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GEOLOGY 12
SEDIMENTARY ROCKS
Assignment # 25
April, 2006
INTRODUCTION
- rock cycle - weathering of igneous rocks (mechanical + chemical) = break down of
igneous rocks into smaller and smaller bits
sediments
discussion = erosion - transportation - sorting - deposition - eventual lithification
- divers origins = difficult to classify sedimentary rocks
3 common sedimentary rock types: 1) shale, 2) sandstone and 3) limestone
geologists study origin of sedimentary rocks studied by: 1) sequence of rock layers, 2)
sediment grains, 3) sedimentary structures i.e. fossils, cross beds, ripple marks, and mud
cracks
- importance of sedimentary rocks = economic: coal, limestone
- ¾ of the surface of continents is covered by thin layer of sedimentary rocks. Economic
importance of sedimentary rocks = coal, trapped crude oil, natural gas, ground water salt,
gypsum, uranium and iron ore; limestone for concrete, sands and gravels for aggregates
and glass, and shales for producing brick and tile
SEDIMENT
sediment = collective name for loose, solid particles that are unconsolidated (loose; i.e.
not bonded together)
- sediments are classified according to size of grains e.g. boulders, cobbles,
pebbles, gravel, sand, silt (most silt is quartz), clay (mineral = group of silicate minerals)
clay sized particles = refers to size > 1/256 mm (mud = wet silt and/or clay)
Page 124, Table 6.1
- when you see pictures of glacial lakes, they are usually milky blue/green in color = result
of ‘rock flour’ = grinding away of rock minerals into clay size particles
- character of sediment is affected by: 1) weathering, 2) erosion and 3) transportation
(rounding and sorting during transportation) and eventual deposition
(remember that weathering of rock material continues as erosion processes occur)
- sediments originate from:
1 - weathering and erosion of preexisting rocks (e.g. intrusives, extrusives)
2 - chemical precipitation from solution
Transportation: effects on weathered material
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rounding = grinding away of any sharp edges during transportation and usually
associated with water transportation, glaciers, wave action (rounding much more important
then speroidal weathering = square weathered at edges first resulting in rounded shapes)
sorting = sediment grains selected and separated according to size, shape, specific
gravity. This is done especially by water transportation. Well sorted sediments are
sediments that consist of same sized grains.
Sorting of material is first done by size: eg river sorting sediments: large pieces at head of
river, smaller pieces along river, fine sediments at deposition
Page 125 Fig. 6.2
Smaller particles are deposited as the energy of any river flow diminishes.
deposition = material comes to rest or settles
deposition = also means and includes accumulation of dead organic materials such
as shells, coral, and vegetation (over millions of years this can be large accumulations of
deposits) Page 126 Fig. 6.4 and Fig. 6.5
environment of deposition = location at which deposition occurs: e.g. sea floors,
desert valley, river channel, coral reef, lake bottom, beaches, etc. NOTE: each
environment has different physical, chemical and biological characteristics that impact
upon and result in varied sedimentary rocks.
- geologists always try to determine the environment in which sedimentary rocks
were formed in
Preservation: sedimentary rocks weather easily when exposed on land. Sedimentary rock
under water can be preserved for much longer periods of time
Lithification = process that converts loose sediments into sedimentary rock. Two main
processes of lithification =
1) compaction = grains squeezed tightly together
2) cementation = precipitation of cement binds the grains together into a rock
- sediment falls to lake bottom
pore space exists between grains (40-50% of top
sediments can be pore space)
more sediments deposited and accumulated on top
overburden weighs down on lower layers
compacts grains together
as underground water slowly moves through compacted layers solid material called cement fills
pore space binding grains together (cementation) e.g. calcite, iron oxide, and silica are
cementing agents; these minerals act as such a natural "glue"
Sandstone and shale are
examples of sedimentary rocks
clastic texture = sedimentary rocks that consist of sediment grains bound by cement
crystalline texture = sedimentary rocks bound together by interlocking crystal structure
Page 127 Fig. 6.7
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TYPES OF SEDIMENTARY ROCKS
3 basic types defined on how the rocks have been formed
1) clastic = formed by lithification of sediment
e.g. breccia and conglomerates page 128, fig. 6.8
sandstone = cementation of grains
? (greywacke < 15% fine grained matrix; dark gray or green; most formed
by transported sediments by turdbidity currents e.g. underwater landslides, surface storms,
sediment laden rivers depositing into lakes or oceans)
shale: consist of fine grained silt and clay (2/3 clay, 1/3 quartz silt); splits
(fissility) into layers (laminations); shales undergo great compaction and great weight forces
minerals to orientate perpendicular to pressure Page 131 Fig. 6.15
2) chemical = precipitation from solution (e.g. rock salt)
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3) organic = consolidation of plant or animal remains (e.g. coal)