# 5 Sedimentary Rock Diagrams

Clastic sedimentary rocks, other than limestone, may be classified on the basis of their mix of grain sizes, as specified by the Wentworth scale. Diagrams show how sedimentary rocks are formed and the materials that created them.

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## Conglomerate, Sandstone, and Mudstone

This diagram is used to classify sedimentary rocks according to the mixture of grain sizes in them. Only three grades are used:

1. Sand is between 1/16 millimeter and 2 mm.
2. Mud is anything smaller than sand and includes the silt and clay size grades of the Wentworth scale.
3. Gravel is anything larger than sand and includes granules, pebbles, cobbles, and boulders on the Wentworth scale.

First, the rock is disaggregated, typically using acid to dissolve the cement holding the grains together. DMSO, ultrasound, and other methods are also used. The sediment is then sifted through a graduated set of sieves to sort out the different sizes, and the various fractions are weighed. If the cement cannot be removed, the rock is examined under the microscope in thin sections and the fractions are estimated by area instead of weight. In that case, the cement fraction is subtracted from the total and the three sediment fractions are recalculated so that they add up to 100 — that is, they are normalized. For example, if the gravel/sand/mud/matrix numbers are 20/60/10/10, gravel/sand/mud normalizes to 22/67/11. Once the percentages are determined, using the diagram is straightforward:

1. Draw a horizontal line on the ternary diagram to mark the value for gravel, zero at the bottom and 100 at the top. Measure along one of the sides, then draw a horizontal line at that point.
2. Do the same for sand (left to right along the bottom). That will be a line parallel to the left side.
3. The point where the lines for gravel and sand meet is your rock. Read its name from the field in the diagram. Naturally, the number used for mud will also be there.
4. Notice that the lines that fan downward from the gravel vertex are based on values, expressed as a percentage, of the expression mud/sand and mud, meaning that each point on the line, regardless of the gravel content, has the same proportions of sand to mud. You can calculate your rock's position that way, too.

It only takes very little gravel to make a rock "conglomeratic." If you pick up a rock and see any gravel clast at all, that's enough to call it conglomeratic. And notice that conglomerate has a 30 percent threshold. In practice, just a few large grains is all it takes.

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## Sandstone and Mudstones

Rocks with less than 5 percent gravel may be classified according to grain size (on the Wentworth scale) using this diagram.

This diagram, based on the Folk classification of sediment, is used to classify sandstones and mudstones according to the mixture of grain sizes making them up. Assuming that less than 5 percent of the rock is larger than sand (gravel), only three grades are used:

1. Sand is between 1/16 mm and 2 mm.
2. Silt is between 1/16 mm and 1/256 mm.
3. Clay is smaller than 1/256 mm.

The sediment in a rock may be assessed by measuring a few hundred randomly selected grains in a set of thin sections. If the rock is suitable — for instance, if it is cemented with easily soluble calcite — the rock can be disaggregated into the sediment using acid, DMSO, or ultrasound to dissolve the cement holding the grains together. The sand is sifted out using a standard sieve. The silt and clay fractions are determined by their settling speed in the water. At home, a simple test using a quart jar will give the proportions of the three fractions.

Use this diagram by drawing a horizontal line to mark the value for sand, then mark your silt to see where the two intersect.

This graph is related to the previous graph for gravel/sand/mud: the center line of this graph is the same as the bottom line of the gravel/sand/mud graph. Imagine taking that bottom line and fanning it out into this triangle to split the mud fraction into silt and clay.

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## Sedimentary Rocks Diagram

This diagram is based on the mineralogy of grains of sand size or larger (on the Wentworth scale). Finer-grained matrix is ignored. Lithics are rock fragments.

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## QFL Provenance Diagram

This diagram is used to interpret the ingredients of sandstone in terms of the plate-tectonic setting of the rocks that produced the sand. Q is quartz, F is feldspar and L is lithics (rock fragments that are not broken down into single-mineral grains).

The names and dimensions of the fields in this diagram were specified by William Dickinson and colleagues in a 1983 GSA Bulletin on the basis of hundreds of different sandstones in North America. As far as I know, this diagram has not changed since then. It is an essential tool in studies of sediment provenance.

This diagram works best for sediment that does not have a lot of quartz grains that are actually chert or quartzite, because those should be considered lithics instead of quartz. For those rocks, the QmFLt diagram works better.

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## QmFLt Provenance Diagram

This diagram is used like the QFL diagram, but it is designed for provenance studies of sandstones that contain a lot of chert or polycrystalline quartz (quartzite) grains. Qm is monocrystalline quartz, F is feldspar, and Lt is total lithics.

Like the QFL diagram, this ternary graph uses the specifications published in 1983 by Dickinson. By assigning lithic quartz to the lithics category, this diagram makes it easier to discriminate among sediments that come from the recycled rocks of mountain ranges.

Source

Dickinson, William R. "Provenance of North American Phanerozoic sandstones in relation to tectonic setting." GSA Bulletin, L. Sue Beard, G. Robert Brakenridge, et al., Volume 94, Number 2, GeoScienceWorld, February 1983.

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