DSDP 367

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Coordinates: 12 ° 29 '12 "  N , 20 ° 2' 48"  W.

Map: Atlantic Ocean
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DSDP 367​
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Atlantic Ocean

DSDP 367 was a research well as part of the Deep Sea Drilling Project with the aim of geological exploration of the Cape Verde Basin in the eastern North Atlantic .

Location description

The bore for DSDP campaign 41 ( Engl. Leg 41 ) counts, was third in the period to 10 March 1975 by the drillship Glomar Challenger driven out. Only a single hole was drilled at drilling position 12 ° 29.2'N, 20 ° 02.8'W, approximately 370 kilometers southwest of Dakar and 460 kilometers southeast of Praia . The sea floor was encountered at a depth of 4,758 meters. The borehole passed through 1,144 meters of sediments and 7 meters of basalt below , of which 174.3 meters of drill core could be used for scientific analysis.

Before the drilling, this section of the Cape Verde Basin was only known from seismic reflection profiles, which had been created by the research vessels Valdivia (Valdivia 10) and Vema (Vema ​​29 and Vema 31). The drilling site is around 200 kilometers west of the African continental slope (southern Senegal ) in the transition area to the actual deep-sea basin . About 60 kilometers further west crosses the anomaly-free, magnetic quiet zone of the Jura (Jurassic Magnetic Quiet Zone) in a north-south direction and 100 kilometers further west the submarine canyon system Cayar , which starts a little north of Dakar.

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
unit 1 255 m Foraminifera- bearing nannofossil marls and embedded quartz sands 41 to 69% Pleistocene to Miocene
Subunit 2a 9.5 +? m Diatoms - leading radiolarian sound Upper Eocene
Subunit 2b 7 m Zeolite-rich clay stones with chert and porcelainite Eocene to Upper Paleocene
Unit 3 237.5 m Colorful, silty clay stones 0% Upper Paleocene to Upper Cretaceous
Subunit 4a 151 m Black slate 0 to 34% Lower Turon to Upper Aptium / Upper Albium
Subunit 4b 9.5 m Colorful clay stones 0% Oberapt to Unteralb
Subunit 5a 57 m Light gray nannofossil limestone , olive-colored marl stones and black slate 92 to 94% Upper apt / lower alve to Hauterivium
Subunit 5b 140.5 m Nannofossil limestone, marl and chert 58 to 97% Valanginium / Hauterivium to Oxfordium / Kimmeridgium
Unit 6 55 m Red-brown, clayey nannofossil limestone, marl, mudstone and chert 51 to 91% Oxfordium to Kimmeridgium
Unit 7 7 +? m basalt 0%

Interpretation of the sedimentological findings

The DSDP profiles 367 and 368. The profile through the Cape Verde Basin can be seen on the right-hand side, for comparison the neighboring profile on the Cape Verde sill on the left.

The borehole passed through the entire Mesozoic and Cenozoic sedimentary shell in order to encounter basalts at a depth of 1,144 meters, which are generally interpreted as oceanic crust .

The unit 1 is the express very rich in calcareous micro-fossils, a continuous sedimentation until the start of the Middle Miocene. Through numerous silty and sandy interferences, it reveals a terrigenous, detritic character that increases more and more towards the hanging wall. In the upper layers of the Pleistocene and Pliocene, the sand inclusions even become turbiditic and lead to reclaimed fossils from the Ceno and Mesozoic times. The sediment accumulation rates in the hanging wall are very high at 90 meters / million years.

The underlying unit 2 from the Eocene and Paleocene, probably separated by a hiatus , is characterized by its richness in silica . This manifests itself in subunit 2a in the form of radiolarians, which are recrystallized into chert tubers in subunit 2b. The chert tubers are accompanied by porcelain layers and abundant zeolites in subunit 2b. The marine sediments of the Eocene are known for their high production of silicic acid, so radiolarians flourished during this period. The clay sediments show a cyclic alternation (in the decimeter range) between black and green layers with a fluctuating organic carbon content of 2.5 to 0.3%. Possible explanations are based either on a cyclically varying, terrigenous entry or in a cyclical nature of the deep current.

