DSDP 368

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Coordinates: 17 ° 30 '24 "  N , 21 ° 21' 12"  W.

Map: Atlantic Ocean
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DSDP 368​
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Atlantic Ocean

DSDP 368 was a research well as part of the Deep Sea Drilling Project with the aim of geological exploration of the Cape Verde sill in the eastern North Atlantic .

Location description

The well was driven from March 13-20, 1975 from the drillship Glomar Challenger . Only one hole was drilled at drilling position 17 ° 30.4'N, 21 ° 21.2'W, approximately 200 kilometers northeast of Sal and 600 kilometers west of Nouakchott . The sea floor was encountered at a depth of 3366 meters. The bore passed a total of 984.5 meters of sediment , of which 327.7 meters were seeded .

With the exception of two seismic reflection profiles, recorded in 1971 by the German research vessel Meteor and by RV Vema of the Lamont-Doherty Earth Observatory and a borehole ( DSDP 141 north of the threshold), very little was known about the geological structure of the central Cape Verde threshold up to this point known.

The Cape Verde Sill is one of the most important threshold regions in the Atlantic. Starting from the West African continental slope , it extends at a width of around 900 kilometers to the western edge of Cape Verde . To the northeast of the island archipelago, it forms a gently arched, dome-like structure that stretches roughly east-northeast with a minimum water depth of 2950 meters. The threshold can also be traced west of Cape Verde in the direction of the Mid-Atlantic Ridge , but here it is much deeper with more than 4000 meters of water. It separates the Canary Basin in the north from the Cape Verde Basin in the south.

Pierced stratigraphy

The evaluation of the drill cores resulted in a stratigraphic profile in which the following units can be distinguished (from young to old):

unit Mightiness Rock type Lime content Age
Subunit 1a 132.5 m Marl and marly mud rich in nannofossils , predominantly foraminifera 50-86% Holocene to Lower Pliocene
Subunit 1b 133 m Light gray nannofossil mud, which merges into green-gray marls and clays on the lying surface . With three layers of volcanic ash > 80% in the sludge of the hanging wall, reduced to 0% in the clay of the lying wall Upper to Lower Miocene
Subunit 2a 389.5 m Cyclically alternating dark green, silty clays and light green clay stones . Turbidite 0-13% Lower Miocene to Lower Eocene ?
Subunit 2b 76 m Alternating red and green slates 0% Paleocene ? to upper chalk
Subunit 2c 219 m Cyclically alternating, olive-colored, silty and sandy mudstones and slate clays. Very similar to subunit 2a. Turbidite. 0% Upper Chalk
Subunit 3 34.5 +? m Black slate . Contains basaltic storage corridors on the lying surface 1 - 19% Upper Cretaceous to Turonium / Albium ?

The Lower Cretaceous limestones to be expected under the black schists, which are exposed, for example, in the neighboring DSDP 367 well and on Maio , could not be found.

Interpretation of the sedimentological findings

The DSDP profiles 367 and 368. The profile through the Cape Verde sill can be seen on the left, for comparison the neighboring profile in the Cape Verde basin on the right.

As can already be seen on the meteor profile, only flat, tectonically undisturbed, hemipelagic sediment layers were found on the approximately 1000 meters drilled. What was surprising, however, was the complete absence of calcareous sedimentation from a drilling depth of around 250 meters. Instead, calceless turbidites of continental origin were encountered (subunits 2a and 2c). Even subunit 1b is also influenced by turbidity, even if only in a fine clastic manner (distal). The turbidite sedimentation provided an average of 8 cycles per meter.

Also noteworthy are the three Miocene ash layers of subunit 1b, which were poured at about the same time as the neogene volcanism that began on the Cape Verde Islands. The basaltic storage tunnels that penetrated much deeper into the black slate of subunit 3 are related to the same cogenetic relationship; they could be dated to 19.1 million years BP (Lower Miocene, Burdigalium ). The Miocene volcanic activities near Dakar in Senegal must also be taken into account in this context.

