Weathering morphologies of the Fontainebleau Sandstone and ...

M. Thiry Weathering morphologies of the Fontainebleau Sandstone and related silica mobilityRIDGE"whale backs" of bare sandstonediscontinuous sand coverESCARPMENTdrawing of the sand - breakup of theridge - labyrinth-like corridorsSLOPErock chaos – dome-likeboulders with silica crusts50-100 mFig. 1: Schematic sketch showing the dismantling of the Fontainebleau Sandstone ridges by vertical and horizontalfractures and development of the dome-like shaped boulders on the sand slopes.Dome-like morphologies with silicacrustsAer break up, the blocks acquire rounded shapes.In this way, at the ridge escarpment, the sandstoneplates resulting from horizontal fracturing becomerounded at their upper edge, whereas their lowerplane show a prominent lipp-like flange. The silicadeposits formed in the fractures are put into reliefby this weathering.The more massive facies also round out and formdome-shaped boulders at the top of the ridges and onthe rockfall on the sandy slopes. Boulders diminishregularly in size away from the escarpment and thedomes become coated with a 0.5 to 2 cm-thick silicacrust, harder than the sandstone body (Fig. 2E &2F). This silica crust is restricted to the top of thedomes, and is lacking on the lower parts and on theoverhanging surfaces. It is put into relief by differentialalteration at the periphery of the domes.Thin sections across the silica crust show that theovergrowthed quartz grains of the sandstone aresplit off by tiny cracks, from 1 to 20 µm-wide,along the grain contacts, mostly parallel to the crustsurface. Quartz grains are only seldom fractured.These cracks are cemented by brown to clear opalwhich forms the harder and less alterable crust at thetop of the domes. Nor empty cracks, nor cracks withclay or organic-rich illuviations are never observed.This may indicate that opal precipation itself bringsup the disjunction of the quartz grains.Polygonal dissolution groovesPolygonal networks of grooves develop on thedome flanks and on the overhanging surfaces(Fig. 2G). Such polygonal grooves never impressin the silica crusts capping the domes and coatingthe fractures. The network is generally isometric,forming polygones of about 5 to 10 cm in diameter,they may become in horizontal elongated. Inplaces, the layout of the overhanging surfaces ofthe sandstone blocks, and even larger pans, clearlypoint out that these surfaces are related to a formerlandsurface supporting a paleosol which has beenstripped off by erosion. Thus development of thepolygonal networks of grooves may be relatedto processes occuring within the soil and not incontact with the atmosphere.Thin sections show development of large intergranularvoids which come with preferentialdissolution of the quartz overgrowths. Thesevoids remain empty, or may contain tiny quartzchips oen blended with iron oxydes. No preexistingstructure within the sandstone is relatedto the development of the dissolution voids, northe polygonal grooves.Nevertheless, if the development of the polygonalgrooves indicates weathering-dissolution rocesses,the mechanism leading to the polygonal structureremains unknown.48 Ferrantia • 44 / 2005