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PETROLOGY OF SEDIMENTARY ROCKS

Dr. Sugeng S Surjono

Lab. Sedimentografi

Jurusan Teknik Geologi, Fakultas Teknik

Universitas Gadjah Mada

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INTRODUCTIONThree Rock Types: Igneous, Sedimentary, Metamorphic rocks

Origin of sedimentary rocks:- Formation of source rocks/sediment source : intrusion, metamorphism, volcanism, tectonic uplift- Weathering : physical and chemical breakdown of source rocks- Erosion and Transportation agent of transportation : water, wind, ice- Deposition material is deposited within depositional basins- Diagenesis sediment is covered by successive layer of younger sediment; increased temperature and pressure leading to consolidation and lithification of the sediment into sedimentary rocks

Sedimentary rocks : rocks formed at the surface of the earth under low-temperature and low-pressure, result from the accumulation and solidification of sediments, material transported in water, air or ice (Raymond, 1995).

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INTRODUCTION

Types of Sedimentary Rocks: (Tucker, 1991)Siliciclastic (fragmental) : - Conglomerates & breccias - Sandstones - Mudrocks Biogenic, biochemical and organic : - limestones & dolomites - cherts - phosphates - coal - oil shaleChemical : - evaporites - ironstonesVolcaniclastic : (e.g.) ignimbrites, tuffs, hyaloclastites

Sedimentary rocks are characterized by :-Presence of layers-Presence of transported grains-Sedimentary structures-Fossils

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Sedimentary rocksSedimentary rocks

• Clastic (siliciclastic) rocks (80-85% of the Clastic (siliciclastic) rocks (80-85% of the stratigraphic record)stratigraphic record)

• Carbonate sediments and rocks (10-15% of the Carbonate sediments and rocks (10-15% of the stratigraphic record) stratigraphic record)

• Volcaniclastic sediments and rocksVolcaniclastic sediments and rocks• Others (< 5% of the stratigraphic record) :Others (< 5% of the stratigraphic record) :

- Organic (carbonaceous) sediments and rocksOrganic (carbonaceous) sediments and rocks- EvaporitesEvaporites

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SILICICLASTIC SEDIMENTARY ROCKS

Clast (from the Greek klastos, meaning ‘broken’) is the technical term for broken fragment within sedimentary rocks. It is also called as terrigenous grains

Siliciclastic sedimentary rocks are composed by clasts that originated from transportation and deposition of pre-existing rocks within depositional environments. Mechanism involved in the transportation include the wind, glaciers, river currents, waves, tidal currents, debris flow and turbidity currents (Tucker, 1991).

DESCRIPTION OF SEDIMENTARY ROCKS (Prothero & Schwab, 2005):

-Color-Sedimentary textures-Sedimentary structures-Composition -Fossil contents-Geometry of sedimentary rocks

Two important features of siliciclastic sediments related to depositional processes and diagenesis are sedimentary textures and structures.

Because most terrigenous grains are composed in part of silica, they are often referred to as siliciclastic grains.

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Color-Color usually reflects some aspect of the rock’s composition (bulk color can reflect the color of major mineralogical components)-Color of rock controlled by color of clast, matrix and cement-Color is not treated as an independent property, however, but as an aspect of sedimentary rock composition

Basic components of siliciclastic sedimentary rock are :-clasts or fragments-matrix-cements

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Sedimentary texturesTextures refers to the size, morphology, and arrangement (fabric) of siliciclastic grains that make up a sedimentary rock.

