Winker, Charles D.¹
(1) Shell International E&P Inc, Houston, TX

ABSTRACT: Diachronous Erosion Surfaces: Theory, Examples, and Implications

Diachronous erosion surfaces (DES), where some deposits above the surface are older than some deposits below, can be generated in a wide variety of depositional environments and over a wide range of spatial and temporal scales; all that is required is a migrating zone of erosion flanked by zones of deposition. One characteristic of DESs is that strata terminating from above and below the surface approach from opposite directions (if they approach from the same direction, the surface is unambiguously isochronous). On a Wheeler time-space diagram, the lacuna corresponding to a DES behaves similarly to a time-transgressive facies belt. A DES has two or more correlative conformities. A DES can be a single, simple surface, or it can be part of complex branching or anastamosing system. Branching/anastamosing complexes can contain both isochronous and diachronous surfaces; the most obvious or prominent surface in the complex (the basal surface, for example) can by diachronous. 
Potentially troublesome occurrences are those associated with possible incised valleys, such that temporal relationships with subjacent, laterally adjacent, or downdip deposits may be ambiguous. From an exploration standpoint, an important implication is that the DES model tends to predict smaller volumes and shorter distances of basinward sand bypass than does the isochronous erosion surface (IES) model usually assumed in sequence stratigraphy. In stratigraphic practice, either DESs must be excluded as possible sequence boundaries (a challenging proposition), or the definition of sequence boundary should be relaxed to include possible diachroneity.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.