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Sunday, 18 January 2015

Rhyolite pebbles on Lundy island -- and limestone too

This map from Rolfe et al, 2012 is generalised but nonetheless interesting, since it suggests an ice limit impinging onto Salisbury Plain not far from Stonehenge. 

I've been looking at these two papers by Chris Rolfe and colleagues -- and am reminded of the presence of rhyolite pebbles in the erratic assemblage.  They appear convinced that the rhyolites are not local, and suggest they might have come from Skomer Island off the coast of Pembrokeshire.   So now we have rhyolites on Lundy Island and on Baggy point, and I'm pretty sure there are rhyolites on Flat Holm as well (that may be confirmed when Sid Howells has completed the identifications of the samples collected there during our autumn 2014 visit.)

There are limestone erratics too.  Could they have come from South Pembrokeshire?

This all starts to look interesting..... but remember that these papers refer to an EARLY DEVENSIAN glaciation of Lundy.  That looks out of synchronisation with the Late Devensian glaciation which reached the Scilly Isles.  Either there are problems with dating techniques, or there was a sort of pulsed behaviour, with different parts of the Irish Sea Glacier reaching their maxima many thousands of years apart.......  No doubt this will be sorted out eventually.

But as ever, what happened in the Devensian can give us some guidance to what happened in the generally much bigger Anglian Glaciation. 

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TIMING OF THE MAXIMUM EXTENT OF LATE PLEISTOCENE GLACIATION IN N.W. EUROPE: EVIDENCE FROM LUNDY
by CHRIS J. ROLFE, PHILIP D. HUGHESAND ANTONY G. BROWN,  Journal of the Lundy Field Society, 4, 2014, pp 7-18.

ABSTRACT
This paper presents geomorphological and cosmogenic isotope evidence for the glaciation of Lundy. 26Al/10Be analyses from glaciated bedrock surfaces reveal an exposure age of c. 35-40ka. This challenges the long-established view that the last glaciation this far south must belong to Middle Pleistocene, such as the Anglian Stage (c. 480-420 ka), when ice reached as far south as London. Instead, the findings suggest glaciation of Lundy during the last ice age (Devensian Stage). However, the ages from Lundy suggest that the ice sheet in this area was at its largest extent well before the global Last Glacial Maximum at c. 26-21 ka.


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Rolfe, C.J., Hughes, P.D., Fenton, C.R., Schnabel, C., Xu, S., Brown, A.G., 2012. Paired 10Be and 26Al exposure ages from Lundy: new evidence for the extent and timing of Devensian glaciation in the southern British Isles. Quaternary Science Reviews 43(2012): 61-73.

ABSTRACT
Lundy lies in a strategic geographical position for understanding the glacial history of the British Isles. The island bears evidence of glaciation, largely in the form of ice-moulded bedrock and glacially- transported boulders e an unusual occurrence this far south in the British Isles. Irish Sea ice penetrated the western Bristol Channel overriding Lundy from the northwest during the last phase of glaciation in this area. The results of paired terrestrial cosmogenic nuclide analyses (26Al/10Be) constrain the timing of this extensive glaciation and provide, for the first time, an age for the exposure of Lundy granite following deglaciation. The results from nine paired samples yield 26Al/10Be exposure ages of 31.4-48.8 ka (10Be) and 31.7-60.0 ka (26Al). This challenges the view that any glaciation this far south must belong to Middle Pleistocene glaciations, such as the Anglian Stage (c. 480-420 ka) and a Devensian age for the last glaciation is consistent with findings from the Isles of Scilly further south. However, the findings suggest early-mid Devensian (marine isotope stage (MIS) 4-3) glaciation of Lundy. It also implies that the island was exposed or covered for a short time by non-erosive cold-based ice at the global Last Glacial Maximum (LGM) during MIS 2 (26-21 ka). The potential exposure of the island throughout MIS 2 contrasts with the evidence from the Isles of Scilly and the Celtic Sea, which were glaciated at the LGM.

EXTRACT

Two (pebble) samples had similar physical properties (fine-grained light blue-grey clasts) with similar geochemistry with SiO2 contents of 75.8% and 79.5% and combined Na2O and K2O of 2.6% and 3.3%. These samples are similar to some rhyolites. On Lundy, rhyolite dykes are known to be present -- including in the north of the island near the gravel spreads where the clasts were found (Thorpe and Tindall, 1992). However, the local rhyolites have much higher values of combined Na2O and K2O (7.8e10.5%). Furthermore, the clasts have much higher iron oxide content (Fe2O3: 6%) than most of the Lundy rhyolites that have otherwise comparable geochemistries (Fe2O3: 1e2%). The clasts have a closer geochemical composition to rhyolites from the thick rhyolite sequences of the Skomer Volcanic Group off the westernmost coast of Pembrokeshire (e.g. Thorpe et al., 1989) although the precise origin remains unknown.

In addition to the siliceous clasts analysed using XRF, several limestone clasts collected from the gravel spreads are clearly erratics since no carbonate bedrock lithologies are found on or near Lundy. Given that rhyolites, quartz/quartzites, siliceous sedimentary and carbonate limestones are common lithologies, a wide range of sources are possible as noted in the previous paragraphs. Nevertheless, the presence of these erratic clasts is consistent with transport and deposition in association with an Irish Sea Ice Sheet. The fact that the clasts are well-sorted into cobbles and gravels and predominantly rounded suggests deposition by water. The presence of these deposits on the watershed is consistent with sediment release at the apex of bedrock obstacles in subglacial channels (cf. Lesemann and Brennand, 2009).


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