III SEDIMENTARY ROCK
- sediment – loose fragments of rock, minerals and organic
material
- most sedimentary rock is made up of combinations of
different types of sediment
- characteristics are determined by the source of the
sediment, the way the sediment was moved and the
conditions under which the sediment was deposited
A. Formation of Sedimentary Rocks
1. source of the sediment determines the sediment’s
composition
2. Two main processes convert sediments into
sedimentary rock
a. compaction – the process in which the volume and
porosity of the sediment is decreased by the
weight of the overlying sediments as a result of
burial beneath other sediments
b. cementation – the process in which minerals
precipitate into pore spaces between sediment
grains and bind sediments together to form rock
3. Classifying Sedimentary Rock
a. process by which the rock forms
b. composition of the rock
4. Three main classes of sedimentary rock
a. chemical
b. organic
c. clastic
b. these classes contain their own classification based
on shape, size and composition
B. Chemical Sedimentary Rock
1. sedimentary rock that forms when minerals
precipitate from a solution or settle from a
suspension
2. evaporation
a. minerals dissolved in water remain when the
water evaporates
b. concentration of minerals in the water remaining
becomes high enough to cause minerals to
precipitate out
c. evaporites – rocks that form from the minerals
left behind
3. examples include gypsum and halite
C. Organic Sedimentary Rock
1. sedimentary rock the forms from the remains of
plants and animals
2. organic limestone
a. marine organisms remove chemicals from sea
water
b. shells are made from calcite and aragonite
c. shells eventually become limestone
3. chalk is an example
D. Clastic Sedimentary Rock
1. sedimentary rock that forms when fragments of
preexisting rocks are compacted or cemented
2. classified by size of sediments
a. conglomerate – rock composed of rounded
fragments that range in size from fine mud to
boulders
b. breccia – rock composed of fragments that are
angular and have sharp corners
c. sandstone – sedimentary rock that is composed of
sand-size grains
d. shale – sedimentary rock that is composed of
clay-size particles
E. Characteristic of Clastic Sediments
- physical characteristics are determined by the way
the sediments were transported
- four major agents of transportation
- water
- ice
- wind
- gravity
- speed of transport affects size and shape of sediments
- distance traveled affects size and shape of sediments
1. Sorting
a. tendency for currents of air or water to separate
sediments according to size
b. well-sorted – all grains are roughly the same size
c. poorly-sorted – grains consists of many different
sizes
d. sorting is a result of changes in the speed of the
agent that is moving the sediment
1d. large grains deposited first
2d. fine grains stay suspended longer
3d. fine grains are deposited farther from shorer
or on top of coarser sediments
2. Angularity
a. particles collide with each other and other objects
as they are transported
b. collisions can cause a change in size and shape
c. the first break tend to be angular and uneven
d. the longer the distance traveled, the more
rounded and smooth the sediment becomes
e. the farther the sediment travels, the smoother the
sediment becomes
F. Sedimentary Rock Features
- deposition environment – setting in which the
sediment is deposited
- rivers, deltas, beaches and oceans
1. Stratification
a. layering of sedimentary rock
b. occurs when the conditions of deposition changes
1b. change in sediment type
2b. change in depositional environment
c. beds
1c. stratified layers
2c. vary in thickness
aa. length of time sediment is deposited
bb. how much sediment is deposited
d. massive beds
1d. beds that have no internal structures
2d. similar sediment is deposited for long periods
3d. large amounts of sediments are deposited at
one time
2. Cross-Beds and Graded Bedding
a. cross-beds
1a. slanting layers of sedimentary rock
2a. generally form in sand dunes or rivers
b. graded bedding
1b. various sizes and kinds of materials are
deposited within one layer
2b. occurs when different sizes and shapes
settle at different levels
3b. largest grains at bottom
4b. smallest grains at top
5b. reverse grading – smallest grains on bottom
and largest grains on top
3. ripple marks
a. caused by the action of wind or water on sand
b. sediment was once part of a beach or river bed
4. mud cracks
a. form when muddy deposits dry and shrink
b. flood plain or dry lake bed are common places
5. fossils and concretions
a. fossils – the remains or traces of ancient plants
and animals
b. concretions
1b. lumps of rock that have a composition that is
different from the main rock body
2b. form when minerals precipitate from fluids
and build up around a nucleus
3b. geode – minerals that crystallize inside
cavities to from a special type of rock