doi:10.1144/gsjgs.147.4.0693 1990; v. 147; p. 693-701 Journal of the Geological Society

 A. J. DIMBERLINE, A. BELL and N. H. WOODCOCK  

A laminated hemipelagic facies from the Wenlock and Ludlow of the Welsh Basin 

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© 1990 Geological Society of London

Journal of the Geological Society, London, Vol. 147, 1990, pp. 693-701, 6 figs. Printed in Northern Ireland

A laminated hemipelagic facies from the Wenlock and Ludlow of the Welsh Basin

A . J . DIMBERLINE, A . BELL & N . H . WOODCOCK

Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

Abstract: A hemipelagic facies from the Silurian succession in Wales consists of finely laminated, dominantly silt-grade sediment. It can be interbedded with sand- or mud-grade turbidites, or can occur as an independent facies. Its occurrence and character was controlled mainly by low bottom water oxicity. The pelagic fauna of the laminated hemipelagite supports this view. Many of the silt-grade particles have an aggregate structure and probably represent the faecal pellets of oceanic zooplankton. The fine lamination is thought to have formed by alternations of planktonic blooms and increased discharges of silt into the basin. By comparison with modern examples this alternation was likely to have been annual or seasonal.

The Silurian Welsh Basin The Welsh Basin was a Cambrian to Early Devonian zone of relatively rapid subsidence on the Avalonian microcon- tinent (Fig. l ) , bordered to the northwest and southeast by shallow or emergent platforms. Recent reviews of its history are given by Woodcock (1984, 1990) and Kokelaar (1988). During most of Ordovician time, represented by rocks of the Gwynedd Supergroup, it was a volcanically active marginal basin, but most volcanism ceased in late Caradoc time. A basin-wide unconformity in the early Ashgill (Pusgillian) marks tectonic activity along the basin-bounding fault systems and the onset of deposition of the Powys Supergroup (Ashgill through Lower Devonian). During this interval the basin was in a non-volcanic active margin or collision zone setting.

The northern and central to southern parts of the Welsh Basin had differing histories during the time of Powys Supergroup accumulation. In Ashgill and Llandovery times, slow hemipelagic deposition in the north contrasted with repeated and voluminous influxes of turbidity currents in mid-Wales. The majority of these sediments were deposited under oxic conditions, which encouraged burrowing and grazing organisms and the destruction of any hemipelagic lamination.

During earliest Wenlock time there was a marked decrease in the oxicity of the basin bottom waters. This anoxic event can be traced throughout the basins bordering the Iapetus Ocean (Kemp 1985, 1990). The early Wenlock also saw increased subsidence and clastic supply in the northern Welsh Basin. A turbidite depocentre developed, termed the Denbigh Trough (Cummins 1957), which remained a discrete sub-basin throughout Wenlock and Ludlow times, largely barred from the continuing mid-Wales depocentre, the Montgomery Trough (Fig. 1).

Inversion of the Welsh Basin probably began during Ludlow time and climaxed in the Acadian shortening late in the Early Devonian. The inversion was accompanied by a rapid transition from marine to non-marine facies in the basin and on its Bounding platforms.

Distribution of laminated hemipelagite

Palaeogeographical distribution Laminated hemipelagite occurs over the whole area of the Welsh Basin where Wenlock and Ludlow rocks are preserved (Fig. 1; Cummins 1959~; Warren et al. 1984). It laps some way onto the basin slopes but is not preserved on the platforms. Laminated hemipelagite in not restricted to those basinal area inundated by the contemporary sand turbidite systems. Two areas exemplify this relationship. On the southeast basin margin, northerly-directed Wenlock turbidity flows were confined laterally by a fault-controlled slope, whilst hemipelagic sediment was deposited on basin floor and slope alike (Dimberline & Woodcock 1987). On the palaeohigh between the Montgomery and Denbigh Troughs, the Derwen Ridge of Cummins (1957), Wenlock turbidites are almost absent yet a continous laminated hemipelagite sequence was deposited.

The distribution of Fig. 1 is compatible with turbidity flows hugging depositional lows and with vertical fallout forming laminated hemipelagite indiscriminately on lows, slopes and highs throughout the basin.

