Field Excursion the Silurian Usk Inlier
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SOUTH WALES GEOLOGISTS ASSOCIATION FIELD EXCURSION THE SILURIAN USK INLIER Saturday 15 June 2019 Field Leader: Lesley Cherns (Cardiff University) 1 2 INTRODUCTION TO THE SILURIAN LIMESTONES OF THE USK INLIER The Usk inlier is a periclinal fold (dome) with N-S axis that exposes <650 m of Silurian rocks laid down on the south-eastern shelf of the Welsh Basin (Figs 1, 2). The shelly and calcareous succession includes a local equivalent of the early Silurian (Homerian) Much Wenlock Limestone of the Welsh Borderland, including small reefs. Soft calcareous shales (Coalbrookdale Formation, >245 m) dominated early Wenlock (Sheinwoodian) sedimentation on the Midland Platform while relative sea levels were high (Fig. 3). Late Wenlock (Mid-Late Homerian) sea level fall was accompanied by widespread deposition of shallow marine carbonate sediments across the eastern shelf (30-60 m), including patch reefs with diverse fossil biotas (Cramer et al. 2011a). The Usk area represents the southern extent of the reef belt. The main reef belt and largest reefs are along the limestone escarpment of Wenlock Edge (~30 m; Scoffin 1971), with smaller and scattered reefs in thicker limestone sequences across the platform interior towards Dudley (62 m) and Walsall (Ratcliffe and Thomas 1999; Cherns et al. 2006). At Dudley, a double-peaked positive stable C isotope excursion (Corfield et al. 1992) correlates with widely reported events at this time (Cramer et al. 2011b; Fig. 4). The ‘Usk Limestone’, a local lithological correlative of the Much Wenlock Limestone of Shropshire (as recognised already by Murchison in 1839), forms the top of the Wenlock Series. Its thickness ranges from <1m in the east to 13 m in the west of the inlier (Walmsley 1959). Where it is thicker, the lower, massive beds with some small reefs (Localities 1, 3; Fig. 2) pass up into nodular limestones. An overlying 1-2 m of decalcified mudstones with a Wenlock fauna were included by Squirrell and Downing (1962) in the basal Elton group of the Ludlow rather than in the Limestone (cf. Walmsley 1959). There has been some debate about the Limestone at Usk since Hurst (1975) proposed an early Ludlow age, although the fauna is characteristically Wenlock and there is no biostratigraphical evidence for an younger age (Bassett 1976). It is overlain by a thick succession of muddy and flaggy calcareous siltstones of the Ludlow Elton Group–Lower Bringewood Formation (<270 m; Lower and Upper Forest Beds of Walmsley 1959; Squirrell and Downing 1962; Sheridan 2002; Fig. 3). Very fossiliferous siltstones of the Upper Forest Beds will be seen at Llandegfedd Reservoir (Locality 4; Fig. 2).Those siltstones grade up into more calcareous, nodular silty limestones of the ‘Aymestry Limestone’ (Upper Bringewood-Lower Leintwardine formations; Lower Llanbadoc beds of Walmsley 1959; Locality 2; Fig. 2). Wenlock reefs Silurian reefs form part of the first carbonate platform deposition in the history of the early Palaeozoic Anglo-Welsh Basin (Fig. 5). By mid-Silurian times, Avalonia had drifted northwards into tropical latitudes as the Iapetus Ocean closed (Fig. 6). Wenlock reefs developed widely in this interval in tropical zones, e.g. Baltica (Gotland, Estonia). In the Anglo-Welsh Basin, early Silurian transgression represented by shales and siltstones was followed by carbonates fairly widely 3 across the inner–mid shelf platform (e.g. Cherns and Bassett 1999; Cramer et al. 2011a). Silurian reefs are typically lenticular patch reefs. On Wenlock Edge they average 12 m high by 4.5 m wide (Scoffin 1971), becoming larger and wider upwards in the sequence and towards the southwest; at Hilltop they form <80% of the Limestone in a barrier-like structure (Scoffin 1971). The Silurian shelf edge was tectonically controlled by the Church Stretton–Pontesford Lineament. Behind this shelf edge barrier, numerous patch reefs developed, decreasing in size and abundance away towards the inner shelf. The Usk succession represents one such inshore shelf setting. There, small patch reefs developed in thin limestone successions, overlying silty mudstones interbedded with impure nodular limestones. Many Wenlock patch reefs originate on lenses of coarse shelly debris, among which crinoid grains are prominent. Typically, once established, upward and outward growth from numerous points produced irregular, overlapping reef lenses, commonly separated by thin shaly or marly layers, in a massive structure. These mushroom-shaped bodies were historically termed ‘ballstones’, and are clearly distinct from the surrounding thinly bedded sediments. The irregular edges of the reefs interfinger with thinly bedded inter- reef and off-reef facies of shelly and crinoidal limestones and nodular shaly limestones. The Usk reefs are tabulate coral-bryozoan frameworks, with rugose corals and brachiopods locally common, and some molluscs (e.g. Fig. 7). On Wenlock Edge, coral-stromatoporoid frames are typical. 4 Figure 1. The distribution of Pre-Cambrian and Lower Palaeozoic rocks of Wales and the Welsh Borderland; Usk inlier in rectangle. 