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Sedimentary rocks
Geology 103
Making sediment
• Weathering = rock breakdown into smaller
rock, or minerals, or chemicals
• Sediment = result of weathering of rocks
• Erosion = movement of sediment
Sediment stages
1. Weathering
2. Erosion
3. Transportation via
water, glaciers and wind
4. Deposition
5. Burial and
compaction
6. Diagenesis
Physical weathering
• Physical:
Breaking apart of
rocks by a
physical force
• Also called
“mechanical”
weathering
Chemical weathering
• A chemical reaction
alters the composition
of the minerals in the
rocks (e.g., dissolving
halite or altering
feldspars into clay
minerals)
• Dissolving limestone
leads to karst
topography and caves
Caves are the product of varying water tables and
limestone dissolution; cave formations (speleothems)
are the result of calcite precipitation
Or a combination
• Physical weathering
may break up
bedrock, then
chemical weathering
may break down the
pieces into a soil
A quick word on soils
• Soils are the
weathered material
at the surface that
include both organic
and mineral
components
• Soils differ due to
the parent material,
time of weathering
and water content
Lithification
• Sedimentary
rocks
become
lithified
when they
are
compacted
then
cemented
Diagenesis is also a term used to describe the
production of sedimentary rocks from sediments
Compaction
squeezes out the
water
Precipitation: addition
of new minerals
cements the sediment
particles
Classification of sed rx
• Clastic = “broken”; sed rx made of broken-up
parts of other rocks
• Chemical sed rx are made from the
precipitation of chemicals from water
(“evaporites”) or the oxidation of chemicals
• Biological sed rx are “born”; that is, they
derive from the remains of creatures
Relative abundance of sediments
Clastic sed rx
• Classified by dominant grain size
• Scale: “boulder”, “cobble”, “pebble”, “granule”,
“sand”, “silt”, “clay”
• Texture and composition are secondary
considerations (e.g., “shale”)
Texture: Grain roundedness
• The roundedness
(that is, how sharply
defined the
“corners” of
individual grains
are) is used to
determine the
transport distance
• Proximal = near;
distal = far
Texture: Grain sorting
• The variation between coarse and fine
particles in the sediment is called sorting
• Sorting is used to infer transport distance;
well-sorted sediments have come far
Mineralogy of sandstone and depositional environments
Layering in clastic sed rx
• Layers are called
beds, unless they
are thin, in which
case they are called
laminae
• Beds and laminae
represent distinct
depositional events,
like floods
Chemical sed rx
• Evaporites: limestone, rock salt
• Oxidation product: taconite (iron ore)
How evaporites are deposited
1. During the Miocene epoch, the
Mediterranean Sea became a shallow
evaporite basin when the Strait of
Gibraltar was above sea level
2. Reduced
water
exchange with
the Atlantic
3. Evaporation
removed water
4. Fresh water
inflow was limited
5. Gypsum and halite crystalize first forming evaporites
The Mediterranean dried up repeatedly between 5.6 and 5.33
Ma; the Zanclean Flood refilled the basin within two years
GarciaCastellanos et
al.,
“Catastrophic
flood of the
Mediterranean
after the
Messinian
salinity crisis”,
Nature 462
(2009), 778781
Biological sed rx
• Plant remains: coal
• Animal remains: limestone, chert
Most biological sed rocks are deposited
on (and make) carbonate platforms
Carbonate platform formation
Within the reef lagoon,
growth of carbonatesecreting organisms,
including forminefera,
coral, algae and molusks,
is rapid, and carbonate
sediments form quickly
Eventually a carbonate
platform grows with steep
sides towards the sea
If sea level rises, the
reef continues to grow
towards the light and
lagoon sedimentation
outpaces sedimentation
in the open ocean
Coral reefs can form an atoll around a
mid-ocean volcano
Bora Bora atoll, South Pacifc
Coral reefs and atolls creation
Process first described by
Charles Darwin
Water energy
• The speed of the
depositing medium
(usually water) can
be inferred from
clastic and some
other sed rx
• This is because
coarser grains settle
first in a suspension