Geology Flashcards
Lt. Matthew Fontaine Maury
1842-1870s Head of US Navy hydrographic office First marine geologist First deep marine bathymetric map (N Atlantic) MAR circa 1855 Telegraph Plateau Soundings for laying telegraph line
Challenger Expedition
1872-1876 Charles Wyville Thomson, prof @ Edinburgh convinced royal society of London to let them go Circumnavigated globe 362 stations 500 (492) soundings dredge, cored rock and sediments collected water samples measured temperature, salinity, currents
WWI
Echosounding helped to hear enemy subs
German meteor expedition
1925-1927 First to use continuous recording echosounding to study the seabed
Maurice Ewing
in 1948 Founded Lamont-Doherty Geological Observatory (LDGO) led to theory of plate tectonics
Bruce Heezen and Mary Tharp
Map of the entire ocean floor published in 1977
Alvin
Research Sub
built in 1964
Max depth 14,000ft (4000m)
Deep Sea Vent Communities
Discovered in 1977
Pacific
JOIDES
Joint Oceanographic Institues of Deep Earth Sampling
1960s-1980s
Glomar Challenger
Drilled 624 sites
confirmed valididty of seafloor spreading and plate tectonics
continued with Ocean Drilling Program (ODP) in 1985
Deepsea Drilling Project
1960s
started with Glomar Challenger
JOIDES Resolution
successor of the Glomar Challenger
operated in the International Ocean Drilling Program (ODP)
1985
Can drill 5 miles below ocean surface
500 wells
Integrated Ocean Drilling Program
IODP
2003-2013
multiple platforms
refurbished JOIDES Resolution, Chickyu
GPS
Fully operational in 1994
DOD funded for missile launches
degraded until 2000, then available to everyone
GLONASS
Russian Global Navigation Satellite system
incomplete coverage until about 2004
Satellites
have a clock set to exactly the same time and know their exact position
trasmits position and time signal
GPS recieves signal, delayed by distance travelled- difference is calculated and the distance to each satellite is calculated
for precise GPS location
need 4 satellites
the atomic clock is on the satellite, not on handheld.
the 4th satellite provides the atomic clock component
Sampling the bottom
grab sample
gravity core and Kasten core
Piston core
vibracore
Pneumatic hammer coring
rotary drill core
Grab sampler
Not representative of the bottom
gravity corer
top of the core gets disturbed, good for bottom seds
Kasten core
type of gravity core, but rectangular
has a liner
have to process on the ship
much larger sample and less disturbance
Piston Coring
method of choice
Freefalls a known distance dependent upon material
creates a vacuum right at sample at the surface so the core isn’t disturbed
Vibracoring
Deep water or shallow
vibrations liquify material and buries it
pneumatic hammer coring
tower of power
very efficient- 20 in one day
IODP coring
Riserless drilling- uses seawater to remove cuttings from the bottom to the top
Giant push core
wireline drilling
concrete re-entry cone
Riser drilling- similar to riserless, uses mud instead of water
casing around drill pupe pumps mud down hole, can go much deeper. Mud keeps the hole from collapsing
Closed system
Bathymetric and Side-Scan Sonar
Corrects for:
Salinity, instrument offset from GPS, boat movement, tide
swath is 10x water depth
“mowing the lawn”
sends out an array
depth is proportionate to swath size (shallow: narrow, deep: wide)
mosaiced together
Microbathymetry Sonar
have to move it around to stitch it together and get rid of shaddows
echodistance
determined by time
Distance = (2-way travel time X sound Velocity)/2
speed of sound in water
1500 m/s
variable depending on water pressure, temp, and salinity
Side scan data
won’t give you depth info
give you info about targets on seafloor- position and height above the bottom
will give you real time info about height of fish above the bottom
can be used for seafloor classification
has a swatch that’s not depth dependent, like a multibeam
low amplitude with smooth bottoms
Side Scan vs mutibeam
elevation vs details
multi channel seismic data
multiple hydrophones
each hydrophone is a channel
low frequency, they travel through the seafloor and reflect off different strata
can have multiples of the same reflection (look for twice the height of the water surface)
air gun data
lower frequency
lower resolution
but can see deeper into seafloor
Convergent Margin
single channel seismic data
used for high res studies of upper few meters of sediment
boomer with a hydrophone streamer
Chirper uses a swept frequency, hydrophone built in
best of both worlds, high res and deep penetration (ew)
hyperbolic trace or diffractions
generated by features that have dimensions comparable to the wavelength of the acoustic signal
end of a fault plane, rough topographic, boulders
sparker vs airgun
sparker attenuates faster than airgun
Vertical resolution
affected by frequency
1/4 lamda = vertical resolution
ex. dominant frequency is 1.5khz, velocity is 1500m/s
(1500/1500)/4 = 0.25m
Layers of Earth
Crust (5-70 km)
Mantle (2900 km)
Core (3486 km)
Earth is mostly mantle
Like a Ferrero Roche
P- waves
Primary waves
travel faster
pressure waves
shear waves
same direction as wave propogation
can travel through liquids and go faster when this occurs, but refracted by the core
compress like a slinky
S-waves
Secondary waves
perpendicular to wave propogation
cannot travel through liquids
squiggly like a slinky
Mohororvicic Disontinuity/ Moho
Between crust and the mantle
Changes is P-wave velocity
can get depth
Age of Earth?
