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The Corallian Group (Upper Jurassic) of Wiltshire, England. 3: Lyneham to Royal Wootton Bassett
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The Corallian Group (Upper Jurassic) of Wiltshire, England. 3: Lyneham to Royal Wootton Bassett John K. Wright Dept. of Earth Sciences, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, United Kingdom
A R T I C L E I N F O
Article history: Received 31 March 2017 Received in revised form 19 May 2017 Accepted 24 May 2017 Available online xxx Keywords: Jurassic stratigraphy Sedimentation and tectonics
A B S T R A C T
New information from boreholes at Lyneham and the construction of a relief road south of Royal Wootton Bassett has been combined with field mapping to produce the first synthesis of the transition between the Upper Jurassic shelf sediments of the Wessex Basin and the laterally equivalent clay facies sediments overlying the East Midlands Microcraton. Periodic uplifts of the Wootton Bassett High at the northern margin of the Wessex Basin saw the repeated attempts to spread clay facies sedimentation southwards into central and southern Wiltshire frustrated by uplifts of the high. This resulted in periods of erosion followed by new episodes of shallow water shelf sedimentation succeeded by deeper water strata, these beds resting upon the eroded edges of the older sediments. © 2017 The Geologists' Association. Published by Elsevier Ltd. All rights reserved.
1. Introduction In previous studies of the Wiltshire Corallian, the author has concentrated on strata deposited in the basinal areas centred on Calne and on Westbury in the Vale of Pewsey (Wright, 2014, 2016). This study carries this work northwards into the area around Lyneham and Royal Wootton Bassett (Fig. 1). Geographically, the area contains two contrasting halves. Lyneham itself sits on a plateau dipping gently to the east, sloping down from between 155 to 125 m in elevation, 6 sq km in area, with valleys deeply cut into the plateau extending southwards toward Hilmarton and northwards towards Tockenham Wick. To southwest and west a marked scarp slope overlooks the Oxford Clay of the vale of the River Avon. To the northeast, the plateau passes down into the broad valley occupied by Brinkworth Brook, before rising into the substantial hill occupied by Royal Wootton Bassett.
to map south ofLyneham in the area around Hilmarton, and it is in fact possible to build up here a sequence of strata which conflicts with Arkell’s interpretation. Subsequent to Arkell’s mapping, more than a hundred shallow borehole records (logs available on the BGS website, bgs.ac.uk/geologyofbritainviewer/boreholescans) obtained during the construction of buildings at Lyneham airfield have again shown that Arkell’s interpretation has to be modified. The area just to the south of Royal Wootton Bassett has more recently attracted attention due to the presence of well preserved ammonite faunas at several levels in the Upper Oxfordian and Lower Kimmeridgian (Harding et al., 2000; Bristow et al., 2000; Wright, 2003). However, uncertainties about the correlation of the Royal Wootton Bassett and Lyneham areas remained, and it has only been with the aid of exposures revealed during construction of the Royal Wootton Bassett Relief Road (RWBRR) in 2016 that some of these uncertainties have been resolved.
2. History of the geological research in the area 3. Notes on the development of th`e members Early authors (Blake and Hudleston, 1877; Woodward, 1895; White, 1925), noted the attenuated nature of the Upper Jurassic succession in the area, but made little comment. Arkell (1933) dismissed the area in only 2 paragraphs. However, Arkell (1951) mapped the Lyneham area in detail, improving our understanding of the geology considerably. However, Arkell failed
E-mail address: [email protected] (J.K. Wright).
During the Oxfordian, the present area was situated at the northwestern margin of the Wessex Basin. As such, the area was subject to intermittent sedimentation, with major episodes of uplift and erosion, so that there is thinning, and eventual absence, of some of the members seen in the basinal areas to the south. The full sequence of strata which can be made out at Lyneham and also at Royal Wootton Bassett is shown in Fig. 2. A summary of the succession at Lyneham is as follows:
http://dx.doi.org/10.1016/j.pgeola.2017.05.008 0016-7878/© 2017 The Geologists' Association. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: J.K. Wright, The Corallian Group (Upper Jurassic) of Wiltshire, England. 3: Lyneham to Royal Wootton Bassett, Proc. Geol. Assoc. (2017), http://dx.doi.org/10.1016/j.pgeola.2017.05.008
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00
01
02
03
04
05
06
07
Railway cutting
V
MAP OF ARKELL (1951)
B406
9
Coral Rag exposure
Bradenstoke
RWB RR
F BR
Greenhill Common n
Brickkiln Copse The Folly Beckett’s Copse
e
Lin
of
A3102
tio sec
IN KW
3B
Templars Firs B.H.
81
O RT H BR O
Tockenham
O
80
K
MAP OF BRISTOW ET AL. (2000)
Cowleaze Copse
79
Lyneham 3A
Former Lyneham Airfield
n
Preston
io e
of
se ct
Catcomb Wood Li
83
82
O Tockenham Wick
09 84
6
EY
1 02 A3
Tockenham Corner Quarry
49
Royal Wootton Bassett
SWINDON SHEET A LL
08
B4
99
MARLBOROUGH SHEET
78
Freegrove Farm
n
Goatacre
77
Catcomb Fig. 1
quarry
76 Beacon Hill
Hilmarton
0
1
2km
Fig. 1. Map of the area showing principal roads, localities mentioned in the text and the areas covered by published geological maps.
Kimmeridge Clay Formation No exposures known
Sandsfoot Formation, Ringstead Clay Member Sandy clay with limonite ooids. Westbury Ironstone Member Pebbly, fine to medium or medium grained, sandy chamositeoolite ironstone Sandsfoot Grit Member Fine grained, argillaceous sand Lyneham Member Stiff, blue–grey, brown-weathering, silty clay with pebbles of limestone and calcareous sandstone.In places a gravel of limestone fragments. Stanford Formation, Coral Rag Member Massive, coralliferous, micritic limestone, in places rubbly, argillaceous and thin bedded.
Many metres
c. 5 m
3–4 m
0–2 m
Calne Freestone Member Shelly, ooidal and pisolitic limestone Newton Clay Member Blue–grey, recrystallized limestone in rather nodular layers, alternating with bands of dark grey clay. Branching corals common. Clay is more prominent at top, often a bed of ooidal pisolite at the base.
0–3 m
0–9 m
Hazelbury Bryan Formation Fine, laminated, cross-bedded sand
0–3 m
Oxford Clay Formation Stiff, dark blue clay
3 m seen
0–4 m
0–5 m
In the northeast, around Royal Wootton Bassett, the post Coral Rag succession is entirely in Ampthill Clay facies, followed by Kimmeridge Clay. All these units are described below in ascending order.
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LYNEHAM
ROYAL WOOTTON BASSETT
KIMMERIDGE CLAY FORMATION
KIMMERIDGE CLAY FORMATION
3
RINGSTEAD CLAY MB. c.5m WESTBURY IRONST. MB. 3-4m
SANDSFOOT FORMATION
SANDSFOOT GRIT MB. c.2m LYNEHAM MB. 0-4m
AMPTHILL CLAY FORMATION c.20m
CORAL RAG MB. 0-5m
STANFORD FORMATION
CALNE FREESTONE MB. 0-3m NEWTON CLAY MB. 0-9m
CORAL RAG MB 3m.
STANFORD FORMATION NEWTON CLAY MB. 7.5m
HAZELBURY BRYAN FORMATION 0-3m
OXFORD CLAY FORMATION
OXFORD CLAY FORMATION
Fig. 2. Table of formal stratal terms used in the present work, with thicknesses. Hiatus indicated by vertical rulings.
3.1. Hazelbury Bryan Formation Due to the strong overlap by the Newton Clay Member, little remains of this predominantly sandy unit which forms such prominent features near Calne and Steeple Ashton. The best section seen was at Tockenham Wick, where White (1925) saw 2.5 m of gently cross-bedded sand. At The Folly, north of Lyneham, Arkell (1927) described a section showing 2.15 m of “yellow sand with ferruginous concretions and interlaminated clay”. At present, there are no natural exposures, but traces remain; for instance, fine grained, well sorted silty sand is dug by badgers in Catcomb Wood [ST 997 779]. 3.2. Kingston Formation No evidence for the presence of any of the members of the Kingston Formation, an important element in the stratigraphy to the northeast at Highworth (Arkell, 1941), and to the south at Calne (Wright, 2014) has been found in the present area. 3.3. Stanford Formation 3.3.1. Newton Clay Member The Newton Clay Member was introduced by Bristow et al. (1995) for a predominantly argillaceous unit with subordinate marly and sandy bands present throughout much of north Dorset. It was traced throughout south Wiltshire by Wright (2014, 2016),
and it is well developed in the present area. Much of the Newton Clay Member is present in limestone facies here, though clay still predominates at the top. The best exposures are either side of the valley draining the plateau south of Lyneham between Beacon Hill and Goatacre (Fig. 3A). At the base here is coarse, very soft, poorly sorted oomicrite, dug by badgers at [SU 017 765], and seen to a thickness of 3 m in Catcomb Quarry by Blake and Hudleston (1877). Overlying this, and seen in the hillsides on both sides of the valley, is soft, white micritic and earthy limestone containing Thecosmilia annularis (Fleming), and this is then overlain, as seen in temporary excavations at the top of Beacon Hill [SU 013 763], by tough, argillaceous micrite with many T. annularis,Nucleolites scutatus Lamarck, Nanogyra nana (J. Sowerby), etc. Topping the sequence is fine, plastic, slightly ooidal, shelly clay revealed by deep ploughing in fields near Goatacre [SU 018 768]. A boring through much of the Newton Clay Member succession was made just east of Lyneham Airfield [SU 026 778]. The log of the boring (BGS Pit/borehole record SU07NW1) covered 4.5 m of the member. The principal lithology consisted of alternations of 5–12 cm beds of blue-grey limestone separated by bands of dark grey clay. A 0.3 m bed of stiff, grey–black clay divided the limestone alternation sequences into two nearly equal halves. Exposures of the Newton Clay Member have been made south of Royal Wootton Bassett. In the railway cutting here, Reynolds and Vaughan (1902) saw 8 m of clay containing “hard” (? = limestone)
Please cite this article in press as: J.K. Wright, The Corallian Group (Upper Jurassic) of Wiltshire, England. 3: Lyneham to Royal Wootton Bassett, Proc. Geol. Assoc. (2017), http://dx.doi.org/10.1016/j.pgeola.2017.05.008
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A
SW A
metres LYNEHAM Freegrove MB. Goatacre Farm
125
Beckett’s Copse
Lyneham SANDSFOOT ? GRIT MB. ?
NE B Tockenham
CORAL RAG MB.
120
CORAL RAG MB.
CALNE FREEST. MB. Beacon Hill
115 110
Tockenham Corner
WESTBURY IRONSTONE MB.
