A facies analysis of the cambrian of wales

Palaeogeography, Palaeoclimatology, Palaeoecology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

A FACIES ANALYSIS OF T H E C A M B R I A N OF WALES T. P. CRIMES

Department of Geology, The University, Liverpool (Great Britain) (Received July 3, 1969)

SUMMARY

The Cambrian sediments of Wales are analysed with particular regard to lithology, directional and non-directional inorganic sedimentary structures and trace fossils. It is deduced that deposition occurred in a narrow, fault controlled, roughly northeast-southwest trending trough. The northwestern margin of the trough was variously situated on and to the northwest of Anglesey while the southeastern margin probably lay in the Welsh Borderlands or English Midlands. The axial terminations of the trough are unknown. Sediment transport within the trough appears to have been mainly by turbidity and other bottom currents, with the coarser sediment generally laterally derived from nearby fault uplifted landmasses and the finer sediment axially derived from a more distant southwesterly source. The biogenic and inorganic sedimentary structures are consistently in agreement in indicating that the sediments did not all accumulate in either shallow or deep water as has been previously claimed but in variable water depth at all localities. The depositional history of the area does not readily accord with the early stages of the generally accepted scheme of geosynclinal evolution and the Cambrian sequence should not therefore be considered as part of the so-called Welsh Lower Palaeozoic geosynclinal sediments as advocated by JONES 0938). INTRODUCTION

In Wales, the type region for the Cambrian System, there are four main outcrops of Cambrian strata covering a total area of approximately 1,250 km 2 (Fig.l). The abundance of coarse clastics and occurrences of cross bedding and ripple marks within these sequences led the early investigators to favour deposition in shallow water. Local or regional facies and thickness changes were used to suggest derivation of most or all of the sediments from an easterly source (NIcHOLAS, 1915; MATLEY and WILSON, 1946; WOODLAND, 1946). Similar criteria, however, led JONES (1933, 1938) to postulate a northerly or northwesterly source. According to Jones the sediments were derived from a landmass situated in the Irish Sea or south Pennine regions and deposited in a shallow-water geosynclinal trough aligned northeast-southwest. Palaeogeography, Palaeoclimatol., Palaeoecol., 7 (1970) I 13-170

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After 1950 comparative facies analyses were neglected in favour of detailed local investigations. In the Harlech Dome, following preliminary work by KVENEN (1953), KOPSTEIN (1954) from a study of grain orientation and directional sedimentary structures, contended that the sediments of almost the entire Cambrian succession were derived from the south-southwest and deposited mainly from turbidity currents in a deep-water geosynclinal trough whose axis lay to the northnortheast of the Harlech Dome and must have been aligned roughly west-northwest-east-southeast. In contrast, BASSETTand WALTON(1960) showed from sole mark orientations that Lower Cambrian greywackes on nearby St. Tudwal's Peninsula were deposited by currents flowing from the northeast. They pointed out that grain orientation in the Harlech Dome, assumed by KOPSTEIN (1954) to be a depositional fabric and used in reconstructing palaeocurrent trends, was largely or entirely deformational. Following the measurement of 80 directional sedimentary structures of unspecified type in 6 square miles of the Harlech Dome, KNILL(1959) suggested deposition of the Lower and Middle Cambrian rocks of Wales took place in a Palaeogeography, PalaeoclimatoL, Palaeoecol., 7 (1970) 113-170

A FACIES ANALYSIS OF THE CAMBRIAN OF WALES

115

northeast-southwest trough in which the greywackes were deposited by axially flowing turbidity currents from the northeast and southwest and the finer grained sediments were laterally derived. More recently, BOSWELL(1961) has denied the existence of a geosynclinal trough in Wales during Lower Palaeozoic times and has suggested that deposition occurred in a number of basins. Subsequent published investigations have dealt mainly with local palaeocurrent data from the Upper Cambrian sequence in north Wales (CRIMES,1966a,b; EVANS et al., 1966; CRIMES and OLDERSHAW,1967). The purpose of this paper is to present a facies analysis of the Cambrian strata of the four isolated outcrops and to attempt to use this information both to provide a viable interpretation of Welsh Cambrian palaeogeography and to document facies changes within what is suggested to be a narrow trough. Reference will also be made to published work relating to the Cambrian exposures to the east of Wales, in the Welsh Borderlands (Fig.l). In this facies analysis both inorganic and biogenic sedimentary structures will be considered. The use of biogenic structures in interpreting depositional environments has been discussed in a series of recent papers by SEILACHER(1963, 1964, 1967). Briefly, he recognizes in Phanerozoic strata four main depth controlled biogenic facies, each named after its characteristic trace fossil. The facies are, in order of increasing water depth: Skolithos, Cruziana, Zoophycos and Nereites. Trace fossils belonging to these facies have been found at a number of horizons in the Welsh Cambrian and will be referred to in the analysis. Detailed trace fossil descriptions and locality lists, however, will be published elsewhere. The palaeogeographical synthesis, which is developed from this facies analysis, depends in the first place on a correlation between the isolated outcrops (Fig.2). The Upper Cambrian and most of the Middle Cambrian strata can be correlated between the four outcrops on combined lithostratigraphic and biostratigraphic criteria (FARSHORI, 1962; CRIMES, 1969b). The difficulties inherent in correlating between the generally unfossiliferous Lower Cambrian sequences have been discussed by BASSETTand WALTON (1960) and reviewed by BASSETT (1963). The largely lithostratigraphic correlation adopted here for the Lower Cambrian sequences in north Wales is supported by the presence of strikingly similar lithological sequences and the use of a manganese ore bed as a marker horizon. It is similar to the correlation proposed by FARSHORI (1962) and is discussed in detail elsewhere (CRIMES, 1969b). Lateral facies changes, however, appear to preclude any detailed correlation between the Lower Cambrian rocks of north Wales and allegedly coeval rocks in south Wales. The Lower Cambrian rocks of south Wales will therefore in the main be considered only as a local entity. THE COMMENCEMENT OF CAMBRIAN SEDIMENTATION

The abundance of radiometric dates within the range 580-600 m.y. already

Palaeogeography, Palaeoclimatol.,Palaeoecol., 7 (1970) 113-170

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A FACIES ANALYSIS OF THE CAMBRIAN OF WALES

117

recorded from Precambrian rocks in Wales, the English Midlands and northwestern France and anticipated in Leinster (CRIMES and DHONAU, 1967) implies that uplift of these rocks above the level of isotopic reconstitution took place roughly synchronously over a wide area at about the close of the Precambrian. Following a period of denudation, deposition recommenced during Early Cambrian times with a surface of marked unconformity separating the partly recrystallised and deformed Precambrian rocks and the basal Cambrian sediments. In Wales the unconformity can only be seen in Pembrokeshire, where there is a visible structural hiatus between the Precambrian Pebidian and Dimetian rocks and the Lower Cambrian Caerfai Series (GREEN, 1908). In the Welsh Borderlands (Fig.l) Lower Cambrian quartzites rest unconformably on Precambrian volcanics (Uriconian) and metamorphics (Rushton Schists). The unconformity can, however, also be inferred in north Wales (GREENLY,1919) where dated Precambrian metamorphics of the Mona Complex are overlain by a sequence of less deformed partially terrestrial volcanics, known as the Clogwyn or Arvonian Volcanics (GREENLY, 1945). These volcanics lie, apparently conformably, more than one thousand metres beneath the earliest fossiliferous horizon which contains high Lower Cambrian trilobites. They have no equivalent in south Wales and are here taken to mark a local volcanic phase at about the base of the Cambrian succession. THE REORIENTATION OF DIRECTIONAL SEDIMENTARY STRUCTURES

The facies analysis of the Cambrian sediments of Wales is concerned in the first place with the presentation of additional palaeocurrent evidence. In many of the sections the strata are folded and dips are locally steep. In reorientating directional sedimentary structures allowance has been made for dip and the plunge of principal folds (F1). These folds are only slightly affected by subsequent sporadic minor structures (CRIMES, 1969b) which need not be considered in this context. F 1 plunge was either measured in the field as a lineation or computed from field measurements of the orientation of planar structures. Folding of the coarser grained sediments containing the directional structures was generally concentric, with strain analysis indicating absence of compressional distortion (CRIMES, 1969b). In the Harlech Dome, however, both concentric and similar folds are present but minor folds and objects suitable for direct strain analysis are uncommon within the sediments containing the directional sedimentary structures. Measurements of the orientation of directional sedimentary structures were therefore restricted, as far as possible, to exposures at which cleavage was poorly developed or absent and visible effects of deformation minimal. "Unfolding" and computation of the depositional azimuth of the sedimentary structures was accomplished for all areas by methods applicable to concentric folds, without allowance for compressional distortion. Thus no significant errors are anticipated from the unfolding procedures in south Wales, St. Tudwal's Palaeogeography, Palaeoclimatol., Palaeoecol., 7 (1970) 113-170

118

T . P . CRIMES

Peninsula or north Caernarvonshire. In the Harlech Dome the structural relationships, together with the few strain measurements so far made, suggest from the tables given by RAMSAY (1961) that the cumulative error, due to the possible incorrect unfolding procedure and compressional distortion, is unlikely to exceed 10 ° at any of the localities investigated. FACIES ANALYSIS

The Lower Cambrian of south Wales The earliest Cambrian strata in south Wales, the basal conglomerate of the Caerfai Series, consist of conglomerates (ca. 1 m thick) interbedded with subsidiary green greywackes, sandstones and siltstones. The ratio of conglomerates to finer sediments decreases upwards. The conglomerates are commonly of pebble grade but include cobbles and some boulders in the lowest beds. The pebbles, cobbles and boulders are composed of quartz, quartzite, schistose quartzite, acid tuff, red shale and less frequently schistose greywacke, green schist, jasper and granophyre. The acid tufts and red shales can be matched with rock types in the underlying Pebidian and the granophyre pebbles were probably derived from the local

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Palaeogeography, PalaeoclimatoL, Palaeoecol., 7 (1970) 113-170

A FACIES ANALYSISOF THE CAMBRIAN OF WALES

1 19

Dimetian granophyres (HIcKs, 1884; GREEN, 1908). However, not all the fragments can be matched with rocks now locally exposed; some of the schistose fragments and the jasper were probably derived from older strata. Deposition of the lowest beds in shallow water is evidenced by the coarse grained conglomerates and the occurrence within the finer conglomerates and sandstones of large scale (i.e., set thicknesses mostly greater then 5 cm) tabular cross bedding of alpha, gamma and epsilon types (ALLEN, 1963; this paper Fig.3A,B). In modern environments epsilon cross-stratification has so far only been described from intertidal areas affected by gullying (ALLEN, 1963). An intertidal environment would also be consistent with the presence of channels (Fig.3C) up to 2 m deep and the occurrence of Skolithos in these beds. Orientation of the large scale tabular cross bedding suggests south-southwesterly sediment transport (Fig.3D) and this agrees with measurements of flute and groove cast orientation on the base of two beds. The overlying Green Sandstone contains conglomeratic horizons but is largely composed of green feldspathic sandstones generally less than ! m thick with subsidiary laminated siltstones and mudstones. Large scale tabular crossstratification occurs in the sandstones and some of the siltstones have ripple drift bedding. The general lithology suggests continuance of shallow water deposition. A sequence of red siltstones and mudstones, collectively known as the Red Shales, overlie the Green Sandstone and are succeeded by the Caerbwdwy Sandstone which consists of 1-3 m thick beds of purple or green muddy feldspathic fine sandstone and subsidiary mudstones. The purple and green colouration is haphazardly distributed in some beds suggesting that it is at least partially diagenetic in origin. Apart from lamination, occasional ripple drift cross bedding, feeding burrows (Teichichnus) and undiagnostic worm burrows in the Red Shales, these units are devoid of interpretable sedimentary structures. Nevertheless, the absence of such structures as large scale tabular cross bedding, channels and symmetric ripple marks, suggests deposition may have occurred below wave base. The Lower Cambrian succession in south Wales thus appears to form a typical transgressive sequence with deposition occurring in a gradually deepening sea.

