A late Ordovician (Hirnantian) karstic surface in a submarine channel, recording glacio-eustatic...

GEOLOGICAL JOURNAL

Geol. J. 41: 1–22 (2006)

Published online 10 November 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/gj.1029

A late Ordovician (Hirnantian) karstic surface in a submarinechannel, recording glacio-eustatic sea-level changes: Meifod,

central Wales

PATRICKJ. BRENCHLEY1, JIMD.MARSHALL1*, DAVIDA.T. HARPER2,CAROLINEJ. BUTTLER3 andCHARLIEJ. UNDERWOOD4

1Department of Earth and Ocean Sciences, University of Liverpool, Liverpool, UK2Geological Museum, University of Copenhagen, Denmark

3Department of Geology, National Museums and Galleries of Wales, Cardiff, UK4School of Earth Sciences, Birkbeck College, London, UK

The growth and decay of the end-Ordovician Gondwanan glaciation is globally reflected by facies changes in sedimentarysequences, which record a major eustatic fall and subsequent rise in the Hirnantian Stage at the end of the Ordovician. However,there are different reported estimates of the magnitude and pattern of sea-level change.

Particularly good evidence for end-Ordovician sea-level change comes from a sequence at Meifod in central Wales, whichhas a karstified limestone unit within a channel incised into marine shelf sediments. Pre-glacial (Rawtheyan) mudstones have adiverse fauna suggesting a mid-to-deep-shelf water depth of c. 60 m. The channel, 20 m deep, was incised into these mudstonesand partially filled with a mixture of fine sand and detrital carbonate. The taphonomy of bioclasts and intraclasts indicates thatmany had a long residence time on the sea floor or suffered diagenesis after shallow burial before being resedimented into thechannel. The presence of carbonates on the Welsh shelf is atypical and they are interpreted as having accumulated as patchesduring a minor regression prior to the main glacio-eustatic fall. Comparison of the carbon stable-isotopic values of the bioclastmaterial with the global isotopic record confirms that most of the material is of Rawtheyan age, but that some is Hirnantian. Theresedimented carbonates lithified rapidly and formed a limestone, several metres thick, in the deepest parts of the channel. Assea-level fell, this limestone was exposed and eroded into karstic domes and pillars with a relief of over 2 m. The overall, glacio-eustatic, sea-level fall is estimated to be in excess of 80 m.

A succeeding sea-level rise estimated to be 40–50 m is recorded in the laminated crust that mantles the karstic domes andpillars. The crust is formed of encrusting bryozoans, associated cystoids, crinoid holdfasts and clusters of the brachiopodParomalomena, which is normally associated with mid-shelf environments. Fine sands buried the karst topography and accu-mulated to fill the channel. In the sandstones at the base of the channel there is a Hirnantia fauna, while in the sandstones high inthe channel-sequence there is cross-stratification characteristic of mid-shoreface environments. This would indicate a fall ofsea-level of c. 30 m. The subsequent major transgression marking the end of the glaciation is not recorded at the Meifod locality,but nearby exposures of mudstones suggest a return to mid-to-deep-shelf environments, similar to those that prevailed beforethe Hirnantian regression.

The Meifod sequence provides strong evidence for the magnitude of the Hirnantian sea-level changes and by implicationconfirm larger estimates for the size of the ice sheets. Smaller oscillations in relative sea-level seen at Meifod may be localphenomena or may reflect eustatic changes that have not been widely reported elsewhere. Copyright # 2005 John Wiley &Sons, Ltd.

Received 14 June 2004; revised version received 29 April 2005; accepted 3 May 2005

KEY WORDS sea-level; facies; karst; Hirnantian; Ordovician; Wales

Copyright # 2005 John Wiley & Sons, Ltd.

* Correspondence to: J. D. Marshall, Department of Earth and Ocean Sciences, University of Liverpool, Liverpool, L69 3GP, UK.E-mail: [email protected]

Contract/grant sponsor: British Geological Survey; contract/grant number: GA/95E/23.

1. INTRODUCTION

The late Ordovician Gondwanan glaciation, which prevailed briefly within a prolonged greenhouse regime,

induced a major sea-level fall at the start of the Hirnantian and a complementary sea-level rise in the mid-

Hirnantian when the ice caps melted (Figure 1). Oxygen stable isotopes reflect changes in ice volume and

temperature of ocean waters and so record the growth and decay of the Gondwanan ice caps (Figure 1). Coeval

effects on the carbon cycle in the oceans are reflected in the carbon isotope record (Figure 1). The timings of the

sea-level and isotopic changes are particularly significant, because they are coeval with the main phases of the late

Ordovician mass extinction, and are relevant to the timing and magnitude of the anomalous late Ordovician

glaciation (Brenchley et al. 1994, 1995).

Hirnantian facies-sequences seen at many localities around the world reflect the eustatic changes (Brenchley 1988).

