The mid-Dinantian stratigraphy of a portion of central Pembrokeshire

The Mid-Dinantian Stratigraphy of a Portion of Central Pembrokeshire by RAYMOND SULLIVAN Received 30 November 1964; taken as read 6 November 1965 CONTENTS 1. 2. 3. 4.

INTRODUCTION THE SEQUENCE IN THE WEST WILLIAMSTON-PEMBROKE DOCK OUTCROP THE SEQUENCE IN THE HAVERFORDWEST-PENDINE OUTCROP CONCLUSIONS ACKNOWLEDGMENTS REFERENCES

page 283 284

292 297 298

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ABSTRACT: The mid-Dinantian movements, whose importance have been demonstrated farther east in South Wales, are shown to be of greater magnitude in central Pembrokeshire than hitherto supposed. The trend of the shoreline of St. George's Land appears to have been controlled by movements along the locally developed Ritec fault. The Ritec fault was a barrier to sedimentation in mid-Dinantian times, causing breaks in sedimentation and the development of widespread coastal fiats. The Upper Caninia Zone, represented by 350 feet of bioclastic limestones at Tenby, south of the fault, 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. INTRODUCTION deals with the mid-Dinantian rocks of central Pembrokeshire described previously by Dixon (in Strahan et al., 1914, 126-48). The area lies astride the Pembrokeshire coalfield. The Dinantian rocks outcrop between Haverfordwest and Pendine along the northern margins of the coalfield and between Pembroke Dock and West Williamston along the Sageston anticline south of the coalfield. The outcrops are terminated 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 synclines that occupy much of southern Pembrokeshire (Fig. 1). Dixon's classic synthesis resulted in a considerable clarification of the Dinantian stratigraphy in the South-Western Province. It was shown that the Dinantian rocks of Pembrokeshire were laid down in east-west facies belts on a depositional shelf south of the landmass of St. George's land. Northward the Dinantian zones thinned, partly because of the increased effects of the mid-Dinantian movements and partly as a result of the nearness of the fluctuating shoreline of the landmass. The developments at Haverfordwest-Pendine and West Williamston-Pembroke Dock outcrops were compared with that of the North Crop of the South Wales coalfield, while

THE PRESENT ACCOUNT

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RAYMOND SULLIVAN

284

Scale

In

Miles

~ 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 the Pembroke area to the south-west Gower. The Bosherston outcrop was unlike any other succession in the South-Western Province and a link with Belgium and Ireland was postulated. The purpose of the present study was to re-examine the effects of the mid-Dinantian movements in central Pembrokeshire in the light of recent work in areas farther east. In Breconshire, George (1954, 283-322 and 1956, 309-22) has shown that the mid-Dinantian movements consisted of two phases represented in the succession by the sub- and intra-Visean unconformities. The two unconformities have been recognised in central Pembrokeshire and demonstrate that the mid-Dinantian movements are of greater magnitude than hitherto supposed. As a result, the study provides a close analogy between eastern South Wales and Pembroke shire and thus simplifies and unifies lithological contrasts that extend over the greater part 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 the West Williamston-Pembroke Dock outcrop to be a conformable but attenuated sequence, though locally, at West Williamston, he recorded a minor break at the base of the Upper Caninia Zone. Since dolomitisation has obliterated most of the fossils in the lower part of the Dinantian and

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made the establishment offaunal subdivision difficult, Dixon (1921, 137-9), in describing the succession, referred the 400 to 500 feet of the Tournaisian Main Limestone collectively to 'ZCl'. Dixon was also unable to separate the thick development of oolites in the Upper Caninia Zone from the oolites in the overlying Seminula Zone. It now appears that Dixon misinterpreted the sequence, for the Upper Caninia 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 by George (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 Limestone

