The borrowdale volcanic rocks of the Seathwaite Fells, Lancashire

Numerous subconcordant andesite sheets occur in the Ordovician Borrowdale Volcanic Group of N.W. England. Many have brecciated tops and bases in which andesite fragments are enclosed in a fine volcaniclastic matrix. Previously, all have been interpreted as block lava flows with interstitial downwashed sediment. Eight well-exposed sheets are described whose field and textural relations indicate that they were intruded into wet sediment with formation of peperite.Criteria by which such sills can be distinguished from block lavas include discordance and fault-controlled geometry, vesiculation and homogenization of sediments adjacent to upper contacts, and the presence of pillows or peperites a t upper contacts. Because sills d o not always show these characteristics in the field, an extrusive origin for a sheet should be demonstrated and not assumed. However, it is not always possible to make the distinction, in which case the non-genetic term sheet can be usefully applied.Field studies suggest that a substantial part of the Borrowdale Volcanic Group is composed of sills. Thus previously erected lithostratigraphic schemes may not correctly indicate the sequence of magmatic events, and the correlation of locally defined stratigraphies will not be possible until the regional importance of high-level intrusive bodies has been established. KEY WORDS Sills Peperites Fluidization Block lavas Ordovician English Lake District I NTR ODUCTIONThe Borrowdale Volcanic Group (BVG) of the English Lake District is a remnant of a subaerial calc-alkaline volcanic system thought to have been part of a Caradocian arc (Fitton et al. 1982). Previous studies (references in Millward et al. 1978;Moseley and Millward 1982) have concentrated on establishing a BVG lithostratigraphy, and formations have been erected, largely based upon the relative proportion of clastic to largely non-clastic volcanic rocks (tuffslsediments and l a m / sills). Correlation of formations between different areas within the BVG has proved difficult, which is to be expected because subaerial volcanic successions are usually characterized by considerable lateral and vertical facies changes with abundant internal unconformities (Cas and Wright 1987), and because the BVG is now known to have undergone considerable volcanotectonic disruption (Branney and Soper in press).

Section: Crinkle Cragsmentioning

confidence: 99%

“…It occurs within subaqueously deposited fine-to coarse-grained bedded andesitic sediments which form part of Mitchell's (1940) 'Yewdale Tuffs'. Mitchell interpreted the sheet as a lava flow and noticed that it appeared to have burrowed down into underlying sediments.…”

Section: Church Beck Conistonmentioning

confidence: 99%

High‐level peperitic sills in the English Lake District: Distinction from block lavas, and implications for borrowdale volcanic group stratigraphy

Branney

Suthren

1988

Vesicles and related structures in lavas

Whitford-Stark

1973

A review of certain late-stage textures in subaerial, submarine and lunar extrusive rocks is presented, together with a genetic classification of vesicles. The term paravesicle is introduced for hollows in lavas caused by the introduction of a gaseous phase into the lava after extrusion. Examples taken from the andesitic lavas of the Borrowdale Volcanic Series are used to illustrate the usefulness of studying such features.

Structure and stratigraphy of the Borrowdale Volcanic rocks of the Kentmere area, English Lake District

Soper¹,

Numan²

1974

The Borrowdale Volcanic rocks of Kentmere have been reinvestigated. A previous structural interpretation (Mitchell 1929) involving Caledonian isoclines superimposed upon pre-Caradocian NNE trending folds is shown to be incorrect; neither set of structures exist. The geometry of the major structural elements in the area, the Nan Bield Anticline and the Caledonoid cleavage, is described. Evidence is adduced which suggests that the anticline was formed in pre-Caradxian time as a large open fold trending approximately E-W. Its present geometry and that of the Wren Gill Monocline result from the superimposition of the regional end-Silurian cleavage obliquely and inhomogeneously across this preexisting fold.The volcanic succession is divided into four compositionally and petrographically defined formations. In upward sequence these are the Nan Bield and Wren Gill formations (andesitic lavas and tuffs), Kentmere Formation (mainly dacitic pryoclastic rocks) and the Garbourn Formation (andesites and dacites), totalling over 2800 m. It is thought that much of this succession was deposited subaqueously.