The Lower Cambrian of north Wales in north Wales deposition commenced subaerially with the accumulation of several thousand feet of acid volcanic rocks (the Arvonian or Clogwyn Volcanics) which include ignimbrite flows (D. Wood, personal communication in BASSETT, 1963; TREMLETT, 1965). The volcanics are succeeded locally in the Nantlle area by the Tryfan Grit G r o u p w h i c h consists of coarse tabular cross-bedded feldspathic sandstones overlain by finely laminated green siltstones. This group is followed with slight discordance (MORRIS and FEARNSIDES, 1926) by the Cilgwyn Conglomerate which Palaeogeography, Palaeoclimatol., Palaeoecol., 7 (1970) 113-170

120

T.P. CRIMES

contains well rounded pebbles of rock types some of which can be matched with the Clogwyn Volcanics and others with the Late Precambrian Mona Complex now exposed nearby on Anglesey (WILLIAMS, 1923; MORRIS and FEARNSIDES, 1926; TREMLETT, 1965). Apparently coeval conglomerates exposed in eight or nine small tracts on the Bangor-Caernarvon Ridge include a similar suite of pebbles suggested to be of local origin (GREENLY, 1945). The conglomerates are overlain by the Glog Grit Group, which consists of coarse and fine sandstones and quartzites some of which display cross-stratification of the xi and omikron types of ALLEN (1963), with subsidiary finer grained sediments. The evidence therefore suggests that the Cilgwyn Conglomerate and the Glog Grit Group were deposited in shallow water. The remainder of the succession up to the Cymffyrch Grit Group, however, consists of well cleaved mudstones generally of green or purple colour alternating with subsidiary coarser (5 cm to 2 m thick) units which include interbedded greywackes, sandstones and siltstones. The complete sequence is preserved in north Caernarvonshire but only the upper part is exposed in the Harlech Dome (see Fig.2). The slates consist mainly of mudstones but thin siltstones, generally 1-5 cm thick, are not uncommon. Many of the siltstones show parallel lamination and an overall grading with clearly defined base and gradational top. Some beds also show parallel lamination both above and below a ripple-drifted division. Within the "grit groups", e.g., Dorothea, Pen-y-Bryn, some individual greywacke beds are well graded and the grading may give way to parallel lamination towards the top of the bed. The occurrence of graded greywackes within a sequence of dominantly fine grained sediments, suggests that the former were deposited by turbidity currents. Some of the siltstones may have been similarly deposited. The sequence of sedimentary structures in the greywackes suggests derivation from a nearby source area while analogous criteria point to a more distant source for the siltstone (see WALKER, 1967). In the Harlech Dome, however, the Dolwen Grits consist of 30-50 cm thick sandstones interbedded with thin mudstones. The sandstones are not graded and show xi-cross-stratification (ALLEY, 1963), wash-outs and wedgingout, implying shallow water non-turbidite deposition (cf. KOPSTEIN, 1954). Few sole markings have so far been observed by the writer within the greywackes of the slate belt in Caernarvonshire and the other directional sedimentary structure seen--ripple drift bedding--is too strongly deformed to give meaningful palaeocurrent trends. The position of the source of these sediments is therefore at present unknown but since within the same sequence some turbidite beds show proximal and others distal characteristics, more than one source may be envisaged. Evidence on the composition of the source rocks comes from the few pebble horizons. According to G~ENLY (1919) these include rounded fragments of quartzite, albite granite, jasper and green schist and could have been derived from the Precambrian metamorphic complex, a part of which is now exposed nearby in Palaeogeography, PalaeoclimatoL, PalaeoecoL, 7 (1970) 113-170

A FACIES ANALYSISOF THE CAMBRIANOF WALES

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Anglesey. Within some of the sandstones, however, blue quartz grains are abundant: they do not occur within either the underlying volcanics or the local Late Precambrian rocks, except in a few places as derived grains (GREENLY, 1919), and must therefore have come from more ancient source rocks. The Lewisian granulitic gneisses of the northwest Highlands of Scotland are known to contain abundant blue quartz. Recent mapping has indicated the presence of Lewisian-like rocks in southern Leinster, probably extending eastwards into the Irish Sea (CRIMES and DHONAU,1967). Rocks of this type may have contributed to these Lower Cambrian sediments but the source need not necessarily have been in the Irish Sea region. In north Caernarvonshire the slate sequence is succeeded, with abrupt facies change, by the Cymffyrch Grit Group ( = Bronllwyd Grits). The contact, which

Fig.4. Pebble conglomerate bed within the Rhinog Grits at Cefn Cam, east side of Harlech Dome.

Palaeogeography, Palaeoclimatol., PalaeoecoL, 7 (1970) 113-170

122

T.P. CRIMES

can best be seen in the Penrhyn Slate Quarry at Bethesda, shows no evidence of angular unconformity and according to WILLIAMS (1923) the Cymffyrch Grit Group rests on the same slate horizon for at least several kilometres along the strike. The lowest few metres of the group do, however, contain an abundance of fragments of the underlying green mudstones. These fragments which measure up to 40 × 10 cm suggest that the contact may be a surface of submarine erosion. In the Harlech Dome, the lateral equivalents of the Cymffyrch Grits--the Rhinog Grits--follow the Llanbedr Slates in a conformable sequence. The vertical facies change is more gradual, with increase in the ratio of greywacke to mudstone beds upwards. On St. Tudwal's Peninsula the base of the Hell's Mouth Grits ( = Rhinog Grits) is not exposed. These three "grits" consist of alternations of grey, grey-green, and grey-blue greywackes, siltstones and mudstones, Beds of pebble conglomerate also occur within the Cymffyrch Grits particularly near the base but are more common within the Rhinog Grits (Fig.4). The greywacke beds range from 10 cm to 6 m in thickness but are generally about 50 cm thick. Within the Cymffyrch Grits the beds of pebble conglomerate are up to several metres thick near the base but are generally only a few centimetres thick; whereas within the Rhinog Grits many are 1-2 m thick. Siltstones range from 1-300 cm in thickness but are normally 5-30 cm thick. Within the Rhinog and Hell's Mouth Grits, the alternation of greywacke and shale beds, constancy in thickness of individual beds, general absence of fossils, presence of graded bedding and sole markings and angularity and poor sorting have already been interpreted as indicating transportation by and deposition from turbidity currents (KUENEN, 1953; KOPSTEIN, 1954; KNILL, 1959; BASSETTand WALTON, 1960). These criteria can also be used to infer deposition of the greywackes of at least part of the Cymffyrch Grits in the same way. Some greywacke beds (Fig.5A) commence with the graded division (division A of the turbidite sequence as described by BOUMA, 1964) but beds beginning with the lower parallel laminated division (division B) are almost equally common in all three successions. Within the Hell's Mouth Grits a few beds show parallel lamination beneath the graded interval (BASSETTand WALTON,1960; and author's personal observation). Although complete turbidite sequences do occur, most beds show only divisions A and B or B alone. Amalgamated units are not uncommon (Fig.5~) and within the Cymffyrch Grits some units have an erosional surface near the top of division B or between B and the ripple drifted division C (Fig. 5C,D). This may be the result of local increased current velocity within the flow or reworking by bottom currents. Calculation of the P factor (WALKER, 1967) for 50 beds in the lower part of the Hell's Mouth Grits at Trwyn-y-ffosle gave a value o f P = 70, where:

P=A+~B and:

Palaeogeography. PalaeoclimatoL,PalaeoecoL, 7 (1970) 113-170

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A = ~ o f beds beginning with division A (graded division). B = ~ o f b e d s beginning with division B (lower parallel laminated division). Calculations at the top o f the sequence at Trwyn-y-Mulfran yielded a higher P value (90). A value o f P = 80 was obtained from measurements on the Cymffyrch Grits at Marchlyn Mawr.

Palaeogeop, raphy, Palaeoclimatol., Palaeoecol.,

7

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113-170

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T . P . CRIMES

The high P values, occurrence of amalgamated units, thickness of individual greywacke beds, thinness of interbedded mudstones and general coarseness of the sediments, suggests that these are proximal turbidites deposited under conditions of high flow regime. Sole markings on the base of the greywacke beds in the Cymffyrch, Rhinog and Hell's Mouth Grits include flute casts (generally of conical linguiform or

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Fig.6. Sedimentary structures in the greywackes and conglomeratic sandstones of the Rhinog and Bronllwyd Grits. A. Bed of tabular cross-stratified well-sorted sandstone between greywackes comprising a lower AB sequence and an ungraded bed. For legend, see Fig.27. B. Tabular cross-stratified unit with individual sets graded and erosional lower and upper surfaces. Rhinog Grits, Llyn Cwm Bychan (SH 646310). C. Xi cross-stratification, base of Bronllwyd Grits. Above Penrhyn Quarry, Bethesda. D. Wash-out. Rhinog Grits, near summit of Rhinog Fawr. E. Wash-outs in consecutive beds. Rhinog Grits, Mynydd Cwm Mynach (SH 680270). F. Washout between amalgamated AB sequences. Bronllwyd Grits, Marchlyn Mawr. Palaeogeography,

PalaeoclimatoL,

Palaeoecol.,

7 (1970) 113-170

A FACIES ANALYSISOF THE CAMBRIANOF WALES

125

elongate-symmetrical type), groove casts, prod marks and bounce casts while load casts and flame structures also occur. The graded bedding includes all the types described by DZULYNSKI and WALTON (1965, p. 171) but simple continuous grading with good separation of different grain sizes and discontinuous grading in which the finer grained portion is missing, are the most common. Lenses of coarse sandstone occur out of sequence within some graded beds. In the Harlech D o m e beds of well sorted sandstone generally 5-30 cm thick are not u n c o m m o n (Fig.6A). Both the lenses and beds of sandstone are in places current bedded with the g a m m a and epsilon types of ALLEN (1963) in solitary sets (Fig.SE, 6A,B), suggesting deposition from or reworking by currents which commonly moved in a direction roughly perpendicular to that indicated by the trend of sole marks. Xi and trough cross-stratification also occur near the base of the Cymffyrch Grits (Fig.6C) and tabular cross bedding occurs within division A of a few greywacke beds (Fig.5F). Ripple drift bedding is also present, constituting division C of some turbidite beds and the laminations in divisions B and C are in some places convoluted. In some amalgamated units the base of the upper bed truncates convolute lamination in the lower bed thus indicating amalgamation by erosion rather than non-deposition. Wash-outs occur at the base of normal sequences and the upper unit of amalgamated sequences (Fig.6D,E,F). Asymmetric, symmetric and interference ripples have been reported from the Hell's Mouth Grits (BASSETTand WALTON,1960) but have not been observed in the other sequences. Inclusions of mudstone or siltstone are common within some greywacke beds and inclusions of laminated sandstone up to 60 × 60 cm occur within the Rhinog Grits. In a few cases complete siltstone beds are slumped with slump fold closures truncated by the overlying bed. Isolated slump fold closures also occur within division A of simple or amalgamated AB units. More commonly, slumping occurs within division C of more or less complete turbidite sequences. The only biogenic structures are burrows (Fig.7) found in both the Hell's Mouth and Rhinog Grits either on the base of greywacke beds or in the mudstone

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Fig.7. Greywacke-infilled feeding burrows in the mudstone beneath a bed of graded greywacke. Hell's Mouth Grits, Trwyn-y-Ffosle, St. Tudwal's Peninsula.

