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A perspective on what is known and not known about seafloor instability in the context of continental margin evolution
Marine and Petroleum Geology 18 (2001) 499±501
www.elsevier.com/locate/marpetgeo
A perspective on what is known and not known about sea¯oor instability in the context of continental margin evolution L.F. Pratson* Division of Earth and Ocean Sciences, Duke University, 103 Old Chemistry Building, Box 90227, Durham, NC 27708-0230, USA Received 1 September 1999; received in revised form 10 February 2000; accepted 25 April 2000 Keywords: Sea¯oor instability; Continental margin evolution; Sea¯oor failures
Sea¯oor instabilities are the source of catastrophic failure events, such as slumps, slides, debris ¯ows and turbidity currents. As a result, they exert a signi®cant in¯uence on the evolution of continental margin morphology and stratigraphy by dictating when, where, how, and what amounts of sediments are transferred from shallow to deep water by gravity-driven transport. Furthermore, since sea¯oor instabilities are an inherent part of the physical makeup of continental margins, they continue to cause modern day failures even in areas where sea level is high, and seismicity and sediment supply are low. Consequently, sea¯oor instabilities pose an important natural threat to the exploration and utilization of continental margins and their resources. No physiographic province within a continental margin appears to be immune to sea¯oor instability. Sea¯oor failures have been found originating in water depths from the shoreline to the continental rise. However, when the number of failures is plotted versus water depth, their greatest occurrence is found to be on the continental slope (Booth, O'Leary, Popenoe, & Danforth, 1993; Pratson & Laine, 1989). One obvious reason for this is that the continental slope is where margins are steepest and thus where the downslope force of gravity is greatest. A less obvious reason though is that the continental slope also tends to be where sediment accumulation along margins is highest. This runs counter to the traditional view of margins, in which continental slopes are considered to be largely regions of erosion and sediment bypassing. However, when the modern surface of a continental margin is compared to one of its pre-existing surfaces (i.e. a stratigraphic horizon), it is commonly seen that the margin has prograded more than it has aggraded
* Tel.: 11-919-681-8077; fax: 11-919-684-5833. E-mail address: [email protected] (L.F. Pratson).
(Fig. 1A), clearly revealing that sediment accumulation has been greatest on the slope. Boreholes from 11 Legs of the Ocean Drilling Project (11, 41, 47, 93, 95, 96, 150, 155, 157, 164 and 174) show sediments on the open continental slope to be predominantly pelagic and hemipelagic in origin, further supporting the slope as a site of sediment accumulation. Where preserved, open slope sediments often record long periods of slow to moderate, but relatively continuous deposition. For example, Site 1073 drilled during ODP Leg 174 appears to contain a nearly complete record of deposition on the New Jersey Slope from 14 to 80 Kya and from 250 to 600 Kya (Shipboard Scienti®c Party, 1998). Interrupting such continuous records though are signi®cant erosional unconformities associated with slope failure. A number of these unconformities appear to correlate to changes in sea level, but not all. Constraints on where sea¯oor instabilities develop on continental slopes to set up these failure events largely come from circumstantial evidence. Along the US Atlantic margin, the greatest number of mapped sea¯oor failures originate on the mid to upper slope (Booth et al., 1993). They are also more common on the open slope than in submarine canyons, and on low sea¯oor gradients (,98) rather than high ones ($98). Circumstantial evidence has also been used along with theoretical and laboratory analyses and a relatively few, rare direct observations to constrain the causes for sea¯oor instability. Among the most important triggers are: oversteepening by non-uniform deposition, erosional undercutting or tectonics; cyclic loading caused by earthquakes and waves; pore-water freshening by discharging gas or hydrate decomposition; and excess pore pressures induced by ¯uid ¯ow. When this information is combined with the detailed images of sea¯oor failures provided by swath mapping and 3D seismic imaging, it would seem that quite a bit is
0264-8172/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0264-817 2(00)00077-5
500
L.F. Pratson / Marine and Petroleum Geology 18 (2001) 499±501
Fig. 1. (A) Interpreted seismic pro®le from offshore Alabama (modi®ed from Greenlee & Moore, 1988). Note the difference between the youngest interpreted horizon and the sea¯oor indicating that the margin has prograded more than it has aggraded. This progradation indicates that sediment accumulation has been greatest on the slope. (B) Illustration of the COST and DSDP drilling strategy on the New Jersey margin (modi®ed from Poag & Low, 1987).
