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Organic sedimentation in deep offshore settings: the Quaternary sediments approach
Marine and Petroleum Geology 18 (2001) 513±517
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Organic sedimentation in deep offshore settings: the Quaternary sediments approach A.Y. Huc a,*, P. Bertrand b, D.A.V. Stow c, J. Gayet b, M. Vandenbroucke d a
IFP School, Ecole du Petrole et des Moteurs, Institut FrancËais du PeÂtrole, 228-232 Avenue Napoleon Bonap., 92852 Rueil-Malmaison, France b Departement de GeÂologie et Oceanographie, Universite de Bordeaux I, Avenue de FaculteÂs, 33405 Talence, France c School of Ocean and Earth Science, Southampton Oceanography Centre, Southampton University, Waterfront Campus, Southampton SO14 3ZH, UK d Geology±Geochemistry Department, Institut FrancËais du PeÂtrole, 1-4 Avenue de Bois-PreÂau, 92852 Rueil-Malmaison, France Received 30 January 2000; received in revised form 18 February 2000; accepted 1 August 2000 Keywords: Organic sedimentation; Deep offshore; Quarternary sediments
1. Introduction A classical view of organic sedimentation in marine environment has emerged during the past few years, and provides accepted guidelines for understanding the distribution of source rocks in sedimentary basins (Huc, 1988a). According to this conceptual model, the accumulation of organic material is the result of the action of favourable factors including high primary productivity (Pedersen & Calvert 1990), enhanced preservation conditions, and is modulated in a complex way by the sedimentation rate (dilution, time of residence of the organics at the surface of the sediment, detritic versus biogenic origin of the mineral phase, water depth, distance from the shore) (Pelet, 1983; Pelet, 1987; Stein, 1986). The main preservation controls are considered to be the development of anoxic conditions in bottom water (Demaison & Moore, 1980) and the water depth (Suess, 1980). Based on an extensive data set derived from sediment trap experiments, Suess (1980) has shown that the ¯ux of organic material which is exported from the euphotic zone in oceanic realm decreases exponentially with depth throughout the water column (such that only very few percent of the organic ¯ux escaping from the euphotic zone is reaching the bottom at a thousand meter depth). Such a paradigm would appear to exclude any possibility of substantial organic accumulation to take place in deep offshore settings. In this paper two examples of recent Quaternary deposition are given that challenge this paradigm and provide sound elements for revisiting the sedimentology of organic matter in deep water environments. This clearly has impor-
* Corresponding author. Tel.: 133-1-4752-6000; fax: 133-1-4752-7000. E-mail address: [email protected] (A.Y. Huc).
tant implications for a better understanding of the distribution of source rocks in deep offshore petroleum systems.
2. The Makassar strait example The siliciclastic sediments deposited in the Makassar strait, between the passive margin of eastern Borneo and the active margin of western Celebes are herein considered along a cored transect off the Mahakam Delta (Kalimantan, Indonesia) that was made during the Misedor II cruise. The transect (Fig. 1) includes four sites located respectively on the continental shelf (Core KS 13, water depth: 50 m), the slope (Core KS 15, water depth: 250 m), the well developed rise (Core KS 19, water depth: 1975 m) and the abyssal plain (Core KS 12, water depth: 2229 m). These cores, which have been extensively studied by FaugeÁres, Gayet, and Gonthier (1989), encompass sediments deposited during the late Quaternary climatic phases, including the last glaciation, the post glacial period and the Holocene. The sedimentation is predominantly controlled by the terrigenous input originating from the Mahakam delta. The sedimentary regime is closely related to gravity ¯ows which are well expressed as turbiditic sequences on the rise and on the abyssal plain, with more distal characters for the latter (Gayet, FaugeÁres, & Gonthier, 1990). A noticeable point regarding these sediments is their high organic content, including the deepest rise (1975 m) and abyssal plain (2229 m) situations. The two latter cores exhibit even individualized intervals, in which rich assemblage of organic remains constituted of sulphur epigenized woody detritus and fragmentary leaf debris are associated with total organic carbon exceeding 5%. The higher content of organics and plant macrodetritus deposited during the ice age and their decreasing abundance in the subsequent
0264-8172/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0264-817 2(00)00069-6
514 A.Y. Huc et al. / Marine and Petroleum Geology 18 (2001) 513±517
Fig. 1. Pro®le of Organic carbon content in Quaternary cores along a transect in the Makassar Strait.
