Carbonate cycles in Aptian-Albian "Black Shales" of the ...
Carbonate cycles in Aptian-Albian "Black Shales" of the Falkland Plateau MARY E. PARKER Department of Geology Florida State University Tallahassee, Florida 32306
MICHAEL A. ARTHUR Department of Geology University of South Carolina Columbia, S.C. 29208
SHERWOOD W. WISE, JR. Department of Geology Florida State University Tallahassee, Florida 32306
Cretaceous cores from Deep Sea Drilling Project (DSDP) leg 71, site 511, drilled on the eastern Falkland Plateau (51° 00.28' S 46° 58.30' W; figure 1), were examined via carbonate, organic carbon, stable isotope, nannofloral, and ultrastructural analysis in an attempt to determine the mode of origin of thin but discrete pelagic limestone beds intercalated among the black mudstones (figure 2) near the top of the extensive Mesozoic "black-shale"
sequence of the Falkland Plateau. The "black shales" are of interest because of their paleoenvironmental significance and their potential as a source rock for petroleum in and around the South Atlantic Basin. This article is a synopsis of a more extended work that has been submitted for publication elsewhere (Parker et al. in press). The Falkland "black-shale" sequence was deposited at shallow, shelf-break water depths, probably no greater than 400 meters (Basov and Krasheninnikov in press; Sliter 1977). Black mudstones are the dominant lithology among the cycles. This is in contrast to Cretaceous "black shales" developed at more northerly DSDP sites, which were deposited at bathyal or abyssal depths. There carbonate beds dominate the cycles in the pre- to mid-Albian before giving way to multicolored claystones above. The periodicity of the Falkland Plateau dark/light cycles is at least 100,000 years, but these cycles do not appear to be as regular as those at many other Atlantic DSDP sites. The pelagic limestone beds in the Falkland Plateau "black-shale" sequence could represent either periodic incursions of normal open ocean floras during brief periods of ventilation of surface waters in an otherwise anoxic sedimentary basin, or, alternatively, episodes of upwelling and overturn of the water column in the basin, which would enrich otherwise nutrient-poor surface waters. Nannotloral analysis revealed that the diversity of the assemblages was rather high in both the black mudstones and the whitish limestones. Opportunistic species were not present in either in numbers beyond normal background levels. Preservation of coccoliths was surprisingly good among the black mudstones and there was little evidence that wholesale dissolu -
Figure 1. Location of DSDP leg 71, site 511, Falkland Plateau. 1983 REVIEW
153
Site 511
Site 511 cont. 33
2 2 Age
Age
2 -J _J
400
11112111_I Turonuan
---2-
4501
50 I
Ti
Cenomanuan
-_'- 5 Albuan
eas ly 2A Oligocene
1001
E
500
0 E
Aptuan)j( Barremian
0
2 1501
550 late Eocene
-- 6
E 26 Eocene— Paleocene
0 200
213
early 3 Maestr. - - - - MactSr.I Campa ' 0
0
0
Late - Jurassic
600
Sn
6501
2501
tion of the carbonate had occurred. The primary variant was in the absolute numbers of coccoliths, with the limestones being coccolith-rich and the black mudstones coccolith poor. Because both the preservation of the nannofossils and the diversity of the assemblages indicate that little biogenic carbonate was lost due to dissolution, the occurrence and abundance of coccoliths in the sequence must be a function of the productivity of the surface waters. The simplest explanation for these variations seems to be short-lived nutrient events resulting from upwelling and overturn in the expanding South Atlantic basin. Stable isotope measurements of carbonate (i°C) do not adequately constrain the origin of the cyclicity. The 8 11 C data suggest that more nannofossil-rich intervals may be due to higher nutrient supply and overturn of deeper waters at the site. The overturn of surface and deeper waters could have oxygenated bottom waters, leading to oxidation of organic carbon and overall lower organic carbon values in the lighter-colored, relatively more carbonate-rich intervals. A corollary of this interpretation is that periods of lower productivity were due to a more stably stratified water column, development of low oxygen conditions in intermediate water masses, and consequent increased preservation of the organic matter that was produced in the surface layers. Episodes of upwelling and overturn of the water column, rather than influxes of well-oxygenated waters into an otherwise anoxic environment are, therefore, suggested by both the nannofloral and isotopic analyses as a possible mode of origin of the thin but discrete pelagic limestone beds intercalated among the Falkland Plateau Aptian-Albian black mudstones. This interpretation seems reasonable and does not violate the constraints of available data. We thank Chiye R. Wenkam at the U.S. Geological Survey in Denver for her assistance in running the geochemical analyses. This research was supported by National Science Foundation grant DPP 80-20382.
