Neoproterozoic (Torridonian) alluvial fan succession, northwest Scotland, and its tectonic setting and provenance

Abstract

The presence of alluvial fan deposits in the lower Neoproterozoic Torridon Group in north-west Scotland illuminates Torridonian basin development at the eastern Laurentian margin. The 450 m thick Cape Wrath Member of the Applecross Formation consists of alluvial fan conglomerate and arkose succeeded by more distal, braidplain feldspathic sandstone. Palaeocurrent data comprising > 2650 measurements on trough cross-bedding are of low variability and show overall eastward flow. The projection upcurrent of regionally divergent flow directions for the lower part of the member indicates a fan of c. 50 km radius with its apex 30 km to the west near a basement (pre-Caledonian) normal fault with downthrow to the east beneath the north Minch Basin. Extensional tectonics controlled deposition of the Applecross Formation. Regional uplift, causing erosion of a youthful topography on the Lewisian Gneiss, was followed by the development of the Applecross extensional basin in two main stages. Uplift of a western source area by movement on basin-bounding normal faults occurred first in the north and caused pediplanation and alluvial fan deposition in the Cape Wrath area, with subsequent uplift of the source area for the main body of the Applecross Formation occurring further to the west and south along the line of the Minch Fault. The bulk of the Applecross Formation was derived from a weathered terrain of felsic crystalline and related supracrustal rocks reaching from the Outer Hebrides region westward for up to c. 250 km onto what are now the continental margins of the North Atlantic. The tectonic events may mark an early phase in the crustal extension that led ultimately to the opening of the Iapetus ocean.

Generation and preservation of continental crust in the Grenville Orogeny

“…The Torridon Group is composed of w6 km of upwardfining, fluvial pebbly arkose to siltstone with striking stratigraphic monotony and lithologic homogeneity across hundreds of square kilometers (Kinnaird et al, 2007). Paleocurrent and U-Pb detrital zircon data imply these sediments were derived predominantly from the Laurentian continent to the west (Stewart, 1991;Williams, 2001). Several workers propose that the Torridon Group was deposited in a series of extensional basins that formed during the late-stages of the Grenville Orogeny (Soper and England, 1995;Williams and Foden, 2011).…”

The Great Grenvillian Sedimentation Episode: Record of Supercontinent Rodinia's assembly

“…These early successions accumulated between 1030 and 980 Ma based on constraints from youngest detrital zircon grains and overprinting by tectonothermal events. Sedimentological studies in the least deformed sequences in Scotland indicate accumulation in high energy fluvial braid-plain environments (Friend et al, 2003;Krabbendam et al, 2008;Nicholson, 1993;Stewart, 2002;Williams, 2001). Depositional settings of other units that accumulated during this late Mesoproterozoic to early Neoproterozoic sedimentary cycle cannot be confidently resolved due to subsequent deformation.…”

The initiation and early tectonic significance of the Outer Hebrides Fault Zone, Scotland

“…Offshore interpretation of geophysical data (Stein, 1992) and onshore interpretation of depositional patterns in the Torridonian (e.g. Williams, 2001) suggest that an additional, regionally significant normal faulting event occurred along the Minch Fault/Outer Hebrides Fault Zone synchronous with the deposition of the Torridon Group sandstones in northwest Scotland c. 1000 Ma (see also Stewart, 2002). There is no unequivocal evidence onshore for this fault movement (but see Section 3.c).…”