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Mcgill Geology > week 4 > Flashcards

week 4 Flashcards

1
Q

name the ways a sedimentary rock forms

A
  • cementing loose clasts (fragments) of preexisting rock
  • cementing together loose shells and shell fragments
  • accumulation of organic matter from living organisms
  • precipitation of minerals dissolved in water
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2
Q

what do the layers of sedimentary rocks tell and where do they occur

A
  • the layers record a history of ancient environments
  • the layers occur only in the upper part of the crust
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3
Q

what do sedimentary rocks cover

A

underlying basement rock

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4
Q

define the four classes of sedimentary rock

A
  • clastic: loose rock fragments (clasts) cemented together
  • biochemical (cemented shells of organisms)
  • organic: carbon-rich remains of once living organisms
  • chemical: minerals that crystallize directly from water
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5
Q

what provides the raw material for all sedimentary rocks

A

physical and chemical weathering

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6
Q

what do detrital (or clastic) sedimentary rocks consist of

A
  • ditritus (loose clasts) ex: mineral grains, rock fragments
  • cementing material (often quartz or calcite)
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7
Q

name all the ways clastic sedimentary rocks are created

A
  • weathering (generation of detritus via rock disintegration)
  • erosion (removal of sediment grains from parent rock)
  • transportation (dispersal by gravity, wind, water, ice)
  • deposition (settling out of the transporting fluid)
  • lithification (transformation into solid rock)
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8
Q

how are clastic sedimentary rocks classified

A

based of texture and composition
- clast (grain) size
- clast composition
- angularity and sphericity
- sorting
-character of cement

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9
Q

what is clast size

A

the diameter of fragments or grains
- range from very coarse to very dine
-boulder, cobble, pebble, sand, silt, and clay

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10
Q

what is the coarsed-grained sed (boulder, cobble, pebble)

A

gravel

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11
Q

what is the fine-grained (silt and clay)

A

mud

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12
Q

clastic sedimentary rocks can be made out of

A

individual minerals or rock fragments

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13
Q

the composition of the clastic sedimentary rock says what

A

the story about the original source rock

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14
Q

what is angularity

A

the degree of edge or corner smoothness

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15
Q

what is sphericity

A

degree to which a clast nears a sphere

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16
Q

sphericity and angularity of fresh ditritus

A

angular and nonspherical

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17
Q

when does grain roundness and sphericity increase

A

with transport:

  • well rounded: long transport distance
    angular: negligible transport
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18
Q

what is sorting

A

the uniformity of grain zie
- well sorted: all clasts have nearly the same grain size
- poorly sorted: clasts show a wide variety of grain sizes

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19
Q

when does sorting occur

A

size sorting occurs along stream flow

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20
Q

character of cement means

A

minerals that fill sediment pores
different clastic sedimentary rocks have different cement: quartz and calcite are the most common cements

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21
Q

what are the most common cements

A

quartz and calcite

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22
Q

what is breccia

A

angular rock fragments:
- angularity indicates the absence of rounding by transport
- deposited relatively close to clast source
ex: talus or Scree under a cliff face

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23
Q

example of coarse clastics - gravel sized clasts

A

breccia, conglomerate

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24
Q

conglomerate - rounded rock clasts

A
  • clasts rounded as flowing water wears off corners and edges
  • deposited farther from the source than breccia
  • ex: river channel
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25
Q

coarse clastics: sand and gravel sized clasts

A

arkose

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26
Q

what is arkose

A

sand and gravel with abundant felspar,
- commonly deposited in alluvial fans
- feldspar indicates short transport

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27
Q

sandstone: clastic rock made of sand-sized particles

A
  • common in beach and dune settings
  • quartz is by far the most common mineral in sandstones
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28
Q

where are fine calstics deposited

A

in quiet water settings: floodplains, lagoons, mudflats, deltas, deep-water basins

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29
Q

silt, when lithified becomes what

A

siltstone

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30
Q

mud when lithified becomes what

A

mudstone or shale

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31
Q

what is lithification

A

to make into rock

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32
Q

what is biochemical sedimentary rocks

A

sediments derived from the shells of living organisms
- hard mineral skeletons accumulate after death

