Transgressive deposits: a review of their variability

Abstract Transgressive deposits accumulate with rising relative sea level during the landward migration of a coastline. Particularly at short time scales (e.g., 4th- to 6th-order cycles), transgressive deposits can be recognised through the evidence of a gradual or irregular landward shift of facies, or an upward deepening of facies that culminates in a surface or zone of maximum flooding. During transgression, the coastline moves landwards and the shelf area enlarges. This is accompanied by a tendency to have more sediment trapped in the alluvial and coastal-plain environments, a reduced sediment influx to the basin, and cannibalization (through ravinement) of previously deposited sediments, including those deposited in the early stages of transgression. The resultant deposits can be fully marine, estuarine/lagoonal or fluvial, and can include other facies such as coal and eolian deposits with a variability driven by changes in rate of sea-level rise, sediment supply, textural character of the sediments, shelf gradient or basin physiography. Transgression may be continuous or punctuated, the latter occurring by alternation of coastal retrogradation and regression despite a longer term, landward-stepping of the shorezone. This commonly results in shoreface retreat, barrier in-place drowning, or a variety of transgressive parasequences whose character depends on the balance between sediment supply and accommodation creation. Any classification effort based on driving force or sedimentary processes tends to be overidealised. We therefore propose a classification based on the recognition of distinctive surfaces (wave and tidal ravinement surfaces, transgressive surface) within the transgressive lithosome. Transgressive scenarios are presented from different settings. Five types of transgressive lithosome, with variable thickness, lateral extent and internal architecture are discussed. (1) Transgressive deposits developed below the lowest ravinement surface (termed T-A) are commonly coal bearing (back-barrier) and alluvial. They accumulate in low-gradient settings where there is divergence between the ravinement trajectory and the surface being transgressed. (2) Transgressive deposits developed above the tidal but below the wave ravinement surface (T-B) accumulate where tidal processes dominate over storm-wave processes. Here too there is, at least locally, a slight landward divergence between the two surface types, allowing the development of sandy estuarine lithosomes. The wave ravinement surface may be absent in the innermost part of transects or if the deposits represent purely tide-dominated high-energy settings (that are not dealt with in detail in this paper). (3) Transgressive deposits developed above the wave ravinement surface in low-gradient settings (T-C). Such deposits derive from shoreface erosion or from longshore drift. They are usually very thin, but thicken where the transgression was punctuated by regressive pulses or where offshore sand ridges form. (4) Transgressive deposits developed above the wave ravinement surface in high-gradient high-sediment supply settings (T-D). In this setting (commonly fault-bounded terraces, valley walls, or simply steep slopes), the transgressive deposits can be much thicker because all eroded and newly supplied sediment is deposited locally. (5) Transgressive deposits without evidence of ravinement surfaces (T-E). These can be characteristic of low-energy settings that are typically mud-dominated.