The rock cycle clastic sedimentary rocks

25 242 0
The rock cycle clastic sedimentary rocks

Đang tải... (xem toàn văn)

Tài liệu hạn chế xem trước, để xem đầy đủ mời bạn chọn Tải xuống

Thông tin tài liệu

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

Ngày đăng: 30/11/2016, 17:09

Từ khóa liên quan

Mục lục

  • EARTH MATERIALS VI

  • PowerPoint Presentation

  • Slide 3

  • SEDIMENTARY ROCKS

  • Slide 5

  • Mineralogy of sedimentary rocks

  •  

  • Conglomerates & Breccias

  • Slide 9

  • Slide 10

  • Slide 11

  • SANDSTONES

  • Maturity

  • Slide 14

  • Slide 15

  • Slide 16

  • Slide 17

  • Slide 18

  • Slide 19

  • Slide 20

Tài liệu cùng người dùng

Tài liệu liên quan