9.12.4: Deltas and Estuaries

Introduction

When rivers reach standing water such as a lake or the ocean, the flow speed slows down dramatically. And when flows slow down, sediment is deposited. Almost all of the sediment transported in a river is thus deposited close to the river mouth, with the exception of grains that are fine enough to remain in suspension. Lacustrine and marine processes can rework the deposited sediment to distribute it along shorelines.

Deltas form at the mouths of rivers that transport enough sediment to build outward. In contrast, estuaries are present where the ocean or lake waters flood up into the river valley. The key difference between the two is where the sediment transported by the river is deposited. It accumulates seaward or lakeward of the average shoreline for a delta and within the river valley for an estuary. Whether any specific river-sea/lake interface is an estuary or a delta depends on the balance between the supply of sediment, the rate at which the sediment is transported away from the river mouth, and changes in sea/lake level.

Deltas require substantial accumulation of sediment at the river mouth, which can happen when the river supplies a large amount of sediment, transport processes in the sea/lake are relatively slow, and/or the sea or lake level is going down through time. In each of these cases, the river extends farther out into the standing body of water through time. River valleys are flooded, creating estuaries, when the sediment supply is low, significant amounts of sediment are transported away from the shoreline by tides and storms, and/or sea level or lake level is going up faster than sediment is accumulating.

The sedimentary rocks deposited in deltas and estuaries are similar, consisting of a mixture of fluvial, shoreline, and shallow marine facies. However, the association of subenvironments and their vertical sequence through time are different.

Video:

Deltas

Distribution of Depositional Environments

I will draw cross section and map views of a delta showing the delta plane, delta slopes, and prodelta. Rivers flow through delta planes and slow when reaching water, producing a mouth bar. Grain size decreases with distance away from the river mouth.

video:

Sediment Transport Processes

General description from river to sea/lake floor, with variations among river-dominated deltas, wave-influenced deltas, and tide-influenced deltas

Video:

Delta Type

All deltas (by definition) have their sediment transported to the delta by rivers. Thus, fluvial deposits are always associated with them. In addition, depending on marine (or lacustrine) conditions, waves and tides can redistribute the fluvial sediment changing the morphology and facies of deltas. There are three main end member categories of deltas when characterized by processes: 1) River dominated; 2) Wave influenced; and 3) Tide influenced.

Videos:

River-dominated examples:

Wave-influenced examples:

Tide-influenced examples:

Progradation Through Time

Because deltas are sites of sediment building outward from the coast, they are progradational; the landward depositional environments move seaward through time, and are deposited over deeper marine/lacustrine deposits. Thus, delta sequences in the rock record start with deep water, marine (or lacustrine), fine grained sediments and grade upward into shallower water, possible more freshwater, coarser grained sediments. This is one of the distinguishing aspects of deltas that let you define them in the sedimentary record. These changes in grain size and environment typically occur over 1’s to 100’s of meters in the rock record and include many beds.

Estuaries

Distribution of Depositional Environments

Fluvial into marsh or tidal environments, with land on either side.

Sediment Transport Processes

Fluvial, settling from suspension, plus or minus tidal currents. Shallow water depositional environments.

Waves rare except at the seaward/lakeward end of the estuary because the shallow environments and narrow valley.