Offshore Marine Environment
-
Upload
kumar-abhijeet -
Category
Documents
-
view
226 -
download
1
Transcript of Offshore Marine Environment
Contents
• Introduction• Sedimentology – concepts• Fluvial environments• Deltaic environments• Coastal environments• Offshore marine
environments
• Sea-level change• Sequence stratigraphy – concepts• Marine sequence stratigraphy• Nonmarine sequence stratigraphy• Basin and reservoir modeling• Reflection
SBSBSBmfsmfsmfsTSTSTS
Establish a framework of geneticallyEstablish a framework of geneticallyrelated stratigraphic facies geometriesrelated stratigraphic facies geometries
and their bounding surfaces toand their bounding surfaces todetermine depositional settingdetermine depositional setting
Offshore marine environments
• Shallow marine environments include pericontinentalseas that occur along continental margins and have a shoreline-shelf-slope profile; and epicontinental seas that cover continental interiors and exhibit a ramp morphology
• Under idealized conditions the offshore-transition and offshore exhibit a systematic decrease in (wave) energy and grain size; however, such an ‘equilibrium shelf’ is commonly not encountered• Tides and ocean currents can strongly complicate shelf
hydrodynamics• Rapid sea-level changes (e.g., during the Quaternary) result in
relict shelf sediments that are genetically unrelated to the present conditions
EaES 455-6 4
Offshore marine environments
• Wave/storm-dominated shelves ideally exhibit a transition from sands in the lower shoreface, to alternating sands and muds below fairweather wave base, to muddy facies below storm wave base
• Storms have a strong imprint (i.e., storm deposits have a high preservation potential), since they wipe out fairweather deposits
• Tempestites form during storm events and exhibit a characteristic facies succession from an erosional basal surface with sole marks, to a sandy unit with hummocky cross stratification overlain by wave-rippled sand, finally giving way to muds
Offshore marine environments
• Wave/storm-dominated shelves ideally exhibit a transition from sands in the lower shoreface, to alternating sands and muds below fairweather wave base, to muddy facies below storm wave base
• Storms have a strong imprint (i.e., storm deposits have a high preservation potential), since they wipe out fairweather deposits
• Tempestites form during storm events and exhibit a characteristic facies succession from an erosional basal surface with sole marks, to a sandy unit with hummocky cross stratification overlain by wave-rippled sand, finally giving way to muds
Offshore marine environments
• Tides lead to circulation around amphidromic points, ranging from circular to almost rectilinear depending on the shape of the water body
• Tide-dominated shelves exhibit a distinct suite of bedformsin relation to current velocity and sediment (sand) supply
• Erosional features, sand ribbons, and sand waves go along with decreasing flow velocities, commonly associated with mud-draped subaqueous dunes; tidal sand ridges (tens of m high, many km across) are characteristic of shelves with a high supplyof sand
Offshore marine environments
• Tides lead to circulation around amphidromic points, ranging from circular to almost rectilinear depending on the shape of the water body
• Tide-dominated shelves exhibit a distinct suite of bedformsin relation to current velocity and sediment (sand) supply
• Erosional features, sand ribbons, and sand waves go along with decreasing flow velocities, commonly associated with mud-draped subaqueous dunes; tidal sand ridges (tens of m high, many km across) are characteristic of shelves with a high supplyof sand
Offshore marine environments
• Ocean current-dominated shelves are relatively rare; geostrophic ocean currents can lead to the formation of bedforms that are somewhat comparable to those of tide-dominated shelves
• Mud-dominated shelves are usually associated with large, tropical rivers with a high suspended load (e.g., Amazon and Yellow Rivers) that can be transported along the shelf if currents are favorable
Offshore marine environments
• Deep marine environments include the continental slope and the deep sea
• Subaqueous mass movements (mostly sediment gravity flows) involve a range of transport mechanisms, including plastic flowsand fluidal flows• Debris flows are commonly laminar and typically do not produce
sedimentary structures• Turbidity currents are primarily turbulent and more diluted; they
commonly evolve from debris flows• Debris-flow deposits are poorly sorted, related to the ‘freezing’
that occurs once shear stresses can not overcome the internal shear strength
