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Understanding Earth Science > Sedimentary > Flashcards

Flashcards in Sedimentary Deck (33):
1

Uses for the study of sedimentary rocks?

The search for hydrocarbons (oil and gas) is an obvious use for sedimentology, but this is not the only use. The search for water and aquifers is dependent on the sedimentology and specifically the pore space (porosity)

Other uses for sediments/sedimentary rocks
BIF’s: Band Ironstone Formation – source of iron ore
Mining for either deep coal or open cast (growth in India and China)
Search for placer deposits such as diamonds and precious stones
Aggregates – limestone sandstone, cement etc……..

2

Sediment Yields?

the total amount of sediment exported from a drainage basin in a given period of time (Km-2 a-1)
The sediment is commonly referred to as clastic, because it consists of individual clasts (grains, pebbles, boulders etc).
Sedimentary rocks derived from this sediment are referred to as clastic rocks (in decreasing grain size: conglomerates, gravels, sandstones, siltstones, mudstones, clays). Limestones and evaporites are different

3

Trends of sediment concentration?

globally the largest sediment supply is associated with the World’s largest rivers draining large actively uplifting Mountain belts.
• Higher in Asia due to altitude
o Himalayas
o Orogenic belt
• Lower in Africa = craton

4

Types of sediment fall?

Mass wasting: downslope movement of soil or rock under gravity (slow – creep)
Landslide: rapid downward and outward movement of soil or rock

5

Basins? Types?

Bowl shaped depressions where sediment accumulates
Main control is tectonics which form them and allow for subsidence
Rift Basin - These basins form where the continental crust is being stretched. During the early phases of rifting the Earth’s surface subsides because it thins as it is stretched.As the rift grows, slip on the border faults drops blocks of crust down, produc-ing low areas and narrow mountain ridges - East Africa Rift

Intracractonic Basin - These tend to develop in the interiors of continents away from tectonic plate margins and mountain belts. They have generally round or oval shapes, and have long geological histories of relatively slow subsidence. One hypoth-esis on their formation is that they are thought to form over areas of previous rifting. When rifting stops, the once hot nd stretched crust starts to cool, contracts and sinks. This sinking is known as thermal sag.

Passive Margin Basin - These form along the margins of con-tinents that are not tectonic plate boundaries. They are usually underlain by a former rift with oceanic crust. Passive margin basins occur because long after the rifting has ceased, the thermal relaxation and subsidence continues. - Gulf of Mexico

Foreland Basins - Foreland basins are associated with regions of compressional tectonics and form adjacent and parallel to mountain belts. They are formed primarily as a result of the downward flexing of the lithosphere in response to the weight of the adjacent mountain belt, though many geological and geodynamic processes combine to control their subsequent evolution. The foreland basin receives sediment that is eroded off the adjacent mountain belt, filling with thick sedimentary successions that gradually decrease in thickness away from the mountain belt. - Persian Gulf

Strike-slip basins - Pull apart basin in areas of strike slip faults - San Andreas

6

Basics of sediments?

• Sediment is the collective term for loose fragments of rocks or minerals formed from erosion and or from direct precipitation
• Initially unconsolidated
• Transportation sees weathering and change of character of sediment
• Rounding
• Sorting
Wentworth classification - based on grainsize - conglomerate, sandstone, siltstone, claystone
Phi is the –log2d where d is grain size in mm.

7

Types of sediments?

Clastic sedimentary rocks
 Conglomerates and breccia (>2mm)
 Sandstones
 Mudrocks, mudstones, shales (>50% silt or clay size)

Biogenic sediments: a sedimentary rock formed of skeletal fragments of calcite or aragonite and commonly known as a limestone. However, the direct precipitation of calcium carbonate from warm supersaturated, shallow highly agitated marine waters can leads to ooids and oolitic limestone.

