A pre-caldera plateau-andesite field in the Borrowdale Volcanic Group of the English Lake District

Abstract

The 6 km thick Ordovician Borrowdale Volcanic Group is readily divisible into a lower 2.2–2.7 km thick predominantly pre-caldera succession dominated by basalt, andesite and dacite sheets, and an upper succession of caldera-related ignimbrites and volcaniclastic sedimentary rocks of c. 3 km thickness. The lower Borrowdale Volcanic Group rocks, here included within a single lithostratigraphical unit, the Birker Fell Formation, affords a well-exposed section through a pre-caldera sequence. The Birker Fell Formation is dominated by andesites which comprise 60% of the stratigraphy. Thin sequences of reworked volcanic detritus are commonly interbedded with the andesites; locally there are thicker units of volcaniclastic sedimentary rocks. Interpretation of the stratigraphy and eruptive history of the formation has been aided by the recognition of some distinctive lithological units including: (1) a 200-600 m thick sequence of weakly parallel-bedded basaltic tuffs resting on the pre-volcanic basement, (2) a 200-600 m sequence of single flow unit aa basalts, (3) dacite flows and ignimbrites up to > 1100 m thick in the middle and upper parts of the formation and (4) aphyric to pyroxene-phyric, simple and compound flows of basalt, 50-400 m thick, locally present in the uppermost parts. Analysis of the facies comprising the Birker Fell Formation indicates that it was emplaced as a sub-aerial, plateau-andesite sequence, formed by the coalescence of products erupted from a number of centres or fissures in an extensional, subsiding volcano-tectonic rift zone. Volcano-tectonic faulting and eruption of thick ignimbrites locally influenced development of this field.

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

“…Ph.D. thesis, Univ. Keele, 1977;Branney, 1988;Petterson et al 1992). The sedimentary features associated with the trace fossil occurrences do not provide conclusive evidence of the type of non-marine environment; a freshwater lacustrine setting is favoured, although a paralic environment with limited marine influence can not be entirely ruled out.…”

The geochemistry and significance of sills within the Ordovician Borrowdale Volcanic Group around Black Combe, SW English Lake District

“…The preservation of the predominantly subaerial Caradoc volcanic rocks was aided by intra-arc extension and volcanotectonic subsidence (Branney & Soper 1988;Petterson et al 1992). Up to 3 km of Eycott Volcanic Group (EVG) rocks aggraded in the northern Lake District and 8 km of BVG rocks in the central Lake District (Millward et al 2000a, b).…”

Very densely welded, rheomorphic ignimbrites of homogeneous intermediate calc-alkaline composition from the English Lake District

“…The lower part of the Borrowdale Volcanic Group comprises an extensive plateau andesite lava field developed by effusion from multiple sources over terrain with low relief, probably within an intra-arc extensional rift (Petterson et al 1992). By contrast, the upper Borrowdale Volcanic Group predominantly comprises volcaniclastic rocks and in particular is characterized by the emplacement of large-scale dacitic and rhyolitic ignimbrites, most notably those associated with piecemeal foundering of the Scafell caldera (Branney & Kokelaar, 1994).…”