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Part III: The Stratigraphic Record |
Ecole des Mines, 60 Bd St Michel, F-75272 Paris, France
Institut Francais du Petrole, 1 Ave. de Bois Préau, F-92506 Rueil-Malmaison, France
, 1 Rue Martignon F-92500 Rueil-Malmaison, France
Collège de France, Station Marcellin Berthelot F-92360 Meudon-la-Forêt, France
Institut de Géologie, 1 Rue Blessig, F-67084 Strasbourg, France
A survey of DSDP black shales from the North Atlantic has been carried out in order to study the evolution of Cretaceous palaeoenvironments in the region. The study involved a reappraisal of the biostratigraphy of the Cretaceous formations and considered accumulation rate, mineralogical and organic geochemical data. Special attention was paid to distinguishing between redeposited and autochthonous sediments. Three main phases of deposition are recognized which are separated by two unconformities:
The Blake-Bahama phase (Valanginian-early Aptian (Bedoulian)) corresponds to relatively uniform depositional conditions over the whole of the North Atlantic with periods of anoxia. The clay mineral assemblages reflect different sedimentary influxes from the continents into the eastern and western parts of the basin which were separated by the mid-ocean ridge.
The transition to the Hatteras phase (Event E1) is marked by a drastic fall in sediment accumulation rate and by a rapid rise in the CCD but not by any appreciable change in clay mineral fluxes.
The Hatteras phase (late Aptian-late Cenomanian) is characterized by distinctly diverse depositional conditions. In the southeastern part of the basin anoxic conditions were persistent, while in the northeast along the European margin there is no, or only very rare, evidence of anoxia. In the west, periods of oxia and anoxia alternated. Over the mid-ocean ridge conditions were oxic. A ‘barrier’ in the region of the present-day Bermuda Rise separated two areas of distinct clay mineral deposition.
The transition to the Plantagenet phase (Event E2) corresponds to low sedimentation rates. It was immediately preceded by, or was coincident with, deposition of a black shale rich in marine organic matter.
The Plantagenet phase (Turonian-Senonian) was a period of uniform sedimentation under oxic conditions.
Possible causes of these changes in sedimentation are outlined.
This article has been cited by other articles:
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  C. E. Barker, M. Pawlewicz, and E. A. Cobabe Deposition of sedimentary organic matter in black shale facies indicated by the geochemistry and petrography of high-resolution samples, Blake Nose, western North Atlantic Geological Society, London, Special Publications, 2001; 183: 49 - 72. [Abstract] [PDF]
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T. Wagner and T. Pletsch Tectono-sedimentary controls on Cretaceous black shale deposition along the opening Equatorial Atlantic Gateway (ODP Leg 159) Geological Society, London, Special Publications, 1999; 153: 241 - 265. [Abstract] [PDF]
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