Palaeogeography, Palaeoclimatol.,Palaeoecol., 7 (1970) 113-170

126

T.P.

CRIMES

immediately beneath. The burrows which are generally ½-1 cm in diameter and filled with greywacke, may be up to several metres in length. They commonly form a bifurcating network but may also occur as horseshoe patterns or branch out from a centre. They are similar to forms described by SEILACHER(1962) from the Spanish ~ysch (Eocene-Cretaceous). A detailed study of directional sedimentary structures within the Hell's Mouth Grits'was made by BASSETT and WALTON (1960) who showed, from the orientation of 367 sole markings from 12 horizons, that the greywackes were deposited from southwesterly flowing turbidity currents and thus postulated a northerly source, perhaps between north and east, possibly even west of north. Sole mark orientation from greywackes of the Rhinog Grits (Fig.8) suggests transport by southward-flowing currents. Wash-outs on the base of graded greywackes are generally oriented parallel to sole markings but those on the base of

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Fig.8. A. Palaeocurrent rose diagrams from sole mark measurements within the Rhinog Grits in the areas indicated on the inset map. B. Palaeocurrent rose diagrams for all measurements of sole marks (a), wash-outs (b), and cross-bedding(c) from the Rhinog Grits in the Harlech Dome. Figures underneath rose diagrams indicate numbers of measurements.

Palaeogeogeaphy, Palaeoclimatol., Palaeoeeol., 7 (1970) 113-170

A FACIES ANALYSIS OF THE CAMBRIAN OF WALES

127

non-graded conglomerate and coarse sandstone beds, show a tendency towards an east-west alignment and give the east-west preferred orientation seen in b of Fig.SB. Measurements of current orientation taken from sets of tabular cross strata in interbedded sandstones also suggest westward transport of these sediments (c in Fig.SB). Thus, although turbidity current flow and bulk sediment transport was from the north, a subsidiary easterly source seems to have supplied conglomerate and sandy traction load. Sole markings were found on only five bedding planes within the Cymffyrch Grits; four sets of groove casts and a single flute cast observation suggest current flow to the south-southwest. The greywackes of the Cymffyrch, Rhinos and Hell's Mouth Grits have a similarity in mineralogical composition and appear to have been derived from a landmass composed of metamorphic and intrusive igneous rocks similar to the Precambrian Mona Complex of Anglesey but perhaps containing more extensive exposures of gneiss, granite, trachyte and keratophyre (WILLIAMS, 1923; WOODLAND, 1939, 1946; BASSETTand W A L T O N , 1960). The pebbles which infiU some of the east-west oriented wash-outs and channels in the Rhinos Grits contain a high proportion of acid volcanic rocks including flow banded rhyolites and ignimbrites. Some of these pebbles possess a cleavage which strikes at various angles athwart the regional cleavage thus suggesting that the rocks were deformed prior to transportation. This implies derivation not from lateral equivalents of the Arvonian Volcanics but from an older succession deformed during the Precambrian. The Uriconian volcanics now exposed to the east in the Welsh Borderlands could have provided such material. Except possibly for the mu cross-stratification at the base of the Cymffyrch Grits, there is little evidence to indicate accumulation of any of these sediments above wave base. It was suggested by BASSETTand WALTON (1960) that the Hell's Mouth Grits may have been deposited in an environment similar to that developed near the foot of the present-day continental slope: an environment characterized by deltas at the foot of submarine canyons. This is consistent with the conclusion that these greywackes are proximal turbidites. The evidence for turbidity currents flowing southwestwards on St. Tudwal's Peninsula and southwards in the Harlech Dome suggests that they were fanning out from north Caernarvonshire. There is, however, no evidence of a submarine canyon in that area: the source may have been where the present Irish Sea lies. Overlying the Rhinog and Hell's Mouth Grits is an unfossiliferous manganese-rich sequence which, as MATLEYand WILSON (1946) suggested, may represent the youngest Lower Cambrian sediments in north Wales. The beds consist of manganiferous shales and mudstones, a bed of manganese ore (ca. 0.5 m thick), and greywackes, generally about 0.5 m thick. The mudstones are generally laminated and contain ripple drift bedding; many greywackes are graded and occur either Palaeogeography, Palaeoclimatol., Palaeoecol., 7 (1970) 113-170

128

T.P. CRIMES

as simple graded beds or AB sequences, and thus appear proximal in character. Groove casts within the few accessible cliff exposures of the Manganese Beds on St. Tudwal's Peninsula are aligned northeast-southwest (five measurements) but no sole markings have been observed in the Harlech Dome. WOODLAND (1939) and MOHR (1955, 1956, 1959, 1964), on the basis of petrographic and geochemical studies respectively, conclude that deposition occurred in an enclosed or partially enclosed, quiet, shallow-water basin with sediment supplied by slow disintegration of largely gneissic landmasses situated to the east and northwest of the Harlech Dome. Thus, apart from a possible shallowing, the environment appears similar to that postulated for the underlying beds. Sediment transport directions may also have been little changed. During Lower Cambrian times north Wales received a substantially greater thickness of sediment than did south Wales. To the east in the Welsh Borderlands the sequence was also condensed with the Lower Cambrian represented by only 200 m of shallow water, fossiliferous quartzites, sandstones and limestones (PoCOCK and WHITEHEAD, 1948). Thus, in general, during Lower Cambrian times, the locus of maximum subsidence lay in north Wales where much of the sediment was deposited in a bathyal environment; in south Wales and the Welsh Borderlands sediment accumulated more slowly in a sublittoral environment. The earlier Middle Cambrian The Barmouth Grits of the Harlech Dome and the Cilan Grits of St. Tudwal's Peninsula succeed the manganiferous sediments conformably and in the absence of fossils are, following MATLEY and WILSON (1946), tentatively placed at the base of the Middle Cambrian. In south Wales fossiliferous Middle Cambrian rocks of the Solva Series overlie the Caerbwdwy Sandstone with disconformity or possibly unconformity (JoNEs, 1940). These three sequences of suggested lower Middle Cambrian age include a high proportion of grey or dark green greywackes; granule conglomerates are also present, particularly in south Wales. The greywackes usually in 0.5-2 m thick beds are interstratified with mudstones commonly only 1-5 cm thick. In the middle part of the Cilan Grits and towards the top of the Middle Solva Series there are also grey, green, red and blue mudstones and red and light-green sandstones. The Barmouth Grits generally lack these more brightly coloured beds and further differ from the other two sequences in the presence of beds of pebble conglomerate, which are generally 0.5-2 m thick. Within the Barmouth and Cilan Grits most of the greywackes are graded with simple continuous grading or discontinuous grading in which the medium sand grade portion is missing. Some of the greywackes within the Barmouth Grits show multiple grading and pen-symmetrical and inverted symmetrical grading also occur; at Llyn Dwlyan roughly 100 cm thick graded and ungraded pebble beds alternate. The prevalence of graded greywackes alternating with thin mudstones, the absence of fossils and occurrence of sole markings (see below) suggest Palaeogeography, Palaeoclimatol., Palaeoecol., 7 (1970) 113-170

129

A FACIES ANALYSIS OF THE CAMBRIAN OF WALES

deposition of these sediments from turbidity currents. The greywackes generally show either A or AB sequences (Fig.9A,B) and amalgamation is common. In many places where division C is encountered it is convoluted. Measurement of

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the ABC index (WALKER, 1967) in the middle and upper parts of the Cilan Grits and the upper part of the Barmouth Grits (see later) gives high values of P indicating proximal character. Within the A and AB sequences and ungraded greywackes, mud pellets and inclusions of mudstone, siltstone and sandstone, up to 1 m in length, are encountered. Many of these inclusions are bent or have jagged margins, suggesting penecontemporaneous erosion. Wash-outs occur within the Cilan Grits but are more frequent within the Barmouth Grits (Fig.9C,D) where they are at the base of graded greywackes and pebble beds and occur infilled with pebble conglomerate between beds of graded greywacke. Palaeogeography, Palaeoclimatol., Palaeoecol., 7 (1970) 113-170

130

T.P. CRIMES T a b u l a r a n d trough cross bedding occur within the middle part of the Cilan

Grits, usually at the top of greywacke beds with A or AB sequences. M u c h of the current-bedded sediment is well sorted a n d c o m m o n l y rests o n an erosion surface (Fig.9E), suggesting reworking of the turbidity current deposited greywacke by traction currents.

