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Turbidite sedimentation.

Turbidity currents are the fastest and most destructive currents in the ocean. The most powerful of them, which can carry hundreds of cubic kilometers of sediment as coarse as gravel, are commonly initiated when earthquakes or storm waves cause submarine landsliding that dislodges sediment on the slopes of continental margins. Hurricane storm surges can initiate turbidity currents from otherwise peaceful atoll and oceanic-island coral reefs. Another important turbidity-current environment lies offshore from Earth's largest rivers, where sediment-laden river water can generate turbidity currents through hyperpycnal flow in which some of the suspended sediment of the river discharge flows along the seafloor and continues downslope as a turbidity current. In addition, the rapidly deposited deltas of these rivers are prone to periodic failure and landsliding.

Rare, very large turbidity currents periodically deposit thick sequences of sediment on oceanic abyssal plains, but their return periods span many thousands of years. However, in some high-discharge fan deltas, several turbidity currents may occur in a single year. Turbidity currents often damage and even destroy human structures, especially submarine telecommunications cables. In fact, our best "observations" of turbidity current velocities are drawn from records of the time elapsed between progressive down-slope cutting of a series of submarine telecommunication cables as a current flows. Velocities of 10 to 20 meters per second are not uncommon. Our understanding of turbidite sediments comes principally from conventional marine-geologic sampling of near-surface sediment and three-dimensional studies of the sediment sequences using seismic-reflection profiling. Ocean drilling allows us to verify this data by sampling the sediment sequences revealed by seismic profiles.

The most fascinating attributes of turbidity currents, their high speeds and their ability to transport coarse sediment into deep water, are also those that make them difficult to study with ocean-drilling techniques. Standard HPC (hydraulic piston core) coring techniques normally used to recover the upper, softer sediment cannot penetrate the thick sand layers left by large turbidity currents. Older, deeply buried sand deposits are easily penetrated by standard rotary drilling, but the sand layers generally are not consolidated (unlike the interbedded mud layers); as a consequence, the sand outside the drill string begins to flow down the hole eventually wedging the drilling pipe in the hole and sometimes causing pipe loss.

Many ocean-drilling scientific objectives require the recovery of continuous and uniform records of deposits for studies of biostratigraphy, past ocean environments, and subsurface geochemical processes. Turbidite deposits typically represent discontinuous or episodic sedimentation and the inclusion of many microfossils transported from shallower water. In thick sediment sequences turbidite sands commonly comprise hydrocarbon reservoirs and thus must be avoided for safety reasons. As a consequence, many ocean-drilling legs are deliberately planned to avoid turbidite deposits.

Nevertheless, turbidite sediment deposits do provide important information on ocean history. They are the most direct record of rapid mountain-belt erosion and provide a high-resolution record of the supply of terrigenous detritus to the ocean. Turbidite sediment derived from volcanic seamounts or oceanic-carbonate platforms provides evidence of the timing of tectonic and volcanic events or faulting on the ocean floor and eruptive activity of seamounts. Predominantly muddy turbidite sediment has been drilled successfully, with high rates of sample recovery on several ocean-drilling legs, with the objective of obtaining continuous stratigraphic sections for interpreting both ocean history and tectonic history of surrounding land areas. Examples of such sections include the Weddell Abyssal Plain (Leg 113) for the glacial history of West Antarctica, the Lau Basin (Leg 135) for volcanic history of adjacent islands, and the Argo Abyssal Plain (Leg 123) for the erosional history of the adjacent continents. Leg 116 drilled the abyssal plain south of the Bengal deep-sea fan, primarily to understand the erosional history of the Himalayas and the character of oceanic-floor tectonism in the area.

In addition to revealing information on oceanic events and tectonic history of the adjacent land areas, the turbidite layers also provide information about some of the flow characteristics of the turbidity currents themselves. Because the precise source area for turbidite-sediment grains can commonly be determined, the Deep Sea Drilling Program and Ocean Drilling Program cores have shown, for example, that sediment from the Columbia River off the Pacific Northwest has been carried by turbidity currents more than 700 kilometers south, then moved west about 150 kilometers before being carried north into an axial valley of a spreading ridge (Leg 5). Turbidity currents generated by large landslides on the flanks of the Hawaiian volcanoes have traveled at least 250 kilometers seaward and moved up and over topographic barriers some 500 meters high (Leg 136).

