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EARTH MATERIALS VI The Rock Cycle: Clastic Sedimentary Rocks Professor Peter Doyle

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Presentation on theme: "EARTH MATERIALS VI The Rock Cycle: Clastic Sedimentary Rocks Professor Peter Doyle"— Presentation transcript:

1 EARTH MATERIALS VI The Rock Cycle: Clastic Sedimentary Rocks Professor Peter Doyle


3 Relative 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

4 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

5 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)

6 Mineralogy of sedimentary rocks Clastic sediments siliciclasticssiliciclastics – quartz, feldspars, clay minerals, rock fragments calciclasticscalciclastics – calcite, dolomite Carbonate (biogenic) sediments aragonite, calcite chalcedony silica (flint) occurs as subsiduary Evaporite (chemical) sediments halite, gypsum and anhydrite Dolomite, also as dolomitisation of calcite

7 Classification of clastic sediments: The Wentworth Scale Grain Size Sediment Grade Rock >256 mm Boulder Conglomerate 256 – 64mm Cobble  (rounded clasts) 64 – 2mm Pebble  Breccia (angular clasts) 2 – 0.063mm Sand Sandstone 0.063 – 0.002mm Silt Siltstone < 0.063mm Mud Mudstone / Shale < 0.002mm Clay Claystone

8 Conglomerates & Breccias ConglomeratesConglomerates - mainly rounded clasts –Ortho conglomerates –Ortho conglomerates - clast supported –Para conglomerates –Para conglomerates - matrix supported BrecciasBreccias - mainly angular clasts Clast typeClast type defines rock type: –Monomict –Monomict - clasts of mainly one composition –Polymict –Polymict - clasts of various compositions –Diamict –Diamict - 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

9 Grains floating in matrixGrains touching Finer grains between major clasts Crystalline cement

10 Quartz clast Calcite cement

11 Orthoconglomerate Monomict breccia Diamict conglomerate

12 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 SiliciclasticsandsSiliciclastic sands have clasts composed usually of quartz, feldspar, fine grained rock fragments or lithics Mud gradematrixMud grade matrix may be present Crystalline diagenetic cements,Crystalline diagenetic cements, e.g. quartz, calcite, clay minerals

13 Maturity MaturityMaturity 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 mature immatureTexturally mature sandstones have well-rounded and well-sorted grains, immature if not Mineralogical maturity refers to the percentage of quartz grainsMineralogical maturity refers to the percentage of quartz grains –Feldspars break down with transport –Quartz grains more resistant Mineralogically matureMineralogically mature sandstones have mostly quartz grains Arkosemineralogically immatureArkose is mineralogically immature

14 Roundness Increasing Roundness=increasing maturity Dry aeolian (airborne) grains Well-rounded & heavily abraded

15 Well sorted Poorly sorted Sorting Increasing sorting=increasing maturity

16 Transport structures Cross-bedding Ripples

17 Mineralogy Increasing quartz=increasing maturity

18 Classifying sandstones: mineralogy >95% Quartz = Quartz arenite Quartz Feldspar Rock fragments (lithics) <95% Quartz - divided on amount of feldspar or lithics

19 Arkose Quartz Feldspar Lithics >25% Feldspar = arkosic composition >25% Feldspar = mineralogically immature

20 PETTIJOHN CLASSIFICATION Based on percentage of matrix Matrix poor (<15%)arenites Matrix poor (<15%) - arenites Matrix rich(>15%)wackes Matrix rich (>15%) – wackes Wackes or (greywackes) Tend to be dark in colour Are poorly sorted

21 Light coloured Dark coloured Examples in hand specimen

22 Quartz sandstone (arenite) Red sandstone Cross-bedded sandstone Polished sandstone Lithic sandstone

23 Fine grained siliciclastic rocks Modal grain size <0.063mm Mudrocks (>50% siliclastic grains, 50% siliclastic grains, <0.063mm) Typical mineralogy –Clay minerals (sheet silicates), –quartz & feldspar MUDROCKS may split (fissile) or not (massive)MUDROCKS may split (fissile) or not (massive) Siltstones non-fissile, coarser grade, mica flakes commonSiltstones non-fissile, coarser grade, mica flakes common MudstonesMudstones - massive, mud-grade ShalesShales - Fissile (splitting), mud-grade

24 Siliciclastic rocks Increasing mud- grade grains

25 Shale

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