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Fig.10. Palaeocurrent rose diagrams for: a = sole marks, Cilan Grits, St. Tudwal's Peninsula (3 flute cast and 41 groove cast measurements); b = sole marks, Barmouth Grits, Harlech Dome (6 flute cast and 55 groove cast measurements); c = wash-outs, Barmouth Grits, Harlech Dome, mainly southeastern part (41 measurements); d ~ large scale tabular crossstratification, Cilan Grits, St. Tudwal's Peninsula (13 measurements); e = large scale tabular cross-stratification, Lower Solva Series, St. David's area (33 measurements). The map shows distribution of sole mark localities and mean current directions, Cilan and Barmouth Grits•

Palaeogeography, PalaeoclimatoL, Palaeoecol., 7 (1970) 113-170

A FACIES ANALYSIS OF THE CAMBRIAN OF WALES

131

Flute casts are rare but groove casts occur on the base of A and AB turbidite sequences, particularly within the Cilan Grits. Biogenic sedimentary structures are uncommon within the Barmouth Grits but Planolites, and less commonly Sinusites occur in the red and green sandstones and mudstones of the Cilan Grits. A form similar to Palaeodictyon occurs on the base of a greywacke at one locality west of Trwyn-Llech-y-doll and there are bifurcating burrows similar to those described from the Hell's Mouth Grits. The trace fossil assemblage is consistent with the bathyal turbidity current-swept environment inferred from the inorganic structures. Sole markings, mainly groove casts, on the base of greywacke beds of the Cilan Grits suggest northeast-southwest trending current flow (Fig.10,a), but many beds have two sets of groove casts, one set aligned roughly northeastsouthwest and the other east-northeast-west-southwest or east-west. This feature is not seen within the Barmouth Grits which show only a north-northwest-southsoutheast alignment of sole marks (mainly groove casts) and northwest-southeast alignment of wash-outs (Fig.10,b,c). Thirteen measurements of tabular current bedding taken from well sorted sandstones within the Cilan Grits, however, suggest either reworking by or deposition from mainly northwest-southeast aligned currents, most of which were moving northwestwards (Fig.10,d). Thus the turbidity current flow direction in the Cilan Grits appears to be perpendicular to that in the Barmouth Grits. This could be explained by postulating either two different sources or a single source to the southeast of the Harlech D o m e giving rise to initially marginally derived turbidity currents, which by the time they reached St. Tudwal's Peninsula, had swung around to flow axially. The lithologic similarity between the two successions, although not eliminating the possibility of two similar but geographically distinct sources, favours the single source hypothesis. Furthermore, if there were two sediment sources, one shedding turbidity currents northwestwards across the Harlech Dome and the other southwestwards across Caernarvonshire, some mixing of sediment from these two sources might be expected on St. Tudwal's Peninsula. The absence of northwest-southeast aligned sole marks in that area shows, however, that this does not occur. Thus the idea of a single source to the southeast of the Harlech Dome seems more probable. The occurrence of thick pebble zones within the Barmouth Grits, but not the Cilan Grits, also suggests that the former were deposited nearer source. More detailed lithological comparisons of the upper 30 beds of the two sequences can be made by studying the exposures in the Barmouth Quarry and adjoining road in the Harlech D o m e and the coastal section west of Trwyn-Llech-y,doll on St. Tudwal's Peninsula. F r o m Table I it can be seen that average greywacke bed thickness and ratio of sand and silt/mud is greater at Barmouth where the P factor is also higher. Grading is much better and more frequently developed at Barmouth and whereas m a x i m u m grain size at the base of graded beds on St. Tudwal's Peninsula is generally 2 mm, it is commonly 2-5 m m at Barmouth. These changes are all

Palaeogeography, Palaeoclimatol.,Palaeoecol., 7

(1970) 113-170

132

T.P.

CRIMES

TABLE I AVERAGE THICKNESS OF DIFFERENT DIVISIONS OF TURBIDITE UNITS AND INTERTLrRBIDITE SEDIMENT IN CM FOR TOP 3 0 BEDS OF BARMOUTH AND CILAN GRITS

Barmouth Grits CilanGrits Division A Division B Division C Division D Interturbidite P factor * =

62 7 * * * 98

17 21 * * 7 77

negligible.

consistent with the Barmouth area being nearer source and if, as seems possible, the sediments were all derived from a landmass southeast of the Harlech Dome, then these data document down-current variations in turbidite deposition from marginal to axial flow. The Lower-Middle Solva Series differs from these units in north Wales in a general absence of graded bedding, although there are a few beds of graded greywacke especially near the top of the sequence. Cross-stratification generally of the beta or g a m m a type of ALLEN (1963) occurs within some of the greywackes and conglomerates. Some of the coarser beds also wedge out laterally. Ripple drift bedding occurs within the siltstones, whereas asymmetric, symmetric and interference ripples occur on the top of some siltstone and mudstone beds within the Middle Solva Series. There are wash-outs at the base of some greywacke and conglomerate beds in the Lower Solva Series but no sole markings have been seen. Skolithos occurs at the base of the Lower Solva Series and is common within the Middle Solva Series which also contains Planolites, Chondrites and, less commonly, transposed burrows and Sinusites. In general, within the Middle Solva Series, Skolithos is restricted to the red sandstones while burrows parallel to the bedding (e.g., Planolites) and feeding burrows (e.g., Chondrites) are confined to the interbedded green siltstones and mudstones. This is consistent with the sandstones being deposited from relatively high velocity currents which led to the vertical burrowing response while the finer green sediments, accumulated quietly with a high organic content which made sediment feeding a satisfactory method of obtaining nutriment. Thus there would be high oxygenation during deposition of the red sandstones and low oxygenation during accumulation of the green silts and muds. Since red sandstones and green silt or mudstones alternate, they represent only temporary environmental changes and have little palaeogeographical significance.

Palaeogeography, PalaeoclimatoL,Palaeoecol., 7 (1970) 113-170

A FACIES ANALYSISOF THE CAMBRIAN OF WALES

133

The occurrence of generally ungraded pebble conglomerates and conglomeratic sandstones, beds wedging out, large scale tabular cross stratification, and symmetric, asymmetric and interference ripples suggests deposition above wave base and this is also supported by the record of Skolithos. Orientation of large-scale tabular cross bedding suggests deposition of the sediments of the Lower Solva Series by west-northwesterly flowing currents (Fig. 10,e). No evidence has been found in these exposures to indicate whether this is an across-slope or down-slope flow. Since the proximal turbidites of the Barmouth Grits in the Harlech Dome show similar flow directions, it may be a down slope orientation with both units having a southeasterly source. The next Middle Cambrian outcrops east of the Harlech Dome are in the Welsh Borderlands where there is a thickness of about 150 m of clastic rocks of a shallow water shelf facies unconformably overlying Lower Cambrian strata (PoCOCK and WHITEHEAD, 1948). A landmass may therefore have been nearby and may possibly have extended south to the Malverns where Middle Cambrian strata appear to be absent. Pebble composition suggests that the landmass consisted mainly of Precambrian metamorphic and intrusive igneous rocks (NICHOLAS, 1915; MATLEY and WILSON, 1946; and author's personal observations). The later Middle Cambrian The earlier Middle Cambrian beds already discussed are overlain on St. Tudwal's Peninsula and in the Harlech Dome by a sequence composed mainly of red, green, purple and grey mudstones, siltstones and sandstones together with green greywackes. This sequence is known as the Caered Mudstones and Flags on St. Tudwal's Peninsula, where it also includes beds of tuff near the base, and the Gamlan Flags and Grits in the Harlech Dome. The greywacke beds, many of which are graded, are more common to the lower, unfossiliferous, part of both sequences, and are generally 20-100 cm thick, but in the Gamlan Flags and Grits reach 4 m in thickness. The sandstones, many of which are brightly coloured, are mostly ungraded and occur Jn 20-50 cm thick beds. The siltstones, which normally occur in 5-30 cm thick beds, often have parallel and convolute lamination and ripple drift bedding. Symmetric and asymmetric ripple marks also occur on the tops of some siltstone and mudstone beds, particularly on St. Tudwal's Peninsula. The sediments are, in general, coarser in the Harlech Dome where several beds of conglomeratic greywacke and, in the east, a bed of pebble conglomerate occur. Slump folds and penecontemporaneously eroded mud and silt pellets and fragments occur within some of the coarser sediments in the Harlech Dome. A few of the greywacke, sandstone and siltstone beds have sole markings, mainly flute and groove casts. In south Wales the equivalent strata (mainly the upper part of the Solva Series) consist of green mudstones and siltstones with occasional thin (1-10 cm) beds of grey and green fine sandstone. Some of these sediments are laminated and

Palaeogeography, Palaeoclimatol.,Palaeoecol., 7 (1970) 113-170

134

T . p . CRIt,l ~

ripple drift bedding occurs but neither graded bedding nor sole markings have been found. Within the Caered Mudstones and Flags and the Gamlan Flags and Grits sediment filled burrows lying parallel to the bedding are common. Some of the burrows bifurcate frequently and form a mat on the base of the siltstone beds. The diameter of the burrows varies from 2 mm to 3 cm, the wider burrows generally being associated with the coarser sediment. This is presumably because the smaller animals cannot digest or pass back the larger grains. The larger non-branching burrows can be referred to the genus Planolites. The Caered Mudstones and Flags also contain other, as yet undescribed, trails, burrows and resting impressions. In south Wales some of the beds of the Upper Solva Series are bioturbated but no diagnostic forms have been recognized. The lower part of the Gamlan Flags and Grits and the equivalent beds of St. Tudwal's Peninsula are unfossiliferous but the upper part is fossiliferous in the Harlech Dome and has yielded a rich trilobite fauna on St. Tudwal's Peninsula (NICHOLAS,1915, 1916; MATLEY and WILSON, 1946). In south Wales the Upper Solva and Lower Menevian strata are also fossiliferous (Cox et al., 1930). The graded greywackes which occur in the lower, unfossiliferous part of the Harlech Dome and St. Tudwal's successions may have been deposited by turbidity currents. In the Harlech Dome the conspicuous grading, common occurrence of AB or amalgamated units and a P factor of 74 (50 bed measurements) suggest that the turbidites were deposited close to the source of the turbidity currents. The siltstones of north Wales, some of which show BCD or CD sequences and sole marks, may have been transported by turbidity currents from a more distant environment. There is no evidence of turbidJty current activity in south Wales. Deposition of the mudstones which dominate the upper part of all three successions, was presumably from suspension without the interference of turbidity currents, while some of the coarser sediments which lack turbidite features may have been introduced by non-turbidity bottom currents. None of the sediments show features to suggest deposition above wave base but the relative abundance of trilobites, many of which show no signs of having been transported, might be taken to preclude accumulation of the upper beds at considerable depth. No measurable directional sedimentary structures were found in south Wales. On St. Tudwal's Peninsula orientation of ripple drift bedding in siltstones, ripple marks and sole marks associated with siltstone, sandstone and greywacke beds (Fig.ll,a-d) show a bimodal distribution with flow towards the northeast and northwest. The coarser grained beds, however, appear to have been deposited mainly by northwesterly flowing currents. Further evidence of southwest-northeast and southeast-northwest current trends comes from magnetic fabric measurements of the siltstones and fine sandstones (CRIMESand OLDERSHAW,1967). Thus all the available data indicate a Palaeogeography, Palaeoclimatol., Palaeoecol., 7 (1970) 113-170

] 35

A FACIES ANALYSIS OF THE CAMBRIAN OF WALES

15 ¸

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Fig.ll. Rose diagrams for: (a-d) Caered Mudstones and Flags, St. Tudwal's Peninsula: a = current directions from ripple drift bedding (69 measurements); b = trend of ripple marks (21 measurements); c = current directions from flute casts (15 measurements); d = current trend from groove casts (18 measurements); e = Gamlan Flags and Grits, Harlech Dome, current directions from sole marks (8 flutes, 33 grooves). Map shows distribution of sole mark localities and mean current directions, Caered Mudstones and Flags, Gamlan Flags and Grits.