Organic sedimentary rocks: form from the accumulation and lithification of organic debris, such as leaves, roots, and other plant or animal material. Good examples include coal, oil shales and shale gas. Bioclastic limestone, and skeletal limestone are also technically organic sedimentary rocks but are usually grouped with the other limestones as being biogenically precipitated.

Chemogenic sediments: Chemical sedimentary rock forms when mineral constituents in solution become supersaturated and inorganically precipitate. Common chemical sedimentary rocks include evaporites such as gypsum, halite, baryte, sylvite

Volcanicastic sediments

8

Facies and Walthers Law

individual layers or strata change their charac-teristics laterally: they are not uniform along their entire extent and undergo a facies change. A faciesis a distinctive packet of sedimentary rock with specific sedimentary features that distinguish it from other facies

Walther’s Law states that the vertical succession of facies reflects lateral changes in environment. Conversely, it states that when a depositional environ-ment ‘migrates’ laterally, sediments of one depositional environment come to lie on top of another

Transgressions (sea level rise) fines upward
Regressions (sea level fall) fines downward

9

Flow hydraulics?

 Grain density and size
 Grain drag
 Grain roughness
 Flow velocity
 Flow viscosity
 Slope
Pebble Clusters
• Shows flow direction
• Larger pebbles on opposite flow direction side

10

Sedimentary structure basics?

Below 1cm = laminae
Above = Bed

Physical processes such as currents are responsible for most sedimentary structures, but chemical reactions and biological activity also contribute

Sedimentary structures: Marks, traces and sediment disturbances preserved in sedimentary strata

Types of sedimentary structures:
 Depositional structures – bedforms
 Erosional structures
 Biogenic structures
 Chemogenic structure
 Internal structures
 Post depositional features

11

Way up?

The technique is particularly important in compressional orogenic belts where sediments can be easily overturned.

12

Geopetal?

Filled in cavities (such as shells) act as sprit levels (Paleohorizontal)
1. Original calcium carbonate shell of living animal
2. Mud that infilled shell after animal had died
3. Crystals that grew in empty space as shell was buried
4. Contact between mud and crystals show the orientation of the ancient seafloor
Pore spaces on top side of grains

13

Bedform structures?

features formed by the interaction of either water or wind and sediment on a bed surface
A bedform is a feature that develops at the interface of fluid and a moveable sediment dominated bed, the result of bed material being moved by fluid flow. Examples include ripples and dunes on the bed of a river or sea floor.

Flute marks - bulbous upstream end that flares downstream - tapered end pointing down-current and the steep end up-current

Current ripples – unidirectional flow
• Steep slopes before eddies show flow direction - steep slope is direction of flow

Wave ripples – bidirectional
• Tapering base
• Truncated top

Cross bedding
• Planar and trough cross bedding are two of the most commonly recognised types of cross-stratification

14

Biogenic structures?

 The action of plants and animals provides many structures
 Trace Fossils (ichnofacies): sediment living organisms

15

Internal structures?

 Graded Bedding: grain size distribution in a regular fashion
 Common – normal grading - decrease in energy during deposition – bigger at bottom – thins up
 Less common – inverse grading - occurs in debris flows and aeolian ripples – bigger at top, thins down

16

Post-depositional structures?

• Desiccation: the evaporation of water and drying up causes shrinkage
and cracks to develop delimiting polygons
• Flame structures: Fluid pressurized by deposition – finds cracks and bursts through

17

Convolted bedding formation processes?