The colorful, silty tones of Unit 3 of the Paleocene and Upper Cretaceous are predominantly of continental origin. They were deposited below the calcite compensation depth (CCD). Their silt content is relatively low, so they represent a rather distal, terrigenous facies. Only relatively little terrigenic material reached the lower continental slope, the majority of which was already deposited in the Senegal Basin and on the shelf.

The black slates of the Oberapts and Cenomaniums (subunit 4a), which sedimented under reducing conditions, are characterized by an increased carbon content of up to 6.7%. They also contain methane , rich autogenous pyrite , as well as dolomite and possibly siderite . However, the deposit conditions could not have been completely anoxic, as evidenced by occasional traces of digging in the sediment. The deep circulation, however, is likely to have been rather restricted with a simultaneous increase in organic primary production.

The slightly thick, underlying colored (red and green colored) clay stones (subunit 4b) from the Oberalb to Unterapt are lime-free, well-ventilated deep-sea deposits that arose below the CCD. They represent a turning point in the reducing conditions in the Cape Verde Basin. It is possible that a relatively strong deep circulation had temporarily established itself at this point in time.

The basal limestone sequence consists of two units: light gray, digested limestone from the Lower Cretaceous to Tithon ( unit 5 ) and red, clay-rich limestone alternating with dark gray, fine-layered marls from the Oxfordian to the Kimmeridgian ( unit 6 ). The fossil-rich limestones are pelagic and were deposited above the CCD, whereby the water depth to the hanging wall increases steadily. The calcareous sediments are heavily digested and consequently formed under oxic conditions. They are amazingly similar to the equivalents found at DSDP 105 in the western North Atlantic, and sediments from the Tethys region such as the Ammonitico Rosso , the Marnes à fucoides or the Maiolica are definitely comparable. This proves that the still young Atlantic in the Upper Jurassic was connected to the Tethys. At the regional level, the lower Cretaceous Morro Formation on Maio is to be mentioned, which can be compared surprisingly well.

Basalt , which is mostly interpreted as oceanic crust, was encountered under the pelagic limestone at a depth of 1146 meters . This is a bit surprising as no oceanic crust can be seen on the profiles at this depth. The possibility of a storage corridor cannot be ruled out, as only the top 7 meters were drilled. Age dating resulted in a Cenomaniac minimum age, which is not very trustworthy because of the weathered condition of the rock.

Fossil content

In addition to benthic and planktonic foraminifera, calcareous nannoplankton ( coccoliths ), dinoflagellates , radiolarians , diatoms , pollen and spores , sponge needles , ammonites and their aptychs, as well as ostracode prints , tintinnids and occasional fish remains were found in the drill cores . The predominance of nannoplankton, the high biodiversity of calcareous shellfish and the aptychs generally suggest deep water facies.

Foraminifera

The following foraminiferous zones could be eliminated (from young to old):

Coccoliths

The following zones were found in the calcareous nannoplankton (coccoliths):

Radiolarians

Radiolarian finds were only made in the Upper Pleistocene, Upper and Lower Eocene and Lower Cretaceous:

Results of the drilling

Probably the most impressive result of the drilling is the astonishing similarity between the Mesozoic sequence encountered in the Cape Verde Basin and the DSDP drilling in the western North Atlantic. This confirms the symmetrical development of the North Atlantic during the Mesozoic Era, which was then prevented from the Paleogene by the onset of a strong, thermohaline circulation and the extensive water exchange with other deep-sea basins.

In contrast to the DSDP 368 borehole located around 550 kilometers further north on the Cape Verde Threshold, Upper Jurassic and Lower Crustacean sediments lying beneath the black shale of the oceanic crust were encountered here - in the lying deep-water limestone, which gave way to clayey sediments below the CCD when subsidence continued. The layer package above the black slate is considerably less thick than DSDP 368.

There is no evidence of any ash layers in the Miocene.

literature

  • The Shipboard Scientific Party (Ed.): Site 367: Cape Verde Basin . 1975, p. 233-326 .

Individual evidence

  1. ^ JI Ewing, CD Hollister: Regional aspects of deep sea drilling in the western North Atlantic . In: Initial Reports of the Deep Sea Drilling Project . tape 11 . United States Government Printing Office, Washington 1972, p. 951-973 .