Origin and emplacement of the loess deposited in northern Brittany and under the English Channel

Abstract

The origin of the loess deposited in northern Brittany can be only understood if its original extension is carefully delineated. Marine sedimentological data show that there are no loess deposits under the English Channel, because they have been removed during the various Pleistocene and Holocene transgressive episodes. However, indirect evidence, and particularly study of the submerged Pleistocene conglomerate which resulted from the cementing of beach deposits overlain by loessic silts when the sea was at a lower level, supports the idea that loess did not extend north of the axis of the English Channel. The northern limit of the loess deposits corresponds with the Hurd Deep. Their southern boundary is located onshore and coincides with the north Armorican coastal loess belt. Comparison between the modern garnet-rich sands sampled in the centre of the western and eastern English Channel with loess of Normandy and Brittany show that the main part of the onshore loess originated in the fluvial sediments accumulated in the Channel River. Transportation of the dust from the valley of this river to Normandy and Brittany indicates southward oriented paleo-winds. The contributions of frost shattering of the offshore chalky formations, the in situ detrital particles produced in northern Brittany, and the effects of the glacial grinding which occurred in southern England seem to be rather limited.