. AY MB ON CL NEWT
HAZELBURY BRYAN FM
RD CLAY FORMATION OXFO
105 100 95 HAZELBURY BRYAN FM
90 85
Chamosite ooid ironstone
B
Limestone gravel
Clay
Micritic lst with branching corals
Fine sand
Micritic lst with massive corals
Fine to medium sand
Pisoidal Limestone
Ooidal Limestone
ESE
WSW WESTBURY IRONSTONE SANDSFOOT GRIT
Templars Firs
Brinkworth Brook
KIMMERIDGE CLAY
120
110
RWBRR (E)
Greenhill Common
Tockenham
130
KIMMERIDGE CLAY
NEWTON CLAY MB.
AMP T C L A HILL Y
INCONSTANS BED
100
Sandy clay MARSTON IRONSTONE HAZELBURY BRYAN FM.
90
OX FO RD
80
CO
RA
LR
AG
CLA Y FO R MA TIO N NEWTON CLAY MB.
70
60
Clay
Sandy Clay
Clay with nodules
Fine sandstone
Fine to medium sand
Ooidal Limestone
Micritic lst with branching corals
Micritic lst with massive corals
Chamosite ooid ironstone
Clay with bivalves
Limestone gravel
Fig. 3. (A) Cross section through the south-western part of the area showing the relationships of the various strata. (B) Cross section through the north-eastern part of the area showing the relationships of the various strata.
bands prominent at the top, underlain by 1.5 m of pisolite and pisolitic clay. Present were spines of Paracidaris florigemma (Phillips), with corals (T. annularis), bivalves and gastropods. These beds were mistakenly allocated to the Coral Rag by Arkell (1951). Exposures on the north side of the RWBRR [SU 0595 8175], though
incomplete, showed several metres of soft, micritic limestone and calcareous clay with T. annularis, Montlivaltia dispar Phillips, Rhabdophyllia phillipsi Milne Edwards and Haime, Trichites ploti (Lluyd), Liostraea quadrangularis Arkell and Lopha gregaria (J. Sowerby). Near the base was a 10 cm bed of poorly sorted oolite.
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3.3.2. Calne Freestone Member This member is only present in the area around and south of Lyneham. In the west, coarse oolite and pisolite were seen to a thickness of 3 m at The Folly (White, 1925), and 0.6 m of tough, pisolitic limestone separated Coral Rag from Hazelbury Bryan Formation sandstone in the very attenuated sequence seen at Tockenham Wick (White, 1925). Eastwards at Goatacre [SU 022 771] and Preston [SU 030 783], field mapping shows the presence of much pisoidal shelly micrite/sparite, some blocks containing a profuse fauna of tiny gastropods. This interesting rock was formerly exposed to 3 m thicknesses at both localities (Blake and Hudleston, 1877), and was referred to as “oolite and pisolite” by Arkell (1951). 3.3.3. Coral Rag Member Tough, shelly, coralliferous micrite is seen at many localities around Goatacre, Catcomb and Bradenstoke, listed by Arkell (1951). Around Lyneham, the Coral Rag is seriously affected by erosion beneath the succeeding Lyneham Member (Fig. 3A), and south and southwest of Lyneham, several borings showed Lyneham Member resting directly on the Newton Clay Member. However, this is only a local effect – just north of Lyneham [SU 023 797], the characteristic highly fossiliferous limestone of the Coral Rag was revealed recently in a temporary excavation. Exposed here were 3 m of tough, micritic limestone with numerous Thamnasteria concinna (Goldfuss) bored by Lithophaga inclusa (Phillips), prolific P. florigemma spines, Chlamys sp., Ctenostreon proboscideum (Sowerby) and Lima zonata Arkell. The lower part of the Coral Rag is still exposed at Tockenham Corner Quarry [SU 039 808]. Here the facies is not reefal, but consists of shelly, rubbly, thin- and cross-bedded, argillaceous limestone. Fragments of branching corals – Rhabdophyllia and Thecosmilia – are abundant, with occasional fragments of Thamnasteria. All are thinly coated with calcite, and at the top this is sufficient to produce numerous pisoids. Arkell (1927) visited this quarry when the junction with the underlying Hazelbury Bryan Formation was visible. His detailed description and section (Arkell, 1927, fig. 8) shows that the argillaceous, pisoidal limestone described above interdigitates with the massive Coral Rag facies. At its base was a 0.3 m bed of pebbly pisolite resting on the cross-bedded yellow sand of the Hazelbury Bryan Formation. Arkell (1951) followed White (1925) in placing the pisolitic beds in the Calne Freestone, but the prolific coral content places these beds in either the Newton Clay Member or the Coral Rag, and as this pisoidal facies is not seen elsewhere in the coralliferous Newton Clay Member, these beds are here included in the Coral Rag Member. Northwards at Wootton Bassett, Arkell (1951) thought that the Coral Rag had passed into the 8 m of fossiliferous, calcareous clay with hard limestone bands seen by Reynolds and Vaughan (1902) in the Wootton Bassett Railway cutting. These beds are much more characteristic of the Newton Clay, and it is now clear that the true Coral Rag is faulted out in the cutting, being recently exposed overlying the Newton Clay Member just to the south in ditches bordering the RWBRR. The following section was seen on the south side of the road at [SU 060 817]: AMPTHILL CLAY FORMATION 1. Grey, silty calcareous clay with small limestone concretions, containing large Liostrea sp., Lopha solitaria (J. Sowerby) and belemnites. STANFORD FORMATION, CORAL RAG MEMBER 5. Pale grey clay, deeply iron-stained in patches, containing numerous tough, white fossiliferous nodules, with bivalves and gastropods and P. florigemma spines, and dark, shelly clay infilling burrows. 4. Pale grey clay parting with P. florigemma spines.
seen to 1.8 m]
5
3. Soft, argillaceous, micritic limestone with scattered rounded bivalve fragments, P. florigemma spines and a layer of thinshelled bivalves, probably Entolium sp., at the base. 2. White micritic limestone containing Rhabdophyllia phillipsi, and 0.20 m with a layer of light-brown weathering on top. 1. Tough, micritic limestone with gastropods, bivalves, including Seen to Ctenostreon proboscideum, Pteria pteropernoides Blake and 0.5 m Hudleston, and Thamnasteria concinna. Reddish, weathered layer on top.
This is one of the few sections of Coral Rag seen in Wiltshire which was not subject to marked erosion before deposition of the overlying beds, and shows that cessation of coral growth was gradual, with conditions gradually becoming more argillaceous. Coral Rag also occurs to the northeast [SU 077 835] and north [SU 074 836] of Royal Wootton Bassett, where typical fossiliferous limestone was exposed in temporary excavations in the 1980s. Numerous borings made in connection with the construction of the M4 Motorway just north of here, viewable on the BGS website, regularly revealed 2–3 m of Coral Rag overlying 8 m of hard grey limestone with bands of clay (Newton Clay Member). 3.4. Sandsfoot Formation 3.4.1. Lyneham Member Throughout the area of Lyneham Airfield, as shown by the upwards of a hundred shallow borings and auger holes tabulated on the BGS website, Stanford Formation limestone is overlain by some 3 m of silty clay containing limestone pebbles, named here the Lyneham Member. This rests erosively on the underlying limestones, and at the base sometimes a complete gravel of limestone pebbles and boulders is present. It might be thought that this is a recent weathering phenomenon, a recent accumulation of gravel or “head”. However, many of the pebbles are rounded and have been abraded by strong currents, and the logs of several of the borings on the east side of the airfield show that the pebbly bed forms part of the local Jurassic stratigraphy. Typical was BGS Pit/ borehole record SU07NW/91 [SU 021 781]; resting directly on argillaceous limestone of the Newton Clay Member was 1 m of soft, silty clay with limestone cobbles. This was overlain by 1.2 m of light and dark mottled clay, and this by 0.7 m of stiff, dark brown, sandy clay with scattered rounded limestone pebbles. The latter may represent the Westbury Ironstone (see below). This boring, and others near here, thus show that the Lyneham Member is the basal member of the Sandsfoot Formation, a very distinctive, silty clay containing limestone pebbles, with a limestone gravel at the base, resting erosively either on the Coral Rag, the Calne Freestone or the Newton Clay members (Fig. 3A), and overlain by stiff, grey clay. Though there are no exposures of the Lyneham Member at present available for study, ploughing of fields underlain by this member near Freegrove Farm [SU 024 774] and Preston [SU 033 782] brings up numerous characteristic rounded, water-worn pebbles of Coral Rag and Calne Freestone. Many of these pebbles have a characteristic thick coating of iron oxide, probably limonite. At Brickkiln Copse [SU 027 799], the clay was formerly dug to make bricks. Arkell’s estimate of 8 m for the thickness of the clay here is however an overestimate – mapping shows that there is only 3–4 m of stiff, grey, slightly silty clay here. Much of what Arkell mapped as clay is weathered Westbury Ironstone (see below).
