Progressive weathering tends toward equilibrium. A completely weathered sediment will consist of quartz and clay minerals: i.e. the siliciclastic sediments. The most intensely weathered material will have clays that are aluminous, eg. kaolinite, and contain no other cations; plus oxides of the most insoluble metals, eg. Fe and Al. Sandstones concentrate the most resistant constituents of the source rock.

The first category of sedimentary rock that we will consider is the one you are looking at in lab right now. Siliciclastic sediments consist of accumulations of two of the main weathering products: rock residues and new minerals.

Rock residues include:

• polymineralic rock fragments ranging in size from sand to boulders

• individual mineral grains, generally in the silt to sand size range.

The primary classification of siliciclastic sedimentary rocks is based on the size of the constituent particles:

Each of these is further subdivided based on composition and in some cases texture.

Because different siliciclastic components have different characteristic sizes, the different kinds of siliciclastic rocks tend to have different compositions. Generally speaking, polymineralic rock fragments are larger than mineral grains which are larger than secondary minerals (clays, oxides).

Look at size distributions for main sedimentary consitiuents: Handout Note: X axis size is given in Phi: -log₂ mm

So conglomerates tend to be dominated by rock fragments, sands by a mixture of RF and mineral grains (although in most cases mineral grains dominate because they are more durable during transport), silts by mineral grains, and muds by clay minerals.

Once you correct for the grainsize effect, the framework-grain composition of siliciclastic rocks is a function of:

• Provenance

• Weathering history

Analysis of composition is used to determine the origin and history of sedimentary material, and hence the paleogeography and paleotectonics of the basin of deposition.

QUESTION: What kind of sediment is best for provenance analysis?

Conglomerates are best because they contain honking great chunks of the source rock. However, conglomerates are a very minor proportion of sedimentary rocks, and so we tend to focus on sandstones, for a variety of reasons

Sandstones are formed from fragmental pieces of pre-existing rocks, mainly mineral grains and polymineralic rock fragments. Sometimes it is easy to identify these constituents in the sandstone; sometimes not so easy.

"Sand" to a sedimentologist is a quantitative term, indicating grains with diameters ranging form 0.063mm to 1mm (4 phi to 0 phi).

Sandstones have a lot of economic significance because they are often highly porous, and they are the major reservoirs for groundwater and petroleum (both oil and gas).

They are generally the rock of choice for provenance studies for sedimentary sequences and paleogeography because their grain size range includes small rock fragments; their size is large enough that individual grains can be identified in thin section with not too much dificulty; but at the same time their grainsize is small enough that a single thin section contains a statistically significant population of grains (whereas with conglomerates you need a large outcrop to get a random count of 500-700 clasts).

The kind of sandstone that is produced depends on a combination of source rock lithology and extent of weathering.

Look at relative contibutions of different source rocks of sand-sized detritus: create diagram on board from pl. 127 of Lewis.

Fine-grained source rocks (low-grade metamorphics and volcanic rocks) tend to produce higher proportions of sand-size rock fragments, and coarser-grained source rocks (high-grade metamorphic rocks and plutonic rocks) tend to contribute more mineral grains.

A major issue in sedimentary petrology is the origin of matrix in sandstones. Many sandstones contain significant amounts of clay-rich material, which was in the past considered to be detrital matrix. However, it was found that there was a correlation between sandstone age and amount of matrix (older sandstones had more matrix).

Q: what is the possible source of this material?

Now realised that labile grains in sandstones...feldspars, heavy minerals, rock fragments...may disintegrate and recrystallise during diagenesis to form a fine-grained phyllosilicate matrix. This complicates interpretations of older, matrix-rich rocks.  

How might you distinguish between primary and secondary matrix?

• homogeneous in composition;

• equally distributed throughout the rock;

• associated with poorly-sorted framework grains

• often inhomogeneous

• associated often (but not always!) with well-sorted framework grains.

Bottom line: if your matrix is inhomogeneous, or there is more than about 10% of it; and/or your framework grains are well-sorted, suspect secondary matrix; but the absence of these clues does not serve as assurance that the matrix is primary.