Lecture 9A: Sedimentary Rocks and Processes

Sedimentary Rocks

A rock is classified as a sedimentary rock if:

• It is composed of sediments — either detrital (rock and mineral fragments) or biochemical / bioclastic (shells and plant material) — that undergo lithification by compaction and cementation; or

• It is composed of minerals that form in situ by precipitation from a solution (chemical) and accumulate and bind into a chemical sedimentary rock.

• Clastic - composed of detrital sediments

  • Silt and Clay

    • Mudstone / Siltstone- made of clay-sized particles if they don’t break into planes

    • Shale - if they break into planes

  • Sand

    • Sandstone - composed of sand (fine, medium, coarse, very coarse) and exhibits lamination or beddings

  • Granule, Pebble, Cobble, and Boulder

    • Conglomerate - has round clasts

    • Breccia - has highly angular clasts

• Biochemical - composed of biologically-derived sediments

  • Coquina - predominantly made of shells

  • Fossiliferous limestone - mostly made of limestone with a lot of fossils (shells and crinoid)

  • Chert - made of radiolarians and is found in deep marine environment, dominate below the CCD or the carbonate compensation depth

  • Diatomites - made of diatoms which can be pennate or centric

  • Chalk - made of coccolithophores

    

• Chemical - precipitated from solution from mineral-rich water

  • Speleothems - cave precipitates

    • The water that drips on caves is mineral-rich, so they precipitate stalactites in the ceiling and stalagmites on the floor and when they connect, it is called a column

  • Travertine - made of terrestrial limestone, precipitate not from the coast or not from the ocean but from terrestrial water

Sedimentary Processes

• Refers to all processes that shape a sedimentary rock — from sediment source to its formation and modification

• Sedimentary Cycle

  • Source area

    • Weathering

    • Erosion

    • Transport

    • Deposition

    • Diagenesis

      • Lithification (compaction and cementation)

      • Modification (rextallization and replacement, dissolution, bioturbation)

  • Sedimentary Rocks

Sedimentary Cycle

• Weathering

  • Involves the in-situ physical, chemical, or biogenic breakdown of rock material into smaller fragments

    • Fragments may be: mineral or rock fragment or dissolved ions

  • Types of weathering:

    • Physical Weathering

      • Disintegration

    • Chemical Weathering

      • Decomposition

    • Differential weathering

      • different rock materials weather at different rates

      • wherein less resistant rocks are weathered and eroded more easily

      • Example: Bouma sequence

Physical Weathering

• Dominant in dry and cold environments

• Arid environments (little to no moisture)

• Disintegration

  • the process of physical weathering: the breakdown of larger and more coherent rock bodies into smaller units. May be due to mechanical or biological processes:

    • Unloading

      • fracturing and jointing in response to decrease in the lithostatic pressure. Decrease in lithostatic pressure results to the erosion and/or faulting of overburden rocks

        • Exfoliation - Spalling off slabs as weathering continues, creating onion-like layers. Material is shed superficially as slabs or scales

    • Frost Action/Shattering

      • pre-existing fractures and weak surfaces are enlarged by the expansion of water

        • Frost Wedging - Occurs due to stresses induced by the expansion of ice when water trapped in existing joints and fractures freezes. Forms talus (debris in the foot of slopes) deposits downslope of bare rock surfaces

    • Crystal Growth

      • expansive force from the growth of crystals weaken the rocks and enlarge cracks. Common in evaporites

    • Abrasion

      • mechanical wearing down, scraping, or grinding away of a rock surface by friction. Results from the action of running water, wind, or ice. Entrained particles enhance the effect abrasion, especially in the case of wind (sandblasting)

        • Sandblasting - occurs when wind blow sand or silt against rocks

        • Glacial striations - very polished area with a lot of scratch marks

        • Pebbles in beaches

        • Ventifacts - natural rock formation from sandblasting when wind that blows on the rock sculps the surface

    • Slaking

      • wetting and drying of rocks that contain micas and clays (minute minerals that expands when wet) that expand when wet, generating stress to adjacent minerals. Common in clay-bearing rocks and soils

    • Insolation

      • volume changes from temperature change that induce stresses that spall (peel-off) small particles from the rock body

      • Repeated expansion and contraction due to rapid T changes

      • Common in clay-bearing rocks and soils

        

    • Evorsion

      • kinetic energy of flowing water alone, without suspended sediments, may be enough to erode rocks and sediments in a stream bed (evorsion), as in waterfall pools, although the effect is enhanced by abrasion.

