The Lower Lias (Lower Jurassic) of the Bridgend area, South Wales

Peter Hodges

HODGES, P. 1986. The Lower Lias (Lower Jurassic) of the Bridgend area , South Wales.Proc. Geol. Ass., 97(3), 237-242 . The Lias in the Bridgend area reveals a facies of marlylimestones and clays in the off-shore Lias and a new locality in the marginal Lias which equateswith the Brocastle Beds of Moore (1867). Re-examination of the marginal Lias along the coastbetween Ogmore -by-Sea and Southern down casts doubt on Tawney's (1866) division of thesedeposits into the Sutton Series and Southerndown Series. A recommendation is made to usethese terms only in a strictly lithological sense , not as a sequence. Recently collectedammonites are used to increase knowledge of the zonal age of the marginal and off-shore Lias.

Department of Geology , University College of Swansea, Singleton Park, Swansea SA2 BPP

3. THE BRIDGEND AREA

The unfossiliferous clays are soft, yellow to lightbrown in colour , and contain a large proportion ofquartz grains under 1 mm in diameter. The marlylimestones are grey, becoming yellow when weath-

During recent excavations at the Ford MotorCompany site (SS 935783) (Fig. 1), sections compris­ing alternations of marly limestones and clays wereexposed. One of these in a drainage channel withinthe northern factory perimeter fence , exposedapproximately 4 m of beds dipping gently eastwardsand comprised :

With the exception of a brief mention by Arkell(1933) little further work was undertaken on theLower Lias until Hallam (1960) carried out asedimentary and faunal analysis of the Lower Lias ofGlamorgan and Dorset. Wobber (1963, 1965, 1966,1968) did a similar study of the South Wales Lias.More recently Beauvais (1976) revised the work ofDuncan on Liassic corals . In her work , Duncan'sstratigraphy of the marginal Lias was used , resultingin a large number of short-lived species given as ofangulata Zone age (as defined by Duncan) . If thework of Moore (1867) is accepted then it is likely thatDuncan 's angulata Zone covers both the bucklandiZone and semicostatum Zone.

0.50m0.15m

0.60mO.15mO.76mO.05m

O.05m

7. Marly limestone, Euagassiceras resupin­atum, Cardinia nilssoni (Koch & Dun­ker), Gryphaea arcuata Lamarck andgastropods .

6. Clay5. Marly limestone, Gryphaea arcuata and

Entolium sp.4. Sandy clay3. Marly limestone2. Clay1. Marly limestone , Euagassiceras resupin­

atum

1. INTRODUCTION

Two distinct facies have long been recognised in theLower Lias of the Bridgend area. One is associatedwith sediments deposited along the marginal areas ofseveral Carboniferous Limestone 'islands' , the otherwith sediments deposited in deeper waters away fromany presumed Liassic land. As it is difficult todetermine accurately sea depths associated with thesefacies, the terms marginal facies and off-shore facieswill be used to define broadly the conditions ofdeposition.

2. HISTORY OF RESEARCH

During the late 19th century numerous geologistsvisisted the Bridgend area to examine the marginalfacies, which Tawney (1866) had divided into theSutton Series and Southerndown Series. Many ofthem differed in their opinion of the age of thesedeposits and much heated debate ensued. A fullsummary of the relevant literature up to the end ofthe nineteenth century was given by Woodward (1893)and mentioned also by Strahan and Cantrill (1904).

Apart from Richardson (1905) little further researchwas undertaken on the Lower Lias of Glamorgan untila series of papers by Trueman (1920, 1922, 1930) inwhich he outlined the stratigraphy of the off-shoreLias, giving detailed sections of both coastal andinland localities, along with a zonal classificationbased on ammonites collected or observed in the field.Trueman agreed with Woodward that there was a lackof any clear separation between the Sutton Stone andSoutherndown Beds. The higher beds of the SuttonStone merged both vertically and laterally into theSoutherndown Beds and the Southemdown Bedsmerged in a similar manner into more normal Liassiclimestones and shales, though this transition was moreabrupt. From his observations on the lithology of themarginal Lias, he offered an explanation of thepossible origins of these deposits . He also constructeda model of the palaeogeography of South Walesduring Lower Liassic times.