is divisible into a thin upper oolitic limestone unit (the Caninia Oolite) and a thicker lower bioclastic limestone unit. Dolomite (Laminosa Dolomite), clearly of secondary origin, locally makes up a considerable part of the succession. The rocks are best exposed near Carew Castle and along the foreshore of Carew River (Fig. 2), where the sequence is as follows: Thickness in feet 5. Well-bedded, light-grey, oolitic limestone: the Caninia Oolite of Dixon ... 40 4. Well-bedded, light-grey, bioclasticlimestones rich in fossils ... SO 3. Thin-bedded,dark bioclasticlimestones with thin interbedded calcareous shales 60 2. Dark-grey,finely crystallinedolomite: the Laminosa Dolomite (beds poorly exposed between Carew Mill and Ford Point along the Carew River). 200 (approx.) 1. Thin-bedded, dark-grey, bioclasticlimestones and thin interbedded calcareous shales. (The base is hidden beneath the estuary but presumably rests conformably on Lower Limestone Shale.) 50 (seen) Only the upper part (units 3 and 4) of the Tournaisian Main Limestone at Carew can be traced northward along the shore of the Carew River to West Williamston (Fig. 3). The lower part of the succession is hidden beneath 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 quarries along the shore of the Carew River at West Williamston. The top of the Caninia Oolite is pocketed and offers lithological evidence of a midDinantian erosion.

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

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

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greatly affected by dolomitisation, and at Bangeston, only twenty feet of Caninia 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 dolomite decreases and 200 feet of bioclastic limestone compose the lower Tournaisian Main Limestone where it overlies the Lower Limestone Shale along the shore, the upper part being unexposed. WEST WILLIAMSTON

CAREW

BANGESTON

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TOURNAI SIAN

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

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RA YMOND SULLIVAN

(ii) Lithology of the Tournaisian Main Limestone. The Caninia Oolite is composed of washed and sorted, uniform sized, ooliths. Many are irregular, their form controlled by the detrital nuclei, which are commonly crinoid fragments, Foraminifera and shell fragments, and around which calcite is concentrically 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 are closely 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 argillaceous matrix. The limestones show much evidence of flushing, shell-banking and current variability. The interbedded shales reflect periodic increases in the influx of mud into the region; the terriginuous mud was probably derived directly from the northern landmass of St. George's Land. Upwards in the succession the limestones become lighter in colour with a decrease in the argillaceous content, the rocks become well sorted and the interbedded shales are absent. Rare ooliths become scattered through the limestones, which passes quickly, in upward sequence, first into shelly oolitic limestone and finally into oolitic limestone proper tCaninia Oolite). The transition is rapid and takes place in ten feet of the sequence. The limestones exhibit varying degrees of dolomitisation, the original texture being replaced by fine granular dolomite (Laminosa Dolomite). (iii) Fauna of the Tournaisian Main Limestone. Fossils are common throughout the bioclastic limestones and include the following species:

Avonia bassa (Vaughan) Buxtonis scrabicula

(Martin) Camarotoechia mitcheldeanensis

(Vaughan) Cleiothyridinia roissyi

(Leveille) Dictyoclostus vaughani

(Muir-Wood) Leptaena analoga

(Phillips)

Rhipidomella michelini (Leveille) Rugosochonetes cf. hardrensis

(Phillips) Schellwienella crenistria

(Phillips) 'Spirifert tornacensis

de Koninck Syringothyris cuspidata

(Martin)

Tylothyris cr. laminosa

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

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Caninia Oolite, of Globovalvulina bristolensis (Reichel), which Dr. R. H. Cummings (personal communication) indicates is also found in the Caninia Oolite in the Avon Gorge and is recorded from the Caninia Oolite by the author in the Pembroke Syncline. It would appear, therefore, that the greater part of the Toumaisian Main Limestone is present below the sub-Visean unconformity in the West Williamston-Pembroke Dock outcrops. (b) Lower Visean Main Limestone (i) Laterial Variation in the Rocks of the Upper Caninia Zone. The

