Introducing Marine Clastic Environments:
Ocean basin schematic from Visual Merriam Webster
- The oceans are beneath base level for flowing streams. Thus, they are the primarily sites of deposition.
- For practical purposes, marine depositional environments begin at wave base.
- Even if sea levels never changed, marine environments of deposition would be enigmatic because of their inaccessibility. The fact of recent rapid sea level change complicates matters further.
- Ultimately, most of their sediments derive from the continents. Few modes of transport are actually able to move these sediments into the deep oceans. Thus, we go from thick sediments near shore to vanishingly thin in the abyssal plain.
- As sea levels change, the primary regions of deposition shift toward and away from the continental slope.
- Thus, periods of extreme sea level lowstand are the most productive for deep ocean sediments because the source of sediments moves toward the ocean basins.
Clastic shelf deposits
Sedimentation on continental shelves (where ocean depths are less than 200 m) are continuous with coastal plain sequences. In clear tropical waters these accumulate abundant carbonates (to be considered later), but in cold water or areas with high siliciclastic inputs, the shelves are covered by fine sands, silts, and muds. During sea level highstands continents are flooded creating large epeiric seas (E.G. Late Carboniferous North America) that may accumulate abundant sediments.
Caveat: Many contemporary continental shelf deposits are continental and marginal marine deposits laid down during Pleistocene lowstand. Although these have been reworked by marine processes, they aren't characteristic of equilibrium marine sediments and may not be good analogs for ancient sedimentation.
Primary considerations: Depositional processes on shelves are dictated by two considerations:
- whether the sediments accumulate above or below storm wave base
- whether tidal currents are strong enough to redistribute particles.
- Where tidal ranges are large (>2 m) and currents are fast (50 to 100 cm/s) asymmetrical sand ribbons or tidal ridges are formed on the continental shelf at oblique angles to strike.
- At tidal currents of less than 50 cm/s, strike elongate sheets or waves of sand develop (right). A tidal sand wave has a crest of 3 to 15 meters and wavelengths of 150 to 500 meters. They are composed of low angle cross-beds (dipping at 5 to 6 degrees, which along with cross sets that are no more than a few meters in thickness, differentiates them from eolian sand dunes).
Coasts have linear sand ridges with variable cross bedding and hummocky cross-stratification, with low-angle curved intersections and upward domed laminae. This bedform is formed at water depths of 5-15m between fair weather and storm wave base. Typically much smaller than tidal sand waves.
Where storm and tidal energy is not sufficient to move sediments, deposition of silt and mud predominates. The typical sequence of a shallow siliciclastic shelf is dominated by storm and tidal processes, but changes in relative sea level are the primary source of sediments and structures on which these forces act. Characteristic stratigraphic profiles are recognized based on whether sea level is regressive, transgressive, or balanced (right).
Continental slope and rise deposits
The continental slope between the shelf and deep ocean floor is relatively narrow (10 to 100 km) and slopes downward at an average angle of 4 to 6 deg. Sediments are moved downslope by gravity, disloged from the shelf/slope break by storms or earthquakes.
The characteristic sedimentary features include:
Turbidites: We have already considered turbidity currents. Let's add some details:
A turbidity current has three parts:
- a head: The head is tall, and thus has the most energy and does most of the erosive work.
- body: The body carries most of the sediment and can be very long-lived and large. It can both erode and deposit.
- tail: The tail is the low concentration part of the flow, and is always decelerating. It deposits most places, but not very much.
As noted, turbidity currents mix with the ambient water, which decrease their concentration thereby slowing the flow and depositing sediment. This feedback process promotes deposition of a turbidite.
Turbidites: We referred to Turbidity currents periodically moving coarse sediments down the delta front. In fact, these are the primary down-slope movers of submarine sediments altogether, and characterize continental slope deposition.
- Earthquakes trigger slumps or other mass wasting events. These kick sediments up into suspension.
- The resulting mixture of water and sediment is much denser than sea water, and goes roaring off downslope. This is a turbidity current. It has a life of its own, separate from the landslide that started it. Turbidity currents are capable of carving canyons in deposits of unconsolidated sediments.
- Eventually, as the turbidity current loses energy, larger, then progressively smaller clasts are deposited. Because progressively finer clasts are deposited in progressively lower energy, grain size and flow conditions are correlated. The result is the Bouma sequence:
- Ta (massive): Structureless basal conglomerates.