Stratigraphical distribution Laminated hemipelagite occurs in the basin in strata from the centrifugus Biozone (lowest Wenlock) to the incipiem Biozone (lower Ludlow) (Fig.2). However, the character of the resulting sediment is controlled strongly by temporal fluctuations in the proportion of interbedded turbidites and in the bottom water oxicity.

At the base of the Wenlock Series (centrifugus Biozone) laminated hemipelagite is interbedded with turbidites but can form >50% of the sequence. Examples of this facies are the Benarth Formation in north Wales and the Nant-y-Sgollen Shales in mid-Wales. In the murchisoni through linnarsoni biozones, turbidites dominate the intrabasinal troughs, forming the Denbigh Grits Group. By contrast, laminated hemipelagite dominates the sequences

693

694 A . J . DIMBERLINE E T A,!,.

on the intervening Derwen Ridge palaeohigh, forming 100% of the sequence near Llangollen. The magnitude and frequency of the turbidites in the troughs decrease in the linnarsoni Biozone, so that laminated hemipelagite makes up about 60% of the sequence. This facies, typical of the Nantglyn Flags Group (ellesae to nilssoni biozones), has a distinctive ‘striped’ or ‘ribbon-banded‘ appearance due to interbedding of 5-40 mm laminated hemipelagite packets with 5-20 mm silt-mud turbidites. Turbidite deposition increasingly affected the Derwen Ridge during late Wenlock time and the basal Ludlow Glyndyfrdwy Group is very similar to the Nantglyn Flags Group.

There are two intervals in the Nantglyn Flags Group, within the ludemis and nassa biozones respectively, which preserve little laminated hemipelagite but contain a benthic infauna as well as epifauna. More oxic bottom waters allowed burrowers and grazers to destroy any lamination that formed. The two oxic episodes have been linked to a continental regression coincident with the development of the Wenlock Limestone on the Midland Platform (McKerrow 1979; Kemp 1990). Both may be a consequence of a drop in sea-level, allowing reef build-up on the shelf and the breakdown of stratification in the basin.

Laminated hemipelagite is rarely preserved in rocks younger than those of nilssoni Biozone age. In north Wales this can be explained by the increasing volume of turbidites supplied to the Denbigh Trough. In mid-Wales the laminated hemipelagite deficiency may have resulted from the breakdown of basin-water stratification, since inter-

Fig. 1. Map of Wales showing outcrop of Wenlock and Ludlow rocks, selected palaeogeographical features and depth- related fauna1 communities for mid- Wenlock time (rigidus Biozone); mod- ified after Hurst et al. (1978). Facies boundaries are conjectural away from outcrop control. LHP, laminated hemi- pelagic facies.

bedded silt and fine sand beds show structures consistent with a storm influence (Tyler & Woodcock 1987). Storms would lead to mixing, to increase of bottom-water oxicity, and in turn to the oxidation of organic matter on the sea-floor. Despite this, some laminated hemipelagite has been described from the storm-influenced Bailey Hill Formation (Tyler 1987: Tyler & Woodcock 1987), the Ludlow ‘turbidites’ of Cummins (1959b). This laminated hemipelagite could have resulted from the re-establishment of water stratification and oxygen depletion of bottom waters between major storm events. Alternatively the storm mobilized sediments could have been deposited rapidly onto organic rich sediments that had not had time to oxidize, isolating the laminated hemipelagite from the now oxygenated surface waters.

The youngest laminated hemipelagite in north Wales is found in the Bont-Uchel Formation of the Elwy Group (Warren et al. 1984), in the incipiens Biozone.

General description and interpretation

Lithology, structure and sequence The laminated hemipelagite consists of silt laminae alternating with organic carbon-rich laminae on a scale of three to four carbon-rich laminae per mm (Cummins 1959a; Warren et al. 1984). The lamination is discontinuous laterally on a 1-5 cm scale in lamination-normal sections (Fig. 3a). The silt laminae contain silt and clay aggregates

HEMIPELAGIC FACIES OF THE WELSH BASIN

3 4

695

Fig. 2. Representative stratigraphical columns for the Silurian of: 1, 2, Denbigh Trough; north flank of Berwyn High (Llangollen Syncline); 4, Montgomery Trough; 5, Midland Platform. DC, Denbigh Grits Group; GDG, Glandyfrdwy Group; LNF, Lower Nantglyn Flags Group; LS, Ludlow shale; PG, Pen-y-Clog Group; UNF, Upper Nantglyn Flags Group; WL Wenlock Limestone; WS, Wenlock Shale; LHP, laminated hemipelagic facies.

up to 2 mm in diameter. Sections cut parallel to the lamination show a distinctive mottled appearance (Fig. 3b) due to the presence of these aggregates. The detailed description and interpretation of the aggregates is covered in a later section.