Palaeogeography, Palaeoclimatol.,Palaeoecol., 7 (1970) 113-170

136

T.P. CRIMES

bimodal dispersal pattern on St. Tudwal's Peninsula. This bimodal distribution of palaeocurrent trends may also exist in the Harlech Dome but three dimensional measurements of ripple drift bedding were not possible, few ripple marks were found and the deformed state of the sediments precluded meaningful magnetic fabric studies. Observations were limited to flute and groove cast measurements (Fig. 11,e and map) which nevertheless tended to suggest deposition of the coarser sediments of the lower part of the sequence from northwestward flowing currents, while the siltstones of the remainder of the succession were apparently transported by northward flowing currents. Thus, in north Wales most of the coarse grained sediment deposited from turbidity currents was being transported northwestwards with the Harlech Dome nearer source and hence receiving coarser sediment. North and northeastward flowing currents were responsible for movement of much of the finer grained material. Middle Cambrian sediments appear to be absent in north Caernarvonshire; in the past this has often been taken to imply that the area was a landmass at that time (e.g., GEORGE, 1963), although no unconformity has been demonstrated between Lower and Upper Cambrian and the contact is usually unexposed or faulted. The facies analysis of areas nearby to the south, discussed above, also fails to provide any convincing evidence of a Middle Cambrian landmass in north Caernarvonshire. Thus, pending remapping of the area, such a suggestion is not adopted here. Higher Middle Cambrian sediments of the Harlech Dome, St. Tudwal's and south Wales consist mainly of black pyritous mudstones, sometimes with interbedded siltstones or silty laminae. In the Harlech Dome, however, the Cefn Coch Grits are much coarser and the top 30 m of the Middle Cambrian in south Wales consist of coarse greywackes and shales. In all areas grading is rare within this part of the sequence and sole markings absent. Ripple drift bedding and convolute lamination occur infrequently. The sediments are commonly bioturbated and in some places highly fossiliferous with an abundant trilobite fauna (NICHOLAS, 1915, 1916; Cox et al., 1930; MATLEY and WILSON, 1946). The position of the source of these sediments is problematical: there are insufficient directional sedimentary structures to infer a transport direction with confidence. Ripple drift bedding orientation measurements were only practicable on the water worn surfaces on St. Tudwal's Peninsula and in oriented cores rejected during magnetic fabric analysis. Only twelve measurements were possible, seven of which indicate current flow from the northwest. Magnetic fabric analysis also showed only a poorly defined northwest-southeast trend of maximum magnetic susceptibility axes from which no reliable palaeocurrent direction could be deduced. From a study of the geochemistry of these sediments in north and south Wales, however, PRICE (1963, p.195) has concluded that they may have been derived not only from northwest but also from southwest and east-southeast. The extensive benthonic fauna associated with these sediments in some parts

Palaeogeography, PalaeoclimatoL, PalaeoecoL, 7 (1970) 113-170

137

A FACIES ANALYSIS OF THE CAMBRIAN OF WALES

of Wales is suggestive of relatively shallow water deposition; this may also be implied by the low cobalt and nickel content (PRICE, 1963, p.199-200). The abundance of organic carbon, uranium, possibly sulphur and vanadium per unit K20 in the rocks of the St. Tudwal's and south Wales areas as compared with the Harlech Dome, implies greater organic activity in the former areas (PRICE, 1963, p. 196). This roughly west--east facies change may have resulted from shallower water conditions in the west. Deposition of these sediments was followed by uplift which was certainly strongest in the northwest on St. Tudwal's where it is indicated by a significant stratigraphical hiatus (NICHOLAS, 1915), and apparently weak or absent in south Wales and the Harlech Dome where the sequence is allegedly conformable (Cox et al., 1930; MATLEYand WILSON, 1946).

The Maentwrog Stage Meantwrog Stage sediments are exposed in all four main outcrops of Cambrian strata. In all parts of Wales except north Caernarvonshire they consist largely of dark blue, buff and grey laminated shales with 6 cm-2 m thick siltstone and fine sandstone beds. On St. Tudwal's Peninsula the lowest metre consists of a calcareous conglomerate and the lowest 40 m of the Maentwrog shows a marked vertical facies change with thickness and frequency of arenaceous beds decreasing upwards (Fig.12). Although conglomerates are lacking, a similar upward facies y.

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7 P factor Fig. 12. Graph showing the varying relationship between P factor and ratio of siltstone and sandstone (turbidite) to shale (interturbidite) at different levels in the Maentwrog Stage sediments of Porth Caered, St. Tudwal's Peninsula. Each point represents average of measurements on 20 beds. The numbers 1-7 refer to the upward stratigraphic order of the 20 bed intervals above the basal disconformity.

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138

T.P. CRIMES

change is evident in the other Maentwrog outcrops. The following 300 m of the sequence, best exposed in the Harlech Dome area, consists of shales with occasional coarser beds (2 cm-1 m thick), which in some places occur in groups, while the topmost 300 m consists almost exclusively of shales. Flute casts are common on the bases of the sandstones and siltstones, and conical, linguiform, comet and bulbous types occur. Longitudinal ridges and furrows, some showing fleur-de-lys pattern and groove casts, prod casts and bounce marks also occur. In addition the bases of some of the thicker beds show load casts. Parallel lamination occurs in at least part of most siltstone beds. Convolute lamination is common in the siltstones and, although it may affect complete beds, generally occurs in the upper part of the bed. Convolution crests sometimes occur on bedding planes as a series of cones but linear crest ridges are more common and on St. Tudwal's Peninsula some are oriented perpendicular to current trend with the folds either facing vertically upwards or down current. In the Harlech Dome crests are generally parallel to current trend. Ripple drift bedding is common in the siltstones and symmetric and rare linguoid ripples occur on the tops of some beds. There are also a few downwardly injected sandstone dykes and structures indicative of diapiric uprise of mud through sand. Fossils are rare in the Maentwrog Beds. Trace fossils are also uncommon but a single specimen of ?Nereites has been found near the base of the succession on St. Tudwal's Peninsula and there are irregular burrows on the base of some siltstones. Rusophycus occurs in profusion on the base of a parallel laminated siltstone supposedly high in the Maentwrog sequence, at Ynys Cyngar, Portmadoc. The sedimentary structures associated with these beds together with the alternation of shales with coarser sediments and the rarity of fossils, suggest deposition by turbidity currents. The record of ?Nereites also indicates a deeper water turbidite facies. The occurrence of Rusophycus, however, suggests that shallower water conditions may have prevailed by high Maentwrog times but the rocks in question, which have yielded no body-fossils, may be of Ffestiniog age. The sequence of sedimentary structures in the coarser sediments also approximates to the turbidite facies model but the lower graded interval is invariably absent. Measurement of the P factor at Porth Caered on St. Tudwal's Peninsula (Fig.12) gives values of P = 48 for the lower beds. Higher in the sequence the turbidite beds are thinner; there is a decrease in P and the ratio sand and silt/mud ( = turbidite/interturbidite) (Fig.12). Some of the lowest beds are amalgamated, particularly on St. Tudwal's Peninsula and bed thicknesses are by no means constant laterally. These observations are consistent with the lower beds having been deposited in a more proximal situation than the higher ones. A swing in palaeocurrent trend is also associated with this vertical facies change. Sole markings indicate that on St. Tudwal's Peninsula the lowest beds were deposited from easterly flowing currents whereas higher beds show northeasterly and then northerly flow directions. A similar swing in current trend but from

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A FACIES ANALYSISOF THE CAMBRIAN OF WALES

139

northeasterly to northerly can also be detected in equivalent sediments within the Harlech D o m e , while in south Wales the lowest M a e n t w r o g sediments were also deposited from easterly flowing currents. D u r i n g the r e m a i n d e r of the M a e n t w r o g

40-

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Fig.13. Palaeocurrent data for Maentwrog Stage sediments of Wales from: a = flute casts (125 measurements); b = groove casts (137 measurements). Map shows distribution of localities and mean palaeocurrent trends.

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140

T . P . CRIMES

Stage sole mark orientation shows that currents flowed to the north-northeast or north in all parts of Wales (Fig.13). The Middle-Upper Cambrian stratigraphic break is strongest in the northwest on St. Tudwal's Peninsula. Thus the widespread occurrence in the lowest part of the Upper Cambrian sequence of proximal turbidites derived from the west, succeeded by more distal turbidites derived from the south, suggests a westwards transgression with the lower beds derived laterally from a nearby but receding western landmass. The higher beds could then have been deposited from axially flowing turbidity currents which originated by lateral derivation from a landmass by then situated further to the west. The trough presumably continued northward into north Caernarvonshire where sediments of suggested Maentwrog age (WrLLIAMS, 1930) consist of black slates, banded mudstones, siltstones and occasional sandstones, with a pisolitic iron ore horizon near the middle of the succession. The siltstones show parallel lamination and ripple drift bedding. The pisoliths, composed largely of chamosite, suggest deposition of at least the middle part of this sequence, in relatively shallow water (PORRENGA, 1967). The position of the margins of the area of deposition during Maentwrog times are uncertain. To the west sediments of possible Upper Cambrian age in Leinster, showing similar palaeocurrent trends to the Welsh deposits, have so far yielded no zonal fossils (CRIMES and CROSSLEY, 1968). To the east shales of Maentwrog age have been reported from the English Midlands (EoMUNDS and OAKLEY, 1958) but there is an absence of equivalent strata in the Welsh Borderlands (PococK and WHITEHEAD, 1948). The lithologic similarity between the Maentwrog Stage sediments in north Wales and those in south Wales and, more particularly, the presence of northeastward and northward flowing currents in both areas, suggests derivation at least in part from a landmass beyond south Wales. The evidence of westwards transgression in the lowest beds suggests that the landmass may have been situated to the southwest rather than southeast of Wales. Study of the mineral assemblages in the siltstones of the Harlech Dome area led WOODLAND(1946) to conclude that the source rocks may have been composed largely of metamorphic and intrusive igneous rocks.

The Ffestiniog Stage Harlech Dome, St. Tudwal's lsland and south Wales. The Ffestiniog Stage sediments of St. Tudwal's Island, the Harlech Dome and south Wales consist of shales together with beds of siltstone and fine sandstone or greywacke, frequently 8-60 cm thick, although thicknesses in excess of 1 m are not uncommon. Some beds, however, show lateral variations in thickness over a few metres. In general the coarser beds are infrequent in the lower part of the succession but dominate the Palaeogeography, PalaeoclimatoL,Palaeoecol., 7 (1970) 113-170

A FACIES A N A L Y S I S OF T H E C A M B R I A N OF W A L E S

141

upper part, except for the top 20 m, which usually consists of interbedded fine siltstones and shales. The coarser grained beds rarely show grading. Some beds, particularly in the lower part of the sequence, have parallel lamination above and below ripple drift bedding but beds composed entirely of unlaminated, parallel laminated or convolute laminated siltstone are more common. Towards the top of the sequence large scale, tabular, often low angle, cross-stratification is common, sometimes of xi type (ALLEN, 1963) and some beds show marked lateral thickness variations. Flow rolls also occur within the siltstones, particularly in the thicker beds in the middle and upper parts of the sequence. To the north of the Harlech Dome intrabed slumping is also common within the siltstones near the top of the sequence. Symmetric and asymmetric ripples are common in all areas but are best developed near Porth-y-rhaw in Pembrokeshire where at one locality most beds are ripple marked (Fig.14). Linguoid and interference ripples also occur, being best seen

Fig.14. Sinuous crested ripple marks, Ffestiniog Stage east of Porth-y-rhaw, Pembrokeshire. near Porth-y-rhaw and at Borth-y-Gest respectively. In many parts of the Harlech Dome, and also in south Wales, tectonic rippling is encountered (Fig.15). This can be distinguished from sedimentary rippling because it generally affects both top and bottom surfaces of the bed. It is also parallel to local tectonic fl lineations. Palaeogeography, Palaeoclimatol., Pa!aeoecoL, 7 (1970) 113-170

142

T . P . CRIMES

Fig.15. Tectonic rippling, parallel to nearby fold axes, Maentwrog Stage, Solva, Pembrokeshire.

