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Monday, 4 December 2017

Ice in the Bristol Channel



One of the figures from the article.   I do not believe that the shown "Late Devensian limit" is well supported by the published evidence, and I am inclined to accept that the so-called Early / Middle Devensian ice limit, drawn largely on the basis of the borehole evidence, is in 
fact of Late Devensian age.

I have been looking again at the 2017 article by Gibbard, Hughes and Rolfe, in which it is argued that Lundy Island was glaciated during the Middle Devensian -- at a time frequently assumed to have been characterised by a mild or interstadial climate. There is a big dispute going on about the dating of rock surface exposure on the island following a glacial transgression -- more of that anon. But what interests me more at the moment is the very strong evidence for thick glacial deposits in the middle section of the Bristol Channel. These deposits (for the most part assumed to be till) are up to 38m thick in logged boreholes -- they may of course be even thicker in other locations. They are not interpreted as glaciomarine sediments dropped from icebergs or from the underside of a flowing glacier; and so this points to heavy glacial incursion from the west. When did this occur? Because the sediments on top of the till are relatively thin (normally just a few metres) a Devensian age is most likely.

Here is an extract from the paper, dealing with the borehole evidence for the area to the east of Lundy.


New insights into the Quaternary evolution of the Bristol Channel, UKPHILIP L. GIBBARD, PHILIP D. HUGHES and CHRISTOPHER J. ROLFE.JOURNAL OF QUATERNARY SCIENCE (2017) ISSN 0267-8179.DOI: 10.1002/jqs.29513 April 2017


Extract:

Swansea Bay and Gower

Boreholes from Swansea Bay indicate tills associated with the southernmost extension of the Welsh Ice Cap which reached ca. 15 km south of Swansea (Bowen, 1970, 1973, 1974; Culver, 1976; Culver and Bull, 1979; BGS, 1986). Sedimentological and microfaunal analyses indicate that these glacial deposits were originally land-based and were progressively submerged by a marine transgression between 10 and 2.5 14C ka BP (Culver and Banner, 1978). A series of rock basins in Swansea Bay were occupied by glacial lakes before marine transgression and are likely to have been present from glacier retreat at ca. 16 14C a BP until marine transgression at ca. 10 14Cka BP (Culver and Bull, 1979). As noted earlier, a clear end moraine is also visible at the sea-bed surface in high-resolution bathymetric maps with a network of meltwater channels visible outside of this moraine (Fig. 3). The eastern part of the moraine arc is covered in thick sand accumulations at Scarweather Sands. Glacial deposits have also been identified beyond the limits of the Swansea Bay moraines by Blackley (1978, his fig. 11), south and south-east of the Scarweather Sands area based on continuous seismic profile records.

The Gower peninsula is well known for its Pleistocene sediment record, with glacial deposits and raised beach deposits recorded and reviewed in numerous publications (e.g. Bowen, 1974; Bowen and Sykes, 1988; Bowen et al., 1985). The Gower is important for glacial research because it marks the southern limit of the Devensian Welsh Ice Cap. A boulder associated with this limit has yielded a 36Cl exposure age of 23.2`2 ka (Bowen et al., 2002). On the south and west of Gower the Paviland Moraine, a marginal complex of meltwater gravels and sands, rests on a red clay till (Bowen, 1974, 1999), rich in Millstone Grit clasts derived from southern Welsh Carboniferous rocks. Raised-beach deposits date to the last interglacial and earlier. Uranium-series ages from a cave stalagmite interbedded with marine beds at Minchin Hole indicate an MIS 5 age (Stringer et al., 1986). Bowen et al. (1985) argued that at least three highstands of sea level are recorded, the older two of which are correlated to the interglacials of MIS 5e (Ipswichian, Eemian Stage) and MIS 7 (Late Wolstonian, Late Saalian Substage).

Atlantic Array and Lundy

Till is present throughout the Atlantic Array survey area (Channel Energy Limited, 2010). This till (unit 3) is overlain by sand and gravel deposits (units 1 and 2). The till is present throughout the Atlantic Array survey area and according to the Channel Energy Limited (2010) report by Gardline Geo- survey reached no more than 4 m thickness. However, a later borehole report (Channel Energy Limited, 2011) indicates a much thicker glaciogenic sequence (see below).

