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Transcript
Sedimentary Rocks
The products of deposition and lithification of weathered
and eroded debris and chemical precipitants.
BY FAR, most of the rocks exposed on
the Colorado Plateau are sedimentary!
• Weathering: the physical breakdown and chemical
alteration of rock at or near the Earth’s surface.
– Mechanical weathering: from physical forces
(e.g. frost wedging, thermal expansion)
– Chemical weathering: chemical alteration of rock
(e.g. dissolution, oxidation)
• Erosion: the physical removal and transportation of
weathered material by water, wind or ice.
Weathering
Erosion
Types of sedimentary rocks:
Clastic (detrital): from deposits of detritus (loose
sediment)
Are lithified by compaction and cementation and tend
to be more permeable with larger grain sizes
Chemical: from reprecipitation (and recrystallization) of
minerals
Crystals grow until they fill all of the pore space, so
these rocks tend to be relatively impermeable until
they are exhumed (brought towards the surface) or
fractured
Weathered material (sediment) has to go
somewhere…
So, Gravity carries the sediment down until it settles
out of air or water
Lithification: the process by which unconsolidated
sediments are changed into solid sedimentary rock
Lithification Processes for clastic rocks:
• Compaction: compression of sediment from the
weight of the overlying material
• Cementation: A chemical change that involves the
precipitation of mineral among the original grains
– Most common cement types:
• Silica: creates a harder, more resistant sedimentary rock
• Calcite: creates a softer rock and will effervesce with HCl
• Iron oxides: causes a red color on weathered surfaces from
oxidation
Common minerals in detrital rocks
Clay: most abundant product of chemical weathering
from feldspars (clay is a mineral name and a size
term)
Quartz: most resistant mineral –when granite is
weathered, chemical weathering attacks the other
minerals and the quartz grains are freed
Less common:
Feldspars: break down easily from chemical
weathering processes
Micas: Soft, breaks down easily from mechanical
weathering processes
Clastic (Detrital) Rocks
• Conglomerate/Breccia
Grain Size
• Sandstone/Arkose
• Siltstone
• Mudstone
• Claystone
Grain Size: gives information about the
energy of the depositional system
Sorting: The degree of similarity in particle size
Gives us information about the depositing current
(e.g. wind blown sand = well sorted, mountain
stream deposits = poorly sorted
Very well
sorted
0.35
Well
sorted
Moderately
sorted
0.5
0.7
Poorly
sorted
2.0
Very poorly
sorted
Rounding
Tells you the distance material was transported
Composition of detritus
Tells us about rapidness of burial and length of
transport, because less stable minerals will
weather quickly at the surface
Classification by % matrix
Sediment size
Sediment Name
Sedimentary Rock
Less than 0.0004 cm
Clay
Shale
0.0004 cm – 0.006 cm
Silt
Siltstone
0.006 cm – 0.2 cm
Sand
Sandstone
Mixed
Mixed
Conglomerate (rounded
sediments)
Mixed
Mixed
Breccia (angular
sediments)
Shale
• Most common
sedimentary rock
• Comprised of silt and clay
sized particles
• Form gradual in a relatively
calm environment
• Compaction: clay minerals
wrap around silt particles
• Darker= more organics
• Low porosity from tightly
packed parallel alignment
of clay and silt
Quartz Sandstone
• Second most common
sedimentary rock
• Contains sand sized
particles of quartz
• Form gradually in a
relatively calm
environment
• Often contains bedding
• Grains cemented
together
• Greater permeability
than shale
Conglomerate/ Breccia
• Usually poorly sorted
• Contains particles larger
than sand size
• Conglomerate has
rounded clasts while
breccia has angular ones
• From a high energy system
near the sediment source
• Can be from glacial activity
• Are rarer than Ss and Sh
because they are usually
deposited in a system that
has not reached
equilibrium
Breccia
Conglomerate
Carbonates
•
•
•
•
Limestone makes up ~10% of all sedimentary rocks
Made up predominantly of calcite and dolomite
Can form by inorganic means or biochemically
Although they are produced under a variety of
conditions, those with a marine, biochemical origin
are the most common
Organic carbonates
Inorganic
Carbonates
Sedimentary Environments
• Continental
–
–
–
–
Fluvial (stream)
Alluvial Fan
Eolian
Playa Lake
• Marine
– Shallow Marine
– Deep Marine
• Transitional
–
–
–
–
Beach
Tidal flat
Lagoon
Delta