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33
Q

-different sedimentary rocks are made from these materials (biochemical sedimentray rocks)

A
  • calcite and aragonite - limestone
  • silica - chert
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34
Q

what is limestone

A

sedimentary rocks made of CaCO3
ex:
- fossiliferous limestone: contains visible fossil shells
- micrite: fine carbonate mud
- chalk: made up of plankton shells

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35
Q

what is chert

A

rock made of cryptocrystalline quartz: Opal

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36
Q

how is chert formed

A
  • silica skeletons of some marine plankton
    -after burial, silica in bottom sediments dissolves
  • silica in pore fluirds solidifies into a gel
  • the silica gel precipitates chert as nodules or beds also fossilized trees
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37
Q

what are organic sedimentray rocks made of

A

made of organic carbon, the soft tissues of living things

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38
Q

what is coal

A

organic sedimentray rock:
altered remains of fossil vegetation
- black, comustible
- over 50-90% carbon
- has fueled industry since the industrial revolution began

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39
Q

what are chemical sedimental rocks comprised of

A

minerals precipitated from water solution:
- have a crystalline (interlocking) texture:
- initial crystal growth in solution
- recrystallization during buriala: neocrystalization

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40
Q

what are the classes of chemical sedimentary rocks

A
  • evaporites
  • travertine
  • replacement chert
  • dolostone: the hard cap of the niagara escarpment
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41
Q

what are evaporites

A

rock from evaporated sea or lake water (chemical sedimentary rock)

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42
Q

how are evaporites formed

A
  • evaporation triggers deposition of chemical precipitates
  • thick deposits require large volumes of water
  • evaporite minerals include halite (rock salt) and gypsum
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43
Q

what does salt tell us

A

of past water

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44
Q

what is travertine

A

calcium carbonate precipitated from ground water where it reaches the surface

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45
Q

how are travertine crerated

A

CO2 expelled into the air causes CaCO3 to precipitate
- ex: thermal springs
- caves: speleothems

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46
Q

what is dolostone

A

limestone altered by Mg-rich fluids
- CaCO3 altered to dolomite by Mg2+ rich water
- diagenisis, dolomitization

47
Q

what is diagenesis

A

physical, chemical and biological changes to sediment
(lithification is one aspect of diagenesis)

48
Q

how does diagenesis come about

A
  • as sediments are buried, pressure and T rise:
  • T between burial and metamorphism (300ºC)
  • interaction with hot groundwater: chemical reactions
    -cements may precipitate or dissolve
  • at higher pressure and T, metamorphism begins
49
Q

what is a replacement of chert (nonbiogenic)

A

cryptocrystalline silica gradually replaced calcite, long after limestone was deposited
ex:
- flint: colored black or gray from organic matter
- agate: precipitates in concentric rings
- petrified woor: wood grain preserved by silica

50
Q

what are some features imparted to sedimetns at or near deposition

A
  • bedding and stratification
    -surface features on bedding layers
  • arragement of grains within bedding layers
51
Q

what do features imparted to sediments aat or near deposition show

A

provide strong evidence about conditions at deposition

52
Q

what is bedding and stratification

A

sedimentary rocks are usually layered or stratified:
- arranged in planar, clase-to-horizontal beds
- the boundary between two beds is a bedding plane
- several beds together constitute strata
- a sequence of beds is called bedding or stratification

53
Q

beds have a definable what

A

thickness that can change

54
Q

bedding forms due to changes in

A
  • climate
  • water depth of deposition
  • current velocity
  • sediment source
  • sediment supply
55
Q

depositional changes vary what

A

the stacking of rock features:
- creates unique packages recognizable over a region
- this distinct rock package is called a formation: - formations are able to be mapped, formations are named for places they are best exposed

56
Q

geologic maps display what

A

the distribution of formations

57
Q

current deposition: water or wind flowing over sediment creates what

A

bedforms

58
Q

what is a bedform character tied to

A

flow velocity and grain size: ripple marks -cm scale ridges and throughs
- develop perpendicular to flow
- ripple marks are frequently preserved in sandy sediments
- found on modern beaches
- found on bedding surfaces of ancient sedimentary rocks