4.6 Billion years
How did Earth form layers?
Single step
1) Cold Accretion of unsorted materials (homogenous) 500-800K
2) Solar Winds- contraction due to gravity, boiled away lighter elements
3) Heated up because of contraction, radioactive decay, and meteorite impacts- and different densities differentiated forming layers
Molten iron and nickel sank to the center
Ocean and atmosphere formed because of differentiation
What is Earth made up of?
Primarily Fe, O, Si, Mg
Radioactivity of young earth
5x more than present
frequency of meteorite impact?
every million years one as big as Kazahkastan meteor
Earth’s core is primarily made up of
Nickle and Iron
Controls Earth’s magnetic field
Earth’s mantle is made up of
Mostly Mg
Crust is composed of lighter materials
K, Na, Si
Differentiation
formation of magnetic field
formation of ocean and atmosphere
formation of layers
Convection Overturn
heat from the interior transferred to crust via convection
dissipates heat rapidly and it cools quickly
Where are oceanic ridges located?
above convection and upwelling in the mantle
deep ocean trenches
descending limb of convection currents
Alfred Wegner
developed theory of continental drift in 1915
supercontinent Pangaea
Evidence for continental drift
continental fit
fossil evidence
Rock sequences and mountain ranges
paleoclimatic- past glacial deposits
age of oceanic crust
older farther from MOR and younger near the middle
oldest is 180 million years old
Earth’s magnetic field
strongest near the poles
displaced ~11.5degrees from actual poles
Geodynamo theory
The magnetic field is generated in the liquid metal region of the outer core
flows for several km/yr
Convecting metal (Fe) creates electrical currents, which creates the magnetic field
paleomagnetism
when magma cools, iron bearing minerals align with Earth’s magnetic field
Seafloor spreading theory
Harry Hess in 1962- continental and ocean crust must move together
confirmed with geomagnetic reversals- normal and reverse
reversals around MOR
should be symmetrical
thickness of sediments decrease near
Ocean ridges
age of oldest ocean crust
less than 180 million years
age of oldest continental crust
3.96 billion
plates move
1-10cm per year
begins with subduction, causes divergent zones (current theory)
Divergent plate boundaries
constructive margins
ocean ridges and seafloor spreading
have axial magma chambers
What is the longest topographic feature on Earth’s surface?
Mid Ocean Ridge
70,000 km
continental rifting
divergent plate boundaries that develop within a continent
landmass splits in two
ex. East africa rift/Red sea
How the oceans were formed
Convergent boundaries
two plates move together
ocean lithosphere goes under plate
eventually re-absorbed into the mantle or collision b/n two continental blocks- mtn building
create deep trenches, slumps
deep ocean trench is how deep?
8-12km
Transform fault boundaries
where two plates grind past each other without production or destruction of lithosphere
accommodates for sphere shape of Earth
Fracture Zones
include active transform faults and inactive extensions into plates interior
EX. San Andreas
Steno’s Laws
1) Law of Horizontality
2) Law of Superposition
3) Law of Lateral Continuity
Not Steno, but still important…
4) Law of Cross-Cutting Relationships
5) Principle of Inclusion
Law of Horizontality
Beds of sediment deposited in water form as horizontal (or nearly horizontal) layers due to gravitational settling.
Law of Superposition
In undisturbed strata, the oldest layer lies at the bottom and the youngest layer lies at the top.
