SedRocks & Fossils 1: Clastic sediments and sedimentary rocks

Describe the origins of clastic sediments

Clastic sediment largely is derived from the weathering and erosion of subaerially exposed pre-existing rock

Made via uplift and exposure, tectonic and volcanic processes

Composition:

• Continental crust: andesitic (intermediate), 20-30% quartz, 20-70% feldspar, 10-30% micas

• Upper mantle: peridotite (ultrabasic), 50-100% olivine

• Oceanic crust: Basaltic – “basic”, 0-50% Olivine, 0-80% pyroxenes and 0-30% plagioclase

Name three types of physical weathering

• Frost wedging (freeze-thaw)

• Exfoliation (heat-cool, peeling)

• Action of plants

Name the types of chemical weathering

• Hydrolysis

• Hydration

• Carbonation

• Oxidation

• Ion exchange

• Chelation

Describe how rates of weathering are mediated

Water and temperature:

• Areas of high rainfall provide the water that is necessary for chemical weathering.

• In areas that are too cold, the water is in the solid phase (ice) and cannot mediate the chemical reactions.

• In areas that are hot, higher temperatures speed up the rate of chemical weathering reactions.

 Setting:

• However, because the development of a thick regolith inhibits access of water to fresh rock rates…

• A mechanism for the rapid (or continuous) removal of the regolith is needed.

• Here, hillslope processes of falls, landslides, creep and surface runoff are important

• But fundamentally, topography and slope are important.

We see, then, that weathering rates (the rate of generation of clastic sediment):

• Is high in areas on abundant rainfall and high temperatures

• Is high in areas of steep slope (and therefore topography)

Describe the starting minerals of clastic sediments

Thinking about our starting ingredients:

• Quartz (20-30%)

• Feldspars (20-70%)

• Micas (10-20%)

Quartz is effectively inert at the Earth’s surface:

• It will not weather chemically and it is hard (Mohs 7) to weather physically

However, feldspars and micas are metastable and react readily with water and weak acids formed in water at surface T and P (“hydrolysis”). 

If we look, then, at the weathered products of our “starting ingredients”, all EXCEPT QUARTZ produce clays

What is a significant property of clays

• They are sheet silicates (phylosilicates) with a single perfect cleavage, that are incredibly soft (Mohs 1-2.5).

• This means they weather physically to form a fine “matrix” (or “mud”) where individual clay minerals cannot be identified in hand specimen or thin section.

• We just call it all “matrix” or “mud”.

Describe the two components of clastic sediment

” Grains” (or “clasts”):

Can be recognised with the naked eye.

Overwhelmingly (1) quartz, but can be (2) felsdpars, (3) micas, or (4) fragments of other rock, or other minerals (“lithics”).

Clay “matrix” (or “mud”):

• Individual particles cannot be discerned with the naked eye

• Looks like a “speckly mush” in thin section.

Where is clastic sediment deposited

• Weathered regolith and soil is eroded via fluvial (river), wind and glacial processes, and can be transported 10s to 1000s kms to a variety of environmental settings

• This can happen locally for short periods of time, anywhere.

• But long-term storage of sediment happens in subsiding sedimentary basins

What is the impact of transport on sediment texture and minerology

However, with increasing distance (D) and time (T) of physical transport, sediment become more “texturally mature”:

• Grainsize decreases

• Grain angularity decreases and sphericity increases

• Grain sorting increases

Also, chemical weathering processes continue during transport, and sediments become more “chemically mature”: There is loss of feldspar, mica and other clasts in favour of clay matrix and quartz

Describe diagenesis

• Diagenesis is the conversion of a sediment into a sedimentary rock through burial.

• The most basic thing that happens is the growth of a cement

• crystals that precipitate from water or a brine that circulates through the pore-space of the sediment.

•  A clastic sediment comprises grains and matrix. Anything else is pore space.

• A clastic sedimentary rock comprises grains, matrix (depositional) and cement (diagenetic). Anything else is pore space.

Describe the Pettijohn classification for sandstones

Based on - composition of the clasts, ratio of grains to matrix

Requires more quantitative analysis than Wentworth

The ternary ‘face’ of the scheme divides all grains into either quartz, feldspar, or ‘anything else’

Useful for analysis of the original source rock of the sediment:

• Sedimentary source rich in quartz

• Igneous, in feldspar

• Metamorphic in ‘other’

Describe “The Toblerone Diagram”:

Depth axis concerns itself with the ratio of ‘matrix’

Amorphous, ‘speckled’ material of clays, and finely broken-down other material

Useful for understanding the ‘energy’ of the depositing processes

Examples:

• 100% quartz and 0% matrix so it is a quartz arenite

• 70% quartz, 20% rock fragments and 10% feldspar so it is a sublitharenite to lithic wacke

• 100% matrix so it is a mudrock/stone

How are classification systems used to describe rocks

• Wentworth (1922) and Pettijohn (1975) classifications can be combined to give a “descriptive name” for hand specimens and thin sections of sandstones

• For example, “a very fine quartz arenite sandstone”, or a medium-to-coarse feldspathic wacke sandstone”.