OR/18/139 Lithostratigraphy and sedimentology

Melchin, M J1, Davies, J R2, De Weirdt, J3, Russell, C4, Vandenbroucke, T R A3, Zalasiewicz, J A. 2018. Integrated stratigraphic study of the Rhuddanian-Aeronian (Llandovery, Silurian) boundary succession at Rheidol Gorge, Wales: a preliminary report. British Geological Survey Internal Report, OR/18/139. 1  Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS, Canada 2  Department of Geography & Earth Sciences, Aberystwyth University, Aberystwyth SY23 3DB, UK (also British Geological Survey, Cardiff Office, Cardiff University, Main Building, Park Place,Cardiff, CF10 3AT, Wales, UK) 3  Department of Geology, Ghent University, Krijgslaan 281 S8, BE-9000, Ghent, Belgium 4  School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK 5  Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK

Although the Rheidol Gorge section is more heavily covered in vegetation than it was at the time it was studied by Jones (1909) 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) and Cullum and Loydell (2011) (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) has been variously placed at either: 1) a layer of prominent calcareous nodules (the “upper nodule layer” of Jones, 1909) (e.g. Cave and Hains, 1986); 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) (Davies et al., 1997).

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)), produces a prominent bedding plane that, together with the “sandy flags” of Jones (1909) 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.

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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.

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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).
* Note that bottom conditions in the Welsh Basin were likely never fully oxic, but remained dysoxic (dysaerobic) at best.