Basa, Tilottama; (1992) The petrology and history of the Holocene sediments of Dungeness, Kent. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The drilling of several deep boreholes through the 40 m thick marine Holocene succession of the Dungeness foreland on the site of the proposed Dungeness "C" Nuclear Power Station provides an opportunity to examine the nature and composition of its sediments. Selected unaltered mollusc shells were used to date the sediment succession at -34.3 m, -32.9 m, -32.4 m and -20.5 m OD to supplement the existing determinations on both wood and shells. The dates obtained showed that the sediments at these depths were deposited between 560±95 and 2755±175 cal. yr BR Study of borehole cores and logs showed that these deposits consist of three distinct divisions: Basal Gravels, Middle Sands and Top Gravels. Grain size analyses indicated that the bulk of the Middle Sands succession (99% - 76%) below the Top Gravels comprised fine sand and silt, the rest being medium sand, clay and a sprinkling of fine to medium gravel. The silt fraction has a predominant peak at 5 to 6 &phis; typical of loess. The detrital mineralogy as a whole consists of 90% quartz, the remainder being feldspar, chert, glauconite and heavy minerals, mainly zircon, tourmaline, garnet, rutile with a variable amount of staurolite, kyanite, amphibole and the epidote group of minerals. The garnets are predominantly Fe-rich with variable amounts of Mn, Ca and Mg. The clay minerals include illite, kaolinite and mixed-clay minerals with a little chlorite and smectite, similar to that reported from brickearths in Kent. The surface of the sand grains have characteristic subaqueous, aeolian and subaerial microtextures and in a few grains there is evidence of high energy glacial features. Most of the pebbles in the Basal Gravels are subangular to subrounded and consist predominantly of flint from the local Cretaceous rocks, with some sandstones, siltstones and limestones. A few exotic pebbles such as granitic angular rock fragments are also present in the Basal Gravels. The sediment constituents are of mixed provenance. The local Cretaceous bedrocks supplied the more resistant heavy minerals and the bulk of the quartz, feldspar, chert and glauconite whereas the less stable heavy minerals along with some quartz and other light minerals were derived from relatively younger Quaternary sources. The surface texture, the size distribution of the silts, the presence of unstable heavy minerals and the clay mineral types indicate an admixture of Pleistocene aeolian loessic sediments which may have been reworked a number of times before they were finally deposited at Dungeness. The Quaternary sediments of the southern North Sea, which contain a large proportion of semistable to unstable heavy minerals, may have supplied sediment to the offshore area near Dungeness at a time when, as the molluscan fauna suggests, Dungeness was evolving under the influence of heavy storms.

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