Law of Lateral Continuity
Horizontal strata extend laterally until they thin to zero thickness (pinch out) at the edge of their basin of deposition.
Law of Cross-Cutting Relationships
An event that cuts across existing rock is younger than that disturbed rock. This law was developed by Charles Lyell (1797-1875).
Principle of Inclusion
Fragments of rock that are contained (or included) within a host rock are older than the host rock.
Unconformities
A surface that represents a very significant gap in the geologic rock record (due to erosion or long periods of non-deposition).
Steps to form an ocean basin
- Extension and formation of normal faults
- Extension and uprise of upper mantle which melts due to heat from increased pressure and volcanism (creates magma)
- Extension, crustal thinning and formation of a graben
- Rifting apart of continents and formation of new ocean crust (MORB- mid ocean ridge basalt)
- Seafloor spreading and rift sequence is buried
Disconformity
A contact representing missing rock between sedimentary layers that are parallel to each other. Since disconformities are parallel to bedding planes, they are difficult to see in nature.
horst and graben
graben- low place where sediment is deposited during continental rifting
horst- block that’s been pushed up
Hoarst and grabben are blocky
Why are rift margins important to oil companies?
they’re prime places for the buildup of Carbon and formation of oil and gas
How Salt domes form
Tons of organic matter deposited via rivers on the shelf
then salt deposited on top of that forming a cap
Then more sediment is buried on top, but salt is low density so it rises through sediment layers towards the surface- pathway through hydrocarbons can move
Nonconformity
A contact in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock.
Paraconformity
A contact between parallel layers formed by extended periods of non-deposition (as opposed to being formed by erosion). These are sometimes called “pseudounconformities”).
Where did the water come from?
From within! with a little help from comets (10-20%)
The Earth outgassed and condensed as the Earth cooled after formation (80-90%)
Offlap
Progradation into deeper water
Stratal Patterns (Slug Diagram)
How much water came from comets?
10-20%
When did the oceans form?
~4 billion years ago
Offlap break
A break in the slope on the depositional clinoform, most often occurring at fairweather wave base.
About equal to the shoreline, indicator of base level activity
separates sediments deposited in shallow water from those deposited in deep water
Eustatic Sea Level
Relative to a fixed datum
Global level that includes tectonics (shape of the bathtub) and volume of water (filling the bathtub)
Longterm changes (thousands to millions of years)
NOT RELATIVE SEA LEVEL <–more localized
What is the continental crust made up of?
Si, and granite
What is the oceanic crust made up of?
Basalt and Mg
Sir John Murray
Published 50 volumes of data from HMS Challenger Expedition
23 years to publish
Geoid
The equipotential surface of the Earth’s gravity field that fits best, in a least squares sense, global mean sea level.
Irregular shape
Make measurements to the ellipsoid (regular shape), then adjust to fit the geoid
In the Archaen
Large meteorite impacts ceased (much lower frequency)
cooling allowed crustal blocks to form
amount of continental crust increased
series of collisions of mini continents making fewer but larger continents
Ophiolite
consists of layers representing part of the upper mantle and the oceanic crust
Key to detecting old subduction zones
Ophiolite Sequence
Pillow basalt
Gabbro- amphibole
basaltic dykes
peridotite- mantel rock didn’t melt all the way
mantle is primarily composed of conglomerates known as…
Peridotite
zenoliths
zenoliths
hitches a ride with the melt
Blueschist
subduction of oceanic crust
first appeared in proterozoic
Isostatic Sea Level
More localized
Affected by changes such as glacial rebound
when did plate tectonics start to occur and how do we know?
late proterozoic (1000-545 mya)
ophiolites, first appearrance of bluescists and high pressure metamorphic rocks
What can gravity tell us?
Structures that exist on the boundaries between oceans and continents
dimensions of MOR magma chambers
the presence and dimensions of offshore sedimentary basins
processes that lead to rifting and formation of ocean basins
Causes of sea level change
Tectonic changes (10-100 Ma; 10-100 m)
Glacial melting/freezing (100 ka; 100 m)
Water storage on continents (lakes, groundwater) (1 day-1 year; 1 cm-1 m)
Temperature (100-1000 years; 1 cm - 1 m)
Gravity anomalies
Difference between measured and expected values
1) expect increase with latitude- g(lat)
2) expect decrease with increasing elevation above sea level- g(fe)
3) expect increase due to the mass of rock between sea level and observation point- g(Boug)
G = g(lat) + g(fe) + g(Boug)
change in g = g(measured) - g(predicted)
When was the highest historic sea level?