Interbedding of laminated hemipelagite with turbidites in the Silurian Welsh Basin has been well described and illustrated by Cave (1979) and Cave & Hains (1986). Logs through the Wenlock of mid-Wales (Fig. 4) show that the laminated hemipelagite always occurs above a structureless turbidite mudstone. It is always overlain by a sharp or erosive contact with the basal silt or sand of a turbidite (Fig. 3f). It never grades up into finer-grained, wispy-laminated or gradedlungraded mud as would be expected if it was a traction current deposit associated with a turbidite (Stow & Bowen 1980). These relationships confirm a non-turbidite origin for the laminated hemipelagite. Moreover, bentonite bands always occur bracketed by hemipelagite, as observed by Kemp (1985).

The same logs (Fig. 4) show that sand-based turbidites tend to be underlain by thinner packets of laminated hemipelagite than do silt-mud turbidites. This suggests that the greater erosive capacity of the coarser turbidity currents removed a greater thickness of accumulated laminated hemipelagite. However, there are examples both of coarse sandstone turbidites underlain by hemipelagite and succes- sive mud turbidites with no intervening hemipelagite, suggesting that the time-gap between turbidite events is also

an important factor, Whilst laminated hemipelagite occurs most commonly between silt-mud turbidites (Figs 3c and d) it also occurs independently of turbidites, for instance in the Llangollen area where there are over 50 m of uninterrupted laminated hemipelagite.

Fauna The laminated hemipelagite is generally unbioturbated, indicating that oxygen levels beneath and just above the sediment water interfaces were too low to support a burrowing infauna (Bromley & Ekdale 1984). At a few localities (e.g. Nantglyn Quarries, SH 977 597 and a roadside quarry, SH 908 663) grazing trails can be seen on the surface of the hemipelagite. These grazing trails do not apparently disturb the lamination.

Graptolites and orthoconic nautiloids are particularly common in the laminated hemipelagite and are often found aligned (Fig. 3e). The aligning currents have not disturbed the lamination. A diverse but sparse shelly fauna has been collected by Warren el al. (1984) from the laminated hemipelagite of the Lower Nantglyn Flags Group of the Denbigh Moors area. This fauna consists of the bivalves Cardiola and Butovicella, pterineids, rare crinoids and ostracodes. The bivalves and ostracodes are found sporadically in ones or twos and only rarely in clusters. Both valves of the bivalves and the ostracodes are commonly found together.

6% A . J . DIMBERLINE ET AL.

b

e

1

Frg. 3. (a) Thin section of laminated hemipelagic showing continuous organic laminae (OL) and silt aggregates (A). Castle Vale (SO 092 747). (b) Section of laminated hemipelagite parallel to lamination, showing mottling due to intersection with organic laminae and tabular aggregate structures (arrowed). Llanbadarn Fynydd (SO 098 775). (c) Laminated hemipelagite (H) with interbedded silt-mud turbidite (centre), Lens cap is 5 cm diameter. Locality as (a). (a) laminated hemipelagite (H) with interbedded silt-mud turbidites. Pencil is 7 mm diameter. Locality as (b). (e) Aligned graptolites on bedding surface of laminated hemipelagite. Bwlch y Sarnau (SO 037 751). ( f ) Thin section of erosive based silt turbidite (T) overlying laminated hemipelagite (H). Locality as (a).

The graptolites, orthocones and the crinoid Scyphocrinites? are assumed to have been pelagic and to have settled through the water column after death (Warren et al. 1984). The crinoids are commonly found articulated. Because crinoids tend to disarticulate soon after death, such good preservation normally implies rapid burial. In the laminated hemipelagite it is more likely that the strongly reducing bottom waters slowed down decomposition.

