Clastic Sedimentary Rocks

Clastic deposition covers about 75% of the Earth’s surface and clastic sedimentary rocks can be categorized into 3 groups based on grain size. Table 1 demonstrates grain size and type of sedimentary rocks.

Table 1 – Grain Size and Sedimentary Rock


Fine Grained Sedimentary Rocks – Mudstone

Mudstone is a fine grain sedimentary rock and this is the most abundant type of sediments. Its grain size is less than 1//16 mm, so people cannot differentiate it with normal eyesight. Typical mudstones are siltstone, claystone and shale and the most common minerals are quartz, feldspar, calcite and clay. The fine grained rock can show the least about its formation because of a very small grain size. Mudstone is defined as sediment with a large component of clay sized material. Mudstone occurs due to slow settling from slow currents. This fine grain sediment is typically formed on blanketing ridges, continental shaves, seafloor, abyssal plain and in trenches.

Mudstone usually contains decayed  organisms, which can be a food source for worms, crustaceans, burrowing clams, etc. These small creatures eat organic matter and then excrete unused inorganic bulk. The process of reworking of organic material in mudstone is called “bioturbation.” This can create black shale, which is a good source of petroleum.

Figure 1 – Shale

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Medium Grained Sedimentary Rocks – Sandstone

Sandstone is classified on grain size, texture and chemicals in a rock and the easiest criteria to differentiate sandstone from other sedimentary rock is the grain size. Sandstone has a grain size range from 1/16 to 2 mm and it can be seen with normal eyesight.  Rocks in this group are quartz, sandstone, arkosic sandstone and greywacke.

Figure 2 – Quartz Sandstone

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Well sorted sandstone is defined when all grains are the same size. On the other hand, poorly sorted sand is sandstone with various grain sizes. The shape of sand grain can tell you how eroded the grain sands are. If you see sandstone’s grain with a more spherical (less angular) shape, you can imagine that this rock was eroded quite heavily and transported over a long distance.

Course Grained Sedimentary Rocks – Gravel and Conglomerate

This type of rock has the biggest grain size, which is bigger than 2.0 mm and rocks in this category are conglomerate and breccia. Large sized rocks must be deposited by strong currents such as mountain rivers because it requires a lot of energy to move big and heavy rocks. Conglomerate (Figure 3) has a rounded shape because it has been transported with a high energy environment for a long period of time. However, breccia (Figure 4) has an angular shape because it is transported quickly.

Figure 3 – Conglomerate

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Figure 4 – Breccia

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Richard C. Selley, 2014. Elements of Petroleum Geology, Third Edition. 3 Edition. Academic Press.

Norman J. Hyne, 2012. Nontechnical Guide to Petroleum Geology, Exploration, Drilling & Production, 3rd Ed.. 3 Edition. PennWell Corp.

Richard C. Selley, 1997. Elements of Petroleum Geology, Second Edition. 2 Edition. Academic Press.

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