~95 mya
Late Cretaceous (the WIS!)
Boug
Changes in density of rock between you and the center of the earth
fe
Free air
has to do with elevation
Sequence Chronostratigraphy (record of sea level changes)
Regressions (basinward movements) are shown to happen really quickly, but is an artifact of the data. The shelf is commonly eroded and open when sea level is low, creating disconformities in the stratigraphy (erases record). Sea level fluctuations began to increase around 37 Mya when Antarctica became glaciated (moved to the south pole).
Why does the MOR have relief?
Because it’s hot, the relief lessens as it cools
What can spreading rates do to sea level?
Fast- increase sea level
Slow- decrease sea level
Last Glacial Maximum
18,000 years ago
Sea level was 125m lower than today
Laurentide ice sheet (3-4 km thick)
How is subsidence increased?
Increasing thickness of sediments and decreasing temperature
What is the most important mode of heat transfer?
Advection of water through rock
How long does it take the entire volume of the oceans to circulate through the crust at spreading ridges?
10 million years
Cesare Emiliani
In 1955 discovered that fluctuations in the oxygem isotope composition of forams in deep sea cores record glacial and interglacial stages.
Oxygen isotope ratio
geologic proxy for sea level change
Oxygen has two stable isotopes, 18 (0.2%) and 16 (99.8%)
Rainfall and ice are very depleted in 18 because 16 evaporates from ocean easiest
Oceans are heavier in 18 when lots of ice/glaciers
benthic forams record ocean water composition in shells
Orbital Eccentricity
The degree to which Earth’s orbit departs from a circle
100,000 year cycle
Axial Tilt
The angle between Earth’s axis and a line perpendicular to the plane of the ecliptic shifts, about 1.5 degrees from its current value of 23.5.
41,000 year cycle
Precession of the equinoxes
Changes in the timing of the equinoxes resulting for a wobble in the Earth’s axial tilt.
22,000 year cycle
Glacial Isostatic Adjustment (GIA)
Glaciers created fore bulges and a depression
loss of heavy mass means bulges go down and middle comes back up
Using corals for paleo-sea level reconstruction
coral data and isotope data agrees
also basal peat data (louisiana) agrees with corals (gulf of mexico)
Increase in rate of sea-level rise
Risen from 0.82 +- 0.02 mm/yr 4,000 years ago to today at 2.8 mm/yr over the last century.
Formation of continents
During the Archean (4.0-2.5 Bya)
intense meteorite impacts, cooling allowed crustal blocks to form, low density first (granites, silicates)
Clinoform
regressive
seaward movement of shoreline
incisions in sequence boundary means erosion from exposure from low sea level, river channels
can be with stable (offlap break moves out, but not up or down), rise (offlap break moves out and up), or fall (offlap break moves out and down) of sea level
Sediments coarsen upwards
transgressive
landward movement of shoreline
with SLR sequence backsteps with sediment supply (offlap break moves up and in)
Stable SL shoreline transgression is not preserved (eroded by waves) or with rising sea level but no sediment supply
sediments fine upwards
Low stand
sedimentation moves basinward, creation of deep sea fans
High stand
Sedimentation moves landward, deep sea sedimentation is more pelagic, no erosion from exposed shelves.