An explanation of the subunit 2b connected between the turbidites is problematic, since its red-green alternation cannot be explained with clastic grain size changes or solution processes. It is likely to be a question of fluctuating redox conditions in the sediment.

The black shales are very rich in organic carbon , they contain up to 11.7% C; At the same time, they have a very high C / N ratio of 74, possibly due to the heating effects of the intrusions in the warehouse corridor. The shale was deposited in a deep basin below the calcite compensation depth (CCD), but not completely anoxic, as evidenced by benthic burrowing traces in the sediment.

There are several explanations for the absence of carbonate sedimentation including calcareous microfossils below 250 meters depth:

  • a low, primary rate of calcification.
  • chemical dissolution processes at the sediment / seabed boundary layer.
  • diagenetic solution processes

Incidentally, similar conditions were found at DSDP 140 in the Canary Basin, here too the lime content increases drastically from the Upper Miocene to the hanging wall. For the Canary Basin, extensive solution processes could be made responsible for the calcium deficit during the Middle Miocene to the Paleogene .

The good state of preservation of calcareous microfossils in the upper section of the borehole (sediments from the Upper Miocene to the Holocene), however, suggests that the CCD either deepened or that the Cape Verde Threshold was lifted above the existing CCD.

Fossil content

In addition to benthic and planktonic foraminifera, calcareous nannoplankton ( coccoliths ), radiolarians and diatoms , sponge needles and occasional fish remains were found in the drill cores .

Foraminifera

Planktonic foraminifera are very common in the first 200 meters. The following foraminiferous zones could be eliminated (from young to old):

Planktonic foraminifera were only found sporadically below a drilling depth of 200 meters, agglutinated benthic forms occurred somewhat more frequently. The taxa encountered reveal a deep marine deposit environment, which, however, was still far above the CCD.

Coccoliths

Coccoliths occur mainly up to a depth of 250 meters. The following zones could be eliminated:

Radiolarians

Radiolarian finds worth mentioning were only made in the Upper Pleistocene, Lower Miocene and Lower Eocene:

Buryella clinata and Bekoma bidartensis are probably assigned to the Paleocene .

Geological evolution

Schematic geological profile through the Cape Verde sill with the position of the borehole

Based on the borehole, the two reflection seismic profiles and the outcrop conditions on Cape Verde, the geological development for the Cape Verde sill can be reconstructed:

In the area of ​​the borehole, the threshold region is underlain by around 149 to 145 million year old oceanic crust ( pillow lavas of the Tithonium ). From the Valanginium onwards, pelagic limestones were deposited over them. The black slate (subunit 3) that adjoins the albium is of deep marine origin; they were approached by the bore in their uppermost section. In the period from the Upper Cretaceous up to and including Aquitanium , a turbidite sequence around 600 meters thick followed (subunit 2). There were then drastic changes in subunit 1b. The turbidite sedimentation gradually gave way to a 200 meter thick, limestone sedimentation (marl and lime sludge of subunit 1). At about the same time, basaltic volcanism set in in Burdigal, which is ultimately responsible for the bulging and lifting of the threshold region. Storage tunnels penetrated the upper black slate and layers of ash are found in subunit 1b. Numerous diapirartig penetrating volcanics ( Engl. Volcanic plugs ) can be seen on the adjacent Vema profile.

literature

  • The Shipboard Scientific Party (Ed.): Site 368: Cape Verde Rise . 1975, p. 233-326 .

Individual evidence

  1. ^ Robert A. Duncan, Everett D. Jackson: Geochronology of basaltic rocks recovered by DSDP Leg 41, Eastern Atlantic Ocean . 1975, p. 1113-1118 .
  2. ↑ R. Muller, M. Sdrolias, W. Roest: Age, spreading rates and spreading symmetry of the world's ocean crust . In: Geochemistry Geophysics Geosystems . 9 (Q04006), 2008, p. 1525-2027 .
  3. ^ CA Williams, IA Hill, RS White: Fracture zones across the Cape Verde Rise, NE Atlantic . In: Journal of the Geological Society . tape 147 . London 1990, p. 851-857 .