the mid-dinantian stratigraphy of a portion of central pembrokeshire

The Mid-Dinantian Stratigraphy ofa Portion of Central Pembrokeshire

by RAYMOND SULLIVAN

Received 30 November 1964; taken as read 6 November 1965

CONTENTS

1. INTRODUCTION2. THE SEQUENCE IN THE WEST WILLIAMSTON-PEMBROKE DOCK OUTCROP3. THE SEQUENCE IN THE HAVERFORDWEST-PENDINE OUTCROP4. CONCLUSIONS

ACKNOWLEDGMENTSREFERENCES

page 283284292297298299

ABSTRACT: The mid-Dinantian movements, whose importance have been demonstrated farther east in South Wales, are shown to be of greater magnitude in centralPembrokeshire than hitherto supposed. The trend of the shoreline of St. George's Landappears to have been controlled by movements along the locally developed Ritec fault.

The Ritec fault was a barrier to sedimentation in mid-Dinantian times, causingbreaks in sedimentation and the development of widespread coastal fiats. The UpperCaninia Zone, represented by 350 feet of bioclastic limestones at Tenby, south of thefault, is represented by only 0 to 36 feet of algal-rich limestones and limestone conglomerates (bounded above and below by unconformities) in central Pembrokeshire.

1. INTRODUCTIONTHE PRESENT ACCOUNT deals with the mid-Dinantian rocks of centralPembrokeshire described previously by Dixon (in Strahan et al., 1914,126-48). The area lies astride the Pembrokeshire coalfield. The Dinantianrocks outcrop between Haverfordwest and Pendine along the northernmargins of the coalfield and between Pembroke Dock and West Williamston along the Sageston anticline south of the coalfield. The outcrops areterminated southward along the line of the Ritec fault. South of the area,and not included in the present account, the Dinantian rocks, as Dixon(1921,64-144) fully showed, are preserved in a number of isolated synclinesthat occupy much of southern Pembrokeshire (Fig. 1). Dixon's classicsynthesis resulted in a considerable clarification of the Dinantian stratigraphy in the South-Western Province. It was shown that the Dinantianrocks of Pembrokeshire were laid down in east-west facies belts on adepositional shelf south of the landmass of St. George's land. Northwardthe Dinantian zones thinned, partly because of the increased effects of themid-Dinantian movements and partly as a result of the nearness of thefluctuating shoreline of the landmass. The developments at Haverfordwest-Pendine and West Williamston-Pembroke Dock outcrops werecompared with that of the North Crop of the South Wales coalfield, while

283

284

Scale In Miles

RAYMOND SULLIVAN

~ CARBONIFEROUS LIMESTONE

Fig. I. Outline map showing the outcrops of the Carboniferous Limestone in Pembrokeshire

that at Tenby corresponded to the south-east Gower and Bristol, and thePembroke area to the south-west Gower. The Bosherston outcrop wasunlike any other succession in the South-Western Province and a linkwith Belgium and Ireland was postulated.

The purpose of the present study was to re-examine the effects of themid-Dinantian movements in central Pembrokeshire in the light ofrecent work in areas farther east. In Breconshire, George (1954, 283-322and 1956,309-22) has shown that the mid-Dinantian movements consistedof two phases represented in the succession by the sub- and intra-Viseanunconformities. The two unconformities have been recognised in centralPembrokeshire and demonstrate that the mid-Dinantian movements are ofgreater magnitude than hitherto supposed. As a result, the study providesa close analogy between eastern South Wales and Pembroke shire and thussimplifies and unifies lithological contrasts that extend over the greaterpart of the southern margins of St. George's Land.

2. THE SEQUENCE IN THE WEST WILLIAMSTONPEMBROKE DOCK OUTCROP

Dixon (1921, 137-42, 147-8) considered the Dinantian succession in theWest Williamston-Pembroke Dock outcrop to be a conformable butattenuated sequence, though locally, at West Williamston, he recordeda minor break at the base of the Upper Caninia Zone. Since dolomitisationhas obliterated most of the fossils in the lower part of the Dinantian and

MID-DINANTIAN ROCKS OF PEMBROKESHIRE 285

made the establishment offaunal subdivision difficult, Dixon (1921, 137-9),in describing the succession, referred the 400 to 500 feet of the TournaisianMain Limestone collectively to 'ZCl'. Dixon was also unable to separatethe thick development of oolites in the Upper Caninia Zone from theoolites in the overlying Seminula Zone.