Calcite Mudstone Group is well displayed at West Williamston in the entrance docks of Point, Tilling Barn, Prinkly and New Dock Quarries on the east bank of the Carew River (Fig. 2). The group maintains a fairly constant thickness of ten to twelve feet. The following sequence is exposed in the New Dock Entrance: Thickness in feet 3. Well-bedded, dark oolitic limestones, with thin shales in the upper few feet. This unit marks the base of the Seminula 45 (seen) Zone 2. Calcite Mudstone Group (iii) Rubbly bedded, calcite mudstones with thin ribs of oolite limestonesrarely exceeding an inch in thickness 2 (ii) Evenbedded, partly dolomitised calcitemudstone 4 (i) Conglomerate-made up of rounded boulders of oolitic limestones derived from the underlying oolites and enclosedin a matrix of yellowmudstone. This is the base of the Upper Caninia Zone 5 1. Caninia Oolite-white weatheringooliticlimestone ... 40 The eroded surface of the Caninia Oolite marks the position of the subVisean unconformity. The conglomerate fills the irregular surface and varies from four to ten feet in thickness. Although the Calcite Mudstone Group retains its general lithology along most of the outcrop, there is considerable variation in detail and individual beds are not readily followed for any great distance. When the group is traced north and south from West Williamston, evidence of erosion at the base disappears. At Garron Pill to the north, twelve feet of the Calcite Mudstone Group is poorly exposed in the banks of the estuary. The Caninia Oolite has an irregular top, but the conglomerate is absent at the base of the Upper Caninia Zone. Traced south and west from West Williamston, the group is poorly exposed in small quarries and along the estuarine flats of the Carew River and Cosheston Pill. There is a variation in thickness, however, in this direction, the Calcite Mudstone Group being five feet thick at Carew, sixteen feet at Milton along Radford Pill, twenty-five feet at Lower Nash

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RAYMOND SULLIVAN

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 Group everywhere lying abruptly on an even surface of Caninia Oolite. (ii) Lithology of the Calcite Mudstone Group. The rocks of the Calcite Mudstone 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 Province and particularly with the Calcite Mudstone Group of this age described by George (1954, 283-322) in Breconshire. The sequence is composed of thin-bedded calcite mudstones, extremely fine-grained and often highly dolomitised, with shaly interbeds. Locally a basal conglomerate is developed, as, for example, at West Williamston. The calcite mudstones are composed of extremely fine amorphous calcite mud, generally structureless, but in part showing mottling that may be of algal origin. The algal origin for these rocks is further supported by occasional algal threads scattered through the rock. Commonly there is a high proportion of organic detritus, which, locally as at Bangeston, for example, 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 the calcite mudstones though they are not so common as in the Breconshire sequence. They are once more dark, very mixed rocks, rarely clean and well sorted, and differ from those of the Caninia Oolite. In these rocks, ooliths, with a thin outer coating of calcite mud, are unevenly distributed in a matrix of extremely fine calcite mud. The ooliths commonly show both radial and concentric structures, and many are composite, the oolithic structure interrupted with thin shells of calcite mud. The cores are commonly crinoid and shell fragments or rounded calcite mud pellets. The calcite mud matrix is commonly highly recrystallised and rarely shows algal mottling. The pellet rocks which occur in the Calcite Mudstone Group 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-Visean relationship along the North Crop of Breconshire leaves little doubt that the Calcite Mudstone group of the West Williamston-Pembroke Dock outcrop belongs to the Upper Caninia Zone. In both areas, the groups are very similar in lithology and fauna and both unconformably overlie the Tournaisian Main Limestone and in turn are unconformably overlain by Seminulan limestones. Fossils are rare in the Calcite Mudstone group except for spirobids, ostracods, calcispheres, algae, and occasional small lamellibranchs and

GARRON PILL I

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TYLLIN BARN

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

... IV \0

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RAYMOND SULLIVAN

brachiopods. The algae include Girvanella sp., Ortonella sp., spongiostromids, and dasycladaceans. Brachiopods include Composita sp., Linoproducts sp. and smalI ryhnchonelIids, these being particularly common in the bioclastic ribs at Bangeston. The fauna of the oolitic limestones overlying the Calcite Mudstone Group, 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 is everywhere abrupt and points to a break in sedimentation (the intraVisean unconformity). The fauna includes numerous brachiopods as listed below: Athyrids undifferentiated Composita 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 the oolitic limestone. Endothyrids in particular are common, together with archaeodiscids, tetrataxids and lituotubellids. The development of Seminulan limestone in the West WilIiamstonPembroke Dock outcrop confirms that the zone is the most constant and most widespread of alI Dinantian zones in Pembrokeshire or South Wales or South-Western Province. The two breaks demonstrated by George (1954,283-322 and 1956, 309-22) in Breconshire are thus shown to extend into Pembrokeshire.

