Les paleocourants marins du bathonien moyen au bathonien superieur dans le nord de la campagne de caen (Normandie)

Abstract

In the northern half of the Campagne de Caen (Normandy), the Bathonian deposits show a succession of four marine carbonate cross-bedded formations: Calcaire de Reviers, Calcaire de Blainville (Middle Bathonian), Calcaire de Ranville and Calcaire de Langrune (Upper Bathonian). The directional sedimentary structures are compiled, described and compared. The evolution of size, shape and direction of the main current-marks are followed through the lithological succession. The close association of large-scale planar cross-bedding and trough-bedding characterized the Calcaire de Reviers, at the base of the column: these structures are interpretated as vestigial patterns of large submarine sand dunes, more than 100 m in wavelength, cut off by meandering stream channels. The formation was laid down under shallowing water conditions on the northeastern margin of the Armorican Massif and resulted from the action of both wave- and tidal-currents. Earlier in Middle Bathonian times, the dimensions of the sand-bodies decreased as well as the depth of the depositional environment, while stream velocities strengthened. During the Late Bathonian, a minor deepening of the basin followed a non-sequence and led to a recurrence of the sedimentary evolution. From the Middle to the Late Bathonian, the resultants of the direction of the main currents reversed following a dextral rotation, so by successive steps it drifts from N 40 to N 180. In the palaeogeographical evolution of this area, this drastic change of the direction of main marine currents appears to be linked to epeirogenic movements and regional climatic modifications, which gave rise to a new pattern of marine currents at the beginning of the Late Bathonian times. By submergence of a swell (or a barrier), epeirogeny would be able to favour the southern spread of colder boreal water on the northern part of the Anglo-Paris Basin, contemporaneous with a distension phase which took place on the North Sea floor at this time.