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White Sands Nat’l Monument, NM
Molly Rogers
Dolomite Mountains
Lea Korsmeyer
SEDIMENTATION AND SEDIMENTARY ROCKS
SEDIMENTARY STRUCTURES
CROSS-BEDDING
Consists of sedimentary layers deposited at an angle to
the underlying set of beds.
Navajo Sandstone
Utah
Coconino Sandstone
Arizona
SEDIMENTATION AND SEDIMENTARY ROCKS
SEDIMENTARY STRUCTURES
CROSS-BEDDING
Form when particles drop from a moving current,
such as a flowing river or wind.
Cross beds always slope towards the down-current
direction.
Consequently, they can be used to indicate direction of
flow.
SEDIMENTATION AND SEDIMENTARY ROCKS
SEDIMENTARY STRUCTURES
CROSS-BEDDING
SEDIMENTATION AND SEDIMENTARY ROCKS
SURFACE SEDIMENTARY STRUCTURES
Surface layers of sedimentary rocks also have structures.
RIPPLE MARKS
Shallow, curving ridges produced by currents
SEDIMENTATION AND SEDIMENTARY ROCKS
SURFACE SEDIMENTARY STRUCTURES
RIPPLE MARKS
SEDIMENTATION AND SEDIMENTARY ROCKS
SURFACE SEDIMENTARY STRUCTURES
MUD CRACKS
Polygonal cracks that form as mud is exposed to air
and then dries out.
Occurs when lakes dry up or evaporate.
SEDIMENTATION AND SEDIMENTARY ROCKS
LITHIFICATION
When sediments are buried they retain heat.
Some heat comes from buried radioactive grains.
Some heat conducted from Earth’s interior.
Sediments are also compressed by overlying sediments.
Sediments are invaded by groundwater carrying ions.
Heat, pressure and ions alter the physical and chemical
nature of the sediments by a processes
collectively known as DIAGENESIS.
SEDIMENTATION AND SEDIMENTARY ROCKS
LITHIFICATION
DIAGENESIS can lead to LITHIFICATION.
LITHIFICATION is the conversion of loose sediments
to solid sedimentary rocks.
Usually occurs in the upper few kilometers of crust.
Temperatures are <200C and at low pressure.
Sediments are COMPACTED and often CEMENTED.
SEDIMENTATION AND SEDIMENTARY ROCKS
LITHIFICATION
COMPACTION - the process by which pressure
reduces the volume of sediment during diagenesis.
Overlying sediment squeezes air and water out of
sediments forcing grains closer together.
Clays have weak attractive forces which causes grains
to adhere and become SEDIMENTARY ROCK.
SEDIMENTATION AND SEDIMENTARY ROCKS
LITHIFICATION
CEMENTATION takes place as water circulates
between grains and compounds originally
dissolved during chemical weathering are
precipitated and bind sediments together.
Types of cement:
Calcium carbonate (CaCO3)
Silica (SiO2)
Iron oxides (Fe2O3, FeOOH)
Iron sulfide (FeS2)
Iron carbonate (FeCO3)
Organic fragments are also compacted and cemented.
SEDIMENTATION AND SEDIMENTARY ROCKS
LITHIFICATION
Increased heat and pressure can also cause
RECRYSTALLIZATION.
Recrystallization is the development of stable minerals
from unstable varieties.
Many marine organisms secrete aragonite in skeletons.
Aragonite recrystallizes to calcite upon the death of
the organism that produced it.
This explains why there is no aragonite in ancient
carbonate rocks.
SEDIMENTATION AND SEDIMENTARY ROCKS
CLASSIFICATION OF SEDIMENTARY ROCKS
Grain-size forms the basis of clastic sedimentary rock
classification.
Sediment Size
(mm)
Particle Name
>256
Boulder
64-256
Cobble
Rock Name
Gravel
Conglomerate
Breccia
4-64
Pebble
2-4
Granule
0.063-2
Sand
Sandstone
0.004-0.063
Silt
Siltstone
Mudstone
Mud
<0.004
Clay
Shale
SEDIMENTATION AND SEDIMENTARY ROCKS
CLASSIFICATION OF SEDIMENTARY ROCKS
CONGLOMERATES AND BRECCIAS
Both have particles sizes > 2mm.
Conglomerates have rounded grains.
Indicates greater weathering
and erosion.
Breccias have angular grains.
Indicates less weathering
and erosion.
SEDIMENTATION AND SEDIMENTARY ROCKS
CLASSIFICATION OF SEDIMENTARY ROCKS
SANDSTONES
Clastic sedimentary rocks with a grain-size between
0.063 (1/16) and 2 mm.
Represent 25% of all sedimentary rocks.
Are usually cemented by carbonate (calcite) or silica.
>90% quartz grains = quartz sandstone
well sorted and rounded grains
>25% feldspar = arkose
poorly sorted and angular grains
>33 1/3 silt, clay and rock fragments = greywacke
poorly sorted and angular grains
SEDIMENTATION AND SEDIMENTARY ROCKS
CLASSIFICATION OF SEDIMENTARY ROCKS
SANDSTONES
SEDIMENTATION AND SEDIMENTARY ROCKS
CLASSIFICATION OF SEDIMENTARY ROCKS
SANDSTONES
Sandstones are useful as building stones
contain oil, gas and groundwater
due to high porosity and permeability
SEDIMENTATION AND SEDIMENTARY ROCKS
CLASSIFICATION OF SEDIMENTARY ROCKS
MUDSTONES
Grain-size is small, < 0.063 (1/16) mm.
Mudstones with particles <0.004 mm in diameter
constitute >50% of all clastic sedimentary rocks.
Usually extremely fine-grained.
Settle out in very quiet waters.
Lakes, lagoons, floodplains and ocean floors.
Generally composed of micas and clay minerals.
These can be oriented parallel to each other.
SEDIMENTATION AND SEDIMENTARY ROCKS
CLASSIFICATION OF SEDIMENTARY ROCKS
MUDSTONES
Parallel alignment of micas and clays is termed
FISSILITY.
SHALE = FISSILE mudstone.
SEDIMENTATION AND SEDIMENTARY ROCKS
CLASSIFICATION OF SEDIMENTARY ROCKS
MUDSTONES
Color in MUDSTONES is variable.
Color usually depends on depositional environment.
Black
Red
Green
= High organic content
Low oxygen content
= Oxidizing environment
Terrestrial settings
= Reducing environment
Marine settings with low O2