It now appears that Dixon misinterpreted the sequence, for the UpperCaninia Zone is thinly represented by five to thirty-six feet of 'lagoon phase'deposits (Calcite Mudstone Group) bounded above and below by unconformities, and the general stratigraphy is similar to that described byGeorge (1954, 283-322; 1956,309-22) on the North Crop of Breconshire.The overlying oolitic limestones are wholly assigned to the Seminula Zone.

(a) Tournaisian Main Limestone

(i) Lateral Variation in the Succession. The Tournaisian Main Limestoneis divisible into a thin upper oolitic limestone unit (the Caninia Oolite) anda thicker lower bioclastic limestone unit. Dolomite (Laminosa Dolomite),clearly of secondary origin, locally makes up a considerable part of thesuccession.

The rocks are best exposed near Carew Castle and along the foreshoreof Carew River (Fig. 2), where the sequence is as follows:

Thicknessin feet

5. Well-bedded, light-grey, oolitic limestone: the Caninia Ooliteof Dixon ... 40

4. Well-bedded, light-grey, bioclasticlimestones rich in fossils ... SO3. Thin-bedded,dark bioclasticlimestones with thin interbedded

calcareous shales 602. Dark-grey,finely crystallinedolomite: the LaminosaDolomite

(beds poorly exposed between Carew Mill and Ford Pointalong the Carew River). 200 (approx.)

1. Thin-bedded, dark-grey, bioclasticlimestones and thin interbedded calcareous shales. (The base is hidden beneath theestuary but presumably rests conformably on Lower Lime-stone Shale.) 50 (seen)

Only the upper part (units 3 and 4) of the Tournaisian Main Limestoneat Carew can be traced northward along the shore of the Carew River toWest Williamston (Fig. 3). The lower part of the succession is hiddenbeneath the estuary. The beds remain constant in thickness and lithology,forty feet of oolitic limestone underlain by eighty feet of light-grey bioclastic limestone being exposed at Point Quarry and the remaining quarriesalong the shore of the Carew River at West Williamston. The top of theCaninia Oolite is pocketed and offers lithological evidence of a midDinantian erosion.

286 RAYMOND SULLIVAN

Fig. 2. Map showing the geology of the West Williamston area

When the Tournaisian Main Limestone is traced westwards from Carewto Pembroke Dock, the proportion of dolomite in the succession increasesand the units defined at Carew are not always recognisable. At Milton,sixty-fivefeet of Caninia Oolite, with thin bioclastic beds in the lower part,is underlain directly by dolomites which continue to the base of theToumaisian Main Limestone (Fig. 3). Farther west, the sequence is rarelyexposed in small quarries along the Cosheston Pill. There too, the beds are


greatly affected by dolomitisation, and at Bangeston, only twenty feet ofCaninia Oolite is preserved, the remainder of the Toumaisian Main Limestone being completely dolomitised. In the extreme west of the outcrop,south of Carr's Rock, Pembroke Dock, the proportion of dolomitedecreases and 200 feet of bioclastic limestone compose the lower Tournaisian Main Limestone where it overlies the Lower Limestone Shale alongthe shore, the upper part being unexposed.

WESTWILLIAMSTON

CAREW MILTON BANGESTON

TOURNAI SIAN MAIN

LIMESTONE

Ifeet

o

SO

100

---------------

~OQrite

ES:5:I Dark bioclastic;~ limestone

Fig. 3. Comparative sections of the Tournaisian Main Limestone between WestWilliamston and Pembroke Dock


(ii) Lithology of the Tournaisian Main Limestone. The Caninia Oolite iscomposed of washed and sorted, uniform sized, ooliths. Many are irregular,their form controlled by the detrital nuclei, which are commonly crinoidfragments, Foraminifera and shell fragments, and around which calcite isconcentrically layered. The ooliths are set in a lighter-coloured groundmass of clear, recrystallised calcite.