3. THE SEQUENCE IN THE HAVERFORDWESTPENDINE OUTCROP Dixon's (1914, 129-35) description of the relationship of the Dinantian rocks along the North Crop of Pembrokeshire requires some emendation in so far as zonal sequence and zonal discontinuities are concerned. It was shown that emergence in mid-Dinantian times had removed much of the Tournaisian Main Limestone along the Haverfordwest-Pendine outcrop except for a thin lens of zaphrentidian limestone present beneath the Visean limestone in the extreme east of the outcrop. The overlying Visean rocks present above the mid-Dinantian unconformity were thought to have been

MID-DINANTIAN ROCKS OF PEMBROKESHIRE

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deposited on an uneven floor. In some places, beds of the Upper Caninia Zone were thought to have been thickly developed; in others, absent. The revised interpretation, first proposed by O. T. Jones in the discussion of George (1954, 317), is that the Upper Caninia Zone is represented only in the extreme east of the outcrop, and once more the succession is thin, represented by 'lagoon-phase' deposits and bounded above and below by unconformities. Westward, it is seen that the Upper Caninia Zone is overlapped by the Seminulan limestones. Moreover, the beds assigned by Dixon to the Upper Caninia Zone in the area west of Castle Ely are shown to be of Seminula Zone age. (a) Tournaisian Main Limestone (i) Lateral Variation in the Succession. The lower Tournaisian Main Limestone preserved below the sub-Visean unconformity is best exposed in the cliffs at Dolwen Point, Pendine, and in Cloygen Quarry, a few miles to the east. The thickness of Tournaisian Main Limestone preserved amounts to only seventy-two feet, the upper sixty feet being light-grey, evenly bedded, ooliticlimestone (Oolite Group), the lower twelve feet thin-bedded, dark, crinoidal limestone and dolomites. The base of the succession is hidden beneath the sands at Pendine, but the Lower Limestone Shales crop out close by. The Oolite Group is similar in lithology to the oolitic limestone developed in the lower Tournaisian Main Limestone below the sub-Visean unconformity in Breconshire. Like the Breconshire rocks, they show cleanness of grain, even size of ooliths, and the presence of rare beds of bioclastic and pellet limestone in succession. In the Pembrokeshire sequence, the oolites commonly 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, and the irregular surface is filled by conglomerates composed of rounded pebbles oflimestones in a matrix of yellow mudstones. The beds below the junction 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 is poorly exposed, it can be traced westwards inland from Pendine; and it appears near Greenbridge, just over a mile north-east of Pendine, where its thickness is little more than twenty feet. The group may be very thinly represented in a section exposed in a stream half a mile north-east of Marros Church, a short distance north-west of Greenbridge, but at Gellihalog, five miles or so north-west of Pendine, the Tournaisian Main Limestone is completely over-stepped by the Visean limestones. The working quarry at Gellihalog exposes Seminulan limestones resting on

RAYMOND SULLIVAN

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haematitised crinoidal limestones of the Lower Limestone Shales. Still father west, the Visean rocks completely overstep the Lower Limestone Shales and rest on Lower Old Red Sandstone beyond Templeton, and ultimately on to Silurian near Haverfordwest. (ii) Age of the Oolite Group. The Oolite Group and underlying crinoidal limestones contain a rich brachiopod fauna, which, together with stratigraphical position, indicates a lower Toumaisian Main Limestone age for these beds. They are probably to be correlated with the Oolite Group of Breconshire which George (1954,283-322) has assigned to the Zaphrentis Zone. The common brachiopods are as follows: Avonia bassa

(Vaughan) Camarotoechia mitcheldeanensis

Vaughan 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 Koninck Spiriferellina 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 of toumayellinids and plectogyrids. Evidence of erosion at the base of the Oolite Group at Pendine appears only local and the erosional break is of no great magnitude. There is no evidence of unconformity at this horizon elsewhere along the North Crop or elsewhere in Pembrokeshire.

