GEOLOGY 12 NOTES Name __________________
SEDIMENTS AND SEDIMENTARY ROCKS
Sediments are unconsolidated accumulations of rock and mineral grains and organic
matter that have been transported and deposited by wind, water, or ice. Sedimentary
rocks are formed when sediments become consolidated into a cohesive mass.
Three main sources for the sediments that form sedimentary rocks are:
1. Clastic – formed from fragments of other rocks such as clay, sand, or gravel.
2. Chemical – formed from mineral grains that precipitate out of solution by
evaporation or chemical action.
3. Organic- formed from remains of plant and animals.
• Fragments range in size from large to small as: pebbles, gravels, sand, silt, and clay.
• Fragments are carried by winds, waves, rivers, and glaciers and form in great layers
• Ocean, lake, and ground water all contain dissolved minerals; silica (from quartz),
lime (from calcite), and iron (from iron containing minerals) may form cements.
• When cement minerals precipitate into pore spaces between sand, gravel, or pebbles,
they bind the fragments together, turning loose sediments into firm cemented rock.
• In fine sediments such as clay or silt, sometimes the pressure of the later deposit is
enough to make the particles stick together, but usually cement is required.
• Silica and lime give white or grey cement; iron cement is red, brown or rust-colored.
• Water (or wind) carrying fragments deposit the fragments when the flow slows down.
The heaviest/largest fragments are deposited first. These are heavy pebbles and
gravels. Next come the lighter sands and finally the silts and clays.
Particle Size Range Sediment Rock
over 25.6 cm boulder conglomerate (if fragments are rounded)*
0.2-25.6 cm gravel breccia (angular fragments)*
0.00625- 0.02 cm sand sandstone
0.00039- 0.00625 cm silt siltstone**
< 0.00039 cm clay claystone**
* if particle size is over 0.2 cm, rock is called conglomerate if the fragments are
rounded, or breccia if the fragments are angular.
** both siltstone and claystone are also known as mudstone, commonly called shale if
the rock shows a tendency to split on parallel planes.
Classification/Grouping is done by:
a) particle size - see chart above.
b) roundness or angularity of fragments - see breccia and conglomerate
c) sorting - well-sorted means all grains are about the same size. The spaces
between the grain particles are usually filled with finer materials called the matrix.
• Precipitates - sediments which have been precipitated from or fallen out of solution.
• Evaporites - formed when salt water is trapped and evaporated, leaving dissolved
minerals like halite. Also note: chert (silica), and banded ironstones (deposits due to
oxidation of iron-bearing groundwaters)
Organic / Biogenic
• Biological sediments that are formed when an organism precipitates a shell or
skeleton from the water. When the organism dies, the soft body parts decay, leaving
a shell or skeleton behind. eg. coral reefs give carbonate rocks, silica skeletons of
diatoms and radiolarians give cherts.
• Coal, peat, and lignite result from the burial of plant materials. These rocks are
distinguished by their texture and percentage carbon content.
From Sediment Into Rock
1. Consolidation is the transformation of loose sediment into a coherent sedimentary
2. Diagenesis is the set of processes by which sediment becomes lithified or turned into
rock. These include:
• Compaction – squeezes sediments closer together
• Cementation – glues the sediments together
• Dolomitization – changing of calcium carbonate into calcium-magnesium
1. Clastic sediments:
• rivers, deltas, beaches, tidal flats
2. Chemical sediments:
• carbonate platforms
• deep evaporites
• siliceous bottoms of ocean
3. Other factors that affect the maturity or formation and type of sediments:
• nature of source rock
• distance from source
• effect of climate
• effect of topography
The Facies Concept
Facies - a set of conditions that leads to the formation of a particular type of sediment or
sedimentary rock. (i.e. transgression vs regression p.123-125)
Paleogeography is ancient geography and is constructed through the interpretation of
Paleocurrents are ancient currents that prevailed when the sediments were deposited.
The direction of these currents can be deduced from sedimentary structures.
Paleoclimates can be deduced from sedimentary rock formations and the fossil evidence
found in sedimentary structures.
• Transgression is when the shoreline moves inland as the ocean level rises and
advances over what was dry land. The sequence of rocks formed in this “sedimentary
facies” would be (bottom to top) sandstone, siltstone, claystone and limestone. The
limestone deposits are created when the ocean moves in and slates, shales and
mudstone deposits are created when the ocean moves out and the area is covered by
silts washing into the retreating ocean (Regression). See page 124.
Stratification (or layering) is the most obvious feature of sedimentary rocks. The layers
(or strata) are visible because of differences in the color or texture of adjacent beds.
Strata thicker than 1 cm are commonly referred to as beds. Thinner layers are called
laminations or laminae . The upper and lower surfaces of these layers are called
bedding planes. Clastic sediments display a wide variety of bed structures that help in
determining such factors as "way-up" and depositional environment.
Bedding - may include
1. Planar bedding has little diagnostic importance
but animal burrows or bed form irregularities
might give an indication of way-up and
2. Cross-bedding forms beneath ripples and
dunes. The layering is inclined, dipping
downward in the downcurrent direction. Hence,
cross-beds may be used as paleocurrent
indicators, or indicators of ancient current flow
usually curve at the bottom edge. The upper
edge of individual inclined cross-beds is usually
at a steep angle. Hence, cross-beds may also
be used as "up indicators".
3. Ripple-marked bedding is a feature which forms on
the surface of a bed of sediment. At the time of
formation, the "surface of a bed" is equivalent to
the sea floor, or the bottom of a lake or river.
Ripples which formed in streams or rivers tend to
be asymmetrical and may be used to determine
ancient current directions or paleocurrent
directions. Symmetrical ripples are produced by
waves or oscillating water.
4. Graded bedding results when a sediment-laden
current begins to slow down. The grain size within a
graded bed ranges from coarser at the bottom to
finer at the top. Hence, graded beds may be used as
5. Varves are a special type of lamination which forms
in glacial lakes. Varves represent deposition over
one year, and their formation is related to seasonal
influences. Varves are generally graded, with the
coarser material at the bottom (silt or sand)
representing the spring and summer meltwater
runoff, and the finer material at the top representing
slow settling of clays and organic matter from
suspension during the winter months when the lake is covered with ice. Counting of
varves has been used to measure the ages of some sedimentary deposits.
Mudcracks are a polygonal pattern of cracks
produced on the surface of mud as it dries. The mud
polygons between the cracks may be broken up later
by water movement, and redeposited in lime muds.
Fossils (more details later)
• trace fossils Several types of burrows.