The bioclastic limestones are generally dark-coloured, thin-bedded,poorly sorted calcarenites with interbedded calcareous shales. They areclosely comparable to the 'zaphrentid-phase' limestones of southern outcrops. The clastic fragments are derived from crinoids, brachiopods,bryozoans, Algae and Foraminifera, and set in a fine-grained argillaceousmatrix. The limestones show much evidence of flushing, shell-banking andcurrent variability. The interbedded shales reflect periodic increases in theinflux of mud into the region; the terriginuous mud was probably deriveddirectly from the northern landmass of St. George's Land. Upwards in thesuccession the limestones become lighter in colour with a decrease in theargillaceous content, the rocks become well sorted and the interbeddedshales are absent. Rare ooliths become scattered through the limestones,which passes quickly, in upward sequence, first into shelly oolitic limestoneand finally into oolitic limestone proper tCaninia Oolite). The transitionis rapid and takes place in ten feet of the sequence. The limestones exhibitvarying degrees of dolomitisation, the original texture being replaced byfine granular dolomite (Laminosa Dolomite).

(iii) Fauna of the Tournaisian Main Limestone. Fossils are commonthroughout the bioclastic limestones and include the following species:

Avonia bassa (Vaughan) Rhipidomella michelini (Leveille)Buxtonis scrabicula Rugosochonetes cf. hardrensis

(Martin) (Phillips)Camarotoechia mitcheldeanensis Schellwienella crenistria

(Vaughan) (Phillips)Cleiothyridinia roissyi 'Spirifert tornacensis

(Leveille) de KoninckDictyoclostus vaughani Syringothyris cuspidata

(Muir-Wood) (Martin)Leptaena analoga Tylothyris cr. laminosa

(Phillips) (McCoy)Megachonetes sp, (broad papilionaceous forms)

Tabulate corals are rare but include Michelinia sp. and Syringopora sp.;rugose corals are represented only by zaphrentids. Foraminifera arecommon and include earlandiids, plectogyrids and tournayellinids.

In the upper part of the succession there is a significant dearth ofcaniniids corals, which are characteristic of upper Tournaisian MainLimestone (Lower Caninia Zone) in other parts of South-Western Province.Upper Tournaisian Main Limestone is indicated by the presence, in the


Caninia Oolite, of Globovalvulina bristolensis (Reichel), which Dr. R. H.Cummings (personal communication) indicates is also found in theCaninia Oolite in the Avon Gorge and is recorded from the Caninia Ooliteby the author in the Pembroke Syncline. It would appear, therefore, thatthe greater part of the Toumaisian Main Limestone is present below thesub-Visean unconformity in the West Williamston-Pembroke Dockoutcrops.

(b) Lower Visean Main Limestone

(i) Laterial Variation in the Rocks of the Upper Caninia Zone. TheCalcite Mudstone Group is well displayed at West Williamston in theentrance docks of Point, Tilling Barn, Prinkly and New Dock Quarries onthe east bank of the Carew River (Fig. 2). The group maintains a fairlyconstant thickness of ten to twelve feet. The following sequence is exposedin the New Dock Entrance:

3. Well-bedded, dark oolitic limestones, with thin shales in theupper few feet. This unit marks the base of the SeminulaZone

2. Calcite Mudstone Group(iii) Rubbly bedded, calcite mudstones with thin ribs of

oolite limestonesrarely exceeding an inch in thickness(ii) Evenbedded,partly dolomitised calcitemudstone(i) Conglomerate-made up of rounded boulders of

oolitic limestones derived from the underlying oolitesand enclosedin a matrixof yellowmudstone.This is thebase of the Upper Caninia Zone

1. Caninia Oolite-white weatheringooliticlimestone ...

Thicknessin feet

45 (seen)

24

540

The eroded surface of the Caninia Oolite marks the position of the subVisean unconformity. The conglomerate fills the irregular surface andvaries from four to ten feet in thickness.

Although the Calcite Mudstone Group retains its general lithology alongmost of the outcrop, there is considerable variation in detail and individualbeds are not readily followed for any great distance. When the group istraced north and south from West Williamston, evidence of erosion at thebase disappears. At Garron Pill to the north, twelve feet of the CalciteMudstone Group is poorly exposed in the banks of the estuary. TheCaninia Oolite has an irregular top, but the conglomerate is absent at thebase of the Upper Caninia Zone.

Traced south and west from West Williamston, the group is poorlyexposed in small quarries and along the estuarine flats of the Carew Riverand Cosheston Pill. There is a variation in thickness, however, in thisdirection, the Calcite Mudstone Group being five feet thick at Carew,sixteen feet at Milton along Radford Pill, twenty-five feet at Lower Nash


and thirty-six feet at Bangeston along Cosheston Pill (Fig. 4). There is,therefore, a westward thickening from West Williamston. The conglomerate at the base of the group is absent, the Calcite Mudstone Groupeverywhere lying abruptly on an even surface of Caninia Oolite.

