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 S• Texture =

– 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 >256mm• Cobble 64 - 256mm• Pebble 4 - 64mm• Gravel 2 - 4mm

• Sand 1/16 – 2mm

• Silt 1/256 – 1/16mm

• Clay <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