known about sea¯oor instabilities. But as Prior (1998) points out, there is still a lot that is not known. For example, it is not known why one region of sea¯oor will fail while neighboring regions remain intact. It is also not known why a sea¯oor region will fail in the way that it does. And it is not known why certain sea¯oor regions exist under conditions that theoretically should result in failure. An excellent case in point is the Northern California continental shelf and slope in the vicinity of the Eel River. This region was selected for detailed study by the US Of®ce of Naval Research STRATAFORM program in part because it's high potential for sea¯oor instability (Nittrouer & Kravitz, 1996). The margin is being deformed by the tectonic movement of the North American Plate over the Juan de Fuca Plate. On average, the margin experiences an earthquake of Richter magnitude $6 every decade (Couch, Victor, & Keeling, 1974). Local rivers annually deliver more than 20 million metric tons of organic-rich sediments to the margin (Field, Clarke, & White, 1980). And the margin sediments have become charged with gas hydrates
(Field & Barber, 1993). Yet despite these numerous agents for destabilizing the sea¯oor, comprehensive surface and subsurface mapping of the area has revealed only two relatively small failures (Driscoll, 1998; Gardner, Prior, & Feild, 1999) Ð an unexpected and surprising result. So while failures provide important constraints on the distributions and causes of sea¯oor instabilities, we are still missing critical information as to why this phenomenon varies from one site to the next. And it is this variability that makes sea¯oor instabilities so dif®cult to understand and ultimately predict. More or new surface measurements are not going to improve upon this situation without greater study of the subsurface sediments in which instabilities form. At present, the standard approach for evaluating sea¯oor stability has been to conduct geotechnical measurements on sediments collected with piston cores, which generally penetrate no more than 6 m below the sea¯oor. Given the large scale over which sea¯oor instabilities develop, this is like trying to diagnose the health of an elephant using pins.
L.F. Pratson / Marine and Petroleum Geology 18 (2001) 499±501
To properly analyze sea¯oor stability, sediments need to be sampled in and around failure-prone areas to depths of at least 50±100 m. This type of penetration would allow detailed examination of the physical and chemical makeup of destabilized sediments and the known or potential failure surfaces they bound. The technology for conducting this research has been in use for years in both the oil industry and ODP. Hydraulic piston coring, in-situ borehole measurements, and loggingand measurement-while-drilling all offer new approaches for measuring sea¯oor stability. However, to date, these tools have been used for other purposes. In the oil industry, the tools have been applied in geotechnical studies of small, targeted areas to identify and avoid sea¯oor instabilities that might jeopardize exploration/production drilling. In ODP, they have been utilized to sample failure deposits on the continental rise or unconformities on the continental slope that might correlate to ¯uctuations in global sea level (Fig. 1B). These and other tools should now be used in a new, systematic sampling program to study sea¯oor instability. In academia, the issues of natural hazards and seascape evolution are receiving increased interest. In the oil industry, exploration and production is being conducted in everdeeper waters on the continental slope. And in both arenas, there is an increasing awareness that important advances in understanding continental margin stratigraphy can come from studying continental margin processes. Sea¯oor instabilities and their role in continental margin evolution are central to all of these issues. It is time to formulate how best to put their study through drilling into action. References Booth, J. S., O'Leary, D. W., Popenoe, P., & Danforth, W. W. (1993). US Atlantic continental slope landslides: their distribution, general attri-
501