(ii) Lithology of the Calcite Mudstone Group. The rocks of the CalciteMudstone Group belong to a 'lagoon phase' in the sense of Dixon (1921,72-3). They are closely comparable in lithology to the 'lagoon phase'deposits at the base of the Visean elsewhere in the South-Western Provinceand particularly with the Calcite Mudstone Group of this age described byGeorge (1954, 283-322) in Breconshire.

The sequence is composed of thin-bedded calcite mudstones, extremelyfine-grained and often highly dolomitised, with shaly interbeds. Locally abasal conglomerate is developed, as, for example, at West Williamston.The calcite mudstones are composed of extremely fine amorphous calcitemud, generally structureless, but in part showing mottling that may be ofalgal origin. The algal origin for these rocks is further supported byoccasional algal threads scattered through the rock. Commonly there is ahigh proportion of organic detritus, which, locally as at Bangeston, forexample, produces thin bioclastic limestones in the calcite mudstones.Particularly abundant are ostracods, calcispheres and crinoid fragments,the latter only found in more bioclastic and oolitic layers.

Occasional thin oolites and pellet rocks are also interbedded in thecalcite mudstones though they are not so common as in the Breconshiresequence. They are once more dark, very mixed rocks, rarely clean and wellsorted, and differ from those of the Caninia Oolite. In these rocks, ooliths,with a thin outer coating of calcite mud, are unevenly distributed in amatrix of extremely fine calcite mud. The ooliths commonly show bothradial and concentric structures, and many are composite, the oolithicstructure interrupted with thin shells of calcite mud. The cores are commonly crinoid and shell fragments or rounded calcite mud pellets. Thecalcite mud matrix is commonly highly recrystallised and rarely showsalgal mottling. The pellet rocks which occur in the Calcite MudstoneGroup are dominantly composed of uniform pellets of fine, sub-opaque,structureless, calcite mud and are probably of faecal origin.

(iii) Age of the Calcite Mudstone Group. The Tournaisian-Viseanrelationship along the North Crop of Breconshire leaves little doubt thatthe Calcite Mudstone group of the West Williamston-Pembroke Dockoutcrop belongs to the Upper Caninia Zone. In both areas, the groups arevery similar in lithology and fauna and both unconformably overlie theTournaisian Main Limestone and in turn are unconformably overlain bySeminulan limestones.

Fossils are rare in the Calcite Mudstone group except for spirobids,ostracods, calcispheres, algae, and occasional small lamellibranchs and

mC.le;" . udll O..

~....t:l,t:l....Z>Z>-l....>Z:::coo~en

o"rl

'"ttl~tl:l:::co~ttlen:I::....:::cttl

8

BANGESTONLOWERNASH

76

MILTON

CO", IO••'OIIi

No4u lor olqo lli.utollla

5

~

~

51 Shale. oftd Il'l orl

CAREW

10

,'f• • '

CoUtIi Qlldpde t rock

Hou iu oolites

NEWDOCK

4

~~

llim

UPPER

ONIN IA

ZONE

--- ---

TYLLINBARN

:5

POINTPARK

2

GARRONPILL

I

Fig. 4. Comparative sections of the Upper Caninia Zone in the West Williamston-Pembroke Dock outcrop between GarronPill and Bangeston

IV\0...


brachiopods. The algae include Girvanella sp., Ortonella sp., spongiostromids, and dasycladaceans. Brachiopods include Composita sp., Linoproducts sp. and smalI ryhnchonelIids, these being particularly common inthe bioclastic ribs at Bangeston.

The fauna of the oolitic limestones overlying the Calcite MudstoneGroup, the basis for assigning them to the Upper Caninia Zone (Dixon,1921,137-42,147-8), establish not an Upper Caninia Zone but a Seminulan(S2) age for these beds. The junction with the Calcite Mudstone Group iseverywhere abrupt and points to a break in sedimentation (the intraVisean unconformity). The fauna includes numerous brachiopods aslisted below:

Athyrids undifferentiatedComposita ficoides

(Vaughan)Davidsonina carbonaria

(McCoy)Dielasma sp.Gigantoproductus giganteus (Martin)

Linoproductus corrugatohemisphericus (Vaughan)

Linoproducus hemisphericus(Sowerby)

Productus garwoodi(Muir-Wood)

rhynchonellidsundifferentiated

Tabulate corals are often common. Rugose corals are locally abundant,particularly Carcinophyllum sp., Lithostrotion affine (Fleming), Lithostrotion martini Edward and Haime, and Lithostrotion pauciradiale (McCoy).Foraminifera are abundant and confirm a Seminula Zone age for theoolitic limestone. Endothyrids in particular are common, together witharchaeodiscids, tetrataxids and lituotubellids.