59
Q

what are dunes

A

similar to ripples but much larger:
- form from water or wind transported sand
- occur in streams and in desert or beach regions
- range in size from tens of cm to hundreds of m
- often preserve large internal cross beds

60
Q

how are cross beds created

A

by ripple and dune migration:
- sediment moves up the gentle side of a ripple or dune
- sediment piles up, then slips down the steep face (the slip face continualy moves downcurrent, added sediment forms sloping cross beds)

61
Q

what are turbidity currents and graded beds

A
  • sediment moves on a slope as a pulse of turbid water
  • as pulse wanes, water loses velocity and grains settle
  • coarsest material settles first, medium next, then fines

this process forms graded beds in turbidite deposits

62
Q

what makes the locations where sediment accumulate differ

A
  • cehmical physical and biological characterirsts
  • sediment delivery tranport and depositional conditions
  • energy regime
63
Q

when do bed surface markings occur

A

after deposition while sediment is still soft

64
Q

examples of bed surface markings

A
  • mudcracks: polygonal desiccation features in wet mud
  • scour marks: throughs eroded in soft mud by curent flow
  • fossils: evidence of past life: footprints, shell impressions
65
Q

locations where sediment accumulate

A
  • terrestrial
    -coastal
    marine
66
Q

where are the sedimetns deposited in terrestrial environments

A

above sea level

67
Q

glacial environments; deposit

A
  • glacial environments due to movement of ice: ice carries and dumps every grain size, creates glacial till; poorly sorted gravel, sand, silt, clay
68
Q

terrestrial environments: mountain stream environment

A
  • fast-flowing water carries large clasts during floods
  • during low flow these cobbles and boulders are immobile
  • coarse conglomerate is characteristic of this setting
69
Q

terrestrial environment: alluvial fan

A

sediments that pile up at a mountain front:
- rapid drop in stream velocity creates a cone-shaped wedgeg
- sediments become conglomerate and arkose

70
Q

terrestrial environments: sand-dune environments:

A

wind-blown, well sorted sand - aeolian deposition:
- dunes move according tot he prevailing winds
- result in uniform sandstones with gigantic cross beds

71
Q

terrestrial environments: river environments

A

channelized sediment transport:
- sand and gravel fill concave-up channels
- fine sand, silt, and clay are deposited on nearby flood plains

72
Q

terrestrial environments: lake

A

large ponded bodies of water:
- gravels and sands trapped near shore
- well sorte muds deposited in deeper water

73
Q

terrestrial environments: delta

A

sediment piles up where a river enters a lake

often topset, foreset, bottomset (glibert-type) geometry

74
Q

marine delta environments are depostied where

A

at sea level

75
Q

marine delta environments : delta

A

sediment accumulates where a river enters the sea:
- sediment carreid by the river is dumped when velocity drops
- deltas frow over time, building out into the basin
- much more complicated than simple lake deltas
- many sub-environments present

76
Q

marine environments: coastal beach sands

A

sand is moved along the coastline;
- sediments are constantly being processed by wave action
- a common result? well-sorte, well- rounded medium sand
- beach ripples often preserved in sedimentary rocks

77
Q

marine envrionments: shallow-marine clastic deposits

A

finer sand, silts, muds:
- fine sediment deposited offshore where energy is low
- finer silts and muds turn into siltstones and mudstones
- usually supports an active biotic community

78
Q

marin environment : shallow water carbonate environment

A

-most sediments are carbonates - shells of organisms
- warm, clear, amrine water, relatively free of clastic sediments
- protetced lagoons accumulate mud
- wave-tossed reefs are made of coral and reef debris
-source of limestones

79
Q

marine environments deep marine deposits

A

fines settle out far from land :
- skeletons of planktonic organisms make chakl or chert
- fine silt and clay lithifies into shale

80
Q

what is subsidence

A

sinking of the land during sedimentation

81
Q

how do sediments vary in thickness

A
  • thin to absent where nonsedimentary rocks outcrop
  • thicken to 10-20 km in sedimentary basins
82
Q

what are basins

A

special places that accumulate sediment

83
Q

where do basins form

A

where tectonic activity creates space

84
Q

rift basin:

