Lab 4 - Sedimentary and Metamorphic Rocks We will examine a few types of sedimentary rocks: -Clastic sedimentary rocks -Biochemical sedimentary rocks -Organic sedimentary rocks -Chemical sedimentary rocks Clastic sedimentary rocks Classify based on: Grain size Grain shape Sorting Mineralogy Sedimentary structures …can be used to describe sediment “maturity” Special Sandstones: Quartz Arenite >90% Quartz Typically well-sorted, rounded grains Indicates long-distance transport, moderate energy, mature sediment Special Sandstones: Arkose >25% Feldspar Typically moderately-to-poorly sorted, angular grains Indicates relatively short-distance transport (near source), moderate energy, immature sediment Special Sandstones: Breccia Mix of clasts (quartz, feldspar and lithic/rock fragments) in a muddy matrix Typically poorly sorted, angular grains Indicates relatively short-distance transport (near source), moderate energy, immature sediment Biochemical Sedimentary Rocks • Limestone – consists of the calcite (CaCO3) shells of organisms • Shells are compacted, cemented and recrystallized Biochemical Sedimentary Rocks Chert – consists of silica (SiO2) that is very fine grained (crypto- crystalline) Formed from skeletal fragments of micro-organisms that settle onto the ocean floor Organic Sedimentary Rocks Coal – Consists of 50-90% carbon It is a black, combustible rock • Why is this considered sedimentary rock? It is made of detritus of plants or other organic matter Chemical Sedimentary Rocks Travertine – Consists of limestone (CaCO3) that is precipitated from solution (out of groundwater) Mammoth hot springs, CA Sedimentary Structures – ripples or dunes Sedimentary Structures – cross beds Occur within ripple or dune structures Fig. 5.21abc W. W. Norton Example of cross beds - Navaho Sandstone, Utah Ancient dunes and cross beds Eolian (wind-blown) sands are very well sorted and well rounded Grains are mostly quartz Walther’s Law Vertical changes in sedimentary rocks register lateral changes in the depositional environment Metamorphic Rocks Classify based on: • Texture – Crystal Size – Foliation – Compositional Banding • Composition – Dominant mineralogy Metamorphic Rock Texture Foliated – Has a planar Non-foliated – No planar fabric (repetition of planar fabric evident surfaces of layers) Metamorphic index minerals – only form in metamorphic rocks and indicate particular metamorphic grades For every Metamorphic Rock, there is a Protolith (or Parent Rock) Granite Gneiss Limestone Marble Common foliated metamorphic rocks originate as a shale Shale Slate Phyllite Schist Gneiss Increasing Temperature and Pressure Mt. Monadnock, NH Slate Protolith: Shale Very fine grained Distinct foliation “slate-y cleavage” Breaks along this cleavage Low-grade metamorphism Phyllite Protolith: Shale, Slate Slightly coarser grained than slate Foliated Greenish sheen (“satin-y luster”) as clays alter to micas and chlorite Low-to-intermediate grade metamorphism Schist Protolith: Shale, Slate, Phyllite, Basalt, Greywacke Coarse Grained Foliated (referred to a “Schist-osity”) Contains micas Intermediate-high grade metamorphism Gneiss Protolith: Shale, Slate, Phyllite, Schist, Greywacke, Igneous Rocks (Felsic) Coarse grained Compositional banding and foliation High grade metamorphism Non-foliated Metamorphic Rocks - Quartzite Protolith: Quartz Arenite, other quartz-rich Sandstone or Siltstone Non foliated Metamorphic Rocks - Marble Protolith: Limestone, Dolomite.