Barclay, W.J., Browne, M.A.E., McMillan, A.A., Pickett, E.A., Stone, P. & Wilby, P.R. 2005. The Old Red Sandstone of Great Britain. Geological Conservation Review Series No. 31, JNCC, Peterborough.
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W.J. Barclay and B.P.J. Williams
Foreshore and cliff exposures at Freshwater West
Descriptions of the section have been given by Dixon (1921, 1933a,b), Williams (1971, 1978), Allen et al. (1981b) and Williams et al. (1982). Marshall (2000a,b) described the Upper Old Red Sandstone in detail. Freshwater West lies in the Pembroke peninsula south of Milford Haven and south of the Ritec Fault
The section can be reached from the B4319. From this road above Little Furzenip
At the northern (Little Furzenip) end of the section, the basal Old Red Sandstone rests unconformably on marine Silurian rocks of Wenlock age, the Gray Sandstone Group (Williams, 1971, 1978; Bassett, 1974; Allen and Williams, 1979b; Allen et al., 1981b; Williams et al., 1982). The Gray Sandstone Group consists of conglomerates, green fine- to medium-grained sandstones, thin limestones and dark grey mudstones. The group is truncated by an irregular scoured, channelled surface that is overlain by the basal Old Red Sandstone rocks, the Freshwater East Formation. This is 18.45 m thick and comprises conglomerates in its lower part, with finer-grained sandstones above. The conglomerates are green to dark grey, of cobble-and pebble-grade, and are interbedded with fine-to medium-grained sandstones and green-grey mudstones. They are framework supported and include clasts up to 450 mm of quartzitic and lithic sandstones, vein quartz and olive-green mudstone in a coarse-grained sandstone matrix, some of the clasts matching lithologies in the underlying Silurian strata. The conglomerates are massive or cross-bedded with internal scour surfaces and overlie erosion surfaces. The upper, finer-grained beds contain lingulids, plant remains, arthropod tracks, Pachytheca and ostracoderm fragments (Dixon, 1921; Richardson and Lister, 1969; Williams, 1978).
The Freshwater East Formation is succeeded by the Moor Cliffs Formation. This is 119.56 m thick and consists predominantly of red and green mudstones in which calcrete glaebules and tubules are abundant, commonly arranged in pseudo-anticlinal structures. Subordinate lithologies include very fine- to fine-grained sandstones, exotic and intraformational conglomerates and airfall tuffs. The Townsend Tuff Bed is the thickest (2.81 m) of the tuffs. It crops out on the foreshore immediately north of Little Furzenip
The abrupt junction between the Moor Cliffs Formation and the overlying Freshwater West Formation is well exposed on the southern side of Little Furzenip headland. The latter is 345.78 m thick and can be examined in great detail on the wave-polished foreshore section between the headland and the base of the Ridgeway Conglomerate Formation to the south (Williams, 1971, 1978). The lower part of the formation (the Conigar Pit Sandstone Member) is sandstone-dominated (e.g. Williams and Hillier, 2004), whereas mudstone predominates in the upper part (the Rat Island Mudstone Member). The Conigar Pit Sandstone comprises intraformational conglomerates, very fine- to medium-grained sandstones and calcrete-bearing mudstones, commonly arranged in upward-fining cycles
The mudstones of the Conigar Pit Sandstone Member are laminated or massive, blue-mottled and contain small burrows. Some very fine-grained sandstone-mudstone complexes contain desiccation cracks and arthropod tracks. Some soft-sediment deformation structures in the form of sandstone balls in mudstone occur locally. Calcrete is common in the mudstones, including nodules, massive limestone and veins, and in pseudo-anticlinal fans. The top of the Conigar Pit Sandstone is placed at the top of the highest thick (5.18 m), green sandstone complex, which has a rare extraformational conglomerate at its base.
The Rat Island Mudstone Member consists mainly of thick, red mudstones that are blue-mottled, laminated and contain calcrete in the form of nodules, pseudo-anticlinal fans and some massive horizons. There are a few very fine-grained, ripple-laminated sandstones with Beaconites antarcticus. There are also some prominent thick (2.05 m), coarse, pebble intraformational conglomerates with burrows and fish scales.
The disconformable junction between the Rat Island Mudstone and the overlying Ridgeway Conglomerate Formation is placed at the base of an intraformational conglomerate-mudstone complex. The mudstones in the Ridgeway Conglomerate are different from those in the underlying Rat Island Mudstone in being brighter red and ill-sorted, with scattered granules and small pebbles of vein quartz and carbonate. The sandstones are also different in that they are phyllarenitic in composition. The Ridgeway Conglomerate Formation (Williams, 1971, 1978) is a broadly upward-coarsening sequence of red, sandy mudstones with calcrete and ill-sorted mudrocks, fine- to coarse-grained litharenites rich in metamorphic and sedimentary rocks, and exotic, mainly clast-supported, petromict conglomerates
The sandstones in the formation are fine- to coarse-grained litharenites, many of them rich in phyllite fragments. They are massive or graded, parallel bedded or cross-bedded and locally burrowed. The mudstones are mainly thick and bright red and are ill-sorted, with quartz grains and granules, as well as seams of vein quartz, carbonate debris and exotic granule-and fine pebble-grade detritus. Calcretes occur as nodular, prismatic, massive and vein horizons.
The Ridgeway Conglomerate Formation is overlain with no local angular discordance, but with regional unconformity, by the Skrinkle Sandstones Group. The abrupt facies change is well exposed in the cliff and foreshore
The Skrinkle Sandstones Group comprises two formations, the Gupton Formation and the West Angle Formation (Marshall, 1977, 1978, 2000a,b). The Gupton Formation is subdivided into the Lower Sandstone Member and overlying Stackpole Sandstone Member. Marshall (e.g. 2000a) carried out a detailed analysis of the facies present and their associations. The Lower Sandstone Member is 55 m thick and crops out in the cliffs and foreshore
The upper part of the West Angle Formation (Red-Grey Member) is predominantly red, but is characterized by the incoming of sporadic grey-green sandstones and pebbly sandstones with well-preserved plant fragments. The sandstones occur at the bases of fining-upward sequences capped by calcretized red mudstones. Grey mudstones with freshwater microfossils, coalified plant debris and lingulids appear higher in the member, along with a few sheets of phosphatized pebble conglomerate. The topmost beds of the formation are interbedded red and grey sandstones with mudstone interbeds capped by a mature, calcareous sandstone with well-rounded quartz pebbles and sparse marine brachiopods and bryozoa.
The unconformity between the Moor Cliffs Formation and the Gray Sandstone Group at the base of the section represents a gap of at least 6 Ma, from the late Wenlock (Homerian) to the Lochkovian. This contrasts with the situation at Albion Sands, where the basal Old Red Sandstone facies has a gradational relationship with the Gray Sandstone Group (Hillier, 2000; Hillier and Williams, 2004). The lower part of the Freshwater East Formation is interpreted as a proximal alluvial deposit. The sheet-like, heterolithic units in the upper part represent deposition in an alluvial coastal-plain environment, with wave ripples and the presence of lingulids suggesting episodic marine influence, perhaps in estuarine tidal-flats or as storm washovers (Williams and Hillier, 2004).
The Moor Cliffs Formation is dominated by red mudstones that were traditionally referred to overbank deposition of a high-sinuosity fluvial systems in an alluvial coastal-plain setting (Allen and Williams, 1978; Williams et al., 1982). Recently, however, detailed examination of the mudrocks of the Lower Old Red Sandstone has revealed a range of possible origins (Marriott and Wright, 1996, 2004; Marriott et al., 2005). These authors propose a depositional environment similar to a modern tropical dryland river system in which moderately sinuous, anastomosing ephemeral rivers reworked muddy floodplain sediment during seasonal flooding. The floodplain sediments were pedifled to varying degrees as calcic vertisols (calcretes) and mud soil aggregates that were reworked by wind and/or water as sand- or silt-sized pelleted aggregates. Infrequent major flood events caused extensive stripping of sediment from the floodplain surface and the deposition of sand sheets. Ephemeral lakes formed in depressions on the floodplain after floods, trapping aeolian dust. Marriott and Wright (2004) recognize four mudrock facies:
Marriott and Wright (1993, 1996, 2004) suggested a dynamic environment in which stable periods with minor flood events produced steady aggradation of the floodplains. These were interrupted occasionally by major flood events when vast amounts of sediment were stripped and deposition in river channels took place.
The alluvial architecture of the sandstones of the Moor Cliffs Formation has been intensively studied (e.g. Williams and Hillier, 2004), particularly in the interval immediately above the Townsend Tuff Bed (Allen and Williams, 1982; Love, 1993; Love and Williams, 2000; Love et ed., 2004). The sheet-like finer-grained sandstones were probably the products of unconfined sheet flooding over the floodplain. The coarser, cross-bedded sandstone 56 m above the Townsend Tuff Bed is an amalgamation of units produced by sheet-flood events, its erosive incision reflecting a drop in sea level that was possibly induced tectonically (Williams and Hillier, 2004).
The common presence of calcrete nodules in the formation is evidence of repeated, seasonal wetting and drying of the floodplain in a semiarid climate. The thick succession of stacked, mature calcretes (Stage III of Machette, 1985) of the Chapel Point Calcretes Member points to a prolonged period of non-deposition, sediment starvation and basin shut-down. Each mature profile is estimated to have taken 10 000 years (e.g. Allen, 1974d, 1986), although Marriott and Wright (2004) note that rates of carbonate formation are highly variable, depending on factors including the porosity of the parent sediment and the carbonate content of the percolating water. Hundreds of thousands to millions of years may have been required for Stage III calcrete formation. The regional extent of these calcretes demonstrates 'shut-down' of the Anglo-Welsh Basin, after which sediment provenance, fish fauna and ichnofacies all changed.