Estimates of the amount of sea-level change range from 50–100 m (Brenchley and Newall 1980) to 30–36 m (Long

1993) or ‘possibly less than 20 m’ (Chen 1984). The fall in sea-level has been interpreted as occurring in two phases

with an intervening rise (Brenchley and Newall 1980; Brenchley and Storch 1989), or as a series of fluctuations (Long

1993; Armstrong and Coe 1996). Hirnantian glacial sequences in North Africa show two cycles of ice sheet advance,

consistent with two phases of lowered sea-level (Sutcliffe et al. 2000) within the full glacial interval.

The shelf sequence on the eastern margin of the Welsh Basin records the Hirnantian sea-level fall with a widespread

erosion surface including channels deeply incised into shelf mudstones (Brenchley and Newall 1980, 1984). Above the

erosion surface are sandstones, possibly deposited during a regressive phase within the subsequent transgression. In

one of the channels, at Meifod in Powys, central Wales, there is a more complex fill, which includes a unit of karstified

limestone below the main sandstone body. The importance of buried erosional topography in establishing the magni-

tude of sea-level change has been emphasized by Johnson et al. (1998) with reference to Silurian sequences. The

implications of the karst in the Meifod channel-fill sequence are discussed in this paper.

2. PALAEOGEOGRAPHIC CONTEXT

Prior to the Hirnantian, throughout the Rawtheyan, deep neritic mudstones with a mixed brachiopod–trilobite

fauna prevailed along the margin of the Welsh Basin across North Wales and along the eastern shelf. From there,

a west-facing palaeoslope descended to deeper basin mudstones (Figure 2a), with a fauna mainly of trilobites

(Price and Magor 1984; Brenchley 1992, p. 32). The Hirnantian glacio-eustatic sea-level changes radically altered

this palaeogeography. The initial regression created a widespread erosion surface across the shelf and incised chan-

nels that apparently fed mainly fine sediment onto the slope and created mud-dominated fans at the base of the

slope (Figure 2b). During the subsequent overall transgression the channels were filled with sand, which also

extended beyond the channel to blanket much of the rest of the erosion surface.

3. STRATIGRAPHY

The upper Ordovician and lower Silurian stratigraphy of the Meifod area was first mapped and described by King

(1928), who identified the Ordovician–Silurian boundary as being at the base of the sandstone unit. The erosion

surface below the sandstone was described and figured, and the sandstone referred to as the ‘Basal Sandstone of

Graig-wen’ in the erroneous belief that it was earliest Silurian in age, rather than latest Ordovician. The discontin-

uous limestones at the base of the sandstones were first recorded by Sedgwick (1843, cited in King 1928) and were

regarded as large concretions, as they were much later by King (1928). Fearnsides, in a discussion of King’s (1928)

paper, erroneously referred to great blocks of limestone, which had been derived from a more continuous lithified

sheet, and later Brenchley (1993) made a rather similar interpretation.

3.1. Lithostratigraphy

A summary of the lithostratigraphy is shown in Figure 3.4. Mudstones form the lowest part of the relevant

stratigraphy in the Meifod area. The upper part of the mudstone sequence is characterized by the presence

2 p. j. brenchley ET AL.

Copyright # 2005 John Wiley & Sons, Ltd. Geol. J. 41: 1–22 (2006)

of thin (< 3 cm) laminated siltstones and dispersed phosphate nodules up to about 1.5 cm in diameter, contain-

ing a diverse fauna of small fossils. The upper surface of the mudstones is erosional, with deep broad channels

and shallower and irregular incisions in many parts of the surface. The overlying limestone fills the depressions

in the topography, where it locally has an estimated maximum thickness of 15 m, but thins to zero at the channel

margins. The limestone is composed of bioclasts, intraclasts and fine-grained sand that, in parts, constitute 50%

of the rock. The upper surface of the limestone is in the form of domes and pillars with an estimated topo-

graphic relief of up to 3 m. Mantling the domes and pillars is a crust, formed predominantly of encrusting

bryozoans. The overlying Graig-wen Sandstone fills the topographic relief of the limestones and accreted

upwards to a form a thickness of about 25 m. The sandstones are massive or thick bedded, with parallel- or

cross-lamination: they are composed of fine-grained, well-sorted, angular quartz sand. The overlying biotur-

bated shales with thin sandstones are not exposed above the sandstones, but are seen in small exposures a few

kilometres away (see below).

3.2. Biostratigraphy

In general the mudstones are sparsely fossiliferous, but the dispersed phosphate nodules have many fossils of

small size constituting a diverse and undeformed fauna including the trilobites Primaspis (Primaspis) cf. evoluta,

Cybeloides sp., Stenopareia sp., Flexicalymene? sp., Remopleurides spp. and Tretaspis sortita, indicating a Zone 7,

youngest Rawtheyan age. Other elements of the fauna from the nodules are brachiopods, Sulevorthis and Chone-

toidea, Plectambonites (now Eoplectodonta) simulans, recorded by King (1928), an indeterminate non-articulated

brachiopod and a torynelasmatid. In addition, there are molluscs including Concavodonta, Sinuites, conulariids,

indeterminate small gastropods and orthoconic nautiloids; indeterminate rare ostracodes, crinoid columnals and

bryozoans are also present.