A.Y. Huc et al. / Marine and Petroleum Geology 18 (2001) 513±517 515
Fig. 2. Distribution of organic content in Quaternary sediments offshore Namibia.
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A.Y. Huc et al. / Marine and Petroleum Geology 18 (2001) 513±517
sediments can be rationalized in terms of glacioeustatic changes. During the last glacial maximum (around 20 kyrs. BP), the largest part of the continental shelf was exposed. Organic bearing ¯uvial and deltaic sediments were deposited on the platform by the Mahakam river, but part of the organic matter was conveyed toward the rise and the abyssal plain where it accumulated together with high energy turbidites. During the post glacial time, rising of the sea tended to trap a part of the ¯uvial sediments, including organic matter at the edge of the shelf, feeding turbidites that become ®ner and organically poorer as the transgression proceeds. 3. The offshore Namibia example The Benguela current offshore of Namibia is controlling one of the present day most proli®c upwelling system in terms of primary organic productivity. The bottom sediments associated with this upwelling system have been sampled at different locations. The super®cial sediments of the shallow platform area (,150 m) contain an average of 9% TOC and exhibit local maxima reaching as much as 25% under the most active and permanent LuÈderitz and Walvis bay cells of the Benguela system (Calvert & Price, 1971). During DSDP Leg 75, sediments were obtained from a more northern location at site 532 (water depth: 1341 m) and yielded an average organic content of 3.5% TOC in the 75 m thick Quaternary sediments. The considered area of the platform represents a huge surface (1200 km long and 100 km wide) of organic rich pelagic/hemipelagic sediments. During the same Leg, another site (530B) located at the foot of the northern ¯ank of the Walvis ridge at a depth of 4639 m penetrated a 120 m thick section of Quaternary sediments with an average organic content of 3% TOC (Fig. 2) (Meyers, Huc, & Brassel, 1981). These sediments show clear features indicative of redeposition by mass transport processes, including turbidite sequences and debris ¯ow. The entire section is interpreted as the redeposition of sediments originating from a shallow setting on the adjacent platform and ridge, where organic-rich unstable mud had been previously laid down by pelagic/hemipelagic processes under the in¯uence of the highly productive Benguela system (Huc, 1988b; Stow, 1987). More recently three 40 m long piston cores have been recovered during the Nausicaa-Images II cruise along a transect corresponding to the LuÈderitz cell. The sampling depth range includes 1028 m (core MD962087), 2910 m (core MD962098) and 3606 m (core MD962086) (Fig. 2). The geochemical data show that the average organic content of these Quaternary pelagic/hemipelagic sediments are respectively 7.7% TOC, 3.81% TOC and 1.77% TOC. These contents are interpreted to be the result of the combined effect of organic input and carbonate dissolution (Bertrand et al., in preparation). They provide clear
evidence for the possibility of accumulation of organic rich pelagic/hemipelagic deposits in deep oceanic setting. A noticeable point is that these sediments do not show indication of complete anoxia, such as lamination or absence of benthic fauna, at the bottom water interface. A similar situation was observed at Site 532 of Leg 75 where burrows re¯ecting bioturbation by benthic organisms can be recognized throughout the cored Quaternary interval (Meyers, Brassel, & Huc, 1984). This implies that strict anoxia is not indispensable for the accumulation of a sediment with an organic content suf®cient to be in the source rock class. The occurrence of substantial amount of organic matter reaching sediments deposited in very deep settings can be explained by the high biological surface productivity. High primary productivity promotes repacking of organic detritus and of small particles by physicochemical (i.e. ¯occulation) and biological processes producing large organomineral particles (faecal pellets, marine snow, aggregates) with rapid sinking rate which can act as ef®cient vehicles for transporting organic matter through the water column (Dagg & Walser, 1986). 