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