The limestones have yielded a small brachiopod fauna with Mirorthis, Sowerbyella?, Eoplectodonta? that are

typical of the Rawtheyan. The brachiopods occur as uncommon, dispersed single clasts; their association with

resedimented bioclasts and intraclasts suggests they too might be resedimented and do not necessarily indicate

the depositional age of the limestone (see discussion below). Within the bryozoan crust on the limestone there

ascensus

persculptus

extraordinarius

pacificus

RA

WT

HE

YAN

HIR

NA

NT

IAN

RH

UD

DA

NIA

N

Stage GraptoliteZone

Extinctions

StableIsotopes

Glaciation Environmental ChangesSea-level

majorice caps

-6 -4 -2 0 4 62 0 40 80m

Pre-extinction fauna

Survival/Recoveryfauna

Survival/Recoveryfauna

Survival fauna

Start of global cooling,change in carbon cycling,start of sea-level fall.

Global warming, change incarbon cycling,sea-level rise,widespread anoxia.

Benthic Faunas

δ18O δ13C

Ord

ovic

ian

Silu

rian

Figure 1. A summary diagram showing late Ordovician events, biotic changes and stratigraphy (based on data as summarized in Brenchleyet al. 2003).

late ordovician karstic surface, central wales 3


are locally common articulated and disarticulated valves of Paromalomena polonica, a species which appears at

the start of the Hirnantian and persists into the Rhuddanian, but is most common in the Hirnantian.

The Graig-wen Sandstone has a well-preserved fauna comprising articulated and disarticulated brachiopods

dominated by Hindella crassa incipiens together with Dalmanella testudinaria, Hirnantia sagittifera, Kinnella

kielanae and Plectothyrella crassicostis (see Figure 12). The assemblage is typical of the ‘Hirnantia fauna’, which

indicates an Hirnantian age. The immediately overlying bioturbated shales have yielded no benthic fossils, but

King (1928) recorded two graptolites from a higher level. One of the graptolites was from greenish-grey mud-

stones, which yielded a small specimen of Normalograptus (formerly Climacograptus) normalis, indicating a

probable Hirnantian age. The second graptolite, from a higher level, is Lagarograptus (formerly Monograptus)

acinaces, which is likely to be of Llandovery age.

3.3. Stratigraphic relationships

The channel and its fill, described in this paper, are exposed in a line of small quarries adjacent to the north side of

the road (A495, Ordnance Survey Sheet 125, grid references SJ102093 to SJ104095) (Figure 3.3). The relevant

sequence comprises upper Rawtheyan mudstones, overlain by discontinuous sandy limestones covered by

Hirnantian sandstones (the Graig-wen Sandstone) (Figure 3.3). The sandy limestones at their thickest are estimated

to be 15 m thick, but probably do not extend laterally for more than a few hundreds of metres at most. They are

a. Rawtheyan Palaeogeography of Wales

Anglesey

MeifodPon

tesfor

d Linley

Fault S

ystem

Church

Stretto

n

Fault S

ystem

50km

Bioclasticcarbonates

Shelf mudstones/siltstones

Land

Llandovery

BasinalMudstones

Figure 2. Palaeogeography of the Welsh Basin and adjacent shelf during (a) late Rawtheyan when sea-level was high and (b) Hirnantian whensea-level was low.



demonstrably absent along strike in the Graig-wen Quarry (locality D, Figure 3.3) and are not recorded elsewhere

in the region, even where the Graig-wen Sandstone is present (King 1928). The upper surface of the limestones is

deeply karstified to form domes and pillars at least 2 m high. These karstic limestones only occupy the deeper parts

of the channel (Figure 4), so that the overlying sandstone not only mantled the limestone, but came to rest on an

erosion surface on the mudstones, in places where the limestone has been removed by erosion or where the lime-

stone had never been deposited (Figure 4).

The complex relationships between the component units of the stratigraphy are commonly difficult to determine

because the rocks dip at c. 85� towards the observer, who consequently has a view nearly perpendicular to the plane

of bedding. The quarrying was for the sandstone, so it ceased when it encountered the stratigraphically older lime-

stones, now seen as the karstic domes and pillars, emerging through the sandstone cover. Locally, quarrying also

ceased where the limestone was missing and it encountered the underlying shales. The sandstones are <20 m thick

and have a very irregular base where they have filled the karstic topography of the limestone and erosional depres-

sions in the shales (Figure 4). The shales into which the main channel was incised are best exposed at both ends of

the exposure in what are interpreted to be the margins of the channel.