4. Conclusions According to conventional view, deep water settings are not favourable for source rock formation because organic material is subjected to intensive degradation during its transit through the water column. However mass transport (via turbidity currents and debris ¯ow for example) of organic sediments previously deposited in shallow water provides one means for the accumulation of terrestrial and marine derived organic rich sediments in deep offshore situations. Moreover, highly biological productive areas, such as active upwelling zone appear to deliver suf®cient quantity of organic charge to outbalance the degradative capacity of the system, leading to the formation of organic-rich sediments even in deep and not strictly anoxic conditions. These conclusions are presented as important hypotheses that require further elaboration, discussion and testing in other present day deep water settings and, if possible, in ancient sediment records. References Bertrand, P., Pedersen, T., Schneider, R., Shimmield, G., Giraudeau, J., MuÈller, P., Foster, J., Venec-PeyreÂ, M. Th., Pierre, C., Massias, D., & Tribovillard, N. (in preparation). SE Atlantic records suggest older occurrence and low latitude control of the paleocean circulation bipolar seesaw. Calvert, S. E., Price, N. B. (1971). Recent sediments of the South West African shelf. The geology of the East Atlantic continental margin. Delany, F. M., London, Her Majesty's Stationery Of®ce 70/16, 175± 185. Dagg, M. J., & Walser, W. E. (1986). The effect of food concentration on faecal pellet size in marine copepods. Limnology and Oceanography, 31 (5), 1066±1071. Demaison, G., & Moore, G. T. (1980). Anoxic environments and oil source
A.Y. Huc et al. / Marine and Petroleum Geology 18 (2001) 513±517 bed genesis. American Association of Petroleum Geologists Bulletin, 64, 1179±1209. FaugeÁres, J. C., Gayet, J., & Gonthier, E. (1989). Microphysicographie des deÂpoÃts quaternaires dans le deÂtroit de Makassar (oceÂan indien). Opposition entre une marge stable (BorneÂo, Kalimantan) et une marge active (CeÂleÁbes, Sulawesi). Bulletin of the Society of Geology, France, 8 (4), 807±818. Gayet, J., FaugeÁres, J. C., & Gonthier, E. (1990). La seÂdimentation Quaternaire reÂcente dans le deÂtroit de Makassar. Oceanologica Acta, 10, 307±320. Huc, A. Y. (1988a). Sedimentology of organic matter. In F. H. Frimmel & R. F. Christman, Humic substances and their role in the environment (pp. 215±243). Chichester: Wiley. Huc, A. Y. (1988b). Aspects of depositional processes of organic matter in sedimentary basins. Organic Geochemistry, 13 (1-3), 433±443. Meyers, P. A., Brassel, S. C., Huc, A. Y. (1984). Geochemistry of organic carbon in South Atlantic sediments from Deep Sea Drilling Project, Leg 75. Initial Reports of the Deep Sea Drilling Project. Washington, U.S. Government Priniting Of®ce, 75, 967±981. Meyers, P. A., Huc, A. Y., Brassel, S. C. (1981). The organic matter
517
patterns in South Atlantic sediments deposited since the late Miocene beneath the Benguela upwelling system. In Bjùroy et al. Advances in Organic Geochemistry, Bergen, Norway. Pedersen, T. F., & Calvert, S. E. (1990). Anoxia versus productivity: what controls the formation of organic rich sediments and sedimentary rocks? American Association of Petroleum Geologists Bulletin, 74, 454±466. Pelet, R. (1983). Preservation and alteration of present-day sedimentary organic matter, New York: Wiley. Pelet, R.(1987). A model of organic sedimentation on present day continental margins. Marine petroleum source rocks. Brooks, J., Fleet, A. J., 26, 167±180. Stein, R. (1986). Organic carbon and sedimentation rate. Further evidence for anoxic deep-water conditions in the Cenomanian/Turonian Atlantic ocean. Marine Geology, 72, 199±209. Stow, D. V. A. (1987). South Atlantic organic-rich sediments: facies, processes and environments of deposition. Marine Petroleum Source Rocks. B. J. and Fleet, A. J., 26, 287±299. Suess, E. (1980). Particulate organic carbon ¯ux in the ocean. Nature, 288, 260±263.