The bivalves and ostracodes may have been pelagic (KM 1969), but it is more likely that they were epibenthic (McAlester in Berry & Boucot 1967). Their association with graptolites suggests that they were adapted to low oxygen environments which excluded most other life forms but which best preserved graptolites (Berry & Boucot 1967). Bivalves such as Cardiola may have been part of an exaerobic biofacies (Savrda & Bottjer 1987) present around

HEMIPELAGIC FACIES OF THE WELSH B A S I N 697

U)

2 4-

E 3 1 2 1 1 . . , . . . . ._. F . . . . .

0 h" l

100

U)

c E E Q) - c C Q) 0

50

0

parallel lamination m cross lamination 0 structureless sandstone

I

l

laminated hemipelagite turbidite mudstone

Fig. 4. Examples of logged sections including laminated hemipelag- ite (a) Packet of laminated hemipelagite/mud turbidites within sand turbidite sequence (Bwlch-y-Sarnau Quarry, SO 037 751). (b) Interbedded laminated hemipelagite, fine sand/silt-mud turbidites (Llanbadarn Fynydd, SO 098 775).

the dysaerobic/anaerobic boundary. This boundary would have shifted its vertical position through time due to changes in primary productivity, sea level, the position of the pycnocline, or the rate of sedimentation. In a slope setting, therefore, only a relatively thin zone of sediments would contain this interface.

The widespread occurrence of laminated hemipelagite indicates that the mid-Silurian floor to the Welsh Basin was generally below the dysaerobic/anaerobic boundary and therefore preserves a predominantly pelagic fauna. At times

this boundary coincided with the sedimentlwater interface, excluding infauna but allowing the establishment of a restricted exaerobic benthos of bivalves and grazing organisms.

Possible modern analogue

An instructive modern analogue for the Wenlock laminated hemipelagite is the varved sediment in the Tertiary basins of the Californian Borderland (Hulsemann & Emery 1961; Isaacs 1984; Thornton 1984). In basins such as the Santa Barbara Basin (Thornton 1984) anoxic bottom water allows the preservation of laminites, together with turbidites and flood suspensate layers. The lamination is due to increased claylsilt discharge into the basin in November to March alternating with plankton fallout during the rest of the year (Emery 1960). The lamina couplets prove to be annual by correlation with rainfall totals and tree rings (Soutar & Crill 1977).

Interpretation

The laminated hemipelagite is interpreted as interturbidite sediment that formed the hemipelagic background deposit in the basin, following similar interpretations by Cummins (19596) and Cave (1979) in Wales and by Kemp (1985) in the Southern Uplands of Scotland. Hemipelagic deposition occurred primarily by slow settling through the water column in the absence of marked traction or turbidity currents. (Fig. 6). A component of current-induced lateral advection of suspended sediment (Drake et al. 1978) is evidenced by aligned graptolites and orthocones (Fig. 3e).

It is proposed that the silt laminae were formed by a combination of vertical fallout of aggregates produced in the water column and the deposition of silt and clay grade sediment by extremely dilute density currents with very low velocities just sufficient to align graptolites. A bottom water nepheloid layer (Gorseline 1984; McCave 1984) is a possible mechanism. The source of the terrigenous sediment for the Wenlock hemipelagite could have been wave-resuspended shelf sediment, aeolian input, or river discharge. The latter source is suggested by the likely delta-fed origin of the Wenlock turbidite system (Dimberline 1987; Dimberline & Woodcock 1987).

The carbon-rich laminae are thought to have originated by the vertical fallout from seasonal phytoplankton blooms. An alternative is that they represent sulphur-oxidizing bacterial mats (cf. Williams & Reimers 1983). Such mats form in dysaerobic sediments between the oxygenated zone and the underlying hydrogen sulphide zone. The laminated hemipelagite contains no direct evidence, such as phosphate cements, for a permanent sulphidicloxic interface at or below the sediment-water interface. However, other features associated with this interface, grazing trails and a limited epifauna, are seen occasionally.

The current alignment of graptolites and orthocones without disruption of laminae is compatible with, though does not require, a bacterial mat binding the sediment surface. The sulphidic/oxic interface may have been usually just above the sediment surface, but at times was lowered to coincide with it and thus allowing the development of a bacterial mat. Such mats may have contributed to the organic carbon in the hemipelagite, but are unlikely to have been the only source.