The development of Seminulan limestone in the West WilIiamstonPembroke Dock outcrop confirms that the zone is the most constant andmost widespread of alI Dinantian zones in Pembrokeshire or South Walesor South-Western Province. The two breaks demonstrated by George(1954,283-322 and 1956, 309-22) in Breconshire are thus shown to extendinto Pembrokeshire.

3. THE SEQUENCE IN THE HAVERFORDWESTPENDINE OUTCROP

Dixon's (1914, 129-35) description of the relationship of the Dinantianrocks along the North Crop of Pembrokeshire requires some emendationin so far as zonal sequence and zonal discontinuities are concerned. It wasshown that emergence in mid-Dinantian times had removed much of theTournaisian Main Limestone along the Haverfordwest-Pendine outcropexcept for a thin lens of zaphrentidian limestone present beneath the Viseanlimestone in the extreme east of the outcrop. The overlying Visean rockspresent above the mid-Dinantian unconformity were thought to have been


deposited on an uneven floor. In some places, beds of the Upper CaniniaZone were thought to have been thickly developed; in others, absent.

The revised interpretation, first proposed by O. T. Jones in the discussionof George (1954, 317), is that the Upper Caninia Zone is represented onlyin the extreme east of the outcrop, and once more the succession is thin,represented by 'lagoon-phase' deposits and bounded above and below byunconformities. Westward, it is seen that the Upper Caninia Zone is overlapped by the Seminulan limestones. Moreover, the beds assigned byDixon to the Upper Caninia Zone in the area west of Castle Ely are shownto be of Seminula Zone age.

(a) Tournaisian Main Limestone

(i) Lateral Variation in the Succession. The lower Tournaisian MainLimestone preserved below the sub-Visean unconformity is best exposed inthe cliffs at Dolwen Point, Pendine, and in Cloygen Quarry, a few miles tothe east. The thickness of Tournaisian Main Limestone preserved amountsto only seventy-two feet, the upper sixty feet being light-grey, evenlybedded, ooliticlimestone (Oolite Group), the lower twelve feet thin-bedded,dark, crinoidal limestone and dolomites. The base of the succession ishidden beneath the sands at Pendine, but the Lower Limestone Shales cropout close by.

The Oolite Group is similar in lithology to the oolitic limestone developedin the lower Tournaisian Main Limestone below the sub-Visean unconformity in Breconshire. Like the Breconshire rocks, they show cleanness ofgrain, even size of ooliths, and the presence of rare beds of bioclastic andpellet limestone in succession. In the Pembrokeshire sequence, the oolitescommonly are highly recrystallised.

There is an abrupt junction at the base of the Oolite Group at Pendine,the contract with the underlying bioclastic limestones being irregular, andthe irregular surface is filled by conglomerates composed of roundedpebbles oflimestones in a matrix of yellow mudstones. The beds below thejunction are dolomitised to a depth of a few feet.

Westwards from Pendine, the Tournaisian Main Limestone is overstepped by Visean limestone. Despite the fact that the Oolite Group ispoorly exposed, it can be traced westwards inland from Pendine; and itappears near Greenbridge, just over a mile north-east of Pendine, where itsthickness is little more than twenty feet. The group may be very thinlyrepresented in a section exposed in a stream half a mile north-east ofMarros Church, a short distance north-west of Greenbridge, but atGellihalog, five miles or so north-west of Pendine, the Tournaisian MainLimestone is completely over-stepped by the Visean limestones. Theworking quarry at Gellihalog exposes Seminulan limestones resting on


haematitised crinoidal limestones of the Lower Limestone Shales. Stillfather west, the Visean rocks completely overstep the Lower LimestoneShales and rest on Lower Old Red Sandstone beyond Templeton, andultimately on to Silurian near Haverfordwest.

(ii) Age ofthe Oolite Group. The Oolite Group and underlying crinoidallimestones contain a rich brachiopod fauna, which, together with stratigraphical position, indicates a lower Toumaisian Main Limestone age forthese beds. They are probably to be correlated with the Oolite Group ofBreconshire which George (1954,283-322) has assigned to the ZaphrentisZone.

