Descriptions of Depositional Environments

Terrestrial Environments

Glacial Environments

All glacial sediments are collectively known as drift. These are divided into glacial tills deposited directly from the ice and glacial outwash deposited by glacial meltwaters.

Glacial Till

Glacial till can consist of mixed particle sizes from clay, to sand-size particles to huge boulders because ice can carry all these sizes mixed together. A moraine is a landform at the side or front of a glacier that typically consists of heaps and ridges of unsorted, unstratified, unconsolidated (loose) glacial till that has not been reworked by meltwater. The terminal moraine is a ridge of till that accumulates at the lower margin of a moving glacier. Lateral moraines form at the sides of glaciers.

Glacial Outwash

Glacial outwash is composed of sands and gravels released into meltwater streams by a melting glacier subsequently that form outwash fans or plains in front of the end moraine. The result is a stratified deposit with the coarsest material accumulating closest to the ice margin. Glacial lakes may form in front of the glacier in which fine grained silts are deposited. One distinguishing feature of these lake sediments that distinguishes them from regular lake sediments are dropstones. These are particles much larger than silt-sized that have melted out of icebergs above, and dropped into the finer silts below. Varves are fine laminations of dark and light sediments. These are often destroyed by burrowing bed dwellers in other lakes, but life is often absent in glacial lakes and so they are better preserved.

Alluvial (River) Environments

Alluvial Fans

Streams flowing down a steeper narrow-sided valley may empty out into a broad flatter valley or onto a plain. The resulting drop in water flow rate causes sediment to be deposited in a fan-shaped structure with a steep apex where the side valley enters, gradually becoming a wide, gentle incline near the bottom. The drop in flow rate is often sudden at the top of the fan, and sediment may drop out quickly. Sorting is poor. Coastal alluvial fans may build directly out into the ocean. Alluvial fans in arid environments are particularly coarse and poorly sorted from flash floods typical in arid environments.

Braided Rivers

A braided river is located close to its source and carries a high sediment load. Channels are actively filling with sediment and when the channel can no longer take the river flow, the river jumps and begins a new channel. This results in a network of many intertwining (braided) channels across a broad flood plain. Close to the sediment source, gravel and boulder deposits are horizontally bedded, and rarely include sand or clay. Bedding may not be evident in the thickest beds. Downstream, braided river deposits may fine upwards from grain supported gravel to sand, mud and plant material.

Meandering Rivers

The gradient of a river (drop in elevation over distance downstream) gradually decreases downstream, as does the grain size of the sediment being carried. The river begins to wind back and forth across the landscape in sinuous curves called meanders. The faster water in the channel scours sediment from the outside of each curve and deposits it downstream in the slow water on the inside of curves. These are known as point bar deposits. The sediments on the inside of a corner will be finest in the slowest, shallowest water, and will gradually coarsen into the centre of the channel. Coarse sediments within the centre of the channel are therefore progressively overtopped by finer sediments as the inside of the corner overruns the former channel. This creates a fining upwards sequence (FUS).

Arid and Semi-Arid Environments

Arid environments have very low average precipitation, but when it does rain, it may come in sudden storms. Streams in arid environments may only run following rainfall, after which they dry up. Alluvial fans are formed where these streams come off the highlands at the edge of a sediment basin. These fans can be very poorly sorted. Playa lakes are lakes that fill after a rainfall event, and then gradually evaporate again during dry periods. Dunes are created by windblown sand. The sediments in arid environments are exposed to air when they are dry between rain events, and so they may develop red colouring due to iron oxides forming in the presence of oxygen.

Transitional Environments


Rivers enter the sea or lakes and their flow velocity drops, which causes suspended sediment to be deposited. Large, nearly flat, triangular or fan-shaped plains of accumulated river-supplied sediment build out into the lake or ocean over time. Coarser sands or gravels are deposited at the inland side in the main channel. Upper delta plains may be traversed by many tributaries originating from the main river channel. Finer sediments are deposited at more distal parts of the fan as the water gets deeper and quieter. A given point away from the fan will therefore start as fine sediment, which becomes progressively coarser as the fan builds out further (progrades) and fills over top. This creates a coarsening upwards sequence (CUS).

