1. Chapter six
1
Somali National University
Puntland-Baran Branch
lecturer: Mr. Mohamed koob
M.Sc. Applied Geology
Depositional Environments
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2. Introduction:
Depositional environments are various sedimentary environments where sediments are
being deposited after the process of transport took place on them.
These environments indicate the mode of sediment deposition, for instance; mud is
very fine grained and indicates a very calm sedimentary environment such as a river
delta.
There are 4 important kinds of continental depositional environments.
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3. Alluvial (alluvial fans and fluvial systems (rivers)),
Lacustrine (lake) environments,
Desert (Aeolian) environment and
Glacial (ice) environments.
However; other depositional environments include; shoreline environments (beaches,
delta and tidal flats), Marine environments (deep sea, continental shelf, organic reefs
and continental margin, lagoons and Barrier islands).
As mentioned earlier, this chapter will focus on continental depositional environments
only.
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5. Depositional environments of some important sedimentary rocks:
Sedimentary rocks are always deposited from oldest to the youngest, and
unless some incredible earth movements (faults and folds) intervenes and
deform the sediments.
The oldest layer will always be at the bottom and the youngest layer will
always be at the top.
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6. Depositional environments of Sandstone:
The sand that commonly forms sandstones is found mainly in 2 important
places, (may also be found in other few marine and continental environments
especially in rivers) and these 2 important places are: beaches and deserts,
so when the sandstone is observed by geologists, they must tell whether it was
deposited in a beach, river or in a desert environment,
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8. Layers of the rock must be properly observed to see whether it mimics the
pattern of sand dunes alone (desert) or whether there are signs of water
deposition (fluvial/beach)
such as preserved ripple marks, flat layers which are representing the swash
zone of the beach (the place where the waves come onto the beach and flatten
the sand out) or evidence of waves from large storms moving the sand
underwater.
Another way; to differentiate between the desert and beach deposits is to
examine the rock above and below the layer you’re looking at.
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9. Depositional environments for Shale
Shale is a rock that form and comes from relatively deep, calm water. Since shale is
made up of very small particles (fine sediments),
It must be deposited in water that is calm enough to no longer suspend such fine
particles.
Any place where there’s tranquility and the water is also calm enough for fine
sediments to settle out, you might find Shale.
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11. Depositional environments for Limestone:
The very deep water ocean is one place where the limestone deposits are mostly found.
Way out in the ocean, where the water is calm and deep and some live multitudes of
tiny marine animals live.
These animals use the calcium carbonate in sea water to make their shells.
The shells then accumulate over millions of years and are eventually buried, cemented
and turned into limestone.
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13. A limestone that is observed with fossils that look like corals or any other reef creature
so that may mean that the environment that deposited that limestone was probably
associated with the reef.
Reefs commonly reside in shallow marine water and are found only in tropical
environments.
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14. Depositional environments for Conglomerate
Conglomerates are rudaceous rocks (coarse grain size fragments/clasts) and can
sometimes have spaces between their clasts (clast-supported) and some other
times, they may be filled with matrix in between their clasts (matrix-supported).
A common place to find conglomerates is near the river.
Rivers move at different velocities from time to time and from season to season,
this makes them to have the ability to transport and deposit rocks of all different
sizes.
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16. CONTINENTAL DEPOSITIONAL ENVIRONMENTS
These are environments this course will pay closer attention to, and they
are:
Alluvial (alluvial fans and rivers) systems
Aeolian (desert)systems
Lacustrine (lake) systems
Glacial (ice)systems
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17. FLUVIAL AND ALLUVIAL SYSTEMS:
These are subdivided into alluvial fans and rivers.
The term “alluvial” refers to both alluvial fans and rivers while “fluvial” is
only used for rivers,
Hence fluvial deposits are also referred to as alluvial deposits.
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18. Alluvial fans:
These are cones of detritus (sediments) that form at a break in slope at the
edge of an alluvial plain.
They are formed where a fluvial trunk with steep gradient enters a slope
break and suddenly drops most of its sediment load.
Alluvial fans comprise of cone-shaped deposits in areas of high relief where
erosion is rapid and where a lot of detritus is available.