Grain size-Grain or siliciclastic particles range in size from clay to boulder

-The grade scale most widely used by sedimentologist is the Udden-Wenthworth scale

-The Udden-Wentworth grain-size scale is based on factors of two: = -log2 d ; where d is grain size in mm

- It extends from <1/256 mm (0.0039) to >256 mm and is divided into four major size categories (clay, silt, sand, and gravel) that can be further subdivided

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Udden-Wenthworth grain-size scale for sediments and the equivalent phi scale

Mu

d

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• Grain-size (particle-size, granulometric) analysisGrain-size (particle-size, granulometric) analysis• The old-fashioned way: direct measurement (gravel) and The old-fashioned way: direct measurement (gravel) and

sieve/pipette analysis (sand and mud) sieve/pipette analysis (sand and mud) • The modern technology: laser particle sizing (sand and mud)The modern technology: laser particle sizing (sand and mud)

Grain size parameters : mean, sorting, skewness, kurtosis

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Graphical method

Graphic Mean

Standard deviation

Skewness

Kurtosis Kurtosis

Grain size parameters

3

845016

(σ1) = 4

1684 + 6,6

595

)1684(2

5021684

+ )595(2

502595

(SK1) =

(Mz) =

)2575(44,2

595

(KG) =

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Moment measures (mathematic method)Moment measures (mathematic method)• First moment: mean (cf. median, mode)

• Premier measure of the grain sizePremier measure of the grain size

x ø =x ø =

Grain size parameters

N

fm

100

)( 2 Xmf

3

3

100

)(

XmfSkø =

• Second moment: variance (cf. standard deviation)• Measure of the degree of sortingMeasure of the degree of sorting

σσø =ø = ( ( = standard deviation) = standard deviation)

• Third moment : Skewness

- Measure of the symmetry of the grain-size Measure of the symmetry of the grain-size distributiondistribution

4

4

100

)(

Xmf

• Fourth moment : Kurtosis

- Measure of the sharpness or peakedness of a grain-Measure of the sharpness or peakedness of a grain-size frequency curvesize frequency curve

Kø =

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SK1 class :+1,0 - +0,3 very fine-skewed+0,3 - +0,1 fine-skewed+0,1 - -0,1 near-symmetrical-0,1 - -0,3 coarse-skewed-0,3 - -1,0 very coarse-skewed

KG class:<0,67 very platykutic0,67 – 0,90 platykurtic0,90 – 1,11 mesokurtic1,11 – 1,50 leptokurtic1,50 – 3,00 very leptokurtic>3,00 extremely leptokurtic

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Grain Morphology

Three aspects of grain morphology are the shape, sphericity and roundness.

• The shape or form of grain is measured by various ratios of the long, intermediate and short axes.

• Sphericity is a measure of how closely the grain shape approaches that of a sphere.

• Roundness is concerned with the curvature of the corners of a grain and six classes from very angular to well rounded.

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Grain shape classification

Roundness and Sphericity

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Grain Fabric

Fabric for grain in sedimentary rock refers to their orientation and packing and to the nature of contacts between them.

• Grain Packing is a function of the size and shape of grains and postdepositional physical and chemical processes that bring about compaction of sediment.

• Grain orientation is mainly a function of the physical processes and condition operating at the time of deposition

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• Sedimentary structures occur at very different scales, Sedimentary structures occur at very different scales, from less than a mm (thin section) to 100s–1000s of from less than a mm (thin section) to 100s–1000s of meters (large outcrops); most attention is traditionally meters (large outcrops); most attention is traditionally focused on the bedform-scale : Microforms (e.g., ripples) focused on the bedform-scale : Microforms (e.g., ripples) ;Mesoforms (e.g., dunes); Macroforms (e.g., bars);Mesoforms (e.g., dunes); Macroforms (e.g., bars)

Sedimentary structures

The majority of structures form by physical processes, before, during and after sedimentation. Other result from organic and chemical processes

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• LaminaeLaminae and and bedsbeds are are the basic sedimentary the basic sedimentary units that produce units that produce stratification; the stratification; the transition between the transition between the two is arbitrarily set at 10 two is arbitrarily set at 10 mmmm

• Normal gradingNormal grading is an is an upward decreasing grain upward decreasing grain size within a single lamina size within a single lamina or bed (associated with a or bed (associated with a decrease in flow velocity), decrease in flow velocity), as opposed to as opposed to reverse reverse gradinggrading

