Sedimentary Rocks

Clastic vs. Non-clastic

• Sedimentary rocks in two major groups• Clastic (detrital)

– Composed of fragments of silicate minerals (mostly quartz and clay)

– Name largely determined by the grain size (see handout)

• Non-clastic– Composed of various minerals (usually just one)

– Name largely determined by the composition

Clastic (detrital)• Coarse grained (> 2 mm)

– Conglomerate (rounded pieces)– Breccia (angular pieces)

• Medium grained (<2 mm but still visible)– Quartz sandstone (mostly quartz)– Arkose (abundant feldspar – often pink)– Greywacke (much clay – often dark in colour)

• Fine grained rocks (cannot see ind. grains)– All “mudrocks”– Siltstone (gritty), shale (splits easily), claystone

(smooth, slippery feel)

Non-clastic• All one mineral (usually)• Often “crystalline” (visibly so or VERY smooth

(with conchoidal fracture)• Named according to mineral present (use hardness

and reactivity to HCl)– H > glass, no HCl reaction: mineral is chalcedony, rock

is chert– H ~ penny, no HCl reaction: mineral is dolomite, rock

is limestone (dolomitic)– H ~ penny, HCl reaction: mineral is calcite, rock is

limestone– Penny > H > fingernail, salty taste, no HCl reaction:

mineral is halite, rock is Rock salt– H < fingernail, no HCl reaction: mineral is gypsum,

rock is Rock gypsum

Doing the lab – CaCO3?

• Test with HCl (acid).

• Fizz means calcite present.– If it’s all calcite – rock is non-clastic

• Be careful to note if the rock is dominantly calcite or just partly (i.e., is the rock all calcite, a limestone, or just random parts, e.g., a calcite cemented sandstone)

Doing the lab – C or NC

• C: is the rock clastic; i.e., composed of discrete particles (grains) of silicate material (quartz, feldspar, clays, etc.)?

• NC: is the rock non-clastic; i.e., all one mineral, probably a non-silicate, and probably crystalline?

Doing the lab – Grain size• Only applicable to clastic rocks (i.e. with non-

clastic rocks say “not applicable” or “n.a.”)• Coarse grained (c.g.): > 2 mm• Medium grained (m.g.): vis. grains to 2 mm• Fine grained (f.g.): gritty on teeth• Very fine grained (v.f.g.): slippery to feel

(probably with an earthy smell)

• HINT: The higher the clay content of clastic rock the darker the colour.

Doing the lab – Components

• What makes up the rock– Grains? (Sand, silt, pebbles, …)– Of what?

• A single mineral?

• Multiple components?

• …

Sorting• Only applicable to clastic rocks (for non-clastic rocks

say “n.a.”)• Fine-grained (and v.f.g.) rocks are too fine to see the

sorting. Say “too fine to see” or “n.d.”• Two kinds of sorting

– Compositional• Well = composed of only one component

• Poor = mix of several different components

– Size• Well = all grains are same size

• Poor = grains are all sizes

• Compositional and Size sorting usually go hand-in-hand

Other features

• Include:– Fossils– Salty taste– Obvious bedding/laminations– Ooids (oolites)– …

Name

• Use– Handout chart– Book– Wall poster

Questions?