A
  • divergent plate boundaries
  • crust thins by stretching and rotational normal faulting
    -thinned crust subsides
    -sediment fills the dwon -dropped trhoughs
85
Q

passive margins

A
  • continental edge far from plate boundary
  • underlain by crust thinned by previous rifting
    -thinned crust subsides as it cools
  • subsiding basin fills with sediment from rivers entering sea
86
Q

intracontinental bassin

A

interiors far from margins
- may be linked to failed crustal rifts
- continue to subside for millions of years after formation

87
Q

foreland basins

A

craton side of collisional mountain belt
- flexure of the crust from loading creates a downwrap
- fills with debris eroded off of the mountains
- fluvial, deltaic, and lake sediments fill foreland basins

88
Q

sedimentary deposition is strongly linked to what

A

sea level:

89
Q

changes in sea level do what

A
  • sea level rises and falls up to hundreds of meters: changes in climate, tectonic processes
  • depositional belts shift landward or seaward in response
  • layers of strata record deepening or shallowwing upward
90
Q

what is transgression

A

flooding due to sea level rise
- sediment belts shift landward ; strata “deepen” upward

91
Q

what is regression

A

exposure due to sea level fall:
- depositional belts shift seaward: strata “ shallow upward”
- regrssion tied to erosion: less likely to be preserved

92
Q

sea level rise and fall creates a predictable pattern T or f

A

T

93
Q

what is deep time

A

the immense span of geologic time

94
Q

who is the father of modern geology

A

james hutton scottish physician and farmer
- identified features in rocks that resembled features froming in modern sedimentray environments
- the first to articulate the principle of unifromitarianism

95
Q

what is principle of unifromitarianism

A

the presnt is the key to the past:
- processes seen today are the same as those of the past
- geologic change is slow: large changes require a long time
- therefore, there must have been a long time before humans

96
Q

what are the two ways of geological dating

A
  • relative ages: based upon order of formation
    -numerical ages: actual number of years snce an event
97
Q

what did sir charles lyell write

A

principles of geology:
- deciphering earth history
- established relative ages of materials

98
Q

principle of original horizontality

A
  • sediments settle out of a fluid by gravity
  • causes sediments to accumulate horizontally
  • sediment accumulation is not favored on a slope
  • hence, titled sedimentary rocks must be deformed
99
Q

principle of superposition

A

undeformed sequence of layered rocks: each bed is older than the one above and younger than the one below

young on top, old on bottom

100
Q

principle of lateral continuity

A
  • strata often form laterally extensive horizontal sheets
  • subsequent erosioin dissects once-continuous layers
  • flat-lying rockc layers are unlikely to have been disturbed
101
Q

principle of cross-cutting relations

A
  • younger features cut across older features
  • faults dikes erosion etc is younger than the material that is faullted, intruded, eroded
102
Q

principle of baked contacts

A
  • igneous intrusion cooks the invaded country rock
    baked rock is older
103
Q

principle of inclusions

A

rock fragment within another

  • inclusions are older than enclosing material:
    weathering rubble come from older rock, fragments are older than igneous intrusion
104
Q

fossils are often preserve in what type of rock

A

sedimentary rock

105
Q

principle of fossil sucession

A

frist appearance of a species, range and extinction are used for dating
- global extinctions are caused by extraordinary events
- fossils succeed one another in aknown order (evolution)
- time period is reecognized by its fossil content

106
Q

what is fossil range

A

first and last appearance
- each fossil has unique range
-range overlap narrows time

107
Q

what do fossils correlate strata:

A
  • locally
  • regionally
  • globally
108
Q

what is an unconformity

A

a time gap in the rock record from erosion or nondeposition

109
Q

three types of unconformity

A
  • angular unconformity
  • nonconformity
    -disconformity
110
Q

angular unconformity represents a huge gulf in time

A
  • horizontal marine sediments deformed by orogenesis
  • mountain eroded completetly
  • renewed marine invasion
  • new sediments deposited
111
Q

ex of angular unconformity

A

hutton’s unconformity: scotland
- vertical beds of ordovician sandstone
- missing time: 50 million years

112
Q

nonconformity

A

igneous/metamorphic rocks capped by sedimentary rocks

113
Q

not done Done diapo 71

A
114
Q
A

Mcgill Geology (5 decks)

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