FORMATION OF SPECTACULARLY FLAT GRANITE OUTCROPS THROUGH HYDROFRACTURING

A series of near-planar subhorizontal bedrock surfaces is developed in Precambrian gneissic granite basement in SW Sweden. These surfaces are located within 0.1 – 10 km of the 150 m-high Halleberg and Hunneberg table mountains, which are comprised of marine sedimentary units capped by a dolerite sill. The near-planar bedrock surfaces have traditionally been interpreted as exhumed remnants of a regional Subcambrian unconformity. We evaluate an alternative hypothesis that these spectacular surfaces are attributable to formation and erosional exploitation of subhorizontal sheeting joints by the late Quaternary Fennoscandian Ice Sheet. We use a combination of landscape analyses in a GIS, field measurements of bedrock jointing, laboratory analyses of basement chemical weathering, ground penetrating radar imaging of subsurface jointing, erosion rates from cosmogenic nuclides, and numerical ice sheet modelling. This diverse dataset supports the following interpretations: (1) marine erosion of weathered rock during the Cambrian transgression was likely important to the formation of the regional SCU; (2) the spectacular flatness of the surfaces studied is attributable to Quaternary glacial erosion, with sheeting joints providing a first-order structural control; (3) subhorizontal hydrofracturing of bedrock in the near-surface (perhaps up to hundreds of meters depth) occurred in either a marine proglacial or a subglacial/ice marginal setting. This hydrofracturing was promoted by episodic drainage of supra-glacial meltwater ponds along crevasses and fractures in the ice that are directly connected to the ice sheet bed, and by ice sheet deformation around the steep sides of the table mountains. Basal meltwater could temporarily decouple the ice sheet margin from its bed and generate subglacial (and/or proglacial) water pressures in the bedrock that are sufficient to generate subhorizontal fractures hundreds of meters long. Glacial ice subsequently exploited these subhorizontal planes of weaknesses to erode blocks (with volumes potentially exceeding 125 m³, based on observations elsewhere in southern Sweden), leaving remnant near-planar bedrock surfaces.