237

238 PETER HODGES

_ Rhaetian

------;fD-- - - Newcastle Hill

- - - -.~ - -- - - -. BRIDGEND

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fOff-shoreLower Lias

MarginalTrias

CarboniferousLimestone

r;:::;:;:::::1 Marginal:.;.:.:-:-:.:.: Lower Li as

Fig. 1. Geological Sketch Map, Bridgend Area.

ered. They are biomicritic with a tendency to becomemore marly at the interface with the overlying andunderlying clays. The presence of burrows andaccumulations of shell debris along the surface ofbedding planes indicates extensive reworking of thesediments by marine organisms. This bioturbation hasresulted in the partial destruction of any originalparallel lamination.

The limestones are very fossiliferous and contain arich fauna of silicified gastropods, bivalves andammonites. The ammonites include Arniocerassemicostatum (Young & Bird) and Euagassicerasresupinatum (Simpson) indicating the resupinatumSubzone of the semicostatum Zone. This establishesthese beds as the youngest Sinemurian exposuresrecorded in the Vale of Glamorgan. (see Cope et al.1980, p. 38).

Temporary exposures at SS 919787 and SS 928793revealed beds of identical lithology and age, also atNewcastle Hill (SS 901801) where 3 m of similar, butgenerally unfossiliferous, beds were exposed whichyielded Gryphaea and Pinna. Thus marly limestonesand clays were deposited in early Sinemurian timesover a large area of what is now the town of Bridgend.

Francis (1959) referred to 3 boreholes sunk in theBridgend area. One of these (borehole No. 2 atSS 922787) gave a total thickness of 152m for the Lias

limestones, shales and clays. The borehole started inbeds of the semicostatum Zone and at a depth of125m was in the liasicus Zone. Another account ofexposures in the Bridgend area was given by Moore(1867) in which he described a section in a Bridgendquarry comprising 108 m of limestones and claysranging from the conybeari Subzone to the sauzeanumSubzone. This section, however, included 'bedscurved and faulted' 15 m, and 'disturbed and coveredbeds' 4.6 m. Thus, there is some doubt as to whetherthis was a continuous sequence. Strahan (1904, p. 72)also referred to a statement by Woodward that Moorehad found Ammonites angulatus in this section, whichadds further doubt as to whether the section showed acontinuous sequence.

4. THE MARGINAL LIAS

There has been much argument concerning the age ofthe marginal Lias and its division into the SuttonSeries and Southerndown Series as suggested byTawney (1866). Although this division has since beenaccepted in the geological literature of the SouthWales Lias, several geologists have questioned itsvalidity (see Bristow, 1867; Woodward, 1888, 1893;Wobber, 1963, 1968; Owen & Rhodes, 1969).Differences in lithology can be seen in the marginal

LOWER LIAS IN S. WALES 239

Lias, but no sharp boundary exists to warrant thedivision by Tawney. However, a clear change inlithology does occur vertically and laterally betweenthe top of these beds and the overlying limestones andshales of the Blue Lias.

The Sutton Stone and Southerndown Beds (ofTawney) are diachronous and cannot be separatedpalaeontologically. Vertical changes in both lithologyand colour are often so great that is is impossible tolocate any boundary. I am in agreement with Wobber(1968) that the terms Sutton Stone and SoutherndownBeds should be retained only in a lithological sense todescribe approximate colour and mineralogicaldifferences, and to recommend the abandonment ofthese terms in attempting to define two stratigraphicalunits with independent characteristics.

Inland localities in the marginal Lias of theBridgend area are rare, but one has been found in adisused Carboniferous Limestone quarry (SS 927773)to the east of Bridgend, known as Longlands Quarry.A section at the west end of the quarry exposesmarginal Lias of Sutton Stone lithology containingmany bivalves and coral fragments. The discovery ofthis locality is important as it equates with theBrocastle Beds of Moore, and is located about 0.5 kmfrom Brocastle. From Moore's description I believethat an overgrown depression in a field oppositeLonglands Quarry (SS 929773) could be his locality.

