Field and biostratigraphic evidence for an unconformity at the base of the Eycott Volcanic Group in the English Lake District

Abstract

The Eycott Volcanic Group of the Lake District Lower Palaeozoic inlier consists of basaltic andesite and andesite sheets and associated, mainly coarse, volcaniclastic rocks. The volcanic rocks have been regarded previously as interdigitated with, and equivalent in age to, mudrocks of the upper part of the Skiddaw Group (Tremadoc-Llanvirn). Microfloral evidence has been quoted in support of this interpretation, but has not been substantiated by re-assessment of the critical data. Furthermore, a recent examination of the base of the Eycott Volcanic Group has shown that it rests with angular unconformity on the Skiddaw Group. Skiddaw Group rocks beneath the unconformity range in age from possible late Cambrian to early Llanvirn. The lowest part of the Eycott Volcanic Group, the Over Water Formation, consists of siltstones and tuffaceous sandstones yielding a diverse microflora, and is intercalated with two andesite sheets interpreted herein as sills. Since the currently accepted Llanvirn age for the Eycott Volcanic Group cannot be confirmed, the volcanism may have been penecontemporaneous with the Llandeilo-Caradoc Borrowdale Volcanic Group episode. There are implications for a pre-volcanic tectonic deformation episode.

The stratigraphy, correlation, provenance and palaeogeography of the Skiddaw Group (Ordovician) in the English Lake District

“…A few sub-vertical cylindrical burrows are also present, as are sporadic circular and meandering burrows. (Millward & Molyneux 1992). The youngest part of the formation proved so far is earliest Llanvirn.…”

“…Moseley (1984) changed its name to the Tarn Moor Mudstone Formation and placed it near the top of the Skiddaw Group as a partly lateral equivalent of the Eycott Group. Recent work by Millward & Molyneux (1992) indicates that correlation with the Eycott Group is unjustified, so the correlation of Jackson (1978) (Wadge 1972); the youngest part of the formation proved is in the Tarn Moor Tunnel (Wadge et al 1969).…”

“…Group (Millward & Molyneux 1992) which unconformably overlies the Northern Fells Belt of the Skiddaw Group in the Lake District. However, the shelly fauna from the Grangegeeth Group has Laurentian affinities, whereas the Eycott Volcanic Group is overlain by the Drygill Shales which have a Longvillian (mid-Caradoc) shelly fauna with Anglo-Welsh, and therefore Avalonian or Gondwanan, affinities (Dean 1963).…”

The Ordovician acritarch genus Coryphidium

“…Dean and Martin 1978), Ireland (Smith 1981, Connery and Higgs 1999, Maziane et al 2000, and Todd et al 2000, the United Kingdom (Downie and Ford 1966, Lister et al 1969, Molyneux 1979, Turner and Wadge 1979, Turner 1982, Downie 1984, Rushton 1985, Molyneux 1987, Rushton and Molyneux 1989, Cooper and Molyneux 1990, Millward and Molyneux 1992, Cooper et al 1995, Molyneux 1996, Molyneux and Leader 1997, and Molyneux 1999, Belgium (Martin et al 1970, Martin 1974, Vanguestaine 1986, Servais 1991, Servais and Maletz 1992 and the Avalonian parts of Germany (Burmann 1970, Maletz and Servais 1993, Servais 1994 The South American occurrences on eastern Gondwana include the localities investigated in Argentina by Rubinstein et al 1999, Rubinstein and Toro 2001, and Rubinstein 2003 …”

Relationship between metamorphism and structure in the Skiddaw Group, English Lake District

“…Soft-sediment deformation and slump folds are widespread . Tilting, uplift and erosion of the Skiddaw Group preceded the Eycott Volcanic Group and Borrowdale Volcanic Group volcanism (Soper, 1970;Wadge, 1972;Millward & Molyneux, 1992). Simpson (1967) recognized three widespread cleavages in the Skiddaw Group.…”