Delta bays are protected bays that may form between the extending channel arms of a delta (see lagoons below).

Tidal Flats

Tidal Flats are the broad flat areas located at shorelines or on deltas.

Supra-Tidal Flats

This is the area behind (onshore from) the tidal zone, just above the reach of normal high tides. This area is normally terrestrial, and can either be wet/marshy or can be arid and evaporative. The supra-tidal area may be inundated during high tides when there are winds from a storm driving the water onshore. The plants and animals that colonize this area must be able to survive brackish (periodically saline) conditions. Waves from storm deposits may create symmetrical ripples, whereas any stream channels crossing the flats will result in asymmetric ripples.

Intertidal Flats

This is the zone of shore front found at elevations between high and low tides. The area is exposed by the daily cycling of the tides (two highs, two lows per day).


Beaches are shallow angled deposits of clastic sediment formed along the shore where loose clastic sediments (from cobbles to silts) are continually re-worked by wave action in the intertidal zone. Beaches occur within the intertidal zone, but not every intertidal zone includes a beach.

Barrier Island

This is a deposit that forms offshore from clastic material (typically sands) that protects the shore from direct wave action. These may be the result of currents parallel to the shoreline created by waves arriving onshore at an angle. The outside of these islands is typically a beach, while the main body may have sand dunes.

Marine Environments

Sub-Tidal Zone

This is the zone near shore found just deeper than the inter-tidal zone. This area is not exposed to air like the intertidal zone, but is affected by wave energy and the sediments here are often re-worked.


Shallow marine lagoons are formed near shore where a carbonate reef, or clastic barrier island (see below) are present offshore. These both provide shelter from the open ocean, and allow the lagoon to be mostly quiet water. Occasional storms will drive water over the offshore barrier. Storms may bring larger sediment, and increased wave energy to the lagoon that re-works sediments.

Delta Bays

A delta bay is a shallow protected bay formed between the main channels of a delta complex. They experience similar depositional conditions as lagoons.


Reefs are accumulations of carbonate occurring offshore where marine organisms that use carbonates in their body skeletons (eg: corals and others) have built up a large mound of carbonate. These may be below water at all times, or parts of the reef may be exposed at low tides.

Shelf (Storm Dominant)

The continental shelf is the shallower portion of the ocean adjacent to the continent that is underlain by continental crust. The ocean floor would usually experience low energy conditions appropriate for deep water, and finer-grained silts or clays would be depostied. During high-energy storm events, part of the continental shelf may be exposed to increased wave energy, and sands may be mobilized.

Shelf (Tide Dominant)

Tidal currents respond to 6.25 hour cycling of the ocean tides from high to low to high to low within a 25 hour period. In certain coastal environments, these tides create currents that can move across the sediment floor with sufficient velocity to transport sands and deposit them in large cross-beds that can be similar to those in desert dunes.

Areas of shelf close to land are sometimes called proximal. Further from land towards the edge of the continental shelf is known as the distal portion of the shelf, and for either storm or tidal inner shelf regions, the distal shelf is deeper and quieter water.

Continental Slope & Sub-Marine Fans

Continental crust eventually gives way to ocean crust. Where the continental crust ends, the depth of ocean increases from 100s of metres up to 1000s of metres. The continental slope is the incline from the shallower shelf down to the deep ocean basin. The incline is shallow (3 to 4 degrees slope).

Sediments from land, and from elsewhere on the continental shelf, will accumulate on the slope as the sediments are moved further from land. These sediments sometimes overload the slope and the slope will collapse in a manner similar to a snow avalanche. These create incut valleys at the top of the slope where the material has slumped, and create fan shaped deposits at the base where the material has come to rest. These sub-marine fans extend from the slope out into the Basin.

Deep Marine Basin

The deep ocean beyond the continents lies upon basaltic crust that forms part of the oceanic lithosphere. The water depth can be kilometres, and only very fine clastic sediments are deposited from wind-blown detritus, or as the very distal parts of sub-marine fans. Carbonates may form from the bodies of microscopic plankton that float within the surface of the ocean.


Laboratory Manual for Earth History Copyright © by kpanchuk. All Rights Reserved.

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