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

THE ROCK CYCLE

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

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

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)

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

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 – 0.002mm Silt Siltstone < 0.063mm Mud Mudstone / Shale < 0.002mm Clay Claystone

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

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

Quartz clast Calcite cement

Orthoconglomerate Monomict breccia Diamict conglomerate

SANDSTONES CLASTSCLASTS have modal grain size in the sand grade ( 2mm – 0.063mm) –Coarse sand mm, –medium m –fine mm 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

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

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

Well sorted Poorly sorted Sorting Increasing sorting=increasing maturity

Transport structures Cross-bedding Ripples

Mineralogy Increasing quartz=increasing maturity

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

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

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

Light coloured Dark coloured Examples in hand specimen

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

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

Siliciclastic rocks Increasing mud- grade grains

Shale