The corals Cyathocoenia dendroidea Duncan andActinastraea gibbosa (Duncan) listed by Moore fromhis Brocastle Beds are identical with those collectedfrom Longlands Quarry. The bivalve Gryphaea isrelatively abundant as are pectinids. Preservation isgenerally poor making it difficult to determine manyof the species. Although no ammonites were found,the presence of Gryphaea is a useful indicator, as inSouth Wales it is generally not found below theangulata Zone. If we accept the view of Moore (1867,p. 524) that ammonites resembling the juveniles ofArnioceras semicostatum were found in his Brocastlebeds, then it is possible that the marginal Lias atLonglands Quarry belongs to the semicostatum Zone.The locality is close to the off-shore Lias describedabove from the Ford Motor Co. site (SS 935783)which is known to be of late semicostatum Zone age.

Another small exposure of marginal Lias of SuttonStone lithology can be seen nearby in it road cuttingon the A48 at Crack Hill (SS 941768) where thegastropods Rimula liasina (Terquem) and Cerithiumpentacostae Moore were collected. Both were listed byMoore (1867) in his Brocastle Beds. Along the coastnear Pant y Slade (SS 872742) the marginal Lias canbe seen resting unconformably on the CarboniferousLimestone and is conglomeratic at its base with clastsof derived Carboniferous Limestone and chert. Thebase of the marginal Lias at this locality is almostcertainly in the planorbis Zone and possibly extendsdown into the upper Rhaetian. Francis (1959, p. 161,

175) actually refers to detrital limestones of SuttonStone lithology in the upper Rhaetian of the Bridgendarea.

(a) Section at Longlands Quarry (SS 927773)

The west side of Longlands Quarry exposes beds ofmarginal Lias, characterised by light grey limestonesof Sutton Stone lithology. These beds dip approxi­mately 40°NW towards Bridgend, resting unconfor­mably on horizontal beds of Carboniferous Lime­stone. The section described below is indicative ofnear-shore high energy conditions exhibited by muchshell and coral debris. Many of the beds containderived Carboniferous Limestone clasts and alsosandstone clasts possibly derived from the localRhaetian. Stylolites-a common feature in beds ofSutton Stone lithology-are seen at the interfacebetween many bedding planes. A section fromLonglands Quarry is shown in Fig. 2 and comprises:

8. Limestone with shell and coral debris,large calcite crystals and cracks withcalcite infill, Atreta sp. 0.30 m

7. Irregular limestone beds with shelldebris, angular and rounded clasts ofCarboniferous Limestone up to 3 erndiameter, rolled clay, calcite crystals. 0.36 m

6. Limestone with shell and coral debris.lower three quarters of bed brecciated

c v vu 01"\ 0 8tJ 0 ')1" c

7.

6.

v 5.OCORAL DEBRIS0

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Fig. 2. Section at Longlands Quarry.

240

5.

4 .

3.

PETER HODGES

2.

1.Fig. 3. Ammonites from the marginal Lias. 1. Psilophyllites cf. hagenowi (Dunker), (NMW.83.22G.140) xz. 2. Calocerasbloomfieldense Donovan, (NMW.83.22G .141) x l. 3. Caloceras bloomfieldense Donovan, (NMW.83.22G.142) xt. 4.Caloceras d. bloomfieldense Donovan, (NMW.83.22G.143) x2. 5. Schlotheimia angulata (Schlotheim), (NMW.83.22G.144)xl.

LOWER LIAS IN S. WALES 241

1. Psilophyllites d. hagenowi (Dunker),NMW 83.22G.140 (Xl)Collected in situ at the base of the cliff path on thesouth side of Trwyn y Witch, (SS 888726), in beds ofSoutherndown Beds lithology, marking the top bed ofthe marginal Lias. Psilophyllites hagenowi has alimited range and is most abundant in the portloeki

(b) Ammonites recorded from the marginal Lias

Although the marginal Lias has yielded many bivalvesand gastropods, ammonites rarely occur. However, afew records have been made during the last centurywhich may be added to here.

The first record of ammonites from the marginalLias was made by Tawney (1866), Ammonitessuttonensis Tawney (considered to be allied to what isnow Caloceras johnstoni (Sowerbyj) was recorded 6 mabove the base of the marginal Lias both at Dunraven(Trwyn y Witch) and at Sutton Quarry, andAmmonites dunravenensis Tawney, was recorded9.1 m above the base of the marginal Lias atDunraven (Trwyn y Witch). Tawney considered thelatter species different from Ammonites suttonensis,but his illustration (pI. 4, fig. 1) has insufficient detailto enable a positive identification.