(b) Lower Visean Main Limestone (i) Lateral Variation in the Lower Visean Succession. The sequence exposed at Pendine spectacularly illustrates the Visean unconformities present along the North Crop in Pembrokeshire. In the cliffs at Dolwen Point, the Upper Caninia Zone, represented by the Pendine Conglomerate, may be shown bounded above by the intra-Visean unconformity and below by the sub-Visean unconformity. The succession exposed is as follows:

MID-DINANTIAN ROCKS OF PEMBROKESHIRE

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Thickness in feet 4. Well-bedded, dark-grey, Seminulan oolites and algal limestones making up the greater part of the Seminula Zone and exposed for a distance in the cliffs 3. Well-bedded, dark-grey, algal limestone, nodular in the lower two 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 highlydolomitised, and with thick, interbedded, yellow mudstones (i) Conglomerate, consisting mainly of boulders and pebbles of oolitic limestone filling the irregularities in the top of the underlyingTournaisian Main Limestone This is the base of the Upper Caninia Zone ... 1. Well-bedded, recrystallised oolitic limestone-the Oolitic Group of the Tournaisian Main Limestone ...

350 (seen) 12

20t 7

7-8 58

The top of the Oolite Group is seen to be pocketed for several feet and the depressions filled with boulders derived from the underlying beds. The boulders are large, rounded, and set in a clay matrix. The bedded calcite mudstones in the middle of the Pendine Conglomerate (unit 2(ii» lie on the irregular surface of the underlying conglomerate and are steeply inclined when compared with the gently dipping Seminulan limestones and Tournaisian limestone above and below. Lying irregularly above the calcite mudstone is a dark conglomerate (unit 2(iii», which is poorly sorted and contains a great variety of boulders and pebbles. The contact of the Pendine Conglomerate and the overlying Seminulan limestones is abrupt, the dark conglomerate (bed 2(iii» being overlain by rubbly, dark, algal limestones, which become regularly bedded upward in the 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 100 yards to the east, it is rapidly reduced to between two and three feet of dark-coloured conglomerates. It would appear that, at Pendine, the conglomerate 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 to seven feet of conglomerate, similar to the lower conglomerate (bed 2(i» at Pendine, being composed of boulders derived from the underlying Oolite Group. 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 is represented by two to three feet of conglomerates (similar to bed 2(iii» PROC. GEOL. ASSOC., VOL. 76, PART 3, 1965

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RAYMOND SULLIVAN

directly overlying the Oolite Group with an irregular contact, and in tum abruptly overlain by dark coloured, Seminulan limestones. Father west from Greenbridge, the Seminula Zone oversteps the Pendine Conglomerate to transgress ultimately the whole of the Tournaisian succession. In a stream section a half-mile north-west of Greenbridge, the Pendine Conglomerate is reported to be absent by Dixon (1921 131), Seminulan limestones resting directly on the Oolite Group. A similar relationship 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 the Pendine Conglomerate are similar to the basal Visean calcite mudstones at West Williamston. They consist of fine calcite mud, in part recrystallised and commonly mottled in appearance, though no identifiable algal tissue has been observed. They also include pellet rocks with abundant ostracods and calcispheres. The boulders in the lower conglomerate at Pendine are all derived from the underlying Oolite Group. The upper conglomerate is more varied; it contains a variety of pebbles including iron-stained crinoidallimestones, bioclastic limestones, pellet rocks and oolites. The pebbles are set in a dark sandy martix, containing angular quartz grains, and heavy minerals including zircon, rutile, garnet and tourmaline. The heavy minerals were probably 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 underlying Oolite Group. The crinoidallimestone pebbles are commonly stained with iron 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 possibly derived from the Oolite Group or from the pelletoid rocks in the lower Visean 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 the matrix of the Pendine Conglomerate or in the calcite mudstones (other than ostracods and calcispheres) to indicate the age of the rocks. By analogy with the sequence in the North Crop of Breconshire, the Pendine Conglomerate is assigned to the Upper Caninia Zone, which, of course, implies westward continuation in the North Crop of Pembrokeshire of the sub- and intra Visean unconformities. The beds directly above the Pendine Conglomerate contain abundant Davidsonina carbonaria (McCoy) and an associated fauna which include Caninia cylindrica (Scouler) and are, therefore, of Seminula Zone age.

