A simulation model of alluvial stratigraphy

TLDR

The study presents a quantitative two‑dimensional model that simulates the development of alluvial sedimentary successions beneath a floodplain traversed by a single major river. The model incorporates laterally variable aggradation, fine‑sediment compaction, tectonic movement at floodplain margins, and channel avulsion to explain the distribution of channel‑belt sand and gravel within overbank fines. Experiments show that channel‑belt deposit interconnectedness and areal density decline with increasing floodplain width, longer avulsion periods, and higher aggradation rates; tectonic movements influence successions only when a consistent tilting direction exists, causing offlap deposits to cluster near active margins while fine‑grained alluvium accumulates on inactive sides; channel‑belt deposits thicken during aggradation but may self‑regulate, and multistorey features from aggradation can be mistaken for those from avulsive superposition.

Abstract

ABSTRACT The quantitative model presented simulates the development of a two‐dimensional alluvial sedimentary succession beneath a floodplain traversed by a single major river. Several inter‐related effects which influence the distribution of channel‐belt sand and gravel bodies within overbank fines are accounted for. These are (a) laterally variable aggradation, (b) compaction of fine sediment, (c) tectonic movement at floodplain margins, and (d) channel avulsion. Selected experiments with the model show how the interconnectedness and areal density of channel‐belt deposits decrease with increasing floodplain width/channel‐belt size, mean avulsion period, and channel‐belt aggradation rate. Separation of stream patterns based on interconnectedness and channel deposit density is difficult. Tectonic movements do not have a significant influence upon the successions unless a preferred direction of tilting is maintained (half‐graben). Then channel‐belt deposits showing offlap tendencies tend to cluster adjacent to the active floodplain margin, leaving dominantly fine‐grained alluvium to accumulate on the inactive side. Individual channel‐belt deposits thicken during aggradation, although a self‐regulating limit to such thickening is likely to operate. ‘Multistorey’features resulting from aggradation may be difficult to tell apart from those arising through superposition of distinct channel‐belt deposits of avulsive origin.