Pelagic Clay Diatom Ooze
-- - - - - Campanian Chart £ £ £ £ £
11Z11 3001
4
Diatom-Rad. or Siliceous Ooze
References
Nannofossil Ooze Calcareous Ooze lrnJ —l -- Looc3I Calcareous Chalk Nannofossil Chalk
Clay/Claystone Sand/Sandstone
3501 Santonian
Santonian11-1ziI1 Coniacian 4001
EEEti= L. .. .1 Basalt
Initial Reports of the Deep Sea Drilling Project, Vol. 71. Washington, D.C.: U.S. Government Printing Office.
Krasheninnikov, et al. (Eds.), = late Zeolite m = middle e = early
Figure 2. Columnar section of site 511.
154
Mud/Mudstone
Basov, I. A., and V. A. Krasheninnikov. In press. Benthonic foraminifers of the Mesozoic and Cenozoic sediments in the southwestern Atlantic and an indicator of paleoenvironment, DSDP leg 71. In W. J . Ludwig, V. Krasheninnikov, etal. (Eds.), Initial Reports of the Deep Sea Drilling Project, Vol. 71. Washington, D.C.: U.S. Government Printing Office. Parker, M. E., M. A. Arthur, S. W. Wise, Jr., and C. R. Wenkam. In press. Carbonate and organic carbon cycles in Aptian-Albian "black shales" at DSDP site 511, Falkland Plateau. In W. J . Ludwig, V.
L z
Sliter, W. V. 1977. Cretaceous foraminifers from the southwestern Atlantic Ocean, leg 36, Deep Sea Drilling Project. In P. Barker, I. W. D. Dalziel, et al., (Eds.),
Initial Reports of the Deep Sea Drilling Project,
Vol. 36. Washington, D.C.: U.S. Government Printing Office.
ANTARCTIC JOURNAL
MICHAEL A. ARTHUR Department of Geology University of South Carolina Columbia, S.C. 29208
SHERWOOD W. WISE, JR. Department of Geology Florida State University Tallahassee, Florida 32306
Cretaceous cores from Deep Sea Drilling Project (DSDP) leg 71, site 511, drilled on the eastern Falkland Plateau (51° 00.28' S 46° 58.30' W; figure 1), were examined via carbonate, organic carbon, stable isotope, nannofloral, and ultrastructural analysis in an attempt to determine the mode of origin of thin but discrete pelagic limestone beds intercalated among the black mudstones (figure 2) near the top of the extensive Mesozoic "black-shale"
sequence of the Falkland Plateau. The "black shales" are of interest because of their paleoenvironmental significance and their potential as a source rock for petroleum in and around the South Atlantic Basin. This article is a synopsis of a more extended work that has been submitted for publication elsewhere (Parker et al. in press). The Falkland "black-shale" sequence was deposited at shallow, shelf-break water depths, probably no greater than 400 meters (Basov and Krasheninnikov in press; Sliter 1977). Black mudstones are the dominant lithology among the cycles. This is in contrast to Cretaceous "black shales" developed at more northerly DSDP sites, which were deposited at bathyal or abyssal depths. There carbonate beds dominate the cycles in the pre- to mid-Albian before giving way to multicolored claystones above. The periodicity of the Falkland Plateau dark/light cycles is at least 100,000 years, but these cycles do not appear to be as regular as those at many other Atlantic DSDP sites. The pelagic limestone beds in the Falkland Plateau "black-shale" sequence could represent either periodic incursions of normal open ocean floras during brief periods of ventilation of surface waters in an otherwise anoxic sedimentary basin, or, alternatively, episodes of upwelling and overturn of the water column in the basin, which would enrich otherwise nutrient-poor surface waters. Nannotloral analysis revealed that the diversity of the assemblages was rather high in both the black mudstones and the whitish limestones. Opportunistic species were not present in either in numbers beyond normal background levels. Preservation of coccoliths was surprisingly good among the black mudstones and there was little evidence that wholesale dissolu -
