Deep Marine Systems (WPF)

The Deep Oceans

Deep ocean basins provide the largest areas of sediment accumulation on Earth, but they are also some of the least accessible.  Expansive regions underlain by basaltic oceanic crust provide space for deposition of sediments that are shed from continental margins and are overlain by up to kilometers of ocean water. Turbidity currents and debris flows often characterize sedimentation in deep marine environments (Nichols, 2009). These processes transport detritus down the continental slope to the deep ocean. Wind blown dust, ash, fine particulate matter, and the shells and skeletons of dead marine life are also all sources of deep marine sediment. Identifying facies as “deep water” is often problematic. Within shallow environments (<200 m depth), waves, tides and storm currents rework sediment to produce features that are absent at greater depths. Therefore, evidence for deposition in deep water may sometimes be based on the absence of shallow water structures as well as (or more than) identifying positive deep water indicators (Nichols, 2009).

Distal Marine Fan Deposits


Distal marine fan deposits composed of sandy and muddy turbidite beds (Nichols, 2009). 

Geomorphology & Modern Analog

The Ocean Basins


Most of the Earth’s surface is covered in oceans, which formed by sea-floor spreading and are underlain by dense basaltic crust. Spreading centers, where hot basaltic magma extrudes onto the sea floor and begins to cool, occur along the mid-ocean ridge and are kilometers below the surface of our oceans. As the hot material cools, it becomes denser and sinks relative to the younger, hotter crust. This relationship of young, hot crust near spreading centers and old, cold crust along the oceanic periphery has developed a characteristic profile to the ocean floor with sloping surfaces and increasing depth further from the spreading centers (Nichols, 2009; Fig. 1). The oceans are bound by continental crust, which contributes huge volumes of sediment to ocean basins and concentrates large clastic depositional systems near the margin of the continents (Nichols, 2009). The issue with deep-water sedimentation is that, although modern systems are easy to identify, they are extremely difficult to access. Study of the deep ocean is therefore largely dependent on bathometric surveys, sonar, and seismic reflection surveys.

Figure 1: Schematic block diagram of ocean floor showing the crust and its relationship to the mid-ocean ridge. 

Sea Floor Morphology


The edges of continental shelves (~200 m below sea level) are connected to the ocean basin floor (~4000-5000 m below sea level) by a series of broad continental slopes, which extend into the deep ocean for up to hundreds of kilometers (Nichols, 2009).  Despite their relatively low gradient, the continental slopes are often cut by steep sided submarine canyons, which can serve as pathways for sediment transport from the continent to the deep ocean. Beyond the continental slopes, the ocean floor is generally a broad flat plain, except in areas where there are occasional subaqueous volcanoes, known as seamounts. These can be significant sources of volcaniclastic material to submarine depositional systems.  The deepest parts of the ocean occur where oceanic plates are actively subducting. Such is the case along the Marianas Trench where the Pacific plate is subducting beneath the Mariana Plate (Hole, 1984). Because that environment is far from the continental margin, it is sediment starved. We can compare that to the subduction along S. America’s western margin, which abundant sediment is available from the continent. In this way, the morphology of the sea floor helps determine the availability of sediment and contributes to the development of deep marine sedimentary environments (e.g. Hole, 1984, Gamberi et al., 2008). 


Depositional Processes and Facies


Debris Flow Deposits and Turbidites


The movement and deposition of clastic material in the deep ocean is primarily achieved through mass movement of material by debris flows and turbidity currents. The mobilization of a poorly sorted sediment mixture down the continental slope and out onto the basin floor is a debris flow. Modern debris flows have been identified off the northwest coast of Africa (Masson et al., 1994) as well as in the stratigraphic record (e.g. Johns et al., 1981; Pauley, 1995). Unlike on land, submarine debris flows can mix with sufficient amounts of water to become dilute mixtures of sediment-laden water which flows by gravity do to a density contrast between the denser water (carrying sediment) and the surrounding fluid. These more dilute, density driven, flows are known as turbidity currents and can rapidly transport huge amounts of material. Masson (1994) showed that a single turbidite deposit off the Canary Islands has a volume of ~ 125 km3.  Fine et al. (2005) used data from severed telegraph lines associated with a turbidity current from 1929 to estimate flow speeds between 60-100 km hr-1. In deep marine settings the primary controls on sedimentation are often the sediment source type, width of the shelf, and the basin/floor gradient (Gamberi et al., 2008). Sediment type and size affects the shape of the resulting submarine fan systems and a variety of different geometries are possible (Fig. 2).  