The common brachiopods are as follows:

Avonia bassa(Vaughan)

Camarotoechia mitcheldeanensisVaughan

Cleiothyridinia roissyi(Leveille)

Dictyoclostus vaughani(Muir-Wood)

Krotovia spinu/osa(Sowerby)

Leptaena analoga(Phillips)

athyrids indet.orthetetids indet.pustulids indet,

Reticu/aria sp.Rhipidome/la michelini

(Leveille)Rugosochonetes cf. hardrensis

(Phillips)Sche/lwienella crenistria

(Phillips)Schizophoria resupinata

(Martin)'Spirifer' cf. tornacensis

de KoninckSpiriferellina octoplicata

(Sowerby)Syringothyris cuspidata

(Martin)

Corals are rare, and the rugosans represented by occasional zaphrentoids.A lower Toumaisian Main Limestone age for the Oolite Group is supported also by the Foraminifera, among which there is a great abundance oftoumayellinids and plectogyrids.

Evidence of erosion at the base of the Oolite Group at Pendine appearsonly local and the erosional break is of no great magnitude. There is noevidence of unconformity at this horizon elsewhere along the North Cropor elsewhere in Pembrokeshire.

(b) Lower Visean Main Limestone

(i) Lateral Variation in the Lower Visean Succession. The sequenceexposed at Pendine spectacularly illustrates the Visean unconformitiespresent along the North Crop in Pembrokeshire. In the cliffs at DolwenPoint, the Upper Caninia Zone, represented by the Pendine Conglomerate,may be shown bounded above by the intra-Visean unconformity and belowby the sub-Visean unconformity. The succession exposed is as follows:


4. Well-bedded, dark-grey, Seminulan oolites and algal limestones making up the greater part of the Seminula Zone andexposed for a distance in the cliffs

3. Well-bedded, dark-grey,algal limestone, nodular in the lowertwo feet, marking the base of the Seminula Zone. '"

2. Pendine Conglomerate(iii) Dark-coloured conglomeratecontaining a large variety

of pebbles set in a dark matrix ...(ii) Thin-bedded,calcite mudstones, in part highlydolomi

tised, and with thick, interbedded, yellow mudstones(i) Conglomerate, consisting mainly of boulders and

pebbles of oolitic limestone filling the irregularities inthe top of the underlyingTournaisian Main LimestoneThis is the base of the Upper Caninia Zone ...

1. Well-bedded, recrystallised oolitic limestone-the OoliticGroup of the Tournaisian Main Limestone ...

Thicknessin feet

350 (seen)

12

20t

7

7-8

58

The top of the Oolite Group is seen to be pocketed for several feet andthe depressions filled with boulders derived from the underlying beds. Theboulders are large, rounded, and set in a clay matrix. The bedded calcitemudstones in the middle of the Pendine Conglomerate (unit 2(ii» lie on theirregular surface of the underlying conglomerate and are steeply inclinedwhen compared with the gently dipping Seminulan limestones andTournaisian limestone above and below. Lying irregularly above the calcitemudstone is a dark conglomerate (unit 2(iii», which is poorly sorted andcontains a great variety of boulders and pebbles.

The contact of the Pendine Conglomerate and the overlying Seminulanlimestones is abrupt, the dark conglomerate (bed 2(iii» being overlain byrubbly, dark, algal limestones, which become regularly bedded upward inthe succession. The algal limestones in turn are overlain by oolitic limestones with some algal beds and shale intercalations.

The Pendine Conglomerate thins rapidly in the cliffs at Dolwen Point.Within 100yards to the east, it is rapidly reduced to between two and threefeet of dark-coloured conglomerates. It would appear that, at Pendine, theconglomerate is filling a marked hollow in the irregular top of the Tournaisian Main Limestone.

The Pendine Conglomerate is also thinly represented at Cloygen Quarry,three and a half miles north-east of Pendine. There it consists of six toseven feet of conglomerate, similar to the lower conglomerate (bed 2(i» atPendine, being composed of boulders derived from the underlying OoliteGroup. It fills irregularities in the top of the Tournaisian Main Limestone.

Similarly, the Pendine Conglomerate is only thinly represented northwest of Pendine. Near Greenbridge, just over a mile north-west, it isrepresented by two to three feet of conglomerates (similar to bed 2(iii»

PROC. GEOL. ASSOC., VOL. 76, PART 3, 1965 20


directly overlying the Oolite Group with an irregular contact, and in tumabruptly overlain by dark coloured, Seminulan limestones.

