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A Primer on Sedimentary Rocks
From Prof. R. Aileen Yingst, University of Wisconsin at Green Bay,
http://www.uwgb.edu/yingsta/
•When rocks are broken down into fragments, either through the mechanical means of
weathering, or through chemical reactions, the fragments are called sediment.
When that sediment is compacted or cemented together, it forms a sedimentary
rock.
•Sedimentary rocks are the most common rocks that we encounter, because the Earth
has so many efficient weathering processes that constantly break down rock and
create sediment.
•Common sedimentary rocks include sandstone (made up, not surprisingly, of sand)
and limestone (made up of calcium carbonate precipitated out of solution).
•Sediments become sedimentary rock through compaction (squeezing sediments
together and forcing out any fluids) and cementation (introduction of a cementing
agent).
•Sediments are either clastic or chemical. That is, rocks are broken down through
either mechanical or chemical means.
Clastic sediment
•Clastic sediment is what one usually thinks of when speaking of sediment. From the
Greek word klastos (broken), it refers to the broken remains of rocks of all types,
broken and altered by weathering processes such as wind, water and ice. Clastic
sediment is also known as detrital sediment.
•Clastic rocks (sedimentary) differ from igneous rocks not by the mineral composition
but by the grain texture that shows its history of mechanical weathering, the
cementation material and the possible presence of fossils (which cannot survive the
high temperatures required to melt igneous rocks).
•Sedimentary layering is also a powerful clue to sedimentary origin. Igneous rocks
have minerals which grow in every direction. Sedimentary rocks, on the other
hand, are often laid down in layers that mimic the horizontal surface of the Earth.
Chemical sediment
•Chemical sedimentary rocks may contain fossils and other sedimentary
characteristics, but their components were not broken up mechanically. Rather,
rocks were dissolved in solution (as salt can dissolve in water) and transported,
then precipitated chemically (as salt can precipitate out of a saturated solution).
•Precipitation can occur biochemically. Example: tiny plants living in seawater can
decrease the acidity of the surrounding water and so cause calcium carbonate to
precipitate.
•Precipitation can also occur through inorganic means. Example: As seawater
evaporates, it often leaves behind salts which have precipitated out. This is how
halite (salt) is formed.
Lithification of a sedimentary rock
•Once either clastic or chemical sediment is formed, it becomes sedimentary rock
through sediment transport, deposition and diagenesis, or the alteration of
sediments that create rock out of deposited sediments.
•Each of these steps occur in many ways, but each of these processes will leave clues
in the characteristics of the rocks.
Transport
•Sediment can be transported in many ways. The main types of transport are through
wind action, water action, glacial or ice action, or mass wasting (mass movement
of Earth, through avalanches, slides, slumps and so on).
•Each of these processes leaves telltale marks on the rocks that are produced.
•Example: Sediments transported by water often have rounded, sorted grains because
of the abrading, polishing action of the sediments transported in the flow.
•Example: Sediments transported by glaciers are often broken and shattered by the
grinding action of the travelling glacier. Thus, the sediments are commonly angular
rather than rounded.
Deposition
•Sediment deposition occurs when the method of transport is unable to carry the
sediment grains.
•This is related to the size and weight of the grains and thus is an indicator of the
power and speed of the transporting mechanism.
•Example: Wind action cannot transport grains much larger than sand.
•Example: Glaciers are slow movers, but because they are so large, they can carry
sediment the size of boulders for great distances, depositing them only when the ice
melts and retreats.
Wind-transported sediment is usually no larger than sand-sized.
Glacial till can be comprised of all sizes of sediment from silt and mud to boulders.
Diagenesis or Lithification
•Changes that sediment undergoes after deposition are referred to as diagenesis. This
includes any transformations during and after formation into a rock. The actual
process of rock formation from sediment is called lithification.
•Diagenesis occurs through the processes of compaction, cementation, recrystallization
and chemical alterations of the sediment.
•Compaction is the simplest change that sediments can undergo.
•Compaction occurs as the weight of accumulating sediment forces the rock and
mineral grains together. This reduces pore space and eliminates some of the
contained water.
•Cementation is a result of water circulating through the pore spaces of a sediment.
•If that water carries in it dissolved substances that then precipitate out during
circulation, those substances left behind in the sediments can act as cementing
agents.
•Calcium carbonate is one of the most common cements because it is found abundantly
in seawater.
