Sedimentary Rocks
Sediment
any loose, solid particles that originate
from either weathering and erosion of
rock, or the chemical precipitation of rock
from solution.
Gravel: particles >2mm
Boulder: basketball sized and up
Cobble: between a tennis ball and a
basketball
Pebble: between a tennis ball and the
thickness of a nickel
Smaller Particles
Sand: grains between 1/16 mm and 2 mm
(between the thickness of a nickel and the
thickness of a human hair)
Silt: grains between 1/16 and 1/256 mm
(too fine to feel gritty to the touch, but still
feels gritty to the teeth)
Clay: grains smaller than 1/256 mm (too
fine to feel gritty to the teeth)
vocabulary
• Weathering- destructive processes that
change the chemical and physical character of
rock near the Earth’s surface.
• Erosion- is the picking up or physical removal
of rock particles by water, wind, ice, etc.
mechanical vs. chemical weathering
Weathering
Soil Erosion
Madison County, AR 2004
Southwestern Arkansas 2005
Transportation of from one place to another can
alter the shape and size of sediment particles.
• Rounding: The grinding away of sharp edges
caused by the grinding and scraping of sediment
against each other.
• Sorting: Sediment grains can be sorted
according to size, shape, or specific gravity by
water or wind.
Sorting
Deposition- as stream velocity slows, it can’t hold the larger
grains in the water column anymore. At each drop in
velocity, progressively smaller particles fall out.
Lithification: The process that compacts,
cements, and/or crystallizes loose
sediment together to form a rock.
note: Not all sediments are preserved where they are first
deposited. For example, a river can deposit sand along its
banks, but a large flood can later remove that sand before
it is preserved as a rock.
The processes of Lithification
Compaction: The rearranging of sediment so there is much less
open space between the grains.
Cementation: The precipitation of a mineral between the grains
that holds the grains together to make a solid rock.
Calcite and silica (quartz) are common cements.
Sedimentary rocks are split into two
groups: clastic and non-clastic.
• Clastic: rocks formed from the
weathered fragments of existing rock.
• Fragments can be large (pebbles) or
tiny grains of a mineral (quartz sand).
non-clastic sedimentary rocks
rocks formed from:
1. the chemical precipitation of minerals from
solution (rock salt)
2. the formation of rock by living organisms
(coral reef)
3. the accumulated and compressed remains
of soft bodied organisms (coal).
clastic sedimentary rocks
marine carbonate deposition
non-clastic sedimentary rocks
evaporites (rock salt)
fossiliferous limestone
Some features found in sedimentary rock that are
formed during or just after deposition, but before
lithification.
Bedding: Individual layers of (mostly) parallel sedimentary rock.
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Some beds are easy to see because of the color differences, others are a
distinguished by a change in texture or weathering patterns.
Cross-Bedding: The preserved tracks of dunes as they migrated
through air or water.
Photo: A. Marshall
Graded Bedding: A single bed that has grain sizes sorted vertically
from large to small.
Ripple Marks: Ripples that have been preserved as a rock.
Photo: A. Marshall
Sedimentary Conglomerate
Identified by the presence of rounded pebbles in a fine grained matrix.
Formation: Forms from the lithification of rounded, but not sorted rock fragments.
Depositional environment(s): Rock that falls into pounding surf or a fast moving
river could be rounded but not sorted. Outcrops are usually the small
scale you would expect from this kind of depositional history.
Sedimentary Breccia
Identified by large angular fragments in a fine grained matrix.
Formation: formed from the cementation of rock fragments that have not been
sorted or rounded, so they probably formed near their source rock.
Depositional environment(s): Steep slopes beneath weathering outcrops could
accumulate angular fragments, or a landslide could collect fragments like this.
This rock is not very common.
Quartz Sandstone
Identified by sandy texture and lack of any other minerals but quartz.
Formation: formed from the cementation of quartz sand, color depends on the
cementing mineral, but can be buff shades of brown, red, yellowish, even
green!
Depositional environment(s): river beds and deltas, sand dunes, beaches
Siltstone
Identified by the very fine grain size without the plated look of shale.
Formation: forms from the cementation of very fine quartz grains. Color is usually
shades of brown or gray, sometimes yellowish.
Depositional environment(s): transitional zones between moving waters such as
rivers, and still, quiet waters such as lakes, and river flood plains.
Shale
Identified by grains too small to be seen with the eye and fissle property.
Formation: forms from very fine grain sediment being compacted and cemented.
Color is usually shades of gray and black, can also be brown, yellowish or green.
Depositional environment(s): very still, quiet waters such as deep ocean basins,
river flood plains, and lakes.
Fossiliferous Limestone
Identified by the abundance of fossils, and strong reaction with HCl acid.
Formation: formed by the cementing of the hard parts of organisms by calcite.
Colors range from white to tan, brown, and gray, sometimes yellowish.
Depositional environment(s): marine setting, warm, shallow water offshore
Dolostone
Identified by the very fine crystalline texture and the weak reaction to HCl.
Formation: forms from the recrystallization of limestone after lithification
Depositional environment(s): Lithified rock where seawater seeping through can
bring in extra magnesium that replaces some of the calcite. This process
often destroys whatever fossils were present, making it nearly impossible
to determine the original environment the rock formed in.
Chalk
Identified by the powdery texture that comes off on your hands.
Formation: forms from the compaction of microfossils
Depositional environment(s): microorganisms in the water column die and pile up
on the ocean floor. Eventually they are compacted together to form a
rock.
Chert
Identified by very fine grain texture and hardness (it is silica, like quartz).
Formation: 2 ways to get chert: 1. replacement of original rock with silica, 2.
accumulation and cementation of very tiny microfossils
Depositional environment(s): 1. groundwater moving through lithified rock
replaces some of the original rock structure with silica. Usually seen as
lumps, nodules, and veins in other types of rock, or 2. tiny marine fossils
build up in the same way chalk is formed, but are more cemented and
usually later replaced by fine grain silica, destroying the fossils.
Geologist are still debating which idea is the best, if either.
Coal
Identified by black color, light weight and softness (hardness of 1.5-2.5).
Formation: formed by the compaction of organic material that has not completely
decayed. Color is black. Anthracite coal is more shiny due to extra heat
and pressure after formation, and bituminous is more dull.
Depositional environment(s): an environment with vigorous organic production
and water with a low oxygen content are needed. So the best environment
to form coal is in a bog or swamp.