698 A . J . DIMBERLINE E T A L

Fig. 5. (a) Thin section of laminated hemipelagite showing large silt aggregate (A). Castle Vale (SO 092 747). (b) Backscatter SEM image of laminated hemipelagite showing chlorite-rimmed aggregate containing clay-grade mica and chlorite. Locality as (a). (c) Chondrites-bioturbated hemipelagite from centrifugzu Biozone, Howgill Fells (SO 699 970). (a) Backscatter SEM image of laminated hemipelagite showing aggregate of clay-grade chlorite and illite-mica. Locality as (a).

I SILT INPUT ~, wave

aeolian resuspended

l

Y Y

ANAEROBIC

organic laminae from

blooms - - - - - I siltlmud 75=-.Z=----.=z- . . seasonal phytoplankton - I* turbidites 0. . . &..' , Q. - - . .

__. - - . . =.c ----L=.- - - - - siltlmudrnatrix ; +LHP silt/clay aggregate

Fig. 6. Cartoon illustrating the origin of laminated hemipelagite and its relationship to sand turbidites.

HEMIPELAGIC FACIES OF THE WELSH BASIN 699

Periodicity

Direct analogy with the California Borderland suggests that the sub-millimetre scale lamination in the laminated hemipelagite is annual. This conclusion must be viewed with some caution. Assessment of the number of laminae in a known number of graptolite zones give periodicities of about four years in the Lake District (Kemp 1985) and about ten years in north Wales. The extent to which the deficit is due to erosional removal of laminae is unknown.

In the Santa Barbara Basin, occasional homogeneous mud layers ( < l cm thick) record increased discharge from floods on land. The mean periodicity for the mud layers is 59 years, and for silt-mud turbidites it is 120 years (Thornton 1984). Similar thin (< l cm) homogeneous mud layers in the Wenlock hemipelagite may also represent flood layers. Silt-mud turbidites in mid-Wales have comparable mean repeat times of 75 years for sand-poor sections and 123 years for sand-rich sections (Dimberline 1987; Dimberline & Woodcock 1987). Warren et nl. (1984) obtained a mean repeat time of 200 years for mud turbidites intercalated with laminated hemipelagite in north Wales, based on the number per graptolite biozone.

Comparison with previous interpretations Carey & Roy (1985) interpreted the Silurian laminated facies in the Jemtland Formation of northern Maine as part of a silt/clay turbidite. They concluded that ‘the laminated shale occurs above non-laminated shale and below siltstone beds’ and recognized a complete Bouma (1962) sequence as: laminated siltstone, laminated shale and uniform shale. They interpreted the laminated hemipelagite as the Piper (1978) E l division. Although they recognized both the aggregate structures and the organic laminae their mechanism fails to explain either feature. Neither does it account for the much higher organic content and pelagic fauna of the laminated hemipelagite compared with associated lithologies of similar grainsize.

Warren (1963 and Warren et al. 1984) envisaged a rather similar turbiditic origin for laminated hemipelagite in the Wenlock of south Scotland. He explained its high organic and fauna1 content by the ‘entrapping of pelagic organisms by drifting clouds of fine turbidite sediment’. There still remains the difficulty of explaining a lamination defined by alternating organic-rich with silt/mud laminae in terms of traction processes in a density current. The ‘slow uniform deposition’ mechanism of Cummins (1959a) also leaves the origin of the lamination unspecified.

In summary, there are four main lines of evidence that the laminated facies is hemipelagic background sediment and not turbiditic.

(a) Its position between but never within individual turbidites, whether sand-, silt- or mud-based.

(b) The ubiquity of sharp upper contacts to packets of laminated hemipelagite rather than an upward gradation into finer-grained mud.

(c) The abundant pelagic fauna in the laminated hemipelagite which is very rare in the associated silt-mud turbidites. The laminated facies is known to palaeontologists as ‘graptolitic shale’.

(d) Bentonite bands always occur bracketed by hemipelagite.

Aggregate structures

Description Aggregate structures are composite grains up to 2 mm in diameter, occurring abundantly in the silt laminae of the laminated hemipelagite (Cummins 1959~). There are two compositional types, those with a high quartz-silt content (Figs 3a and 2a) and those entirely of clay size chlorite and illite (Figs 5b and c). Original pellet shape is hard to determine, but bedding parallel sections (Fig. 3b) show ellipsoidal to cylindrical shapes. In bedding normal sections (Figs 2a and 3a) pellets are elongated in the plane of the lamination.