Figure 2. Continued



4. ENVIRONMENTAL ANALYSIS

4.1. Rawtheyan Mudstones

Up to 15 m of Rawtheyan Mudstones are identifiable at outcrop, but only 2 m are continuously exposed at any one

location in the line of quarries. The mudstones represent the uppermost levels of the Rawtheyan sequence that are

preserved below the sub-Hirnantian erosion surface. The mudstones are silty and micaceous and their lack of fis-

sility suggests they may have been extensively bioturbated. Interbedded are thin, fine sandstone and siltstone beds,

rarely more than 3 cm thick, with parallel or small-scale, unidirectional cross-lamination. Within the mudstones

are phosphate nodules widely dispersed along several horizons within a metre or more of the sequence. The

nodules (<1.5 cm diameter) yield a diverse and unfragmented fauna, with common trilobites, brachiopods, mol-

luscs, echinoderms and bryozoans (see above). The diverse fauna, with unfragmented individuals of small size,

suggests a broadly outer shelf environment, with a fauna associated with the inner part of Benthic Assemblage

(BA 5) or the outer part of BA 4, which would suggest a water depth of about 60 m (Brett et al. 1993). However,

this depth might be an overestimate, as the trilobite association indicates that it was not an outermost shelf envir-

onment (A.W. Owen, personal communication, 1998): furthermore, the small-scale, unidirectional ripple-lamina-

tions in the thin sandstone beds suggest deposition by storm-generated currents, below storm wave-base (>50–

60 m; Brenchley and Harper 1998). Based on the combination of the faunal and sedimentological evidence we

suggest that a depositional depth of 50–60 m is likely.

Figure 3. Location maps of the field area (1, 2 and 3) and Upper Ordovician lithostratigraphy (4).



4.2. The erosion surface and channel morphology

The estimated relief on the erosion surface is between 15 m and 25 m, based on the exposed or inferred depth below

the top surface of the Rawtheyan Mudstones. Near location C (Figure 3.3), a channel depth of a minimum of 15 m

is indicated by a slope discontinuously exposing a sequence of 15 m of mudstones overlain by limestone at the base

of the slope. The maximum estimate of the depth of the channel (c. 25 m) is based elsewhere on the thickness of the

sandstone and limestone sequence. The estimate assumes that the base of the limestone is at the break of slope at

the back of the ridge into which the quarries were cut and the top of the unquarried sandstone is the top of

the channel fill. The channel is about 350 m wide and is exposed intermittently, between locations A and C on

Figure 3.3. The topography of the base of the channel is irregular, with steep-sided troughs and hollows, varying

in scale from c. 1 m to larger incisions <15 m deep (seen between B1 and B2 on the profile in Figure 4). The orien-

tation of the channel is uncertain, because the axis of the channel could be orthogonal or oblique to the cross-sec-

tion that is exposed. The NE/SW orientation of channels on the palaeogeographic map on Figure 2b is inferred

from the estimated direction of the palaeoslope. Late Ordovician channels on the shelf rimming the Welsh Basin

are not commonly recorded, and no others contain a limestone in the channel fill. A channel has been recognized at

Garth (Brenchley 1988), and Groom and Lake (1908) recognized the channel-shaped cross-section of the Glyn Grit

at Glyn Ceiriog where the sedimentology is consistent with a channel environment (Brenchley and Cullen 1984).

The channels may have been major fluvial conduits of fine sediment across the shelf and onto the slope where it

accumulated as large slope aprons. There is no evidence of the slope being incised by submarine canyons from

which headward erosion could have created shelf channels. We therefore envisage the Meifod channel as being a

major fluvial channel, which was swept clear of fine sediment until headward erosion of feeder tributaries on the

shelf tapped a source of coarse carbonate sediment in carbonate shoals (see below), from where it was transported

and resedimented in the main channel.

4.3. The morphology and composition of the karstified limestones

The sandy limestone unit occurs as a series of domes and pillars (Figures 5 and 6) with a primary relief of as much

as 3 m. The karstic surface is within a nearly vertically bedded sequence from which the tops of the domes and

pillars emerge through the stratigraphically succeeding sandstones. No pillar is fully exposed, but a few are partly

exposed down to their base. Locally, it is possible to see that the limestone extends continuously beneath the

troughs separating the domes, suggesting that the domes rise from a limestone sheet (Figures 6 and 7). Their

sides were originally vertical or steeply sloping and some had nearly horizontal clefts that created overhangs,

whilst in others, irregular clefts were vertical (Figures 6 and 7). The surfaces of the domes are commonly unevenly

20m

NSandstone

Channel baseLimestoneLimestone

(inferred)

Near verticalquarry face

Log S1

Shale Shale

B1 B2

Figure 4. Map of the shale–limestone–sandstone sequence exposed between B1 and B2 in the line of quarries shown on Figure 3.3. The mainroad runs alongside the top of the sandstones. The map is an aerial view of vertical beds, younging SSE and exposed by quarrying. Log S1,

located here, is shown in Figure 11.



undulating, suggestive of an erosional or corrosional surface. A laminated crust, mainly composed of encrusting

bryozoans, covers the surfaces and obscures any internal structures that might be within the mounds. Consequently,

the limestone appears to be internally massive in outcrop, and any variability in its lithology is obscured. Most of

the limestone succession comprises a mixture of very fine quartz sand, up to a proportion of about 50%, with abun-

dant carbonate bioclasts and fine sand to granule grain-size intraclasts. Bryozoan fragments are the commonest

bioclasts (up to about 60% of the bioclasts) followed by pelmatozoan and brachiopod fragments (Figure 8a).

