Sedimentary Rocks

Types of Sedimentary Rocks Sedimentary rocks can be broken up into two groups. As you can see in Figure 1, sedimentary rocks are either considered inorganic land-derived (clastic) or chemically and/or organically formed. As you can also see from Figure 1, clastic means fragmental or another way of saying that is clastic refers to smaller broken up pieces of rock. Figure 1. This table titled Scheme for Sedimentary Rock Identification is from your Earth Science Reference Tables (ESRT).

Clastic Sedimentary Rocks As you can see from Figure 2, clastic sedimentary rocks are formed from the compaction and cementation of sediments. Sediment is broken up pieces of rock that is formed when rocks break down. The process of rocks breaking down into smaller pieces is known as weathering.

Sediments Sediments are classified by their size. Figure 3 below shows the three largest sized sediments. Boulders Cobbles Pebbles Figure 3. This rock garden is composed of sediments ranging in boulder in size to pebbles.

Sediments The picture on the right is a jar containing sediment ranging from sand to clay and water. After the jar was shook, it was placed on the table and was undisturbed for a period of 24 hours. Notice the larger sediment was deposited first followed by the silt and clay. Because clay is so small and unlike the other sediments is flat, it takes about an entire day for the clay to settle.

Relative Size of Gravel-Sized Sediment and Smaller

Earth Science Reference Tables – Clastic Sedimentary Rocks Notice, the rocks arranged in this part of the reference table are arranged by grain (sediment) size.

Formation Turning loose sediment into solid rock is known as lithification. Sediment is created when bedrock is weathered creating sediment (Figure 4). Those pieces are then transported by moving water, wind, or glaciers in a process called erosion. When rivers carrying sediment flow into a large body of water (lake or ocean) the sediment is deposited. Deposition is the settling of sediment on earth’s surface. Figure 4. Above is bedrock that is breaking down creating the loose boulders on the base of the slope. Notice from the zoomed in picture on the left that the sediment created is large and angular.

Stream Erosion Streams like the two seen here will move sediment down stream. The stream on the left is moving very fast and therefore will move smaller and larger sediment downstream. However, the stream on the right is located in the dry climate of Arizona and because the water isn’t moving very fast it is unable to move the larger boulders, cobbles, and pebbles. During heavy rainfall flash floods will turn this quiet stream into a raging river. When this occurs the largest sediment in this picture will move moved downstream and will later be deposited as the water recedes. Also notice that unlike the sediment on the previous slide, this sediment is rounded. This is a result of the sediment rolling down the stream bed.

Deposition Much of the larger sediment moved by rivers is deposited upstream because it is too large to be carried downstream by the current. However, smaller sediment like sand, silt, and clay usually reach a rivers final destination which are lakes and oceans (Figure 5). When a river reaches a large body of water, the velocity of the water slows dramatically and the current can no longer hold the sediment it was carrying. As a result large amounts of sediment is deposited. deposition Figure 5. This aerial photo shows a river carrying a lot of sediment causing the muddy appearance of the water. You can also see the locations in which deposition is occurring.

Compaction As the diagram on the right shows, over thousands to millions of years, deposited sediment will built up into thick layers. The weight of the overlying sediment will squeeze or compact the bottom layers. In some cases, like shale, this will convert sediment like mud into solid rock. However, most sedimentary rocks require an additional process to glue the loose sediment together.

Cementation Many sedimentary rocks are formed in marine environments, however sediments that are deposited on land will be buried over time. Either seawater or ground water on land will fill up the pore spaces located between the sediment (Figure 6). Over time, the dissolved minerals in the water will begin to precipitate. When this occurs crystals will grow until they have filled up all of the empty pore spaces. This process of cementation will turn loose sediment into solid rock because the precipitated minerals act as glue and cement the sediment together (Figure 7). Click here to see an animation that demonstrates this process. Figure 6. Space between sand and silt grains are known as pore spaces. Figure 7. Diagram showing the cementation process.

Crystal Growth – Precipitation of a Solution As the salt water evaporates, the solution becomes more concentrated. When it becomes saturated the mineral halite begins to precipitate. This same process occurs in the pore spaces between sediment. This process glues or cements sediment together.