Caswell Bay, Gower, West Glamorgan

[SS 594 877]–[SS 598 869]

Introduction

The Caswell Bay GCR site lies on the south coast of the Gower Peninsula, 8 km south-west of Swansea. The site includes the cliffs along the east side of Caswell Bay from high water mark at the head of the bay [SS 594 877] southeastwards to Whiteshell Point [SS 598 869]. The site thus includes cliff and raised beach exposures along a shoreline nearly 1 km in length. The section is exposed across a faulted anticline (Figure 9.23) and comprises one of the finest and most accessible sections of Courceyan to Holkerian age in South Wales. The site is particularly valuable for the superb exposures of a variety of sedimentary features relating to deposition and diagenesis, as well as for studying characteristic faunas.

Aspects of the geology of Caswell Bay have been described by many workers. General accounts include those of Strahan (1907a) and Dixon and Vaughan (1911). The sedimentology of the whole exposed succession has been examined as part of a larger study of the Carboniferous Limestone in Gower by Ramsay (1987). George (1978b) provided a detailed account of the Chadian part of the succession and the effects of early subaerial exposure were described by Spalton (1979, 1982). The Arundian succession has been described by Simpson (1985a) and the Holkerian succession by Scott (1988).

Description

Structural complexities (folding and thrusting) relating to the Variscan Orogeny have resulted in the repetition of several sections of the Penmaen Burrows Limestone Group, Caswell Bay Oolite and Caswell Bay Mudstone, thus providing a unique opportunity for the examination of lateral facies changes in these units, a circumstance that is precluded in the complementary section at Three Cliffs Bay (see GCR site report, this chapter) where significant repetitions are not apparent.

The stratigraphical thickness of the beds is about 250 m, but individual units are repeated along the exposure (Figure 9.23). The oldest unit exposed is the Penmaen Burrows Limestone Group (also known as the 'Black Rock Limestone') which occurs in the core of the Langland Anticline and is repeated by faulting on both limbs (Figure 9.23). It is strongly dolomitized, particularly at its top where it is referred to as the Laminosa or Langland Dolomite. Mitchell et al. (1986) established that the Courceyan–Chadian boundary lies within the Penmaen Burrows Limestone Group (their Black Rock Limestone) farther west on Gower, but it has not been recognized at Caswell Bay. However, Ramsay (1987) attempted to extrapolate the position of the boundary on lithological grounds and placed it about 20 m below the contact with the Caswell Bay Oolite, with approximately another 20 m of Penmaen Burrows Limestone Group seen below the boundary in the exposure nearest the head of the bay. Despite dolomitization, Ramsay (1987) was able to recognize bioturbated bioclastic packstones in the Penmaen Burrows Limestone Group.

The Caswell Bay Oolite, which is the lateral equivalent of the Gully Oolite of the Bristol area (and is so-called on Gower by some workers), is a massive pale-grey limestone about 40 m thick, composed mainly of cross-stratified oolitic grainstone (Figure 9.24). It is of Chadian age (George et al., 1976). The field characteristics have been described in detail by Spalton (1979, 1982) and Ramsay (1987), the petrography and chemistry by George (1978b), and the diagenesis by Searl (1988a,b, 1989a). The Caswell Bay Oolite shows considerable lateral variation, which is evident in the various sections exposed at this site logged by Ramsay (1987). In particular, an erosion surface that is present midway through the unit in sections near the head of the bay is represented by thick channel deposits in the seaward sections. The surface at the top of the Caswell Bay Oolite is a distinctive erosive feature, consisting of a pot-holed surface with associated calcrete (Spalton, 1979, 1982). Fossils are not abundant in the Caswell Bay Oolite and are concentrated at certain horizons. Most characteristic are bellerophontid gastropods, schuchertellid brachiopods and the corals Syringopora cf. reticulata, Michelinia megastoma and Koninckophyllum praecursor (George, 1978b).

The Caswell Bay Mudstone is particularly well seen at this site and consists of a thin succession (4–6 m) of well-bedded, impure fine-grained limestones and dolomites that show significant lateral variation within the site (Figure 9.25). It has been called the 'Modiola phase' or 'lagoon phase' by Dixon and Vaughan (1911). A detailed description of the lithologies is provided by George (1978b), Spalton (1979) and Ramsay (1987). Laminated dolomitic mudstones are the most common rock types, but disrupted, brecciated and slumped beds also occur. Fossils are limited but calcispheres, ostracodes, formainifera and crinoid fragments are characteristic.

