Sediments and sedimentary rocks - lecture 2

Sediments

loose material, uncemented

sedimentary rocks

lithified, cemented

volcaniclastic

any rock or sediment made mostly of broken volcanic fragments

Clastic (mud, sand, gravel)

sedimentary rocks from pieces of older rocks and sorted by particle size

Carbonates

Kalksteen etc. Non-clastic: coral reef. Clastic carbonate: coral sand

Evaporites

overgebleven delen na verdamping zoals zouten.

Mud includes…

Silt and clay

General division of sizes into

mud, sand, gravel

Boundary particle sizes (we will adhere to the soil sceince classes)

Silt-sand boundary 63 μm (50 μm in soil science) and Clay < 4 μm (< 2 μm in soil science)

Gravel

> 2 mm

Granules, pebbles, cobbles and boulders

When consolidated (cemented): conglomerate

Types of conglomerates

Monomict (all clasts same material), polymict (many different materials)

Clastic sedimented types

If mixed with sand and/or mud admixed to the gravel (matrix): sandy gravel, muddy gravel/sandy conglomerate, muddy conglomerate. Clast-supported or matrix-supported

Clast imbrication

That all pebbles are aimed in the same direction due to flow resistance

Sand

63 (50) – 2000 μm in diameter
very fine, fine, medium, coarse and very coarse

composed of detrital mineral grains (separate minerals) and/or lithic fragments (pieces of eroded rock)

common: quartz, feldspar, mica and heavy minerals

Clay and Silt

Clay: < 4 (2) μm
Silt: 63 (50) – 4 (2) μm

grains largely invisible to the nake eye; tasting

quartz most common mineral in silt

clay consists of phyllosilicatae minerals

when silt and/or clay are consolidated the resulting rock is called mudrock

Clay and silt buildup

clay minerals have a layered chemical structure:

two groups of clay minerals; kandite (2 layers) and smectite (3 layers)

Important clay minerals: kaolinite (warm, humid), montmorillonite (temperate; swelling clay), illite (temperate), chlorite (arid)

Analysis

scanning electron microscopy(SEM) or X-ray diffractometer (XRD)

Roundness

reflects transport history

sphericity

inherited, formed during weathering, this is how long the sediment is stretched

Fabric

anisotropic-isotropic (preferred orientation of clasts causing preferred directions of fractures of consolidated rock)

analysis sediments in field

gravel: counting in quadrant, sand: sieving, Silt and clay: settling tube and pipette

Mineralogical maturity

proportion of reistant to labile clast types

textural matural

sorting shape

Non-clastic sediments

carbonaceous (organic) deposits. peat