Passive Margin Morphology
Continental Shelf- 75 km wide 0.1⁰
Continental Slope- 10-100 km wide, 4.0⁰
Continental Rise- 0-600 km wide, 0.05⁰-0.6⁰
Abyssal Plain- 0.05⁰
Canyons are conduits for sediment flows
Abyssal Plain
Flat, nearly level
Thick sediments blanketing the rugged ocean crust
Sedimentation through lateral movement or snowing down
Sediments thicken towards the margins
Very little mixing because of no waves, but there are distal turbidite deposits
Hot Spots
Stationary in upper mantle
mantle plumes burn through plates while plates move over top
E.g. Hawaii, Iceland, Yellowstone
Indicate movements of plates
Atolls
Volcano–> fringing reef–> barrier reef–> atoll–> guyot–> seamount
reef gets larger, is bigger at atoll stage than fringing
islands subside due to cooling
reefs affected by sea level and erosion
Sediment locations
Neritic: Shoreface or shelf
Pelagic: fine-grained accumulating in open ocean far from land through settling particles
Hemipelagic: >25% of the fraction coarser than 5 um is of terringenous volcanogenic and/or neritic origin
Types of sediments
Terrigenous, biogenic, authigenic, volcanogenic, cosmogenic
Terrigenous sediments
Produced by weathering and erosion of rocks on land
Transported by rivers, wind (dominant), and icebergs
Biogenic sediments
CaCO3 and SiO2 oozes composed of hard parts of organisms
Fine grained, make up <30% of pelagic sediments
controlled by productivity and dissolution
Ocean is never saturated with SiO2, highest deposition in areas of upwelling and high productivity
CaCO3 oozes cover about 50% of the ocean floor
CCD
Calcite Compensation Depth
The depth where the rate of supply and dissolution of CaCO3 are in balance
Below CCD Calcite dissolves
Aragonite dissolves at shallower depths than calcite
CCD is more shallow in the Pacific (4200-4500 m) than Atlantic (5000 m) because of higher CO2 (old water)
Buried CaCO3 (forams) do not dissolve
Mechanisms for deep-sea sedimentation
Settling from water column
Bottom transport by gravity flows
Transport by geostrophic bottom currents
Chemical and biochemical precipitation on the ocean floor
Authigenic Sediments
formed by precipitation of minerals in seawater
manganese nodules are concentric layers accreting 1.7-8.7 mm per million years
Lysocline
The depth at which the rate of calcite dissolution rapidly increases.
Due to increased corrosiveness of water
High pressure, low temperature, high CO2
Volcanogenic sediments
Stuff ejected from volcanoes
Ash and tephra
Can use as a dating marker, creates a layer in the sediment record
only helpful near volcanoes
Cosmogenic Sediments
Pieces of meteorites that survive the trip through the atmosphere
microtektites
small glassy meteorites
not a big component at all
Sediment thickness is controlled by what?
Rates of deposition of Lithogenic, biogenic, abyssal clays, and Mg nodules?
Lithogenic: 1 m/1000 years
Biogenic: 1 cm/1000 years
Abyssal clays: 1mm/1000 years
Manganese nodules: 0.001 mm/1000 years
Controlled by age of underlying crust, tectonics, structure of basement, nature and location of sources, sediment delivery process.
Distribution and average thickness of marine sediments
High latitude shelf topography
Very deep
rugged topography
landward sloping profile
glacio-isostatic imbalance
Accounts for 150m of the continental shelf depression at the grounding line (~25%)
When was the Messinian Salinity Crisis?
End of the Miocene, about 5 Mya
How much of the Antarctic ice sheet discharges into the Ross Sea
25%
What’s the average shelf depth in Antarctica
500m
What was the Messinian Salinity Crisis?
Global salinity dropped
Mediterranean Sea was isolated from the rest of the ocean
1 million km3 of evaporites deposited over
Took about 1000 years to dry up, 700k years to form
Shoreline moved basinward rapidly, noticeable at human time scales
Evaporated ~40 times
What’s the maximum shelf depth in Antarctica
1200m
Is the inner shelf or outer shelf deeper at high latitudes
inner shelf
Why are glaciers difficult to deal with?
Hard to date
They’re efficient at removing the sedimentary record
map geologic units around the continent
Messinian Crisis setting
20 Mya Arabian plate impinged upon the Eurasian plate blocking connection to the Indian Ocean
Connection to Atlantic temporarily closed when Africa moved North
Led to drier climate in entire region
5 Mya Combined effects of uplift in west
sea levels fell, not all tectonic driven
Evaporite history varies per basin in the Mediterranean
drumlins
streamlined hills made of till; steeper on the side from which the ice came
Evaporite deposition sequence
CaCO3–> CaSO4 (gypsum)–> NaCl–> K and Mg
How much of the west antarctic ice shee (WAIS) drains into Pine Island Bay
1/3
Mega Scale glacial lineations
corrugation ridge formation
sediment squeezed into ridges by the trailing edge of a portion of a broken up ice shelf rising and settling to the sea floor under tidal influence as it drifts seaward
How much evaporites are produced from 1000 m of seawater?
15 m
Evaporiates turn into…
Salt diapirs, domes, and turtle structures
Low density so they migrate up in high pressure areas
Evidence for the Messinian Crisis
Rivers cut canyons on the shelf when dried, velocity increased, and eroded the steeper slope.