Possible modern anaIogues Clay and silt grade particles need such small currents to keep them in suspension that particle by particle deposition in a pelagic setting is virtually impossible (Pryor 1975). The vertical flux of sediment in the oceans is dominated by relatively large aggregated particles (McCave 1984). The aggregates are mainly faecal pellets of pelagic organisms and mucus feeding sheets. Pellets have a greater preservation potential that the sheets. The importance of faecal pellets as a vehicle for transporting sediment to the sea floor has been recognized widely (e.g. Smayada 1971; McCave 1975; Roth et al. 1975; Dunbar & Berger 1981).

The best modern analogues for the Welsh Wenlock aggregates occur in the California Borderland basins. In the Santa Barbara Basin vertical fallout of tunicate and crustacean pellets accounts for 50% of the sediment flux to the central basin floor and for 60-90% of trapped material (Dunbar & Berger 1981). The pellets are tabular, ellipsoidal and cyclindrical, but rarely spherical. They range up to 1 mm in diameter. The composition of the pellets reflects the composition of suspended matter, suspension-feeding organisms cleaning the water of edible and indigestible material alike. Most California Borderland pellets consists of clay minerals and quartz silt (Dunbar & Berger 1981).

Other modem pellet producers are shallow water, benthic, filter-feeding organisms and polychaete worms. Benthic faecal pellets are robust and hydrodynamically equivalent to fine quartz sand (Pryor 1975). Some may have been transported into the Welsh Silurian basin from adjacent platforms. Pioneering polychaetes pelletize sedi- ment with low dissolved oxygen contents (<0.1 ml/l-l), leaving no worm tubes or disrupted lamination (Cuomo & Rhoads 1987). However, very low pore-water oxygen contents will not support a polychaete assemblage. Cuomo & Rhoads (1987) describe such a sediment with extremely fine lamination and abundant pyrite, similar to the Wenlock laminated hemipelagite, and suggest that its contained pellets could be polychaete pellets transported from more oxic sediments upslope or be from a zooplankton source.

Interpretation It is concluded that the aggregate structures in the Welsh Silurian laminated hemipelagite are primarily the faecal pellets of pelagic organisms. Transported pellets of benthic organisms are a possible component. It is proposed that the clay pellets were produced by a different organism to the silt pellets, perhaps one unable to ingest silt-sized particles.

700 A . J . DIMBERLINE E T A L .

There is no literature on the faecal pellets produced by Silurian pelagic filter feeders. Modern arthropods are important pellet producers and could also have been so during Wenlock and Ludlow times (S. Conway-Morris, pers. comm.). Organisms providing a food source for filter feeders could have included algae, dinoflagellates, zoofl- agellates, acritarchs, and chitinozoans.

Comparison with previous interpretations A faecal pellet origin for the Welsh Silurian aggregates has been suggested previously by Jones (1954) and Llewellyn (1965). Kemp (1985) rejected this origin on two accounts.

(a) the pellets of benthic burrowers could not have been transported throughout the Iapetus Ocean. This is not a serious objection in the Welsh Basin with its closely bordering platforms, nor in similar basins marginal to Iapetus. (b) Planktic organisms would not have produced faecal pellets as large as the aggregates seen. This objection is not supported by the data from the California Borderland basins (Dunbar & Berger 1981).

Other proposed origins for the aggregate structures in the Welsh Silurian must be considered. They are not the result of bioturbating organisms (Jones 1954; Cummins 19590; Llewellyn 1965) for the following reasons.

(a) The interface between the laminated hemipelagite and the underlying turbidite mud is always sharp and this mud is rarely bioturbated. (b) Discrete lensoid or tabular aggregate geometries dominate over cylindrical shapes that would be compatible with burrows as well as with a pelletal origin. (c) Undoubted bioturbation of laminated hemipelagite does not produce pellets, but rather destroys both the organic-rich laminae and the aggregate structures. Examples occur in the the ludensis and nussa biozones of the Denbigh Trough (Warren et al. 1984), the upper Llandovery of mid-Wales (Smith 1987, 1988), and the basal Wenlock of the Howgill Fells (Fig. 5d).