www.elsevier.com/locate/marpetgeo
Organic sedimentation in deep offshore settings: the Quaternary sediments approach A.Y. Huc a,*, P. Bertrand b, D.A.V. Stow c, J. Gayet b, M. Vandenbroucke d a
IFP School, Ecole du Petrole et des Moteurs, Institut FrancËais du PeÂtrole, 228-232 Avenue Napoleon Bonap., 92852 Rueil-Malmaison, France b Departement de GeÂologie et Oceanographie, Universite de Bordeaux I, Avenue de FaculteÂs, 33405 Talence, France c School of Ocean and Earth Science, Southampton Oceanography Centre, Southampton University, Waterfront Campus, Southampton SO14 3ZH, UK d Geology±Geochemistry Department, Institut FrancËais du PeÂtrole, 1-4 Avenue de Bois-PreÂau, 92852 Rueil-Malmaison, France Received 30 January 2000; received in revised form 18 February 2000; accepted 1 August 2000 Keywords: Organic sedimentation; Deep offshore; Quarternary sediments
1. Introduction A classical view of organic sedimentation in marine environment has emerged during the past few years, and provides accepted guidelines for understanding the distribution of source rocks in sedimentary basins (Huc, 1988a). According to this conceptual model, the accumulation of organic material is the result of the action of favourable factors including high primary productivity (Pedersen & Calvert 1990), enhanced preservation conditions, and is modulated in a complex way by the sedimentation rate (dilution, time of residence of the organics at the surface of the sediment, detritic versus biogenic origin of the mineral phase, water depth, distance from the shore) (Pelet, 1983; Pelet, 1987; Stein, 1986). The main preservation controls are considered to be the development of anoxic conditions in bottom water (Demaison & Moore, 1980) and the water depth (Suess, 1980). Based on an extensive data set derived from sediment trap experiments, Suess (1980) has shown that the ¯ux of organic material which is exported from the euphotic zone in oceanic realm decreases exponentially with depth throughout the water column (such that only very few percent of the organic ¯ux escaping from the euphotic zone is reaching the bottom at a thousand meter depth). Such a paradigm would appear to exclude any possibility of substantial organic accumulation to take place in deep offshore settings. In this paper two examples of recent Quaternary deposition are given that challenge this paradigm and provide sound elements for revisiting the sedimentology of organic matter in deep water environments. This clearly has impor-
* Corresponding author. Tel.: 133-1-4752-6000; fax: 133-1-4752-7000. E-mail address: [email protected] (A.Y. Huc).
tant implications for a better understanding of the distribution of source rocks in deep offshore petroleum systems.
2. The Makassar strait example The siliciclastic sediments deposited in the Makassar strait, between the passive margin of eastern Borneo and the active margin of western Celebes are herein considered along a cored transect off the Mahakam Delta (Kalimantan, Indonesia) that was made during the Misedor II cruise. The transect (Fig. 1) includes four sites located respectively on the continental shelf (Core KS 13, water depth: 50 m), the slope (Core KS 15, water depth: 250 m), the well developed rise (Core KS 19, water depth: 1975 m) and the abyssal plain (Core KS 12, water depth: 2229 m). These cores, which have been extensively studied by FaugeÁres, Gayet, and Gonthier (1989), encompass sediments deposited during the late Quaternary climatic phases, including the last glaciation, the post glacial period and the Holocene. The sedimentation is predominantly controlled by the terrigenous input originating from the Mahakam delta. The sedimentary regime is closely related to gravity ¯ows which are well expressed as turbiditic sequences on the rise and on the abyssal plain, with more distal characters for the latter (Gayet, FaugeÁres, & Gonthier, 1990). A noticeable point regarding these sediments is their high organic content, including the deepest rise (1975 m) and abyssal plain (2229 m) situations. The two latter cores exhibit even individualized intervals, in which rich assemblage of organic remains constituted of sulphur epigenized woody detritus and fragmentary leaf debris are associated with total organic carbon exceeding 5%. The higher content of organics and plant macrodetritus deposited during the ice age and their decreasing abundance in the subsequent
0264-8172/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0264-817 2(00)00069-6
514 A.Y. Huc et al. / Marine and Petroleum Geology 18 (2001) 513±517
Fig. 1. Pro®le of Organic carbon content in Quaternary cores along a transect in the Makassar Strait.