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19. Schematic diagram of alluvial fans with a steep depositional slope.
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20. 12/12/2023 20
(a) Schematic representation of a typical alluvial fan, and (b) representation of a typical
drainage basin Alluvial fan characterization is concerned with the determination of the
role of the fluvial sediment supply for the evolution of fan deltas.
21. The term alluvial fan has been used in geological and geographical literature to describe
a wide variety of deposits.
Modern fans are particularly common in areas of high relief at the base of a mountain
range where abundant sediment supply is available
In arid and semi-arid regions alluvial fans may pass down slope into desert floor
environments with internal drainage.
These fans can also develop in humid climatic conditions, if relief and detritus are both
plenty, e.g: the Kosi River fan adjacent to the Himalayas
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22. Depositional processes of alluvial fans:
Sedimentary processes on alluvial fans are determined by the availability of water,
the amount and type of sediment being carried to an alluvial fan.
Three main types of processes in which transport and deposition take place on
alluvial fans can be by: Debris flow, sheet flood and stream flow.
Debris flows and sheet flood deposits are the main building blocks while stream-
flow deposits are the products of reworking in between the major events.
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23. Debris flows
where there is a dense mixture of water and sediment. Alluvial fan cone will be
built up if this is the dominant process.
Due to the high density and viscosity, the flow will be laminar and it will
continue to flow over the land surface as a viscous mass until it runs out of
momentum or the flow loses water along the way.
Beds that are deposited by debris flows maybe tens of centimeters to meters
thick.
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24. Debris flow deposits on an alluvial fan in Panamint Valley at the base of Panamint
Butte in the Cottonwood Mountains.
Note how the stream channel is incised near the top of the fan, but the debris flow
deposit is spreading out on the lower fan surface.
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25. Characteristics of beds normally deposited by debris flow include:
Conglomerates that are matrix supported; sorting of the conglomerate into different
clast sizes within or between the beds is very poor; outsize clasts that may be
meters across may occur within the debris flow.
Debris flow deposits illustrating generation of coarsening-upward trend in terms
of grading.
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26. Sheet flood:
Refers to the loose detritus which is moved as bed load onto the fan surface when
the catchment area of an alluvial fan is covered with water probably by heavy
rainstorms, the flow then spreads out over a portion of the fan as a sheet flood.
This type of flow occurs on slopes of about 30 and 50, under these conditions most
pebbles, cobbles and boulders are carried as bed load, but finer pebbles and
granules may be partially in suspension along with sand and finer sediment.
The formation and destruction of these standing waves occurs repeatedly during a
sheet flood event.
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27. The most common style of facies seen in sheet flood facies are depositional couplets of
coarse gravel deposited as bed load when standing waves are forming, overlain by finer
gravel and sand deposited from suspension as the wave is washed out,
These couplets are typically 5-20cm thick and occur in packages tens of centimeters to
a couple of meters thick formed by individual flow events.
Individual sheet flood deposits maybe hundreds of meters wide and stretch from the
apex to the toe of the fan, but individual couplets within them are typically few meters
across.
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29. Characteristics of sheet flood deposits are as follows:
Beds are very well stratified with distinct couplets of coarser gravel and sandy, finer
gravel.
Sheet geometry of beds are tens of centimeters to a couple of meters thick.
The sediment is poorly sorted, but silt and clay sized material is largely absent.
Imbrication of clasts is common and cross stratification formed by antidunes may
be preserved on the rocks.
Beds may show normal grading.
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30. Incised channel flow-
this is a flow that takes place
through channels, 1-4m high,
cutting into the upper fan,
these channels facilitates
downslope movement of
sediment-gravity flows and
sheet floods.
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31. Fluvial (river) systems:
Rivers act as the principal mechanism of transporting weathered material
away from the upland areas and carrying it to a lake or the sea.
This material gets to be deposited into a lake, the sea, in the river channel
itself or in the floodplain.
River deposit is preserved under certain conditions to become part of the
ancient sedimentary record.
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32. Erosional zone: This is a zone where the erosion of the weathered material occur.
The river flows through the weathered material and removes it from the site of
weathering.
When it rains, the rain water in the soil can entrain the weathered material
into the river.
