5.2: Conglomerates and Breccias

Conglomerates (rounded clasts) and breccias (angular clasts) are composed of >50% gravel-sized particles (>2 mm). Although they are the most coarse-grained clastic rocks, they are relatively rare in the rock record because they are derived from tectonically active areas and have relatively little preservation potential unless transported into the subsiding part of a basin.

Composition

For our purposes, we will consider conglomerates and breccias to have more than 50% clasts. When describing these rocks, clasts are the larger particles that make up the rock; they are >2 mm diameter and are analogous to the framework grains in a sandstone. The smaller particles between the clasts are called matrix; they are <2 mm diameter and are analogous to the muddy matrix in a sandstone. In terms of describing these rocks, we can consider the following characteristics:

Clast composition

Extraformational clasts are composed of older fragments of sedimentary, igneous, and/or metamorphic rocks that formed outside of the sedimentary basin. We use the term oligomict to describe a conglomerate/breccia that is composed of one or a very few types of lithic clasts and the term polymict to refer to one that is composed of many types of lithic clasts.

Intraformational clasts are composed of partially lithified clasts that formed within the sedimentary basin. Common examples of intraformational clasts include caliche nodules, siderite nodules, mud chip rip-up clasts, bones, scales, or a variety of other materials.

Matrix composition

The matrix of a conglomerate/breccia can essentially be described in the way that you would describe a sandstone (they will be overwhelmingly composed of sand and gravel). Properties like composition, sorting, cement, etc. Because mud beyond what one would expect as clayey matrix is difficult to explain from a hydrodynamic perspective, rocks with a mixture of sand, mud, and gravel are termed diamictites (discussed later on in this chapter).

Support and Fabric

Clast-supported conglomerates are exactly that … the clasts generally touch one another, they typically contain less than 15% matrix, and if the matrix was removed the clasts would not collapse or significantly shift. Matrix-supported conglomerates typically contain 15-50% matrix, which means that the clasts appear to be “floating” within the matrix and would collapse if the matrix was removed. Some conglomerates and breccias have clasts that are imbricated; they were aligned by flowing water and have “shingled” appearance with the long axis dipping in an upstream direction.

 

Figure \(\PageIndex{1a}\): Characteristics of conglomerates and breccias, part 1 (all images from Michael C. Rygel via Wikimedia Commons; CC BY-SA 4.0). A) Breccia with angular clasts and B) conglomerate with rounded clasts. C) Extraformational polymict conglomerate with a variety of clast lithologies. D) Oligomict breccia with clasts composed almost entirely of basalt clasts derived from the nearby (~25 m) basin margin. E & F) Intraformational breccias composed of mud-chip rip clasts and poorly lithified carbonate, respectively.

 

Figure \(\PageIndex{1b}\): Characteristics of conglomerates and breccias, part 2 (all images from Michael C. Rygel via Wikimedia Commons; CC BY-SA 4.0). A) Conglomerate with a sandstone matrix that could be described using the descriptive terminology and techniques discussed in the previous chapater. B) Matrix-supported breccia. These basalt clasts tumbled from the basin margin into eolian sand dunes. C) Clast-supported polymict conglomerate. D) Congomerate with imbricated clasts that record paleoflow from left to right.

Additional Resources and Readings