A.Y. Huc et al. / Marine and Petroleum Geology 18 (2001) 513±517 515
Fig. 2. Distribution of organic content in Quaternary sediments offshore Namibia.
516
A.Y. Huc et al. / Marine and Petroleum Geology 18 (2001) 513±517
sediments can be rationalized in terms of glacioeustatic changes. During the last glacial maximum (around 20 kyrs. BP), the largest part of the continental shelf was exposed. Organic bearing ¯uvial and deltaic sediments were deposited on the platform by the Mahakam river, but part of the organic matter was conveyed toward the rise and the abyssal plain where it accumulated together with high energy turbidites. During the post glacial time, rising of the sea tended to trap a part of the ¯uvial sediments, including organic matter at the edge of the shelf, feeding turbidites that become ®ner and organically poorer as the transgression proceeds. 3. The offshore Namibia example The Benguela current offshore of Namibia is controlling one of the present day most proli®c upwelling system in terms of primary organic productivity. The bottom sediments associated with this upwelling system have been sampled at different locations. The super®cial sediments of the shallow platform area (,150 m) contain an average of 9% TOC and exhibit local maxima reaching as much as 25% under the most active and permanent LuÈderitz and Walvis bay cells of the Benguela system (Calvert & Price, 1971). During DSDP Leg 75, sediments were obtained from a more northern location at site 532 (water depth: 1341 m) and yielded an average organic content of 3.5% TOC in the 75 m thick Quaternary sediments. The considered area of the platform represents a huge surface (1200 km long and 100 km wide) of organic rich pelagic/hemipelagic sediments. During the same Leg, another site (530B) located at the foot of the northern ¯ank of the Walvis ridge at a depth of 4639 m penetrated a 120 m thick section of Quaternary sediments with an average organic content of 3% TOC (Fig. 2) (Meyers, Huc, & Brassel, 1981). These sediments show clear features indicative of redeposition by mass transport processes, including turbidite sequences and debris ¯ow. The entire section is interpreted as the redeposition of sediments originating from a shallow setting on the adjacent platform and ridge, where organic-rich unstable mud had been previously laid down by pelagic/hemipelagic processes under the in¯uence of the highly productive Benguela system (Huc, 1988b; Stow, 1987). More recently three 40 m long piston cores have been recovered during the Nausicaa-Images II cruise along a transect corresponding to the LuÈderitz cell. The sampling depth range includes 1028 m (core MD962087), 2910 m (core MD962098) and 3606 m (core MD962086) (Fig. 2). The geochemical data show that the average organic content of these Quaternary pelagic/hemipelagic sediments are respectively 7.7% TOC, 3.81% TOC and 1.77% TOC. These contents are interpreted to be the result of the combined effect of organic input and carbonate dissolution (Bertrand et al., in preparation). They provide clear
evidence for the possibility of accumulation of organic rich pelagic/hemipelagic deposits in deep oceanic setting. A noticeable point is that these sediments do not show indication of complete anoxia, such as lamination or absence of benthic fauna, at the bottom water interface. A similar situation was observed at Site 532 of Leg 75 where burrows re¯ecting bioturbation by benthic organisms can be recognized throughout the cored Quaternary interval (Meyers, Brassel, & Huc, 1984). This implies that strict anoxia is not indispensable for the accumulation of a sediment with an organic content suf®cient to be in the source rock class. The occurrence of substantial amount of organic matter reaching sediments deposited in very deep settings can be explained by the high biological surface productivity. High primary productivity promotes repacking of organic detritus and of small particles by physicochemical (i.e. ¯occulation) and biological processes producing large organomineral particles (faecal pellets, marine snow, aggregates) with rapid sinking rate which can act as ef®cient vehicles for transporting organic matter through the water column (Dagg & Walser, 1986). 