10.4: Clastic Marine Environments
Nature of Ocean Basins
The depth of ocean basins is largely controlled by tectonic activity and the geological setting in which they form. Pericontinental seas are found at the edges of continents, adjacent to very deep oceanic waters. These seas contrast with epicontinental seas, which occur when continental areas become submerged. Epicontinental seas are relatively rare in the modern world, with Hudson Bay being one of the few examples. However, they were far more prevalent during the Cretaceous and other greenhouse climate phases when ocean basins held less water (rapid seafloor spreading made them warmer and more buoyant) and because there was more liquid water present because there was little/no glacial ice. This lead to widespread, but relatively shallow flooding of continental landmasses.
Shelf Environments
The shelf is a relatively shallow, very gently dipping area along the submerged margin of a continent or in a foreland basin . It extends from the boundary with the lower shoreface to the more steeply dipping shelf break. The width of the continental shelves varies greatly, ranging from a few kilometers to over 1,500 kilometers wide and ~100 to 200 m deep.
Inner Shelf
Waves are an important influence on sedimentation in many inner shelves. Fair-weather wave base is the depth at which waves influence the seafloor under normal conditions ... the depth is equal to 1/2 of the spacing between waves. Waves are larger during storms and their influence extends to greater depth ( storm wave base ). The offshore transition is the innermost portion of a wave-dominated inner shelf; it lies between fair weather wave base and storm wave base. It is characterized by admixed sand and mud, intense bioturbation and the resulting paucity of primary sedimentary structures. Hummocky cross-stratified (HCS) sandy layers deposited during storm events may be present locally if they were buried before they could be homogenized via bioturbation.
In relatively shallow seas with significant tidal currents sediment movement is more continuous and large, straight- to sinuous-crested sand ridges may be present. These features are elongate parallel to current direction and can be ornamented with bi-directional cross beds the reflect ebb and flood currents.
Outer Shelf
Beyond the inner shelf, the outer shelf lies at depths too great for wave and tidal influence. Here, variably bioturbated mudrock via suspension deposition is the most common lithology. Weak currents exist, mainly influenced by oceanic circulation or upwelling. Additionally, relict sand bodies may also be present - they represent remnants of coarser sediments deposited when sea levels were lower and sediment transport was more active. Carbonate deposition can also occur in deeper water portions of some clastic-dominated shelves.
Deep Marine Environments
Slope, Rise, and Abyssal Plain
Deep marine depositional settings include the continental slope, continental rise, and abyssal plain. The continental slope dips seaward at approximately 4 degrees and serves as a major conduit for sediment transport from the continental shelf to the deep ocean. The slope generally extends from depths of around 200 meters at the shelf break to approximately 3,000 meters where it transitions into the rise. This region also marks the transition from continental to oceanic crust. Sediment is delivered through suspension deposition and storm-driven transport (via gravity and exceptional currents), but the relatively steep gradient makes the slope prone to soft-sediment deformation, failure, and gravity mass movement. Slope failures lead to the development of turbidity currents (turbulent mixtures of sediment and water) which can carve submarine canyons and transport sediment downslope under the influence of gravity.
At the base of the continental slope, the continental rise consists of sediment transported by turbidity currents. This region, which slopes more gently at around 0.5 degrees, extends from approximately 3,000 to 4,000 meters in depth. These currents form channelized systems with levees, resembling subaerial floodplain channels. As turbidity currents lose energy, they deposit sediment in a characteristic sequence described by the Bouma model, which details the transition from high-energy turbidity currents to lower-energy traction currents and eventual suspension deposition. This results in a fining-upward sequence, with coarser material deposited first and finer sediments settling out as the current dissipates.
Beyond the continental rise lies the abyssal plain , one of the most extensive depositional environments on Earth (it covers nearly 50% of the planet’s surface!). The abyssal plain extends from depths of about 4,000 to 6,000 meters and has an extremely low gradient, typically less than 0.05 degrees. Sedimentation here is almost exclusively pelagic , with fine-grained sediments settling out from suspension. These deep-sea sediments are primarily composed of siliceous or calcareous ooze and deep-sea clays.
Carbonate Compensation Depth
The carbonate compensation depth (CCD) plays a crucial role in determining the composition of abyssal sediments. At shallow depths, surface waters are supersaturated with calcite, allowing for relatively abundant carbonate deposition. However, as depth increases, cold temperatures and high pressure cause the water to become undersaturated, which contributes to the dissolution of calcareous material. At the CCD, the rate of carbonate dissolution exceeds the rate of supply, and below this depth carbonate sediment is not present and siliceous oozes or deep sea clays become dominant. Features like mid-ocean ridges and volcanic seamounts can create significant topographic relief in the deep ocean and the calcareous sediment deposited in these shallow waters may be eventually be buried with bedded cherts or shales as plate motion causes the plate to move, cool, and subside.