Three boreholes (BH 10, 12, 13), out of 15 proposed in a Senergy report for RWE npower renewables (2010), encoun- tered glacial sediments in the Atlantic Array survey area. They include clay and/or sands interpreted as glacial (Pleistocene) with a thickness of 2–8 m, underlying 2–18 m accumulations of sand. Data from four of the 15 proposed borehole locations (BH 6, 11, 12, 15) are presented in a later report by the Danish Geotechnical Institute (GEO) for Channel Energy Limited (2011) and in three of these bore- holes glacial deposits are reported (BH 6A, 12, 15A). The locations of the GEO boreholes are given in Fig. 1. Summa- ries of the borehole logs are illustrated in Fig. 5. The thickest glacial deposits in the Bristol Channel are present between Lundy and the mainland (BH 15A) where 37.9m of glacial deposits are logged (Fig. 5) (Channel Energy Limited, 2011). In this borehole the top 1.9m is composed of sand. This is underlain by three units of silt/clay deposits with occasional subangular and subrounded cobbles. The silt/clay units (1.9– 12.7, 17.2–23.0 and 24.5–39.0m depths) are separated by sand/silt and gravel units. The middle silt/clay units (17.2– 23.0m) and also the sand and gravels separating this unit from the upper silt/clay unit contain ‘rare seams of fine to medium gravel and lignite’. The upper silt/clay unit is predominantly grey although grades upwards into reddish and dark brown gravelly/sandy/silty clays. The lowermost silt/ clay unit is reddish grey and olive-grey in colour.


The borehole log from BH 15A records the entire sediment sequence below 1.9 m as from a glacial environment (Gl) and of glacial age (Gc) (Fig. 5) (Channel Energy Limited, 2011). Some units are logged exclusively as glacial (1.9–5 m), while below this the sediments are recorded as both glacial and marine (Ma) or freshwater (Fw), sometimes all three simultaneously. However, the lithostratigraphy of the borehole log is interpreted in this paper as representing three till units separated by sand/gravel units. The middle till unit and overlying sand and gravels contain organic materials (lignite). Neither the upper nor lower tills contain any references to lignite. While nogeochronology is available, it is possible that the lower till unit is Middle Pleistocene in age and the middle and upper tills are of Late Pleistocene (Devensian Stage) age. The middle till may be Early or Middle Devensian in age, with lignite seams being derived by reworking of interglacial deposits, possibly of freshwater origin. However, further research is required to determine the age and nature of these deposits. Nevertheless, it does appear that multiple till units are recorded in BH 15A.

In BH 12 the clay unit, described as exclusively glacial, is much less thick than in BH 15A and is present for nearly 4 m between 14.6 and 18.5m and is characterized by many colour variations (Channel Energy Limited, 2011). The clay unit is overlain by 14.6 m of sand with shell fragments. BH 12 is associated with a prominent asymmetrical ridge. This has smaller ridges superimposed onto it (ripples) but the main core of the ridge itself could be a moraine associated with ice coming from the west. The prominent ridges in the area of BH 12 (and to the east) look different from other large ripples and large sand waves of the area. The steep western slope of the ridge is consistent with a steeper ice-contact moraine slope.

BH 6A records just over 3 m of clays/silts between 3.5 and 7.6 m. They are described as mudstone and siltstone but are considered glacial in environment and age (with codes Gl and Gc), although codes indicating marine (Ma) and Jura[- ssic] (Ju) are also used (Channel Energy Limited, 2011). These fine-grained deposits are overlain by 3.5 m of sand and gravel mixed with cobbles and boulders. In the borehole log notes these deposits are considered marine/recent or glacial envi- ronment/age and the interpretation is ‘very uncertain’.

The upper contacts of glacigenic silt and clay deposits in all three cores are at similar elevations of 49.1, 48.9 and 48.6 for boreholes 6, 12 and 15A. Sand and gravel with shell fragments overlies this lower unit (Fig. 5) (Channel Energy Limited, 2011).