0.15 m
0.06 m 0.15 m
3.4.2. Sandsfoot Grit Member In a pit near the centre of Lyneham (BGS Pit/borehole record SU07NW/6), Sandsfoot Grit was exposed resting on Lyneham Member clay [SU 023 788]. Fine sand, becoming argillaceous
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below, was seen to a thickness of 2.5 m, resting on blue-grey, “stiff”, closely fissured clay. This member also crops out immediately southeast of Lyneham, where argillaceous, largely fine grained sand with a small proportion of medium grained quartz sand, was dug in an excavation for cables [SU 030 783] and at a field entrance [SU 031 778]. Eastwards and northeastwards, the cross-cutting Westbury Ironstone Member rapidly removes the Sandsfoot Grit and the Lyneham Members (Fig. 3A). 3.4.3. Westbury Ironstone Member It has become clear during the course of this work that the Westbury Ironstone, formerly thought to be present only near Westbury (Wright, 2016), is present in the Lyneham area. The key exposures are near Beckett’s Copse [SU 032 794], where a recently cut 1.2 m deep ditch exposed a 0.2 m layer of highly weathered sandy iron-ooid ironstone containing fragments of ammonites (? Ringsteadia sp.) and eroded pebbles of limonite-ooid limestone. This rests on argillaceous, fine to medium and medium grained quartz sand, seen to 0.5 m, with 0.5 m of similar, argillaceous sand overlying the ironstone. This alternation of argillaceous sand with chamosite oolite ironstone is here all included in the Westbury Ironstone Member. The Westbury Ironstone Member makes a large spread on the ridge between Cowleaze Copse [SU 025 792] and Brickkiln Copse [SU 027 799]. Ditches here reveal brown sandy clay. Arkell (1951) found this sandy clay exposed in numerous pipe trenches and ditches near Tockenham, and thought it belonged to what is referred to here as the Lyneham Member, but the distinctive ironstone clasts, limonite ooids and 10–20% medium and coarse quartz sand shows that much of this is weathered Westbury Ironstone. At Preston, Blake and Hudleston (1877) saw “2 ft (0.6 m) of rotten, ferruginous rock containing fragments of plicatiloid ammonites” resting directly on Coral Rag. This was discounted by Arkell (1951) as “hillwash or solifluction debris” yet it is now clear that this was Westbury Ironstone cutting down to rest directly on Coral Rag. North-eastwards, the Westbury Ironstone Member becomes less coarse-grained, passing into pebbly, iron-rich, fine or fine to medium grained sand, probably no more that 3 m thick. It makes a big spread through fields north and east of Tockenham, covering 1 sq km, where deep ploughing reveals characteristic argillaceous, fine to medium-grained sand (Fig. 3B). 3.4.4. Ringstead Clay Member In a ditch section northeast of Tockenham, above the Westbury Ironstone outcrop [SU 045 801] is exposed sandy clay with scattered limonite ooids, very similar to the sandy clay containing abundant limonite ooids typical of the member as seen in the Steeple Ashton area (Wright, 2016). Similar sediment is ploughed up in neighbouring fields [SU 045 798]. 3.5. Ampthill Clay Formation Interesting new exposures of this unit were made when the RWBRR (now officially termed Wannell Way) was constructed in 2016. The relief road follows the valley of Brinkworth Brook south of the town. Following low ground for nearly 1 km from [SU 059 817] to [SU 068 817], much of the course of the road was raised to avoid flooding, and there were no cuttings. As such, it might seem that there was little of interest to the geologist. However, substantial, 2 m deep ditches were cut either side, reaching down to bedrock, and though some sections of ditch were subsequently filled with gravel, over much of the course of the road, semipermanent exposures of Middle and Upper Oxfordian and Lower
Kimmeridgian strata became available for study. By Spring 2017 however, the sections had deteriorated considerably. The full succession of these beds as seen in the RWBRR and nearby in the Templars Firs Borehole (Bristow et al., 2000) and at Greenhill Common (Wright, 2003) may be summarised as follows: KIMMERIDGE CLAY FORMATION 8. Fine clay 7. Shelly clay with micrite nodules – [Inconstans Bed] AMPTHILL CLAY FORMATION 6. Fine, silty clay with occasional limonitised ammonite casts. fault – indeterminate gap 5. Khaki coloured clay (0.3 m) overlying buff, sandy clay. indeterminate gap 4. Soft, plastic clay with pyritic ammonites. indeterminate gap 3. iron-ooid clay with micritic, iron-shot oolite [Marston Ironstone] 2. Grey, silty clay with scattered siderite nodules indeterminate gap 1. Shelly silty clay with Liostrea and belemnites –
seen to 0.5 m 0.15–0.30 m seen to 0.7 m
seen to 0.7 m probably several metres 0.2 m seen to c. 1 m seen to 1.8 m
STANFORD FORMATION, CORAL RAG MEMBER [1–6] Several beds of argillaceous, micritic limestone underlain by tough, coralliferous micrite.]
Bed 3 is only seen at Greenhill Common. Evidence for Bed 4 is only seen at Templars Firs. The correlation of these beds with those seen at Lyneham is given in Fig. 4. At the western end of the RWBRR, the transitional uppermost Coral Rag beds 1–6 were described above. The lowest Ampthill Clay Bed 1 has a fully marine fauna. It is succeeded by poorly fossiliferous clays yielding limonite nodules (Bed 2). These were dug in a deep excavation close to the RWBRR [SU 061 817], and were seen to a thickness of 1 m at Greenhill Common (Wright, 2003). They are characteristic of the lower part of the Ampthill Clay both here and near Swindon. (Wright, 2003). Unfortunately, there was then a gap in the exposures, and the fossiliferous Marston Ironstone (Wright, 2003) and the clays yielding excellently preserved pyritic ammonites at Templars Firs (Bristow et al., 2000) were not exposed. When the Ampthill Clay was seen again to the east, the excavation continued in a long section of pale buff, sandy clay containing between 10% and 20% fine and medium quartz sand and phosphatic and limonitic clasts, overlain by sandy khaki coloured clay (Bed 5). A small fault then separated this sequence from a long dip section showing only a small thickness of poorly fossiliferous Ampthill Clay (Bed 6). This was overlain by a layer of clay 15–30 cm thick full of Deltoideum delta and small limestone nodules containing micrite-infilled ammonites (Pictonia spp), bivalves (Lima zonata Arkell, Pleuromya alduini (Brongniart)), gastropods (Pleurotomaria sp.) and Torquirhynchia inconstans (J. Sowerby) (Bed 7). This clearly represents the Inconstans Bed marking the base of the Kimmeridge Clay (Harding et al., 2000). 3.6. Correlation and zonal stratigraphy of the beds 3.6.1. Lithological correlation Correlation across the area has largely to be lithological (Fig. 4), as ammonite faunas have not been found in the Middle Oxfordian beds, and only occur sporadically in the Upper Oxfordian beds. The lithological correlation of the Middle Oxfordian is simple. The Newton Clay Member maintains its distinctive character of alternations of coralliferous micrite and clay, with oolite at the base, right across the area, from Beacon Hill to Royal Wootton
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Tockenham - Lyneham KIMMERIDGE CLAY FORMATION
?Inconstans Bed
7
Inconstans Bed strata not exposed
AMPTHILL CLAY FORMATION
6
? MARSTON IRONSTONE
RINGSTEAD CLAY MB.
not exposed
WESTBURY IRONSTONE MB. SANDSFOOT GRIT MB.
5 not exposed Templars Firs only Greenhill Common only
?
4 3 2 1
L. KIMMERIDGE
Royal Wootton Bassett Relief road
not exposed
? ?
UPPER OXFORDIAN
Templars Firs BH
7
LYNEHAM MB.
?
CORAL RAG MB.
5
m
CALNE FREESTONE MB.
0
MIDDLE OXFORDIAN
CORAL RAG
HAZELBURY BRYAN FM. OXF ORD CLAY FM .
Chamosite Limestone ooid ironstone gravel
Clay
Sandy Clay
Clay with nodules
Fine sandstone
Fine to medium sand
L. OXFORDIAN
NEWTON CLAY MB.
Ooid Micritic lst with Micritic lst Limestone branching with massive corals corals
Fig. 4. Proposed correlation of the Middle and Upper Oxfordian successions near Lyneham and Royal Wootton Bassett. In the Middle Oxfordian, the erosive episodes were extreme at Royal Wootton Bassett, resulting in a thin sequence. Though the total sequence of strata at Lyneham appears thicker, the erosive episodes at here were locally quite marked so that members come and go, often leading to a similarly attenuated sequence.
Bassett (Fig. 3A, B). The Coral Rag equally maintains its massive, coralliferous nature everywhere. The earliest Ampthill Clay is a fully marine, silty clay, the Liostrea clay (Bed 1, Fig. 4). It is placed immediately above the Coral Rag in Fig. 4 as there is a transition from limestone facies into clay facies here. A similar transition from Coral Rag into Ampthill Clay was seen in the Swindon Borehole (Gallois and Cox, 1994). The Lyneham Member is then placed above the Liostrea Clay, the limonite-coated pebbles at its base matching similar pebbles found in the base of the Steeple Ashton Coral Bed, which is somewhat younger (Wright, 2016). Above the Lyneham Member clays one would expect to find at Lyneham representatives of beds 2–4 of the Ampthill Clay succession, particularly the distinctive iron-ooid micrite of the Marston Ironstone. However, eroded fragments derived from these beds are found in the transgressive Westbury Ironstone Member near Lyneham, showing erosion of these middle Ampthill Clay beds here. The medium grained quartz sand which forms a prominent constituent of the Westbury Ironstone is then found in the upper Ampthill Clay of the RWBRR, the sandy Bed 5, and firmly correlates the two.
3.6.2. Correlation based on ammonite zones and subzones Fig. 5 lists the ammonite zones and subzones of the Boreal sequence, as set out by Sykes and Callomon (1979) and Wright (2003), which might be expected to be present in the area. No Middle Oxfordian ammonites have been recorded from the Lyneham district, and allocation of strata to zones and subzones given here has to be based on evidence from elsewhere in Wiltshire (Wright, 2014, 2016). Concerning the Upper Oxfordian, the placing of the Ampthill Clay beds within this sequence of Zones in part can only be tentative. The ferruginous clay (Bed 2) is placed at the Serratum/ Regulare junction based on the dating of similar beds in the Swindon Borehole (Gallois and Cox, 1994). The Marston Ironstone rests erosively on the underlying clays both at Swindon (Gallois and Cox, 1994), and at South Marston (Wright, 2003). An abundant ammonite fauna is known, and its dating as belonging to the lower Rosenkrantzi Zone (Marstonense Subzone) is clear (Wright, 2003). The ammonite fauna of the Templars Firs mudsprings (Bed 4) appears to be younger. Much of the evidence for allocation of the
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LOWER KIMMERIDGIAN (PARS)
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KIMMERIDGE CLAY FORMATION
Bauhini
Inconstans Bed Rosenkrantzi
Rosenkrantzi
UPPER OXFORDIAN
Marstonense
Fine clay
Sandy clay
RINGSTEAD CLAY MB. WESTBURY IRST. MB. SANDSFOOT GRIT MB.
Pyrite-ammonite clay Marston Ironstone Bed
Regulare Limonite nodule clay Serratum
Serratum Koldeweyense Glosense
LYNEHAM MB.
Glosense
LOWER OXFORDIAN (PARS)
MIDDLE OXFORDIAN
Ilovaiskii Blakei
Tenuiserratum
Liostrea clay CORAL RAG MB.
Tenuiserratum Maltonense
CALNE FREESTONE MB. NEWTON CLAY MB.
Densiplicatum Vertebrale Cordatum
Cordatum
HAZELBURY BRYAN FM.
SEEND CLEEVE MB.
SANDRIDGE MB.
Costicardia OXFORD CLAY FORMATION Bukowskii
Fig. 5. Oxfordian zones and subzones of the Boreal Province after Sykes and Callomon (1979) and Wright (2003), with a chart showing the approximate agerange of the formations and members. Also shows informal units.
Ampthill Clay strata in the area to ammonite zones came from the Templars Firs Borehole (Bristow et al., 2000). However, the borehole seems to have provided a very unsatisfactory view of the succession. Most of the core consisted of “very soft mud” containing pyritised ammonites (Bristow et al., 2000, p. 238), yet when the Ampthill Clay succession was exposed in the RWBRR, it did not consist of soft mud, but of a harder, silty and even sandy clay. Thus not all the Ampthill Clay in this area consists of soft mud. That which has this consistency, one of the most interesting parts of the succession, unfortunately was not exposed in the relief road. Mud from this part of the sequence appears to have been injected throughout the strata in the borehole. The intense artesian pressure which would be required to result in these Ampthill Clay fossils being carried right up to the surface at Templars Firs is clearly demonstrated by Fig. 3B. Thus, though Bristow et al. (2000) state that small, pyritised ammonite nuclei occur throughout the sequence, the extensive exposures of up to 30% of the Ampthill Clay succession in the RWBRR yielded only occasional fragments of pyritised ammonites – these can only be common at one level. Ammonites from the mudsprings sufficiently large enough to identify with certainty are rare, but Harding et al. (2000) figure several, the distinctive Amoeboceras rosenkrantzi Spath, and Prorasenia sp. showing the appearance of secondary ribs beneath the final, overlapping whorl. Both forms are characteristic of the upper part of the Rosenkrantzi Zone, the Rosenkrantzi Subzone, Allocation of the mudsprings fauna to the Serratum/Regulare Subzones by Bristow et al. (2000) is thus clearly erroneous.
