offshore environments

Question 1

The sea floor environment
Continental shelve

Answer 1

Why study the sea floor and its sediments?
– navigation safety
– coastal erosion and sea level rise,
– Fisheries
– earthquake and tsunami hazards
– Offshore infrastructures (e.g., windfarms, submarine cables)

neritic zone:
-corresponds to the shallow marine environment (mean low water to about 200-metre depths)
-generally corresponds to the continental shelf
-proximal to land, sunlight reaches ocean floor, permits photosynthesis -> relatively abundant nutrients and biologic activity
- Bottom sediments either of terrigenous and/or carbonate particles and reefs (in warm waters of low latitude regions)

Sediments along ocean margins
-1/5 ocean = margins
- thickest sediment
- highest volume sediment (>90%)

Siliciclastic shelves and their terrigenous sediments
-90% sediment coverage lithogeneous

Subdivisions of the continental shelf
-shoreface, inner shelf, middle shelf, outer shelf, slope

Margins of continental shelves
-basement ridge, folded belt, fault block, reef, volcanic ridge, diapirs

Question 2

Wave- and storm-dominated shelves

Answer 2

Sediment facies of sand ridges
A:
side: trough cross laminations, planar lamination, hummocky cross laminations, planar lamination
front: ripple cross stratification, planar lamination, hummocky cross stratification
B:
side: hummocky cross stratification & swaley bedding
front: hummocky cross stratification & swaley bedding, med-high angle cross bedding

Question 3

Tide-dominated shelves

Answer 3

Shelves affected by ocean currents

Ancient siliclastic shelf sediments
Distinguishing characteristics:
– Sheet-like (tabular) sandstones, sandy siltstones, and
mudstones
* Well-developed, even bedding
* Cross-bedded sandstone (tide-dominated shelf)
* Reactivation surfaces
* Storm beds and hummocky xs-beds or HCS (storm-dominated shelf)
– Extensive lateral dimensions (> 1000 km 2 ) and great
thickness (> 100 m)
– Moderate compositional maturity (QFR)
– Diverse and abundant marine fossil organisms
– Bioturbation; trace fossils ( e.g. Cruziana)

“idealized” profiles
A: transgressive tide dominated shelf
B: transgressive storm-dominated shelf
C: regressive storm dominated shelf
- Regression produces coarsening-upward successions

Question 4

Evaporites and carbonates

Answer 4

Hydrogenous sediment (shallow seas)
<1% ocean floor
precipitation: crystals form from supersaturated water as dissolved ions get together and form solid material

Modern carbonate environments
Warm-water vs. cool-water assemblages
* Cool-water assemblages
– Heterozoan associations: Foraminifers and molluscs
* Warm-water assemblages
– Photozoan associations
* Reef-building corals
* Calcareous green algae

Coral reef environment
Coral reefs cover 250,000 km2
Typically grow in upper 0-30 m of water
where temperatures >18 C (1-25mm/yr)

mounds: Structures built by small delicate organisms in low energy env. (shallow or deep water)
Microbial mounds (stromatolites)
Skeletal mounds (reef-building organisms)
Mud mounds
Occur at various scales

Frame builders and binders
- plate corals
- hard corals
- giant limpet
- red algae
Bafflers and binders
- green seaweeds
- soft coral
- encrusting yellow coral
-Bowl coral
- red algae

Question 5

The oceanic (deep-water) environment

Answer 5

Features of passive margins

Sand-rich fan

Mixed sand/mud system

Mud-rich submarine fan

Architectural elements of deepwater systems

Deep water currents and contourites
* Some deepwater bottom currents flow parallel to
bathymetric contours near the base of the continental
slope and across the continental rise
– Thermohaline-induced and geostrophic bottom-currents
– Flow velocity may be intensified by the Coriolis effect driving
currents west along continental margins in the northern
hemisphere enhanced during storms
– “Benthic storms”:
* At the lower continental rise, south of Halifax, Nova Scotia, these
velocities may reach up to 73 cm/s

Calcareous and siliceous oozes
-autotrophs require nutrients in the water
- Calcareous oozes: warm, moderate nutrient water
- siliceous oozes: nutrient rich cold water

Biogenous ooze = mud sized shells
- 55% surface coverage (10% by volume)

Carbonate Compensation Depth (CCD)
depth of the CCD depends on
- the solubility of calcium carbonate
-> determined by temperature, pressure, amount of dissolved CO2 in the water.
Calcium carbonate is more soluble at lower temperatures, at
higher pressures and lower pH.

Question 6

Distribution of deep ocean sediment

Answer 6

Abyssal (red) clay
Hydrogenous and diagenetic metal nodules (deep
sea)
- Dissolved Mn and Phosphate can become supersaturated and precipates on the ocean floor as nodules

Meteorites and tektites
&laquo_space;1% of surface sediments

Answer 7

– Source
* Terrigeneous
* Marine organisms (fallout)
* Volcanoes
* Cosmic fallout

– Means of transport / deposition
* Wind
* Gravity flows
* Ocean currents
* Ice rafting
* Settling

– Factors influencing rate of supply

– Potential for dissolution or change on the sea floor