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GEOLOGY 12
SEDIMENTARY ROCKS
Assignment # 26
April, 2006
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
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)
Chemical Sedimentary Rocks:
- are precipitated (separated out from a solution) out from a water environment
a) either directly from inorganic processes or
b) by the actions of organisms
- chemical rocks include: carbonates, cherts and evaporates
Carbonate rocks:
- two main types = limestone and dolomite
limestone: composed mostly of calcite
2 main types of limestone are: biochemical limestone or
inorganic limestone
biochemical limestone:
- form the majority of limestones
- precipitated through actions of organisms e.g. coral
- formed on continental shelves
- formed in warm, shallow waters
- formed of algae, shells, corals
- may be any size but are broken down & rounded as they are transported
along the ocean floor by currents and wave actions
- may be coarse grained with recognizable fossils or
- may be fines grained and dense from accumulation of algae
Page 133 Fig. 6.18 - 6.20
chalk = biochemical sedimentary rock formed from microscopic marine organism
Page 134 Fig. 6.21
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Inorganic limestone: formed from the accumulation of and cementation of very
small grains of sand or other precipitants in the water
- limestones are very susceptible to recrystallization = process by which new crystals are
formed in the developing limestone due to increasing pressure due to growing top layers
of accumulated sediments. Recrstallization is usually composed of same composition as
parent material.
- crystals are usually large and due to pressure, destroy visible fossil remnants
note: not the same as recrystallization within mantle as a result of heat and
pressure
- chert fine grained sedimentary rock formed mostly of silica
- evaporites sedimentary rocks from crystals formed due to evaporation of water
such as rock salt and rock gypsum
Page 136 Fig. 6.24
third basic type of formation of sedimentary rocks = organic sedimentary rocks
(first two types were clastic (formed by lithification) and second type is chemical formed
sedimentary rocks)
coal = sedimentary rock formed from the compaction of completely decayed plant
material.
- partial decay of abundant vegetation uses up oxygen in swamp water and bogs
- without oxygen material stops decomposing
- material is compressed, water squeezed out of rotting vegetation
- coal eventually results
- coal usually found in horizons/seams or beds that are horizontal or slightly tilted
- because of how coal is formed its not as compacted as igneous rock
- when mining for coal, always danger of tunnel collapse since not as compacted
as igneous rocks. (much safer to work in an iron or nickel mine compared to a
coal mine)
- what can cause thick beds of coal = ancient swamps, bogs and jungle areas
- the coal beds mined in Cape Breton formed in tropical environments in warm
temperate or tropical swamps and bogs
- as crustal plates moved, beds of coal moved with the plates resulting in their
present location
possible internet search topic = research origins of coal deposits in Cape Breton,
describe the coal seams i.e. shape, how large, type of coal, how deep, and dangers of
coal mining
Assignment Question Page 149 #3, 4, 7, 9, 10
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ORIGINS OF OIL AND GAS
- oil and natural gas originate from organic matter (microscopic organisms, one celled
algae
settle on sea floor
accumulate in marine mud : best areas were
in basin types of environments with little of no water circulation e.g. continental shelves
- because oil and gas originate from ancient plant and animal life, they are called fossil
fuels
as material decomposes - uses up oxygen = stops decaying process
material undergoes increased pressure and heat
converts material to oil or
natural gas
as compaction continues
porous and permeable layers
droplets of oil squeezed out and accumulate in more
oil and gas less dense then water
water saturated rock layers
then oil and gas generally tend to rise above
SEDIMENTARY STRUCTURES
- sedimentary structures formed during or shortly after deposition but before lithification
- sedimentary structures are important to geologists because:
1 - provide clues about how material was transported
2 - provide clues to final resting place of material
SEDIMENTARY STRUCTURES
6 structures briefly described = 1) bedding, 2) cross bedding, 3) graded beds, 4) mud
cracks, 5) ripple marks, 6) fossils
1) bedding = - most prominent structure
- visible layering in rock
- most bedding horizontal = deposits of sedimentary material deposited in