Father west from Greenbridge, the Seminula Zone oversteps the PendineConglomerate to transgress ultimately the whole of the Tournaisian succession. In a stream section a half-mile north-west of Greenbridge, thePendine Conglomerate is reported to be absent by Dixon (1921 131),Seminulan limestones resting directly on the Oolite Group. A similarrelationship would appear to be present at Blaencilcoed, farther west,where Seminulan limestones rest directly on iron-stained crinoidal limestone beds in the Lower Limestone Shales.

(ii) Lithology of the Pendine Conglomerate. The calcite mudstones of thePendine Conglomerate are similar to the basal Visean calcite mudstones atWest Williamston. They consist of fine calcite mud, in part recrystallisedand commonly mottled in appearance, though no identifiable algal tissuehas been observed. They also include pellet rocks with abundant ostracodsand calcispheres.

The boulders in the lower conglomerate at Pendine are all derived fromthe underlying Oolite Group. The upper conglomerate is more varied; itcontains a variety of pebbles including iron-stained crinoidallimestones,bioclastic limestones, pellet rocks and oolites. The pebbles are set in a darksandy martix, containing angular quartz grains, and heavy minerals including zircon, rutile, garnet and tourmaline. The heavy minerals wereprobably derived from the Old Red Sandstone.

Oolitic limestones are the most common pebbles in the upper conglomerate and the oolites are indistinguishable from those of the underlyingOolite Group. The crinoidallimestone pebbles are commonly stained withiron and contain abraded fragments of crinoids, brachiopods and ostracods.Foraminifera in the pebbles confirm derivation from a Tournaisian source.Ostracods are sufficiently abundant in some pebbles to constitute ostracodal limestones. Pellets rocks are also fairly common and were possiblyderived from the Oolite Group or from the pelletoid rocks in the lowerVisean beds in the immediate area. In addition, pebbles of algal limestones,containing mainly Ortonella sp., are rare.

(iii) Age of the Pendine Conglomerate. No fossils could be found in thematrix of the Pendine Conglomerate or in the calcite mudstones (other thanostracods and calcispheres) to indicate the age of the rocks. By analogywith the sequence in the North Crop of Breconshire, the Pendine Conglomerate is assigned to the Upper Caninia Zone, which, of course, implieswestward continuation in the North Crop of Pembrokeshire of the sub- andintra Visean unconformities.

The beds directly above the Pendine Conglomerate contain abundantDavidsonina carbonaria (McCoy) and an associated fauna which includeCaninia cylindrica (Scouler) and are, therefore, of Seminula Zone age.

MlD-DINANTIAN ROCKS OF PEMBROKESHIRE 297

Identification of the Upper Caninia Zone (S1) along the North Cropwest of Castle Ely was made by Dixon (1914, p. 133), on the assumptionthat Caninia cylindrica (Scouler) was diagnostic of this zone. It is nowknown that the species continues into the overlying Seminula Zone, as seenat Pendine.

Dixon's interpretation of the diachronous rise of the 'lagoon phase'deposits northwards, the Pendine Conglomerate being of basal Seminulanage, is probably too simple, for he did not appreciate the existence of thesub-Seminulan breaks. The comparison with Breconshire provides a closesimilarity in Seminulan overlap and simplifies and unifies the lithologicalcontrasts, which can now be extended over a great part of the southernflanks of S1. George's Land.

4. CONCLUSIONS

The main palaeogeographical control on Dinantian sedimentation inPembrokeshire was the massif of St. George's Land. The trend of thenorthern shoreline appears to have been effected by the incipient growth ofthe Ritec fault, which controlled sedimentation at intervals during theevolution of the depositional shelf. The pre-Tournaisian shoreline may beplaced accurately along the line of the Ritec fault, where Tournaisianrocks overstep the Upper Old Red Sandstone, to rest, with littoral sediments at the base, on Lower Old Red Sandstone.