Heinrich Event
Armadas of the N Atlantic
huge influx of iceburgs
More lithic during heinrich events, more forams between heinrich events
What did Heinrich events due to thermohaline circulation?
Shut it down
How large were iceburgs during heinrich events?
keel depths 50-310m
megaburgs were >650m
What happens when 6% of the Earth’s salt is removed?
Lower freezing point of water
More sea ice
lower sea level
moves CCD up because CaCO3 saturation lowers
Refilling the Mediterranean
Sea level rose >10 m a day
90% of water transferred in a short period ranging from months to two years
Discharge of ~10^8 m3 s-1 (3-orders of magnitude larger than the present Amazon River).
Walther’s Law
Depositional environments beside each other in map view will be superimposed on top of each other in a conformable vertical succession of strata.
where do glaciers form?
precipitation as snow, snow must accumulate
high latitutes and high elevations
ice is flowing
efficient agent of erosion
interrupts hydrologic cycle by locking up water
Valley glaciers
colder conditions in high altitude mtns keep snowfall from melting away during summer
Found in mountainous areas
lengths greater than widths
only cover a small region
transform V-shaped river valleys to U-shaped
Ice Sheets
continental glacier
covers vast areas and are unconstrained by underlying topography
Greenland, WAIS, EAIS
Found in polar regions
Facies
The aspect, appearance, and characteristics of a rock unit, usually reflecting the conditions of its origin.
Transitions between subenvironments
May shift so that the deposits of an adjacent environment lies directly on top of a laterally related environment.
Use modern environments to evaluate facies
Glacier movement
Gravity primary force
Entire ice sheet moves 5-50 m/yr
Plastic flow and basal slip
fastest movement in the center
friction slows down the slides
Zone of accumulation
snow accumulates and forms ice
Zone of ablation
general term for loss of ice or snow from a glacier
sublimation, melting, evaporation, calving
Controls on facies
- Sedimentary processes
- Sediment supply
- Climate
- Tectonics
- Sea level change
Glacial budget
Delta
Delta is at the end of a river and is the sedimentary record of deposition into deeper water
plucking
loosen and lift blocks of rock- mechanical weathering
Abrasion
sediment in ice acts as giant sandpaper
creates rock flour and striations
Rock flour
very fine-grained material
Striations
grooves scratched in bedrock that indicate direction of ice movement
Drift
general term applied to any deposit associated with glaciers
Till
sediment deposited directly from melting ice; till is unsorted and massive
Stratified drift
deposits from glacial meltwater streams
Moraines
ridges made of till that form at margins of a glacier
End (terminal) moraine
forms at the bottom end of glacier
lateral moraine
forms at side of glacier
Medial moraine
when two glacial valleys merge
What is sequence stratigraphy?
A method to impose the dimension of time on the relationships of rock units in space (area and depth)
By understanding how rock units are related in time and space, we can better interpret how they are connected.
Basically transgressive and regressive cycles
East Antarctic Ice Sheet
Continental ice sheet
Ice Stream
corridors of fast flow within an ice sheet
They discharge most of the ice and sediments from ice sheets
fed by tributaries that extend up to 1000km into interior of ice sheet
Ice shelf
Floating platform of ice
forms where ice sheet flows onto ocean surface
thickness ranges from 100-1000m
NOT sea ice
Grounding line
The boundary between floating ice shelf the grounded ice that feeds it (resting on rock)
What is limestone composed of
CaCO3
Calcite, Aragonite
control of carbonate sediment production
Temperature- warm water (18-36 degrees C)
Salinity- normal salinities (27-40 ppt)
Light intensity- abundant light (shallow water), clear water
What kind of slopes can carbonates hold? Why?
Steep
they bind themselves together much more easily- lithify into rock
Clinoforms
Topset <0.1 degree, alluvial, deltaic, and shallow marine
Foreset >1.0 degree deeper water depositional processes
Bottomset, low gradients, deep water depositional systems
Offlap break, break in slope between topset and foreset
Controlled by the rate of sediment supply vs. rate of creation of accommodation space on the shelf.
Carbonate banks are isolated platforms that are surrounded by deep or shallow water? Do they receive terrigenous clastic supply?
Deep
Yes
Carbonate atoll is a type of carbonate bank formed above what?