Geophysical surveys reported by Channel Energy Limited (2010) and RWE npower renewables (2010) have revealed the presence of more than 22 000 boulders (>0.3 m height) in the Atlantic Array survey areas. Each of these boulders has been identified and measured for height with data on each available in the report by Channel Energy Limited (2010, appendix D). This includes the cable routes through Barnstaple Bay. Two concentrated clusters of boulders occur in the Atlantic Array survey area, one in the west and a larger cluster in the east. The extent of the eastern boulder cluster can be extrapolated southwards since concentrations of boulders are also revealed in the three cable routes between Lundy and the mainland (Fig. 1). 

The origin of the boulder clusters can be attributed to either marine or glacial origin, or both. In the former case boulder concentrations may be related to transient shoreline positions. The boulders could also be glacially transported. The fact that till is reported in boreholes in areas where no surface boulders are present, whereas no till is reported in areas where boulders are found, suggests that either the boulders are not of glacial origin or that they represent boulder remnants of a pre-existing till layer, where the fine matrix has been winnowed out. Pleistocene tills are often relatively thin in the central Bristol Channel and no more than 2–8 m in the cores reported from the Atlantic Array survey area (although 16.1m thick between Lundy and mainland Devon – see above). Another possibility is that the boulders are ice-rafted, since erratic boulders are commonly found on shorelines along the coast of Devon and elsewhere in the Bristol Channel, including as far east as Flat Holm (see ‘Cardiff and the Vale of Glamorgan’ below).



The borehole records referred to in the text. There is clearly some variety in the glacial units revealed in the borehole logs -- there may be up to three till units, and some sands and gravels and freshwater deposits / marine deposits. How many glacial episodes are represented?

Boreholes from the BGS north and west of Lundy also report glacial deposits. These include reports from Borehole 72/49 (51 ̊0.860 N, 4 ̊54.820 W), Borehole 72/52 (51 ̊23.030 N, 4 ̊54.450 W) and Borehole 73/60 (51 ̊24.930 N, 4 ̊44.800 W). Borehole 72/49 is situated ca. 25km due west of Hartland Point in Devon. This borehole revealed sand and gravel (0–3 m) underlain by poorly sorted unconsolidated sediments (3–6m), including clay with numerous rounded, subangular and angular rock fragments. The latter includes ‘broken cobbles and pebbles of igneous material’. These unconsolidated sediments are considered ‘Recent and Pleistocene’, while underlying bedrock revealed in the borehole is Carboniferous mudstone. Boreholes 72/52 and 73/60 are situated west of the Atlantic Array survey area between Lundy and Pembrokeshire. In Borehole 72/52, deposits are entirely ‘Pleistocene and Recent’ (i.e. Pleistocene and Holocene), the top 0.5 m recorded as sand, underlain by 3 m of brown-grey ‘Boulder Clay’. In Borehole 73/60 the top 0–2.5 m is ‘Recent’ sand and gravel, below which ‘Pleistocene: Till, red-brown, stiff with small rounded pebbles’ is recorded between 2.5 and 9.5 m.

The offshore evidence clearly illustrates that the Outer Bristol Channel was glaciated during the Pleistocene (Fig. 6). As noted above, Lundy also shows clear evidence of glaciation (Mitchell, 1968; Rolfe et al., 2012, 2014; Fig. 1). The island has large areas of ice-scoured granite bedrock with perched boulders (local lithology) and extensive areas of erratic gravels. Rolfe et al. (2012) dated the granite bedrock surfaces using paired 10Be/26Al analyses, the results demonstrating that there was no evidence of long-term complex exposure (i.e. exposure, burial and then re-exposure etc). However, the island had been exposed for ca. 40ka, i.e. since the Middle Devensian.
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Note that the text also refers to great concentrations of boulders in places. No less than 22,000 of them have been logged -- most of them NOT in conjunction with the glacial deposits found in the boreholes.
Note how similar this proposed ice edge position is to that proposed on this blog as the approx ice edge of the Devensian Celtic Sea piedmont glacier. Maybe things are coming together.......





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