Turbidity currents and turbidites
Turbidity currents are underwater gravity flows triggered by the mobilization of sediments on a slope. These phenomena are common in deep marine environments close to the continental scarp, but can occur also in lakes and coastal environments or be associated with carbonatic sedimentation in deep water basins close to reefs. Their formation requires the presence of large amounts of loose sediments close to a scarp, for example next to a delta. Once triggered, turbidity currents may travel for tens to hundreds of kilometers and distribute millions of cubic meters of sediments on the seafloor, forming deep sea fans. The lithified product of a turbidity current is a turbidite, which is a graded layer that generally contains a coarse-grained base and a fine-grained top (conglomerate to coarse sandstone to mudrock).
What are turbidity currents?
Turbidity currents originate when water-rich clastic sediments are destabilized, for example by a submarine slump, and start to move downslope in underwater environment. As the sediment starts to slide on the seafloor, it incorporates water and progressively lose cohesion, becoming a mixture of water and suspended particles that move together in density flow. Perhaps, the best way to understand what a turbidity current is, is to show one recreated in the lab (courtesy: Western Washington University):
As you can see, turbidity currents are fairly different from other gravitational movements like landslides or slumps that terminate when the slope (hence the effect of gravity) ends. Turbidity currents are density flows driven by the difference in density between the turbulent sediment-loaded water and the surrounding marine water. For this reason they can travel on the seafloor for tens to hundreds of km at speeds like 10 to 30 km/h before stopping. A nice story that led to the discovery of these processes years later is related to the M 7.1 earthquake that struck Newfoundland on November 18, 1929. After the earthquake, the telegraph cables laid on the Atlantic floor were progressively sheared off, one after another, over the next 13 hours and 17 minutes, up to some hundreds of kilometers away from the epicenter. At the time, geologists hypothesized that a fault continued to propagate. Today, we know that the seismic event triggered a submarine landslide that produced a turbidity current that travelled on the abyssal plains of the Atlantic, interrupting all communications. You can read the story in detail on Earth Magazine.
Turbidites and Bouma sequence
Turbidites were first described in the field by Arnold H. Bouma (1962), who defined what today we call Bouma sequence in deep water marine sediments. Bouma recognized that many of the layers he was investigating consisted of typical intervals with homogeneous structures. In particular, he recognized five characteristic intervals from the base to the top of the turbidite
A – a graded interval with gravel to sand size particles lacking any structure, starting with an erosional surface
B – sandstone with plane parallel lamination
C – sandstone with cross-lamination and convolute lamination
D – laminated silt to mud
E – laminated to homogeneous mud.
The interpretation of this structure is related to the progressive decrease in speed that turbidites experience as they travel on the seafloor. When the turbidity current is not fast enough anymore to hold all its sedimentary load in turbulent suspension, the coarser grains begin to settle all at once generating the first graded bed (A). Finer-grained sand can continue to move by traction while there is still a current flowing and speeding down: this forms first parallel lamination (B), then cross lamination with ripples and climbing ripples (C). Laminated fine-grained silt and mud (D) deposit when the flow is very slow and close to stop. Finally, layer E deposits when the turbidity current is over and the mud particles that remained suspended in water start to slowly decantate. This layer represents also the inter-layer between two turbiditic events, as it contains also the mud that slowly settles down from the water column in underwater environments. Therefore, while layers A to D represent a single event that may have occurred in less than a few hours, layer E testifies both the settling of the mud from the tail of the turbidite and the slow pelagic sedimentation of mud from the water column that takes place between a turbiditic event and the next one. The resulting architecture of a turbidite is characterized by a fining upward sequence, with sediments that are progressively more and more fine-grained towards the top, as a result of decreasing velocity and energy of the flow.
Other types of turbidites
Turbidites in nature show a wide range of structures, beyond the Bouma sequence. In first order, Bouma sequences themselves may be incomplete, as some grain sizes can be missing and therefore do not develop the associated layers. More generally, mixtures of water and sediments produce a wide range of non-cohesive density currents, with different viscosity and active mechanisms that allow the mixture to flow. Finally, turbidites may evolve an incorporate more water as they flow and this causes lateral variations in depositional facies and structures.
Debris flows (which are not turbidity currents) are cohesive flows where sediments move suspended in a muddy/sandy matrix, producing unsorted, massive, and chaotic deposits. When a certain amount of water starts to be present in the flowing sediment, water infiltrates between grains reducing the overall matrix strength and making the flow non-cohesive. High density currents like hyperconcentrated and concentrated density flows are sustained by buoyancy and grain- to grain support. Their relatively high viscosity and density hinders the development of Bouma sequences, since grain sizes are not free to move and separate within the flow. Hyperconcentrated flows are still chaotic and poorly organized, while in concentrated density flows we may see graded beds (to a certain degree) topped with cross and parallel laminae, even if the separation of the various grain sizes is not perfect. Turbidity currents in a strict sense are low-density flows that can develop Bouma sequences or surge-type flows (when only fine-grained sediments are available), as the high water content allows sediments to organize and move more freely within the flow, suspended in the turbulent flow.
Turbidites and erosion
Turbidity currents and associated density flows commonly show erosional surfaces at their base. This happens because the currents, travelling on the seafloor at relatively high speed, have enough energy to erode the underlying beds. Interlayer mudstones (layer E of Bouma) are often victims of erosive processes and rip-up clasts of mudstone, tore off the seabed, can be incorporated in the overlying bed. Channels and scours may develop at the base of the turbidite, leading to complex architectures characterized by truncated and amalgamated sandstone beds.
Turbidity currents deposit water-saturated sediments. After deposition, sediments start to compact into sedimentary rocks and water is expelled upwards. The presence of sequences of sand (permeable) and mud (impermeable) produces an architecture with barriers that hinder water escape from the sediment. Water tends, therefore, to concentrate at the top of turbidites, causing liquefaction to occur. This deforms original sedimentary structures. The convolute lamination that occurs so frequently at the top of turbidites is the product of liquefaction over original parallel to cross lamination. Dish, flame, and ball-and-pillow structures constitute other very common liquefaction-related structures.
Bouma, A. H. (1962). Sedimentology of some flysch deposits. Agraphic approach to facies interpretation, 168.
Lowe, D. R. (1982). Sediment gravity flows; II, Depositional models with special reference to the deposits of high-density turbidity currents. Journal of sedimentary research, 52(1), 279-297.
Mulder, T., & Alexander, J. (2001). The physical character of subaqueous sedimentary density flows and their deposits. Sedimentology, 48(2), 269-299.
Mutti et al. (2003). Deltaic, mixed and turbidite sedimentation of ancient foreland basins. Marine and Petroleum Geology, 20(6-8), 733-755.