Presentation on theme: "Sedimentary Rocks. Clastic vs. Non-clastic Sedimentary rocks in two major groups Clastic (detrital) –Composed of fragments of silicate minerals (mostly."— Presentation transcript:
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 – CaCO 3 ? 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