Finally, fragments of the Upper Oxfordian Ringsteadia place the Westbury Ironstone in the Rosenkrantzi Zone, as elsewhere in Wiltshire. 4. Structure and sedimentation My previous studies of the Wiltshire Corallian (Wright, 2014, 2016) have concentrated on the Calne Sub-basin and the Pewsey Sub-basin (Fig. 6) where thicker accumulations of sediment took place as the result of syndepositional movements of bounding faults during episodes of crustal extension. The situation in the Lyneham-Royal Wootton Bassett area was very different. Being situated north of the Variscan Front Thrust on the Variscan Foreland (Chadwick, 1986), and being close to the margin of the East Midlands Microcraton (Woods and Chacksfield, 2012), there are no bounding faults (Fig. 6). The country between Lyneham and Royal Wootton Bassett, and on to Purton, consisted of an area subject to periodic episodes of uplift and erosion, separated by periods when typical marine shelf sedimentation took place. This area is thus a small-scale version of one of the rigid blocks which remained buoyant at intervals in parts of England throughout Jurassic time, in a similar manner to the operation of the Market Weighton ‘Block’ (Kent, 1980) or Market Weighton High (Rawson and Wright, 2000), but on a smaller scale. Arkell (1933, p. 395) introduced the “Purton Axis”. However, as the area of reduced sedimentation is centred on Royal Wootton Bassett, Arkell (1933, p. 80) favoured calling the structure the Wootton Bassett Axis, and this is the term, as the Wootton Bassett High, which will be used here. The first major uplift of the high which is evident from the succession occurred prior to deposition of the Newton Clay Member. Some 30 m of predominantly sandy Hazelbury Bryan Formation and probable Kingston Formation was removed in a gently arching uplift (Fig. 3A, B), and a normal sequence of the Newton Clay Member was then laid right across. A rise in relative sea level saw shallow water ooid shoals replaced by deeper water clay seas which regularly cleared sufficiently to allow deposition of argillaceous limestone with the delicate branching coral Thecosmilia. This is seen to south at Hilmarton and to the north at Royal Wootton Bassett. The shelly limestone and oolite of the Calne Freestone was then deposited in the south, extending up to Preston, east of Lyneham, where it wedges out. This may be due to the second uplift which saw the Calne Freestone and Newton Member largely removed, particularly near Tockenham Wick, where at one point Coral Rag rests on Hazelbury Bryan sand (White, 1925). Deposition of the Coral Rag then proceeded normally right across from Lyneham to Royal Wootton Bassett without being interrupted by an area of clay facies sedimentation as postulated by Arkell (1951). The initial rubbly, shelly, argillaceous limestone was succeeded by extremely fossiliferous reefal limestone with massive Thamnasteria colonies. In the Upper Oxfordian, on at least 3 occasions Ampthill Clay sedimentation spread down to the south, but was interrupted by an episode of uplift and erosion and the deposition of shallow water sediments. Thus, though there is a gradual upwards transition from Coral Rag into Ampthill Clay at Royal Wootton Bassett, at Lyneham the basal Lyneham Member rests very erosively on the Stanford Formation. An area of 1–2 sq km south of Lyneham was uplifted and transformed into a shallow rock platform with a gravel beach and low cliffs carved in Coral Rag and Calne Freestone (Fig. 3A). Normally, when such things happened in the Jurassic, there was time for the sea to cut right through, and the subsequent strata to rest on a planar erosion surface cut in the older beds. Rapid subsidence at Lyneham prevented this, and saw the beach gravel coated in limonite
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EAST MIDLANDS SHELF
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WOOTTON BASSETT HIGH
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N IO S N TE X CA E
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SUB-BASIN SE
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VARISCAN FOLDBELT AT DEPTH SEEND HIGH
PEWSEY SUB-BASIN
WESSEX BASIN 0
10
20km
40 Fig. 6. Diagram showing the situation of the area in relation to structures in the basement. Faults after BGS (1925, 1974, 2011) and Wright (2014). CA = Calne, HW = Highworth, LY = Lyneham, PU = Purton, RWB = Royal Wootton Bassett, SA = Steeple Ashton, SE = Seend, SM = South Marston, SW = Swindon, WE = Westbury. VFT = Variscan Front Thrust at depth, after Chadwick (1986).
and preserved in sandy marl, passing rapidly upwards into light and dark mottled clay at maximum 4 m thick. It is interesting to note that very similar rolled pebbles of Coral Rag and Calne Freestone, coated with limonite, occur at the base of the Steeple Ashton Coral Bed at Steeple Ashton (Wright, 2016), and one is tempted to correlate the Lyneham Member and the coral bed on this basis. To the northeast at Greenhill Common (Wright, 2003), Swindon (Gallois and Cox, 1994) and South Marston (Wright, 2003), the Bed 2 clay with limonite nodules is overlain by the micritic ironshot oolite of the Marston Ironstone, and then by fine, plastic clay with pyritised ammonites. It seems almost certain that the Marston Ironstone and the pyrite-ammonite clay (Beds 3 and 4) spread south over much of Wiltshire, succeeded by Sandsfoot Grit, but were removed by the widespread erosion beneath the Westbury Ironstone. Frequent eroded clasts of limonitic oomicrite typical of the nodules present in the Marston Ironstone are scattered through the Westbury Ironstone at Lyneham.
The Westbury Ironstone clearly correlates with the sandy Ampthill Clay of Bed 5 in the relief road section, marking the point where the Westbury Ironstone Member passes laterally into the Ampthill Clay Formation. The situation was that the persistently subsiding East Midlands Microcraton was occupied by an area of moderately deep shelf sea across which the Westbury Ironstone was unable to encroach. Subsidence now affected the whole of southern England, and clay facies, as the Ringstead Clay, spread right across to the Dorset coast. Callomon (in Wright, 2003, p. 104) noted that the shelly mudstone of the Inconstans Bed rested erosively on the Ampthill Clay west of Swindon, and that uppermost Oxfordian sediments are absent here. Acknowledgements The kind cooperation of the many farmers over whose land the author has tramped during the course of this survey is gratefully
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acknowledged. Grateful thanks are due to Professor J.C.W. Cope and other anonymous referees for their close scrutiny of the original manuscript, which has enabled many significant improvements. The figures were drawn by Lynne Blything. References Arkell, W.J., 1927. The Corallian rocks of Oxford, Berkshire and north Wiltshire. Philosophical Transactions of the Royal Society of London B216, 67–181. Arkell, W.J., 1933. The Jurassic System in Great Britain. Clarendon Press, Oxford. Arkell, W.J., 1941. A map of the Corallian beds around Highworth. Proceedings of the Geologists’ Association 52, 79–109. Arkell, W.J., 1951. The geology of the Corallian Ridge near Wootton Bassett and, Lyneham, Wilts. Wiltshire Archaeological and Natural History Magazine 54, 1–18. BGS, 1925 (reprinted 1974). Marlborough. England and Wales Sheet 281. Solid and Drift Geology. 1:63,360. British Geological Survey, Keyworth, Nottingham. BGS, 1974. Swindon. England and Wales Sheet 252. Solid and Drift Geology. 1:63,360. British Geological Survey, Keyworth, Nottingham. BGS, 2011. Bath. England and Wales Sheet 265. Bedrock and Superficial Deposits. 1:50,000. British Geological Survey, Keyworth, Nottingham. Blake, J.F., Hudleston, W.H., 1877. On the Corallian rocks of England. Quarterly Journal of the Geological Society of London 33, 260–405. Bristow, C.R., Barton, C.M., Freshney, E.C., Wood, C.J., Evans, D.J., Cox, B.M., IvimeyCook, H.C., Taylor, R.T., 1995. Geology of the Country Around Shaftesbury. Memoir of the British Geological Survey. HMSO, London xii+182 pp. Bristow, C.R., Gale, I.N., Fellman, E., Cox, B.M. (with Wilkinson, I.P., Riding, J.B. 2000). The lithostratigraphy, biostratigraphy and hydrogeological significance of the mud springs at Templars Firs, Wootton Bassett, Wiltshire. Proceedings of the Geologists’ Association 111, 231–245. Chadwick, R.A., 1986. Extension tectonics in the Wessex Basin, southern England. Journal of the Geological Society of London 143, 465–488.
Gallois, R.W., Cox, B.M., 1994. The Kimmeridge Clay and underlying strata (Upper Jurassic) at Swindon, Wiltshire. Proceedings of the Geologists Association 105, 99–110. Harding, I.C., Armitage, J., Hollingworth, N., Ainsworth, N., 2000. Sourcing mudsprings using integrated palaeontological analyses; an example from Wootton Bassett, Wiltshire, England. Geological Journal 35, 115–132. Kent, P.E., 1980. Subsidence and uplift in East Yorkshire and Lincolnshire: a double inversion. Proceedings of the Yorkshire Geological Society 42, 505–524. Rawson, P.F., Wright, J.K., 2000. Geologists’ Association Guide No 34, The Yorkshire Coast. The Geologists’ Association, London vi+130 pp. Reynolds, S.H., Vaughan, A., 1902. On the Jurassic strata cut through by the South Wales Direct line between Filton and Wooton Basssett. Quarterly Journal of the Geological Society of London 58, 719–752. Sykes, R.M., Callomon, J.H., 1979. The Amoeboceras zonation of the Boreal Upper Oxfordian. Palaeontology 22, 838–903. White, H.J.O., 1925. The Geology of the Country Around Marlborough. Memoir of the Geological Survey of Great Britain. HMSO, London xi+112 pp. Woods, M.A., Chacksfield, B.C., 2012. Revealing deep structural influences on the Upper Cretaceous Chalk of East Anglia (UK) through inter-regional geophysical log correlations. Proceedings of the Geologists’ Association 123, 486–499. Woodward, H.B., 1895. The Jurassic Rocks of Britain, 5. The Middle and Upper Oolitic Rocks of England (Yorkshire Excepted). Memoir of the Geological Survey of Great Britain. HMSO, London xiv+499 pp. Wright, J.K., 2003. New exposures of the Ampthill Clay near Swindon, Wiltshire, and their significance within the succession of Oxfordian/Kimmeridgian boundary beds in southern England. Proceedings of the Geologists’ Association 114, 97–121. Wright, J.K., 2014. A new section through the Corallian Group (Oxfordian, Upper Jurassic) at Calne, Wiltshire, southern England. Proceedings of the Geologists’ Association 125, 83–95. Wright, J.K., 2016. The stratigraphy and geological setting of the Oxfordian Corallian Group around Steeple Ashton and Westbury, Wiltshire, U.K. Proceedings of the Geologists’ Association 127, 266–279.
