Photo by W. W. Little

Conglomerate is defined as a clastic sedimentary rock in which at least 30% of the clasts are of gravel size.

Conglomerate (rudite/rudaceous)

Textural classification is based upon whether or not framework grains are in contact with one another and reflects the relative abundance of framework grains to matrix (sand and/or mud).

Textural Classification

Clast-supported Matrix-supported

Clast-supported conglomerates are further subdivided into open- and closed-framework, depending upon whether the intergranular pore space is mostly empty or mostly filled with matrix

Clast-supported Conglomerate

Open-framework Closed-framework

Matrix-supported Conglomerate

Matrix-supported conglomerates consist of framework grains “floating” in matrix and are often referred to as “diamictites.”

A clast-supported conglomerate with less than 15% matrix is referred to as an orthoconglomerate. A conglomerate with more than 15% matrix is a paraconglomerate. Most paraconglomerates are diamictites.

Ortho- vs. Paraconglomerate

Orthoconglomerate Paraconglomerate

Compositional classification is based upon whether framework grains are composed of a single (oligomictic) or multiple (polymictic or petromictic) compositions.

Compositional Classification

Oligomict(ic)

Polymict/petromict(ic)

Significance if Matrix

The relative abundance of matrix can be a reflection of the rate or mode of deposition. Matrix-rich deposits suggest rapid, simultaneous deposition by aqueous flow (flash flood, debris flow, turbidity flow) or deposition by a highly viscous fluid (ice). A lack of matrix suggests winnowing by continuous aqueous fluid flow.

Source classification is based upon whether the clasts were derived from without (extraformational/exotic) or within the basin (intraformational/rip-up). The terms intra- and extra-basinal are also used to describe these rocks.

Source Classification

Extrabasinal Intrabasinal

Photo by W. W. Little

Breccia

Breccia is conglomerate in which the majority of clasts are angular and indicates rapid deposition and burial. Angular gravel is sometimes referred to as rubble.

Photo by W. W. Little

Clast roundness can be an indicator of relative transport time. Angular clasts represent rapid burial shortly after sediment production. Rounded clasts suggest relatively long periods of surface exposure prior to burial. Because of the longer surface exposure, rounded clasts tend to have been transported longer distances than angular clasts. Clast composition also plays a significant role in its rate of rounding.

Significance of Roundness

Photo by W. W. Little

• Solution breccia: groundwater movement

• Tectonic breccia : fault movement

• Volcanic breccia: explosive volcanism

• Cataclastic breccia: mass wasting

• Collapse breccia: sink hole collapse

• Ejecta: meteorite impactBreccias are not always indicative of sedimentary processes but can be produced in a number of ways. Non depositional breccieas are typically classified according to the process responsible for their formation.

Non-sedimentary Breccias

Provenance of Conglomerate

The primary control on gravel composition is the source rock. Because of the large grain size, conglomerates are particularly useful as provenance indicators.