The bryozoan fauna is particularly diverse (Figure 9), with at least 10 species including six trepostome genera,

Hemiphragma, Eridotrypa, Hallopora (two species), Anaphragma?, Peronopora and Batostoma. There are also

fragments of Cystoporata and Cryptostomata (both Ptilodictyina and Rhabdomesina). Solitary, conical rugose cor-

als occur dispersed in small numbers, but two denser concentrations are also recorded. In addition, there is abun-

dant algal material, some in the form of irregularly shaped bioclasts with a shadowy internal tubular structure and

some as oncoids.

Bioclasts are commonly rounded and many have been bored; some have pyritic and phosphatic coatings that

may be truncated (Figure 8a). Such material evidently spent considerable time on the sea-floor and may have suf-

fered early marine suboxic and anoxic diagenesis prior to transportation.

Intraclasts are common and often well rounded (Figure 8b and c). They include micrites, silty or sandy micrites,

micrites containing bioclasts or oosparite grains, all indicating that the carbonate sediment had been cemented

prior to its erosion to form intraclasts. Some intraclasts have a rim of pyrite, which suggests marine diagenesis

in anoxic water below the sea-floor. Oosparite intraclasts have suffered pervasive dissolution and have been cemen-

ted by sparry calcite cement, both features of meteoric diagenesis (Figure 8c). Pyrite rims and meteoric cements in

the bioclasts and lithoclasts are commonly truncated (Figure 8b and c) indicating that the material was exhumed

and eroded before being resedimented. Also present are intraclasts of phosphate, which were initially probably

small concretions, that were exhumed and then transported and resedimented.

Some individual bioclasts in the sandy limestones have fringing fibrous cement (Figure 8d) that is interpreted as

an early marine cement. The fringing cement is post-dated by sparry calcite within and between the grains. The

Figure 5. (a) Photograph of the near-vertical quarry face and (b) sketch of the central part of exposure shown in (a) illustrating the distributionof limestones and sandstones in the quarry face. Bedding is near vertical. The limestones are interpreted as cross-sections through karstic domes

that have been surrounded by later sandstone deposits.



Figure 6. Morphology of the karstic domes: (a) the top of domes emerging from the sandstone cover, seen on a vertical surface; (b) a cross-section X–Y showing the topography of the top of the domes and the sandstone cover.

Figure 7. Reconstruction of the limestone domes and pillars projecting above the limestone surface.



fibrous fringing cement appears to be restricted to individual bioclasts that have not resedimented; if this is the case

they imply marine cementation at the site of deposition in the channels.

The mixture of grains with very different taphonomy and diagenetic history suggests that the sources included

both loose shell material, particularly bryozoans, on the sea-floor and lithified oolitic and bioclastic carbonates

eroded from the substrate. The associated phosphate clasts were probably derived from the Rawtheyan mudstones

undergoing deeper erosion.

The high relief of at least 3 m and steep sides to the limestone surface topography suggest that the domes and

pillars were rigid structures. The wholly clastic and bioclastic composition of the sandy limestone, including

reworked phosphate nodules, strongly suggest that the domes are unlikely to be bioconstructional features such

as bioherms. The alternative interpretation, proposed here, is that the domes are erosional, karstic features formed

at the top of the sandy bioclastic limestone that had accumulated at the bottom of the channel. This would imply

Figure 8. Photomicrographs of the sandy limestones. (a) Bioclasts in a sandy matrix: note boring (b), and pyritized (p) and possiblyphosphatized rims. Field of view 6 mm. (b) Rounded reworked intraclast (i), Field of view 4 mm. (c) Intraclast (i) of oolitic and crinoidallimestone in sandy limestones. Ooids have suffered meteoric diagenesis, prior to erosion and redeposition of the intraclast. d, Dissolutiontextures; c, cement that is truncated at the margin of the grain. Field of view 3.5 mm. (d) Bryozoan bioclast with internal isopachous fringing

cements (f). Field of view 2.5 mm.

