Terminal Distributary Channels and Delta Front Architecture of River-Dominated Delta Systems

TLDR

Terminal distributary channels are the most distal channelized features of river‑dominated deltas, typically 100–400 m wide, 1–3 m deep, rarely incised, and associated with mouth‑bar deposits that are infilled by aggradation and migrate laterally or upstream, exhibiting unidirectional flow structures and wave/tide reworking, with orientations ranging from 123° to 248°. Recognizing terminal distributary channels is critical in interpretation of river‑dominated deltas. In shallow‑water river‑dominated deltas, multiple coeval terminal distributary channels at different scales produce a lobate delta‑front shape opposite to the digitate Mississippi type yet similar to wave‑dominated deltas, with shallow‑water deposits capped by small channels whose cross‑sectional area is only a small fraction of the main trunk channel, and without deep distributary channels despite typical coarsening‑upward facies.

Abstract

Abstract Using modern and ancient examples we show that river-dominated deltas formed in shallow basins have multiple coeval terminal distributary channels at different scales. Sediment dispersion through multiple terminal distributary channels results in an overall lobate shape of the river-dominated delta that is opposite to the digitate Mississippi type, but similar with deltas described as wave-dominated. The examples of deltas that we present show typical coarsening-upward delta-front facies successions but do not contain deep distributary channels, as have been routinely interpreted in many ancient deltas. We show that shallow-water river-dominated delta-front deposits are typically capped by small terminal distributary channels, the cross-sectional area of which represents a small fraction of the main fluvial trunk channel. Recognizing terminal distributary channels is critical in interpretation of river-dominated deltas. Terminal distributary channels are the most distal channelized features and can be both subaerial and subaqueous. Their dimensions vary between tens of meters to kilometers in width, with common values of 100–400 m and depths of 1–3 m, and are rarely incised. The orientation of the terminal distributary channels for the same system has a large variation, with values between 123° (Volga Delta) and 248° (Lena Delta). Terminal distributary channels are intimately associated with mouth-bar deposits and are infilled by aggradation and lateral or upstream migration of the mouth bars. Deposits of terminal distributary channels have characteristic sedimentary structures of unidirectional effluent flow but also show evidence of reworking by waves and tides.