Sedimentary Rocks
Sedimentary Rocks
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Products of mechanical and chemical weathering
Account for about 5% of Earth’s crust
Contain evidence of past environments
 Texture
 Fossils
Sedimentary Rocks
Texture and composition - the keys to classification.
Texture:
Clastic or detrital
Non-clastic
Chemical (or crystalline)
Biochemical
Sedimentary Rocks
Clastic or detrital rocks are composed of particles of preexisting rocks which have been weathered, eroded,
transported, deposited, and cemented together.
Grain size (energy)
Rounding - energy and length of transport
Sorting - uniformity of grain size
Sedimentary Rocks
Chemical sedimentary rocks: precipitated from solution.
Texture also referred to as crystalline- readily
recognized in moderately coarse-grained examples.
Very fine-grained chemical rocks appear massive.
Some limestone, halite, sulfates, etc.
Evaporites are a common source.
Also caves, hot springs.
Sedimentary Rocks
Biochemical rocks: formed by the processes of organisms
Calcite or silica shells or skeletons.
Form directly (coral reefs) or
Cemented together after organisms die & accumulate
Energetic wave action may break up shells and produce
clastic textures
= clastic or biochemical??
We'll consider it biochemical because the fragments are
of biochemical origin
If shell fragments are clearly recognizable, the texture is
called skeletal
Sedimentary Rocks
Sedimentary Rocks
Clastic/Detrital Rocks
Shale- fine grained (mud, silt, clay)
Fissile (splits in layers)
Fissile Shale
Shale with leaf fossils
Low-energy environment:
Offshore shallow marine
Lake
Sedimentary Rocks
Clastic/Detrital Rocks
Siltsone/mudstone- fine grained (mud, silt, clay)
Massive- breaks in clumps
Low-energy environment:
Offshore shallow marine
Lake
Shale with leaf fossils
Fissile Shale
Sedimentary Rocks
Clastic/Detrital Rocks
Sandstone- Composed of sand-sized particles
Well-sorted: water or wind
Forms in a variety of environments: beach, floodplain…
Quartz is the predominant mineral
Thin-section of sandstone under the petrographic microscope
Sedimentary Rocks
Clastic/Detrital Rocks
Graywacke
“Immature” sandstone
Contains lots of easily-weathered material:
Mafic minerals, volcanic glass/ash
Typically occurs in subsiding basin
near an active volcanic arc
(uplift/erosion and burial > chemical
weathering)
Sedimentary Rocks
Clastic/Detrital Rocks
ConglomeratePebbles/cobbles
Well-rounded: long transport
High-energy environments:
Beach, River
Sedimentary Rocks
Clastic/Detrital Rocks
BrecciaPebbles/cobbles
Poorly-rounded: short transport
High-energy environments:
Beach, River, Fault zone
Breccia
Sedimentary Rocks
Chemical Rocks
Some Limestone is precipitated
directly from water
Also true for some dolostone
(Ca-Mg-carbonate) and chert
(SiO2)
Sedimentary Rocks
Chemical Rocks
Some Limestone is precipitated
directly from water
Travertine is typically precipitated from
groundwater and is seen in caves
Oolitic limestone contains small
spherical ooids
Sedimentary Rocks
Chemical Rocks
Rock salt
Evaporites are precipitated directly
from seawater when a large
quantity evaporates
Rock Gypsum
Sedimentary Rocks
Biochemical Rocks
Most Limestone is organic:
precipitated as shells, reefs,
and even as tiny shells from
planktonic life
Fossiliferous Limestones
Sedimentary Rocks
Biochemical Rocks Most Limestone is organic:
precipitated as shells, reefs, and even
as tiny shells from planktonic life
Chalk fossils
under electron
microscope
Limestone Reef
Chalk Cliffs
Sedimentary Rocks
Biochemical Rocks
Coquina is composed of broken-up
shell fragments due to marine wave
action in the near-tidal zone
It also qualifies as a detrital rock
Sedimentary Rocks
Biochemical Rocks
Chert, like chalk, is composed of tiny shells
from planktonic life, but the organisms involved
secrete SiO2 shells.
Diatoms and radiolaria
have microscopic SiO2
shells
Sedimentary Rocks
Biochemical Rocks
Coal is organic: buried and un-oxidized plant remains
Sedimentary Rocks
Biochemical Rocks
Coal is organic: buried and un-oxidized plant remains
Sedimentary Rocks
Sedimentary Environments
Using sedimentary rocks to interpret Earth history
Sedimentary Rocks
Sedimentary Structures
Large-scale cross-beds: wind-blown
sands in an arid environment
Many sedimentary structures may be
used to determine original upward
direction in vertical or overturned strata
Ripple-marks: running water- beach or river
Using sedimentary rocks to interpret Earth history
Sedimentary Rocks
Sedimentary Structures
Graded beds- offshore turbidites
Using sedimentary rocks to interpret Earth history
Sedimentary Rocks
Sedimentary Structures
Mud-cracks and raindrop imprints
Using sedimentary rocks to interpret Earth history