There is scanty evidence of the positions of Tournaisian shorelines inPembrokeshire, for Tournaisian seas extended northwards beyond thepresent residual outcrops. The northward thinning of the TournaisianMain Limestone is accentuated by Visean overstep along the border of thefluctuating St. George's landmass. Accompanying the attentuation, thereare widespread facies changes and the evidence in Pembrokeshire confirmsthe palaeogeographical conditions envisaged by George (1958, 249-51) ofshallow-water conditions gradually extending southwards in Tournaisiantimes with uplift of the coastal flats. It is evident that oolitic sedimentationin the lower part of the Tournaisian Main Limestone is the first evidence ofthis regional uplift on the flanks of St. George's Land. Emergence occurredlocally at Pendine, as evidenced by the slight break in succession at the baseof the Oolite Group.

The highly saline flats were flanked on the seaward side by winnowedcrinoid and shell banks, now forming the bioclastic limestones through thegreater part of the Tournaisian Main Limestone south of Pendine. It wasnot until late Tournaisian times that oolitic sedimentation extended southwards in the West WiIliamston-Pembroke Dock outcrops.

The mid-Dinantian movements brought sedimentation to a close in theareas north of the Ritec fault. Movement along the fault resulted in the


deposits being uplifted and eroded. Emergence was greatest along theNorth Crop, where approximately 400 feet of Tournaisian Main Limestone was removed between Tenby and Pendine. Despite the magnitude ofthe erosion, uplift need not have been great; a gentle tilt would have beensufficient to have caused a widespread retreat of the sea. An estimated slopeof 1 in 80, normal to the depositional strike, would be required to removethe 400 feet of strata between Pendine and Tenby,

Visean transgression took place northward in Pembrokeshire across abevelled surface of Tournaisian rocks. Only in the Pendine and WestWilliamston areas, where the Tournaisian surface is irregular, are therelittoral deposits at the base of the Visean sequence. Elsewhere in theCentral Pembrokeshire, the Upper Caninia Zone deposits lie with aneven, though abrupt, junction on the underlying Tournaisian Main Limestone. The Upper Caninia Zone beds in the northern outcrops are, nevertheless, composed of shallow-water 'lagoon phase' deposits, providing alithological pointer to the break.

It is not unlikely that the line of the present Ritec fault continued to beimportant throughout Upper Caninian times, and defined a persistentshallow area to the north. In contrast, to the south lay an extensive opensea, the floor of which had a gentle southerly tilt and which receiveduniformly bioclastic sediments. However, evidence of the palaeogeographical conditions of Upper Caninian times is greatly reduced by Seminulanoverstep, which blankets the Caninian beds. It is certain that the maximumadvance of the Upper Caninian sea was to the north of Pendine, but thereis doubt whether the shallow-water coastal flats were flooded by the opensea from the south during Upper Caninian times, and there deposits removed by later pre-Seminulan erosion.

The northern boundary of the Dinantian sea in Pembrokeshire was alongthe line of the Ritec fault, when the intra-Visean movements began at theend of the Upper Caninian times. The movements were followed by a widespread marine transgression, which resulted in the Seminulan sea spreadingnorthwards across the bevelled surface of older rocks.

ACKNOWLEDGMENTS

I am much indebted to Professor T. N. George of the University ofGlasgow for the direction and supervision of my research. Dr. W. G. E.Caldwell of the University of Saskatchewan and Dr. R. H. Cummings ofRobertson Research Corporation read the manuscript and made helpfulsuggestions. I extend my thanks to Mr. Thomas Phillips of West Williamston for his interest shown in the work. The work was pursued during thetenure of a Maintenance Award from the Department of Scientific andIndustrial Research, for which I express my gratitude.


REFERENCESDIXON, E. L. 1921. The Country around Pembroke and Tenby. Mem. geol. Surv, U.K.GEORGE, T. N. 1954. The Pre-Seminulan Main Limestone of the Avonian Series in

Breconshire. Quart. J. geol. Soc. Lond., 110, 283-322.---. 1956. Carboniferious Main Limestone of the East Crop in South Wales.

Quart. J. geol. Soc. Lond., 111, 309-22.---. 1958. Lower Carboniferous Palaeogeography of the British Isles. Proc.

Yorks. geol. Soc., 13,227-318.STRAHAN, A. et al. 1914. The Country around Haverfordwest. Mem. geol. Surv. U.K.VAUGHAN, A. 1905. The Palaeontological Sequence in the Carboniferous Limestone

of the Bristol Area. Quart. J. geol. Soc. Lond., 61, 181-305

Raymond SullivanDepartment of GeologySan Francisco State CollegeSan Francisco, California

the mid-dinantian stratigraphy of a portion of central pembrokeshire

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