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?