• Fining-upward Fining-upward successionssuccessions and and coarsening-upward coarsening-upward successionssuccessions are the are the products of vertically products of vertically stacked individual bedsstacked individual beds

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Cross stratificationCross stratification

• Cross lamination (Cross lamination (small-scale cross stratificationsmall-scale cross stratification) is ) is produced by ripplesproduced by ripples

• Cross bedding (Cross bedding (large-scale cross stratificationlarge-scale cross stratification) is ) is produced by dunesproduced by dunes

• Cross-stratified deposits can only be preserved when a Cross-stratified deposits can only be preserved when a bedform is not entirely eroded by the subsequent bedform is not entirely eroded by the subsequent bedform (i.e., sediment input > sediment output)bedform (i.e., sediment input > sediment output)

• Straight-crested bedforms lead to Straight-crested bedforms lead to planar cross planar cross stratificationstratification; sinuous or linguoid bedforms produce ; sinuous or linguoid bedforms produce trough cross stratificationtrough cross stratification

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GED2008GED2008Low angle planar cross-bedding, Kali Ngalang-Gunung Kidul, YK

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Cross stratificationCross stratification

• The angle of climb of cross-stratified deposits increases with The angle of climb of cross-stratified deposits increases with deposition rate, resulting in ‘deposition rate, resulting in ‘climbing ripple cross climbing ripple cross laminationlamination’’

• Antidunes form cross strata that dip upstream, but these are Antidunes form cross strata that dip upstream, but these are not commonly preservednot commonly preserved

• A single unit of cross-stratified material is known as a A single unit of cross-stratified material is known as a setset; a ; a succession of sets forms a succession of sets forms a co-setco-set

Planar stratificationPlanar stratification

• Planar lamination (or planar bedding) is formed under both Planar lamination (or planar bedding) is formed under both lower-stage and upper-stage flow conditionslower-stage and upper-stage flow conditions

• Planar stratification can easily be confused with planar cross Planar stratification can easily be confused with planar cross stratification, depending on the orientation of a section (strike stratification, depending on the orientation of a section (strike sections!)sections!)

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• Cross stratification produced by wave ripples can be Cross stratification produced by wave ripples can be distinguished from current ripples by their symmetry distinguished from current ripples by their symmetry and by laminae dipping in two directionsand by laminae dipping in two directions

• Hummocky cross stratificationHummocky cross stratification (HCS) forms during (HCS) forms during storm events with combined wave and current activity storm events with combined wave and current activity in shallow seas (below the fair-weather wave base), and in shallow seas (below the fair-weather wave base), and is the result of aggradation of mounds and swalesis the result of aggradation of mounds and swales

• Heterolithic stratificationHeterolithic stratification is characterized by is characterized by alternating sand and mud laminae or bedsalternating sand and mud laminae or beds

• Flaser bedding is dominated by sand with isolated, thin Flaser bedding is dominated by sand with isolated, thin mud drapesmud drapes

• Lenticular bedding is mud-dominated with isolated ripplesLenticular bedding is mud-dominated with isolated ripples

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Gravity-flow depositsGravity-flow deposits

• Debris-flow depositsDebris-flow deposits are are typically poorly sorted, matrix-typically poorly sorted, matrix-supported sediments with random supported sediments with random clast orientation and no clast orientation and no sedimentary structures; thickness sedimentary structures; thickness and grain size commonly remain and grain size commonly remain unchanged in a proximal to distal unchanged in a proximal to distal directiondirection

• TurbiditesTurbidites, the deposits formed , the deposits formed by turbidity currents, are typically by turbidity currents, are typically normally graded, ideally composed normally graded, ideally composed of five units (Bouma-sequence with of five units (Bouma-sequence with divisions ‘a’-‘e’), reflecting divisions ‘a’-‘e’), reflecting decreasing flow velocities and decreasing flow velocities and associated bedformsassociated bedforms