Transfer zone: This is a zone where the transport of material as bed load, in
suspension or in solution take place. This material is transferred by the river.
During the times of flood, the water can flow outside the river banks, this is
known as overbank flow.
Depositional zone: This is a zone along the river where sediments are deposited as
the flow regime decreases.
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33. Channel forms of river systems:
For one to understand the processes of sediment transport and deposition within
rivers.
it is vital to completely be aware and understand various channel forms that
rivers consist of.
Rivers in the depositional tract can have a variety of forms which are described
as follows:
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34. The main types of rivers that are mostly known and used by geologists
include
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35. Braided river- also known as multiple channel as it comprises of mid-channel bars
that are covered with water at bank-full flow due to heavy rainfalls.
Braided rivers are characterized by coarse materials (gravel, sand), they’re dominated
by bed load transport, they have comparatively high gradients (up to decimals of a
degree) and floodplains are readily reworked by channel shift.
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36. Transport and deposition on braided rivers
Sediments are transported by rolling and saltation along the channel floor, then
deposited as bars of sand, gravel or even pebbles on mid-channel bars.
The bars in a braided river channel are exposed at low-flow stages but they are
covered when the flow is at bank-full level.
The bars within the channel vary in shape, they are either; elongate along the axis of
the channel and are called longitudinal bars.
or they can be wider than they are long, spreading across the channel and are called
transverse bars.
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38. Meandering river-
also known as single channel river, it is a sinuous (winded) river with depositional
bars only on the insides of the bends.
Some geologists call this type of a channel a curvaceous channel form, as it develops
curves along its journey.
Meandering rivers are often characterized by finer materials (fine sand and mud (clay
and silt)), they’re dominated by suspended transport.
They have comparatively low gradients (less than a decimal of a degree) and
floodplains are more stable and are reworked at slower rates.
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39. Transport and deposition on meandering rivers
Meanders develop by the erosion of the river bank, accompanied by the
deposition on the opposite side of the channel where bed load cannot be carried.
Meandering rivers transport and deposit a mixture of suspended and bed load
(mixed load).
The bed load is carried by the flow in the channel, with the coarsest material
carried in the deepest parts of the channel.
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40. showing the point bars, where sediment deposition takes place and also showing the cut
banks.
Point Bars
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41. Anastomosing river
also known as Ana branching rivers, which consists of multiple, interconnected
channels that are separated by areas of floodplain.
Anastomosing rivers are characterized by very fine materials (few sands and mud),
they’re dominated by suspended transport.
They have very low gradients (almost flat plains).
Flood plains on anastomosing rivers are very stable and lateral channel migration takes
place at very slow rates.
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43. Deposition on anastomosing rivers.
These are mostly seen in places where the banks are stabilized by vegetation.
Positions of its channels are fairly fixed, but the new ones forms due to
flooding.
Anastomosing river encompasses a number of upward aggrading sandstone
lenses within predominant floodplain mudstones.
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44. Straight river
This is a type of a river with extremely low sinuosity.
They are often characterized by both fine and coarse materials, bed load is often
dominant.
Straight rivers have high gradients. Flood plains are poorly defined on not
existing at all.
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45. Illustration of a straight river in Colorado, River Grand, Arizona.
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46. Deposition of sediments on a flood plain of a river
A flood plain refers to a strip of land adjacent to the rivers, and is covered by
water during seasonal floods.
When the discharge exceeds the capacity of the channel, water overflows over
the banks and out onto the floodplain where deposition occurs.
Natural levee deposits form predominantly on the concave or steep bank side of
the meander loops and they typically containing horizontally stratified fine
sands overlain by laminated mud.
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47. Schematic diagrams showing deposition on the floodplain and formation of natural levee (left), and a diagram
showing the development of the floodplain in a stream
(right).
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48. Primary depositional structures that are commonly observed in flood plain sediments are:
Very thin beds that are normally graded firm sand to mud.
Evidence of soil formation.
Evidence of initial rapid flow (plane parallel lamination) quickly waning and
accompanied by rapid deposition (climbing ripple lamination).
Thin sheets of sediment, often only a few centimeters thick to tens to hundreds of
meters.
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