content and I regard them as unnecessary because they largely duplicate existing current terminology, in part formalized by the stratigraphic code of the North American Commission of Stratigraphic Nomenclature. This code has the flexibility for use by all workers using stratigraphic terms and for change as needed. Moreover, the nomenclature allows for separation of objective observational data from interpretations, a goal essential to all scientific studies.

When viewed within the historic framework of stratigraphic analyses, sequence stratigraphy in a strict sense is a specialized study of lithostratigraphy, which emphasizes unconformities or key surfaces, condensed sections, and related facies associations. In a broad sense, sequence stratigraphy is the same as stratigraphy except with more emphasis placed on explaining sedimentary

cycles caused by relative sea level changes, syndepositional tectonics, or autocyclic depositional processes. However, without recognition of unconformities, a sequence stratigraphic study would be no different from a traditional stratigraphic analysis in describing and interpreting sedimentary cycles on all scales.

In some basins, e.g., Mesozoic and Cenozoic continental margin basins, subsurface stratal patterns derived from multifold seismic profiles also are used in establishing sequences. However, such expensive seismic data are not always available to stratigraphers for analysis and interpretation. Furthermore, the cratonic and some foreland basins seldom exhibit stratal patterns on seismic sections to identify sequences. For these reasons, stratigraphic terminology derived from seismic data should not be established to guide other types of stratigraphic analysis.

In petroleum exploration within shelf areas of foreland and continental margin basins and cratonic basins, two types of unconformities are particularly important, both related to sea level changes. The first type of unconformity, a subaerially exposed lowstand surface of erosion (LSE, sequence boundary), is caused by relative sea level lowering. The boundary is recognized by incised paleovalleys, paleosols, and missing facies. The second type of unconformity is a transgressive surface of erosion (TSE, sometimes called a ravinement surface), and occurs where shoreface erosion moves over coastal plain deposits during a relative sea level rise.

Examples of subtle stratigraphic traps in siliciclastic rocks associated with unconformities are discussed for the Lower Pennsylvanian strata of the mid-continent region. Also reviewed are the problems of applying the new sequence stratigraphic terminology in relation to established terminology.

AAPG Search and Discovery Article #90991©1993 AAPG Mid-Continent Section Meeting, Amarillo, Texas, October 10-12, 1993.