Flute casts and load casts are found occasionally; groove casts are more common (Fig.16). Care must, however, be taken to distinguish true groove casts from fine tectonic lineations. The former are generally less regular in trend and relief than the latter and may also be accompanied by other sole markings, such as prod marks. Trilobites are rare but Lingulella occurs at all levels and is particularly abundant in the top 20 m where many bedding planes are crowded with complete valves. This horizon, mapped by several authors as the "Lingulella Band", occurs on St. Tudwal's Island, all around the Harlech Dome and in south Wales. Biogenic sedimentary structures are particularly abundant in these sediments. Irregular, non-diagnostic trails, and burrows generally with a diameter less than 0.25 cm, are preserved on the base of many of the siltstone beds. Rusophycus and Dimorphichnus occur in the middle and upper parts of the Ffestiniog sequence while the higher beds also contain Cruziana, Diplichnites and other arthropod tracks. Skolithos also occurs, particularly in association with some of the Lingulella-rich beds. These trace fossils, which are described in greater detail in Cmrcms (1970), indicate that most of the sediments of the Ffestiniog Stage belong to the sublittoral Cruziana facies while the occurrence of Skolithos suggests that the highest sediments may have accumulated in an infralittoral or littoral environ-

Palaeogeography, PalaeoclimatoL,PalaeoecoL, 7 (1970) 113-170

A FACIES ANALYSISOF THE CAMBRIANOF WALES

143

Fig.16. Flute casts with later groove casts. The groove casts within the arrowed flute cast show the current flow pattern produced by the flute cast hollow. Upper Cambrian (low Ffestiniog Stage?), Tan Fannau, southwest of Harlech Dome. ment. Deposition above wave base is also supported by the complex interference ripples, symmetric ripples and large scale, tabular cross bedding found in the middle and upper parts of the sequence and the occurrences of Lingulella, generally accepted as a shallow water form. Shallower, intertidal, conditions are implied by the mudcracks found by FEARNSIDES (1912), near the top of the succession in the Skolithos bearing beds. Current flow to the north is indicated by sole mark orientation in the Harlech D o m e and St. Tudwal's Peninsula (Fig.17). In south Wales sole mark orientation measurements (2) and ripple mark orientation measurements (90) also indicate northerly flowing currents. Southerly derived Ffestiniog siltstones in the Portmadoc area, however, have slump folds and flow rolls which are aligned approximately northeast-southwest and where determinable face southeastwards, possibly indicating a palaeoslope in that direction (Fig. 18, 19). The concept of a southeasterly inclined slope on which northerly moving material would collect and slip down is consistent with the palaeogeography of north Caernarvonshire to be described shortly. The palaeocurrent evidence therefore indicates derivation of the siltstones and fine sandstones, in Pembrokeshire at least, from beyond south Wales (Fig. 17).

Palaeogeography, PalaeoclimatoL, Palaeoecol., 7 (1970) 113-170

144

T . P . CRIMES

15

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Fig.17. Palaeocurrent data for Ffestiniog Stage sediments, a and b: rose diagrams for, respectively, flute cast directions (50 measurements) and groove cast trends (149 measurements) from Wales excluding north Caernarvonshire; c and d: rose diagrams for current direction from asymmetric ripple marks (14 measurements) and symmetric ripple mark trends (76 measurements) from the St. David's area, south Wales. Map shows distribution of all sole mark localities, numbers of measurements and mean current trend.

Palaeogeography, Palaeoclimat oL, P alaeoeeoL, 7 (1970) 113-170

A FACIES ANALYSISOF THE CAMBRIAN OF WALES W <

E

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Fig.18. The use of the concept of facing in determining the direction of transport of slumped beds. A fold faces in a direction normal to its axis, along the axial plane and towards the younger beds (SHACKLETON,1958). A. The slumped graded bed faces west in the direction of sediment transport. B. The geometrically identical slumped graded bed faces east also in the direction of sediment transport. C. Slumps in current bedded siltstone faces southeast, Ffestiniog Stage, Borth-y-Gest. D. An isolated slump fold closure composed of graded siltstone faces southeast, Ffestiniog Stage, Borth-y-Gest. 15

10

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& A Fig.19. A. Trend of slump fold axes (26 measurements), Ffestiniog Stage, Borth-y-Gest. Arrow indicates facing direction where determinable (3 measurements). B. Trend of elongation of flow rolls in a single bed, Ffestiniog Stage, Borth-y-Gest.

L i t h o l o g i c a l affinities with the M a e n t w r o g sediments suggest derivation f r o m the same source, possibly situated southwest o f south Wales.

Palaeogeography, Palaeoclimatol.,Palaeoecol., 7 (1970) 113-170

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A FACIES ANALYSISOF THE CAMBRIANOF WALES

147

The method of transportation raises problems. KOPSTEIN (1954) and also EVANS et al. (1966) have suggested that the siltstones were carried by turbidity currents. This may be correct for some of the beds from the lower part of the sequence, but higher-up few beds show many turbidite characteristics. The biogenic and inorganic sedimentary structures show that in all probability the higher part of the Ffestiniog sequence accumulated in a sublittoral environment above wave base. It is difficult to conceive of material being transported by turbidity currents from beyond south Wales to north of the Harlech Dome in such shallow water. Furthermore, some bedding planes have up to 20 Rusophycus (trilobite resting impressions) oriented parallel to, but not affected by, current markings. Most of the impressions indicated that the animals faced upstream. Such a trilobite lifeorientation is likely in response to a slow moving bottom current but not a turbidity current. In general then, the evidence militates against movement of most of these sediments by turbidity currents; less spasmodic bottom currents appear a more likely alternative.

North Caernarvonshire. When traced northwest from the Harlech Dome, the Ffestiniog Stage sediments show a marked facies change (Fig.20). Beds of orthoquartzite up to 2 m thick appear in the sequence, and on the slopes of Moel Hebog three main quartzite horizons can be mapped (SHACKLETON, 1959). Traced further

Fig.21. Symmetric ripple marks, wavelength about 50 cm, in granule conglomerate. Ffestiniog Stage, northern wall of Cwm Graianog, Bethesda, north Wales.

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148

T . P . CRIMES

northwards into north Caernarvonshire the sediments become coarser and between Bethesda and Llyn Cwellyn 30-180 cm thick, beds of granule conglomerate, medium-coarse sandstone and greywacke, dominate the succession. Some beds thicken and thin, or disappear, laterally within a few metres and often beds of coarser sediment follow one another with little or no intervening mud. Within these outcrops few beds are graded; indeed some of the sandstones are well sorted throughout. Parallel lamination occurs within some of the siltstones and orthoquartzites in the Hebog area but is uncommon within the coarser clastics further north.

Fig.22. Symmetric ripple marks in granule conglomerate. Ffestiniog Stage, northern wall of Cwm Graianog. Ripple drift bedding and convolute lamination are also uncommon except in the siltstones of the Hebog area. Large scale cross-bedding occurs within the orthoquartzites but is common within the conglomerates, sandstones and greywackes. Alpha, beta and xi types are most abundant but g a m m a and epsilon types are also encountered (ALLEN, 1963). Flute and groove casts are not uncommon, particularly on the base of some of the fine-medium grained sandstones and greywackes. Ripple marks occur on the tops of the coarser grained beds throughout the area but are particularly evident in the northeastern outcrops. Symmetric, asymmetric, flat topped and interference forms are common and linguoid and cuspate

Palaeogeography, PalaeoclimatoL,Palaeoecol., 7 (1970) 113-170

A FACIES ANALYSIS OF THE CAMBRIAN OF WALES

149

types also occur (Fig.21-24). Ripple wavelength varies from a few centimetres to over 75 cm with amplitudes from a fraction of a centimetre to l0 cm. The larger ripples are associated with the coarser sediment. Many ripples have internal cross bedding and appear to have been formed under conditions of net deposition but some show an internal structure clearly unrelated to the ripple form and are presumably erosional features.

Fig.23. Interference ripples in coarse sandstone, Ffestiniog Stage, northern wall of Cwm Graianog. Numerous bedding planes in Cwm Graianog are covered with straight crested ripple marks which can be followed over many tens of metres (Fig.25). The best exposed ripples, of symmetric, asymmetric and flat topped type, affect sediment of medium-coarse sand or granule conglomerate grain size, and have wavelengths of about 60 cm and amplitudes of about 5 cm. Measurements on thirteen extensively exposed ripple marked sandstone beds gave an average ripple index of 12. A few blocks of mud-cracked sediment have also been found lying near the top of the succession in Cwm Graianog, The sandstones and interbedded mudstones yield Lingulella which are abundant in the higher beds in the Hebog area. The only other body fossil yet recorded is a trilobite (Olenus micrurus) from Marchlyn Mawr (RAMSAY, 1866), although trilobite resting impressions have been found by the writer above Cwm Graianog (CRIMES, 1970).

Palaeogeography,Palaeoclimatol.,Palaeoecol., 7 (1970) 113-170

150

T. l'. CRIMES

Biogenic sedimentary structures increase in abundance and variety northeastwards. Cruziana semiplicata occurs north and northeast of Moel Hebog (CRIMES, 1969a) but can be found in abundance between Llyn Peris and Bethesda (CRIMES, 1968). In these eastern exposures Rusophycus is common and Phycodes, Diplichnites, Dimorphichnus, and Planolites occur and are described in CRIMES (1970). Skolithos occurs locally, especially near the top of the succession. From this assemblage it is clear that the sediments belong to the Cruziana facies, although the Skolithos facies may be represented in the higher beds.

Fig.24. Slightly asymmetric ripples with internal cross bedding in granule conglomerate, Ffestiniog Stage, northern wall of Cwm Graianog. The coarser sediments in the eastern outcrops near Bethesda have, by implication, been considered as turbidites by EVANS et al. (1966), who postulated that they may have been deposited near the axial zone of an undefined trough. Sole markings are not uncommon at certain horizons and some of these sediments are greywackes but they differ from typical turbidites in the following respects: (1) The beds are not usually graded nor do they show a typical turbidite sequence of sedimentary structures. (2) Some of the sediments are well sorted. (3) Both bottom and top surfaces of beds are often sharply defined. (4) Sediment dispersal patterns are quadrimodal (see below). (5) Symmetric and asymmetric ripple marks are common and affect coarse

Palaeogeography, PalaeoclimatoL,PalaeoecoL, 7 (1970) 113-170

A FACIES ANALYSISOF THE CAMBRIANOF WALES

151

Fig.25. Symmetric and flat topped ripple marks, wavelength about 60 cm, in granule conglomerate, Ffestioniog Stage, northern wall of Cwm Graianog. grained sediment. (6) Interference and flat topped ripple marks occur. (7) Beds may be of variable lateral thickness. (8) Large-scale tabular cross bedding is common and includes alpha, beta, xi, epsilon and g a m m a types (ALLEN,1963). (9) The biogenic sedimentary structures are characteristic of the Cruziana and Skolithos facies, not of the Nereites facies with which turbidites are usually associated. These sediments apparently were not transported by turbidity currents. The well sorted nature of some of the sediments, abundance of ripple marks and largescale cross stratification, together with variations in lateral bed thickness, suggest deposition in a sublittoral environment. Interference ripples, although reported from deep water, are more commonly formed above wave base and marine studies suggest that flat topped ripples are restricted to deposits laid down within reach of tidal scour. The occurrence of Lingulella further suggests shallow water conditions and the designation of most of these sediments to the Cruziana facies also implies deposition above wave base under conditions of frequent current scour which led trilobites to adopt a burrowing or furrowing habit for protection. Preservation of extensive ripple marked surfaces requires calm conditions following ripple mark

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152

T.P.

