Download Sedimentary Rocks

Sedimentary Rocks
Sedimentary rocks are formed when pieces of pre-existing rock or parts of onceliving 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 (Boggs, 1995).
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 (Boggs,
1995).
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. The depositional
environment for these rocks would be marine or evaporite basins (Boggs, 1995).
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 (Fowler, 1990).
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. In ocean basins, sediment depths can vary from 0 at the active ridge plate
boundary to 0.5 – 1.0 kilometers in the basins (Fowler, 1990).
Strike-slip or transform plate boundaries occur when two plates move laterally
with respect to each other. In many cases, these transform boundaries form transform
fault valleys. These valleys become sediment filled grabens that can have either marine
or non-marine sediments depending on the depositional setting. The thick deposits form
from the erosion of materials at higher elevations above the valley. This can affect how
quickly and what types of sediments are deposited. In the transform valleys, numerous
localized facies changes can be found. In addition, transform faults can displace stream
channels and this can also affect sedimentation in these features.
Plate tectonics control sedimentation through its influence on the sediment source
area. The affects include: 1) influencing the composition by controlling the lithology of
the source rocks; 2) influencing the sediment denudation rates and the size of the basins
that sediments accumulate in, which influences sedimentation rates and sediment
thickness; and, 3) control on sea level which affects sediment processes and
environments. This has an impact on sediment characteristics and stratigraphic
relationships. Sedimentary rocks cannot be discussed without considering the
depositional environment and the significant impacts that plate tectonics can have on
those environments (Fowler, 1990).
References
Boggs, Sam Jr., 1995. Principles of Sedimentology and Stratigraphy, Second Edition.
Prentice Hall: Englewood Cliffs.
Fowler, C.M.R., 1990. The Solid Earth: An Introduction to Global Geophysics.
Cambridge University Press: Cambridge.