Eocene sequence stratigraphic siliciclastic sedimentation model in the Southern Bight, North Sea
Jacobs, P. (1994). Eocene sequence stratigraphic siliciclastic sedimentation model in the Southern Bight, North Sea, in: 14th International Sedimentological Congress: Abstracts. pp. 215-215 In: (1994). 14th International Sedimentological Congress: Abstracts[s.n.]: Recife, more |
| Abstract | The Belgian sector of the Southern Bight of the North Sea stands as a copy-book example of a shallow marine siliciclastic intracratonic sedimentary basin with a ramp type margin setting, characterized by low subsidence rates, relative tectonic quiescence, and a low gradient relief. Clastic sediment supply resulting of erosion by constant southern Alpine uplift, origins from a proto-Rhine-Meuse-Scheldt river drainage system, gradually infilling the basin. The mainly deltaic sedimentation system gives rise to (offshore and onshore) Tertiary onlapping and shallowing upward sedimentary series of metric to decametric thickness with a dip to the NNE of less than 5%.To identify the offshore seismostratigraphical units introduced on basis of 16,000 km of high resolution reflection seismic lines shot in the Southern Bight, four cored wells have been drilled in front of the Belgian coast, roughly forming a dip section and displaying a marine sediment series of Eocene age with a composite thickness of about 200m. Sedimentary facies analysis, grain size trends, sediment genetic interpretation and sequence analysis of the four wells were used to reconstruct the sedimentation model.Two major transgressive/regressive cycles can be discerned, the lower one consisting of Lower and Middle Eocene sediments evidencing 3 sedimentary environments. Lowermost Eocene sediments are mainly clayey, display an overall fining upward trend and are deposited on a mud shelf. A prograding delta outbuilding on the continental shelf deposits middle to upper Lower Eocene sediments, gradually becoming more sandy. Middle Eocene sediments are mainly sandy, contain calcarenite horizons, coarsen upward and are mostly of tidal and lagoonal origin. The upper transgressive/regressive cycle is only composed of 2 sedimentary environments. Upper Eocene strata consist of an alternation of sandy and clayey sediments of distal deltaic origin, gradually fining upward, with slow sedimentation rates, while the Uppermost Eocene is more sandy and of tidal sand flat origin.A schematic relative sea level curve valid for the Southern Bight of the North Sea has been constructed on basis of the detailed facies analysis of the sediments in the 4 wells (biostratigraphy is in progress). In total 7 'lithostratigraphical' sequences can be recognized: the lowermost 4 sequences belong to the 1st cycle, while the 3 following sequences correspond with the 2nd cycle. The lowermost sequence of all has a large areal extension and an overall occurrence in a more basinward position. The three following sequences are deposited more landward, display an aggradational pattern and are mainly characterized by an overall fall in relative sea level: sediment nature gradually coarsens upward from stiff clay towards medium sands as an expression of overall progradation and considerable shallowing upward of the basin. The fifth sequence (2nd cycle) occurs in again a more basinward position due to deposition after an important rise in relative sea level, and is characterized by a large areal extension and lateral continuity due to deepening of the basin. The last two sequences again show progradation and aggradation, and gradually coarsen and shallow upward as a result of renewed overall relative sea level fall.The schematical (Palaeogene) clastic wedge has a dip architecture consisting of stretched metric to decametric systems tracts with a distinct facies signature. Sequence boundaries are mostly of type 2 nature as type 1 is generally restricted to incision below shelf edge, only occurring in response of rapid relative sea level drop, generating Lowstand Systems Tracts as incised valley infills with fluvial and tidal character during late lowstand and early transgression times. Most sedimentary units consist of stacked elongated Transgressive Systems Tracts or TST-parasequences as transgressions backstep over vast areas on the continental shelf in response of minor relative sea level rises. Proximal or distal deposition determines sediment nature due to basin configuration. Highstand Systems Tracts sometimes tend to have a restricted occurrence as a result of lower preservation potential because subject to erosion during the next relative sea level fall. Strike architecture may show juxtaposition of sediment series due to lateral shift of the multiple sediment feeder system as proximal, medial and distal delta parts migrate or interfinger. |
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