butes, and implications. Submarine landslides: selected studies in the US exclusive economic zone. W. C. Schwab. US Geological Survey Bulletin, 2002, 14±22. Couch, R. W., Victor, L. P., & Keeling, K. M. (1974). Coastal and offshore earthquakes of the Paci®c Northwest between 39 and 498 N latitude and 123 and 1318 W longitude, Corvallis, OR: Oregon State University Press, School of Oceanography (pp. 67). Driscoll, N. (1998). High resolution side-scan and seismic images of landslides on the northern California continental shelf (abs.). EOS transactions American Geophysical Union. Field, M. E., Clarke, S. H. Jr., & White, M. E. (1980). Geology and geologic hazards of offshore Eel River Basin, northern California continental margin. US Geological Survey Open-File Report 80-1080 (80 pp.). Field, M. E., & Barber Jr., J. H. (1993). Submarine landslide associated with shallow sea¯oor gas hydrates off northern California. Submarine landslides: selected studies in the US exclusive economic zone. W. C. Schwab. US Geological Survey Bulletin, 2002, 151±157. Gardner, J. V., Prior, D. B., & Field, M. E. (1999). Humboldt Slide Ð a large shear-dominated retrogressive slope failure. Marine Geology, 154, 323±338. Greenlee, S. M., & Moore, T. C. (1988). Recognition and interpretation of depositional sequences and calculation of sea-level changes from stratigraphic data Ð offshore New Jersey and Alabama Tertiary. Sea-level changes: an integrated approach. C. K. Wilgus, et al. SEPM Special Publication, 42, 329±353. Nittrouer, C. A., & Kravitz, J. H. (1996). strataform: a program to study the creation and interpretation of sedimentary strata on continental margins. Oceanography, 9, 146±152. Poag, C. W., & Low, D. (1987). Unconformable sequence boundaries at deep sea drilling project site 612, New Jersey transect: their characteristics and stratigraphic signi®cance. In C. W. Poag & A. B. Watts, Initial reports of the deep sea drilling project, Vol. 95. (pp. 453±498). Pratson, L. F., & Laine, E. P. (1989). The relative importance of gravityinduced versus current-controlled sedimentation during the Quaternary along the mideast US outer continental margin revealed by 3.5 kHz echo character. Marine Geology, 89, 87±126. Prior, D. B. (1998). Sea¯oor instability and evolution of slope morphology. SEPM-IAS Research Conference: Strata and Sequences on Shelves and Slopes, September 15±19, Sicily, Italy. Shipboard Scienti®c Party. (1998). Site 1073. In J. A. Austin, N. ChristieBlick, M. J. Malone et al. (Eds.), Proceedings of the Ocean Drilling Program, Initial Reports, (Vol. 174A). (pp. 153±191).
www.elsevier.com/locate/marpetgeo
A perspective on what is known and not known about sea¯oor instability in the context of continental margin evolution L.F. Pratson* Division of Earth and Ocean Sciences, Duke University, 103 Old Chemistry Building, Box 90227, Durham, NC 27708-0230, USA Received 1 September 1999; received in revised form 10 February 2000; accepted 25 April 2000 Keywords: Sea¯oor instability; Continental margin evolution; Sea¯oor failures
Sea¯oor instabilities are the source of catastrophic failure events, such as slumps, slides, debris ¯ows and turbidity currents. As a result, they exert a signi®cant in¯uence on the evolution of continental margin morphology and stratigraphy by dictating when, where, how, and what amounts of sediments are transferred from shallow to deep water by gravity-driven transport. Furthermore, since sea¯oor instabilities are an inherent part of the physical makeup of continental margins, they continue to cause modern day failures even in areas where sea level is high, and seismicity and sediment supply are low. Consequently, sea¯oor instabilities pose an important natural threat to the exploration and utilization of continental margins and their resources. No physiographic province within a continental margin appears to be immune to sea¯oor instability. Sea¯oor failures have been found originating in water depths from the shoreline to the continental rise. However, when the number of failures is plotted versus water depth, their greatest occurrence is found to be on the continental slope (Booth, O'Leary, Popenoe, & Danforth, 1993; Pratson & Laine, 1989). One obvious reason for this is that the continental slope is where margins are steepest and thus where the downslope force of gravity is greatest. A less obvious reason though is that the continental slope also tends to be where sediment accumulation along margins is highest. This runs counter to the traditional view of margins, in which continental slopes are considered to be largely regions of erosion and sediment bypassing. However, when the modern surface of a continental margin is compared to one of its pre-existing surfaces (i.e. a stratigraphic horizon), it is commonly seen that the margin has prograded more than it has aggraded
* Tel.: 11-919-681-8077; fax: 11-919-684-5833. E-mail address: [email protected] (L.F. Pratson).
