Micropalaeontological evidence for the age of the Borrowdale Volcanic Group

Molyneux, Stewart G.

doi:10.1017/s0016756800013273

Cited by 19 publications

(13 citation statements)

References 6 publications

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“…2;British Geological Survey, 1991 b). The dark grey mudstones and siltstones of the Holehouse Gill Formation are considered to be marine because they have yielded an acritarch flora (Molyneux, 1988). The marine microfossils in this formation were first discovered by Numan (unpub.…”

Section: B Stratigraphy and Sedimentologymentioning

confidence: 99%

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Non-marine arthropod traces from the subaerial Ordovician Borrowdale Volcanic Group, English Lake District

Johnson

Briggs

Suthren³

et al. 1994

Geol. Mag.

119

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Arthropod trace fossils are described from the Borrowdale Volcanic Group, a sequence otherwise devoid of faunal evidence. Two forms, both made by the same probably myriapod-like organism, are assigned to the ichnogenera Diplichnites and Diplopodichnus. The lithologies preserving the trace fossils are non-marine and may have been deposited in a freshwater lacustrine environment; some of the traces were probably made in temporarily emergent conditions. The change from one form of trace to the other reflects drying out of the substrate. The trace fossils probably record some of the earliest freshwater arthropods, before the widespread colonization of land by the group.

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Section: B Stratigraphy and Sedimentologymentioning

confidence: 99%

“…The locally developed Holehouse Gill Formation has yielded an acritarch flora indicating marine conditions of probable Caradoc age (Molyneux, 1988). The only evidence of a contemporary fauna is confined to trace fossils ( Fig.…”

Section: Introductionmentioning

confidence: 99%

Non-marine arthropod traces from the subaerial Ordovician Borrowdale Volcanic Group, English Lake District

Johnson

Briggs

Suthren³

et al. 1994

Geol. Mag.

119

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“…Whilst emplacement of the EVG had traditionally been correlated with the lower part of the BVG (Marr, 1900), a much earlier age was considered likely for a number of years (Downie & Soper, 1972). The recognition of microfauna comparable with taxa currently not known to occur below the Caradoc (Molyneux, 1985;Millward & Molyneux, 1992) has restored the possibility that the EVG and BVG are essentially coeval.…”

Section: Magnetostratigraphy and Age Of The Borrowdale And Eycott Volmentioning

confidence: 99%

Palaeomagnetism of the Borrowdale and Eycott volcanic groups, English Lake District: primary and secondary magnetization during a single late Ordovician polarity chron

1997

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Late Ordovician volcanic rocks of the English Lake District typically have a magnetic remanence dominated by a single characteristic component. Previous investigations have interpreted this remanence as both of primary (pre-folding) and secondary origin. Palaeomagnetic field tests have been conducted on (a) andesite blocks from an autobrecciated lava top, (b) andesite blocks in mass-flow breccias, and (c) fault-blocks tilted during Ordovician caldera collapse to establish the time of remanence acquisition. All three tests show that the lavas retain a magnetization acquired during initial cooling: magnetizations of the breccias are coherent within clasts and random between clasts, whilst magnetizations of the tilted fault blocks converge with better that 95 % confidence when corrected for the effects of caldera collapse. In contrast, the volcaniclastic sedimentary and pyroclastic rocks possess an Ordovician secondary remanence acquired after strata had been tilted by volcano-tectonic subsidence. A distributed sample of 65 andesite and basalt sheets through the Borrowdale Volcanic Group has a mean remanence direction D/I = 341.9/-48.9°( α 95 = 4.0°) yielding a positive fold test and a palaeomagnetic pole at 12.7°E, 4.3°S (dp/dm = 3.5/5.3°). A progressive steepening of the palaeofield direction is recorded during emplacement of the Borrowdale Volcanic Group (~I = -39°to I = -51°) which continued into the interval of volcanotectonic overprinting (I = -62°); the equivalent motion of Eastern Avalonia is ~20°into higher southerly latitudes.Both the Eycott and Borrowdale volcanic groups exhibit uniform normal polarity throughout. Correlation with the geomagnetic time scale for the Ordovician restores the broad correlation between the two groups by constraining their emplacement and partial overprinting to a single long normal polarity chron occupying the Nemagraptus gracilis and earlier part of the Diplograptus multidens biozones (late Llandeilo and early Caradoc). All the volcanism, therefore, occurred within a period of no more than ~5 Ma. The palaeomagnetic evidence confirms that the Borrowdale Volcanic Group was affected by both syn-volcanic deformation (caldera collapse) and regional compressive deformation prior to deposition of the (late Ordovician-Silurian) Windermere Supergroup. The succession of primary and secondary Ordovician palaeomagnetic poles from the Lake District inlier defines an anticlockwise apparent polar wander (APW) loop with the apex correlating with 'soft' closure of the Iapetus Ocean and late Ordovician deformation. The APW paths from Avalonia and Baltica converge at this point as subduction ceased and the arc subsided beneath the sea after mid-Caradoc times.

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“…The age of the Borrowdale Volcanic Group must lie somewhere between that of the underlying Tarn Moor mudstones (late Llanvirn, Wadge et al 1969) and the Coniston Limestone Formation (early-mid Ashgill, Ingham, McNamara & Rickards, 1978). Recently, Molyneux (1988) concluded from microfloral evidence that at least the upper part of the Borrowdale Volcanic Group was probably of Caradoc age. In contrast, examination of microfloras from siltstones near the base of the Eycott Volcanic Group had earlier led Downie & Soper (1972) to conclude that volcanism began in the northern Lake District during earliest Llanvirn time.…”

Section: Introductionmentioning

confidence: 99%

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

Millward

Molyneux

1992

Geol. Mag.

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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.

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Micropalaeontological evidence for the age of the Borrowdale Volcanic Group

Cited by 19 publications

References 6 publications

Non-marine arthropod traces from the subaerial Ordovician Borrowdale Volcanic Group, English Lake District

Non-marine arthropod traces from the subaerial Ordovician Borrowdale Volcanic Group, English Lake District

Palaeomagnetism of the Borrowdale and Eycott volcanic groups, English Lake District: primary and secondary magnetization during a single late Ordovician polarity chron

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

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