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Distribution of Clay Minerals In the Northern North Sea Basin During the Paleogene and Neogene: A Result of Source-Area Geology and Sorting Processes
Clay-mineral compositions have been analyzed in samples from the Danish, Norwegian, British, and Dutch North Sea sectors and from onshore Denmark and Germany, comprising both wells and outcrops (clay pits and cliff sections). The time slices investigated comprise from the Paleogene, the post-Ekofisk Fm. interval of the Paleocene, the entire Eocene, and the entire Oligocene. The Neogene time slices investigated comprise the following Miocene intervals: the Vejle Fjord Fm., the Klintinghoved Fm., the Arnum Fm., and the Gram Fm. There were not enough samples from offshore wells to make a detailed stratigraphic subdivision of the Paleogene. It was, however, possible to refer Miocene samples to specific lithostratigraphic units. The onshore Danish nomenclature was applied, and seismic sections were used to correlate between wells and outcrops. Existing biostratigraphic data were used to control the stratigraphic interpretation. The distribution of the most dominant clay minerals—smectite, chlorite, kaolinite and illite—are shown on maps comprising the Paleocene, Eocene, Oligocene, and the formations Vejle Fjord Fm., Klintinghoved Fm., Arnum Fm., and Gram Fm. from the Miocene.
The investigation has shown that the source-rock composition plays an important role in the clay-mineral content of the sediments. For example, smectite makes up a higher proportion in periods with substantial supply of volcanic material, especially in the Paleocene and early Eocene. In source areas, where metamorphic rocks dominate, chlorite is present in the sediment adjacent to these areas. The illite content in sediments increases where the source area is dominated by granites and/or gneisses. Sorting of the supplied suspended material generally controls the depositional area for the minerals. For example, the larger particles of kaolinite are generally more abundant in nearshore areas, whereas the relatively small smectite particles normally dominate in the central parts of the sedimentary basins. Illite and chlorite are relatively abundant in areas between those rich in smectite, present in the central part of the basin, and those rich in kaolinite, present in the marginal parts of the basin. Climatic conditions during erosion and deposition seem to play an important role, especially in the distribution of chlorite, which is rather sensitive to chemical weathering. This means that chlorite makes up a greater proportion of the sediment in periods with colder climate, such as in the Oligocene and late Miocene. The uplift of surrounding areas seems to be an important factor for the amount of detrital material transported to the North Sea. The elevation of marginal areas also played an important role in the reworking of Paleogene sediments during the Neogene. Mass flows, including turbidity currents, have redistributed marginal kaolinite-rich sediments, deposited in marginal areas of the basin, to more basin-central locations, especially in the Paleocene Viking Graben. Diagenetic modification of the originally supplied clay minerals seems to be minor since the highest contents of smectite occur in the oldest and deepest areas with more than 3 km of overburden. The reason for this is believed to be the extremely low permeability of the very fine-grained smectite-dominated clays hindering water from being expelled from the sediments, combined with low contents of K-bearing minerals necessary for the formation of illite from smectite.
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