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The Corallian Group (Upper Jurassic) of Wiltshire, England. 3: Lyneham to Royal Wootton Bassett John K. Wright Dept. of Earth Sciences, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, United Kingdom
A R T I C L E I N F O
Article history: Received 31 March 2017 Received in revised form 19 May 2017 Accepted 24 May 2017 Available online xxx Keywords: Jurassic stratigraphy Sedimentation and tectonics
A B S T R A C T
New information from boreholes at Lyneham and the construction of a relief road south of Royal Wootton Bassett has been combined with field mapping to produce the first synthesis of the transition between the Upper Jurassic shelf sediments of the Wessex Basin and the laterally equivalent clay facies sediments overlying the East Midlands Microcraton. Periodic uplifts of the Wootton Bassett High at the northern margin of the Wessex Basin saw the repeated attempts to spread clay facies sedimentation southwards into central and southern Wiltshire frustrated by uplifts of the high. This resulted in periods of erosion followed by new episodes of shallow water shelf sedimentation succeeded by deeper water strata, these beds resting upon the eroded edges of the older sediments. © 2017 The Geologists' Association. Published by Elsevier Ltd. All rights reserved.
1. Introduction In previous studies of the Wiltshire Corallian, the author has concentrated on strata deposited in the basinal areas centred on Calne and on Westbury in the Vale of Pewsey (Wright, 2014, 2016). This study carries this work northwards into the area around Lyneham and Royal Wootton Bassett (Fig. 1). Geographically, the area contains two contrasting halves. Lyneham itself sits on a plateau dipping gently to the east, sloping down from between 155 to 125 m in elevation, 6 sq km in area, with valleys deeply cut into the plateau extending southwards toward Hilmarton and northwards towards Tockenham Wick. To southwest and west a marked scarp slope overlooks the Oxford Clay of the vale of the River Avon. To the northeast, the plateau passes down into the broad valley occupied by Brinkworth Brook, before rising into the substantial hill occupied by Royal Wootton Bassett.
to map south ofLyneham in the area around Hilmarton, and it is in fact possible to build up here a sequence of strata which conflicts with Arkell’s interpretation. Subsequent to Arkell’s mapping, more than a hundred shallow borehole records (logs available on the BGS website, bgs.ac.uk/geologyofbritainviewer/boreholescans) obtained during the construction of buildings at Lyneham airfield have again shown that Arkell’s interpretation has to be modified. The area just to the south of Royal Wootton Bassett has more recently attracted attention due to the presence of well preserved ammonite faunas at several levels in the Upper Oxfordian and Lower Kimmeridgian (Harding et al., 2000; Bristow et al., 2000; Wright, 2003). However, uncertainties about the correlation of the Royal Wootton Bassett and Lyneham areas remained, and it has only been with the aid of exposures revealed during construction of the Royal Wootton Bassett Relief Road (RWBRR) in 2016 that some of these uncertainties have been resolved.
2. History of the geological research in the area 3. Notes on the development of th`e members Early authors (Blake and Hudleston, 1877; Woodward, 1895; White, 1925), noted the attenuated nature of the Upper Jurassic succession in the area, but made little comment. Arkell (1933) dismissed the area in only 2 paragraphs. However, Arkell (1951) mapped the Lyneham area in detail, improving our understanding of the geology considerably. However, Arkell failed
E-mail address: [email protected] (J.K. Wright).
During the Oxfordian, the present area was situated at the northwestern margin of the Wessex Basin. As such, the area was subject to intermittent sedimentation, with major episodes of uplift and erosion, so that there is thinning, and eventual absence, of some of the members seen in the basinal areas to the south. The full sequence of strata which can be made out at Lyneham and also at Royal Wootton Bassett is shown in Fig. 2. A summary of the succession at Lyneham is as follows:
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00
01
02
03
04
05
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07
Railway cutting
V
MAP OF ARKELL (1951)
B406
9
Coral Rag exposure
Bradenstoke
RWB RR
F BR
Greenhill Common n
Brickkiln Copse The Folly Beckett’s Copse
e
Lin
of
A3102
tio sec
IN KW
3B
Templars Firs B.H.
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O RT H BR O
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K
MAP OF BRISTOW ET AL. (2000)
Cowleaze Copse
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Lyneham 3A
Former Lyneham Airfield
n
Preston
io e
of
se ct
Catcomb Wood Li
83
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O Tockenham Wick
09 84
6
EY
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Tockenham Corner Quarry
49
Royal Wootton Bassett
SWINDON SHEET A LL
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MARLBOROUGH SHEET
78
Freegrove Farm
n
Goatacre
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Catcomb Fig. 1
quarry
76 Beacon Hill
Hilmarton
0
1
2km
Fig. 1. Map of the area showing principal roads, localities mentioned in the text and the areas covered by published geological maps.
Kimmeridge Clay Formation No exposures known
Sandsfoot Formation, Ringstead Clay Member Sandy clay with limonite ooids. Westbury Ironstone Member Pebbly, fine to medium or medium grained, sandy chamositeoolite ironstone Sandsfoot Grit Member Fine grained, argillaceous sand Lyneham Member Stiff, blue–grey, brown-weathering, silty clay with pebbles of limestone and calcareous sandstone.In places a gravel of limestone fragments. Stanford Formation, Coral Rag Member Massive, coralliferous, micritic limestone, in places rubbly, argillaceous and thin bedded.
Many metres
c. 5 m
3–4 m
0–2 m
Calne Freestone Member Shelly, ooidal and pisolitic limestone Newton Clay Member Blue–grey, recrystallized limestone in rather nodular layers, alternating with bands of dark grey clay. Branching corals common. Clay is more prominent at top, often a bed of ooidal pisolite at the base.
0–3 m
0–9 m
Hazelbury Bryan Formation Fine, laminated, cross-bedded sand
0–3 m
Oxford Clay Formation Stiff, dark blue clay
3 m seen
0–4 m
0–5 m
In the northeast, around Royal Wootton Bassett, the post Coral Rag succession is entirely in Ampthill Clay facies, followed by Kimmeridge Clay. All these units are described below in ascending order.
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LYNEHAM
ROYAL WOOTTON BASSETT
KIMMERIDGE CLAY FORMATION
KIMMERIDGE CLAY FORMATION
3
RINGSTEAD CLAY MB. c.5m WESTBURY IRONST. MB. 3-4m
SANDSFOOT FORMATION
SANDSFOOT GRIT MB. c.2m LYNEHAM MB. 0-4m
AMPTHILL CLAY FORMATION c.20m
CORAL RAG MB. 0-5m
STANFORD FORMATION
CALNE FREESTONE MB. 0-3m NEWTON CLAY MB. 0-9m
CORAL RAG MB 3m.
STANFORD FORMATION NEWTON CLAY MB. 7.5m
HAZELBURY BRYAN FORMATION 0-3m
OXFORD CLAY FORMATION
OXFORD CLAY FORMATION
Fig. 2. Table of formal stratal terms used in the present work, with thicknesses. Hiatus indicated by vertical rulings.
3.1. Hazelbury Bryan Formation Due to the strong overlap by the Newton Clay Member, little remains of this predominantly sandy unit which forms such prominent features near Calne and Steeple Ashton. The best section seen was at Tockenham Wick, where White (1925) saw 2.5 m of gently cross-bedded sand. At The Folly, north of Lyneham, Arkell (1927) described a section showing 2.15 m of “yellow sand with ferruginous concretions and interlaminated clay”. At present, there are no natural exposures, but traces remain; for instance, fine grained, well sorted silty sand is dug by badgers in Catcomb Wood [ST 997 779]. 3.2. Kingston Formation No evidence for the presence of any of the members of the Kingston Formation, an important element in the stratigraphy to the northeast at Highworth (Arkell, 1941), and to the south at Calne (Wright, 2014) has been found in the present area. 3.3. Stanford Formation 3.3.1. Newton Clay Member The Newton Clay Member was introduced by Bristow et al. (1995) for a predominantly argillaceous unit with subordinate marly and sandy bands present throughout much of north Dorset. It was traced throughout south Wiltshire by Wright (2014, 2016),
and it is well developed in the present area. Much of the Newton Clay Member is present in limestone facies here, though clay still predominates at the top. The best exposures are either side of the valley draining the plateau south of Lyneham between Beacon Hill and Goatacre (Fig. 3A). At the base here is coarse, very soft, poorly sorted oomicrite, dug by badgers at [SU 017 765], and seen to a thickness of 3 m in Catcomb Quarry by Blake and Hudleston (1877). Overlying this, and seen in the hillsides on both sides of the valley, is soft, white micritic and earthy limestone containing Thecosmilia annularis (Fleming), and this is then overlain, as seen in temporary excavations at the top of Beacon Hill [SU 013 763], by tough, argillaceous micrite with many T. annularis,Nucleolites scutatus Lamarck, Nanogyra nana (J. Sowerby), etc. Topping the sequence is fine, plastic, slightly ooidal, shelly clay revealed by deep ploughing in fields near Goatacre [SU 018 768]. A boring through much of the Newton Clay Member succession was made just east of Lyneham Airfield [SU 026 778]. The log of the boring (BGS Pit/borehole record SU07NW1) covered 4.5 m of the member. The principal lithology consisted of alternations of 5–12 cm beds of blue-grey limestone separated by bands of dark grey clay. A 0.3 m bed of stiff, grey–black clay divided the limestone alternation sequences into two nearly equal halves. Exposures of the Newton Clay Member have been made south of Royal Wootton Bassett. In the railway cutting here, Reynolds and Vaughan (1902) saw 8 m of clay containing “hard” (? = limestone)
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A
SW A
metres LYNEHAM Freegrove MB. Goatacre Farm
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Beckett’s Copse
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NE B Tockenham
CORAL RAG MB.
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CALNE FREEST. MB. Beacon Hill
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WESTBURY IRONSTONE MB.
. AY MB ON CL NEWT
HAZELBURY BRYAN FM
RD CLAY FORMATION OXFO
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90 85
Chamosite ooid ironstone
B
Limestone gravel
Clay
Micritic lst with branching corals
Fine sand
Micritic lst with massive corals
Fine to medium sand
Pisoidal Limestone
Ooidal Limestone
ESE
WSW WESTBURY IRONSTONE SANDSFOOT GRIT
Templars Firs
Brinkworth Brook
KIMMERIDGE CLAY
120
110
RWBRR (E)
Greenhill Common
Tockenham
130
KIMMERIDGE CLAY
NEWTON CLAY MB.
AMP T C L A HILL Y
INCONSTANS BED
100
Sandy clay MARSTON IRONSTONE HAZELBURY BRYAN FM.
90
OX FO RD
80
CO
RA
LR
AG
CLA Y FO R MA TIO N NEWTON CLAY MB.