The aggregates could not have been produced entirely by compaction around irregularly distributed carbonaceous flakes (Rickards 1964, 1965) because:

(a) the aggregates are discrete structures that are occasionally bound and rimmed by organic matter (Figs 2a and 3a); (b) compaction could not have produced the observed segregation into silt and clay aggregates; (c) the uncompacted aggregates were probably not spherical, countering the argument that compaction was not sufficient to produce the present lensoid shapes; (d) plastically deformed rip-up clasts of laminated hemipelagite showing well developed aggregate struc- tures occur in turbidite sandstones.

The aggregates are not thought to be the mucus feeding sheets of gelatinous zooplankton, so-called marine snow (Kemp 1985), because of the following.

(a) Marine snow consists of large (0.5-1000mm) fragile aggregates and sheets of mucus (Gilmer 1972) which are unlikely to remain as discrete structures after deposition. They will break up and release their bound sediment at the water surface and during descent (I. N. McCave, pers. comm.). The Wenlock aggregates are clearly robust enough to have survived compaction.

(b) Marine snow does not contain the tabular and cylindrical shapes seen in many of the aggregates. (c) Marine snow could not promote the observed differentiation into silt and clay aggregates.

General relevance of laminated hemipelagite The main conclusions accepted from previous work or derived from this study are:

(a) The laminated hemipelagite of Wales was deposited during prolonged episodes of low bottom water oxygen content in Wenlock and Ludlow times: (b) laminated hemipelagite is a hemipelagic back- ground facies, not deposited itself by turbidity currents although commonly interbedded with turbidites: (c) the fauna of the laminated hemipelagite is predominantly pelagic; a sparse; low-diversity benthic fauna represents an exaerobic epifauna; (d) the aggregate structures with the laminated hemipelagite are primarily the faecal pellets of pelagic organisms, with subordinate transported pellets from benthic organisms; (e) the lamination within the laminated hemipelagite is approximately annual.

These conclusions are of more than local relevance. A facies similar to the Welsh laminated hemipelagite is found in many Silurian sequences throughout the world (Kemp 1985, 1990; R. B. Rickards pers. comm.). Good examples occur in basins now adjacent to the Welsh Basin: the Lake District (Rickards 1964, 1965: Llewellyn 1965), the Southern Uplands (Warren 1963; Kemp 1985) and Ireland (Archer 1981). Its widespread development probably reflects sluggish circulation of the world's oceans and a global 'greenhouse' state (Leggett et al. 1981). However, it is clear that the spatial and temporal distribution of laminated hemipelagite has been controlled by local tectonic factors as well as by global oceanographic effects, particularly events that cause sand turbidite input to basins.

The hypothesis of a near-annual lamination within the laminated hemipelagite has wide implications, allowing estimates of accumulation rates and turbidite repeat times throughout much of the Silurian. The spacing of organic laminae in the Wenlock laminated hemipelagite is one area in which there is general agreement. Cummins (1959a) found 3-4 organic carbon-rich laminae per millimetre; Kemp (1985) measured 3-3.5; and a figure of 3 has been determined in this study. The sedimentation rate of the hemipelagite is therefore approximately 0.33 mm a-', 33 cm ka-', 333 m Ma-'. Both this rate and the relative coarseness of the hemipelagite is consistent with that of present day continental margin settings (Stow et al. 1985).

Where the hemipelagite is interbedded with thin silt-mud turbidites, packets of laminated hemipelagite as thin as 0.5-1.5 mm have been traced laterally as much as 25 m, suggesting little or no erosion by the turbidity flows (e.g. Nantglyn Flags, Deeside Slab Quarry SJ 137 404). If erosion was minimal then the thickness of the laminated hemipelagite interbeds will be proportional to the time period between the turbidites, and it is possible to calculate accumulation rates for the turbidite-laminated hemipelagite interbeds. The potential of such quantitative studies is currently being explored using the sequences in North Wales.

HEMIPELAGIC FACIES OF THE WELSH BASIN 701

This work was funded by NERC research studentships (A. J. D. and A. B.) and a NERC research grant (N. H. W.). Discussions with A. Kemp, N. McCave, B. Rickards, R. Smith, J . Tyler, and P. Warren are gratefully acknowledged. A constructive review by R. Cave substantially improved the paper.

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Received 15 May 1989; revised typescript accepted 10 December 1989.