Back Bay, Monreith

[NX 3678 3947]–[NX 3694 3930]

J.E. Treagus

Highlights

Back Bay, Monreith has the most dramatically exposed cliff section of large-scale folding in the Southern Uplands (Figure 2.13). Two large-scale, D₁ fold pairs are completely exposed in the 30 m-high cliffs, with an excellent example of D₂ structures superimposed. The geometry of these features is characteristic of the deformation style in the Central Belt of the south-west Southern Uplands.

Introduction

The rocks of this site display a typical range of lithologies in the Hawick Rocks, including some massive greywackes, as well as thinner-bedded greywackes, siltstones, and mudstones. The principal feature of the site is the refolding of the north-westerly fold pairs by a D₂ fold pair with a flat-lying south-easterly dipping axial surface and crenulation cleavage. The style of this D₂ deformation is typical of its local development throughout the Central Belt, but here is part of a 2.3 km-wide zone that crosses the Whithorn Peninsula and can be traced to the north-east coast of Wigtown Bay (Figure 2.1). The D₂ deformation is locally associated here with minor north-west directed thrusting, but regionally appears to result from subvertical shortening of the D₁ fold stack.

The site is part of the Whithorn area described by Rust (1965) in which he recognized four phases of deformation related to folds and cleavage. The F₁ folds of Rust are the D₁ folds of the West Burrow Head site 10 km to the south, and of the present description. Superimposed on this phase Rust recognized two sets of folds, his F₂ and F₄ phases, and the Back Bay locality is quoted (Rust, 1965, p. 14 and Plate 2) as an example of the F4 folding with accompanying cleavage. The distinction of F₂ and F₄ appears to be partly on the flatter dips of the axial surfaces of the latter and also on the superimposition of F₄ on the locally developed, steeply plunging F₃ folds.

Stringer and Treagus (1980, 1981), recognized one dominant phase of deformation, D₁, with its associated (but non-axial planar) cleavage. This is locally affected by a D₂ deformation (incorporating Rust's F₂ and F₄) particularly in one 2 km-wide, NE-trending belt (labelled A on (Figure 2.1)), on which the Back Bay site lies in a median position.

Description

The north-western half of the section (Figure 2.14) is dominated by a spectacular profile of a large open D₂ fold (minimum amplitude 5 m) superimposed upon a tight upright D₁ anticline at [NX 3682 3942] and a syncline at [NX 3685 3940] of c. 30 m amplitude (Rust, 1965, Figure 1 and Plate 2). The folds plunge gently to the north-east. At the north-west end of the section, steep NW-dipping strata and S₁ cleavage are deformed by smaller-scale, D₂ 'step' folds with axial surfaces and sporadic, D₂ crenulation cleavage inclined gently to the south-east (Stringer and Treagus, 1981, figure 3A). Minor thrust faults, with small northwesterly displacements and a zone of quartz veins, are associated with the D₂ folds.

The south-eastern half of the section (Figure 2.14) comprises a D₁ anticline at [NX 3691 3937] and syncline at [NX 3693 3936], see (Figure 2.13) forming an asymmetrical D₁ fold pair (Folds 3 and 4 in Stringer and Treagus, 1981, figure 3A) which verges to the south-east. The folds are tight (interlimb angle 20–35°); overturned strata in the common short limb (15 m wide, measured horizontally) dip steeply to the north-west, and strata in the long limbs (50 m wide) dip moderately to steeply to the north-west. The D₁ fold pair (wavelength c. 75 m, amplitude c. 30 m) is probably intermediate in scale to larger scale, 0.25–3.0 km, D₁ folds. Gentle curvature of the north-westerly-dipping axial surfaces indicates large-scale D₂ folding. The D₁ cleavage in mudstone and sandstone beds strikes close to 070° and 090° respectively, markedly oblique to the north-easterly strike of the D₁ fold axial surfaces.

The site generally offers the opportunity to examine all aspects of the D₁ and D₂ deformations, including the relative thickening of fold hinges compared with limbs, boudinage of limbs, the variable development of the S₂ cleavage, and the associated quartz veining and thrusting.

Interpretation

The interest of the site lies in the scale and geometry of both D₁ and D₂ folds, and the fact that it affords the rare opportunity to observe one phase superimposed on the other.

The D₁ folds give a glimpse of the amplitude and wavelength of this phase of folding, although the first-order D₁ folds in the Central Belt are thought by Stringer and Treagus (1983) to be of the order of 0.25–3.0 km. The section (see (Figure 2.14)) also reveals the south-east vergence of these second-order folds, as seen by the relative thickness of the north-west- and south-east-younging limbs respectively. In fact, the north-west-younging limb, between the north-west and south-east fold pairs, is uncharacteristically gently dipping, pre sumably as a consequence of D₂ deformation. A 30° sheet-dip to the north-west would probably represent the pre-D₂ value, seen here interrupted by major folding. In the accretionary model, these folds would be interpreted as having a primary attitude with gentle landward-dipping (NW) axial planes and south-east vergence. Their rotation to their approximate present attitude would have taken place during thrusting, possibly exemplified at the West Burrow Head site. Finally, the transecting cleavage would have been impressed on the folds during the late Silurian closure of the Iapetus and before dykes were emplaced, the latter dated by Rock et al. (1986) at 418–395 Ma.

The D₂ folding shows the typical (but often steeper) dip to the south-east of the axial surfaces of the folds and their consequent south-easterly vergence. Like Rust (1965), but for different reasons, Knipe and Needham (1986), elsewhere, distinguished two generations of folds with this geometry. The earlier, with steeper axial surfaces, they ascribe to pre-accretion rotation and associate with continuing oceanward rotation from D₁ (Knipe and Needham, 1986, Figure 11). The second set, with flatter, south-easterly-dipping axial surfaces like those at Back Bay, they attribute to post-accretion rotation and note the association with minor thrusting directed to the north-west. They associate this structure with shortening in the collision related to the closure of the Iapetus. Stringer and Treagus (1980, 1981) were unable to distinguish these two sets of post-D₁ structures, either on the basis of interference of one set with another or from differences in morphology or mineral growth related to the respective crenu-lation cleavages. The present description shows that all structures here called D₂ must post-date the S₁ cleavage and must reflect very late (post-395 Ma) post-collision adjustments, possibly related to the adjacent granite bodies of this age. The Back Bay site would be an obvious locality for further work to unravel this contentious post-D₁ deformation history.

Conclusions

This site is important as the best-known locality where the Caledonian D₂ folds of the Southern Uplands can be clearly seen to be superimposed D₁ folds, formed earlier in that mountain-building phase. Large, upright asymmetrical folds with amplitudes up to 30 m, associated with steeply inclined cleavage (fine, closely spaced, parallel fractures), are refolded by a second generation of smaller folds orientated almost at right-angles to the first. The first generation (D₁) of folds is thought to result from the subduction of oceanic crust during the closure of the Iapetus Ocean, whereas the cleavage probably relates to eventual continental collision in the end-Caledonian mountain-building episode. The second generation of folds (D₂) is possibly related to uplift. As well as providing an unequalled view of the style and scale of D₁ folds, this locality must rank as one of the most dramatic large-scale exposures of major refolded folds anywhere in the British Isles.

References