    • Cavitation

      • breaking of rocks by ocean waves from bubble implosion

      • creates sea caves and sea arches

        

    • Biological Activities

      • interaction of organisms with rocks and sediments

        • Root Wedging - roots of plants tend to erode or weather rocks

        • Bioturbation (Animal Activities) - animals tend to burrow or disturb the sediments

          • Pholas dactylus (clams) - tend to burrow in rocks

Chemical Weathering

• Dominant in wet and hot environments (humid and tropical environments)

• Decomposition - breakdown of rock materials that involves changes in chemical composition

• Chemical weathering attacks less stable minerals by leeching or alteration to more stable minerals

• Incomplete in steep slopes; complete in gentler slopes

  • Dissolution

    • occurs when minerals are wholly or partly dissolved

    • breakdown of minerals into ions

    • dominant process in limestones and evaporites

    • Example: Halite reacting with water and it dissolves to sodium ions and chlorine ions

  • Carbonation

    • carbonic acid reacts with calcite to produce Ca²+ and HCO³-

    • Example: Calcite (calcium carbonate) reacts with aqueous solution (carbonic acid) —> calcium ion and bicarbonate ion

    • They are responsible for karst environment

    • Pinnacle karst - towering structures made of limestone that has been dissolved

    • Sinkholes - when limestones that are dissolved and the overlying layer is an unconsolidated layer

  • Ion Exchange

    • occurs when ions are directly exchanged between a mineral and a solution

    • Example: Potassium Feldspar (s) + Water with lots of dissolved hydrogen ions —> clay minerals and ions

  • Hydrolysis

    • chemical reaction between mineral and water in which dissolved [H+] ions and/or [OH-] are added to form one or more new minerals, typically silicates to hydroxides/clay minerals

    • Commonly results to clay minerals

    • Example: Potassium Feldspar + ions + water —> Kaolinite (clay mineral) + Orthosilicic acid + ions

  • Hydration / Dehydration

    • involves addition of water to a crystal structure during reaction between a mineral and an aqueous solution, converting into another mineral (reverse of this is Dehydration)

    • Addition of water molecule(s) in mineral structures. See wetting and drying

    • Examples:

      • Anhydrite + H2O = Gypsum

      • Hematite + H2O = Goethite and Limonite

  • Oxidation

    • chemical reaction in which one or more electrons are transferred from a cation to an anion, increasing the valence of the cation

    • Alteration of minerals by the exchange of electrons with other elements or compounds

    • Oxidation acts with hydrolysis to decompose ferromagnesian minerals (i.e. olivine, pyroxene, amphibole, biotite) to form hematite and limonite

      • Reduction - involves the gain of electron

    • Example:

      • Olivine —> Hematite

      • Rhodonite (pink, manganese-bearing silicate Mn²+) —> Manganite (Mn³+) + Orthosilicic acid

    • Laterite soil - reddish soil from the weathering of ultramafic rock

      • Bedrock (ultramafic)

      • Saprolite (reddish, nickel, Zone of accumulation)

      • Limonite (yellowish, fe and al, Zone of Eluviation)

      • Ferricrete

  • Organic-Decomposition

    • biological activity causing decomposition

      • Chelation - highly soluble organic molecules bind with metallic elements that remove them from the mineral

    • Example: Lichens also have significant impact in the chemical weathering of rocks by the excretion of various organic acids, particularly oxalic acid, which can effectively dissolve minerals and chelate metallic cations.

• Goldich Stability Series - method of predicting relative stability or weathering potential of various minerals on the surface

  

  • Dunites - rocks made of >90% olivine

Soil

• a product of weathering

• unconsolidated surficial deposits produced by weathering processes and capable of supporting rooted plants

• composed of regolith, humus, water, and air:

  • Regolith - layer of sediments derived from the weathering of bedrock

  • Humus - organic matter

• the formation of soil, given time, produces a soil profile (distinct horizons)

• Factors controlling soil formation:

  • Sediment source - source rock composition determines how effective physical and chemical weathering processes

  • Climate

    • Precipitation - governs the extent of hydrolysis, hydration, and dissolution

    • Temperature Fluctuations - fluctuations determine importance of ice-wedging and insolation

  • Topographic Relief - controls the exposure of minerals to weathering agents

    • No soil development because of very steep slope

    • Transported soil developed on unconsolidated stream deposits

    • Thinner soil on steep slope because of erosion

    • Residual soil is developed on bedrock

    • Thicker soil develops on flat terrain

Erosion

• Action of surface processes that wear away and transport rock particles and dissolved ions from the source

• Agents:

  • Gravity

  • Wind

  • Water

  • Ice

  • Biological Activity

Transportation

• Involves the mobilization and moving along of sediments

• The relocation of weathering products from its source to its depositional site, via agents of transport

Erosion and Transportation

• Agents:

