Summary
Deltas are depositional environments that form where the mouths of rivers meet oceans, inland seas, large lakes, and lagoons. These rivers carry and deposit sediment more rapidly than it can be evenly distributed by typical basinal processes, causing sediment to fan out into a deltoid-like shape (Wright, 1977). Sediment in deltaic environments can be deposited both subaerially (on the surface), and subaqueously (underwater). However, the vast majority of sediments are deposited subaqueously (Boggs, 2014). While all deltas share certain characteristics, there are multiple classes of delta, each having their own unique depositional processes and drivers. These include fluvial-dominated deltas, tide-dominated deltas, wave-dominated deltas, fan deltas, and mixed-process deltas (Boggs, 2014).
Geographic Dimensions
10s of km to 100s of km in size
Largest delta: Ganges Delta, South Asia: 350km wide
Key Depositional Processes
Fluvial-dominated deltas, which behave differently depending on the conditions of its source river.
Tide-dominated deltas, where tidal currents overpower river currents causing redistribution of sediments within the deltaic basin (Ramon-Duenas et al., 2021).
Wave-dominated deltas, where strong waves cause restrict the outflow of water and sediment from the mouth of the river.
Fan deltas, which form when the basin of an alluvial fan is deposited into open water.
Mixed-process deltas, which combine 2 or more of the aforementioned processes.
Principle Sedimentary Structures & Distinguishing Characteristics
Fluvial-dominated deltas: Turbidites, far-reaching fine sediments, lamination (Maloney et al., 2018)
Tide-dominated deltas: "Dendritic structure" formed by sand bars and ridges. New distributaries form with storm surge or floods.
Wave-dominated deltas: Sediments are redistributed to laterally by longshore currents to form beaches and barrier bars (Boggs, 2014).