Discussion of ‘A late Ordovician/early Silurian non-depositional slope and perched basin along the Tywi Anticline, Mid Wales’ by D. M. D. James
✍ Scribed by R. A. Waters; J. R. Davies; C. J. N. Fletcher; D. Wilson
- Book ID
- 102225484
- Publisher
- John Wiley and Sons
- Year
- 1992
- Tongue
- English
- Weight
- 418 KB
- Volume
- 27
- Category
- Article
- ISSN
- 0072-1050
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✦ Synopsis
In Volume 26, 7-26 of the Journal, Dr D. M. D. James described the stratigraphy, sedimentology and structure of the late Ordovician to earliest Silurian succession in the core of the Tywi Anticline between the Sugar Loaf and the Wye Valley. He has used these new data to consolidate his previous model of a non-depositional slope, established by Ashgill progradation of the shelf margin, separating basinal from shelf facies during the latest Ashgill and early Silurian (James 1983).
Over the past four years, we have mapped the northern region of the Tywi Anticline, north-east of the Chwefru Valley, as part of the survey of the 1:50000 Sheet 179 (Rhayader) (British Geological Survey (a) in press). Our results (Barron et al. 1989; Davies et al. 1991; Smith et al. 1991; Waters et al. in press; Wilson et al. in press) lead us to question aspects of James's Ashgill stratigraphy and the viability of his non-depositional slope model.
ASHGILL STRATIGRAPHY; THE PROBLEMS WITH JAMES' CORRELATIONS
The onset of the late Ashgill glacio-eustatic regression and subsequent latest Hirnantian (intra-persculptus Biozone) eustatic transgression are reflected throughout the Welsh Basin by widely recognized lithological changes (Brenchley and Cullen 1984; Brenchley 1988; Temple 1988). Our correlation across the Tywi Anticline of these major events (Figures 1 and2) differs from that implied by James and reflects a very different structural, biostratigraphical and lithological assessment of the Ashgill succession (British Geological Survey (a) in press). The following are key points.
The onset of the glacio-eustatic regression, within the Ashgill successions of both limbs of the Tywi Anticline, is marked by the change from predominantly burrow mottled mudstones to thinly silt laminated, silty mudstones, in which slumped units are common. This lithological change records an increase in the rate of sedimentation and concomitant exclusion of burrowing benthos (Brenchley 1988). On the northwest limb, we place this change at a level about 40m below the top of James's Cefn Cynllaith Formation, e.g. at SN 9758 6444. Underlying this boundary, interbedded with strata which we regard as pervasively burrow mottled, but ascribed by James to his Cefn Cynllaith Formation rather than his Craig Las Formation, we have recognized three distinctive mappable units of dark grey laminated graptolitic mudstone, e.g. at SN 9785 6279 and from SN 9432 6050 to SN 9420 6082 (Figures 1 and2). These laminites, akin to Silurian laminated hemipelagites, record deposition under anoxic bottom conditions (Cave 1979; Dimberline et a/. 1990). They contain a low diversity graptolite fauna that is probably anceps Biozone in age. Palynomorph assemblages from these laminites further suggest that they equate with the three graptolitic units of the Red Vein in the Corris area (Pugh 1923; Jehu 1926), as suggested by Roberts (1929).
Correlation of both the laminites and the lithological change marking the onset of the glacio-eustatic regression across the Tywi Anticline is hindered by severe faulting. Nevertheless, it can be shown that