Lecture 11 - Sedimentary Rocks - Classifications

Chapter 7

Sedimentary rocks may be classified as clastic, biochemical, and chemical.

Clastic, or "detrital," sedimentary rocks consist of solid sedimentary fragments. Sedimentary clasts ("detritus") are classified on the basis of grain size:

boulder ... coarse particles about 25 cm (~10") or more in diameter
cobble
pebble
sand ... particles about one millimeter in diameter
silt
clay ... fine particles smaller than .004 mm in diameter.

The absolute size divisions of clasts is not really important for our purposes - you know already that a boulder is bigger than a pebble, etc. - but it is important to note that size is proportional to the energy associated with transport (stronger currents transport larger clasts) and size gives a rough indication of the distance to the source area.

Major types of clastic sedimentary rocks are (from fine to coarse constituent clasts):

mudstone: clay ("blocky") and shale (splits easily)
siltstone
sandstone
sedimentary conglomerate or breccia

Clastic sedimentary rocks are classified mainly on the basis of texture, with sensible names that refer to the size of material they contain. Breccias are similar to conglomerates, in that they contain large clasts, but differ in that the clasts are angular, not rounded as is typical of transportation (breccias usually form from collapsed or fallen rocks - such as within a cave or along a steep mountainside).

Biochemical sedimentary rocks consist of minerals precipitated from solution by organic processes:

limestone ... formed mainly of calcite, precipitated as skeletal material (in molluscs, corals, etc.)
chert ... formed of skeletal SiO2 - as from diatoms (a planktonic, marine micro-organism)

Biochemical sedimentary rocks generally form in basins with ample supplies of sunlight and nutrients. Similar to sedimentary rocks, and contained within them, coal is a biochemical deposit composed of compressed, reduced organic remains - typically of plants.

Chemcial sedimentary rocks, sometimes referred to as "orthochemical" sedimentary rocks, originate from materials taken into solution and reprecipitated, without the influence of organic activity:

evaporites ... (rock halite and rock gypsum)
dolostone ... (made of the mineral dolomite)
iron formations ... (made mostly of hematite)

Note that the biochemical and chemical sedimentary rocks are named for their composition, by the mineral they contain, whereas the clastic rocks are named mainly by texture.

Many important natural resources, most from a global economic standpoint or a nation's gross national product, are associated with sedimentary rocks. Most of Earth's energy resources, in terms of percentage of energy produced today, is produced from fuels in sedimentary rocks. Fossil fuels, coal and petroleum, are generally formed from organic remains buried in sedimentary basins, and hosted in sedimentary rocks. Uranium ores are also generally formed from weathering of uranium-rich igneous or metamorphic rocks, with the uranium deposited along with clasts to form sandstone. Therefore, with the exceptions of hydroelectric power and buring wood, the significant production of our world's energy depends on understanding and interpreting sedimentary rocks, and using that information to recover fuels.