Sedimentary Rocks

Sedimentary rocks are rocks that are produced through the accumulation and lithification of the weathered remains of other rock or through precipitation of dissolved minerals from water. Rocks formed from the lithification of sediment derived from the weathering of other rocks are classified as clastic sedimentary rocks, whereas those produced through precipitation from water or through biological processes are classified as chemical sedimentary rocks. 

Since the movement and final deposition of sediment derived from other rocks is dependent 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 climate and environmental condition of the past.

Formation of Clastic Sedimentary Rocks

Clastic sedimentary rocks form through the lithification of accumulated sediment and involves both cementation and compaction. During compaction the weight of overlying materials causes grains to be squeezed together decreasing the pore space between grains and increasing friction between individual sediment grains. Compaction has a much greater affect on sediment of smaller grain sizes such as silts and clays (imagine squeezing a handful of gravel together, would the gravel maintain its shape when you open your hand? What about clay?)

Cementation is a process by which other minerals such as silica, calcite, or iron oxides precipitate out of ground water into the pore space between deposited clasts. These mineral cements than act like a glue holding individual clasts together. Since this cement is composed of other minerals, those minerals' properties greatly affect the strength of the resulting sedimentary rock. 

Classifying Clastic Sedimentary Rocks

When studying sedimentary rocks, one of the most obvious aspects will be the particle or clast size and shape. Determining whether a sedimentary rock is composed of sand, silt or clay, or gravel is the first step in classifying sedimentary rocks. The following table defines these terms based on average particle diameter. For general purposes of identification, it is not usually necessary to measure individual sediment grains since the texture of the rock and visual observation will help identify the grain size.

Another sediment property that can be detected by the texture of the rock is the grain shape. Grain shape stores a tremendous amount of information about the geologic history of the sediment. Whether particles in the sediment are rounded or angular is determined by the amount of abrasion the particles have undergone. Abrasion occurs as sediment is moved throughout the environment and it bounces or rubs against other clasts. This abrasion slightly wears down the edges of the sediment. Therefore, the further a clast travels from its point of origin, the more abrasion occurs and thus the more rounded the clast should be.

Chemical Sedimentary Rocks

Another type of sedimentary rock is the chemical sedimentary rock. This type of rock is formed when minerals, dissolved in water, begin to precipitate out of solution and deposit at the base of the water body. This can occur in hot springs, such as those in Yellowstone, where changes in water chemistry initiate precipitation of calcium carbonate in the form of travertine, in areas where sea water evaporates, depositing rock salt or gypsum, or in oceans where changes in water chemistry causes minerals to precipitate.

Due to the manner in which they are formed, these types of rocks may exhibit a crystalline texture. This most often occurs in rocks produced by the accumulation of siliceous or calcareous tests (or shells) of microorganisms. During the burial process, water may react within the small pores and recrystallize into fine-grained texture. Because most chemical sedimentary rocks are formed in marine environments, it is not unlikely to find fossils within chemically precipitated rocks.

In shallow marine environments a specific type of precipitation, called an ooid, can occur. In these environments material is often under constant motion due to tidal or wave action. As these sediment roll back and forth on the sea or lake floor, precipitating minerals will crystallize only on the exposed surface. Eventually, thin spherical layers of precipitated minerals will develop on the original particle (or nucleus). Sedimentary rocks composed of ooids are described by the precipitated mineral, for example, ooilitic limestone or ooilitic hematite.