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 (wellpoor) 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 Silt Clay >256mm 64 - 256mm 4 - 64mm 2 - 4mm 1/ 16 – 2mm 1/ 1/ mm – 256 16 <1/256mm Grain size 2 • Sand – divided into coarse, medium and fine • SIEVING to separate grain fractions (technique demonstration) • Then weigh and calculate percentages • Geoscience page 89 figure 5.22 examine frequency plots for dune sand, beach sand, tidal mud, glacial till and river sand Sorting • Well sorted = particles nearly all the same size – Current strength constant and for long periods of time • Poorly sorted = particles of a great mix of sizes – Current strength suddenly drops and material is dumped Porosity/Permeability • Obviously sorting links well to poroperm • Well sorted, rounded, medium grain size has good pore spaces and will allow water to fall through quickly • Poorly sorted, angular sediments have small pore spaces and trap water reducing permeability 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 us? they 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 of deposition?” • Sediment colour • Bed thickness (how long conditions persisted for) • 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 • 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 (dissolved and re-precipitated) • Temperature increase with depth – Diagenesis/Metamorphism boundary 300C (temperature increases at 1C 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 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