Applicability of Sequence Stratigraphic Models to Thick Fluvial Successions in Tectonically-Active Basins

Abstract

Sequence stratigraphic models for thick fluvial successions continue to evolve to account for controlling factors other than base-level fluctuation. For instance, many models place a sequence boundary at the base of amalgamated channel-belt deposits that cap coarsening-upward accumulations, relating this surface to a drop in base level. However, this surface is often characterized by features more indicative of lateral channel-belt migration under conditions of aggradation. These successions commonly develop significantly inland of likely influence by marine shoreline fluctuations and may not respond to eustatic base-level controls, particularly when factoring lag time for effects to propagate upstream. Additionally, these deposits are typically found in settings of relatively continuous subsidence accompanied by high sedimentation rates, such as foreland basins, in which accommodation is produced proximally to the source, trapping much of the sediment before it reaches a position where it can be impacted by eustatic base-level controls. Deposits that accumulate during early phases of foreland development do not have a connection to the marine realm, yet demonstrate similar patterns to those that do. Other models suggest accommodation is produced by tectonically-induced subsidence, with filling in response to either a slowing of space production or to simple progradation, as coarser proximal deposits accumulate over finer distal deposits. Other factors include variability in discharge relative to sediment supply and distributive vs. contributive channel patterns. With each addition comes new terminology that, in the end, still ties successions to “sequence boundaries,” which, by definition, are “unconformities and their correlative conformities.” Part of the complexity may arise from applying concepts where they don’t fit. One model might work for passive margins, another for foreland basins, and another for rift basins, yet there will always be exceptions, even between one foreland basin and another or within the same basin. Sequence stratigraphy is an effective tool for analyzing sedimentary basins, but we might be handicapping ourselves by forcing it into situations for which it was not designed. I propose it would be more effective to refrain from all-encompassing formal labels and return to a simple descriptive terminology, such as “coarsening upward interval” and “gradational contact” to describe and interpret thick fluvial successions.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018