—————————————————————————————————————————————"Figure 9. Bryozoans from karstic limestones. (A) Hemiphragma sp., NMW 2004.1G.4.2i. (B) Eridotrypa sp., NMW 2004.1G.7.2i. (C)Hallopora sp. A, NMW 2004.1G.5i. (D) Hallopora sp. B, NMW 2004.1G.14i. (E) Batostoma sp., NMW 2004.1G.9.2i. (F) Batostoma sp.,NMW 2004.1G.12.2i. (G) Batostoma sp., NMW 2004.1G.12.2i. (H) Peronopora sp., NMW 2004.1G.14ii. (I) Fistuliporina sp, NMW2004.1G.1.2i. (J) Ptilodictyina sp, NMW 2004.1G.14iii. (A, B, C, D, F, H, I) all longitudinal sections; (E) and (J) transverse sections; (G)

tangential section. All scale bars 500mm. All specimens housed at the National Museum of Wales (NMW).



that there must have been a phase of extensive lithification in the channel prior to karstification. The petrographic

evidence for the environment of lithification is, however, equivocal. Some individual bryozoan bioclasts have a

thin fringe of non-ferroan fibrous calcite internal cement (Figure 8d), which is likely to have been marine in origin,

whilst others have more blocky non-ferroan and later ferroan cements which might suggest shallow meteoric

diagenesis. The sandy matrix is cemented by a mixture of ferroan and non-ferroan calcite cements of uncertain

origin. Despite the evidence for karstification, it is worth noting that there is no evidence for extensive vadose

cementation.

4.4. The laminated crust

A laminated limestone crust, up to 3 cm thick (Figure 10), closely mantles the irregular microtopography of the

upper surface and most of the lateral surfaces of the domes. The contact between the laminated crust facies and

underlying sandy limestones (Figure 10a and c) is sharp, highly irregular and truncates bioclasts. It is interpreted to

be the karstic dissolution surface. The crust itself is superficially similar to laminated micritic crusts on Carboni-

ferous palaeokarsts which have been interpreted to be subaerial (e.g. Walkden 1974; Wright 1982), but the crust at

Meifod appears to have a very different origin. It has an in situ marine fauna dominated by bryozoans and is appar-

ently wholly of marine origin.

The bryozoans in the crust are mostly cystoporate forms, although some trepostome fragments can be identified.

The bryozoans appear to mantle the karstic surface and most are probably in situ encrusting forms, in life position

(Figure 10). Most of the bryozoans appear to have been abraded. A few of the skeletal clasts are downward-facing

(for example the specimen of Fistuliporina sp. in Figure 9 I); suggesting that they have been transported.

The sediment matrix between the bryozoans is much more micritic than in the underlying sandy limestones

(Figure 10b and c). The micrite fills most of the zooecia within the bryozoans and the spaces between the bryozoan

colonies. Thin irregular seams of organic residue are common within the crust. These might indicate a microbial

origin, although they appear to have been substantially modified by pressure dissolution during burial.

The crust, locally, contains large rounded quartz grains, similar to those that sporadically occur in the sandstone

adjacent to the pillars and domes. It seems probable that these grains became incorporated in the crust as the coarse

sand accreted within a turbulent microenvironment between the pillars and domes.

The biota both within and on the crust is clearly indicative of colonization of a lithified surface in a submarine

environment. As well as the bryozoans, there are pelmatozoan holdfasts, including a whole pluricolumnal with a

holdfast and pseudocirriferous attachment structures (Donovan 1995). The cystoid Gomphocystitis asper (Paul

1984) occurs partly embedded in the crust, as do clusters of the brachiopod Paromalomena polonica. The marine

waters must have been deep enough to submerge the karst topography and to allow colonization by the fauna.

Paromalomena polonica has an ecological range from middle BA3 to BA5. The small size of the brachiopods

and the low diversity of the assemblage in which they are found is typical of BA 4–5 which would suggest a water

depth of c. 50–60 m. With such water depths in the channels the shelf, outside the channel, would then have been

flooded to depths of around 40 m. However, the channel location and the hard substrate on which the Paramalo-

mena fauna appears to have lived are atypical for the species. Consequently, direct comparisons with its normal

depth location may be invalid.

4.5. The sandstones

Sandstones fill both the irregular topography of the karstic limestone surface and the depressions in the underlying

mudstones, where the limestone is missing. They accreted to fill the channel and are patchily spread across the

Hirnantian shelf. The sandstone is fine-grained, angular-grained and well sorted, except where the fine sand is

mixed into accumulations of coarse well-rounded grains at the bottom of depressions in the karst surface or adja-

cent to the side of domes, where large grains may be incorporated into the laminated crust. The sandstone sequence

filling the channel has massive beds commonly interbedded with horizontal and low-angle cross-stratified units

(Figure 11, S1). In addition, there are both tabular and lenticular cross-stratified beds (Figure 11, S1) with planar