•After burial, occasionally less stable minerals may change to more stable forms
through recrystallization. That is, the minerals begin to grow again, interlocking
with each other.
•This process is important in the alteration of more porous limestone to harder, more
compact versions of this mineral.
•Chemical alterations can also affect sediments.
•If oxygen is present, organic remains are quickly converted into carbon dioxide and
water. This is called an oxydizing environment.
•In the absence of oxygen to bond with, organics will be transformed into solid carbon,
such as peat and coal.
•From a planetary perspective, chemical alterations can also occur. For example, the
wind-transported sediments of Mars have been subjected to the chemical process of
rust.
Classifying sedimentary rocks
•Sediments can be distinguished from other rock types by the presence of stratification,
by the size and texture of the grains, and by the presence of grain sorting.
•Although each of these characteristics may not be present, those that are, are usually
excellent clues as to the processes that formed the sedimentary rocks in question.
Stratification (layering)
•Because sediments are typically laid down at the Earth’s surface, they tend to form in
horizontal layers. These are called strata (singular = stratum).
•This layered arrangement of strata in a body of rock is referred to as bedding. Each
bed may be different from those around it, in thickness, in typical grain size, or in
other characteristics.
•The differences in beds are clues to changes in the way sediments were deposited or
transported.
Cross-bedded standstone near Kanab, Utah, consists of ancient sand dunes that have
been converted to sedimentary rock. Cross strata are inclined to the right, in the
direction toward which the winds were blowing.
•Planar, or parallel bedding usually occurs in water, where currents or waves are
minimal.
•Ripple marked bedding indicates wave or wind action.
•Cross bedding are the work of turbulent flow in water or wind (dunes).
•Graded bedding, or sorting of grains with large ones near the bottom, is usually
formed from standing water in which sediments have been able to settle out.
Grain size
•Often sedimentary rocks are classified by grain size.
•Grain size is an indicator of the strength or speed of the transport mechanisms.
•An abundance of smaller grains indicates a weaker or slower transport mechanism
(wind or a slow moving stream). Conversely, larger grains of sediment would
result from stronger or faster forces (floods, glaciers).
•Example: Wapatki sandstone, with grains almost too small to be seen. These
sediments were laid down by slow-moving water.
Grain texture
•Grain texture refers to the angularity or roundedness of the grains in a sedimentary
rock.
•Angular grains are commonly broken from bedrock, while rounded grains have been
abraded by other fragments.
•Typically the more rounded a grain is, the more time that grain has come into contact
with other grains, and thus the longer the distance of travel for that grain.
Sorting
•Sorting is a conspicuous feature of many sedimentary rocks. Sorting is the process by
which grains of like size or weight are grouped together in a rock. This process is
most common in air or water transported sediments.
•Sorting is often an indicator of the energy of the transport mechanism: poor sorting
means lower energy or glacial transport.
•Grains can be sorted by specific gravity, for instance, as heavier particles carried in a
stream may fall out of the flow before lighter particles.
•Grains in a rock or bed may also be sorted by size, which is more important when
most grains are of the same mineral (and thus similar specific gravity).
•Finally, if transport if sediment is particularly long, grains may be sorted by
durability. Thus, in some cases, the only grains that survive the distance are the
most resistant to weathering and fracture — typically quartz.
Well sorted grains in a sandstone sample.
Poorly sorted grains in a conglomerate.
Other features
•Other features that are found commonly in sedimentary rocks include fossils and
various surface features.
•Fossils, the remnants of animals and plants, are often buried with sediments, protected
against oxydation and erosion, and are converted to rock. Fossils are indicators of
the climate, surface or sea temperature, and precipitation at the time of burial.
•Ripple marks from waves and mud cracks of dry lake beds can also be preserved and
lithified to create sedimentary rock.
Sedimentary environments
•Sediments (and thus sedimentary rocks) are formed in many environments, which can
be grouped into four large categories: non-marine water environments, ice or
glacial environments, eolian environments and ocean environments.
•Non-marine water sediments are formed in streams and lakes. Both mechanical and
chemical sediments are important. Lakes or dry lake beds, especially, are places to
look for chemically deposited sediment.
•Sediments eroded and transported by glaciers are deposited at the base of the glacier
or released at its edge during melting.
•Wind-transported sediments are typically found in eolian environments, better known
as deserts.
•Ocean sediments are formed either in near-shore environments such as beaches,
lagoons and deltas, offshore on the continental shelf (like reefs), or in the deep sea.