Coastal Plain Incised Valley Complexes: Their Time Transgressive Nature and What They Can Tell Us about Paleo-Sea Level Fluctuations

Incised valleys preserved in the stratigraphic record are valley-form erosional surfaces that resemble incisional valleys observed on the Earth's surface today. So it is natural to think that the erosional surfaces we see preserved stratigraphically represent buried valleys, i.e., buried topographic surfaces whose geometries might help us partially reconstruct the environmental conditions that led to their formation . Here we explore this assumption via physical experiments in which we can observe and measure valley formation on the surface as well as the resultant deposit. We distinguish between topographic valleys, valleys defined by surface topography at some instant in time, and stratigraphic valleys, which are preserved valley-form erosional surfaces in the stratigraphic record. Both are produced by incision, and so either could be termed an “incised valley”. We a find that valley basal erosional unconformities form over most of the duration of a sea-level cycle, do not represent topographic surfaces, and do not constitute time lines. Also, because valleys form through a continuous process of channel incision, backfill, and channel migration (avulsion) during relative sea level fall, earlier fluvial fills can lie on top of the extended erosional surface, which overrides successively younger delta fronts as it develops. Thus, although locally the deposits above the unconformity are always younger than those below it, the unconformity spans so much time that some of the deposits above it end up being older than some of the deposits below it. The net result is that there are numerous, though relatively small-scale, deviations from one of the frequently quoted fundamental characteristics of a sequence boundary, which is that rocks above it be everywhere younger than rocks below it.