Deep structural control on distribution of raised beaches and Quaternary sedimentation in Brittany? A review

Abstract

This paper is a review of the possible influences of deep structures on Quaternary sedimentation and the distribution of raised beaches, a problem which has never been raised before in any paper. The distribution of the Quaternary outcrops is very patchy in Brittany and seems to be organized in a random way, partly because the control by the short wavelength basement structures is not easy to recognize but also because the most efficient control is associated with long wavelength structures which are far beyond the scale of the usual geological maps and the immediate observations. Gravity data and terrain-levelling give far better results than the other geophysical techniques when the Moho undulations are compared with the surficial data. The two main domains involved in the long wavelength deformations are (a) the Alpine domain where all the Moho undulations are concentric with the Alps; and (b) the Armorican domain in which all the undulations are west–northwest–east–southeast oriented, parallel with the axis of the initial opening of the Bay of Biscay. Study of the topography and of the superficial structures associated with these undulations reveals that the undulations delineated in the Alpine Domain result from compression (buckling). These associated with the Armorican Domain result from extension (boudinage). Both phenomena show a typical wavelength of 200–250 km, which is in agreement with the results of the physical and numerical modeling currently available. These two types of deformations are crosscutting in Brittany. The general uplift of Western and Central Brittany is a well-controlled geological phenomenon. This is shown by the incision of the rivers which increases towards the west, suggesting larger uplift in the west than in the east. This uplift has been also demonstrated by two different terrain-levelling techniques. The distribution of the Pleistocene/Holocene sediments is compared with the deep structures. The westernmost Alpine undulation controlled the north–south deposition of the recent loess and swamps in eastern Brittany. The Biscay Bay related structure controlled the deposition of loess along the northern coast of Brittany, and also the location of the Pleistocene raised beaches. The gradient in beach age results from the preferential uplift of the westernmost part of Brittany.