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
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
3
Q
what do sedimentary rocks cover
A
underlying basement rock
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
5
Q
what provides the raw material for all sedimentary rocks
A
physical and chemical weathering
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)
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)
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
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
10
Q
what is the coarsed-grained sed (boulder, cobble, pebble)
A
gravel
11
Q
what is the fine-grained (silt and clay)
A
mud
12
Q
clastic sedimentary rocks can be made out of
A
individual minerals or rock fragments
13
Q
the composition of the clastic sedimentary rock says what
A
the story about the original source rock
14
Q
what is angularity
A
the degree of edge or corner smoothness
15
Q
what is sphericity
A
degree to which a clast nears a sphere
16
Q
sphericity and angularity of fresh ditritus
A
angular and nonspherical
17
Q
when does grain roundness and sphericity increase
A
with transport:
- well rounded: long transport distance
angular: negligible transport
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
19
Q
when does sorting occur
A
size sorting occurs along stream flow
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
21
Q
what are the most common cements
A
quartz and calcite
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
23
Q
example of coarse clastics - gravel sized clasts
A
breccia, conglomerate
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
25
coarse clastics: sand and gravel sized clasts
arkose
26
what is arkose
sand and gravel with abundant felspar,
- commonly deposited in alluvial fans
- feldspar indicates short transport
27
sandstone: clastic rock made of sand-sized particles
- common in beach and dune settings
- quartz is by far the most common mineral in sandstones
28
where are fine calstics deposited
in quiet water settings: floodplains, lagoons, mudflats, deltas, deep-water basins
29
silt, when lithified becomes what
siltstone
30
mud when lithified becomes what
mudstone or shale
31
what is lithification
to make into rock
32
what is biochemical sedimentary rocks
sediments derived from the shells of living organisms
- hard mineral skeletons accumulate after death
33
-different sedimentary rocks are made from these materials (biochemical sedimentray rocks)
- calcite and aragonite - limestone
- silica - chert
34
what is limestone
sedimentary rocks made of CaCO3
ex:
- fossiliferous limestone: contains visible fossil shells
- micrite: fine carbonate mud
- chalk: made up of plankton shells
35
what is chert
rock made of cryptocrystalline quartz: Opal
36
how is chert formed
- 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
37
what are organic sedimentray rocks made of
made of organic carbon, the soft tissues of living things
38
what is coal
organic sedimentray rock:
altered remains of fossil vegetation
- black, comustible
- over 50-90% carbon
- has fueled industry since the industrial revolution began
39
what are chemical sedimental rocks comprised of
minerals precipitated from water solution:
- have a crystalline (interlocking) texture:
- initial crystal growth in solution
- recrystallization during buriala: neocrystalization
40
what are the classes of chemical sedimentary rocks
- evaporites
- travertine
- replacement chert
- dolostone: the hard cap of the niagara escarpment
41
what are evaporites
rock from evaporated sea or lake water (chemical sedimentary rock)
42
how are evaporites formed
- evaporation triggers deposition of chemical precipitates
- thick deposits require large volumes of water
- evaporite minerals include halite (rock salt) and gypsum
43
what does salt tell us
of past water
44
what is travertine
calcium carbonate precipitated from ground water where it reaches the surface
45
how are travertine crerated
CO2 expelled into the air causes CaCO3 to precipitate
- ex: thermal springs
- caves: speleothems
46
what is dolostone
limestone altered by Mg-rich fluids
- CaCO3 altered to dolomite by Mg2+ rich water
- diagenisis, dolomitization
47
what is diagenesis
physical, chemical and biological changes to sediment
(lithification is one aspect of diagenesis)
48
how does diagenesis come about
- 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
what is a replacement of chert (nonbiogenic)
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
what are some features imparted to sedimetns at or near deposition
- bedding and stratification
-surface features on bedding layers
- arragement of grains within bedding layers
51
what do features imparted to sediments aat or near deposition show
provide strong evidence about conditions at deposition
52
what is bedding and stratification
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
beds have a definable what
thickness that can change
54
bedding forms due to changes in
- climate
- water depth of deposition
- current velocity
- sediment source
- sediment supply
55
depositional changes vary what
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
geologic maps display what
the distribution of formations
57
current deposition: water or wind flowing over sediment creates what
bedforms
58
what is a bedform character tied to
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
what are dunes
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
how are cross beds created
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
what are turbidity currents and graded beds
- 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
what makes the locations where sediment accumulate differ
- cehmical physical and biological characterirsts
