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SEDIMENTARY ROCKS
A Brief Review
THERE ARE 4 BASIC COMPONENTS OF
SEDIMENTARY ROCKS
• sediments & sed. rocks are usually described according to 4 basic components:– a. grains (larger particles)– b. matrix (fines)
– c. pores (air, H2O, oil, gas)
– d. cement (usually post-depositional)
cement matrix
grains
pores
CLASSIFICATION OF SEDIMENTARY ROCKS
• Classified as either:– Detrital = composed of broken fragments & weathering products of
other rocks &/or minerals
• Shales
• Sandstones
• conglomerates
– Chemical = composed of minerals precipitated from water solutions
• Inorganic (direct ppt)– evaporites like rock salt which is halite
– some cherts, such as at Ross Creek
• Organic (indirect ppt)– fossiliferous limestones (organisms secrete CaCO3, not seawater)
– some cherts (with silica skeletons of micro-organisms, diatoms)
DETRITAL SEDIMENT GRAIN SIZES• Grain-size is the primary basis of distinguishing different kinds of detrital sedimentary rocks:
SIZE RANGE (mm)
PARTICLENAME
SEDIMENTNAME
ROCKNAME
>256 Boulder
64 to 256 Cobble
4 to 64 Pebble
2 to 4 Granule
GravelConglomerate orBreccia
1/16 to 2 Sand Sand Sandstone
1/256 to 1/16 Silt
<1/256 Clay
Silt,Mud, orClay
Siltstone, Shale,Mudstone, orClaystone
CLASSIFICATION OF DETRITAL SEDIMENTARY ROCKS
TEXTURE SEDIMENTNAME &GRAIN SIZE
FEATURES ROCKNAME
rounded rock fragments ConglomerateGravel(>2 mm) angular rock fragments Breccia
quartz predominates Quartz Sandstone
quartz with lots of feldspar ArkoseSand(1/16 to 2 mm)
dark colour; quartz with lots offeldspar, clay, rock fragments
Graywacke
splits into thin layers Shale
Clastic
(=brokengrains)
Mud(<1/16 mm) breaks into clumps or blocks Mudstone
CONGLOMERATE• Cemented sedimentary rock
whose grain-size diameters are greater than 2 mm
• Four categories:
– 2-4 mm = granule conglomerate
– 4-64 mm = pebble "
– 64-256 mm = cobble "
– >256 mm = boulder "
• Cements vary. Common ones are:
• calcite (calcium carbonate)
• quartz (silicon dioxide)
• hematite (iron oxide)
CONGLOMERATE
• Conglomerates are common in continental stream channel deposits
BRECCIA
• A conglomerate whose grains are angular
SANDSTONE• Grain size = 1/16th mm to 2 mm• Cements vary, commonly: calcite
quartz
or iron oxides• Quartz sandstone = quartz grains
predominate• Arkose
– rich in feldspar grains– pink or red colour from feldspars
• Graywacke– >15% clay matrix– gray to black colour– lots of feldspar & rock fragments
SILTSTONE
• Grain size:– less than fine sand (<1/16th mm)– larger than clay sizes (<1/256th mm)– feels "gritty between your teeth :(
• Quartz dominates
• Structures:– finely laminated– rippled– cross-bedded
SHALE
• Subparallel parting splits into thin layers
• Clay and silt sizes dominate
• Clay minerals dominate
• Colours vary:– Grey (contains organic matter)– Black:
• may be rich in organic matter
• some associated with coals
– Red (contains iron oxides)
• Preserves fossils well
CLASSIFICATION OF CHEMICAL SEDIMENTARY ROCKS
GROUP TEXTURE COMPOSITION ROCK NAME
clastic ornonclastic
calcite(Ca carbonate)
Limestone
clastic ornonclastic
dolomite(Ca + Mg carbonate)
Dolostone(Dolomite)
nonclastic microcrystallinequartz
Chert
nonclastic halite, NaCl Rock salt
Inorganic
nonclastic gypsum Rockgypsum
clastic ornonclastic
calcite (shells) Limestone
nonclastic microcrystallinequartz (micro-shells)
ChertBio-chemical
nonclastic altered plant remains(organic matter)
Coal