(Fig. 1A), clearly revealing that sediment accumulation has been greatest on the slope. Boreholes from 11 Legs of the Ocean Drilling Project (11, 41, 47, 93, 95, 96, 150, 155, 157, 164 and 174) show sediments on the open continental slope to be predominantly pelagic and hemipelagic in origin, further supporting the slope as a site of sediment accumulation. Where preserved, open slope sediments often record long periods of slow to moderate, but relatively continuous deposition. For example, Site 1073 drilled during ODP Leg 174 appears to contain a nearly complete record of deposition on the New Jersey Slope from 14 to 80 Kya and from 250 to 600 Kya (Shipboard Scienti®c Party, 1998). Interrupting such continuous records though are signi®cant erosional unconformities associated with slope failure. A number of these unconformities appear to correlate to changes in sea level, but not all. Constraints on where sea¯oor instabilities develop on continental slopes to set up these failure events largely come from circumstantial evidence. Along the US Atlantic margin, the greatest number of mapped sea¯oor failures originate on the mid to upper slope (Booth et al., 1993). They are also more common on the open slope than in submarine canyons, and on low sea¯oor gradients (,98) rather than high ones ($98). Circumstantial evidence has also been used along with theoretical and laboratory analyses and a relatively few, rare direct observations to constrain the causes for sea¯oor instability. Among the most important triggers are: oversteepening by non-uniform deposition, erosional undercutting or tectonics; cyclic loading caused by earthquakes and waves; pore-water freshening by discharging gas or hydrate decomposition; and excess pore pressures induced by ¯uid ¯ow. When this information is combined with the detailed images of sea¯oor failures provided by swath mapping and 3D seismic imaging, it would seem that quite a bit is
0264-8172/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0264-817 2(00)00077-5
500
L.F. Pratson / Marine and Petroleum Geology 18 (2001) 499±501
Fig. 1. (A) Interpreted seismic pro®le from offshore Alabama (modi®ed from Greenlee & Moore, 1988). Note the difference between the youngest interpreted horizon and the sea¯oor indicating that the margin has prograded more than it has aggraded. This progradation indicates that sediment accumulation has been greatest on the slope. (B) Illustration of the COST and DSDP drilling strategy on the New Jersey margin (modi®ed from Poag & Low, 1987).
known about sea¯oor instabilities. But as Prior (1998) points out, there is still a lot that is not known. For example, it is not known why one region of sea¯oor will fail while neighboring regions remain intact. It is also not known why a sea¯oor region will fail in the way that it does. And it is not known why certain sea¯oor regions exist under conditions that theoretically should result in failure. An excellent case in point is the Northern California continental shelf and slope in the vicinity of the Eel River. This region was selected for detailed study by the US Of®ce of Naval Research STRATAFORM program in part because it's high potential for sea¯oor instability (Nittrouer & Kravitz, 1996). The margin is being deformed by the tectonic movement of the North American Plate over the Juan de Fuca Plate. On average, the margin experiences an earthquake of Richter magnitude $6 every decade (Couch, Victor, & Keeling, 1974). Local rivers annually deliver more than 20 million metric tons of organic-rich sediments to the margin (Field, Clarke, & White, 1980). And the margin sediments have become charged with gas hydrates
(Field & Barber, 1993). Yet despite these numerous agents for destabilizing the sea¯oor, comprehensive surface and subsurface mapping of the area has revealed only two relatively small failures (Driscoll, 1998; Gardner, Prior, & Feild, 1999) Ð an unexpected and surprising result. So while failures provide important constraints on the distributions and causes of sea¯oor instabilities, we are still missing critical information as to why this phenomenon varies from one site to the next. And it is this variability that makes sea¯oor instabilities so dif®cult to understand and ultimately predict. More or new surface measurements are not going to improve upon this situation without greater study of the subsurface sediments in which instabilities form. At present, the standard approach for evaluating sea¯oor stability has been to conduct geotechnical measurements on sediments collected with piston cores, which generally penetrate no more than 6 m below the sea¯oor. Given the large scale over which sea¯oor instabilities develop, this is like trying to diagnose the health of an elephant using pins.