Figure 10. Laminated crust. (a) A cut face through the top of a dome, showing the laminated crust, largely constructed by encrusting bryozoanswhich mantle the corroded surface at the top of the dome. Arrow marks the contact between the sandy limestones and the overlying laminatedcrust. (b) Photomicrograph of laminated crust. Much of the crust is composed of cystoporate bryozoans which have been infilled with micrite.Many of the bryozoans are in growth position. Scale bar 2 mm. (c) Photomicrograph. Contact between the crust and underlying sandy limestones(arrowed). The bryozoan (Br) is in life position. The abundant irregular black seams of insoluble residue apparent in photographs (b) and (c)

have been extensively modified by pressure dissolution. Scale bar 2 mm.



and trough cross-stratification. The stacked sandstone sets suggest that the deposition took place in a relatively

high-energy environment with a copious supply of fine, angular sand that is likely to have come from a relatively

local source. In an open shelf situation, the thick cross-stratified bed-sets would indicate large dunes typical of an

environment below the beach zone, but above the lower part of the shoreface, where interbedded muds would be

expected. The depositional environment is therefore interpreted as being within the mid-part of the shoreface that

is likely to be in a depth range of 5–10 m. This is less than the depth suggested by the presence of a well-preserved,

disarticulated but unfragmented Hirnantia fauna near the base of the channel. The particular assemblage that is

present is characteristic of the Dalmanella–Hindella Association of Rong and Harper (1988) and is regarded as

having belonged to the upper part of Benthic Assemblage 3; this is interpreted as occupying the shoreface to inner

shelf transition with an estimated depth of 15–30 m (Brenchley and Harper 1998). However, the sedimentary evi-

dence suggests that the depositional depth of the Graig-wen Sandstone was shallower than this. One resolution to

this apparent disparity between the bathymetric evidence is that the Hirnantia fauna occupied the bottom of the

channel when it was submerged during a transgression and that the sedimentary structures, present in the upper

part of the channel-fill, formed when sand had accreted to fill the channel. The cross-stratified sandstones are in the

upper part of the Graig-wen Sandstone and may have formed in a shoreface or within a tidal channel within a

nearshore area. There is, however, no evidence of opposed flow direction, so a mid-shoreface environment that

developed above the filled channel is more likely, with depths of 5–10 m.

4.6. The uppermost Graig-wen transition and overlying shales

A continuous sequence through the top of the sandstones into the succeeding shales is not currently exposed in the

Meifod area, but King (1928) recorded the presence of a group of blue-black micaceous shales with thin hard bands

of siltstone (up to 2 cm thick), commonly with small-scale cross-lamination. A short sequence of what is believed

to be the upper part of the Graig-wen Sandstone is present c. 6 km north of Graig-wen (grid reference SJ1040 1520)

The sequence has thick-bedded sandstones and interbedded siltstones (Figure 11, S2). Some of the sandstones are

parallel laminated and others have hummocky cross-stratification (HCS). The thick bedding and presence of large-

scale HCS suggest deposition on a shoreface, but the interbedded siltstones suggest periodic quieter conditions

such as occur low on the shoreface or inner shelf. Shales similar to those above the Craig-wen Sandstone, described

by King (1928), occur as loose material in a bank adjacent to the sandstone. The loose material of bioturbated

shale and thin sandstones apparently belong to the overlying shale unit and reflect a substantial change in envir-

onment. A small section of similar shales is exposed nearby (SJ 0950 1400). Here, interbedded within the biotur-

bated shales, are thin, very fine sandstone beds with unidirectional cross-lamination and other beds with wispy silt

laminae (Figure 11, S3); the fine sandstones are interpreted as being deposited by unidirectional storm currents in

an environment below storm wave-base. The facies is very similar to that in the upper part of the Rawtheyan in the

outcrops at Graig-wen, suggesting that sea level had risen to a similar level as that before the Hirnantian regression,

though the absence of a benthic shelf fauna suggests the depth might have been greater (i.e. 60 mþ), unless the lack

of fossils is a taphonomic phenomenon.

5. ISOTOPIC CONSTRAINTS ON THE TIMING OF EVENTS

The Hirnantian marks a period of global change in marine carbon isotope signatures. Analysis of the fossils from

end-Ordovician and early Silurian sequences in Baltica and Laurentia (Brenchley et al. 2003) consistently show that

whilst Rawtheyan and early Silurian �13C values range from �2 to þ2% elevated carbon values, greater than

þ2%PDB (Pee Dee Belemnite international standard), are exclusively restricted to the early part of the Hirnantian

stage (see Introduction and Figure 1). Stratigraphic differences in maximum carbon isotopic values have been main-

tained in successions in Norway and southern Scotland, where burial diagenesis or even low-grade metamorphism

has affected the sediments (Brenchley et al. 1997; Brenchley and Marshall 1999; Underwood et al. 1997).

