sedimentary textures a jo conway

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GL4 E1 KI 2c • Sedimentary rocks exhibit differences in texture: – – – – Grain angularity Sphericity Size Sorting • Which reflect: – Derivation (original rocks) – Climate (during formation) – Post-depositional factors Texture • T O S S • Texture = – – – – S Orientation (random/lined up), Size (measurements, all same?), Shape (rounded/angular), Sorting (wellpoor) Differences in texture • Sedimentary rocks show great differences in their texture • This relates back to their mode of formation Grain shape • Angular – little evidence of wear, sharp corners, little transport • Increased sphericity - more spherical, rounded, corners smoothed off to broad curves, great amount of transport Grain shape • Defined by ratio of dimensions of the fragment • Length, breadth, thickness (a, b, c axes) • Zingg classified shape into tabular, equant, blade and rod • Some unusual: DREIKANTER/VENTIFACT – wedge shaped (wind transport, desert) Size • • • • Boulder Cobble Pebble Gravel • Sand >256mm 64 - 256mm - 64mm - 4mm /16 – 2mm • Silt • Clay [...]...Quick practical 1 Measure a, b, c axes of 10 pebbles (from puddingstone conglomerate, best “guestimate” in some cases!) describe shape – Zingg analysis 2 Using hand lens, 30 grains of at least 3 sands (desert, glacial, beach from jars on windowsill) – shape analysis, size analysis, sorting analysis (NB: replace sand into correct jars after examination!) So what do shape, size, sorting tell... reflect: – Derivation (what were the original rocks) – Climate (during formation) – Post-depositional factors • Geoscience page 87 • Copy figure 5.20 Characteristics of sedimentary grains Derivation • what were the original rocks? • Lots of different rock particles mean the HINTERLAND was a big area and very diverse Climate • Which existed during particle accumulation • Sedimentary logs “what was the environment... Burrowing animals before loose sediment is turned into rock • DIAGENESIS/LITHIFACTION • Turning the loose sediment into a rock (hardening) • Compaction - pressure – weight of sediments above, squeezes grains together (decrease in porosity) • Loss of water (volume changes, water escape structures etc) Diagenesis 2 • Mineral forms can change (recrystallisation eg aragonite to calcite) • Minerals can be exchanged/replaced... be exchanged/replaced (dissolved and re-precipitated) • Temperature increase with depth – Diagenesis/Metamorphism boundary 300°C (temperature increases at 1°C per 30mdepth; pressure increases 1atm per 4.4m depth) • Sand – fairly well compacted on deposition • Muds – high water content Addition of CEMENT • “glue” grains together • Calcite, silica etc • Deposition of minerals in pore spaces • Produces... • Grain size – Hjulstrom – current strength • Coal – swamp • Limestone – cwsmas • Shale – slow currents (fine material) • Red well sorted fine sandstone (desert) • Conglomerate – dumped material, sudden slowing of current Also need to observe structures • Sole structures - base of bed – eg erosion features (potholes) seen at Trevor quarry • Current bedding (direction of flow) Post depositional factors... together • Calcite, silica etc • Deposition of minerals in pore spaces • Produces rigidity in the rock • Can be simultaneous with deposition (penecontemporaneous) or introduced later • Geoscience page 86 • Copy figure 5.19 major diagenetic processes Finally… • Read - Chapter 5 in Geoscience as a summary
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