The change in basin architecture instigated deposition of the more proximal Freshwater West Formation. The common cyclic arrangement of the facies in fining-upward sequences in the lower part (the Conigar Pit Sandstone Member) points locally to a fluvial origin in high-sinuosity meandering streams, as evidenced by common lateral accretion surfaces (Williams, 1978; Allen et al., 1982). However, the basin infill architecture is variable, with unconfined sheet-flood fades also being abundant (Williams and Hillier, 2004). The basal part of the member here is more mud-rich than elsewhere in Pembrokeshire, perhaps due to its elevated position in the hanging-wall of the Ritec/Benton Fault (Williams and Hillier, 2004). A northerly provenance is indicated by palaeocurrent directions, in contrast to the southerly derived Ridgeway Conglomerate Formation above. The Rat Island Mudstone Formation is of muddy floodplain origin similar to that of the Moor Cliffs Formation (Marriott and Wright, 1993, 1996, 2004; Marriott et al., 2005).
The Ridgeway Conglomerate Formation is interpreted as the deposits of an alluvial-fan system that prograded northwards over fringing alluvial mudflats or playas from an emergent southern source (Williams, 1971; Williams et al., 1982; Tunbridge, 1986; Ekes, 1993). The source area may have formed as a result of uplift in the footwall of a fault in what is now the Bristol Channel during Acadian deformation (Marshall, 2000a). At least some of the red mudrock inter-beds in the formation owe their origin to deposition from bedload of pedogenic mud aggregates reworked from muddy floodplains (Ekes, 1993). Other mudrock facies represent mudflows on the fan surface (Williams et al., 1982).
The Skrinkle Sandstones Group is interpreted as the fill of a post-Acadian synrift, fault-bounded basin in which alluvial-fan, lacustrine, braidplain delta and high-energy braidplain deposition took place successively in two superimposed sequences, the first an axial basin-fill, the second transverse fill (Marshall, 2000a,b). The Ritec Fault bounded the basin to the north, the succession thickening southwards in the hanging-wall of a fault in the present-day Bristol Channel. The first phase of the axial basin filling commenced with deposition of the Lower Sandstone Member of the Gupton Formation. Marshall (2000a,b) interpreted the three facies associations of the member respectively as small sheet-flood systems in a lacustrine or floodplain setting; isolated channel-fills; and meandering channel deposits. The coarsening- and thickening-upward nature of the succession suggested to Marshall a distal to proximal change and a SE-prograding terminal fan system. The lower part of the Stackpole Sandstone Member (mudstone-rich heterolithic association) is interpreted as lacustrine and lake-delta deposition, the upper (quartz arenite association) as high-energy, fluvial braidplain deposition. The textural maturity of the sandstones suggests long axial transport paths rather than local sourcing from the footwall of the Ritec Fault.
The West Angle Formation represents the second, transverse, basin-fill succession. Palaeocurrent directions in the lower part (the Conglomerate Member) indicate south and south-west drainage. Lateral accretion sets indicate point-bar formation and meandering river channels, with thin sheets of intraformational conglomerate and overlying rippled sandstone in the mudstone floodplain fades probably representing sheet-flood events. The upper part of the West Angle Formation (Red-Grey Member) is interpreted by Marshall (2000b) as recording the start of the Carboniferous transgression. The sporadic green beds and plant fragments point to a higher water-table on the alluvial floodplain. The progressive incoming of grey mudstones with freshwater microfossils, coafified plant debris and lingulids higher in the member suggests lacustrine deposition. The sheets of phosphatized pebble conglomerate contain bryozoa and sharks' teeth and indicate sporadic marine inundation prior to the main Carboniferous transgression. The calcareous sandstone at the top of the sequence has north-directed palaeocurrents and is interpreted by Marshall as a coastal barrier sand, with the underlying beds being lagoonal and washover fan deposits.
The importance of this site lies in the magnificent exposure of the entire Old Red Sandstone succession and the ease of accessibility of most of the rocks, which attracts large numbers of students and researchers. It is the classic section of the Lower Old Red Sandstone in southern Britain. The site has played a crucial role in the understanding of Old Red Sandstone-basement relationships in the Anglo-Welsh Basin, and of the tectonic and sedimentary history of the basin. The detailed sedimentological studies carried out here have vastly increased our knowledge of the terrestrial depositional environments of the Devonian. Also of prime importance is the part the site has played in the elucidation of the effects of contemporaneous faulting in controlling deposition. Along with the West Angle Bay (North) and Freshwater East-Skrinkle Haven sites, this site provides a complete picture of the transition into the overlying Carboniferous marine succession. Recent palynological study has added to the stratigraphical importance of the site by yielding an Early Devonian plant spore assemblage. Exceptionally well-preserved trace fossils provide an insight into the early animals that inhabited dry land for the first time.