Page 179 - Laboratory Manual in Physical Geology

COMPOSITIONAL CLASSIFICATION OF SEDIMENT AND SEDIMENTARY ROCKS

A. DETRITAL (SILICICLASTIC) SEDIMENT AND SEDIMENTARY ROCK IS MOSTLY ONE OR MORE OF THESE:

Rock fragments: Quartz grains: Feldspar grains: Clay:
may be angular or angular grains freshly large angular grains commonly forms from
rounded; can include broken from their source freshly broken from their chemical decay of
detrital chert grains (see and pebbles rounded source and small suban- feldspars and micas
“chert” below) during transportation gular grains

B. BIOCHEMICAL SEDIMENT AND SEDIMENTARY ROCK IS MOSTLY EITHER OR BOTH OF THESE:

Bivalve Coral
mollusk shell
(clam)

Brachiopod
shell Stem parts of a crinoid
(“sea lilly”) that is an animal
related to starfish
Shell bioclasts: Plant fragments:
broken and whole animal are brown in peat and
shells black in coal

C. CHEMICAL SEDIMENT AND SEDIMENTARY ROCK IS MOSTLY MADE OF ONE OR MORE OF THESE:

Gypsum: Calcite spar (crystals): Dolomite: Halite:
white or gray, easily reacts with dilute HCl, usually cryptocrystalline; gray to red cubic crystals
scratched with your breaks into rhombohedral reacts with dilute HCl only (often intergrown as rock
fingernail shapes if it is powdered salt); salty taste

Ooids: Limonite: Hematite: Chert:
tiny (< 2 mm) spheres of opaque brown to yellow opaque brick red to silver a gray, red, brown or black
calcite or aragonite that rusty-looking crusts, layers; gray layers; cements cryptocrtstalline variety of
resemble miniature pearls; cements sediment, making sediment, making it look quartz (may contain fossils,
reacts to dilute HCl it look yellow to brown red including silica microfossils

FIGURE 6.2 Composition of sedimentary rocks. Scale for all images is × 1 unless noted otherwise.

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