The Many Faces of Erosion: Theory Meets Data in Sequence Stratigraphic Analysis

William E. Galloway and Dennis A. Sylvia: Department of Geological Sciences and Institute for Geophysics, The University of Texas at Austin, Austin, Texas

Abstract

Sequence stratigraphic application has emphasized the recognition and use of subaerial (fluvial entrenchment) or shallow marine/shoreface (regressive ravinement) surfaces as critical boundaries for defining sequences. These surfaces are variously objectively or conceptually associated with times of onset, maximum rate, and/or lowest position of relative sea level fall. However, well-dated Quaternary analogues demonstrate that the fluvial entrenchment surface is neither inherently synchronous nor regional, and that low-stand facies associations and their bounding surfaces are highly dependent upon the vagaries of paleogeography and sediment supply. Furthermore, a growing array of examples display stratigraphies in which demonstrable subaerial or ravinement surfaces correlative to fall events are poorly preserved or entirely lacking, but in which sequences can be defined by use of various combinations of transgressive ravinement, marine deflation, and marine starvation surfaces.

These surfaces may not and need not correspond to a relative fall (or rise) of sea level. Selection of stratigraphic surfaces as sequence boundaries and interpretation of sequence systems tract compositions and relationships both require understanding of the overall depositional systems tract and of the full array of regime variables: sediment supply, sediment composition, base level change, and energy regime. Functional, reproducible, and chronostratigraphic "genetically related successions of strata bounded by unconformities or their correlative conformities" can be defined, correlated, mapped, dated, and interpreted through the use of a variety of regional stratigraphic surfaces of non-deposition and erosion