The Problem with Sequence Boundaries: Insights from Late Quaternary Systems and Experimental Studies

The sequence boundary was defined as an unconformity and its correlative conformity, a surface of chronostratigraphic significance that everywhere separates all strata above from all strata below. Within incised valleys, the sequence boundary is commonly defined by the base of fluvial incision, which demarcates a basinward facies shift. Use of this surface is derived from a model for fluvial incision and sediment bypass during relative sea-level fall: fluvial deposits that rest on the sequence boundary are commonly assumed to have filled an empty container during late lowstand or early transgression.

We suggest the sequence boundary is a useful concept, but needs corrective surgery to repair misconceptions about fluvial response to sea-level fall, and links between fluvial and deltaic/shorezone facies. First, the model of fluvial incision and sediment bypass during sea-level fall has never been verified in Quaternary systems or experiments, and can be falsified. Second, sediment bypass and deposition within incised valleys are not mutually exclusive: erosion and channel-belt deposition continuously redefines the valley shape over the duration of a base-level cycle, and at any one point the basal valley-fill surface is the same age as fluvial deposits that rest on top of it. Third, extension of valleys across a newly emergent shelf is accompanied by linked channel-belt deposition and delta progradation.

In the coastal plain segments of incised valleys, valley evolution is stepwise, with periods of incision followed by periods of lateral migration and channel-belt deposition. The resulting basal surface is time-transgressive and never existed in the landscape at any one time: however, the basal surface is an unconformity, and meets the original criteria for a sequence boundary. By contrast, deltaic and shoreface facies on the shelf, which reside below the classically-defined sequence boundary, are commonly fed by, and the same age as, fluvial deposits above the same surface, which violates criteria for definition of an unconformity. In the cross-shelf part of the system, the base of fluvial deposits is a facies boundary, a surface with practical mapping implications that demarcates the basinward extent of fluvial systems in response to relative sea-level fall, but has no sequence stratigraphic significance. The surface equivalent to the sequence boundary farther updip is a composite downlap surface at the base of prograding clinoforms.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California