• A key mechanism in turbidity currents is ‘autosuspension’(turbulence --> suspended load --> excess density --> flow --> turbulence)
EaES 455-6 23
EaES 455-6 24
EaES 455-6 25
EaES 455-6 26
Velocity profile of debris flowsBingham fluid
shear stress
yield strength
dynamic viscosity
shearrate
⎟⎟⎠
⎞⎜⎜⎝
⎛∂∂
+=yu
y μττ
Plug layer
Shear layer
Yield strength: constant during flow
Animation
Offshore marine environments
• Deep marine environments include the continental slope and the deep sea
• Subaqueous mass movements (mostly sediment gravity flows) involve a range of transport mechanisms, including plastic flowsand fluidal flows• Debris flows are commonly laminar and typically do not produce
sedimentary structures• Turbidity currents are primarily turbulent and more diluted; they
commonly evolve from debris flows• Debris-flow deposits are poorly sorted, related to the ‘freezing’
that occurs once shear stresses can not overcome the internal shear strength
• A key mechanism in turbidity currents is ‘autosuspension’(turbulence --> suspended load --> excess density --> flow --> turbulence)
Animation 1
Animation 2
EaES 455-6 32
EaES 455-6 33
EaES 455-6 34
EaES 455-6 35
EaES 455-6 36
EaES 455-6 37
EaES 455-6 38
EaES 455-6 39
EaES 455-6 40
EaES 455-6 41
EaES 455-6 42
EaES 455-6 43
EaES 455-6 44
EaES 455-6 47
EaES 455-6 48
EaES 455-6 49
EaES 455-6 50
EaES 455-6 51
EaES 455-6 52
Offshore marine environments
• Contrary to debris flows, turbidites exhibit a distinct proximal to distal fining
• The idealized Bouma sequence, consisting of divisions A-E, is most useful for medium-grained, sand-mud turbidites, but it must be applied with care• A: Rapidly deposited, massive sand• B: Planar stratified (upper-stage plane bed) sand• C: Small-scale (climbing ripple) cross-stratified fine sand• D: Laminated silt• E: Homogeneous mud
• High-density and low-density turbidity currents give rise to incomplete, coarse-grained (A) and fine-grained (D-E) turbiditesrespectively
• Contourites are formed by ocean currents and commonly represent reworked turbidites
EaES 455-6 56
Bera, Sarkar et al., 2008,Bull. Geol. Soc. Amer.
Offshore marine environments
• Contrary to debris flows, turbidites exhibit a distinct proximal to distal fining
• The idealized Bouma sequence, consisting of divisions A-E, is most useful for medium-grained, sand-mud turbidites, but it must be applied with care• A: Rapidly deposited, massive sand• B: Planar stratified (upper-stage plane bed) sand• C: Small-scale (climbing ripple) cross-stratified fine sand• D: Laminated silt• E: Homogeneous mud
• High-density and low-density turbidity currents give rise to incomplete, coarse-grained (A) and fine-grained (D-E) turbiditesrespectively
• Contourites are formed by ocean currents and commonly represent reworked turbidites
Offshore marine environments
• Submarine canyons at the shelf edge (commonly related to deltas) are connected to submarine fans on the ocean floor
• The size of submarine fans is inversely related to dominant grain size (i.e., mud-dominated submarine fans are 104–106
km2, sand or gravel-dominated submarine fans are 101–102
km2)• Submarine fans share several characteristics with deltas; they
consist of a feeder channel that divides into numerous distributary channels bordered by natural levees (‘channel-levee systems’) and are subject to avulsions• Proximal fan (trunk channel)• Medial fan (lobes)• Distal fan
Animation
Offshore marine environments
• Submarine canyons at the shelf edge (commonly related to deltas) are connected to submarine fans on the ocean floor
• The size of submarine fans is inversely related to dominant grain size (i.e., mud-dominated submarine fans are 104–106
km2, sand or gravel-dominated submarine fans are 101–102
km2)• Submarine fans share several characteristics with deltas; they
consist of a feeder channel that divides into numerous distributary channels bordered by natural levees (‘channel-levee systems’) and are subject to avulsions• Proximal fan (trunk channel)• Medial fan (lobes)• Distal fan
Offshore marine environments
• Hemipelagic sediments consist for at least 25% of fine-grained (muddy) terrigenous material that is deposited from suspension, commmonly after transport by hemipelagicadvection• Distal, muddy turbidites merge gradationally into hemipelagic
deposits• Eolian dust is an important component (~50%) of hemipelagic
(and pelagic) facies• Black shales have a 1–15% organic-matter content and form in
anoxic bottom waters, sometimes in shallow seas (e.g., Western Interior Seaway)
• Pelagic sediments are widespread in the open ocean and primarily have a biogenic origin