DSDP 366
Coordinates: 5 ° 40 ′ 41 ″ N , 19 ° 51 ′ 5 ″ W.
DSDP 366 was a research well as part of the Deep Sea Drilling Project with the aim of geological exploration of the Sierra Leone threshold in the eastern North Atlantic .
Location description
The drilling, which is part of the DSDP campaign 41 ( English Leg 41 ), was driven between February 22nd and March 1st, 1975 from the drilling ship Glomar Challenger . Two holes were drilled - DSDP 366 at drilling position 5 ° 40.7'N and 19 ° 51.1'W and 366 A not far to the northwest. The two wells are located around 800 kilometers southwest of Freetown . The sea floor was encountered at a depth of 2,860 meters. Hole 366 passed 850.5 meters of sediment (582 meters of which were seeded), 366A only 367 meters.
The Sierra Leone threshold separates the Cape Verde Basin in the north from the Sierra Leone Basin in the south. Prior to drilling, nothing was known about the nature of the crust in the deeper subsurface of the threshold and magnetic anomalies were missing. It was assumed that a complete, sedimentary succession of the Tertiary and Upper Cretaceous could be found on the threshold, which was not influenced by turbidite currents and, due to the relatively shallow water depth, was deposited above the calcite compensation depth (CCD). In addition to creating the most complete profile possible, the aim of the drilling was to explore the subsidence profile of the threshold region in order to influence its influence on deep circulation (the threshold may have prevented deep circulation between the North and South Atlantic in the geological past). It was also hoped to hit the (oceanic?) Crust.
Before the drilling, the DSDP 13 of Campaign 3 ( leg 3 ) had already been drilled 180 kilometers further east-northeast , which hit a small elevation on the eastern flank of the Sierra Leone Ridge at a depth of 4,588 meters and penetrated into layers of the Upper Cretaceous . The Neogene consisted of carbonate muds and clays , the Eocene of radiolarian muds and chert and the Upper Cretaceous of chert, lime and shale. Reflexionsseismisch close to the planned drilling by the research ship was Vema a profile (Vema 30) was created, which was to recognize the horst-like nature of the threshold (with a steep southeastern termination).
The Sierra Leone Sill extends from the shelf area of the Guinea Plateau, which protrudes far to the south-west, in a south-westerly direction to the east-west trending Saint Paul Fracture Zone . A north-west-south-east oriented deep sea channel, which probably marks the course of the Guinea Fault Zone , clearly separates it from the shelf of West Africa . Along the 4,000 meter depth line , the sill has a length of around 800 kilometers and a maximum width of 400 kilometers. Morphologically, it is divided into two sub-areas at its northeast end by a south-westerly trenching channel - into a north-east section, which is severely fissured due to the eruption of the deeper subsoil, and a slightly north-westerly tilted, shed-like south-west section, which lies between 2700 and 2900 meters water depth and on which the Bore was sunk.
Pierced stratigraphy
The evaluation of the drill cores resulted in a stratigraphic profile in which the following units - mainly pelagic carbonates - can be distinguished (from young to old):
| unit | Mightiness | Rock type | Lime content | Age |
|---|---|---|---|---|
| unit 1 | 136 m | Nannofossil marl and nanno sludge | 62 to 83% | Pleistocene to Middle Miocene |
| Unit 2 | 344 m | Cyclically alternating nanno sludge / chalk and marl / pelagic clays | 14 to 83% | Middle Miocene to Middle Eocene |
| Unit 3 | 199.5 m | Cyclically alternating nanno chalks with porcelain lime / cherts or silica lime | 14 to 92% | Middle Eocene to Lower Eocene |
| Unit 4 | 171 m | Limes and marls | 4 to 72% | Lower Eocene to Upper Cretaceous |
Interpretation of the sedimentological findings
Like DSDP 13, the borehole passed through the entire Cenozoic and Upper Cretaceous. The oceanic crust was not encountered.
The unit 1 consists of 18.5 meters thick Nannoschlämmen the Pleistocene , 57 meters thick Nannoschlämmen, light gray and greenish Nannomergeln and Foraminiferenschlämmen the Pliocene and 50.5 meters thick, light gray and greenish Nannomergeln, Nannoschlämmen and Foraminiferenschlämmen the Upper Miocene. It shows no diagenetic overprints and only minimal compaction .
