1. 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

2. Texture T O S SS Texture = –Orientation (random/lined up), –Size (measurements, all same?), –Shape (rounded/angular), –Sorting (well  poor)

3. Differences in texture Sedimentary rocks show great differences in their texture This relates back to their mode of formation

4. 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

5. 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)

7. Size Boulder>256mm Cobble64 - 256mm Pebble4 - 64mm Gravel2 - 4mm Sand 1 / 16 – 2mm Silt 1 / 256 – 1 / 16 mm Clay< 1 / 256 mm

8. 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

9. 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

10. 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

11. 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!)

12. So what do shape, size, sorting tell us? they reflect: –Derivation (what were the original rocks) –Climate (during formation) –Post-depositional factors

13. Geoscience page 87 Copy figure 5.20 Characteristics of sedimentary grains

14. Derivation what were the original rocks? Lots of different rock particles mean the HINTERLAND was a big area and very diverse

15. 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

16. Coal – swamp Limestone – cwsmas Shale – slow currents (fine material) Red well sorted fine sandstone (desert) Conglomerate – dumped material, sudden slowing of current

17. Also need to observe structures Sole structures - base of bed – eg. erosion features (potholes) seen at Trevor quarry Current bedding (direction of flow)

18. 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)

19. 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 300  C (temperature increases at 1  C per 30mdepth; pressure increases 1atm per 4.4m depth)

20. Sand – fairly well compacted on deposition Muds – high water content

21. 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

22. Geoscience page 86 Copy figure 5.19 major diagenetic processes

23. Finally… Read - Chapter 5 in Geoscience as a summary