Clastic rock types in detail: Conglomerates

Because of their formation requirements (coarse detritus, strong current transport) and their physical properties (high porosity and permeability), the majority of sedimentary research has focused on sandstones and conglomerates. This is even though they represent, at most, 25% of the stratigraphic record. Below are sets of criteria and classifications that are handy in describing these lithologies.

Review clast characteristics and the clastic rocks they form:

Clast diameter (mm): Clast name: If rounded to subangular: If angular: Rock type if rounded to subangular: Rock type if angular:
>256 Boulder
64 - 256 Cobble Gravel Rubble Conglomerate Breccia
4 - 64 Pebble
2 - 4 Granule
1/16 - 2 Sand Sand Sand Sandstone Sandstone
1/256 - 1/16 Silt Mud Silt Mudrock Siltstone
< 1/256 Clay Mud Clay Mudrock Claystone

Conglomerate and Breccia

Lithified gravel and rubble are called conglomerate and breccia, respectively. Conglomerates are very special in the sedimentary record for two reasons:

As such, they are very helpful in reconstructing provenance of the hinterland, as they consist of sediments that generally have not been transported far, and are often linked to nearby tectonic activity.

Together these comprise 1 - 2% of sedimentary rocks.

Breccia (right)

Conglomerate (right)

For now, we will treat conglomerates and breccias together as "conglomerates." Remember, though, that if the major clasts are angular, the rock must be described as a breccia. This fact is significant when considering its origin,as breccias are not necessarily sedimentary.

Parameters of conglomerate composition

Grain size: Conglomerates (and breccias!) typically display two grain size classes:

Compositionally, we distinguish:

Clast stability:

Clast origin: Finally, we distinguish clasts depending on their origin as:

Using this information, we can begin to classify conglomerates according the classification scheme provided by your text:

When clasts are extraformational, we have two general categories:

Paraconglomerates are further broken down based on the internal structure of the matrix:

Textural issues:

Sorting and modality: Conglomerates are very poorly sorted. We see two general classes:

Imbrication: The systematic orientation of clasts in a conglomerate is termed imbrication. (A common example is overlapping.) This usually indicates stream or glacier transport in which grains are aligned with current.

Still confused about the agent of transport for your conglomerate? Look for hints such as sedimentary structures, or the texture of clasts. Basal flute casts indicate a turbidity current. Parallel striations of clasts (right) suggests they came in contact with a glacier.

Note: Diamictite is another term for a paraconglomerate, and is often used to denote glacial rocks.

Finally, in addition to the foregoing, breccias can form through exotic processes unrelated to weathering and erosion including: