SEDIMENTARY ROCKS SEDIMENT

SEDIMENTARY ROCKS
SEDIMENT –unconsolidated (“loose”
loose”)
granular geologic material
SEDIMENTARY ROCK —lithified granular
geologic material
Sedimentary Processes
SEDIMENT
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Comes from:
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– Physical weathering which creates particles
– Chemical weathering which creates solutions
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Sediment is unconsolidated
Weathered and eroded material transported from
higher to lower places
Sediment sorting by a river
SEDIMENT
z
z
Particles are transported and deposited;
solutions are transported and precipitated
Transportation
– Grain size
– Rounding
– Sorting
http://www.uwrf.edu/~iw00/101gr/rounding.gif
Sorting = process by which particles having a
particular size or characteristic are selected or
concentrated. Which processes result in poorlysorted sediment?
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Sedimentary Processes
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Common Depositional Environments
ENVIRONMENTS of deposition
– MarineMarine- most abundant
– Terrestrial
River
Marine shoreline and nearnear-shore
environments
Braided stream
Alluvial fan deposits (terrestrial)
Sediment moves downstream via migrating bars and
in suspension during flood events
Badwater
alluvial fan
Marli Miller
Anastamosing channels—caused by sediment overload
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SEDIMENT
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Clastic
Particle size
– Gravel (> 2 mm)- Pebbles, cobbles, boulders
– Sand (2 mm to 1/16 mm)
– Silt (1/16 mm to 1/256 mm)
– Clay (< 1/256 mm)
• Deposition ClayClay-sized particle vs. clay mineral
TYPES OF SEDIMENTARY
ROCKS
Clastic – fragments accumulate
z Chemical – precipitates accumulate
z Organic – biogenic, lumped w/ chem in book
z
Common Sedimentary Rocks (Clastic)
z
CONGLOMERATECONGLOMERATE-
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Breccia
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SANDSTONESANDSTONE-
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SHALESHALE-
– From GRAVELGRAVEL- coarse rounded fragments…
fragments…
– From GRAVELGRAVEL- coarse angular fragments…
fragments…
– From SANDSAND- (1/16 mm to 2 mm)…
mm)…
– From SILT or CLAY (mud)
– Most common sedimentary rock
Conglomerate
Breccia
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Sandstone
Bondi Beach
<=Ss
thin
section
Note: can’t see grains—feels smooth to the touch; if it contains some silt
particles, then call it either a silty shale or mudstone (if 50/50 silt/clay)
Lithification
z
z
Lithification (compaction and cementation)
Compaction
Cementation
– Cement deposited by groundwater
– Cement most commonly calcite (CaCO3)
– Less commonly silica (SiO2)
– Iron oxide (hematite) when red
Lithification of Shale
Shale outcrop
Bedford Shale, Cuyahoga River valley,
Ohio, Mississippian age
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Chemical sedimentary rocks
Bioclastic limestone
Other Sedimentary Rocks
Chalk
z
COAL
– partial decay of plants
– important fossil fuel
A soft marine limestone formed
from the tests of many tiny
protozoans, mainly foraminifers
(aka “forams”)
SEDIMENTARY STRUCTURES
z
BEDDINGBEDDING– Bedding plane—
plane—an interuption in deposition
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z
Principle of Original Horizontality
Principle of Superposition
Bedding in sandstone and shale, Utah
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Still sediment—not yet lithified!
Bedded chert from CA coast ranges
Peat layers, lahar deposits, and tephra east of Enumclaw
Coal beds in Puget Group sedimentary rocks near Ashford
Channel fill
Sharon Conglomerate, Cuyahoga River valley,
Ohio
~315 ma
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Lava over sediment:
Tieton Andesite lava flow (1
Ma) from Goat Rocks
volcano over fluvial gravel
along US 12 west of Naches
Tilted beds
What does the principle of “original
horizontality” tell you about the
history of these sedimentary strata?
SEDIMENTARY STRUCTURES
These features are indicators of processes and
environments
z
z
How crossbeds form—
form—migrating dunes!
