The rock cycle clastic sedimentary rocks

EARTH MATERIALS VI The Rock Cycle: Clastic Sedimentary Rocks Professor Peter Doyle P.doyle@imperial.ac.uk Profdoyle@btinternet.com 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 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 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 Lithification due to: Compaction (squeezing out water) Diagenesis (precipitation of cements) Transport shown by sedimentary structures Mineralogy of sedimentary rocks Clastic sediments • siliciclastics – quartz, feldspars, clay minerals, rock fragments • calciclastics – 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 >256 mm 256 – 64mm 64 – 2mm – 0.063mm 0.063 – 0.002mm < 0.063mm < 0.002mm Rock Boulder  Cobble  Pebble  Conglomerate (rounded clasts) Breccia (angular clasts) Sand Silt Mud Clay Sandstone Siltstone Mudstone / Shale Claystone Conglomerates & Breccias • Conglomerates - mainly rounded clasts – – Ortho conglomerates - clast supported Para conglomerates - matrix supported • Breccias - mainly angular clasts • Clast type defines rock type: – Monomict - clasts of mainly one composition – Polymict - clasts of various compositions – 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 Finer grains between major clasts Grains floating in matrix Crystalline cement Grains touching Ca lc i te ce m Quartz clast en t Diamict conglomerate Orthoconglomerate Monomict breccia SANDSTONES • CLASTS 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 • Siliciclastic sands have clasts composed usually of quartz, feldspar, fine grained rock fragments or lithics • Mud grade matrix may be present • Crystalline diagenetic cements, e.g quartz, calcite, clay minerals Maturity • Maturity is a function of sediment transport • Textural maturity refers to: – The degree of roundness of the grains – The amount of sorting of the grain sizes • Texturally mature sandstones have well-rounded and well-sorted grains, immature if not • Mineralogical maturity refers to the percentage of quartz grains – Feldspars break down with transport – Quartz grains more resistant • Mineralogically mature sandstones have mostly quartz grains • Arkose is mineralogically immature Roundness Dry aeolian (airborne) grains Well-rounded & heavily abraded Increasing Roundness=increasing maturity Sorting Well sorted Poorly sorted Increasing sorting=increasing maturity Transport structures Cross-bedding Ripples Mineralogy Increasing quartz=increasing maturity Classifying sandstones: mineralogy Quartz >95% Quartz = Quartz arenite 25% Feldspar = arkosic composition >25% Feldspar = mineralogically immature Quartz Feldspar Lithics PETTIJOHN CLASSIFICATION Based on percentage of matrix • Matrix poor (15%) – wackes • Wackes or (greywackes) • Tend to be dark in colour • Are poorly sorted Examples in hand specimen Light Light coloured coloured Dark coloured Light coloured Dark coloured Cross-bedded sandstone Red sandstone Polished sandstone Lithic sandstone Quartz sandstone (arenite) Fine grained siliciclastic rocks • Modal grain size 50% siliclastic grains, [...]... grained siliciclastic rocks • Modal grain size 50% siliclastic grains, 95% Quartz = Quartz arenite 25% Feldspar = arkosic composition >25% Feldspar = mineralogically immature Quartz Feldspar Lithics PETTIJOHN CLASSIFICATION Based on percentage of matrix • Matrix poor