Transcript SEDIMENATRY ROCKS sedimentary rocks

SEDIMENATRY ROCKS
sedimentary rocks = form when sediments become
pressed or cemented together or when sediments
precipitate out of solution
1. sediments = rock, minerals, and organic matter
broken into fragments
2. carried away and deposited by wind, water, and ice
3. sediments are produced by weathering and erosion
a. weathering produces clastic sediments = can be
huge boulders to microscopic fragments with
worn surfaces and rounded corners
b. erosion = removal and movement of surface
materials from one location to another
c. deposition = when sediments settle down on
ground or sink to bottoms of bodies of water
d. sedimentary basins = depressions in Earth
where sediments are usually
deposited
e. lithification = physical and
chemical processes that
transform sediments into rock
Lithification Types
1. compaction = layers of sediment become
compressed by weight of layers above
a. pushes fragments together
b. squeezes out air and water
c. can reduce volume by 50%
d. causes grains to interlock
2. cementation = sediments are glued together by
minerals deposited between sediments
a. type of cement affects the nature of rock
1) quartz cement makes toughest rocks
2) red iron oxide cement creates red sandstone
b. time required depends on availability of cement
and water to carry the cement
3. often form as layers
a. older layers usually on bottom
b. younger layers on top
Features:
1. stratification = visible horizontal layering that
develops as sediment is deposited
a. also called bedding
b. cross-bedding = arrangement of small beds lying
at an angle to main sedimentary layering
c. graded bedding = largest grains collect at bottom
of layer and grain size decreases toward top
Cross bedding
2. ripple marks = wavy features on sandstones
a. formed by action of wind or water on sand
3. mud cracks = wet clay dries and contracts, filled
with materials and fossilized
a. result when muddy deposits dry and shrink
b. usually a polygonal shape
4. fossils = remains, impressions, or any other
evidence of plants and animals preserved
a. most commonly found in limestone
b. can also be found in shale,
coal, and sandstone
5. concretions = nodules of rock
with different composition
of main rock body
6. geodes = ball-like objects that have
small hollow where
minerals grow inward
from hard outer rim of
silica
Classification: based on kind and size of sediments
that form them
1. clastic rocks = separate fragments become
compacted and cemented together
a. clastic particles = rock and mineral fragments
that have been physically transported and
deposited
b. bioclastic rocks = made up of broken shells and
other organic fragments (plants/animals)
c. named based on shape and size of sediments
d. make up more than 80% of sedimentary rocks
e. coarse-grained clastics = contain gravel-size rock
and mineral fragments
1) conglomerate = rounded gravel sized
fragments
2) breccia = fragments are angular with sharp
corners
f. medium-grained clastics = contain sand-sized rock
and mineral fragments
1) sandstone = lithified sand grains
a) mostly rounded quartz grains
b) layers useful in mapping old stream and river
channels
2) porosity = percentage of open space between
grains in rock
a) pores through which
liquids can move
b) makes sandstone valuable
as underground reservoir for
oil, natural gas, and
groundwater
g. fine-grained clastics = contain silt and/or mud
1) siltstone = mostly silt-sized grains
2) shale = mostly silt and clay-sized minerals with
quartz and feldspar
a) makes up ~70% of all sediment rock
b) usually gray to black
c) contains decayed remains of plants and
animals
d) pressed into flat layers that easily split apart
3) rock have very low
porosity → forms barriers
that hinder movement of
groundwater and oil
2. chemical rocks = form from minerals that have
been dissolved in water
a. evaporites = water evaporates leaving dissolved
minerals behind
Ex: halite, and gypsum
b. precipitates = dissolved minerals that settle out
of water due to change in temperature or
chemical reaction
Ex: calcite → limestone
c. most commonly form in arid regions, oceans,
drainage basins on continents with low water
flow
3. organic rocks = forms from remains of organisms
a. organic limestone = shells
of marine organisms
b. coal = decayed plant remains
buried and compacted
c. chalk = composed
of tiny, one celled
marine organisms
d. bioclastic limestone = calcite shells compacted and
cemented
1) bits and pieces of shells appear as fossils in the
rock
e. coquina = coarse-grained shell fragments
cemented together
Importance
1. used to provide geologic evidence of surface
conditions in Earth’s past
2. indicate location and direction of flow of ancient
rivers, wind or wave direction over lakes and
deserts, and ancient shoreline positions
3. reservoir for natural resources such as oil
4. many are mined for use in industry