Contribution à l’etude géologique et sédimentologique de la Manche orientale

More than 1500 km of high definition continuous seismic profiling have been recorded by the company BEICIP-Geotechnip, for the CNEXO, since the publication of the geological map (Boulogne-Rouen) of the eastern English Channel. Latest information has improved our knowledge of the geological structures and the quaternary sedimentation west of the Boulonnais. (1) Geology. The southern border of the Boulonnais Jurassic anticlinorium is composed of a succession of faulted anticlines and synclines, oriented NW-SE, in which the Portlandian and a Cretaceous series, from Wealden to Cenomanian age, are outcropping alternately. The Bassurelle bank rests on an upper Kimmeridgian anticline which plunges near the French coast west of Le Touquet. To the north, lying parallel to this high zone, a Cretaceous basin extends far to the northwest. The existence of these structures seems to be the consequence of a late-Cenomanian tectonic phase already known on land. (2) Quaternary sedimentation. The sand banks of the eastern English Channel are of two types: those out to sea (Vergoyer) present a prograded structure oriented west—east; those by the coast (Bassurelle de Somme) present a thin stratified structure inclined east-west. The bed-rock topography over which the Flandrian transgression advanced may explain these differences.

Evolution of the estuary of the Seine since the last glaciation

The study of many core samples taken in the estuary of the Seine complemented by seismic data have permitted an understanding of the history of this region during the Flandrian transgression and during the last glaciation. The Quaternary deposits comprise three units, from top to bottom we distinguish: (1) Flandrian sediments (brackish and marine formations); (2) Fresh-water postglacial deposits, and (3) Basal gravels and pebbles on the bed-rock. The palaeovalley presents a varied morphology with palaeomeanders and two successive erosion-levels. Upon these erosion-levels rest gravels and pebbles (figure 1). The upper level (I) situated between - 3.7 m and - 20.7 m n.g.f.^[ is dated as early in the Weichselian glaciation while the lower level (II) which slopes downward from — 24 m n.g.f. at Tancarville to - 50 m n.g.f. off Le Havre is dated from the last part of the Weichselian glaciation. The gravels and pebbles of the fluvial basin of the Seine have been reworked in part by the Flandrian transgression or contaminated per descensum by the shelly-marine sediments characterized by the mineralogic association: hornblende, epidote and garnet. The Holocene deposits include various freshwater sediments: clays, silts, sands, calcareous tufas and peats deposited during the Boreal and the Atlantic stages before the marine influence and during the regressive stages. The brackish and marine sediments comprise silts associated with organic muds, shelly sediments and pebble bands. Brackish and marine deposits have been dated by pollen analysis and radiocarbon dated as from Boreal to Subatlantic. During the Flandrian transgression two regressive periods have been displayed: one before 8000 b.p. and one about 7200 b.p.

Sediment Distribution in Christchurch Bay, S. England

The area of Christchurch and Poole Bays has been cut by the sea in the period since the breaching of the Chalk ridge between the Isles of Wight and Purbeck late in the Flandrian transgression. Erosion of the soft Tertiary rocks must have been fast and is still active. In the Barton area the cliffs have been receding at about 1 m/year since 1895 (May, unpublished). There the cliffs are composed of badly cemented sands and sandy clays, capped with plateau gravels about 3 m thick, and erosion of them has released large quantities of sand and gravel, sufficient to maintain features such as Hurst Spit. This Spit approaches within 1.3 km of the Isle of Wight, but is separated from it by the tidal channel of the West Solent, in this area up to 60 m deep. In these narrows the south west flowing ebb current reaches 2.25 m/sec, whereas the flood current only reaches 2 m/sec.

Sur la mise en place des depots grossiers du plateau continental dans la partie nord du golfe de Gascogne

Relic deposits of cold periods of Quaternary Flandrian transgression, covered with fine sediment, France

Etude des depots flandriens de l'anse Duguesclin, pres de Cancale (Ille-et-Vilaine)

The Flandrian (Pleistocene) stratigraphy and the intermittent character of the Flandrian transgression in the Cancale area (Ille-et-Vilaine, France) are established on the basis of information from a series of borings in a sand bar in the Duguesclin bay. The present topography is an exact reflection of the Flandrian topography; the sand bar and the marsh behind it have retreated inland, however.

Formations quaternaires du havre de Blainville (Manche)

Analysis of the heavy mineral and granulometric composition of the shore and harbor sands of the Blainville area, France, shows that an early Quaternary marine phase was followed by a period of emergence, then the Monastirian transgression, regression, the Flandrian transgression, and finally by the formation of the present dunes.

Atlantic Gulfs, Estuaries, and Cliffs

The morvan theory with slight modification fits the structure of the coasts of south-western Britain better than the fault theory of Atlantic coasts advanced by Suess.The English Channel and St. George's Channel have been long in existence, and are more probably of tectonic than erosional origin.Erosional deepening of the valleys which were drowned by the Flandrian transgression to become estuaries must have taken place almost entirely in glacial ages when the level of the ocean has been low. The tempo of coastal erosion, both subaerial and marine, has been rather slow, however, owing to the great width of the continental shelf. Thus shoreline details survive from an interglacial age.In regions with a narrower continental shelf, deepening and also extensive enlargement of valleys across coastal lands have taken place in each glacial age of low ocean level, and interglacial aggradations have determined terrace levels at consistent heights in the inland valleys of large rivers, as exemplified in Portugal.The cliffs of Cornwall and Wales show traces of two cycles or marine retrogradation separated by a withdrawal of the sea. Thus the “bevel”, or “coastal slope” above modem cliffs can be accounted for as an ancient sea-cliff graded subaerially during the period of low ocean level.