Moore (1867) recorded juvenile specimens ofArnioceras semieostatum in the marginal Lias atBrocastle near Bridgend. He also recorded Ammonitesangulatus from the marginal Lias in beds of SuttonStone lithology and in beds of Southerndown Bedslithology at Dunraven. Another record of anammonite was made at approximately 5.6 m above thebase of the marginal Lias.

Trueman (1922) recorded Sehlotheimia d. thalassiea(Quenstedt) (considered a synonym of Sehlotheimiaangulata (Schlotheim) by Lange (1951», about 3 mabove the base of the marginal Lias at Pant y Slade.In addition to the above, the following specimenswere collected by the author. These are shown in Fig.3 and are deposited in the Department of Geology,National Museum of Wales.

with derived Carboniferous Limestoneclasts.

5. Fairly massive limestone with someshell and coral debris.

4. Irregular limestone beds with shell andcoral debris, calcite crystals, Gryphaeaand Chlamys.

3. Limestone densely packed with shelland coral debris, sandstone clasts,rolled clay, calcite crystals.

2. Limestone with some shell debris,galena crystals

1. Massive limestone sparsely populatedwith shell debris and CarboniferousLimestone clasts, calcite crystals.

0.16m

0.40m

0.60m

0.45m

0.30m

0.60m

Subzone of the liasieus Zone, in northwest Europe(see Dean et al., 1961).

2. Caloeeras bloomfieldense Donovan,NMW 83.22G.141 (Xl)Collected in situ, south side of Trwyn y Witch(SS 888726), 0.5 m above the last specimen, in theBlue Lias. Caloeeras bloomfieldense is known to occurin the laqueus Subzone of the Bristol area (seeDonovan, 1952).

3. Caloeeras bloomfieldense Donovan,NMW 83.22G.142 (Xl)Collected in situ, 1 m below the top of Black Rocksquarry (SS 869742), in beds of Sutton Stone lithology.The specimen is a xenomorphic impression of anammonite on the attachment area of the oysterTerquemia. This indicates that the Sutton Stone at thislocality belongs to the laqueus Subzone of the liasieusZone.

4. Caloeerasef bloomfieldense Donovan,NMW 83.22G.143 (X2)Collected in situ in the same bed as the last specimen.

5. Schlotheimia angulata (Schlotheim),NMW83.22G.144 (Xl)This specimen was found in material on the floor ofBlack Rocks quarry (SS 869742), and has probably

ZONES SUBZONES BRIDGE NO AREA

MicroderocerasCaenisites hirchiturneri C. brooki 7 7 ? ?

z Euagassiceras

\III< >-- resupinatum '"0:: III U

::> Arnioceras Agassiceras 01 -0J: c:semicostatum scioionianum 0 c:

UJ ?c.nti '"z Coroniceras III

V'l reynesi /<0101

~~c:s0:: A. bucklandi --'-0 III III

UJ7 < c: 01 01

~ E -o Arietites Coroni ceras z'" '"\GOI - .c--' bucklandi rotiforme >- III

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... -0

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S.complanata

Ic, UJ 01

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0-z angulata S.extranodosa u :I: III< V'l 01- .. 7 I E<:l \

u, .-~ Alsatites

A.laqueus u, --'0

I-liasicus W~,ehneroc,rasI- ortlock IUJ

:I:..

CaiocerasPsiloceras -johnstoni

/planorbis P. planorbi s ...0N

? ~Pre-p~nOrbi s beds001

RHAETIAN..-

\Penarth group...

Q.

Fig. 4. Stratigraphical Column, Bridgend Lias.

242 PETER HODGES

fallen from the s lopes above the quarry, higher in thesuccession . The matrix around the specimen is ofSutton Stone lithology.

A stratigraphical column of the Bridgend Lias isshown in Fig. 4.

ACKNOWLEDGEMENTS

I am indebted to Dr. J. C. W. Cope and Dr. A. T . S.Ramsay for the ir help and constructive criticismduring the preparation of th e manuscript , also to Dr.