- sediment delivery tranport and depositional conditions
- energy regime
63
when do bed surface markings occur
after deposition while sediment is still soft
64
examples of bed surface markings
- 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
locations where sediment accumulate
- terrestrial
-coastal
marine
66
where are the sedimetns deposited in terrestrial environments
above sea level
67
glacial environments; deposit
- glacial environments due to movement of ice: ice carries and dumps every grain size, creates glacial till; poorly sorted gravel, sand, silt, clay
68
terrestrial environments: mountain stream environment
- 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
terrestrial environment: alluvial fan
sediments that pile up at a mountain front:
- rapid drop in stream velocity creates a cone-shaped wedgeg
- sediments become conglomerate and arkose
70
terrestrial environments: sand-dune environments:
wind-blown, well sorted sand - aeolian deposition:
- dunes move according tot he prevailing winds
- result in uniform sandstones with gigantic cross beds
71
terrestrial environments: river environments
channelized sediment transport:
- sand and gravel fill concave-up channels
- fine sand, silt, and clay are deposited on nearby flood plains
72
terrestrial environments: lake
large ponded bodies of water:
- gravels and sands trapped near shore
- well sorte muds deposited in deeper water
73
terrestrial environments: delta
sediment piles up where a river enters a lake
often topset, foreset, bottomset (glibert-type) geometry
74
marine delta environments are depostied where
at sea level
75
marine delta environments : delta
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
marine environments: coastal beach sands
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
marine envrionments: shallow-marine clastic deposits
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
marin environment : shallow water carbonate environment
-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
marine environments deep marine deposits
fines settle out far from land :
- skeletons of planktonic organisms make chakl or chert
- fine silt and clay lithifies into shale
80
what is subsidence
sinking of the land during sedimentation
81
how do sediments vary in thickness
- thin to absent where nonsedimentary rocks outcrop
- thicken to 10-20 km in sedimentary basins
82
what are basins
special places that accumulate sediment
83
where do basins form
where tectonic activity creates space
84
rift basin:
- divergent plate boundaries
- crust thins by stretching and rotational normal faulting
-thinned crust subsides
-sediment fills the dwon -dropped trhoughs
85
passive margins
- 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
intracontinental bassin
interiors far from margins
- may be linked to failed crustal rifts
- continue to subside for millions of years after formation
87
foreland basins
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
sedimentary deposition is strongly linked to what
sea level:
89
changes in sea level do what
- 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
what is transgression
flooding due to sea level rise
- sediment belts shift landward ; strata "deepen" upward
91
what is regression
exposure due to sea level fall:
- depositional belts shift seaward: strata " shallow upward"
- regrssion tied to erosion: less likely to be preserved
92
sea level rise and fall creates a predictable pattern T or f
T
93
what is deep time
the immense span of geologic time
94
who is the father of modern geology
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
what is principle of unifromitarianism
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
what are the two ways of geological dating
- relative ages: based upon order of formation
-numerical ages: actual number of years snce an event
97
what did sir charles lyell write
principles of geology:
- deciphering earth history
- established relative ages of materials
98
principle of original horizontality
- 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
principle of superposition
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
principle of lateral continuity
- strata often form laterally extensive horizontal sheets
- subsequent erosioin dissects once-continuous layers
- flat-lying rockc layers are unlikely to have been disturbed
101
principle of cross-cutting relations
- younger features cut across older features
- faults dikes erosion etc is younger than the material that is faullted, intruded, eroded
102
principle of baked contacts
- igneous intrusion cooks the invaded country rock
baked rock is older
103
principle of inclusions
rock fragment within another
- inclusions are older than enclosing material:
weathering rubble come from older rock, fragments are older than igneous intrusion
104
fossils are often preserve in what type of rock
sedimentary rock
105
principle of fossil sucession
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
what is fossil range
first and last appearance
- each fossil has unique range
-range overlap narrows time
107
what do fossils correlate strata:
- locally
- regionally
- globally
108
what is an unconformity
a time gap in the rock record from erosion or nondeposition
109
three types of unconformity
- angular unconformity
- nonconformity
-disconformity
110
angular unconformity represents a huge gulf in time
- horizontal marine sediments deformed by orogenesis
- mountain eroded completetly
- renewed marine invasion
- new sediments deposited
111
ex of angular unconformity
hutton's unconformity: scotland
- vertical beds of ordovician sandstone
- missing time: 50 million years
112
nonconformity
igneous/metamorphic rocks capped by sedimentary rocks
113
not done Done diapo 71
114