 Liquefaction: A sudden loss of shear resistance associated with a collapse of grain supported fabric
 Framework collapse: increase in pore fluid pressure
 resediments from base up; fluid escape rapid but does not destroy primary lamination
 Fluidisation: the vertical fluid escape from a granular aggregate e.g. sand, grains are supported momentarily against gravity
 0.1-.05 mm
 destroys primary structures

18

Alluvial fans:

• Discrete
• Semi-conical shape
• Stream emerges from a mountain belt and deposit broad cone shaped bodies of sediment
• Radius typically 2-15 km
• Conglomerates, breccia, sandstones and mudstones
• Coarsest sediments found closest to mountain front (proximal) and finest further away (distal)
Fan Size
• Drainage area is the main influence
• Climate and geology cause differences in fan area
• Fan slope decreases with size - as slope decreases coarser materials are dropped
• Gradation in fan size, catchment area, and transport
Alluvial fans are associated with moun-tain fronts that may have long been eroded away, and faulted sedimentary basins where associated subsidence can yield high preservation potential of the fan sediments (

19

Some debris types?

Flash Flood Debris:
• Angular, Poorly sorted
• Picked up, transported and deposited very quickly

Glacial:
• Ice transported and deposited sediments, shapes the landscape
• Till, moraine, striations commonly preserved in rock record

800Myr – Snowball Earth

20

Aeollian environments?

• Only 20% of modern deserts are sandy
• eroding mountains (40%)
• “stony‟ deserts (10-20%)
• desert flats (10-20%)
• Deserts can be (semi)arid, warm and cold:
• (sub)polar climatic zones: glacial processes dominate
• (semi)arid climatic zones: aeolian processes dominate

Desert Rock surfaces:
• Very well rounded
• Desert concrete
• Desert Varnish

21

Lacustrine?

• Lakes make up 1% of the earth’s continental surface and store less than 0.02% of water in the hydrosphere
• Important source rocks and climate indicators
• Coarse sediments deposited near margins
• Finer grained sediments carried out to lake centre
• Laminations and varves

22

River environments?

• Important for controlling supply of sediment
• Range of grain sizes – conglomerates, sandstones, mudstones etc
• Meandering, braided and straight channels

Braided River
• Multi-thread channels, high energy, steep valley gradients (<0.5o), large and variable discharges, non-cohesive banks, bedload transport
• Materials found in the banks are what’s found in the channels

Meandering Stream
• Single channel, high sinuosity
• Migrate by selective bank erosion, point bar deposition, meander cut off and avulsion
• Channels are sandy, banks are finer grained

Point bar – filing upward sequence

23

Source area?

Sediments are continually being produced by the processes of weathering and erosion and subsequently transported by water (fluvial processes), wind (aeolian pro-cesses) and ice (glacial processes)

24

Deltas?

• A delta is a shoreline proturbance formed at the point where a river enters an ocean basin or other large standing body of water.
• The flow of sediment laiden water from a channel and into an unconfined, standing body of water is a process unique to deltas
• Tectonic settings of deltas can be varied - accommodation space vs sediment supply (fluvial)

• Sea level rise = growth of continental shelf + prograding deltas
• Sea level drop = shrinking of continental shelf + shrinking deltas

25

Continental Shelves?

• Staging post between erosion of continents and final deposition in deep ocean
• Sediment dispersed by a complex mix of tides, waves and ocean currents
• Wide spread sands, muds and carbonates
• Sediments on shelf regularly reworked and replaced

26

Carboante shelves?

• Factors leading to carbonate precipitation: Warm temperature; Low pressure; Agitation (Well oxygenated)
= promote the formation of carbonates in shallow tropical marine environments.
• Carbonate sediment tends to be autochthonous - forms where it is deposited
• Warm shallow seas often thought as Carbonate factories due to carbonate production
• From carbonate settings allochthonous carbonate material can be exported to adjacent environments
• All of these promote the formation of carbonates in shallow tropical marine environments. Thus, carbonates form in water that is not deep to begin with, and carbonate environments tend to shoal upward toward the surface, forming carbonate platforms.
• Most carbonate production is biogenic. Because carbonate secreting critters tend to be intolerant of muddy waters, carbonate dominated environments tend to be distinct from clastic environments.
• Carbonate factories frequently shallow to the surface where they may be exposed due to sea level fluctuations on various time scales, resulting in erosion and karstification.

27

Chemical precipitation?