Figure 2. Schematic diagram showing the submarine fan geometries that result from varies sediment types (Nichols, 2009).


Pelagic Sedimentation


Fine suspended material in the ocean can include dust blown from the continents by wind, small particles of ash from volcanic eruptions, particulate matter from fires, and bioclastic material from the remains of numerous varieties of marine organisms. Pelagic sediments of biogenic origin are the most abundant. The hard, calcium carbonate parts of marine organisms can create fine-grained deposits of calcareous ooze on the ocean floor, which accumulates at rates between 3 and 50 mm/kyr (Einsele, 2000).  That said, pelagic sediments only form significant accumulations in areas that are not dominated by sediment from other sources. Along continental margins, the contribution of sediment derived from land makes pelagic sediments less significant, while further from the margin pelagic sediments are more significant due to the relative lack of typical detritus (Nichols, 2009). Relative warmth and nutrient supply also play a role in the viability of calcareous material. Deep ocean pelagic deposits occasionally bear fossils, the most common of which are the skeletons of microscopic organisms (ex. Foraminifers, coccoliths and Radiolaria; Nichols, 2009). Most biogenic pelagic material is therefore typically very fine grained, but occasionally the shells of large swimming organisms like cephalopods and the bones of aquatic animals can be incorporated into deep sea sedimentary packages and are used to help estimate depths. 

The solubility of calcium carbonate in pelagic sediments is dependent on both temperature and pressure. Higher pressures and lower temperatures increase the amount of calcium carbonate that can be dissolved in the same mass of water.  The calcite compensation depth, the depth at which calcite will have completely dissolved, is ~4000 m (Nichols, 2009). These depth related pressure conditions lead to concentrations of silica, which is less soluble, and clay minerals below the calcite compensation depth. Above the calcite compensation depth, calcareous oozes are able to form carbonate mudstones, while below it siliceous oozes form chert. In portions of the ocean that are sufficiently deep, the only sediments deposited are pelagic clays because they are below both the calcite and silica compensation depths, meaning that essentially all material has moved into solution apart for these special pelagic clay materials (Fig. 3).

Figure 3: Schematic cross section of the ocean showing different depositional zones based on the calcite compensation depth (Nichols, 2009).


Chemogenetic Sedimentation


The unique conditions of the deep ocean also allow for the direct precipitation of some silicates, sulphates, and metal oxides (normally of Fe and Mn). Manganese ions derived from hydrothermal sources or the weathering of continental rocks can produce nodules that grow very slowly in deep marine settings but are sometimes common (Calvert, 2003). Volcanic vents on the sea floor can facilitate unique chemical reactions which allow special communities of organisms and hydrothermal deposits which have been occasionally seen in the stratigraphic record of ophiolites and observed directly by submersibles (Nichols, 2009). 


Controls on Depositional System Evolution


Sedimentation in the extreme deep ocean is predicated upon several factors:

1.   Depth: below 200 m is considered the deep ocean, where tides and waves are no longer a primary driver of depositional style. Zones of sedimentation exist based on the depth at which there is partial and full dissolution of various minerals:~3000 m-partial carbonate dissolution, ~4000 m total carbonate dissolution, ~6000 m silica dissolution.

2.   Proximity to continental margin: Determines the contribution of continental sediment and type of sediment available.

3.   Gradient/Slope: Affects debris-flows and turbidites; dictates the way material is contributed to some basin. Deposition on proximal and distal portions of a fan are very different and determined by the slope.

