Turbidity Currents

Turbidity currents are becoming and increasingly well known occurrence in marine sediments. Turbidity currents occur when fine sands on a continental slope become mixed with the surrounding water to become a sediment laden, dense liquid layer. The dense material then flows down the slope at a high velocity, cutting a submarine canyon as it flows (Mastbergen, Van den Berg 2003). While the classic model of turbidity currents may be well known and understood, there is also geological evidence of non-stereotypical turbidity currents occurring that require a much deeper explanation. Turbidity currents can be formed in different varieties due to deflection and reflection of the current when it reaches a bounding slope.

Methods

Most methods for researching differentiations in turbidity currents involve observing ancient sediment deposits that can be viewed on land. There is evidence of altercations in deposits in areas around the Mediterranean in areas such as the South Central Pyrenees in Spain (Schuppers, Martinius 1994), the Sorbas Basin in Southeastern Spain (Haughton), and Western Central Italy (Kneller, McCaffrey 1999). By observing sedimentary rock formations on land that were created by turbidity these geologists were able to gather enough evidence to show that not all turbidity currents behave in the same manner.

Results

Schuppers and Martinius (1994) observed rocks formed by turbidity currents with sediments that had not settled in the usually fashion of grading. These sediments contained fine-grained material at the top and bottom of the rock, with coarser grained material sandwiched in between. In common turbidity currents the coarse grained material would have settled first and be on the bottom of the formation (Mastbergen, Van den Berg (2003). Schuppers and Martinius (1994) postulated that a turbidity current that had not flowed in a common fashion caused the formations. These turbidity currents, they theorized, were caused when coarse-grained material had been deposited on top of fine-grained material through some process like a volcanic event or by a previous turbidity current scraping away fine sediment to expose coarse sediment.

The fine sediments below would then become mixed with the surrounding water and create a turbidity current that would flow down the slope. Rather than sink down, the coarse sediment would ride on top of this flowing layer and be deposited on top of it once settled “much as in hover crafting” (Schuppers, Martinius 1994). Haughton observed deposits in Southeast Spain. Haughton (1994) stated, “Turbidity currents are important in distributing sediment in many basins. They are capable of long-distance sediment transport on low slopes or flat abyssal plains, flowing for hundreds and sometimes thousands of kilometers. There are, however, a number of types of basin which, while being particularly prone to turbidity currents, have widths of only a few tens of kilometers.”

He found rock formations that showed sediment deposits that had been deflected off of a bounding slope, causing them to move perpendicular to the slope rather than with it as usual turbidity currents do. Haughton (1994) also observed sediments that had pooled, or gathered in one location at the base of a slope rather than continue to move and eventually dissipate. This pooling effect couples in well with results gathered by Kneller and McCaffrey (1994). Kneller and McCaffrey (1994) used many examples of deflected or reflected turbidity currents to provide solid explanations of how they were formed. They stated “Low [velocities] result in decoupling of the flow into a lower, denser part that moves around the topography and an upper part that moves up or over it” (Kneller, McCaffrey 1994). They observed a turbidity currents interaction with a bounding slope and discussed the relationship between the height of a bounding slope and the velocity of a sediment flow. If turbidity currents have a high velocity and approach a relatively low slope, they have enough velocity to move up and over the slop. If a high velocity current reaches a steep slope, it will travel up the slope initially but will be reflected backwards causing a backwards flow and create a pooling effect similar to that observed by Haughton (1994).

In some cases the denser particles in the flow will be reflected while the less dense fluids have enough force to travel completely over the slope. If the velocity of a turbidity current is low and it reaches a low bounding slope, the less dense fluids will be able to travel directly over the slope while the heavier fluids will be deflected and will travel laterally to the slope. In the case of a low velocity current coming to a steep slope, the less dense material will travel up the slope initially but will eventually be reflected and pool while the more dense material is deflected from the start and will travel laterally to the slope (Kneller, McCaffrey 1999).

Kneller, McCaffrey 1999

This diagram from Kneller and McCaffrey (1999) is an excellent visual representation of their work. From left to right there is an increase in turbidity current velocity and from top to bottom there is an increase in slope size.

Discussion

While the research in these projects all comes from the Mediterranean, it could be assumed that such formations must occur elsewhere in the world; however, the Mediterranean provides a rather unique setting for turbidity currents due to it having both steep slopes with a narrow basin. The lack of a deep-sea trench also helps these formations as sediments in locations such as the Indian Ocean would most likely be lost to a trench before they encountered a bounding slope.

The data gathered here is all from archaeological evidence in sedimentary rock formations on land. Research from sediments currently in the Mediterranean could provide more modern evidence. This would aid research in determining whether turbidity currents such as these still flow in the Mediterranean as well as specific locations where they occur. If such data were gathered then turbidity flows could be mapped in a river-like system which would help researchers determine where sediment deposited by a turbidity current originally came from.

Research from Schuppers and Martinius (1994) on the abnormal layering of turbidity current deposits could void other theories made as to how coarser sediments settle on top of finer sediments. Again, examples of “hover crafting” sediment flows in modern data would also be extremely helpful as one can be sure their theories would be challenged by the scientific community for their unusual and somewhat fanciful nature.

Conclusion

This research certainly adds new depth to an already interesting topic. Turbidity currents seem to be gathering a lot of attention recently as many journals seem to have articles covering new and fantastic and occasionally far-fetched ways turbidity currents might form the ocean floor and explain the previously unexplainable. Archaeological evidence can only help so much however and much more research needs to be done in the field as these events are occurring, so that the scientific community can fully understand what processes are involved in making a dense layer of sediment laden liquid to hurtle down a slope to the deeper waters beyond…or in some cases hovercraft down.