Stone run (block stream) formation in the Falkland Islands over several cold stages, deduced from cosmogenic isotope (¹⁰Be and ²⁶Al) surface exposure dating

Abstract

S. 2008. Stone run (block stream) formation in the Falkland Islands over several cold stages, deduced from cosmogenic isotope ( 10 Be and 26 Al) surface exposure dating.ABSTRACT: Cosmogenic isotope ( 10 Be and 26 Al) surface exposure dating has been applied to valley-axis and hillslope stone runs (relict periglacial block streams) and their source outcrops in the Falkland Islands, South Atlantic. The data indicate that stone runs are considerably older landforms than previously envisaged and afford no evidence that they are a product of the Last Glacial Maximum; the samples range in apparent 10 Be age from 42k to 731k yr BP, but some of these are minima. The results indicate that valley-axis stone runs may be up to 700-800k yr old, have simple exposure histories and are composite landforms that developed over several cold stages. Analyses of some hillslope and outcrop samples also demonstrate simple exposure histories with 10 Be ages from 42k to 658k yr BP. In contrast, isotopic ratios from other hillslope and outcrop samples reveal they have had a complex exposure history involving periods of burial or shielding; the samples range in 10 Be age from 59k to 569k yr BP and these are regarded as minimum age estimates. Larger stone runs may be older than smaller runs and there is a possibility that stone runs older than 800k yr exist in other parts of the Falklands. The assertion that glaciation in the Falklands was restricted to the highest uplands is supported by the data, and the potential for age determination of other boulder-strewn and bedrock landforms, using cosmogenic isotope analysis, in order to extend the geochronology of Quaternary events and processes is noted.

Periglacial geomorphology of summit tors on Bodmin Moor, Cornwall, SW England

“…Hence the landsystem signature may have been generated over the last glacial cycle or longer. Nevertheless, landforms that indicate extreme periglacial landscape maturity elsewhere, for example the stone runs of the Falkland Islands (Hansom et al, 2008;Wilson et al, 2008), are not well developed around the lower slopes of the Bodmin Moor tor summits. Their occurrence in surrounding valleys, beyond the mapping areas presented in this paper, could potentially verify extreme periglacial landsystem age.…”

“…It is surprising, therefore, that little systematic mapping of periglacial landforms has ever been undertaken on the granite uplands, an important exception being the assessment of Cox Tor and its environs by te Punga (1956Punga ( , 1957, Gerrard (1988) and Miller (1990), where altiplanation terraces and spectacular examples of 'clitter' (blockfield) landforms such as boulder lobes and boulder runs were identified and used as exemplars of periglacial landscape maturity. Boulder runs and castellated tors in particular are definitively indicative of mature periglaciation in settings that have never been glaciated, such as the Falkland Islands for example (Clapperton, 1975;Clark, 1972;Hansom, Evans, Sanderson, Bingham, & Bentley, 2008;Joyce, 1950;Wilson, Bentley, Schnabel, Clark, & Xu, 2008). More recently Evans et al (2012aEvans et al ( , 2012b and Harrison and Evans (2014) have highlighted that the Cox Tor and neighbouring Staple Tor area, with its castellated tors, altiplanation terraces and mature boulder lobes and runs, contrasts markedly with the core of the north Dartmoor landscape where such apparently mature periglacial landforms are not as well developed; instead, this core area, the highest terrain of the Dartmoor uplands, is characterized predominantly by summits with subdued or no tors and a sparsity of mature boulder lobes and appears to have been formerly glaciated.…”

The distribution and paleoclimate implications of periglacial landforms in eastern Australia

“…The widespread evidence for clay weathering products and rounded nature of boulders associated with some block deposits, do suggest that block deposits in part owe their formation to chemical weathering (Caine 1983, Rea et al 1997. The principle driver of formation of block deposits has been debated particularly in the Falkland Islands (Andrè et al 2008, Hansom et al 2008, Wilson et al 2008, however studies of the deposits in the Falklands and similar studies from Scandinavia which present evidence for chemical weathering still acknowledge the role Late Quaternary periglacial conditions have played on block deposit development (Rea et al 1997, Whalley 1997, Van Steijn et al 2002, Andrè et al 2008, Goodfellow et al 2009). Block deposits with obvious headwall sources are clearly products of mechanical weathering with chemical weathering playing a secondary role in formation.…”

“…These deposits, also known as boulder streams or stone runs (Wilson et al 2008) form by the same process as block slopes, however they can be distinguished by their formation on a range of slopes from shallow valley floors with slope angles close to 0° to hill-slopes greater than >25°, with common valley floor confinement (Eaton et al 2003) and downslope elongation of the deposits (Jennings 1969, White 1976. Many block streams display evidence of slow movement in the form of irregular topography, perched boulders, lobes and depressions in the surface of the deposits (Wilson 2013).…”

“…Oguchi and Tanaka 1998, Boelhouwers 1999a, b, Van Steijn et al (2002, Wilson et al 2008). In Australia there is no evidence of modern block deposit activity, however, researchers, notably Webster (1974), Caine (1983) and Barrows et al (2004), have documented numerous relict block deposits in the Australian Alps and highlands of Tasmania.…”