70
60
Clay
Sandy Clay
Clay with nodules
Fine sandstone
Fine to medium sand
Ooidal Limestone
Micritic lst with branching corals
Micritic lst with massive corals
Chamosite ooid ironstone
Clay with bivalves
Limestone gravel
Fig. 3. (A) Cross section through the south-western part of the area showing the relationships of the various strata. (B) Cross section through the north-eastern part of the area showing the relationships of the various strata.
bands prominent at the top, underlain by 1.5 m of pisolite and pisolitic clay. Present were spines of Paracidaris florigemma (Phillips), with corals (T. annularis), bivalves and gastropods. These beds were mistakenly allocated to the Coral Rag by Arkell (1951). Exposures on the north side of the RWBRR [SU 0595 8175], though
incomplete, showed several metres of soft, micritic limestone and calcareous clay with T. annularis, Montlivaltia dispar Phillips, Rhabdophyllia phillipsi Milne Edwards and Haime, Trichites ploti (Lluyd), Liostraea quadrangularis Arkell and Lopha gregaria (J. Sowerby). Near the base was a 10 cm bed of poorly sorted oolite.
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3.3.2. Calne Freestone Member This member is only present in the area around and south of Lyneham. In the west, coarse oolite and pisolite were seen to a thickness of 3 m at The Folly (White, 1925), and 0.6 m of tough, pisolitic limestone separated Coral Rag from Hazelbury Bryan Formation sandstone in the very attenuated sequence seen at Tockenham Wick (White, 1925). Eastwards at Goatacre [SU 022 771] and Preston [SU 030 783], field mapping shows the presence of much pisoidal shelly micrite/sparite, some blocks containing a profuse fauna of tiny gastropods. This interesting rock was formerly exposed to 3 m thicknesses at both localities (Blake and Hudleston, 1877), and was referred to as “oolite and pisolite” by Arkell (1951). 3.3.3. Coral Rag Member Tough, shelly, coralliferous micrite is seen at many localities around Goatacre, Catcomb and Bradenstoke, listed by Arkell (1951). Around Lyneham, the Coral Rag is seriously affected by erosion beneath the succeeding Lyneham Member (Fig. 3A), and south and southwest of Lyneham, several borings showed Lyneham Member resting directly on the Newton Clay Member. However, this is only a local effect – just north of Lyneham [SU 023 797], the characteristic highly fossiliferous limestone of the Coral Rag was revealed recently in a temporary excavation. Exposed here were 3 m of tough, micritic limestone with numerous Thamnasteria concinna (Goldfuss) bored by Lithophaga inclusa (Phillips), prolific P. florigemma spines, Chlamys sp., Ctenostreon proboscideum (Sowerby) and Lima zonata Arkell. The lower part of the Coral Rag is still exposed at Tockenham Corner Quarry [SU 039 808]. Here the facies is not reefal, but consists of shelly, rubbly, thin- and cross-bedded, argillaceous limestone. Fragments of branching corals – Rhabdophyllia and Thecosmilia – are abundant, with occasional fragments of Thamnasteria. All are thinly coated with calcite, and at the top this is sufficient to produce numerous pisoids. Arkell (1927) visited this quarry when the junction with the underlying Hazelbury Bryan Formation was visible. His detailed description and section (Arkell, 1927, fig. 8) shows that the argillaceous, pisoidal limestone described above interdigitates with the massive Coral Rag facies. At its base was a 0.3 m bed of pebbly pisolite resting on the cross-bedded yellow sand of the Hazelbury Bryan Formation. Arkell (1951) followed White (1925) in placing the pisolitic beds in the Calne Freestone, but the prolific coral content places these beds in either the Newton Clay Member or the Coral Rag, and as this pisoidal facies is not seen elsewhere in the coralliferous Newton Clay Member, these beds are here included in the Coral Rag Member. Northwards at Wootton Bassett, Arkell (1951) thought that the Coral Rag had passed into the 8 m of fossiliferous, calcareous clay with hard limestone bands seen by Reynolds and Vaughan (1902) in the Wootton Bassett Railway cutting. These beds are much more characteristic of the Newton Clay, and it is now clear that the true Coral Rag is faulted out in the cutting, being recently exposed overlying the Newton Clay Member just to the south in ditches bordering the RWBRR. The following section was seen on the south side of the road at [SU 060 817]: AMPTHILL CLAY FORMATION 1. Grey, silty calcareous clay with small limestone concretions, containing large Liostrea sp., Lopha solitaria (J. Sowerby) and belemnites. STANFORD FORMATION, CORAL RAG MEMBER 5. Pale grey clay, deeply iron-stained in patches, containing numerous tough, white fossiliferous nodules, with bivalves and gastropods and P. florigemma spines, and dark, shelly clay infilling burrows. 4. Pale grey clay parting with P. florigemma spines.
seen to 1.8 m]
5
3. Soft, argillaceous, micritic limestone with scattered rounded bivalve fragments, P. florigemma spines and a layer of thinshelled bivalves, probably Entolium sp., at the base. 2. White micritic limestone containing Rhabdophyllia phillipsi, and 0.20 m with a layer of light-brown weathering on top. 1. Tough, micritic limestone with gastropods, bivalves, including Seen to Ctenostreon proboscideum, Pteria pteropernoides Blake and 0.5 m Hudleston, and Thamnasteria concinna. Reddish, weathered layer on top.
This is one of the few sections of Coral Rag seen in Wiltshire which was not subject to marked erosion before deposition of the overlying beds, and shows that cessation of coral growth was gradual, with conditions gradually becoming more argillaceous. Coral Rag also occurs to the northeast [SU 077 835] and north [SU 074 836] of Royal Wootton Bassett, where typical fossiliferous limestone was exposed in temporary excavations in the 1980s. Numerous borings made in connection with the construction of the M4 Motorway just north of here, viewable on the BGS website, regularly revealed 2–3 m of Coral Rag overlying 8 m of hard grey limestone with bands of clay (Newton Clay Member). 3.4. Sandsfoot Formation 3.4.1. Lyneham Member Throughout the area of Lyneham Airfield, as shown by the upwards of a hundred shallow borings and auger holes tabulated on the BGS website, Stanford Formation limestone is overlain by some 3 m of silty clay containing limestone pebbles, named here the Lyneham Member. This rests erosively on the underlying limestones, and at the base sometimes a complete gravel of limestone pebbles and boulders is present. It might be thought that this is a recent weathering phenomenon, a recent accumulation of gravel or “head”. However, many of the pebbles are rounded and have been abraded by strong currents, and the logs of several of the borings on the east side of the airfield show that the pebbly bed forms part of the local Jurassic stratigraphy. Typical was BGS Pit/ borehole record SU07NW/91 [SU 021 781]; resting directly on argillaceous limestone of the Newton Clay Member was 1 m of soft, silty clay with limestone cobbles. This was overlain by 1.2 m of light and dark mottled clay, and this by 0.7 m of stiff, dark brown, sandy clay with scattered rounded limestone pebbles. The latter may represent the Westbury Ironstone (see below). This boring, and others near here, thus show that the Lyneham Member is the basal member of the Sandsfoot Formation, a very distinctive, silty clay containing limestone pebbles, with a limestone gravel at the base, resting erosively either on the Coral Rag, the Calne Freestone or the Newton Clay members (Fig. 3A), and overlain by stiff, grey clay. Though there are no exposures of the Lyneham Member at present available for study, ploughing of fields underlain by this member near Freegrove Farm [SU 024 774] and Preston [SU 033 782] brings up numerous characteristic rounded, water-worn pebbles of Coral Rag and Calne Freestone. Many of these pebbles have a characteristic thick coating of iron oxide, probably limonite. At Brickkiln Copse [SU 027 799], the clay was formerly dug to make bricks. Arkell’s estimate of 8 m for the thickness of the clay here is however an overestimate – mapping shows that there is only 3–4 m of stiff, grey, slightly silty clay here. Much of what Arkell mapped as clay is weathered Westbury Ironstone (see below).
0.15 m
0.06 m 0.15 m
3.4.2. Sandsfoot Grit Member In a pit near the centre of Lyneham (BGS Pit/borehole record SU07NW/6), Sandsfoot Grit was exposed resting on Lyneham Member clay [SU 023 788]. Fine sand, becoming argillaceous
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below, was seen to a thickness of 2.5 m, resting on blue-grey, “stiff”, closely fissured clay. This member also crops out immediately southeast of Lyneham, where argillaceous, largely fine grained sand with a small proportion of medium grained quartz sand, was dug in an excavation for cables [SU 030 783] and at a field entrance [SU 031 778]. Eastwards and northeastwards, the cross-cutting Westbury Ironstone Member rapidly removes the Sandsfoot Grit and the Lyneham Members (Fig. 3A). 3.4.3. Westbury Ironstone Member It has become clear during the course of this work that the Westbury Ironstone, formerly thought to be present only near Westbury (Wright, 2016), is present in the Lyneham area. The key exposures are near Beckett’s Copse [SU 032 794], where a recently cut 1.2 m deep ditch exposed a 0.2 m layer of highly weathered sandy iron-ooid ironstone containing fragments of ammonites (? Ringsteadia sp.) and eroded pebbles of limonite-ooid limestone. This rests on argillaceous, fine to medium and medium grained quartz sand, seen to 0.5 m, with 0.5 m of similar, argillaceous sand overlying the ironstone. This alternation of argillaceous sand with chamosite oolite ironstone is here all included in the Westbury Ironstone Member. The Westbury Ironstone Member makes a large spread on the ridge between Cowleaze Copse [SU 025 792] and Brickkiln Copse [SU 027 799]. Ditches here reveal brown sandy clay. Arkell (1951) found this sandy clay exposed in numerous pipe trenches and ditches near Tockenham, and thought it belonged to what is referred to here as the Lyneham Member, but the distinctive ironstone clasts, limonite ooids and 10–20% medium and coarse quartz sand shows that much of this is weathered Westbury Ironstone. At Preston, Blake and Hudleston (1877) saw “2 ft (0.6 m) of rotten, ferruginous rock containing fragments of plicatiloid ammonites” resting directly on Coral Rag. This was discounted by Arkell (1951) as “hillwash or solifluction debris” yet it is now clear that this was Westbury Ironstone cutting down to rest directly on Coral Rag. North-eastwards, the Westbury Ironstone Member becomes less coarse-grained, passing into pebbly, iron-rich, fine or fine to medium grained sand, probably no more that 3 m thick. It makes a big spread through fields north and east of Tockenham, covering 1 sq km, where deep ploughing reveals characteristic argillaceous, fine to medium-grained sand (Fig. 3B). 3.4.4. Ringstead Clay Member In a ditch section northeast of Tockenham, above the Westbury Ironstone outcrop [SU 045 801] is exposed sandy clay with scattered limonite ooids, very similar to the sandy clay containing abundant limonite ooids typical of the member as seen in the Steeple Ashton area (Wright, 2016). Similar sediment is ploughed up in neighbouring fields [SU 045 798]. 3.5. Ampthill Clay Formation Interesting new exposures of this unit were made when the RWBRR (now officially termed Wannell Way) was constructed in 2016. The relief road follows the valley of Brinkworth Brook south of the town. Following low ground for nearly 1 km from [SU 059 817] to [SU 068 817], much of the course of the road was raised to avoid flooding, and there were no cuttings. As such, it might seem that there was little of interest to the geologist. However, substantial, 2 m deep ditches were cut either side, reaching down to bedrock, and though some sections of ditch were subsequently filled with gravel, over much of the course of the road, semipermanent exposures of Middle and Upper Oxfordian and Lower
Kimmeridgian strata became available for study. By Spring 2017 however, the sections had deteriorated considerably. The full succession of these beds as seen in the RWBRR and nearby in the Templars Firs Borehole (Bristow et al., 2000) and at Greenhill Common (Wright, 2003) may be summarised as follows: KIMMERIDGE CLAY FORMATION 8. Fine clay 7. Shelly clay with micrite nodules – [Inconstans Bed] AMPTHILL CLAY FORMATION 6. Fine, silty clay with occasional limonitised ammonite casts. fault – indeterminate gap 5. Khaki coloured clay (0.3 m) overlying buff, sandy clay. indeterminate gap 4. Soft, plastic clay with pyritic ammonites. indeterminate gap 3. iron-ooid clay with micritic, iron-shot oolite [Marston Ironstone] 2. Grey, silty clay with scattered siderite nodules indeterminate gap 1. Shelly silty clay with Liostrea and belemnites –
seen to 0.5 m 0.15–0.30 m seen to 0.7 m
seen to 0.7 m probably several metres 0.2 m seen to c. 1 m seen to 1.8 m
STANFORD FORMATION, CORAL RAG MEMBER [1–6] Several beds of argillaceous, micritic limestone underlain by tough, coralliferous micrite.]