  • Gravity

    • Mass Wasting - downslope movement of rock particles driven by gravity

  • Water

    • Traction - large sediments that roll and slide along the streambed (gravel)

    • Saltation - small sediments that bounce along the streambed (sand)

    • Suspension - fine sediments suspended by turbulence (silt)

    • Solution - dissolved minerals and ions in the water (clay)

    • Hjulström Curve

      • dictates how a particle size will behave at a certain river velocity

  • Water

    • Stream Capacity - measure of the total sediment a stream can carry

    • Stream Competence - maximum size of particles a stream can transport

  • Wind (Aeolian)

    • Traction - fine sediments suspended by turbulence

    • Saltation - small sediments that bounce along the bottom

    • Suspension - fine sediments suspended by turbulence

  • Ice

    • Glacial Sediments (Till) - derived by plucking and abrasion along the rock-glacier interface or by deposition of mass wasting and superglacial streams

    • Glaciers may carry sediments as an internal load or as surface load

• Sediments

  • unconsolidated earth materials that are produced by weathering

  • can be small grains of rocks, minerals, plant-, and animal-remain

  • Texture

    • Refers to the size, shape, and arrangement of constituent particles and the void spaces in between

    • used in classification and interpretation

    • Includes:

      • Grain size

      • Sorting

        • arrangement or packing of grains with respect to each other

      • Roundness

      • Angularity

  • Transport Distance is inversely proportional with grain size, energy, angularity

  • Transport Distance is directly proportional to roundness and sorting

Deposition

• When transportation agents can no longer carry the sediments, the sediments get deposited

• The conditions of the depositional site environments contributes to the overall properties of sed rocks

Diagenesis

• Refers to physical, chemical, and biological processes which collectively result in:

  • transformation of sediments into sedimentary rock (lithification); and

  • modification of the texture and mineralogy of the rock

  

• Compaction

  • As sediment accumulates, the weight of overlying material compresses the deeper sediments

• Cementation

  • Dissolved ions in groundwater may precipitate within pore spaces between sediment grains, forming cement

  • New minerals form among the spaces between the sediment grains which glue (cement) them together

• Other diagenetic processes:

  • Recrystallization - unstable minerals undergo changes in their crystal structures to attain equilibrium (e.g. aragonite to calcite)

  • Replacement - new minerals are formed by altering the composition of existing minerals, or completely fill voids left by dissolution

  • Dissolution - unstable minerals are broken down into ions, which are leached away by pore fluids

  • Bioturbation - the disturbance of sediments or rocks by burrowing organisms

Sedimentary Rocks

• Composition

  • may be made up of:

    • Detrital (siliciclastic rock)

      • Rock fragments

      • Quartz grains

      • Feldspar grains

      • Clay

      • Quartz Sandstone - quartz creating sandstone

      • Arkosic Sandstone - feldspar creating sandstone

    • Biochemical grains, and other common minerals

      • Gypsum

      • Calcite spar (crystals)

      • Dolomite

      • Halite

      • Ooids

      • Limonite

      • Hematite

      • Chert

• Grain Size

  • For sedimentary rocks that are composed of clasts (detrital or biochemical rock and mineral fragments), the Udden-Wentworth scale is used to describe grain size.

    • Gravel (> 2mm)

    • Sand (1/16-2 mm)

    • Silt (1/256 - 1/16 mm)

    • Clay (<1/256 mm)

  • For sedimentary rocks made up of interlocking crystals instead of grains, the size maybe described as such:

    • Microcrystalline - crystals NOT visible under 10x hand lens

    • Macrocrystalline - crystals are visible under 10x hand lens

• Roundness and Sphericity

  • The shape of grains in sedimentary rocks may be described based:

    • Sphericity - how close a grain’s shape is to a sphere

    • Roundness - the absence of angular corners in a grain; opposite of angularity

• Sorting

  • The sorting of grains refer to how uniform the grain sizes are in a sediment sample

Sedimentary Structures

• Bedding - one unit

  • Very thickly bedded if >100 cm

  • Thinly bedded if 3-10 cm

  • Laminae <1 cm

    

• Cross-bedding

• Mud cracks

• Flute marks/casts

Importance of Sedimentary Rocks

• Sedimentary rocks serve as hosts biologic record

  • Solnhofen - limestone where archaeopteryx are found

    • Birds = Avian dinosaurs

  • Gshelia species - oldest fossil in the Philippines and is composed of rugose coral (extinct)

    • Scleractinid corals - living corals

• Sedimentary rocks serve as hosts for many mineral and energy resources

  • If permeable (like sandstone) , they can host oil and gas deposits

• Sedimentary rocks serve as record of the Earth’s geological history