LECTURE#14
Sedimentary Processes
Transportation
Most sediment is transported some distance by gravity, wind,
water, or ice before coming to rest and settling into layers. During
transportation, sediment continues to weather and change in
character in proportion to the distance the sediment is moved.
How transportation affects sediment?
1. It sorts it,
2. shapes it and
3. winnows out less stable components.
Each process depends on the transport agent and the
sediment being transported.
1. Sorting
• Sorting is the degree of similarity in particle size in a sedimentary
rock.
• It is the arrangement of particles in a particular order according to
their type or to separate particles of type from other
• The sorting of sediment is primarily a result of the variable carrying
capacity of water, which is a strong function of current velocity.
• These currents are common in most rivers. Strong currents (> 50
cm/s) can move gravel and even boulders on occasion. Moderate
currents (20-50 cm/s) move sand and smaller clasts. Weaker currents
can only transport silts and clays, the smallest size clasts.
• Desert winds also move sand. Wind usually can only move these
small clasts like sand and clay particles due to its low viscosity but
strong wind can carry larger grains like pebble, cobble etc.
• Glaciers, which move very slowly compared to liquid water, have
enormous carrying capacity due to the much higher viscosity of ice.
• Sediments of mixed size are sorted by carrying capacity. If a
stream's capacity decreases, then the larger clasts it is carrying
will no longer be transported. The suspended load then consists
only of fines.
Porosity and Permeability
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Porosity is the number of pore space present in the total volume of rock.
Permeability means through which something can pass. The interconnected
pores are termed as permeability.
For permeability, porosity must exist.
2. Shaping or Roundness
 Rounding is the grinding away of sharp
edges and corners of rock fragments
during transportation.
 Rounding occurs in when streams, winds, or
waves move sand and other larger
sedimentary particles, the grains lose their
sharp edges and corners and become more
rounded as they collide with other particles
during transport.
 The degree of rounding indicates the
distance
or
time
involved
in
the
transportation of sediment by currents of
air or water. Highly rounded grains indicate
that a great deal of abrasion and hence a
great deal of transport has occurred.
Deposition
• Mechanical - involves loss of velocity
• Chemical – involves evaporation, precipitation.
• Sediment eventually gets put somewhere.
transported and is deposited.
It ceases to be
• Deposition of detrital sediment occurs when energy necessary for
transport is no longer available. Chemical deposition (precipitation)
occurs when environmental (chemical) conditions change.
• Depositional environment:
deposited.
• Continental
• Transitional
• Marine
geographic area where sediment is
Depositional Environments
A. Continental Environments:
a) Fluvial - Sediment was deposited by a
stream.
b) Eolian - Sediment deposited by wind (deserts).
c) Lacustrine - Lake sedimentary deposits.
d) Glacial - Sediment deposited by ice and meltwater.
e)Alluvial Fan- sediment deposited near hill slope by
stream and gravity
B. Marine Environments- Sediment accumulates on the
ocean floor:
a) shallow (continental shelf and reef)
b) deep (abyssal plain)
C. Transitional Environments- between land (terrestrial)
and marine environments.
a) Deltaic - Deposits at the mouth of a major stream.
b) Beaches, lagoons and barrier islands - Sediment
deposited by wind or water.