Sedimentary Rocks

Sedimentary rocks can be classified as either clastic sedimentary rocks or chemical sedimentary rocks. Chemical sedimentary rocks form through either precipitation of dissolved minerals as a result of biologic activity. Clastic sedimentary rocks are those rocks formed from the lithification of sediment derived from the weathering and erosion of other rocks and material. Since the movement and final deposition of this sediment is dependant on the environmental and climatic aspects of a geographic region, understanding how these factors influence the composition of sediment and therefore the resulting rock gives us great insight into the past climate and environmental condition of the area where the sediment was deposited.

Tectonic Settings

Ever since the Earth formed sediment, eroded from exposed rock, has been accumulating on Earth's surface. Although the rate of accumulation varies in each environment, sediment is constantly accumulating on the land, the sea floor, lakes, and streams. Over time it is these sediments that eventually become sedimentary rocks. The rocks that form from these sediments preserve many features indicative of the environment in which they form. By studying and interpreting the features preserved within the sedimentary rock record we are able to decipher Earth's geologic history. But what determines the type of sedimentary rock that forms?

One factor that determines which type of sedimentary rock forms is the tectonic setting. The tectonic setting refers to tectonic properties of the geographic region in which the sediment is accumulating. Active settings are regions found along active convergent plate boundaries. In these regions tectonic forces are physically deforming previously formed rock. This process causes some areas to rise in elevation which increases erosion and some areas to experience sinking which will accumulate sediment.  Areas that become elevated are referred to as sources and those areas that sink are referred to as basins.

The type of rock being weathered in source regions has a major impact on sediment formation. Rocks composed of hard minerals tend to weather at a slower rate than those composed of soft minerals. Therefore, in general when a source rock is composed predominantly of soft minerals more sediment can be produced during equivalent periods of time.

The medium of transport and the processes occurring within the depositional environment also plays a large role in determining what type of sediment is deposited. Water, wind, ice (glaciers), gravity are all capable of transporting sediment, however each is not capable of carrying the same sediment types. For example a stream containing very slow moving water may be capable of carrying silts and clays but if gravel or larger-sized particles are introduced into the stream they fall right to the bottom. These particles are only capable of being transported by very fast moving water.

Finally, the climate of the area and the amount of time available to accumulate sediment also plays a crucial role sediment deposition. Wet climates tend to increase weathering due to the greater amount of water and other dissolved ions available to react with exposed rock. In general wet environments will produces more sediment than dry environments and therefore tend to produce thicker beds.

By understanding how each of these factors influence sediment production and final deposition we can use simple observations to make relatively accurate interpretations of past environments.   

Cratons, Shields, Platforms, and Orogenic Belts

Continents are composed of two tectonic components: a craton and orogenic belts. The craton is the stable interior of a continent that has remained relatively undisturbed by tectonic events since Precambrian Time. Cratons are composed of two distinct components; a shield and a platform. The shield is an area of ancient exposed crystalline rock. The platform is the area of the craton where the shield has been buried by flat or gently warped sedimentary rock.

Orogenic Belts are elongated regions that surround the craton and have been deformed by compressional forces such as those experienced at convergent plate boundaries. The term orogeny means mountain building so orogenic belts are areas where mountains are formed.