Sedimentary Rocks

Introduction: What is the importance of studying sedimentary rocks?

1. Contain important clues to the history of the continents and the Earth's climate history (global warming? global cooling?).

2. Contain important clues to the history of life (fossils).

3. Contain much of the Earth's important and valuable natural resource deposits (oil, natural gas).

I. Processes Involved in the Formation of Sedimentary Rocks

1. weathering - The decomposition and disintegration of rocks and minerals at the Earth's surface by mechanical and chemical processes.

mechanical (physical) weathering - The disintegration of rock into smaller pieces by physical processes (water, wind, ice, heat).

chemical weathering - The chemical decomposition of rocks and minerals by exposure to air, water and other chemicals in the environment.

2. transportation - The movement of sediment by flowing water, ice, wind or gravity.

sediment - solid rock or mineral fragments transported and deposited by wind, water, gravity or ice, precipitated by chemical reactions, or secreted by organisms.

3. deposition - the laying down of rock-forming minerals by any natural agent (water, wind, ice, gravity)

Depositional Environment - The physical environment (for example floodplain, beach, or desert) in which sediments are deposited.

4. lithification - the conversion of loose sediment to solid rock by compaction and cementation.

compaction - Tighter packing of sedimentary grains causing a decrease in porosity, usually resulting from the weight of overlying sediment.

porosity - The proportion of the volume of a material that consists of open spaces.

cementation - The process by which sediment is lithified by precipitation of a mineral cement among the grains of sediment.  The chemical and mineralogical composition of the cement is usually dependent upon the mineralogical and chemical composition of the sediment. The most common cements are calcite, silica, and Fe-oxide.

II. Classification of Sedimentary Rocks (Make sure you see an example of each type in lab!)

1. detrital (clastic) sedimentary rocks - Composed of fragments of weathered rocks (clasts)   Examples: conglomerate, breccia, sandstone, siltstone, shale.   (Fig 3.16, 3.17, 3.18, 3.19).

2. organic sedimentary rocks -  Composed if the lithified remains of plants or animals. Example: coal (Fig 3.22, 3.24), chert

3. chemical sedimentary rocks - Composed of minerals precipitated from solution. Examples: rock salt (halite) (Fig 3.23), gypsum

4. biochemical sedimentary rocks - composed of broken shell fragments and similar remains of living organisms. Example: fossiliferous limestone, coquina (Fig 3.20), chalk (Fig 3.21).

Most abundant sedimentary rocks: shale and siltstone (70%), sandstone (15%), limestone (10%). All others (< 5%)

III. Sedimentary Structures

sedimentary structure - feature that develops during or shortly after deposition of the sediment. Helps understanding of how the sediment was transported and deposited. Important clues to depositional environment.

1. bedding (stratification) - nearly horizontal layering that develops as sediment accumulates layer by layer. The most common type of sedimentary structure. (Fig 3.15) example: The Grand Canyon

2. ripple marks - small, nearly parallel ridges and troughs formed by unidirectional (example: stream bed) or oscillating (example: near-shore waters of oceans and lakes) water or wind currents.  Are used to determine direction of current flow. (Fig 3.25A)

3. mud cracks - polygonal cracks that form as mud shrinks and dries. Indicate sediment deposition in a shallow water  environment that periodically dried up (example: intertidal flat). (Fig 3.25B)

5. fossils - any remains or traces of a plant or animal preserved in rock (example: shells, bones, teeth)

IV. Other Important Sedimentary Rock Characteristics

1. Clast size - 1. Can be used to determine the type of transporting agent. 'High energy' transporting agents (example: river with rapids) can transport large-sized  particles. 'Low energy' environments (example: slow moving river) can transport only small-sized particles. Can also be used to estimate the distance sediment has traveled from its source rock before being deposited. In general, large clasts are deposited closer to their source rock than small and fine-grained clasts. (Example: A stream emerging from a mountainous region into flat landscape will deposit large-sized particles near the base of the mountain as it loses much of its transporting energy. Smaller-sized particles will be carried by lower energy waters to greater distances from the mountain source.)

2. Clast shape - Can be used to estimate the distance over which sediment has been transported before being deposited. In general, rounded clasts have been transported for a greater distance than angular clasts.
 

 


3. Mineralogical composition - Can be used to estimate time, mechanism of sediment transportation, and/or climate. In general, quartz-rich rocks represent long periods of sediment transport during which minerals unstable at the Earth's surface (for example pyroxenes, amphiboles) are destroyed by chemical weathering processes.
(General rule: Silicate minerals which crystallize at highest temperatures on Bowen's Reaction Series are most unstable at the Earth's surface, and will 'disappear' first as a result of weathering processes.)

Study Questions - Sedimentary Rocks