CRIMES

development. Such conditions are most likely to occur well below the intertidal zone where waves are only effective periodically, probably under storm conditions. The combined evidence of inorganic and biogenic sedimentary structures therefore suggests deposition of most of the Ffestiniog Stage sediments of north Caernarvonshire between wave base and low water mark.

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D Fig.26. Rose diagrams for Ffestiniog Stage sediments in north Caernarvonshire showing: A. Current direction from flute cast orientation (32 measurements). B. Current trend from orientation of groove casts (31 measurements). C. Current directions from orientation of cross-strata (95 measurements). D. Trend of symmetric ripple marks (85 measurements). E. Current direction from asymmetric ripple marks (23 measurements).

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A FACIES ANALYSIS OF THE CAMBRIAN OF WALES

153

The shallow water features and coarse grain size of these sediments implies a local source. The restriction of this facies to the northwestern part of north Wales and the northwestward increase in grain size, suggest that the landmass lay in that direction, with an associated southeasterly palaeoslope. It certainly could not lie to the south where finer grained sediments were deposited from northerly flowing currents. Palaeocurrent measurements were taken from four types of directional sedimentary structure: flute casts, groove casts, ripple marks and cross bedding. Most of the flute casts were in fine-medium grained sandstone near the middle of the succession while ripple mark and cross bedding orientation measurements were mainly from the coarser sediments above and below. Groove casts occur infrequently throughout the succession. Flute cast and groove cast orientation measurements (Fig.26A,B) indicate local transport of sand generally to the southwest. Current bedding measurements (Fig.26C) suggest that the coarser sediments were transported by currents of variable orientation with a preference for west-northwesterly and east-southeasterly movement. Ripple mark crests show a northeast-southwest preferred orientation with asymmetric types suggesting sediment transport and erosion by southeasterly and northwesterly flowing currents (Fig.26D,E). The quadrimodal dispersal pattern indicated by these results provides further evidence in favour of shallow-water, non-turbidite deposition. The trend of the sedimentary structures is also consistent with the southeasterly dipping palaeoslope postulated from the regional facies distribution. On this model, ripple mark trend in the coarser sediments would approximately parallel the shore line, while the finer sands with evidence of southwesterly flow might have been transported by across slope bottom currents. This is consistent with modern marine studies which show a common parallelism between offshore ripples and coast line and also across slope bottom current flow (see VAUSE, 1959; POTTERand PETTIJOHN, 1963; KLEIN, 1967). Thus the Ffestiniog Stage sediments of the Harlech Dome, St. Tudwal's Island and south Wales appear to have been derived from the south, deposited in a shallowing sea, whereas the equivalent sediments in north Caernarvonshire appear to have been derived from a nearby landmass to the northwest and deposited in a sublittoral environment. Neither Maentwrog nor Ffestiniog Stage strata are exposed in the Welsh Borderlands.

The Dolgelley Stage and Tremadoc Autochthonous Dolgelley Stage and Tremadoc strata are only exposed in the vicinity of the Harlech Dome and the Welsh Borderlands. The rocks consist mainly of grey and black slates and shales together with subsidiary siltstones, some beds of which are calcareous and show parallel lamination and ripple drift bedding. Beds of greywacke, occasionally graded, also occur in the upper part of the Tremadoc in north Wales. No inorganic sole markings or measurable directional Palaeogeography, Palaeoclimatol., Palaeoecol., 7 (1970) 113-170

154

T . P . CRIMES

structures have been found. Fossils, particularly graptolites, brachiopods and trilobites, are abundant and burrows are also common. Deposition oftlie graptolitic mudstones must have occurred under calm water conditions but the trilobitebrachiopod fauna suggests no great depth of water. The total thickness of sediment does not exceed 400 m in the Harlech Dome but it is at least 1,000 m in the Welsh Borderlands. Clearly the locus of maximum sedimentation and, probably, the trough axis, had shifted eastwards away from north Wales. t H E SEDIMENT SOURCES AND TROUGH MARGINS

From the foregoing account the Welsh Cambrian sediments seem to have been derived from source areas situated to the northwest, east or southeast and southwest. A landmass to the northwest was postulated by JONES (1938) and named the "Irish Sea landmass". It is proposed to refer to the easterly source area as the Welsh Borderlands landmass; and the southwesterly one as St. Georges landmass, after St. Georges Channel which at present time probably occupies at least part of the area of the ancient landmass. The Irish Sea landmass was most active as a sediment source during late Lower Cambrian and middle Upper Cambrian (Ffestiniog Stage) times when substantial thicknesses of coarse clastic sediment were distributed over northwestern Wales. In the former case the sediment was transported into deep water by turbidity currents but in the latter example deposition was in shallow water by non-turbidity bottom currents. The onset of supply of sediment from the Irish Sea landmass was in each case marked by a sudden vertical facies change with the arrival in the trough of a great volume of coarse grained immature sediment (Fig.27). These sudden changes suggest rapid, probably fault controlled, uplift of the source area. In north Caernarvonshire during the Ffestiniog Stage, the northwesterly facies change, evidence of southeasterly palaeoslope and trends of directional sedimentary structures, are all consistent with a northeast-southwest trending shoreline only a few kilometres to the northwest. The situation and alignment of such a shoreline is in agreement with it being controlled by uplift along one of the major northeastsouthwest trending faults on Anglesey such as the Berw fault, which GREENLY (1919) has suggested to have a history of movement from the Precambrian to the Upper Palaeozoic. A fault controlled landmass in this position or, more likely, to the northwest is also consistent with the evidence on the source of the high Lower Cambrian proximal turbidites. Similarly oriented faults, up-throwing to the southeast during Lower Palaeozoic times have also been recorded on the other, northwestern, flank of the Irish Sea landmass in County Wexford (CRIMES and CROSSLEY, 1968). An abrupt vertical facies change at the base of the Middle Cambrian sequence in north Wales, with the arrival of mostly turbidity current transported, apparently southeasterly derived, immature coarse grained clastic sediment, Palaeogeography, PalaeoclimatoL, PalaeoecoL, 7 (1970) 113-170

A FACIES ANALYSIS OF THE CAMBRIAN OF WALES

155

suggests rapid probably fault controlled, uplift of the Welsh Borderlands landmass. Poor exposure and the possibility of cumulative unconformities preclude accurate location of the landmass but the thinning of the Lower and Middle Cambrian strata from a thickness of over 3,000 m in the dominantly turbidite sequence in the Harlech Dome to less than 500 m in the shallow water succession of the Welsh Borderlands suggests the presence of a landmass in the vicinity of the latter area. The nearby northeast-southwest trending Church Stretton fault was repeatedly active during the Palaeozoic (RAsT, 1969) and since Lower and Middle Cambrian strata are absent immediately to the west of it, intra-Cambrian uplift may have occurred along it. The northeast-southwest alignment of the margins of at least the Irish Sea landmass and probably also the Welsh Borderlands landmass therefore tends to confirm the suggestions of JONES (1938) and KNILL (1959) that deposition of the Welsh Cambrian sediments occurred in a northeast-southwest trending trough. It is thus apparent that much of the coarse grained sediment was derived from the northwestern and southeastern trough margins and transported down the trough sides. In contrast, the slltstones, which constitute much of the higher Middle Cambrian and Upper Cambrian sequences, were generally transported approximately northeastwards, roughly parallel to the trough axis, either by distantly derived turbidity currents or other bottom currents. Some, at least, of these finer grained sediments were derived from a landmass southwest of south Wales (St. Georges landmass). The situation of the St. Georges landmass in the ocean to the southwest of south Wales precludes any discussion on its precise location or method of uplift: it could well be along the same northeast-southwest fault lineament as the Irish Sea landmass. DISCUSSION

Attention has recently been focussed on the relationship between depositional environment and sediment dispersal pattern (KLEIN, 1967). Dispersal patterns within the Welsh Cambrian turbidite sequences were generally unimodal but bimodal patterns with the modes roughly 90 ° apart occur within the Rhinog Grits, Cilan Grits and Caered Mudstones and Flags. In the first two units one mode is associated with turbidity current flow directions and the other mode represents the orientation of non-turbidity bottom currents. In the Caered Mudstones and Flags, however, the bimodal dispersal pattern is the result of mixing of sediment deposited from axially and laterally flowing turbidity currents. Bimodal dispersal is allegedly unusual in turbidite sequences (KLEIN, 1967) but not unexpected in the relatively narrow Welsh trough (see also BAILEY, 1969). The shallow-water fine-grained Upper Cambrian (Ffestiniog) sediments show a unimodal dispersal pattern with bottom current transport parallel to the Palaeogeography, Palaeoclimatol., Palaeoecol.. 7 (1970) 113-170

156

T.P. CRIMES

trough axis over a distance of 4- 150 km. In contrast the shallow water, locally derived, coarse grained, Ffestiniog Stage sediments of north Caernarvonshire have a quadrimodal dispersal pattern. Modal classes are approximately 90 ° apart with, in general, finer grained sands transported parallel to the trough axis and coarser sands and conglomerates perpendicular to the axis. Accurate correlation at formation level between the Middle and Upper Cambrian rocks of south Wales, the Harlech Dome and St. Tudwal's Peninsula also allows documentation of downcurrent facies changes. It has been shown that when traced about 40 km downcurrent and probably essentially down the trough margin the coarse grained proximal turbidites of the Barmouth-Cilan Grits show a reduction in individual bed thickness, decrease in grain size of the coarsest detritus and lower P values, consequent upon reduced occurrence of the graded interval. The sand/mud ratio also decreases. The same downcurrent variations have been noted by CONNOLLYand EWING (1967) in a study of recent turbidites in the Puerto Rico Trench and are consistent with the transition from a proximal to a slightly more distal environment (see WALKER,1967, p.32). The axially transported fine grained distal turbidites of the main part of the Upper Cambrian Maentwrog Stage sequence, however, show only a slight reduction in grain size of the coarsest fraction and a corresponding decrease in the frequency of occurrence of the graded interval when traced from south to north Wales, a downcurrent distance of 4- 150 km. Similarly, in the Polish Carpathian flysch DZULYNSKI et al. 0959) suggest that turbidites deposited by axially flowing currents show little or no downcurrent lithological change whereas coarser sediments deposited on the trough margins show a downcurrent reduction in grain size. If, at least some, axially flowing turbidity currents are laterally derived, these observations suggest that they may deposit much of their coarse grained sediment and hence produce the most marked facies changes before, during or shortly after they swing into an axial orientation. Presumably, deceleration would start as the current approached the axial zone and the gradient decreased. Once the turbidity current changed its flow direction rapid deceleration would result in deposition of thick beds of relatively coarse grained sediment. Rapid sedimentation can be expected to continue for some distance as the current adjusts itself to the decreased gradient. As the flow travels further down what might be a relatively constant and gentle axial plunge, the rate of deceleration will be reduced. In an elongate trough deposition of the finer grained sediment will then give a fairly constant lithology over a considerable distance. Thus, as evidenced in the Welsh Cambrian and the Polish Carpathian flysch, proximal turbidites carried down the flanks of a trough can be expected to be more areally restricted and show more rapid downcurrent facies changes that distal turbidites carried by axially flowing turbidity currents. Most of the turbidity current transported coarse grained sediment appears to have been introduced into the Welsh Trough from fault controlled landmasses. Palaeogeography, PalaeoclimatoL, PalaeoecoL, 7 (1970) 113-170