5 Figure 2. The geology of the Usk inlier, indicating the localities to be visited. 6 Figure 3. The stratigraphy of the Usk inlier, with enlargement for the two limestone units and localities marked. Relative sea level curve illustrates Homerian and mid-Ludlow falls through limestone deposition. 7 Figure 4. Generalised δ13C surve for the Silurian (from Cramer 2011b). Note double peak in Homerian (Mulde). 8 Figure 5. Mid Silurian Palaeogeography (http://www.scotese.com) Avalonia (and Baltica) lay in tropical zones when Wenlock reef carbonate ramps developed Figure 6. Late Wenlock Palaeogeography (Cope et al. 1992) 9 Figure 7. Some typical Wenlock fossils 10 ITINERARY We will visit two SSSI localities south west of Usk in the Silurian Much Wenlock Limestone Formation to examine reef and inter-reef facies. We will also visit two RIGS sites, Llanbadoc to look at the Ludlow Upper Bringewood Aymestry Limestone facies and Llandegveth where underlying very fossiliferous Lower Bringewood (Upper Forest) calcareous siltstones are exposed. Note: Just S of Usk and NW of Llanbadoc is Kensington House (‘Cottage’) where Alfred Russel Wallace (1823-1913) was born. 1. Cwm Ton SSSI, Glascoed Park at the Church (SO 3315 0194). The SSSI site is a long narrow strip along the Usk Limestone escarpment running NNW-SSE through the gardens of several cottages, exposed in cuttings, quarries and mines. This is the local equivalent of the Much Wenlock Limestone. The mines used the pillar-and-stall method, leaving long underground galleries. We will first examine the limestones in two of the private gardens, and can then also look in a disused quarry that lies outside the SSSI. Ty-Bryn cottage cliff exposes a succession (<5m) of fossiliferous nodular limestones and silty mudstones, with several thin bentonite bands. Brachiopods are notably common in some beds. Towards the northern end a small coral- bryozoan reef is weathering out from the face. Thicker and more massive limestones form the lower part of the face, where there are also two mine entrances and caverns that continue into the next property, Annsfield. Those caverns include a further 4-5m of the thicker limestones. Annsfield is more overgrown but the limestones display some good sedimentary structures, and the extent of the underground workings is more evident. There are fossiliferous blocks lying loose. The disused quarry opposite Ton cottages (and outside the SSSI) is in the nodular limestone and mudstone facies. 2. Old Quarry at Llanbadoc (SO 3748 0019) Park in car park behind Llanbadoc church (SO 3760 0007) to E of the Usk- Caerleon road. Large disused quarry/cliff to W of road. [Note: Alfred Russel Wallace (1823-1913) was born in the white house NW of the section.] The vertical section here in Ludlow silty carbonates (Aymestry Limestone facies sensu Holland et al. 1963) is the most extensive in the Usk Inlier, comprising >100 m of Upper Bringewood Formation–Lower Leintwardine Formation silty limestones and calcareous siltstones, with very thin bentonites (< 2 cm). The northern end of the quarry has c. 15 m of grey and green, generally 11 thinly to medium bedded argillaceous limestones with nodular bands and interbedded calcareous siltstones. Most beds are heavily bioturbated, but occasional beds show wavy and cross lamination and there are small-scale, thinly bedded graded units with scoured bases and wave-rippled surfaces. Thin lenticular and nodular shelly beds are common, and locally grade up into calcareous siltstones. The upper beds in the section are increasingly siliciclastic. An abundant skeletal fauna (see blocks on quarry floor) commonly includes the brachiopods Atrypa reticularis, Isorthis orbicularis, Leptaena depressa, Strophonella euglypha and Sphaeridiorhynchus wilsoni, with bivalves e.g. Pteronitella inexpectata, gastropods e.g. Poleumita globosa and Bembexia lloydi, nautiloid cephalopods e.g. Kionoceras angulatum, trilobites e.g. Dalmanites, and solitary rugose and tabulate corals e.g. Favosites. The Aymestry Limestone occurs throughout the southern sector of the Welsh Borderland, and records an interval of late Gorstian, relatively low-energy carbonate deposition (e.g. Cherns and Bassett 1999). Thin shelly limestones with well-preserved fossils represent storm events leading to rapid burial of shells. The highest beds of the Upper Bringewood Formation at Usk are significantly siltier than in other Welsh Borderland inliers. Green, thin to medium bedded, bioturbated calcareous siltstones are interspersed with thin shelly beds. The Upper Bringewood–Lower Leintwardine boundary in this section is placed at the first of two very thin limestone conglomerate lags exposed in the roadside section opposite Llanbadoc Church (Sheridan unpublished PhD 2002). These conglomerates are attributed to breaks in sedimentation. An inclined footpath up the bank (from SO 3756 0008) has exposures in the lower beds of the Lower Leintwardine Formation, which are lithologically similar to the underlying Upper Bringewood calcareous siltstones. Pale green-grey, blocky and increasingly micaceous calcareous siltstones (< 19 m) contain occasional thin cross-laminated units with scoured bases.