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

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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

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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)

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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

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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

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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