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A FACIES ANALYSIS OF THE CAMBRIAN OF WALES

165

This association between fault controlled landmasses and turbidity currents may not be fortuitous; not only would rapid uplift provide the sediment but the intense earthquake activity which would be associated with such faults, may have provided t h e triggering mechanism for the turbidity current flows. According to the classic interpretation of JONES (1938) the Lower Palaeozoic rocks of Wales were deposited in a geosyncline. This has been more recently referred to by KAY (1951) as a eugeosyncline. If one accepts a geosyncline simply as "a surface of regional extent subsiding deeply during accumulation of succeeding rocks" (KAY, 1951, p.107) and eugeosynclines as long and narrow geosynclines which "lie in belts of active vulcanism in association with relatively rapid subsidence" (KAY, 1951) then the Welsh Lower Palaeozoic sequence can be just about made to fit both sets of requirements. Eugeosynclines, however, are normally characterized by ophiolites and these are absent in Wales. Also many authors (e.g., AUBOUIN, 1965) accept that geosynclines have a definite pattern of evolution involving an initial deepening of the trough and ending with a period of immediately pre-tectonic flysch-like sedimentation. The Welsh Lower Palaeozoic rocks do not easily fit into this pattern, especially since deeper water flysch-type sedimentation occurs not only at the end of the cycle (Silurian) but also near the beginning (Cambrian). Thus, Welsh Cambrian sedimentation, terminated as it was by strong preArenig movements, does not readily fit into a Lower Palaeozoic "geosynclinal" pattern.

CONCLUSIONS: THE EVOLUTION OF THE T R O U G H

Marine Cambrian sedimentation commenced in Wales with the deposition of a shallow water clastic transgressive sequence (Fig.27). Shallow water conditions persisted in south Wales throughout Lower Cambrian times but in north Wales rapid subsidence subsequently allowed the accumulation in deep water of a thick sequence composed mainly of interbedded mudstones and turbidity current deposited siltstones and greywackes. At least some of the greywackes were apparently initially derived from the fault controlled Irish Sea landmass nearby to the northwest (Fig.28A). At the same time, however, sand and conglomerate was transported by bottom currents westwards possibly from the Welsh Borderlands landmass and deposited within the turbidite sequence in the Harlech Dome area. By now the trough had a roughly northeast-southwest trend which appears to have persisted throughout Cambrian times with the Irish Sea and Welsh Borderlands landmasses generally limiting it to the northwest and southeast respectively. These landmasses appear to have suffered a period of peneplanation towards the close of Lower Cambrian times. Thereafter the supply of coarse clastics was temporarily reduced and manganese rich sediments were deposited in a quiet water environment. Palaeogeography, Palaeoclimatol., Palaeoecol., 7 (1970) 113-170

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Palaeogeography, Palaeoclimatol., Palaeoecol., 7 (1970) 113-170

A FACIES ANALYSIS OF THE CAMBRIAN OF WALES

167

During late Lower or early Middle Cambrian times there was emergence in south Wales where subsequent Middle Cambrian sedimentation commenced with a second transgressive sequence and continued in shallow water with some of the sediment transported by west-northwestward flowing non-turbidity bottom currents. In north Wales, however, deeper water conditions persisted into Middle Cambrian times (Fig.28B) and a further rapid, probably fault controlled, uplift, this time on the southeastern trough margin, led to shedding of greywacke charged turbidity currents over at least the Harlech Dome and probably also St. Tudwal's. Penetration, as far west as St. Tudwal's Peninsula, of turbidity currents derived from the southeastern margins of the trough would imply that during this period the trough axis was near or to the northwest of St. Tudwal's. The northwestern trough margin therefore probably lay well to the northwest of St. Tudwal's Peninsula. During higher Middle Cambrian times (Fig.28C) supply of coarse grained sediment from the southeast was reduced and an increasing contribution was made by turbidity currents from a distant source, and bottom currents, which transported finer grained sediments northeastwards parallel to the trough axis. At the close of the Middle Cambrian there were profound changes in regional palaeogeography and after a period of non-deposition or erosion in the northwest, Upper Cambrian (Maentwrog Stage) sedimentation commenced with the supply of turbidity current transported silts over the whole of Wales from a nearby landmass to the west. During later Maentwrog Stage times (Fig.28D) turbidity currents of more distant origin, travelling axially, distributed southerly derived silts from at least south Wales to north of the Harlech Dome. Supply of silt to this area from the south or southwest continued during the Ffestiniog Stage (Fig.28E) but transport was mostly by non-turbidity bottom currents and deposition occurred in shallower water. Coevally, there was a quadrimodal palaeocurrent system in north Caernarvonshire which was receiving coarse grained sediment from the nearby fault-uplifted Irish Sea landmass. The margin of the landmass may have lain nearer north Caernarvonshire than previously, thus restricting the trough. Cambrian sedimentation was completed by deposition of a sequence of shallow water quietly accumulated and generally fine grained sediments over at least the Harlech Dome area and the Welsh Borderlands during the Dolgelley Stage and the Tremadoc. At this time sedimentation and subsidence were more rapid in the Welsh Borderlands than in north Wales. The sediments in the trough were then gently folded along roughly northeast-southwest trending axes and there was a period of uplift, strongest in the west, before sedimentation recommenced during Lower Ordovician (Arenig) times. The pattern of Cambrian sedimentation therefore appears to have been more complex than has previously been envisaged and does not readily accord with the early stages of the generally accepted scheme of geosynclinal evolution.

Palaeogeography, PalaeoclimatoL, Palaeoecol., 7 (1970) 113-170

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T.V. CRIMES

ACKNOWLEDGEMENTS

The author is grateful to Drs. R. J. Bailey, J. C. Harper and N. Rast for constructive criticism of the manuscript, to Mr. J. Lynch for redrawing the diagrams, and to Mr. D. Smart for photocopying them. The author also acknowledges receipt of an N.E.R.C. grant from 1963-1966, when part of the field work for this study was undertaken. REFERENCES ALLEN, J. R. L., 1963. Classification of cross-stratified units. Sedimentology, 2: 93-114. AUBOUIN, J., 1965. Developments in Geotectonics, 1. Geosynclines. Elsevier, Amsterdam, 335 pp. BAILEY, R. J., 1969. Ludlovian sedimentation in south central Wales. In: A. WOOD (Editor), The Precambrian and Lower Palaeozoic rocks of Wales. Univ. Wales Press, Cardiff, pp.283-304. BASSETT, D. A., 1963. The Welsh Palaeozoic geosyncline: a review of recent work on stratigraphy and sedimentation. In: M. R. W. JOHNSON and F. H. STEWART (Editors), The British Caledonides. Oliver and Boyd, Edinburgh, pp.35-69. BASSETT, D. A. and WALTON, E. K., 1960. The Hell's Mouth Grits: Cambrian greywackes in St. Tudwal's Peninsula, north Wales. Quart. J. Geol. Soc. London, 116: 95-110. BOSWELL, P. G. H., 1961. The case against a Lower Palaeozoic geosyncline in Wales. Liverpool Manchester Geol. J., 2: 612-625. BOUMA, A. H., 1964. Ancient and recent turbidites. GeoL Mijnbouw, 8: 347-388. CONNOLLY, J. R. and EWING, M., 1967. Sedimentation in the Puerto Rico Trench. J. Sediment. PetroL, 37: 44-59. Cox, A. H., GREEN, J. F. N., JONES, O. T. and PmNGLE, J., 1930. The geology of theSt. David's district, Pembrokeshire. Proc. Geologists Assoc. Engl., 41: 412-438. CRIMES, T. P., 1966a. The relative age of some concretions in Cambrian sediments of St. Tudwars Peninsula, north Wales. GeoL J., 5: 33-44. CRIMES, T. P., 1966b. Palaeocurrent directions in the Upper Cambrian of north Wales. Nature, 210: 1246-1247. CRIMES, T. P., 1968. Cruziana: a stratigraphically useful trace fossil. GeoL Mag., 105: 360-364. CRIMES, T. P., 1969a. Trace fossils from the Cambro-Ordovician rocks of north Wales, U.K., and their stratigraphical significance. GeoL J., 6: 333-338. CRIMES, T. P., 1969b. The Stratigraphy, Structure and Sedimentology of some of the Precambrianand Cambro-Ordovician Rocks Bordering the Southern Irish Sea. Thesis, Univ. Liverpool, Liverpool, 196 pp. CRIMES, T. P., 1970. Trilobite tracks and other trace fossils from the Upper Cambrian of north Wales. GeoL J., 7, in press. CRIMES, T. P. and CROSSLEY, J., 1968. The stratigraphy, sedimentology, ichnology and structure of the Lower Palaeozoic rocks of part of northeastern County Wexford. Proc. Roy. Irish Acad., Sect. B, 67: 185-215. CRIMES, T. P. and DHONAU, N. B., 1967. The Precambrian and Lower Palaeozoic rocks of southeast County Wexford, Eire. GeoL Mag., 104: 213-221. CRIMES, T. P. and OLDERSHAW. M. A., 1967. Palaeocurrent determinations by magnetic fabric measurements on the Cambrian rocks of St. Tudwal's Peninsula, north Wales. GeoL J., 5: 217-232. DZULYNSKI, S. and WALTON, E. K., ] 965. Developments in Sedimentology, 7. Sedimentary Features of Flysch and Greywackes. Elsevier, Amsterdam, 274 pp. DZULYNSKI, S., KSIAZKIEWlCZ, M. and KUENEN, PH. H., 1959. Turbidites in flysch of the Polish Carpathians. Bull. Geol. Soc. Am., 70:1089-1118. EDMUNDS, F. H. and OAKLEY, K. P. 1958. British Regional Geology: The Central England District. H. M. Stationery Office,: London, 80 pp. EVANS, A., GARRETT, P. and WHITTAKER, J. H. McD., 1966. Palaeocurrents in the Upper Cambrian of north Wales. Nature, 209: 1230.

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