Cossey, P.J., Adams, A.E., Purnell, M.A., Whiteley, M.J., Whyte, M.A. & Wright, V.P. 2004 British Lower Carboniferous Stratigraphy. Geological Conservation Review Series, No. 29, JNCC, Peterborough.
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The Odynau Bont GCR site is a disused quarry
The key feature of this site is the development of a set of calcrete profiles in the Clydach Halt Member of the Llanelly Formation. The top of the underlying Abercriban Oolite displays minor nibbling, a characteristic of the top of this unit and its equivalent to the east, the Clydach Valley Group (Barclay et al., 1988; Barclay, 1989). This rubbly horizon in the Clydach Valley Group at sites such as Llanelly Quarry (see GCR site report, this chapter) and Clydach Halt Lime Works (see GCR site report, this chapter) is hosted in the Gilwem Oolite, which is probably of Chadian age (Barclay, 1989). However, due to the progressive erosion and overstep of the Abercriban Oolite Group–Clydach Valley Group by the Llanelly Formation (George, 1954; Barclay et al., 1988), the rubbly horizon is here at a lower level in the stratigraphy and may equate to the top of the Clydach Beds of George et al. (1976). The Clydach Halt Member, 6.2 m thick, is distinguished from other exposures at this level by the strong development of the palaeosols, named the Tyle'r Bont Pedocomplex by Wright (1981a, 1982b). A pedocomplex is a sequence of soil profiles in close vertical succession. The palaeosols occur as nodular and massive fine-grained carbonates hosted in green and purple-red clays
The carbonate nodules and beds of the palaeosols consist of microsparitic calcite and exhibit a range of typical calcrete microfabrics such as floating grains, circum-granular crystallaria and micro-nodules (Wright, 1982b). Although calcretes can be found at this level in other localities, the unique aspect of this site is that so many calcrete horizons occur, providing information of the likely length of subaerial exposure represented by this regional disconformity.
Although Courceyan brachiopod and conodont faunas have been recorded from the Abercriban Oolite, this unit is believed to span the Courceyan–Chadian boundary (Barclay et al., 1988), the younger Chadian Gilwern Oolite having been removed by intra-Carboniferous erosion. The Llanelly Formation has produced foraminifera (Barclay, 1989) and conodonts (Stone, 1991) indicating an Arundian age.
The rubbly horizon at the top of the Abercriban Oolite has been interpreted as a palaeokarstic effect caused by the subaerial exposure of the shallow-water oolites under a humid climate (Wright, 1982a). The overlying palaeosols of the Clydach Halt Member can be compared in terms of macro-structure, microfabric and geochemistry to modern calcrete soils found today in semi-arid to sub-humid regions (Wright, 1982b; Wright et al., 1997). Thus a climate change took place between these two phases. Such soils develop from an initial stage with dispersed concentrations of carbonate, precipitated from downward-moving soil waters, through to the growth of nodules, which become so numerous and large that they coalesce to form continuous beds of soil-formed carbonate. These horizons would most likely have formed between 0.5 m and 1.5 m below the actual land surface at the time of formation. The fact that several horizons occur suggests that the land surface aggraded in stages. Although the rates at which calcretes form vary, soils with the horizons of stage 3–4 calcrete comparable to those seen in the middle part of the member, are generally only found associated with land surfaces that are many tens to several hundreds of thousands of years old. The presence of so many calcrete horizons in the member is a clear indication that the area remained as a land surface for probably half a million years or more.
The fact that the horizons are so well developed also indicates that the sedimentation rate was virtually zero for very long periods, followed by periods of aggradation, after which soil formation was renewed at a slightly higher position. Such settings are found in landscapes that are isolated from deposition or erosion on terraced surfaces. The post-Chadian land surface was a complex of stable terraces, with areas between having been eroded by ephemeral rivers that deposited stream and sheet-flood deposits (such as at the nearby site of Baltic Quarry, see GCR site report, this chapter). By integrating this site with other exposures of the Clydach Halt Member it is possible to begin to understand the complexity of a long-lived landscape preserved within the Lower Carboniferous rocks of the region.
Above the Clydach Halt Member, the Cheltenham Limestone Member represents deposition in restricted shallow lagoons and intertidal flats, with occasional influxes of more fully marine waters (Wright, 1986a).
This site, with the best exposure of Lower Carboniferous fossil soils in the region, provides evidence that the sub-Arundian unconformity in the area represents a profoundly long period of time, during which multiple palaeosols developed. Together with other sites containing sections at this level, such as Clydach Halt Lime Works (see GCR site report, this chapter), it is possible to reconstruct a complex and dissected early Carboniferous landscape which developed during what was mainly a period of shallow marine limestone deposition.