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{{OR/18/139}}
Although the Rheidol Gorge section is more heavily covered in vegetation than it was at the time it was studied by Jones (1909)<ref name="Jones 1909">Jones, O T. 1909. The Hartfell-Valentian succession in the district around Plynlimon and Pont Erwyd (north Cardiganshire). Quarterly Journal of the Geological Society, 65, 463–537, pls 24, 25. </ref> and Sudbury (1958), a stratigraphic interval of 20 m in thickness is almost continuously exposed through the upper Rhuddanian and lower Aeronian strata (Figure 2). Our detailed new log of this part of section (Figure 3) supersedes the earlier logs of Jones (1909)<ref name="Jones 1909"></ref> and Cullum and Loydell (2011)<ref name="Cullum 2011">Cullum, A A, and Loydell, D.K. 2011. The Rhuddanian-Aeronian transition in the Rheidol Gorge, mid Wales. ''Proceedings of the Yorkshire Geological Society'', '''58''', 261–266. </ref> (all levels referred to elsewhere in this report refer to this figure). The strata in this succession are assigned to the upper part of the Cwmere Formation and the lower part of the overlying Derwenlas Formation. The boundary between these two formations (or there earlier equivalents; see Davies et al., 1997<ref name="Davies 1997">Davies, J R, Fletcher, C J N , Waters, R A , Wilson, D , Woodhall, D G, and Zalasiewicz, J A., 1997. Geology of the country around Llanilar and Rhayader. Memoir of the British Geological Survey. British Geological Survey, Keyworth, Nottingham (Sheets 178 and 179 (England and Wales)).</ref>) has been variously placed at either: 1) a layer of prominent calcareous nodules (the “upper nodule layer” of Jones, 1909<ref name="Jones 1909"></ref>) (e.g. Cave and Hains, 1986<ref name="Cave 1986">Cave, R, and Hains, B A. 1986. Geology of the country between Aberystwyth and Machynlleth. British Geological Survey Memoir for 1:50 000 geological sheet 163 (England & Wales). HMSO, London.</ref>); or 2) at the base of a succession more continuously dominated by massive mottled mudstones just below the top of our measured section, above the “''magnus ''Band” of Sudbury (1958)<ref name="Sudbury 1958">Sudbury, M. 1958. Triangulate monograptids from the Monograptus gregarius Zone (Lower Llandovery) of the Rheidol Gorge (Cardiganshire). Philosophical Transactions of the Royal Society of London, 241, 485–555.</ref> (Davies ''et al.'', 1997<ref name="Davies 1997">Davies, J R, Fletcher, C J N, Waters, R A, Wilson, D, Woodhall, D G, and Zalasiewicz, J A. 1997. Geology of the country around Llanilar and Rhayader. Memoir of the British Geological Survey. British Geological Survey, Keyworth, Nottingham (Sheets 178 and 179 (England and Wales)).</ref>).

Our studied stratigraphic interval consists of at least three cycles of laminated, graptolitic black shales, grading upward through weakly bioturbated into moderately to strongly bioturbated mudstones. The base of the second of these cycles is a sharp contact of black shales overlying light grey mudstones at the 9.37 m level (marked as the “prominent bedding surface”, Figures 2 and 3). This contact, interpreted as a flooding surface (see Davies ''et al.'', 2016)<ref name="Davies 2016">Davies, J R, Waters, R A, Molyneux, S G, Williams, M, Zalasiewcz, J A, and Vandenbroucke, T R A. 2016. Gauging the impact of glacioeustasy on a mid-latitude early Silurian basin margin, mid Wales, UK. ''Earth Science Reviews'', 156, 82–107. </ref>), produces a prominent bedding plane that, together with the “sandy flags” of Jones (1909)<ref name="Jones 1909"></ref> and two conspicuous nodule horizons, provides an important reference marker for the section. Siltstone to fine sandstone laminae and beds (interpreted as turbidites) occur interbedded with the mudstones and these are thickest and most abundant in the lower half of the section.

Detailed lithological logging (by JRD) has permitted recognition of four distinct facies within the mudstone, based on their style and intensity of bioturbation, colour, and degree of preservation of lamination (Table 1, Figure 4). These can be interpreted in terms of the relative oxicity (RO) of the basin floor conditions at the time of deposition and the distribution of these facies through the section is shown in Figure 3.

[[Image:OR18139fig2.jpg|thumb|center|500px| '''Figure 2'''    View of the upper part of the Rheidol Gorge section; note the prominent bedding surface (by rucksack) 0.8 m below the first appearance of ''D. triangulatus'' Biozone graptolites. ]]

[[Image:OR18139tab1.jpg|thumb|center|600px| '''Table 1'''    Relative Oxicity (RO): Features displayed by deep-water Rhuddanian and Aeronian slope apron mudstone facies in the Welsh Basin that are inferred to record the impact of differing levels of basin floor oxicity (see also Figures 3 and 4; compare with Cave & Hains, 1986; Davies ''et al.'', 1997).<br><span style="font-size:86%">''* Note that bottom conditions in the Welsh Basin were likely never fully oxic, but remained dysoxic (dysaerobic) at best.''</span> ]]

==References==
{{reflist}}
[[Category:OR/18/139 Integrated stratigraphic study of the Rhuddanian-Aeronian (Llandovery, Silurian) boundary succession at Rheidol Gorge, Wales: a preliminary report | 04]]

OR/18/139 Lithostratigraphy and sedimentology - Revision history

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Geosource>Ajhil at 13:34, 4 October 2018