Sedimentary Rocks and Plate Tectonics

Sedimentary rocks are formed when pieces of pre-existing rock or parts of once-living organisms accumulate in deposits on the Earth's surface. There are three main types of sedimentary rocks. They are clastics, biological, and chemical sedimentary rocks. Each category of sedimentary rock has distinct characteristics and depositional environments. In addition, the way the Earth's plates move, or plate tectonics, can significantly affect sedimentation and the formation of sedimentary rocks.

Clastic sedimentary rocks are those rocks that are formed by the accumulation of pieces of pre-existing rocks. They are often deposited in layers or bedding. The pieces of pre-existing rock, or clasts, can range in size from clay to boulders. The clasts are cemented together with some sort of matrix. Typical clastic sedimentary rocks include shale, siltstone, and sandstone. The depositional environment would be any basin where the sediment that has eroded from pre-existing rock can accumulate. This could include river systems, lakes, deserts, and glaciers, as well as beaches, deltas, estuaries, and tidal flats.

Biological (or organic) sedimentary rocks are those that form when the remains of living things accumulate, become compacted and lithified. Ancient plant deposits become coal, and deposits of animal shells may become limestone or coquina. The two dominant minerals in carbonate rocks such as limestone are calcite and dolomite.

Chemical sedimentary rocks are those that are formed by chemical precipitation. These rocks include carbonate rocks such as limestone (both a biological and chemical sedimentary rock) and evaporates such as halite and gypsum.

Plate tectonics not only affects sedimentation and the formation of sedimentary rocks, but it is used to describe how rocks move through the rock cycle to be igneous, sedimentary, and metamorphic rocks.

At convergent plate boundaries, plates are moving toward each other and one plate is subducted beneath the other. As the plate that is being subducted moves beneath the other plate, it is taking with it the accumulated sediment and rocks. The subducted rocks and sediments are heated to the point that they melt to become magma and eventually, igneous rock. In some areas where subduction is occurring, not all of the subducted plate is actually subducted. These special cases refer to ophiolites. Sequences of rock from the subducted plate are actually shoved up onto the over-riding plate. An additional way that sedimentation and sedimentary rocks are affected by plate tectonics at convergent boundaries is with the intrusion of magma into the country rock of the overlying plate. The intrusion causes sedimentary rock to be exposed to heat and pressure and may change the sedimentary rock into metamorphic rock.

At divergent boundaries such as ocean spreading ridges, sedimentation is also affected. In theory, sediments are laid down in horizontal beds and at any place in that bedding layer, the sediments are the same age. At divergent plate boundaries, because the plates are moving away from each other, new plate material is coming up at the boundary. Therefore, the youngest sediments would be found closest to the plate boundary.

Boggs, Sam Jr., 1995. Principles of Sedimentology and Stratigraphy, Second Edition. Prentice Hall: Englewood Cliffs.