Figure 1. Location of DSDP leg 71, site 511, Falkland Plateau. 1983 REVIEW
153
Site 511
Site 511 cont. 33
2 2 Age
Age
2 -J _J
400
11112111_I Turonuan
---2-
4501
50 I
Ti
Cenomanuan
-_'- 5 Albuan
eas ly 2A Oligocene
1001
E
500
0 E
Aptuan)j( Barremian
0
2 1501
550 late Eocene
-- 6
E 26 Eocene— Paleocene
0 200
213
early 3 Maestr. - - - - MactSr.I Campa ' 0
0
0
Late - Jurassic
600
Sn
6501
2501
tion of the carbonate had occurred. The primary variant was in the absolute numbers of coccoliths, with the limestones being coccolith-rich and the black mudstones coccolith poor. Because both the preservation of the nannofossils and the diversity of the assemblages indicate that little biogenic carbonate was lost due to dissolution, the occurrence and abundance of coccoliths in the sequence must be a function of the productivity of the surface waters. The simplest explanation for these variations seems to be short-lived nutrient events resulting from upwelling and overturn in the expanding South Atlantic basin. Stable isotope measurements of carbonate (i°C) do not adequately constrain the origin of the cyclicity. The 8 11 C data suggest that more nannofossil-rich intervals may be due to higher nutrient supply and overturn of deeper waters at the site. The overturn of surface and deeper waters could have oxygenated bottom waters, leading to oxidation of organic carbon and overall lower organic carbon values in the lighter-colored, relatively more carbonate-rich intervals. A corollary of this interpretation is that periods of lower productivity were due to a more stably stratified water column, development of low oxygen conditions in intermediate water masses, and consequent increased preservation of the organic matter that was produced in the surface layers. Episodes of upwelling and overturn of the water column, rather than influxes of well-oxygenated waters into an otherwise anoxic environment are, therefore, suggested by both the nannofloral and isotopic analyses as a possible mode of origin of the thin but discrete pelagic limestone beds intercalated among the Falkland Plateau Aptian-Albian black mudstones. This interpretation seems reasonable and does not violate the constraints of available data. We thank Chiye R. Wenkam at the U.S. Geological Survey in Denver for her assistance in running the geochemical analyses. This research was supported by National Science Foundation grant DPP 80-20382.
Pelagic Clay Diatom Ooze
-- - - - - Campanian Chart £ £ £ £ £
11Z11 3001
4
Diatom-Rad. or Siliceous Ooze
References
Nannofossil Ooze Calcareous Ooze lrnJ —l -- Looc3I Calcareous Chalk Nannofossil Chalk
Clay/Claystone Sand/Sandstone
3501 Santonian
Santonian11-1ziI1 Coniacian 4001
EEEti= L. .. .1 Basalt
Initial Reports of the Deep Sea Drilling Project, Vol. 71. Washington, D.C.: U.S. Government Printing Office.
Krasheninnikov, et al. (Eds.), = late Zeolite m = middle e = early
Figure 2. Columnar section of site 511.
154
Mud/Mudstone
Basov, I. A., and V. A. Krasheninnikov. In press. Benthonic foraminifers of the Mesozoic and Cenozoic sediments in the southwestern Atlantic and an indicator of paleoenvironment, DSDP leg 71. In W. J . Ludwig, V. Krasheninnikov, etal. (Eds.), Initial Reports of the Deep Sea Drilling Project, Vol. 71. Washington, D.C.: U.S. Government Printing Office. Parker, M. E., M. A. Arthur, S. W. Wise, Jr., and C. R. Wenkam. In press. Carbonate and organic carbon cycles in Aptian-Albian "black shales" at DSDP site 511, Falkland Plateau. In W. J . Ludwig, V.
L z
Sliter, W. V. 1977. Cretaceous foraminifers from the southwestern Atlantic Ocean, leg 36, Deep Sea Drilling Project. In P. Barker, I. W. D. Dalziel, et al., (Eds.),
Initial Reports of the Deep Sea Drilling Project,
Vol. 36. Washington, D.C.: U.S. Government Printing Office.
ANTARCTIC JOURNAL