The High Tor Limestone is Arundian in age (George et al., 1976) and consists of roughly 100 in of bioturbated bioclastic and peloidal carbonate sands, resting with sharp, erosive contact on the Caswell Bay Mudstone. About one-third of the way up the succession, trough cross-bedded and planar laminated carbonate sands are overlain by a mudstone with pedogenic features (Spalton, 1979; Ramsay, 1987). Above this are a few metres of dolomitic mudstones and wackestones, before a return to the more characteristic bioclastic and peloidal packstones. As well as large amounts of crinoid debris, the High Tor Limestone contains abundant gastropods, Syringopora cf. geniculata, Michelinia grandis, Siphonophyllia cylindrica and Palaeosmilia (Owen and Rhodes, 1969).

The lower part of the Hunts Bay Oolite is seen at Whiteshell Point at the south-eastern extremity of the site. There is a gradational contact with the High Tor Limestone. Bioturbated packstones at the base give way to cross-bedded oolitic and mixed grain packstones and grainstones with horizons rich in the brachiopods Linoprotonia corrugato-hemispherica and Composita ficoidea (Owen and Rhodes, 1969). Koninckopora is a particularly common element of the microflora (Spalton, 1979).

Interpretation

The exposed Dinantian succession at Caswell Bay consists entirely of shallow marine limestones deposited on a southerly dipping carbonate ramp (Wright, 1986a) (Figure 9.3). The succession dates probably from latest Courceyan (Ramsay, 1987) to early Holkerian age. A variety of shallow marine and marginal environments are represented, and detailed interpretations of these are provided by Ramsay (1987). The following are brief summaries based mostly on his work.

The Penmaen Burrows Limestone Group records storm-influenced offshore environments perhaps deeper than the rest of the succession. Bioturbation and dolomitization have destroyed many of the primary sedimentary structures.

The Caswell Bay Oolite marks a shallowing of the sea and the establishment of shoreline environments in the area. Ramsay (1987) interpreted the Caswell Bay Oolite as the deposits of ebb-tidal deltas and beaches of a barrier-island coastline cut by storm-generated channels. He noted that the deep channel seen in the seaward exposures at this site records a catastrophic event that resulted in the erosion of at least 4000 m³ of sediment. The top of the Caswell Bay Oolite is marked by subaerial exposure and soil formation.

The Caswell Bay Mudstone is a heterolithic assemblage representing a carbonate tidal-flat environment. George (1978b) used the presence of desiccation features and evaporite pseudomorphs as indicators of arid sabkha-type deposits, but Ramsay (1987) noted the paucity of evaporite-related features and interpreted the climate as humid rather than arid or semi-arid. Although George (1978b) and Ramsay (1987) regarded the Caswell Bay Mudstone to be of Chadian age, Riding and Wright (1981) considered the formation to be Arundian in age on sedimentological grounds. The erosive contact of the High Tor Limestone with the Caswell Bay Mudstone is attributed to marine transgression and migration of barrier, shoreface and tidal-channel environments across the tidal-flat complex (Ramsay, 1987). The High Tor Limestone beds record the existence of a barrier-lagoon-inlet complex that was prone to occasional periods of subaerial exposure and soil development.

Initial deposits of the Hunts Bay Oolite record a gradual deepening of the sea, with facies resembling those of the Penmaen Burrows Limestone Group. An oolitic sand-belt was then established in the area, suggesting a return to shallower water environments.

Conclusions

Caswell Bay provides spectacular exposures of a major part of the Dinantian limestone succession in Gower and is the type locality for both the Caswell Bay Oolite and the Caswell Bay Mudstone. A range of environments are represented, from tidal flats and beaches, through shallow marine sand-bodies, to offshore fair-weather and storm deposits. It is thus an extremely valuable teaching and research resource. Repetition of units through folding and faulting allows some lateral variation in these units to be studied. The site is also excellent for studying the effects of subaerial exposure during deposition of a limestone succession, and for studying the effects of later diagenesis. Caswell Bay also displays many faunal elements regarded as typical of the Chadian, Arundian and Holkerian stages in Gower.

References