Bed 3 is only seen at Greenhill Common. Evidence for Bed 4 is only seen at Templars Firs. The correlation of these beds with those seen at Lyneham is given in Fig. 4. At the western end of the RWBRR, the transitional uppermost Coral Rag beds 1–6 were described above. The lowest Ampthill Clay Bed 1 has a fully marine fauna. It is succeeded by poorly fossiliferous clays yielding limonite nodules (Bed 2). These were dug in a deep excavation close to the RWBRR [SU 061 817], and were seen to a thickness of 1 m at Greenhill Common (Wright, 2003). They are characteristic of the lower part of the Ampthill Clay both here and near Swindon. (Wright, 2003). Unfortunately, there was then a gap in the exposures, and the fossiliferous Marston Ironstone (Wright, 2003) and the clays yielding excellently preserved pyritic ammonites at Templars Firs (Bristow et al., 2000) were not exposed. When the Ampthill Clay was seen again to the east, the excavation continued in a long section of pale buff, sandy clay containing between 10% and 20% fine and medium quartz sand and phosphatic and limonitic clasts, overlain by sandy khaki coloured clay (Bed 5). A small fault then separated this sequence from a long dip section showing only a small thickness of poorly fossiliferous Ampthill Clay (Bed 6). This was overlain by a layer of clay 15–30 cm thick full of Deltoideum delta and small limestone nodules containing micrite-infilled ammonites (Pictonia spp), bivalves (Lima zonata Arkell, Pleuromya alduini (Brongniart)), gastropods (Pleurotomaria sp.) and Torquirhynchia inconstans (J. Sowerby) (Bed 7). This clearly represents the Inconstans Bed marking the base of the Kimmeridge Clay (Harding et al., 2000). 3.6. Correlation and zonal stratigraphy of the beds 3.6.1. Lithological correlation Correlation across the area has largely to be lithological (Fig. 4), as ammonite faunas have not been found in the Middle Oxfordian beds, and only occur sporadically in the Upper Oxfordian beds. The lithological correlation of the Middle Oxfordian is simple. The Newton Clay Member maintains its distinctive character of alternations of coralliferous micrite and clay, with oolite at the base, right across the area, from Beacon Hill to Royal Wootton
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Tockenham - Lyneham KIMMERIDGE CLAY FORMATION
?Inconstans Bed
7
Inconstans Bed strata not exposed
AMPTHILL CLAY FORMATION
6
? MARSTON IRONSTONE
RINGSTEAD CLAY MB.
not exposed
WESTBURY IRONSTONE MB. SANDSFOOT GRIT MB.
5 not exposed Templars Firs only Greenhill Common only
?
4 3 2 1
L. KIMMERIDGE
Royal Wootton Bassett Relief road
not exposed
? ?
UPPER OXFORDIAN
Templars Firs BH
7
LYNEHAM MB.
?
CORAL RAG MB.
5
m
CALNE FREESTONE MB.
0
MIDDLE OXFORDIAN
CORAL RAG
HAZELBURY BRYAN FM. OXF ORD CLAY FM .
Chamosite Limestone ooid ironstone gravel
Clay
Sandy Clay
Clay with nodules
Fine sandstone
Fine to medium sand
L. OXFORDIAN
NEWTON CLAY MB.
Ooid Micritic lst with Micritic lst Limestone branching with massive corals corals
Fig. 4. Proposed correlation of the Middle and Upper Oxfordian successions near Lyneham and Royal Wootton Bassett. In the Middle Oxfordian, the erosive episodes were extreme at Royal Wootton Bassett, resulting in a thin sequence. Though the total sequence of strata at Lyneham appears thicker, the erosive episodes at here were locally quite marked so that members come and go, often leading to a similarly attenuated sequence.
Bassett (Fig. 3A, B). The Coral Rag equally maintains its massive, coralliferous nature everywhere. The earliest Ampthill Clay is a fully marine, silty clay, the Liostrea clay (Bed 1, Fig. 4). It is placed immediately above the Coral Rag in Fig. 4 as there is a transition from limestone facies into clay facies here. A similar transition from Coral Rag into Ampthill Clay was seen in the Swindon Borehole (Gallois and Cox, 1994). The Lyneham Member is then placed above the Liostrea Clay, the limonite-coated pebbles at its base matching similar pebbles found in the base of the Steeple Ashton Coral Bed, which is somewhat younger (Wright, 2016). Above the Lyneham Member clays one would expect to find at Lyneham representatives of beds 2–4 of the Ampthill Clay succession, particularly the distinctive iron-ooid micrite of the Marston Ironstone. However, eroded fragments derived from these beds are found in the transgressive Westbury Ironstone Member near Lyneham, showing erosion of these middle Ampthill Clay beds here. The medium grained quartz sand which forms a prominent constituent of the Westbury Ironstone is then found in the upper Ampthill Clay of the RWBRR, the sandy Bed 5, and firmly correlates the two.
3.6.2. Correlation based on ammonite zones and subzones Fig. 5 lists the ammonite zones and subzones of the Boreal sequence, as set out by Sykes and Callomon (1979) and Wright (2003), which might be expected to be present in the area. No Middle Oxfordian ammonites have been recorded from the Lyneham district, and allocation of strata to zones and subzones given here has to be based on evidence from elsewhere in Wiltshire (Wright, 2014, 2016). Concerning the Upper Oxfordian, the placing of the Ampthill Clay beds within this sequence of Zones in part can only be tentative. The ferruginous clay (Bed 2) is placed at the Serratum/ Regulare junction based on the dating of similar beds in the Swindon Borehole (Gallois and Cox, 1994). The Marston Ironstone rests erosively on the underlying clays both at Swindon (Gallois and Cox, 1994), and at South Marston (Wright, 2003). An abundant ammonite fauna is known, and its dating as belonging to the lower Rosenkrantzi Zone (Marstonense Subzone) is clear (Wright, 2003). The ammonite fauna of the Templars Firs mudsprings (Bed 4) appears to be younger. Much of the evidence for allocation of the
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LOWER KIMMERIDGIAN (PARS)
J.K. Wright / Proceedings of the Geologists’ Association xxx (2017) xxx–xxx
KIMMERIDGE CLAY FORMATION
Bauhini
Inconstans Bed Rosenkrantzi
Rosenkrantzi
UPPER OXFORDIAN
Marstonense
Fine clay
Sandy clay
RINGSTEAD CLAY MB. WESTBURY IRST. MB. SANDSFOOT GRIT MB.
Pyrite-ammonite clay Marston Ironstone Bed
Regulare Limonite nodule clay Serratum
Serratum Koldeweyense Glosense
LYNEHAM MB.
Glosense
LOWER OXFORDIAN (PARS)
MIDDLE OXFORDIAN
Ilovaiskii Blakei
Tenuiserratum
Liostrea clay CORAL RAG MB.
Tenuiserratum Maltonense
CALNE FREESTONE MB. NEWTON CLAY MB.
Densiplicatum Vertebrale Cordatum
Cordatum
HAZELBURY BRYAN FM.
SEEND CLEEVE MB.
SANDRIDGE MB.
Costicardia OXFORD CLAY FORMATION Bukowskii
Fig. 5. Oxfordian zones and subzones of the Boreal Province after Sykes and Callomon (1979) and Wright (2003), with a chart showing the approximate agerange of the formations and members. Also shows informal units.
Ampthill Clay strata in the area to ammonite zones came from the Templars Firs Borehole (Bristow et al., 2000). However, the borehole seems to have provided a very unsatisfactory view of the succession. Most of the core consisted of “very soft mud” containing pyritised ammonites (Bristow et al., 2000, p. 238), yet when the Ampthill Clay succession was exposed in the RWBRR, it did not consist of soft mud, but of a harder, silty and even sandy clay. Thus not all the Ampthill Clay in this area consists of soft mud. That which has this consistency, one of the most interesting parts of the succession, unfortunately was not exposed in the relief road. Mud from this part of the sequence appears to have been injected throughout the strata in the borehole. The intense artesian pressure which would be required to result in these Ampthill Clay fossils being carried right up to the surface at Templars Firs is clearly demonstrated by Fig. 3B. Thus, though Bristow et al. (2000) state that small, pyritised ammonite nuclei occur throughout the sequence, the extensive exposures of up to 30% of the Ampthill Clay succession in the RWBRR yielded only occasional fragments of pyritised ammonites – these can only be common at one level. Ammonites from the mudsprings sufficiently large enough to identify with certainty are rare, but Harding et al. (2000) figure several, the distinctive Amoeboceras rosenkrantzi Spath, and Prorasenia sp. showing the appearance of secondary ribs beneath the final, overlapping whorl. Both forms are characteristic of the upper part of the Rosenkrantzi Zone, the Rosenkrantzi Subzone, Allocation of the mudsprings fauna to the Serratum/Regulare Subzones by Bristow et al. (2000) is thus clearly erroneous.