After a hiatus follows discordant the unit 2 from the period Mean Miozän than average Eozän. The 16-meter-thick Middle Miocene is made up of nano sludges. It is underlain by the 106 meters thick Lower Miocene, consisting of calcareous, light gray, greenish and yellowish clays, nanno sludges and nannomer gels. There is also a discordance between the Middle and Lower Miocene. The Oligocene with a thickness of 161 meters and the 57 meters thick Upper Eocene are made up of Nanno sludges and pelagic clays. Overall, the unit 2 is characterized by a cyclic alternating bearing . The cycles, which are between 10 and 50 centimeters thick, are composed of nanno sludges in the hanging wall and nannomer gels in the lying wall or nanno sludge in the hanging wall and pelagic clays in the lying wall, with the more calcareous nanno sludges developing a greater thickness. The contacts between the individual cycles are sharp, but the transition inside is gradual. The Nanno sludges solidify in the Lower Oligocene (below 366 meters depth) and then turn into chalk. Occasional evidence of compaction is present.
Also, unit 3 from the period Medium to lower Eozän characterized by cyclicality. In this case, nanno chalks are laid over silica limestone and porcelain lime with the occasional chert bulb. Individual cycles are between 25 and 40 centimeters thick, of which the clayey, chalky limestone of the hanging wall takes up two thirds and the cherts, porcelain or siliceous limestone a third. Chert tubers, if any, appear in the bottom 5 centimeters. With increasing depth (from 567 meters), silica limes gain in importance. Signs of compaction accumulate, calcite coatings form on the nannofossils, and cementation from carbonate and silica permeates the sediment.
Finally, unit 4 from the period from the Lower Eocene to the Maastrichtian begins with green-gray, clayey limestone and light-gray silica limestone, and then at a depth of 776 meters it changes into thin-layer, olive-gray, more cemented marls. The marls have been ransacked and have 2 to 4 centimeters thick clay interlayers. At the beginning of the Paleocene thin liners and pocket-like insertions appear in the depth range from 718 to 746 meters for the first time from the rest of sediments (Engl. Lay deposits ) interpreted, clayey calcarenites that are very rich in rearranged by flow processes foraminifera. Unit 4 is already intensively compacted and cemented, bulky calcite fills the interior of foraminifera and the nannofossils show extensive overgrowth of calcite.
Results of the drilling
The drilling produced a very complete and continuous, high-marine pelagic sequence of the Cenozoic . The deposited sediments are very homogeneous and consist largely of calcareous microfossils . The overall rather high lime content (generally more than 60%) suggests that the Sierra Leone threshold above the CCD remained during the Cenozoic. Terrigene entries are insignificant and are limited to the clay fraction. This underlines the permanent isolation of the Sierra Leone threshold from the African mainland. The sediment accumulation rates are generally somewhat higher on the threshold than in neighboring deep-sea basins (e.g. in the Cape Verdean Basin). The highest accumulation of 56 meters / million years took place in the Lower Eocene and is accompanied by a blooming of the silica organisms - an indication of an increased organic production in the photic zone. Low rates of accumulation are found in the Lower Paleocene, Lower Oligocene, and Middle Miocene.
The steady sedimentation is only interrupted by two discordances in the Middle Miocene and by calcarenitic residual sediments during the Upper Paleocene. The discordances can be explained by the complete dissolution of the lime peel in connection with an increase in the CCD, which was observed for this period. Erosion and loosening by deep currents is also conceivable.
The profile is a prime example of the increasing compaction of the sediment package with the depth. The cyclical fluctuations in the lime content of units 2 and 3 were also encountered in the South Atlantic on the Walvis Ridge ( DSDP 363 ) and on the Ceará threshold ( DSDP 354 ).
Fossil content
The drill cores contained abundant microfossils consisting of benthic and planktonic foraminifera, calcareous nannoplankton ( coccoliths ), dinoflagellates , radiolarians and diatoms .
Individual evidence
- ^ WH Berger, U. von Rad: Cretaceous and Cenozoic sediments from the Atlantic Ocean . In: DE Hayes, AC Pimm u. a. (Ed.): Initial Reports of the Deep Sea Drilling Project . tape 14 . United States Government Printing Office , Washington 1972, p. 787-954 .