Buttermere Village

[NY 170 176]

F. Moseley

Highlights

Several exposures, adjacent to Buttermere Village, reveal all the small-scale structures generally observed within the Skiddaw Group. At least three phases of deformation are recorded in the rocks here: F₀ folds, generated by slumping, are refolded by F₁ folds of the main D₁ late-Caledonian deformation; crenulation cleavage of D₂ age is also present.

Introduction

The easily accessible exposures around Buttermere Village illustrate well, many of the structural problems of the Skiddaw Slates. In the past, attempts have been made to explain most of the structures hereabouts in tectonic terms (Simpson, 1967; Moseley, 1972; Webb, 1972; Soper and Moseley, 1978). Although obvious sedimentary slumps, typical of many turbidite systems, have been recognized for many years (see Introduction, Chapter 1) the slates have seemed to have three distinctive phases of folding (all presumed to be tectonic), which have been previously labelled 'F₁', 'F₂', and 'F₃' (see (Table 3.1)). Folds identified as 'F₁' are small-scale, complex and have no associated cleavage. They have a variable trend, often N–S, and until recently they were believed to represent a pre-volcanic phase of minor folding (Jeans, 1972; Moseley, 1972; Webb, 1972). The F₂ folds are similar small-scale structures which resemble the F₁ structures, except that they have a generally NE to ENE trend and often have axial-planar cleavage. Most authors have attributed them to the main end-Silurian phase of the Caledonian Orogeny.

The F₃ folds are open recumbent structures with a subhorizontal crenulation cleavage which have also been thought to belong to the main Caledonian deformation.

Description

This site shows the muddy siltstones of the Buttermere Formation of the Skiddaw Group. At Buttermere Quarry [NY 1733 1727], the strata are inclined about 70°SE, with load casts near the top of the quarry showing the way-up. S₁ cleavage and bedding are parallel, but there are near horizontal F₂ crenulations.

On Long How [NY 1725 1730], outcrops of pelites with silty layers expose steeply plunging folds. These folds, once thought to be tectonic (Moseley, 1972) are now thought to be slumps refolded by the main D₁ movements.

Alongside Millbeck [NY 1700 1717], an F₁, vertically plunging fold is seen to be refolded by an F₂ recumbent fold (Moseley, 1972). Intrafolial folds are seen along one of the F₁ fold limbs.

The fourth locality [NY 1765 1703]; (Figure 3.4) is at Buttermere Church, where the gentle surface of a roche moutonée displays steeply plunging folds. The steep plunge is thought to represent a steeply dipping limb of a slump fold (F₀), whereas the folds themselves are largely the product of the D₁ late-Caledonian deformation. These tight folds are related to an S₁ cleavage. The vertical surface alongside the road reveals open recumbent folds (F₂) with a weak crenulation cleavage (Moseley, 1981).

Interpretation

Recent changes in interpretation by Webb and Cooper (1988; and see below — Hassness and Gasgale Crags) are that the F₁ and even some F₂ small-scale folds originated as slumps, those with the main Caledonoid north-easterly trend having been subsequently tightened and developing a cleavage during the main orogenic phase (now considered to be Early Devonian). The difficulty in the field arises from the lack of unambiguous criteria for assessing the origin of the F₁ and F₂ folds. In this description, the F₁ folds, considered by Webb and Cooper (1988) to be of slump origin, are designated F₀, while the F₂ folds, which seem to be coeval with cleavage of late-Caledonian age, are designated F₁. Later folds associated with a flat crenulation cleavage are consequently designated F₂.

Although interference between F₀ and F₁ folds in these localities is limited, it is clear that three sets of structures are represented. The F₀ and F₁ folds are both tight and steeply plunging, but F₀ has S₁ cleavage superimposed. The steep plunge of F₁ can be attributed to a steep dip, produced by large-scale, slump folds (F₀). The interpretation of the F₀ minor structures and steep pre-F₁ dips as indications of sedimentary slump processes rests largely on the arguments of Webb and Cooper (see above). These authors show that the folds are related to major folds and olistostromes which have variable trend and vergence, but which predate the D₁ folds and cleavage.

The Buttermere outcrops show, particularly clearly, that the third set of structures are superimposed on the earlier two. The D₂ affects steeply dipping surfaces to give open and recumbent F₂ folds and a related flat S₂ crenulation, although attitudes depend on the dip of the affected surface.

The D₁ and D₂ structures are considered (Webb and Cooper, 1988) to be a product of the late-Caledonian deformation. It now appears that there are no significant tectonic folds that pre-date the Borrowdale Volcanic Group.

Conclusions

The Buttermere site is important in that, in adjacent outcrops, all three sets of structures that are common to much of the Skiddaw Group can be demonstrated. The first of these (isoclinal folds) were produced by slumping, that is, movement and deformation of masses of sediment either contemporaneous with, or relatively soon after, their deposition on a sloping sea-bed, which means that these folds are of Ordovician age. The second generation of folds (main phase, tight steeply plunging folds) were formed during the main Caledonian mountain-building 'storm', during the early Devonian, at about the time of the closure of the Iapetus Ocean through the convergence of the landmasses to its north and south. The third-generation (open recumbent) folds are taken to be late Caledonian. All three categories are important in the context of the Caledonian evolution of the Lake District.

References