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Identification of the Upper Caninia Zone (S1) along the North Crop west of Castle Ely was made by Dixon (1914, p. 133), on the assumption that Caninia cylindrica (Scouler) was diagnostic of this zone. It is now known that the species continues into the overlying Seminula Zone, as seen at Pendine. Dixon's interpretation of the diachronous rise of the 'lagoon phase' deposits northwards, the Pendine Conglomerate being of basal Seminulan age, is probably too simple, for he did not appreciate the existence of the sub-Seminulan breaks. The comparison with Breconshire provides a close similarity in Seminulan overlap and simplifies and unifies the lithological contrasts, which can now be extended over a great part of the southern flanks of S1. George's Land. 4. CONCLUSIONS The main palaeogeographical control on Dinantian sedimentation in Pembrokeshire was the massif of St. George's Land. The trend of the northern shoreline appears to have been effected by the incipient growth of the Ritec fault, which controlled sedimentation at intervals during the evolution of the depositional shelf. The pre-Tournaisian shoreline may be placed accurately along the line of the Ritec fault, where Tournaisian rocks 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 in Pembrokeshire, for Tournaisian seas extended northwards beyond the present residual outcrops. The northward thinning of the Tournaisian Main Limestone is accentuated by Visean overstep along the border of the fluctuating St. George's landmass. Accompanying the attentuation, there are widespread facies changes and the evidence in Pembrokeshire confirms the palaeogeographical conditions envisaged by George (1958, 249-51) of shallow-water conditions gradually extending southwards in Tournaisian times with uplift of the coastal flats. It is evident that oolitic sedimentation in the lower part of the Tournaisian Main Limestone is the first evidence of this regional uplift on the flanks of St. George's Land. Emergence occurred locally at Pendine, as evidenced by the slight break in succession at the base of the Oolite Group. The highly saline flats were flanked on the seaward side by winnowed crinoid and shell banks, now forming the bioclastic limestones through the greater part of the Tournaisian Main Limestone south of Pendine. It was not 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 the areas north of the Ritec fault. Movement along the fault resulted in the

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deposits being uplifted and eroded. Emergence was greatest along the North Crop, where approximately 400 feet of Tournaisian Main Limestone was removed between Tenby and Pendine. Despite the magnitude of the erosion, uplift need not have been great; a gentle tilt would have been sufficient to have caused a widespread retreat of the sea. An estimated slope of 1 in 80, normal to the depositional strike, would be required to remove the 400 feet of strata between Pendine and Tenby, Visean transgression took place northward in Pembrokeshire across a bevelled surface of Tournaisian rocks. Only in the Pendine and West Williamston areas, where the Tournaisian surface is irregular, are there littoral deposits at the base of the Visean sequence. Elsewhere in the Central Pembrokeshire, the Upper Caninia Zone deposits lie with an even, 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 a lithological pointer to the break. It is not unlikely that the line of the present Ritec fault continued to be important throughout Upper Caninian times, and defined a persistent shallow area to the north. In contrast, to the south lay an extensive open sea, the floor of which had a gentle southerly tilt and which received uniformly bioclastic sediments. However, evidence of the palaeogeographical conditions of Upper Caninian times is greatly reduced by Seminulan overstep, which blankets the Caninian beds. It is certain that the maximum advance of the Upper Caninian sea was to the north of Pendine, but there is doubt whether the shallow-water coastal flats were flooded by the open sea 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 along the line of the Ritec fault, when the intra-Visean movements began at the end of the Upper Caninian times. The movements were followed by a widespread marine transgression, which resulted in the Seminulan sea spreading northwards across the bevelled surface of older rocks. ACKNOWLEDGMENTS I am much indebted to Professor T. N. George of the University of Glasgow for the direction and supervision of my research. Dr. W. G. E. Caldwell of the University of Saskatchewan and Dr. R. H. Cummings of Robertson Research Corporation read the manuscript and made helpful suggestions. I extend my thanks to Mr. Thomas Phillips of West Williamston for his interest shown in the work. The work was pursued during the tenure of a Maintenance Award from the Department of Scientific and Industrial Research, for which I express my gratitude.

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REFERENCES DIXON, 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 Sullivan Department of Geology San Francisco State College San Francisco, California