horizontal layers parallel to earths surface
bedding plane = nearly flat surface of deposition separating two layers
of rock
- each new layer buries the older layer
principle of superposition = layers stacked oldest at the bottom, youngest at the top
Page 138 Fig. 6.26
2) cross-bedding
- thin inclined layers within a larger rock
- display distinct angles to horizontal bedding planes
- most often found in sandstone
how it develops - sand is deposited on slopes
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- sand dunes deposited by wind
- sand ridges deposited by ocean currents
- sand bars deposited by rivers
- rivers deposit sand in river channels and in deltas
Page 138 Fig. 6.27 Page 140 Fig. 6.28
3) graded beds (another sedimentary structure) - vertical change in particle size in bed
- fine grain material at top of bed, coarse grained material at bottom
- may have gravel at the bottom of the bed
- formed by gradually slowing water currents
Page 140 Fig. 6.29
4) mud cracks (another sedimentary structure) - polygonal pattern as mud crack when it dries
(similar pattern to columnar jointing discussed in North Mountain Basalt)
- mud cracks form only in sediments above water (air needed for drying)
- formed when lakes dry up
- water deposited sediments as they dry
- mud cracks lithify to form shales Page 141 Fig. 6.31
5) ripple marks - small ridges formed on top of sediment layers by moving water or wind and
these marks are preserved in the rock
- form perpendicular to motion of water
- wave caused marks = symmetric ridges
- current caused marks = asymmetric
Page 142 Fig. 6.32
6) fossils - traces of plants or animals preserved in rock
- hard parts of plants or animals most likely fossilized
- NOTE: original shells or other fossil parts are not the actual plant
or animal - original parts decompose - what happens is that calcite or
silica fill the pores of the plant or animal and replace it, the original plant
or animal is replaced, not preserved (bugs in amber = hardened tree sap are preserved as sap turns into rock type material
- sometimes plant or animal completely dissolves leaving a mould in the
rock - then silica or calcite fills in this mould forming a cast e.g. pertified
wood.
Page 143 Fig. 6.34
Assignment Questions: Page 149 #11, 12, 13, 14
SEDIMENTARY FORMATIONS:
formation = a body of rock of considerable thickness with characteristics that distinguish it from
adjacent rock units and is large enough to be mapped
- usually based on rock type
- has a visible characteristic that makes it a recognizable unit
- given proper names
- first part of name usually location of formation
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- second name usually the rock type
e.g. Navajo Sandstone
INTERPRETATION OF SEDIMENTARY ROCKS
- interpreting sedimentary rocks give geologists clues about the source of the rock, the deposition
environment, and time of deposition
source area of sedimentary rocks = the locality that was eroded and provided the sediment
- the most important thing to know about a source area = type of original rock, its
location and distance from eventual deposition site
- original rock type determines character of resulting sediment
- the finer and more rounded sediment grains are, the farther they are away from the
source
Page 144 Fig. 6.36
environment of deposition:
1 - continental environments include alluvial fans, river channels, floodplains, lakes,
dunes
2 - glacial environments include ridges, widespread sheets of deposition, and till
3 - alluvial fans include fan shaped piles of sediments
4 - river channels and floodplains include elongated deposits, cross bedding, ripple
marks, mud cracks, and footprints
5 - lakes
6 - deltas
7 - beaches, sand dunes
8 - lagoons
9 - shallow marine shelves
10 - reefs
Page 146 Fig. 6.38 examples of above
Plate Tectonics and Sedimentary Rocks
- plate tectonics distribute sedimentary rocks (e.g. coal distribution around planet)
- along convergent boundary plates erosion is usually rapid
thick accumulations of
clastic materials
- many mountain ranges have fossil remains at their tops due to uplifting along convergent
boundaries due to uplift
- along tranform boundaries rapid erosion and deposition due to fault basin opening and
subsiding due to plate motions
- rapid deposition and burial of organic matter along transform boundaries result in petroleum
deposits in these areas (convergence = too much physical activity i.e. uplifting, subduction) but
transform = sideways movement
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- along divergent boundaries on continents
creation of lakes e.g. Red Sea,
which leads to evaporitic rock formation, and sea deposits
Assignment Questions Page 149 #15 to 28 (multiple choice)