CROSSCROSS-BEDDING (\
(\\\\\\):
– Deposited by sand dunes via wind (terrestrial)
– Deposited by river dunes (ex: in deltas)
GRADED BEDDING
– Caused by turbidity currents or by pulsing flow (glacial
outwash)
z
z
z
MUD CRACKS
RIPPLE MARKS ≈ ≈ ≈ ≈
FOSSILS
Crossbedded sandstone
MSOffice1
Planar cross beds
Sharon Conglomerate,
lower Pennsylvanian
Age (~315 Ma),
Cuyahoga River valley,
Ohio; braided stream
environment
Zion National Park. Festooned cross beds deposited by wind.
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Slide 39
MSOffice1
, 2/13/2008
Crossbeds in dipping layers of Ohanapecosh formation,
volcaniclastic rocks; SR 165 west of Mt. Rainier
Turbidity currents
Deposited by one
turbidity current
D:\digital_content\animations_library\0009.swf
Miocene Hoh Fm. Beach 4, Olympic Coast
Upside down graded beds!
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Mud up to 3 ft thick sits in fields, around homes and on roads in and around Curtis —
and it's starting to dry up. Occupants of this Ceres Hill Road home are burning their
furniture and belongings destroyed by the flood. STEVE RINGMAN / SEATTLE TIMES
Ancient mud
cracks in shale
Modern mud
cracks
Current ripples in wet sediment,
Baja CA
Ripples
Ripple marks in sandstone, Capitol
Reef National Park, Utah
Pat Pringle, photo
Ripple marks in tilted Puget Group sedimentary rocks
of Eocene age near Wilkeson WA
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Fossil
z
…the remains of an organism
preserved in rock. May consist of
original mineral matter, petrified
organic material, or an impression left
behind.
www.fossilmuseum.net
Above: Fossil fish from
Eocene Green River Shale in
western Wyoming
Trilobite - species
Chancia, order Ptychopariida,
Cambrian Addy Quartzite,
WA; David Morgan photo;
www.westernta.com
Fossil flower:
Eocene, Republic
WA
Florissantia uilchenensis
Brachiopod
Inoceramus
Ammonites (Cephalopods)
Wiwaxia, Pam
Gore, gpc.edu
cyanocacteria
http://www.mmf.selcuk.edu.tr/personel/falkaya
/paleolab/bivalvia.htm
Marrella: Burgess
Shale, Univ. of Calgary
www.fossilmuseum.net
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Photos courtesy of R, Weller,
Cochise College
Fossil metasequoia leaf from rocks
several miles east of Packwood,
Oligocene age
Dinosaur footprints,
USGS
http://paleo.cc/paluxy
/ovrdino.htm Glen
Kuban’s site
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Misc. Sedimentary Features
z
z
z
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Sorting
Roundness
Orientation: random vs. preferred
Color
Subfossil Douglas fir buried by Round Pass lahar
from Mount Rainier along the Nisqually River (2.6 ka)
Sorting
Preferred orientation—shows current direction
Imbrication = strong current
<= Current direction
= right to left
Color
A function of size and composition
Finer grained = truer colors
Weathering & matrix cement also influence
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z
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Red = oxidation (ex: hematite, “red beds”
beds”—
most of these are subaerial)
Green = near oxidation/reduction margin
(ex: glauconite,
glauconite, chlorite)
Black = reducing conditions (ex: pyrite,
MnO2, organic matter)
Rock Units
z
FORMATIONFORMATION– A geologic unit extensive enough to show on a
map
– Distinctive from neighboring rock units
– Named locally, ex: Ohanapecosh Formation
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CONTACTS
– Sedimentary contacts – bounding surfaces
between two sedimentary units
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Unconformities—missing time!
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z
z
Angular unconformity – horizontal beds
atop tilted beds…
beds…
Disconformity – horizontal beds atop other
horizontal beds, but missing time in
between…
between…
Nonconformity – sedimentary rocks atop
igneous or metamorphic rocks
Monument Creek, Grand Canyon
Angular
unconformity
Bob Webb, photo, 1986
Granite Rapid, Grand Canyon
Grain size,
sorting, and
angularity in
Tenino
Sandstone
Bob Webb, photo, 1986
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Troutdale Formation
Wildcat Creek beds
US 12 near Rimrock Lake
Centralia Coal Mine—
Skookumchuck
Formation
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