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Sedimentary Rocks
John Day Fossil Beds Natl. Mon.,Oregon
Sedimentary rocks  composed of sediment
(particles derived by weathering)
• Detrital sediments  particles derived by physical
weathering
• Non-detrital sediments  minerals precipitated from
solution by inorganic chem. processes or activities of
organisms
Classification of Sedimentary Particles
Size
>2 mm
1/16-2 mm
1/256-1/16 mm
<1/256 mm
Sediment Name
gravel
sand
silt
clay
Why Should You Study Sedimentary Rocks?
• Sediments are the most common Earth materials.
• Rock Features record environmental conditions at the
time sediment was deposited.
(impt. in deciphering Earth history)
• Fossils  knowledge of pre-existent life preserved in
sedimentary rocks
• Some sed. rocks contain resources (i.e. water,
petroleum, natural gas, etc.).
Transport & Deposition
Ice
-- carries particles of any size.
Wind
-- transports sand & smaller particles.
Water (most prolific agent)
Larger particles require more vigorous current
for transport.
• Depositional environment  geographic area
where sediment accumulates
well-rounded, poor-sorted gravel
angular, poor-sorted gravel
• Rounding -- abrasion during transport reduces particle
size & smoothes sharp corners
• Sorting -- variety of particle sizes present in sediment
or sed. rock (influenced by transport & depositional
processes)
Processes (Physical, chemical, & biological) operating
in the depositional environment impart distinctive
charac. to accumulating sediment.
How Are Sediments Transformed
into Sedimentary Rocks?
Compaction
– due to weight of overlying sediment
– particles pack more closely & pore space is reduced
– deposit’s volume reduced
Cementation
– binds one particle to another by chemical precipitation of
minerals in sediment pore space.
– {Common cements include quartz, calcite, & hematite.}
Lithification
– involves compaction & cementation
– converts sediments to sedimentary rocks
Detrital sedimentary rocks
comprised of solid particles derived from
parent material by phys. weathering
Classification of Detrital Sed. Rocks
Based on:
Sediment Size
Composition
Conglomerate = rounded gravel (>2 mm)
Breccia = angular gravel
A
B
Sandstone = sand-size (1/16 - 2 mm) particles
• classified by mineralogy
Quartz sandstone  most common
Arkose sandstone  25% feldspar
Mudrock  any combination of silt & clay
Siltstone  silt (1/16 – 1 / 256 mm)
Claystone  clay (< 1 / 256 mm)
•
Shale is fissile clay.
(splits along many
closely spaced planes)
•
Mudrocks = most
common sed. rock
(quiet-water environ)
shale
Chemical sedimentary rocks
 ions taken into soln by weathering of parent material
**crystalline texture (interlocking mineral grains)
Biochemical sedimentary rocks
 Organisms aid in the precipitation of minerals.
Classification of Chemical Sed. Rocks
Based on:
Mineral composition
Texture
Evaporites  formed by precipitation of minerals
from evaporating water
\
B
Rock salt {halite (NaCl)} (A) &
Rock gypsum {gypsum (CaSO4 . 2H2O)}
A
(B) are most common.
Limestones
composed of Calcite
(CaCO3)
Coquina
contains much pore space
Clastic texture
 shell fragments
Fossiliferous limestone
skeletal fragments of marine invertebrates
-- pores filled w/ cement & mud --
A
B
Chert
Coal
crystalline texture
silica
(layers of microscopic shells
of marine organisms)
crystalline texture
carbon
(compressed, altered plant
remains that occupied
swamps & bogs)
Strata (beds)  layers differ in color, texture,
& comp. from rock layers above & below.
Sedimentary Facies
-- sets of sed. rock w/ distinct attributes imparted by
their depositional environment
Coastal areas  facies accumulate simultaneously on
various areas of seafloor (w/ unique charac.).
1
Regression
(seaward shift in the shoreline)
• Drop in sea level
2
Nearshore deposits overlie
offshore deposits.
3
Transgression
(landward shift in shoreline)
• Rise in sea level
Nearshore seds overlie old
land surfaces, & offshore
seds are stacked on top.
Determining the Depositional Environment
Grand Canyon
Muav Limestone (offshore)
Bright Angel Shale
Tapeats Sandstone (onshore)
(deposition record of laterally adjacent environ.)
Transgression  Sed structures, fossils & ripple
marks
Sedimentary Structures
- formed by physical processes at the time of deposition
• Cross bedding
• Graded bedding
• Oscillation ripple marks
• Current ripple marks
• Mudcracks
Cross bedding
 inclined layers w/in a bed
 Formed by wind or water
 slopes downward in flow direction
Graded bedding
 upward decrease in grain size w/in a bed
{deposits of turbidity currents}
Ripple marks
-- small ridges separated by intervening troughs
Oscillation ripple marks (A) symmetric crosssections generated by the back-&-forth motion of waves.
A
B
Current ripple marks (B) asymmetric crosssections record flow in one direction (i.e., streams).
Mudcracks
 shrinkage (polygonal patterns of intersecting fractures)
• clay-rich sediment
• depositional environ.  periodic drying
(i.e., a river floodplain, lake shore, or tidal flat)
Fossils are NOT Sedimentary Structures.
 remains or traces of ancient organisms
Morphology of organisms
reveals info about
environment the
organisms lived in.
1. Heavy-shelled clams =
shallow-water, turbulent
2. Thin, fragile-shelled clams =
low-energy
3. Filter-feeding organisms
(corals) = clear seawater
(suspended sediment clogs
their feeding organs.)
Resources in Sedimentary Rocks?
•
•
•
•
Sand & gravel  building & road construction
Limestone  cement
Gypsum  wallboard/plaster
Phosphate-bearing sedimentary rock  fertilizer
• Sand-sized quartz  glass
• Carnotite (uranium mineral)  fuels nuclear reactors
[assoc w/ plant remains in sandstones formed in ancient stream
channels]
• Hematite & magnetite (banded iron formations)  iron
ores