4. Conclusions According to conventional view, deep water settings are not favourable for source rock formation because organic material is subjected to intensive degradation during its transit through the water column. However mass transport (via turbidity currents and debris ¯ow for example) of organic sediments previously deposited in shallow water provides one means for the accumulation of terrestrial and marine derived organic rich sediments in deep offshore situations. Moreover, highly biological productive areas, such as active upwelling zone appear to deliver suf®cient quantity of organic charge to outbalance the degradative capacity of the system, leading to the formation of organic-rich sediments even in deep and not strictly anoxic conditions. These conclusions are presented as important hypotheses that require further elaboration, discussion and testing in other present day deep water settings and, if possible, in ancient sediment records. References Bertrand, P., Pedersen, T., Schneider, R., Shimmield, G., Giraudeau, J., MuÈller, P., Foster, J., Venec-PeyreÂ, M. Th., Pierre, C., Massias, D., & Tribovillard, N. (in preparation). SE Atlantic records suggest older occurrence and low latitude control of the paleocean circulation bipolar seesaw. Calvert, S. E., Price, N. B. (1971). Recent sediments of the South West African shelf. The geology of the East Atlantic continental margin. Delany, F. M., London, Her Majesty's Stationery Of®ce 70/16, 175± 185. Dagg, M. J., & Walser, W. E. (1986). The effect of food concentration on faecal pellet size in marine copepods. Limnology and Oceanography, 31 (5), 1066±1071. Demaison, G., & Moore, G. T. (1980). Anoxic environments and oil source
A.Y. Huc et al. / Marine and Petroleum Geology 18 (2001) 513±517 bed genesis. American Association of Petroleum Geologists Bulletin, 64, 1179±1209. FaugeÁres, J. C., Gayet, J., & Gonthier, E. (1989). Microphysicographie des deÂpoÃts quaternaires dans le deÂtroit de Makassar (oceÂan indien). Opposition entre une marge stable (BorneÂo, Kalimantan) et une marge active (CeÂleÁbes, Sulawesi). Bulletin of the Society of Geology, France, 8 (4), 807±818. Gayet, J., FaugeÁres, J. C., & Gonthier, E. (1990). La seÂdimentation Quaternaire reÂcente dans le deÂtroit de Makassar. Oceanologica Acta, 10, 307±320. Huc, A. Y. (1988a). Sedimentology of organic matter. In F. H. Frimmel & R. F. Christman, Humic substances and their role in the environment (pp. 215±243). Chichester: Wiley. Huc, A. Y. (1988b). Aspects of depositional processes of organic matter in sedimentary basins. Organic Geochemistry, 13 (1-3), 433±443. Meyers, P. A., Brassel, S. C., Huc, A. Y. (1984). Geochemistry of organic carbon in South Atlantic sediments from Deep Sea Drilling Project, Leg 75. Initial Reports of the Deep Sea Drilling Project. Washington, U.S. Government Priniting Of®ce, 75, 967±981. Meyers, P. A., Huc, A. Y., Brassel, S. C. (1981). The organic matter
517
patterns in South Atlantic sediments deposited since the late Miocene beneath the Benguela upwelling system. In Bjùroy et al. Advances in Organic Geochemistry, Bergen, Norway. Pedersen, T. F., & Calvert, S. E. (1990). Anoxia versus productivity: what controls the formation of organic rich sediments and sedimentary rocks? American Association of Petroleum Geologists Bulletin, 74, 454±466. Pelet, R. (1983). Preservation and alteration of present-day sedimentary organic matter, New York: Wiley. Pelet, R.(1987). A model of organic sedimentation on present day continental margins. Marine petroleum source rocks. Brooks, J., Fleet, A. J., 26, 167±180. Stein, R. (1986). Organic carbon and sedimentation rate. Further evidence for anoxic deep-water conditions in the Cenomanian/Turonian Atlantic ocean. Marine Geology, 72, 199±209. Stow, D. V. A. (1987). South Atlantic organic-rich sediments: facies, processes and environments of deposition. Marine Petroleum Source Rocks. B. J. and Fleet, A. J., 26, 287±299. Suess, E. (1980). Particulate organic carbon ¯ux in the ocean. Nature, 288, 260±263.