Valleys That Never Were: Time Surfaces Versus Stratigraphic Surfaces

摘要

Using experimental data, we show how erosional unconformities (sequence boundaries) form and evolve in response to changes in global sea level (eustasy), given passive margin style subsidence and constant conditions of supply of sediment and water. We distinguish between two types of erosional unconformities; broad planar erosional surfaces that form during relatively slow sea-level fall, and incised-valleys that form during relatively rapid sea-level fall. We find that both types of unconformities evolve continuously throughout both sea-level fall and rise, producing erosional surfaces that are highly diachronous and amalgamated. We focus mostly on the role of change in relative sea level (RSL) on the formation of incised valleys and their preservation in the stratigraphic record. We find that there is an ongoing interplay of erosion and deposition that continuously redefines the shape of an incised valley, such that valleys both narrow and widen as they deepen during RSL fall and then continue to widen and fill during RSL rise. Due to this dynamic reshaping, what is preserved in stratigraphy may resemble a valley in shape, but its geomorphic form likely never existed in the fluvial landscape. We also find that these erosional valleys tend to be most diachronous along lateral margins of valley fill in proximal areas of the basin and become somewhat younger on average landward along their axial parts. Overall, the basal erosional unconformity forms over most of the duration of the sea-level cycle, does not represent a topographic surface, and is therefore not a time line. Finally, because valleys form through a continuous process of channel incision, backfill, and channel migration (avulsion) during RSL 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.