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EARTH MATERIALS VIEARTH MATERIALS VI
The Rock Cycle: The Rock Cycle: Clastic Sedimentary RocksClastic Sedimentary Rocks
Professor Peter [email protected]
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THE ROCK CYCLE
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Relative proportions of rock groupsRelative proportions of rock groups
Sedimentary rocks: larger proportion of land surface
The greater proportion created by weathering, erosion & deposition
These are known as clastic sediments
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SEDIMENTARY ROCKS• Clastic sedimentsClastic sediments from weathering & erosion
of pre- existing rocks• Weathering can be physical (freeze-thaw) or
chemical• Erosion & subsequent transportation by
water, wind and ice before deposition• Biogenic and chemical sedimentsBiogenic and chemical sediments form as
biochemical or inorganic chemical precipitates • E.g. shelly fragments in limestones; evaporites
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Sediments to sedimentary rocks
Weathering structures seen in ancient terranes
Transport shown by sedimentary structures
Lithification due to:1. Compaction
(squeezing out water)2. Diagenesis
(precipitation of cements)
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Mineralogy of sedimentary rocks
Clastic sedimentsClastic sediments • siliciclasticssiliciclastics – quartz, feldspars, clay
minerals, rock fragments • calciclasticscalciclastics – calcite, dolomiteCarbonate (biogenic) sedimentsCarbonate (biogenic) sediments• aragonite, calcite • chalcedony silica (flint) occurs as subsiduary Evaporite (chemical) sedimentsEvaporite (chemical) sediments• halite, gypsum and anhydrite • Dolomite, also as dolomitisation of calcite
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Classification of clastic sediments: The Classification of clastic sediments: The Wentworth ScaleWentworth Scale
Grain SizeGrain Size Sediment GradeSediment Grade Rock Rock
>256 mm Boulder Conglomerate 256 – 64mm Cobble (rounded clasts) 64 – 2mm Pebble Breccia (angular clasts)
2 – 0.063mm Sand Sandstone0.063 – 0.002mm Silt Siltstone< 0.063mm Mud Mudstone / Shale< 0.002mm Clay Claystone
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Conglomerates & BrecciasConglomerates & Breccias• ConglomeratesConglomerates - mainly rounded clasts
– Ortho conglomeratesOrtho conglomerates - clast supported– Para conglomeratesPara conglomerates - matrix supported
• BrecciasBreccias - mainly angular clasts • Clast typeClast type defines rock type:
– MonomictMonomict - clasts of mainly one composition– PolymictPolymict - clasts of various compositions – DiamictDiamict - clasts poorly sorted of various types
• Matrix (sediment between major clasts)– sand and mud grade sediments
• Cement binds clasts/matrix (Crystalline, diagenetic, typically calcite, silica
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Grains floating in matrix Grains touching
Finer grains between major clasts
Crystalline cement
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Quartz clast
Calcite
cem
ent
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Orthoconglomerate
Monomict breccia
Diamict conglomerate
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SANDSTONES
• CLASTSCLASTS have modal grain size in the sand grade ( 2mm – 0.063mm)– Coarse sand 2- 0.5mm, – medium 0.5-0.25m – fine 0.25 - 0.063mm
• SiliciclasticSiliciclastic sandssands have clasts composed usually of quartz, feldspar, fine grained rock fragments or lithics
• Mud gradeMud grade matrixmatrix may be present • Crystalline diagenetic cements, Crystalline diagenetic cements, e.g. quartz,
calcite, clay minerals
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MaturityMaturity• Maturity Maturity is a function of sediment transport• Textural maturityTextural maturity refers to:
– The degree of roundness of the grains– The amount of sorting of the grain sizes
• Texturally matureTexturally mature sandstones have well-rounded and well-sorted grains, immatureimmature if not
• Mineralogical maturityMineralogical maturity refers to the percentage of refers to the percentage of quartz grainsquartz grains– Feldspars break down with transport– Quartz grains more resistant
• Mineralogically matureMineralogically mature sandstones have mostly quartz grains
• ArkoseArkose is mineralogically immaturemineralogically immature
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RoundnessRoundness
Increasing Roundness=increasing maturityIncreasing Roundness=increasing maturity
Dry aeolian (airborne) grainsWell-rounded & heavily abraded
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Well sorted Poorly sorted
SortingSorting
Increasing sorting=increasing maturityIncreasing sorting=increasing maturity
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Transport structuresTransport structures
Cross-bedding
Ripples
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MineralogyMineralogy
Increasing quartz=increasing maturityIncreasing quartz=increasing maturity
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Classifying sandstones: mineralogyClassifying sandstones: mineralogy
>95% Quartz =Quartz areniteQuartz arenite
QuartzQuartz
FeldsparFeldspar
Rock Rock fragments fragments (lithics)(lithics)
<95% Quartz - divided on amount of feldspar or lithics
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ArkoseArkose
QuartzQuartz
FeldsparFeldspar LithicsLithics
>25% Feldspar = arkosic composition
>25% Feldspar = mineralogically immature
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PETTIJOHN CLASSIFICATIONPETTIJOHN CLASSIFICATION
Based on percentage of Based on percentage of matrixmatrix
• Matrix poor (<15%)Matrix poor (<15%) - arenitesarenites• Matrix richMatrix rich (>15%)(>15%) – wackeswackes
• WackesWackes or (greywackes) or (greywackes)• Tend to be dark in colour• Are poorly sorted
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Light coloured Light colouredLight coloured
Dark coloured Dark coloured
Examples in hand specimenExamples in hand specimen
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Quartz sandstone (arenite)
Red sandstoneCross-bedded sandstone
Polished sandstone
Lithic sandstone
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Fine grained siliciclastic rocks• Modal grain size <0.063mm• Grouped as Mudrocks Mudrocks (>50% siliclastic (>50% siliclastic
grains, <0.063mm) grains, <0.063mm) • Typical mineralogy
– Clay minerals (sheet silicates),Clay minerals (sheet silicates),– quartz & feldsparquartz & feldspar
• MUDROCKS MUDROCKS may split may split (fissile) (fissile) or notor not (massive)(massive)
• Siltstones Siltstones non-fissile, coarser grade, mica non-fissile, coarser grade, mica flakes commonflakes common
• MudstonesMudstones - massive, mud-grade• ShalesShales - Fissile (splitting), mud-grade
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Siliciclastic rocksIncreasing mud-grade grains
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Shale