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