Rushton, A.W.A., Owen, A.W., Owens, R.M. & Prigmore, J.K. 2000. British Cambrian to Ordovician Stratigraphy. Geological Conservation Review Series No. 18, JNCC, Peterborough, ISBN 1 86107 4727. The original source material for these web pages has been made available by the JNCC under the Open Government Licence 3.0. Full details in the JNCC Open Data Policy
Chapter 7 Tremadoc Series in Wales and England
J.K. Prigmore, A.W.A. Rushton and R.M. Owens
Introduction
The Tremadoc Series stems from the lithostratigraphically conceived Tremadoc Group of Sedgwick (1852), who named it after the village (now Tremadog) in North Wales. This group of strata was characterized palaeontologically by Salter (1866b), who recognized the significance of the fauna as forming a transition between the 'primordial' (Cambrian) and 'lower Silurian' (Ordovician) faunas. Whittard (1960) discussed in detail the historical development of the Tremadoc Series.
The Tremadoc Series, though formerly often treated as the uppermost part of the Cambrian, is now internationally accepted as one of the designations of the lowest series of the Ordovician System. The base of the Ordovician and the definition of the primary divisions of the system are under international debate, and the Cambrian—Ordovician Working Group has recommended a stratotype for the base at a level close to the base of the Tremadoc Series as recognized in Wales. Whether the Tremadoc is to be adopted as the lowest primary international series division of the Ordovician or merely a secondary (regional) series, it remains of worldwide significance, and the sites that exemplify it are accorded corresponding importance.
In Britain the Tremadoc is divided biostratigraphically into a composite sequence of zones (see
The Tremadoc is developed in two main settings: the Welsh Basin, particularly in North Wales, and the English cratonic area, especially the Welsh Borderland.
Wales
In the North Wales basin there are cleaved mudstones with significant but only locally developed sandstone units, of total thickness up to about 500 m. Many stratigraphical terms have been applied in the past (Cowie et al., 1972, pl. 3), and those employed by Fearnsides (1905, 1910) are listed here because they remain useful labels for local lithostratigraphical divisions in the Arenig and Tremadog (or Ynyscynhaiarn) areas. However, wider mapping of Fearnsides' units has proved impracticable, and recent work refers all the slaty mudstones of Tremadoc age in North Wales to the Dol-cyn-afon Formation (Pratt et al., 1995, p. 14) and leaves the sandstone divisions as unnamed members (Howells and Smith, 1997). The divisions may be correlated approximately as given in
The whole sequence was laid down in a fairly shallow open marine setting with some divisions showing striking local changes in thickness across synsedimentary faults (Howells and Smith 1997).
The succession follows the Cambrian conformably, or locally with slight non-sequence, and the boundary is marked by a subtle change in sedimentation linked to oceanic changes such as an increase in marine oxygenation. The stratigraphical succession in North Wales is nearly complete, from the lowest part of the flabelliformis Zone where it overlies the highest Cambrian Zone of Acerocare to the sedgwickii Zone; however, the top of the latter is not seen and there is a hiatus of uncertain magnitude between the youngest Tremadoc and the oldest Arenig in North Wales.
The sites that exemplify the North Welsh Tremadoc succession are indicated in
The lithology and palaeontology of the upper parts of the Tremadoc succession are exemplified by Amnodd Bwll, which shows the Upper Mudstone Member and includes the intended base of the Migneintian Stage, and Y Garth, which exemplifies a different facies of the Upper Mudstone Member. The Upper Sandstone Member exposed at the coast around Porthmadog and inland at Tan-y-Grisiau is not yet represented by a GCR site.
The Tremadoc rocks of South Wales have been described only relatively recently (Owens et al., 1982; Cope and Rushton, 1992). They somewhat resemble the Shineton Shales of the Welsh Borderlands, though the full succession and its aggregate thickness are unknown. The site at Cwm Crymlyn near Llangynog
England
The Tremadoc of the Welsh Borderland and the English Midlands is represented mainly by the Shineton Shale Formation and its equivalents, to which several local names have been given (Cowie et al., 1972, pl. 1). These are unmetamorphosed mudstones and siltstones deposited across the Midlands microcraton, partly in grabens or half-grabens associated with local rifting, in which their thickness may be 2000 m or more (Smith and Rushton, 1993). The succession is generally incomplete at the base, because of either non-sequence or (as at Cherme's Dingle) faulting. Even where there is sedimentary continuity, as in the subcrop in central England, faunas of the topmost Merioneth and lowest Tremadoc remain unproved and the transition beds are of uncertain age (Old et al., 1987; Bridge et al., 1998). At the top of the Shineton Shale Formation there is widespread erosion and an unconformity with overlying beds. Only in the Shelve area is there an approach to continuity between the Tremadoc and the overlying Arenig.
The Shineton Shales are generally poorly exposed, but much of the Cressagian, represented by the flabelliformis and tenellus zones, is exposed in the Cherme's Dingle site, and the lower parts of the Migneintian, namely the salopiensis Zone, are exposed in Sheinton Brook and, at a slightly higher level at Cound-moor Brook, Evenwood see
In northern England, Tremadoc rocks have now been proved at a few places in the Skiddaw Group of the Lake District (Molyneux and Rushton, 1985). Typical acritarch floras of various Tremadoc ages are known from clasts within an olistostrome deposit, the Buttermere Formation of Cooper et al. (1995, p. 201), and a similar derivation may apply also to the strata at the River Calder site (see