11.7: How Do I Recognize a Clastic Sedimentary Rock?

How Do I Recognize a Clastic Sedimentary Rock?

Sedimentary rocks can be subdivided into two major groups based on the processes responsible for their formation: clastic (detrital) sedimentary rocks Links to an external site. and non-clastic sedimentary rocks. Non-clastic (chemical, organic, biochemical) rocks are determined by their major chemical composition; however, if the sedimentary rock is composed of the broken pieces of other rocks, largely produced via physical weathering, the rocks are referred to as clastic or detrital sedimentary rocks. In a clastic sedimentary rock, the classification will primarily focus on the clast properties, including the clast size, overall shape (well-rounded to angular), and sorting of the clasts within the rock (uniform clast size to a range of sizes) (Figure 11.14).

Recall that weathering and erosion generally occur in areas of high elevation, such as the mountains, while deposition occurs in lower areas such as valleys, lakes, or the ocean. Sediment is transported to the area of deposition via the agents of transportation: ice, water, air or gravity. During transportation, sediment changes, often significantly. Geologists (and you!) can recognize the amount of change and the distance the material has traveled, and the transport mechanism, by looking at the maturity of the sediment. Sediment maturity refers to the size, shape, and the general mineral composition of the sediment.

The size of sediments can be described using three broad categories: gravels, sands, and muds (Figure 11.15). Gravels and sands are visible, whereas muds are too small to see with the naked eye and typically need magnification to distinguish individual clasts.

The further sediments travel from their original source, the smaller, more rounded Links to an external site.,  sorted Links to an external site., and uniform Links to an external site. they become (Figure 11.16). Consider your own life or educational journey: the more experiences you collect, the more well-rounded you become. For example, prior to graduation, most community colleges require you to complete numerous general education courses; this requirement is to facilitate your development, much like weathering and erosion, and to help you become a more well-rounded student and person. Secondly, much like each of us, our differences are in part a result of our experiences. The differences of clastic rocks are related to the various depositional environments, and these environments, and the distance traveled, can be interpreted based on the characteristics of the sediment.