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• ImbricationImbrication commonly occurs in water-lain gravels and commonly occurs in water-lain gravels and conglomerates, and is characterized by discoid (flat) clasts conglomerates, and is characterized by discoid (flat) clasts consistently dipping upstreamconsistently dipping upstream

• Sole marksSole marks are erosional sedimentary structures on a bed are erosional sedimentary structures on a bed surface that have been preserved by subsequent burialsurface that have been preserved by subsequent burial

• Scour marks (caused by erosive turbulence)Scour marks (caused by erosive turbulence)• Tool marks (caused by imprints of objects)Tool marks (caused by imprints of objects)

• Paleocurrent measurements can be based on any Paleocurrent measurements can be based on any sedimentary structure indicating a current direction (e.g., sedimentary structure indicating a current direction (e.g., cross stratification, imbrication, flute casts)cross stratification, imbrication, flute casts)

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• Trace fossilsTrace fossils (ichnofossils) are the tracks, trails or burrows (ichnofossils) are the tracks, trails or burrows left behind in sediments by organisms (e.g., feeding traces, left behind in sediments by organisms (e.g., feeding traces, locomotion traces, escape burrows)locomotion traces, escape burrows)

• Disturbance of sediments by organisms is known as Disturbance of sediments by organisms is known as bioturbationbioturbation, which can lead to the total destruction of , which can lead to the total destruction of primary sedimentary structuresprimary sedimentary structures

• Since numerous trace fossils are connected to specific Since numerous trace fossils are connected to specific depositional environments, they can be very useful in depositional environments, they can be very useful in sedimentologic interpretationssedimentologic interpretations

• Soft-sediment deformation structures are sometimes considered to Soft-sediment deformation structures are sometimes considered to be part of the initial diagenetic changes of a sediment, and include:be part of the initial diagenetic changes of a sediment, and include:

• Slump structures (on slopes)Slump structures (on slopes)• Dewatering structures (upward escape of water, commonly due to Dewatering structures (upward escape of water, commonly due to

loading)loading)• Load structures (density contrasts between sand and underlying wet Load structures (density contrasts between sand and underlying wet

mud; can in extreme cases cause mud diapirs)mud; can in extreme cases cause mud diapirs)

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Clastic (siliciclastic) rocksClastic (siliciclastic) rocks

• SandstonesSandstones (20-25% of the stratigraphic record) can be (20-25% of the stratigraphic record) can be subdivided according to the Pettijohn classification, based subdivided according to the Pettijohn classification, based on texture and composition (relative proportions of quartz, on texture and composition (relative proportions of quartz, feldspar, and lithic fragments)feldspar, and lithic fragments)

• Quartz arenite: quartz-dominatedQuartz arenite: quartz-dominated• Arkosic arenite: feldspar-dominatedArkosic arenite: feldspar-dominated• Lithic arenite: dominance of lithic fragmentsLithic arenite: dominance of lithic fragments• Wacke: significantly matrix-supported (>15% mud)Wacke: significantly matrix-supported (>15% mud)

• Quartz wackeQuartz wacke• Greywacke (feldspathic or lithic wacke)Greywacke (feldspathic or lithic wacke)

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Classification of Sandstone (Pettijohn, 1975)

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Clastic (siliciclastic) rocksClastic (siliciclastic) rocks

• MudstonesMudstones (60% of the stratigraphic record) are also (60% of the stratigraphic record) are also known as mudrocks or shales and commonly exhibit a known as mudrocks or shales and commonly exhibit a distinct fissilitydistinct fissility

• ClaystoneClaystone• SiltstoneSiltstone

• ConglomeratesConglomerates are consolidated gravels; are consolidated gravels; brecciasbreccias are are conglomerates with dominantly angular clastsconglomerates with dominantly angular clasts