• Oolite is a sedimentary rock made up of ooids (ooliths) that are cemented together.
• Ooids are spheroidal grains with a nucleus and mineral cortex accreted around it. Nucleus is usually either mineral grain or biogenic fragment.
• Directly ppt from warm seawater
• The term “ooid” is grains < 2 mm in diameter.
• Larger grains pisoids (pisoliths)

28

Reefs?

• Piles of intergrown organismal skeletons and sediment that form a rigid wave-resistant interlocking framework.
• Framework tends to represent a minority of actual reef volume, most of which is carbonate sediment infilling pore space.
• Type of reef depends upon the topography of the continental shelf, degree of exposure to wind, waves, and currents.
Fringing reef - between continent and sea (one side on land
Atoll - reef surround lagoon with sea on exterior side
Barrier reef - reef forms a lagoon between continent and sea

29

Parts of reef and how they have changed?

 Forereef: Sediment shed off of the reef front (includes reef slope and proximal talus)
 Reef framework and crest: The densest zone of in situ framework growth. The crest receives the most wave energy.
 Backreef: Including reef flat, backreef sands, and lagoonal muds. Sheltered from wave energy by reef front. (sheltered, fine sediments)

How reefs have changed:
• Mostly due to extinction
• Framework builders (corals) are generally only 10% of the total volume of the reef other components:
o Carbonate mud (micrite)
o Skeletal elements
o Cement
• Reefs have been around since the Cambrian

30

Conditions and what they indicate?

Sedimentary structures such as ripples and cross-bedded strata indicate the organism lived in moving water where sediment was moved by currents.
§Broken shells may indicate pounding by waves as on a beach or created during storm conditions.
§Diagenesis including the rate of burial, the starting pore fluid chemistry and later mineral replacements within sediment can significantly influence the preservation potential of a fossil or enhance it
§Murky waters, with low light levels and lots of suspended sediment particles can be determined by the type of sedimentary rock and associated sedimentary structures. Filter feeding organisms such as corals are not usually found in muddy waters because the suspended sediments clog their filters.
§Oxygen levels are critical to all organisms. If there is insufficient oxygen in the water, relatively few organisms may have lived in such an environment. However, in oxygen deficient waters organic material in sediments can be well preserved.

31

Deep Sea Sediment classification?


• Location:
o Neritic: On the continental Margin
o Oceanic (Pelagic): overlaying the oceanic crust
• Water depth:
o Pelagic: below 3000m
o Hemi pelagic: 200-3000m
• Grain size:
o Sand, silt, clay
• Origin:
o Authigentic or autochthonous: precipitation from solution
• Hydrogenous: abiogenic (inorganic)
• Biogenous: produced by biological activity
o Allochthonous: carried into the oceans as solid phases
• Luthogenous/terrigenous: from Earth’s crust
• Cosmogenous: extraterrestrial materials
• Sedimentation rate:
o Non-pelagic: form at rates higher than 1cm/1000y
o Pelagic: from at rates lower than 1cm/1000y
o Relic: sedimentation rates of – or less (net dissolution)

32

Turbidity currents and turbidites?

Turbidity Currents:
• Main agent for transporting shallow-water sediment into deep water dwon submarine canyons on to submarine canyons on to submarine fans and the ocean floow
• Speed depends on slope and amout of sediment (current density)

Turbidities:
• Fining upwards sequence in deep sea setting
• Bouma Sequence
o Muds – often bioturbated
o Parallel laminated silts
o Cross laminated fine sands
o Plannar laminated fine-to-medium sands
o Sand and any larger grains the turbidity current was carrying at the time of deposition

33

Cosmogenous Sediments?


• Cosmic dust found in red clay
• Composed of iron-nickel and magneitie formed by oxidation during passage through Earth’s atmosphere –are 50-200 microns in diameter
• Aound 300x103 tonnes of cosmic/meteoritic dust falls on the Eath’s surface each year