M . G . Bassett for his editorial comments. I would lik eto th ank Mrs . C. Bryant for help in the preparation ofphotographs and Mrs. V. M . Jenkins for preparat ionof the typescript.

Thanks are also due to Dr. H. C. Ivimey-Cookwho , in consultation with Professor D . T. D onov an ,helped identify the am monite s, and to th e Ford MotorCompany for access to their site during constructi on .Part o f the research was supported by an awardreceived by the author from the Sylvester-BradleyFund of the Palaeontological A sso ciation .

References

ARKELL, W. J. 1933. The Jurassic System in Great Britain.Clarendon Press, Oxford.

BEAUVAIS, L. 1976. Revision des Madreporaires Liasiquesdecrites par Duncan (1867). Mem. Soc. geol. Fr. (n.s.)126,44-78.

BRISTOW, H. W. 1867. On the Lower Lias orLias-Congolomerate of a part of Glamorganshire . Q. J.geol. Soc. London, 23, 199-207.

COPE, J . C. W. et al. 1980. A correlation of Jurassic rocksin the British Isles, Part One: Introducti on and LowerJur assic. Spec. Rep. geol. Soc. London. 14.

DEAN, W. T., D. T. DO NOVA N & M. K. HOWARTH.1961. The Liassic Ammonit e Zones and Subzones of theNorth -West European Province . Bull. Br. Mus. nat. Hist.Ser. Geol. , 4, 435- 505.

DONOVAN, D. T. 1952. The Ammonites of the Blue Liasof the Bristol district. 1. Psilocer atidae and Schlotheimi­idae. Ann. Mag. nat. Hist . London, 5, 629-55.

FRANCIS, E. H. 1959. The Rhaetic of the BridgendDistrict , Glamorganshire. Proc. Geol. Ass. 70, 158-78.

HALLAM, A. 1960. A sedimentary and faunal study of theBlue Lias of Dorset and Glamorgan . Phil. Trans. R. Soc.London , 8.243, 1-44.

MOORE, C. 1867. On abnormal conditions of secondarydeposits when connected with the Somersetshire and SouthWales Coal-Basins, and on the age of the Sutton andSoutherndown series . Q. J. geol. Soc. London, 23,449-568.

OWEN, T. R. & F. H. T. RHODES. 1969. Geology aroundthe University Towns: Swansea, S. Wales. Geol. Assoc.

Guides. No. 17, 32- 45.RICHARDSON, L. 1905. The Rhaetic and contiguous

deposits of Glamorganshire. Q. J. geol. Soc. London . 61,385-424.

STRAHAN, A. & T. C. CANTRILL. 1904. The geology ofthe S. Wales Coalfield. Part 6, The country aroundBridgend. Mem . Geol. Surv. G.B.

TAWNEY, E. B. 1866. On the Western limit of the Rhaeticbeds in South Wales and on the position of the SuttonStone. Q. J. geol. Soc. London , 22, 68-93.

TR UEMAN, A . E . 1920. The Liassic rocks of the Cardiffdistrict. Proc. Geol. Ass . 31, 93-107.

- - 1922. The Liassic rocks of Glamorgan. Prac. Geol. Ass.33, 245-84.

-- 1930. The Lower Lias (Bucklandi Zone ) of Nash Point ,Glamorgan. Proc. Geol. Ass. 41, 148-59.

WOODWARD, H. B. 1888. Notes on the Rhaetic beds andLias of Glamorganshire. Proc. Geol. Ass. 10, 529-38.

- - 1893. Jurassic rocks of Great Britain (Yorks . excepted),vol. 3. The Lias of England and Wales. Mem. Geol. Surv.G.B.

WOBBE R, F. J. 1963. The Stratigraphy and Palaeontologyof the Liassic Rocks of Glamorgan. Unpublished Ph.D.Thesis, Univ. Wales.

-- 1965. Sedimentology of the Lias (Lower Jura ssic) of S.Wales. J. Sed. Petr., 35, 683- 703.

-- 1966. A study of the depositional area of theGlamorgan Lias. Proc. Geol. Ass. 77, 127- 37.

-- 1968. Microsedimentary analysis of the Lias in SouthWales. Sediment. Geol., 2, 13-49.