• Clast-supported conglomeratesClast-supported conglomerates• Matrix-supported conglomeratesMatrix-supported conglomerates

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Halang Formation, Panujah, Slawi, Central Jawa

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Alternating of siltstone and claystone, Sambipitu Formation-Wonogiri

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Black shale (upper part) overlying above thickly bedded fine sandstone

Sambipitu Formation-Wonogiri

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Normal grading sandstone, from coarse sst to fine sst, ambipitu Formation-Wonogiri

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Conglomerate in Kali Ngalang, Gunung Kidul. Nglanggran Formation

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Siliciclastic rocks classification (Pettijohn, 1975)

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• Principal minerals: calcite, aragonite (unstable), and Principal minerals: calcite, aragonite (unstable), and dolomite (diagenetic)dolomite (diagenetic)

• Principal rocks: Principal rocks: limestonelimestone (>50% CaCO (>50% CaCO33) and ) and dolomitedolomite (dolostone) (CaMg(CO(dolostone) (CaMg(CO33))22))

• Formation of carbonate sediments and rocks occurs by Formation of carbonate sediments and rocks occurs by means of two main processes:means of two main processes:

• Biomineralization of CaCOBiomineralization of CaCO33 by organisms by organisms• Direct chemical precipitation Direct chemical precipitation

32332 COHCaCO2HCOCa

CARBONATE SEDIMENT AND ROCKS

Biogenic carbonate formation occurs by a wide range of organisms Biogenic carbonate formation occurs by a wide range of organisms (e.g., molluscs, corals, forams, algae, bacteria, and many others)(e.g., molluscs, corals, forams, algae, bacteria, and many others)

• Most organisms initially form unconsolidated carbonate sedimentsMost organisms initially form unconsolidated carbonate sediments• Coral reefs and microbial mats (e.g., stromatolites) are examples of Coral reefs and microbial mats (e.g., stromatolites) are examples of

more solid carbonate structuresmore solid carbonate structures• Chemical precipitation produces non-skeletal carbonate grains Chemical precipitation produces non-skeletal carbonate grains

of various sizes (e.g., ooids, pisoids, micrite)of various sizes (e.g., ooids, pisoids, micrite)

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• Biogenic carbonate formation occurs by a wide range of Biogenic carbonate formation occurs by a wide range of organisms (e.g., molluscs, corals, forams, algae, bacteria, and organisms (e.g., molluscs, corals, forams, algae, bacteria, and many others)many others)

• Most organisms initially form unconsolidated carbonate sedimentsMost organisms initially form unconsolidated carbonate sediments• Coral reefs and microbial mats (e.g., stromatolites) are examples of Coral reefs and microbial mats (e.g., stromatolites) are examples of

more solid carbonate structuresmore solid carbonate structures• Chemical precipitation produces non-skeletal carbonate grains Chemical precipitation produces non-skeletal carbonate grains

of various sizes (e.g., ooids, pisoids, micrite)of various sizes (e.g., ooids, pisoids, micrite)

CARBONATE SEDIMENT AND ROCKS

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After Scholle, 2003

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Carbonate sand usually consists either of Carbonate sand usually consists either of (fragmented) skeletal remains or non-(fragmented) skeletal remains or non-skeletal grainsskeletal grains

Carbonate mud (micrite) is commonly the Carbonate mud (micrite) is commonly the product either of chemical precipitation or product either of chemical precipitation or algal/bacterial activityalgal/bacterial activity

Dunham classification of carbonate rocks:Dunham classification of carbonate rocks: Texturally-based subdivision (cf. clastics): Texturally-based subdivision (cf. clastics):

mudstone, wackestone, packstone, mudstone, wackestone, packstone, grainstone, rudstonegrainstone, rudstone

Organically bound framework during Organically bound framework during formation: boundstoneformation: boundstone

CARBONATE SEDIMENT AND ROCKS

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Modified of Dunham Classification