Finally, fragments of the Upper Oxfordian Ringsteadia place the Westbury Ironstone in the Rosenkrantzi Zone, as elsewhere in Wiltshire. 4. Structure and sedimentation My previous studies of the Wiltshire Corallian (Wright, 2014, 2016) have concentrated on the Calne Sub-basin and the Pewsey Sub-basin (Fig. 6) where thicker accumulations of sediment took place as the result of syndepositional movements of bounding faults during episodes of crustal extension. The situation in the Lyneham-Royal Wootton Bassett area was very different. Being situated north of the Variscan Front Thrust on the Variscan Foreland (Chadwick, 1986), and being close to the margin of the East Midlands Microcraton (Woods and Chacksfield, 2012), there are no bounding faults (Fig. 6). The country between Lyneham and Royal Wootton Bassett, and on to Purton, consisted of an area subject to periodic episodes of uplift and erosion, separated by periods when typical marine shelf sedimentation took place. This area is thus a small-scale version of one of the rigid blocks which remained buoyant at intervals in parts of England throughout Jurassic time, in a similar manner to the operation of the Market Weighton ‘Block’ (Kent, 1980) or Market Weighton High (Rawson and Wright, 2000), but on a smaller scale. Arkell (1933, p. 395) introduced the “Purton Axis”. However, as the area of reduced sedimentation is centred on Royal Wootton Bassett, Arkell (1933, p. 80) favoured calling the structure the Wootton Bassett Axis, and this is the term, as the Wootton Bassett High, which will be used here. The first major uplift of the high which is evident from the succession occurred prior to deposition of the Newton Clay Member. Some 30 m of predominantly sandy Hazelbury Bryan Formation and probable Kingston Formation was removed in a gently arching uplift (Fig. 3A, B), and a normal sequence of the Newton Clay Member was then laid right across. A rise in relative sea level saw shallow water ooid shoals replaced by deeper water clay seas which regularly cleared sufficiently to allow deposition of argillaceous limestone with the delicate branching coral Thecosmilia. This is seen to south at Hilmarton and to the north at Royal Wootton Bassett. The shelly limestone and oolite of the Calne Freestone was then deposited in the south, extending up to Preston, east of Lyneham, where it wedges out. This may be due to the second uplift which saw the Calne Freestone and Newton Member largely removed, particularly near Tockenham Wick, where at one point Coral Rag rests on Hazelbury Bryan sand (White, 1925). Deposition of the Coral Rag then proceeded normally right across from Lyneham to Royal Wootton Bassett without being interrupted by an area of clay facies sedimentation as postulated by Arkell (1951). The initial rubbly, shelly, argillaceous limestone was succeeded by extremely fossiliferous reefal limestone with massive Thamnasteria colonies. In the Upper Oxfordian, on at least 3 occasions Ampthill Clay sedimentation spread down to the south, but was interrupted by an episode of uplift and erosion and the deposition of shallow water sediments. Thus, though there is a gradual upwards transition from Coral Rag into Ampthill Clay at Royal Wootton Bassett, at Lyneham the basal Lyneham Member rests very erosively on the Stanford Formation. An area of 1–2 sq km south of Lyneham was uplifted and transformed into a shallow rock platform with a gravel beach and low cliffs carved in Coral Rag and Calne Freestone (Fig. 3A). Normally, when such things happened in the Jurassic, there was time for the sea to cut right through, and the subsequent strata to rest on a planar erosion surface cut in the older beds. Rapid subsidence at Lyneham prevented this, and saw the beach gravel coated in limonite
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80
90
00
10
20
9
30
40
50
00
EAST MIDLANDS SHELF
VARISCAN FORELAND AT DEPTH HW 90 PU
C
DI
MIDLANDS MICROCRATON AT DEPTH
SM SW
RWB
O RI
PE
80
LY
C
I SS
WOOTTON BASSETT HIGH
RA
JU
70
N IO S N TE X CA E
CALNE
Fig 1
SUB-BASIN SE
60 SA WE 50
VFT
VARISCAN FOLDBELT AT DEPTH SEEND HIGH
PEWSEY SUB-BASIN
WESSEX BASIN 0
10
20km
40 Fig. 6. Diagram showing the situation of the area in relation to structures in the basement. Faults after BGS (1925, 1974, 2011) and Wright (2014). CA = Calne, HW = Highworth, LY = Lyneham, PU = Purton, RWB = Royal Wootton Bassett, SA = Steeple Ashton, SE = Seend, SM = South Marston, SW = Swindon, WE = Westbury. VFT = Variscan Front Thrust at depth, after Chadwick (1986).
and preserved in sandy marl, passing rapidly upwards into light and dark mottled clay at maximum 4 m thick. It is interesting to note that very similar rolled pebbles of Coral Rag and Calne Freestone, coated with limonite, occur at the base of the Steeple Ashton Coral Bed at Steeple Ashton (Wright, 2016), and one is tempted to correlate the Lyneham Member and the coral bed on this basis. To the northeast at Greenhill Common (Wright, 2003), Swindon (Gallois and Cox, 1994) and South Marston (Wright, 2003), the Bed 2 clay with limonite nodules is overlain by the micritic ironshot oolite of the Marston Ironstone, and then by fine, plastic clay with pyritised ammonites. It seems almost certain that the Marston Ironstone and the pyrite-ammonite clay (Beds 3 and 4) spread south over much of Wiltshire, succeeded by Sandsfoot Grit, but were removed by the widespread erosion beneath the Westbury Ironstone. Frequent eroded clasts of limonitic oomicrite typical of the nodules present in the Marston Ironstone are scattered through the Westbury Ironstone at Lyneham.
The Westbury Ironstone clearly correlates with the sandy Ampthill Clay of Bed 5 in the relief road section, marking the point where the Westbury Ironstone Member passes laterally into the Ampthill Clay Formation. The situation was that the persistently subsiding East Midlands Microcraton was occupied by an area of moderately deep shelf sea across which the Westbury Ironstone was unable to encroach. Subsidence now affected the whole of southern England, and clay facies, as the Ringstead Clay, spread right across to the Dorset coast. Callomon (in Wright, 2003, p. 104) noted that the shelly mudstone of the Inconstans Bed rested erosively on the Ampthill Clay west of Swindon, and that uppermost Oxfordian sediments are absent here. Acknowledgements The kind cooperation of the many farmers over whose land the author has tramped during the course of this survey is gratefully
Please cite this article in press as: J.K. Wright, The Corallian Group (Upper Jurassic) of Wiltshire, England. 3: Lyneham to Royal Wootton Bassett, Proc. Geol. Assoc. (2017), http://dx.doi.org/10.1016/j.pgeola.2017.05.008
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acknowledged. Grateful thanks are due to Professor J.C.W. Cope and other anonymous referees for their close scrutiny of the original manuscript, which has enabled many significant improvements. The figures were drawn by Lynne Blything. References Arkell, W.J., 1927. The Corallian rocks of Oxford, Berkshire and north Wiltshire. Philosophical Transactions of the Royal Society of London B216, 67–181. Arkell, W.J., 1933. The Jurassic System in Great Britain. Clarendon Press, Oxford. Arkell, W.J., 1941. A map of the Corallian beds around Highworth. Proceedings of the Geologists’ Association 52, 79–109. Arkell, W.J., 1951. The geology of the Corallian Ridge near Wootton Bassett and, Lyneham, Wilts. Wiltshire Archaeological and Natural History Magazine 54, 1–18. BGS, 1925 (reprinted 1974). Marlborough. England and Wales Sheet 281. Solid and Drift Geology. 1:63,360. British Geological Survey, Keyworth, Nottingham. BGS, 1974. Swindon. England and Wales Sheet 252. Solid and Drift Geology. 1:63,360. British Geological Survey, Keyworth, Nottingham. BGS, 2011. Bath. England and Wales Sheet 265. Bedrock and Superficial Deposits. 1:50,000. British Geological Survey, Keyworth, Nottingham. Blake, J.F., Hudleston, W.H., 1877. On the Corallian rocks of England. Quarterly Journal of the Geological Society of London 33, 260–405. Bristow, C.R., Barton, C.M., Freshney, E.C., Wood, C.J., Evans, D.J., Cox, B.M., IvimeyCook, H.C., Taylor, R.T., 1995. Geology of the Country Around Shaftesbury. Memoir of the British Geological Survey. HMSO, London xii+182 pp. Bristow, C.R., Gale, I.N., Fellman, E., Cox, B.M. (with Wilkinson, I.P., Riding, J.B. 2000). The lithostratigraphy, biostratigraphy and hydrogeological significance of the mud springs at Templars Firs, Wootton Bassett, Wiltshire. Proceedings of the Geologists’ Association 111, 231–245. Chadwick, R.A., 1986. Extension tectonics in the Wessex Basin, southern England. Journal of the Geological Society of London 143, 465–488.
Gallois, R.W., Cox, B.M., 1994. The Kimmeridge Clay and underlying strata (Upper Jurassic) at Swindon, Wiltshire. Proceedings of the Geologists Association 105, 99–110. Harding, I.C., Armitage, J., Hollingworth, N., Ainsworth, N., 2000. Sourcing mudsprings using integrated palaeontological analyses; an example from Wootton Bassett, Wiltshire, England. Geological Journal 35, 115–132. Kent, P.E., 1980. Subsidence and uplift in East Yorkshire and Lincolnshire: a double inversion. Proceedings of the Yorkshire Geological Society 42, 505–524. Rawson, P.F., Wright, J.K., 2000. Geologists’ Association Guide No 34, The Yorkshire Coast. The Geologists’ Association, London vi+130 pp. Reynolds, S.H., Vaughan, A., 1902. On the Jurassic strata cut through by the South Wales Direct line between Filton and Wooton Basssett. Quarterly Journal of the Geological Society of London 58, 719–752. Sykes, R.M., Callomon, J.H., 1979. The Amoeboceras zonation of the Boreal Upper Oxfordian. Palaeontology 22, 838–903. White, H.J.O., 1925. The Geology of the Country Around Marlborough. Memoir of the Geological Survey of Great Britain. HMSO, London xi+112 pp. Woods, M.A., Chacksfield, B.C., 2012. Revealing deep structural influences on the Upper Cretaceous Chalk of East Anglia (UK) through inter-regional geophysical log correlations. Proceedings of the Geologists’ Association 123, 486–499. Woodward, H.B., 1895. The Jurassic Rocks of Britain, 5. The Middle and Upper Oolitic Rocks of England (Yorkshire Excepted). Memoir of the Geological Survey of Great Britain. HMSO, London xiv+499 pp. Wright, J.K., 2003. New exposures of the Ampthill Clay near Swindon, Wiltshire, and their significance within the succession of Oxfordian/Kimmeridgian boundary beds in southern England. Proceedings of the Geologists’ Association 114, 97–121. Wright, J.K., 2014. A new section through the Corallian Group (Oxfordian, Upper Jurassic) at Calne, Wiltshire, southern England. Proceedings of the Geologists’ Association 125, 83–95. Wright, J.K., 2016. The stratigraphy and geological setting of the Oxfordian Corallian Group around Steeple Ashton and Westbury, Wiltshire, U.K. Proceedings of the Geologists’ Association 127, 266–279.
Please cite this article in press as: J.K. Wright, The Corallian Group (Upper Jurassic) of Wiltshire, England. 3: Lyneham to Royal Wootton Bassett, Proc. Geol. Assoc. (2017), http://dx.doi.org/10.1016/j.pgeola.2017.05.008