4.   Proximity to submarine canyons: submarine canyons can serve as sediment pathways and the morphology of the canyon helps determine how sediment is transported.

5.   Sediment type: Gravel-rich, sand-rich, mud-rich and mixed systems behave very differently based on the grain size and cohesion of material.

6.  Proximity to seamounts/underwater volcanos: can be important sources of volcanoclastic material and cause unique chemical reactions in the deep ocean.

7.  Tectonics: Subduction zones are some of the deepest portions of the ocean and their relative position can affect how sedimentation occurs. Tectonic cycles can isolate marine basins for periods of geologic time and change the morphodynamics of the system through time. Such was the case with the Mediterranean Sea (Ryan and Hsu, 1972). 

Facies Models


Facies models of the deep ocean are based upon the sediment source type and the gradient/migration pathway used by the sediment. Below are a series of models that represent various deep marine depositional systems. The following facies models show the relationship between the factors discussed above and the distribution of various deep water facies. 

Figure 4: Facies model for gravel-rich submarine fan- small coarse fan delta composed mainly of debris flows (Nichols, 2009).

Figure 5: Facies model for a sand-rich submarine fan- sand rich turbidites formed into lobe shapes that stretch out to the basin floor (Nichols, 2009).
Figure 6: Facies model for a mixed sand-mud fan- lobes are a mix of mud and sand and build further out into the basin (Nichols, 2009).
Figure 7: Facies model for a muddy submarine fan- elongate lobes stretch very far out into the deep basin, sand in some channels (Nichols, 2009). 
Figure 8: Facies model for slope apron deposits- debris flow, slumps, and spill over sands form along the continental slope (Nichols, 2009). 






References:
Calvert, S.E. (2003) Iron-manganese nodules. In: Encyclopedia of Sediments and Sedimentary Rocks (Ed. Middleton, G.V.). Kluwer Academic Publishers, Dordrecht; 376–379. 

Einsele, G. (2000) Sedimentary Basins, Evolution, Facies and Sediment Budget (2nd edition). Springer-Verlag, Berlin.

Gamberi, F., and Marani, M., 2008, Controls on Holocene deep-water sedimentation in the northern Gioia Basin, Tyrrhenian Sea: Sedimentology, v. 55, p. 1889-1903, doihttp://dx.doi.org/10.1111/j.1365-3091.2008.00971.x.

Hole, M. J., 1984, Subduction of pelagic sediments: implications for the origin of Ce-anomalous basalta from the Mariana Islands: Journal of the Geological Society, v. 141, p. 453; 453-472. 

Johns, D.R., Mutti, E., Rosell, J. & Seguret, M. (1981) Origin of a thick, redeposited carbonate bed in Eocene turbidites of the Hecho Group, south-central Pyrenees, Spain, Geology, 9, 161–164

Masson, D.G. (1994) Late Quaternary turbidite current pathways to the Madeira Abyssal Plain and some constraints on turbidity current mechanisms. Basin Research, 6, 17–33. 

Masson, D.G., Kidd, R.B., Gardner, J.V., Huggett, Q.J. & Weaver, P.P.E (1992) Saharan continental rise: facies distribution and sediment slides. In: Geologic Evolution of Atlantic Continental Rises (Eds Poag, C.W. & de Graciansky, P.C.). Van Nostrand Reinhold, New York; 327–343.

Nichols, G., 2009, Sedimentology and Stratigraphy, Wiley and Sons, New York, 225-246. 

Pauley, J.C. (1995) Sandstone megabeds from the Tertiary of the North Sea. In: Characterization of Deep Marine Clastic Systems (Eds Hartley, A.J. & Prosser, D.J.). Special Publication 94, Geological Society Publishing House, Bath; 103–114.

Ryan, W.B.F., and Hsu, 1972, The Pliocene record in deep-sea Mediterranean sediments: Pubblicazione - Milan, Universita, Istituto Di Geologia de Paleontologia, 427.





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