Geology 101: Chapter 6, Sedimentary Rock

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Sedimentary rock

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rock that forms at or near the surface of the Earth

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Three ways sedimentary rock forms

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cementing of pre-existing sediment (clastic), growth of skeletal material (biochemical), or precipitation of minerals (chemicals)

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Explain sedimentary rocks in the crust

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By volume, there are substantially more igneous and metamorphic rock, but sedimentary drapes over all of it.

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What is sediment?

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Loose fragments of rocks, minerals, and shells

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How do we get sediment?

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Weathering, either physical or chemical (reactions that destroy minerals if rock contacts air or water)

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Physical weathering: Jointing

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cracking due to removal of overburden or cooling

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Physical weathering: Exfoliation

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jointing in onion-like sheets (Characteristic of granitic plutons)

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Physical weathering: frost wedging

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Ice expansion during freeze / thaw cycles

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Physical weathering: root wedging

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Plant roots push joints apart

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Physical weathering: salt wedging

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salt from ground water or sea spray cause crystals to grow in porous rock and splits the rock.

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How does physical weathering help chemical weathering?

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It breaks it down into smaller pieces, increasing surface area substantially.

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Chemical weathering: dissolution

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dissolving of minerals in water (CaCO3 + H2CO3 Ca2+ + 2HCO3-)

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Chemical weathering: hydrolysis

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Reaction of minerals with water to produce secondary minerals. (K Feldspar makes Kaolinite / Clay)

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Chemical weathering: oxidation

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"Elemental loss of electrons, commonly due to oxygen. For example: 4Fe2+ + 7O2 + 4H+ => 4(Fe3+)2O3 + 2H2O

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Climate controls of chemical weathering

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Water (volume and movement), temperature (reaction rates), and latitute (weathering is faster in the tropics)

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Rock controls of chemical weathering

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Mineral resistance (highly variable), and texture (available surface area)

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Mineral resistance to chemical weathering with silicates

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Olivine and feldspars are the least stable, while quartz is the most. Amphiboles and pyroxene's are in the middle.

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Secondary minerals (most resistance to chemical weathering)

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Iron oxides, aluminum oxides, and clay

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Evaporites, carbonates, sulfates (least resistent to chemical weathering)

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Dolomite, calcite, pyrite, gypsum, halite

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Soil

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sediment produced by chemical and physical weathering that is subjected to additional processes.

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Three additional processes that occur in soil

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1. Addition of organic matter, ions from rainwater, dust; 2. movement of material downward by groundwater percolation; 3. bioturbation (mixing and disturbing by plants and animals)

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How does soil develop (four steps)

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1. Bedrock begins to disintegrate; 2. Organic materials facilitate disintegration; 3. Horizons form (layers) primitive plants like lichen and moss are degrading the surface; 4. Developed soil supports thick vegetation (so now you have both an A and B-horizon)

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Soil horizons- temperate climate

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O: organic layer, no minerals; A: topsoil, dark, organics; E: transition layer as iron leaches, light color; B: subsoil, clays and ions collect, orange; C: weathered rock; R: regolith, unweathered.

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Soil variation will be due to:

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climate, substrate composition, slope steepness, drainage, time, and vegetation type

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Clastic transport rocks

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transport via water, wind, and ice. The higher the velocity of the transport, the larger the sediment it moves. Also, the further, the finer the material gets sorted.

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Clastic Deposition

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Sediment settles out of transport medium due to velocity change

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Clastic Lithification (2)

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Compaction: minerals are squeezed together and the water / air in pore spaces is expelled; Cementation: circulating waters precipitate cements into the void spaces (CaCO3, silica, oxides, etc.)

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Conglomerate

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pebble to boulder, clasts well-rounded, usually poorly sorted, deposted with high energy currents (flooring rivers, steep mountains, etc.)

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Breccia

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Poorl sorted coarse grains, but with angular braings. Has a short transport

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Sandstone

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deposted by moderately active transport processes (rivers, beaches, wind / sand dunes). Ends up being mostly quartz, feldspar, micas, and rock fragments

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Shale and siltstone

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silt and clay-sized particles, over 1/2 of all sedimentary rocks, particles not visible, deposited in quiet water (deep ocean, lakes, floorplains)

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biochemical sedimentary rocks

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limestone (CaCO3 derived mostly from shells of organisms)

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Formation of Sandstone

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CaCO3 precipitates in warm water and biochemical formation occurs

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Chert

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Silica-rich marine plankton oozes (diatoms, radiolarians) deposited on deep seafloor are recrystallized.

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Coal

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Plant remains that are deposited in layers, along ancient rivers, swamps, over 50% carbon

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Chemical rock: evaporites

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water evaporates and dissolved materials precipitate, usually in arid marine shorelines, examples are: halite and gypsum.

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Chemical rock: precipitates

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travertine (CaCO3) and cert (SiO2)