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Sedimentary Rocks

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)

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Sedimentary Rocks

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  1. Sedimentary Rocks

  2. 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

  3. 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)

  4. 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

  5. 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)

  6. 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?

  7. 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.

  8. Doing the lab – Components • What makes up the rock • Grains? (Sand, silt, pebbles, …) • Of what? • A single mineral? • Multiple components? • …

  9. 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

  10. Other features • Include: • Fossils • Salty taste • Obvious bedding/laminations • Ooids (oolites) • …

  11. Name • Use • Handout chart • Book • Wall poster

  12. Questions?

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