SEDIMENTOLOGICAL AND GROUND PENETRATING RADAR STUDIES OF SUBGLACIAL DEPOSITS IN THE JOUX VALLEY (SWITZERLAND): EVIDENCE FOR LARGE SCALE BACKSET ACCRETION IN AN ESKER

During the Wurmian glaciation, the Joux Valley (Switzerland) was covered by the Jura ice sheet. Many elongated hills, made of till or sand/gravel, formed during this period and cover the valley bottom. In an attempt to determine their origin, we studied a quarry outcrop of one of the sand/gravel hills. An outcrop perpendicular to the hill elongation shows sequences of alternating fine sand to coarse gravel layers indicating fluvial transport with strong and rapid flow variations. They dip toward the hill flank and are truncated at the top by the present topography.

In order to understand the 3-D geometry of the deposits, we conducted 100 MHz ground penetrating radar (GPR) survey on the hill. These GPR data show a penetration depth of up to 15 m and allow good correlation with the nearby outcrop. The transversal profiles show an arched, convex-upward structure of the sediments, whereas longitudinal profiles show a large-scale backset (up-current) stacking of the sediments. The backset stacking is confirmed by the paleo-current indicated by current ripples in the outcrop. The erosional surface seen in the outcrop extends throughout the whole hill.

The arched structures indicate formation within a subglacial closed meltwater conduit (esker), whereas the backset accretion would indicate a large chute-and-pool (high flow energy backset accretion) macroform with dimensions comparable to those of the conduit. The macroform migration backward implies a progressive ice-wall melting contemporaneous to sediments deposition. The erosion level cannot be linked to ice action of the glacier, given the lack of shearing structures in the underlying sediments. Very high-energy subglacial meltwater floods (jökulhlaups), probably due to the drainage of subglacial or supra-glacial lakes, are the more likely cause of the erosion.

This research is supported by the Swiss National Science Foundation (grant no. 20-68091.02)

The two processes inferred from the GPR data (backset accretion in a subglacial conduit and meltwater erosion) underline the importance of meltwater in sedimentological processes under the Jura ice sheet.