Page 185 - Laboratory Manual in Physical Geology

SEDIMENTARY ROCK ANALYSIS AND CLASSIFICATION

STEP 1: Composition. STEP 2: What are the rock’s texture STEP 3: Name the rock
What materials and other distinctive properties? based on your analysis
comprise most of in steps 1 and 2.
the rock?
Rock fragments Mostly angular and/or subangular BRECCIA*
and/or gravel (grains larger than 2 mm) CONGLOMERATE*
quartz grains
rocks and/or silicate mineral crystals Detrital (Siliciclastic) sediment grains: fragmented clay minerals Mostly sand Mostly quartz sand QUARTZ SANDSTONE SANDSTONE Detrital (Siliciclastic) sedimentary rocks
Mostly subround and/or well rounded
and/or
gravel (grains larger than 2 mm)
feldspar grains
and/or
(1/16–2 mm
(e.g., kaolinite)
grains). May
Mostly feldspar sand
ARKOSE
contain fossils
Detrital sediment is
derived from the
Mostly rock fragment sand
LITHIC SANDSTONE
mechanical and
chemical weathering
of continental (land)
WACKE (GRAYWACKE)
Sand is mixed with much mud
rocks, which consist
mostly of silicate
May contain
minerals. Detrital
sediment is also
fossils
called terrigenous No Mud (< 1/16 mm) Mostly silt. Breaks into blocks or layers SILTSTONE MUDSTONE
Mostly clay.
(land derived) visible May contain Fissile (splits easily into layers) SHALE
sediment.
grains fossils Crumbles into blocks CLAYSTONE clastic rocks
Brown porous rock with visible plant fragments that are easily PEAT
broken apart from one another
Biochemical (Bioclastic) sediment grains: fragments/shells of organisms Shells and Black, layered, brittle rock; may be sooty or bright BITUMINOUS COAL LIMESTONE Biochemical (Bioclastic) sedimentary rocks
Plant fragments
LIGNITE
Dull, dark brown, brittle rock; fossil plant fragments may be visible
and/or charcoal
Mostly gravel-sized shells and shell or coral
COQUINA
fragments; (Figure 6.6)
CALCARENITE
Mostly sand-sized shell fragments; often
(FOSSILIFEROUS
shell/coral
contains a few larger whole fossil shells
LIMESTONE)
fragments,
and/or
Silty, earthy rock comprised of the microscopic
calcareous
shells of calcareous phytoplankton (microfossils);
CHALK
microfossils
No visible grains in most of the rock. May break with
No
conchoidal fracture. May contain a few visible fossils
grains
in the micrite
Mostly spherical grains that resemble miniature pearls
Calcite crystals rocks that effervesce in dilute HCl rocks that effervesce in dilute HCl ocks that e fervesce in dilute HCl visible may contain a few visible fossils MICRITE carbonate/calcareous rocks
OOLITIC LIMESTONE
and/or calcite (< 2 mm), called ooliths or ooids
spheres and/or LIMESTONE
microcrystalline Masses of visible crystals and/or microcrystalline; may have
calcite/aragonite cavities, pores, or color banding (Figure 6.8); usually light colored TRAVERTINE
Mineral crystals (inorganic) or chemical residues (e.g., rust) Halite mineral crystals Visible cubic crystals, translucent, salty taste (Figure 6.7) ROCK SALT evaporite rocks Chemical sedimentary rocks
Effervesces in dilute HCl only if powdered. Usually light colored.
Microcrystalline
DOLOSTONE
(Commonly forms from alteration of limestone)
dolomite
Gypsum mineral
Gray, white, or colorless. Visible crystals or microcrystalline.
ROCK GYPSUM
Can be scratched with your fingernail
crystals
Dark-colored, dense, amorphous masses (e.g. limonite),
Iron-bearing minerals
crystals or residues
microcrystalline nodules or inter-layered with quartz or red chert
(banded iron formation)
Microcrystalline, may break with a conchoidal fracture. Hard IRONSTONE
Microcrystalline (scratches glass). Usually gray, brown, black, or mottled mixture
varieties of quartz of those colors. Chert can be regarded as biochemical if its CHERT (a siliceous rock)
(flint, chalcedony, silica came from dissolution of siliceous plankton (diatoms,
chert, jasper)
radiolaria).
*Modify name as quartz breccia/conglomerate, arkose breccia/conglomerate, lithic breccia/conglomerate, or wacke breccia/conglomerate as
done for sandstones.
FIGURE 6.9 Sedimentary rock analysis and classification. See page 166 for steps to analyze and name a sedimentary rock.
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