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ORGANIC (CARBONACEOUS) SEDIMENTS AND ROCKSORGANIC (CARBONACEOUS) SEDIMENTS AND ROCKS

• Peat and organic-rich clastic sediments form in relatively Peat and organic-rich clastic sediments form in relatively anaerobic (reducing) environments (e.g., mires, lakes, anaerobic (reducing) environments (e.g., mires, lakes, oceans)oceans)

• Minerotrophic peatMinerotrophic peat: mostly nutrient-rich, groundwater-fed : mostly nutrient-rich, groundwater-fed mires (e.g., floodplains, delta plains, coastal plains)mires (e.g., floodplains, delta plains, coastal plains)

• Ombrotrophic peatOmbrotrophic peat: mostly nutrient-poor, rainwater-fed mires : mostly nutrient-poor, rainwater-fed mires (e.g., relatively high, flat terrains)(e.g., relatively high, flat terrains)

• GyttjaGyttja: organic-rich lake sediment: organic-rich lake sediment• SapropelSapropel: organic-rich marine sediment: organic-rich marine sediment• CoalCoal consists primarily of solid organic matter; the consists primarily of solid organic matter; the

remainder is known as ‘ash’remainder is known as ‘ash’• Carbonaceous shalesCarbonaceous shales have a lower proportion of solid have a lower proportion of solid

organic matterorganic matter• Oil shalesOil shales (may be formed in anaerobic lake and (may be formed in anaerobic lake and

marine environments) contain organic matter that can marine environments) contain organic matter that can be driven off as liquid or gas by heatingbe driven off as liquid or gas by heating

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EVAPORITESEVAPORITES

• Dissolved salts precipitate out of sea water due to Dissolved salts precipitate out of sea water due to concentration (brine formation) during evaporation (1 km of concentration (brine formation) during evaporation (1 km of sea water --> 12 m of evaporites)sea water --> 12 m of evaporites)

• Evaporites commonly lithify into consolidated rocks upon Evaporites commonly lithify into consolidated rocks upon formationformation

• Least soluble compounds precipitate first:Least soluble compounds precipitate first:• CaCOCaCO33 (calcium carbonate) (calcium carbonate)• CaSOCaSO44 (calcium sulphate: gypsum or anhydrite) (calcium sulphate: gypsum or anhydrite)• NaCl (halite: rock salt)NaCl (halite: rock salt)• Other, less stable (highly soluble) chloridesOther, less stable (highly soluble) chlorides

• Dissolved salts precipitate out of sea water due to Dissolved salts precipitate out of sea water due to concentration (brine formation) during evaporation (1 concentration (brine formation) during evaporation (1 km of sea water --> 12 m of evaporites)km of sea water --> 12 m of evaporites)

• Evaporites commonly lithify into consolidated rocks Evaporites commonly lithify into consolidated rocks upon formationupon formation

• Least soluble compounds precipitate first:Least soluble compounds precipitate first:• CaCOCaCO33 (calcium carbonate) (calcium carbonate)• CaSOCaSO44 (calcium sulphate: gypsum or anhydrite) (calcium sulphate: gypsum or anhydrite)• NaCl (halite: rock salt)NaCl (halite: rock salt)• Other, less stable (highly soluble) chloridesOther, less stable (highly soluble) chlorides

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VOLCANICLASTIC SEDIMENTS AND ROCKSVOLCANICLASTIC SEDIMENTS AND ROCKS

• Lava (cooled magma flows) produces volcaniclastic sediment Lava (cooled magma flows) produces volcaniclastic sediment upon weatheringupon weathering

• Pyroclastic material or tephra (ejected particulate material) can Pyroclastic material or tephra (ejected particulate material) can be subdivided into different compositional categories:be subdivided into different compositional categories:

• Mineral grainsMineral grains• Lithic fragmentsLithic fragments• Vitric material (volcanic glass or pumice)Vitric material (volcanic glass or pumice)

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