A subsiding volcanic island
Isolated Platforms
Have flat tops, steep sides
can be several km thick
can extend over many 100s of km^2
Margins are strongly influenced by wind-driven currents- usually grow higher than reef interior
Base level
The level above which deposition is temporary and erosion occurs
Is an imaginary surface
everything above erodes, below deposits
Platform Evolution
Response to sea level rise
Accommodation
The space available for sediment to accumulate at any point in time. Below base level, above sed surface. Controlled by base level.
Δ accommodation = Δ eustasy + Δ subsidence + Δ compaction.
Δ water depth = Δ eustasy + Δ subsidence + Δ compaction - sediment deposited.
Keep-up reefs
maintain their crests at or near sea level
catch-up reefs
either began as shallow reefs that get deeper when they couldn’t keep up with slr, but then later grew fast OR
started deep and quickly grew upward
Depositional Sequence
Cycle of deposition bounded above and below by erosional unconformities (disconformities).
Controls on durations of sequence
Creation and destruction of accommodation
Tectonics, subsidence, and eustacy
Prograding deposits
Once builtup to sea level, reefs can only grow through prograding
Sediment supply
The rate controls both how much and where accommodation is filled.
Rivers are the principle means of transporting material from the continental interior to the depositional basin.
Give-up reefs
first grew as others did but stopped b/c changes in envrionmental conditions ex.
dropping below the photic zone
ooids
spherical carbonate grains
requires carbonate supersaturation- presence of nuclei and agitation
Usually in shallow depths b/c of wave agitation
Rimmed Carbonate shelf
Energy decreases from outer shelf to shoreline- reefs and CaCO3 sand bodies occur along high energy shelf margin, restricting circulation on the shelf lagoon
Debris from rimmed-shelf margin is shed onto adjacent slope and into the basin
Ex. Belize, Queensland, Australia, Florida
Controls on volume and types of sediments
- Hinterland physiography
- Tectonics
- Climate
- Drainage basin area
- Erosion rate (climate, relief, rock type)
Spur and Groove
passive response of corals to wave action near edge of a platform
Progradation
- Occur when sediment supply exceeds the rate of creation of accommodation space on the shelf.
- Basinward migration of the offlap break.
- Regression - basinward movement of the shoreline.
black mangroves
occupy drier ground
pneumatophores
roots that turn around and come back out at the surface
Once carbonate becomes subaerial
desiccation occurs
Carbonate ramp
gently sloping to deeper water with no break in slop
shoreline might be a beach barrier-tidal delta complex with lagoons and tidal flats behind, or a beach-ridge/strandplain system
Aggradation
- Occur when sediment supply and rate of creation of accommodation space on the shelf are roughly balanced.
- Facies belts stack vertically, offlap break does not migrate landward or basinward.
Retrogradation
- Occur when sediment supply is less than the rate of creation of shelfal accommodation space.
- Facies belts migrate landward.
Former offlap break becomes a relict feature
Submarine Fans and Canyon-Channel System
Sediment transfer zone b/n terrestrial source area and deep-sea depositional sink
Classic slug diagram
Changes in base level and sediment supply through time creates these types of repeating patterns.
How often to deep sea fans get sediment
Not continuously
during mass wasting events
gravity drive
Deep sea fan morphology
Radial, cone, fan
Canyon -channel system
Climatic forcing of deep sea fan formation
subglacial meltwater
monsoonal pulses
Information gained from deep sea fan sedimentology
tectonic formation
climate change
erosion
How much of the global burial of organic carbon is the Bengal Fan responsible for?
10-20%
Slide
large intact blocks moving on a well-defined slippage plane
Slumps
break up into smaller blocks and exhibit some internal deformation of original bedding
Debri flows and turbidity currents
sedimetary gravity flows
mixtures of sediment and water
Turbidites
A turbidite is the geologic deposit of a turbidity current, which is a type of sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean.
What does a turbidite sequence look like?
Fining upward
(not like cereal, more like nuts)
Too much sediment for accomodation space
deposition occurs past the slope and onto the deep sea fan
Contourites
Sediment deposite by contour currents or thermohaline-induced deep water bottom currents and geostrophic currents
Continental rise to lower slope
Horizontal Resolution
affected by trigger rate, speed of boat, wavelength of source
Maurice Ewing’s 5 divisions of Lamont Geological Survey
- Bathymetry
- Siesmics
- Gravity
- Heat Flow
- Magnetics