Ocean-drilling targets selected primarily for stratigraphic or tectonic significance also provide opportunities to determine what turbidite sequences are typical of particular submarine environments. However, many of the fundamental questions concerning the processes of turbidite deposition cannot be addressed on basin floors, reached only by occasional turbidity currents. Cores from deep-sea fans that are crossed by channel/natural levee complexes offer the most continuous record of turbidite deposition and allow us to unravel the complex interplay between seabed morphology and turbidity-current processes.

Glomar Challenger's last cruise (Leg 96) drilled the Mississippi Fan, one of the largest modern turbidite deposits, with the express purpose of learning about the history and processes of deep-water sedimentation in an area where the paleoclimatic effects on sediment supply were relatively well known. The Leg 96 program confirmed extremely rapid rates of deposition on the mid fan (11 meters per thousand years at a distance of nearly 500 kilometers from the river mouth) and that large-scale landsliding also provides major contributions to deep-sea fan sequences. Core samples from the major fan-valley areas further demonstrated a marked change in sedimentation (rate and type of sediment) as sea level rose after the last glacial period.

The next major program for turbidite study will be in early 1994 on the Amazon Fan, which is even larger than the Mississippi Fan. The Amazon Fan exhibits a complex series of meandering channels built by basinward-flowing turbidity currents. A lobe-like deposit of sediment builds up from turbidity currents flowing through and exiting the channels. The channels periodically change course and build new lobes. The Amazon Fan leg aims to further define the sediment types and ages of deposits that have been identified by seismic-reflection profiling, and to relate this information to controls on sediment supply for turbidity currents, such as sea-level change and river discharge.

The thick turbidite sequences on the Mississippi and Bengal submarine fans and other abyssal plains drilled by the Deep Sea Drilling Program and the Ocean Drilling Program are in areas underlain by oceanic crust. The closing of ocean basins through subduction means that the ultimate fate of these turbidite sequences is to be highly fragmented and deformed in subduction zone accretionary wedges and eventually to form part of collisional orogenic belts, and become welded into the crystalline metamorphic fabric of continental crust. Indeed, many of the accretionary wedges drilled on the ocean margin contain a high proportion of turbidite deposits.

The stratigraphic record provided by ocean drilling has brought better understanding of some of the external controls on the accumulation of turbidite deposits. For example, turbidite deposits are more common when sea levels are low worldwide, particularly at mid and high latitudes, and there is a marked increase in turbidite abundance with the onset of extensive continental glaciation in the late Tertiary (during the last 5 million years). The detailed relationship between sea-level change and turbidite deposition remains unclear and is one of the major objectives of the Amazon Fan leg planned for spring 1994.

Being reared near Ocean Lake, Wyoming, is one of the more plausible excuses for Bill Normark's keen desire to go to sea whenever possible. He has been a loyal fan of deep-sea turbidite fans ever since his thesis advisor at Scripps Institution of Oceanography suggested that he choose between global marine excursions and a career in research. When he is not actively involved in the study of modern turbidites or doing his duty as Assistant Chief Geologist for the US Geological Survey, he dreams about continuing his other research interests, including the submerged parts of the Hawaiian volcanoes where humongous submarine landslides dominate the seafloor.

David Piper was educated as a traditional land geologist at Cambridge University. During his Ph.D. studies, he spent a sabbatical year at Scripps Institution of Oceanography, where he met Bill Normark, and has enjoyed working at sea ever since. His interests are in using marine geology to understand the processes involved in depositing rocks seen on land. He is a Research Scientist with the Geological Survey of Canada at Bedford Institute of Oceanography.
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Title Annotation:25 Years of Ocean Drilling
Author:Normarck, William R.; Piper, David J.W.
Date:Dec 22, 1993
Previous Article:Terrigenous sediments in the pelagic realm.
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