L.F. Pratson / Marine and Petroleum Geology 18 (2001) 499±501
To properly analyze sea¯oor stability, sediments need to be sampled in and around failure-prone areas to depths of at least 50±100 m. This type of penetration would allow detailed examination of the physical and chemical makeup of destabilized sediments and the known or potential failure surfaces they bound. The technology for conducting this research has been in use for years in both the oil industry and ODP. Hydraulic piston coring, in-situ borehole measurements, and loggingand measurement-while-drilling all offer new approaches for measuring sea¯oor stability. However, to date, these tools have been used for other purposes. In the oil industry, the tools have been applied in geotechnical studies of small, targeted areas to identify and avoid sea¯oor instabilities that might jeopardize exploration/production drilling. In ODP, they have been utilized to sample failure deposits on the continental rise or unconformities on the continental slope that might correlate to ¯uctuations in global sea level (Fig. 1B). These and other tools should now be used in a new, systematic sampling program to study sea¯oor instability. In academia, the issues of natural hazards and seascape evolution are receiving increased interest. In the oil industry, exploration and production is being conducted in everdeeper waters on the continental slope. And in both arenas, there is an increasing awareness that important advances in understanding continental margin stratigraphy can come from studying continental margin processes. Sea¯oor instabilities and their role in continental margin evolution are central to all of these issues. It is time to formulate how best to put their study through drilling into action. References Booth, J. S., O'Leary, D. W., Popenoe, P., & Danforth, W. W. (1993). US Atlantic continental slope landslides: their distribution, general attri-
501
butes, and implications. Submarine landslides: selected studies in the US exclusive economic zone. W. C. Schwab. US Geological Survey Bulletin, 2002, 14±22. Couch, R. W., Victor, L. P., & Keeling, K. M. (1974). Coastal and offshore earthquakes of the Paci®c Northwest between 39 and 498 N latitude and 123 and 1318 W longitude, Corvallis, OR: Oregon State University Press, School of Oceanography (pp. 67). Driscoll, N. (1998). High resolution side-scan and seismic images of landslides on the northern California continental shelf (abs.). EOS transactions American Geophysical Union. Field, M. E., Clarke, S. H. Jr., & White, M. E. (1980). Geology and geologic hazards of offshore Eel River Basin, northern California continental margin. US Geological Survey Open-File Report 80-1080 (80 pp.). Field, M. E., & Barber Jr., J. H. (1993). Submarine landslide associated with shallow sea¯oor gas hydrates off northern California. Submarine landslides: selected studies in the US exclusive economic zone. W. C. Schwab. US Geological Survey Bulletin, 2002, 151±157. Gardner, J. V., Prior, D. B., & Field, M. E. (1999). Humboldt Slide Ð a large shear-dominated retrogressive slope failure. Marine Geology, 154, 323±338. Greenlee, S. M., & Moore, T. C. (1988). Recognition and interpretation of depositional sequences and calculation of sea-level changes from stratigraphic data Ð offshore New Jersey and Alabama Tertiary. Sea-level changes: an integrated approach. C. K. Wilgus, et al. SEPM Special Publication, 42, 329±353. Nittrouer, C. A., & Kravitz, J. H. (1996). strataform: a program to study the creation and interpretation of sedimentary strata on continental margins. Oceanography, 9, 146±152. Poag, C. W., & Low, D. (1987). Unconformable sequence boundaries at deep sea drilling project site 612, New Jersey transect: their characteristics and stratigraphic signi®cance. In C. W. Poag & A. B. Watts, Initial reports of the deep sea drilling project, Vol. 95. (pp. 453±498). Pratson, L. F., & Laine, E. P. (1989). The relative importance of gravityinduced versus current-controlled sedimentation during the Quaternary along the mideast US outer continental margin revealed by 3.5 kHz echo character. Marine Geology, 89, 87±126. Prior, D. B. (1998). Sea¯oor instability and evolution of slope morphology. SEPM-IAS Research Conference: Strata and Sequences on Shelves and Slopes, September 15±19, Sicily, Italy. Shipboard Scienti®c Party. (1998). Site 1073. In J. A. Austin, N. ChristieBlick, M. J. Malone et al. (Eds.), Proceedings of the Ocean Drilling Program, Initial Reports, (Vol. 174A). (pp. 153±191).