- Tb (planar bedded): Plane bed deposition - Fr > 1.
- Tc (current rippled): Ripples/bedforms - Fr < 1.
- Td (planar laminated): Laminated silt and mud
- Te (suspension fallout only): Laminated mud
- Turbidites can be up to 10 m thick, however most are much smaller. Their bases are often characterized by sole marks such as flute casts.
Flysch deposit, Homer, AK.
Caveat: You generally don't see the entire Bouma sequence in a single turbidite!
- If the turbidite in your outcrop formed some distance from the main axis of the current, basal portions may not be present.
- It may have been scoured and decapitated by subsequent turbidity currents, removing the upper portions.
Ignition: A flow may additionally erode its substrate thus adding mass to its body. This would increase sediment concentration, accelerating the flow and increase erosion. This positive feedback loop is called ignition and promotes erosion. Due to this effect, continental slopes are generally places of sediment bypass, while adjacent basin floors (continental rise) are sites of sediment aggradation.
Remember the Bouma sequence -- Ta (massive), Tb (planar bedded), Tc (current rippled), Td (planar laminated), and Te (suspension fallout only).
- These can vary in proportion, and not all parts of the Bouma sequence are likely to be present.
- Material may also move below the current as bed load. Bed load material will form dunes, ripples, and imbricated clasts, just like bed load material elsewhere.
- carve channels
- transport sediments as channelized flow over long distances
- deposit sediments at a "base-level" determined by topographic gradient
In the modern world, deep-water systems begin below the continental shelf edge. When sea level was at the shelf edge, "deep water" meant deposition below storm wave base farther down the continental slope. Basins may be 100's or 1000's of meters deep. Commonly, they are characterized by margins steeper than 0.5 deg., and commonly 1 deg.-3 deg. gradients. Deep-water systems are one of the few depositional environments where significant volumes of massive (i.e.structureless) sand accumulate (most other environments have cross-bedded sands).
Submarine fans: Fan-shaped bodies of coarse sediment accumulate where turbidity current flow is unconfined. This usually occurs at prominent decreases in local gradient, usually to less than 0.5 deg. As confinement and gradient decrease, flows decelerate and drop their load. This means that proximal fans are dominated by Ta beds whereas distal fans can have more Tb and Tc beds. Deceleration also decreases erosional capacity of flows. This means that fans are commonly more channelized in their proximal positions and more sheet-like in their distal positions. Channel networks on fan avulse regularly, shifting the short term site of deposition.
Leveed channels: When turbidity currents are tall enough to overflow their channels, they become unconfined and decelerate. However, only the low concentration portion of the flow does so. As flows move away from the channel, they get progressively finer grained & lower in concentration. This effect builds levees with the following characteristics:
- they are generally fine grained
- beds thin away from the channel
- beds fine away from the channel.
Many other kinds of sediments can be found in deep-water environments.
Slope mudstones: Silts and clays are brought into the deep water in two ways:
- Pelagic sedimentation, or settling out of the water column
- fine-grained turbidity currents. Many of these turbidites can be associated with small or muddy rivers as well as very large ones. Much of this material can be overbank to channels of various kind, but can also be re-distributed by weak flows on the sea floor. Grain-size and composition will vary with distance from the source rivers.
Abandonment/Drape: During transgression, sediments are sequestered high on the continental shelf. As such, almost no sediment reaches the deep-ocean. This leads to abandonment of active portions of the continental slope and abyss. Abandonment successions are enriched in fine pelagic sediments and are often assoiciated with organic enrichment.
- muddy plumes at delta fronts
- low concentration turbidity currents that flow into the water column itself.
- very fine sediments can be carried into the ocean by wind energy
- biological sediments (e.g., diatoms, foraminiferans) that are born in the water column also contribute to pelagic "rain".
- Volcanic ash falls
- Tektites - impact ejecta (rare)
Depending on the state of ocean chemistry and circulation, carbonate will dissolve at very great depths. This level is called the carbonate compensation depth or CCD. Think of this like a snow line in the ocean, which due to lower pH below (rather than higher temperatures) carbonate dissolves. The ocean floor deeper than about four kilometers is
corrosive to carbonate, so these sediments do not accumulate there.