Alternative Views for Common Assumptions: Reassessing the Origin and Significance of Sequence Boundaries Using Field and Flume
Field data and recent experimental studies independently question long-held paradigms regarding the origin and time significance of fluvially carved sequence boundaries as well as genetic relationships between these surfaces and the strata they bind. These field data derive from an up-dip to down-dip transect through the Cretaceous Dakota Group of the U.S. southern High Plains. The experimental data derive from repeated basin-scale runs of sequence development during relative sea-level change simulated in the Jurassic Tank at the University of Minnesota, St Anthony Falls. Both experimental and field data show that fluvial sand above sequence boundaries are deposited coexistent with the carving of the underlying sequence boundary. The field data do this by inference from mapped cross-cutting relationships within observed stratigraphy and the experimental data through scaled reproduction of the processes inferred and products observed from the field. Both sources converge to reinforce assertions regarding sequence boundaries that require readjustment of some commonly held views. Namely, surfaces commonly mapped as sequence-bounding unconformities weren’t necessarily synchronously exposed, record no common age, and may not consistently separate older from younger strata. Also, fluvial strata above sequence boundaries do not necessarily reflect passive burial of these surfaces during subsequent transgression. Instead these strata may record co-generation of fluvial reservoir architecture and the underlying sequence-boundary over the full duration of the transgressive/regressive cycle because of close genetic links between the two. Furthermore, valley incision and sequence-boundary erosion need not reflect up-dip knickpoint migration from the shore, thus valleys and sequence-boundary continuity may commonly be lost down depositional dip.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009