In order to further constrain the timing of the events at Meifod, bioclast samples from the sandstones and sandy

limestones were analysed for their stable isotopic composition. Many of the samples analysed have very low �18O



values indicating a substantial, probably thermal, diagenetic overprint. The carbon isotopic values of the least

altered material are shown in Figure 12. Most of the resedimented carbonates have carbon isotope values indicative

of a Rawtheyan age. Brachiopods and other bioclasts with �13C value ranging from �1.1% to þ1.1%, have char-

acteristic Rawtheyan compositions, but the three bioclasts and a brachiopod with �13C> þ 2.0% are likely to be

0

2

4

6

8

10

12

14

0

0

2

4

1m.

m.

m.

S1

S2

S3.

25cm

c

c

st

st

fs

fsc

Bioturbation (mostly burrows)Chondrites

Beddingand cross-bedding

st fs

c st fs

Figure 11. S1 and S2 are sedimentary logs through parts of the fine sandstones, showing the interbedding of cross-stratified beds with parallellaminated and massive units (unornamented). S1: Log located in Figure 4; unidirectional cross-bedding; deposition is interpreted to have takenplace in mid shoreface. S2: Hummocky cross-bedded sandstones interbedded with siltstones, Graig-wen Sandstone c. 6 km north of Graig-wen.Depositional environment is interpreted to be at the lower shoreface/inner shelf transition. S3: Beds in the transition between sandstone andmudstone at the top of the sandstone unit near to S2. Thin cross-stratified beds of sandstone are interbedded with mudstones and very thin,laminated siltstones. These sediments are interpreted as deeper water deposits formed below storm wave-base. key: c, clay; st, silt; fs, fine sand.



Figure 12. Hirnantian brachiopods. (A, B) Part and counterpart of small block containing Hirnantia sagittifera (M’Coy), internal (A) andexternal (B) of ventral valve, centre left and centre right, Plectothyrella crassicostis (Dalman), external (A) and internal (B) of ventral valve, topright and top left; Hindella crassa incipiens (Williams), external (A) and internal (B) of ventral valve, bottom right and bottom left, MGUH27058,� 1.5. (C) Hirnantia sagittifera (M’Coy), internal mould of dorsal valve, MGUH 27059,� 1.8. (D) Hindella crassa incipiens (Williams)internal mould of dorsal valve, MGUH 27060,� 2. (E) Hindella crassa incipiens (Williams), internal mould of ventral valve, MGUH27061,� 3. (F) Small block containing Hirnantia sagittifera (M’Coy), internal mould of dorsal valve, left and external mould of ventral valve,right, MGUH 27062,� 3. (G) Plectothyrella crassicostis (Dalman), external mould of dorsal valve, MGUH 27063,� 3. All material reposited

in the Geological Museum, University of Copenhagen (prefix MGUH).



Hirnantian shells (Figure 13). The isotopic data therefore suggest that the population of bioclasts is of mixed ages;

this is consistent with the presence of detrital limestone and phosphate clasts (see above). Latest Rawtheyan lime-

stones are not recorded from the Meifod region, but bioclastic limestones form the basal member of the Glyn For-

mation near Glyn-Ceiriog in North Wales, where it is thought to reflect a relatively small latest Rawtheyan

regression (Cullen 1986). A possible minor regression of a similar age has been widely identified in the Oslo

region, Norway (Brenchley and Marshall 1999; Nielsen and Harper 2003). It seems likely, therefore, that the Raw-

theyan bioclasts in the channel were derived from localized patches of latest Rawtheyan shelf carbonates. The

phosphate clasts probably came from deeper in the Rawtheyan sequence as the shelf became exposed and more

deeply incised in the earliest Hirnantian. The Hirnantian bioclasts in the sandy limestones may have been derived

from even more restricted patches of carbonate that formed temporarily as sea-level fell or may reflect a fauna that

lived in the channel itself. The brachiopod with �13C value of þ4 reflects marine values towards the height of the

regression (Brenchley et al. 2003) so the channel must have been receiving sediment well into the Hirnantian.

The occurrence of the brachiopod Paromalomena polonica attached to the surface of the karstic mounds indi-

cates a Hirnantian age (see Rong et al. 2002) for the transgression that flooded the karstic surface and thereby

constrains the age of lithification and karstification of the limestones to being Hirnantian. The presence of a

Hirnantia fauna in the overlying sandstones is diagnostic of a Hirnantian age for these sediments and this is con-

firmed by the consistently high �13C values in brachiopod shells in the sandstones (Figure 13).

Figure 13. Histograms of stable isotopic compositions of bioclasts from Meifod, on the basis of data from other sections (Brenchley et al. 2003).Rawtheyan carbonates typically have �13C values <þ2 whilst Hirnantian carbonates have higher �13C values. Note that the bioclasts from thesandy limestones at Meifod have a range of �13C values. Most of the shells are apparently Rawtheyan and thus reworked. The four shells with

�13C values >þ2 are Hirnantian. The upper histogram for shells from the sandstones exclusively record Hirnantian values.



2.

3.

1.

4.

5.

6.

7.

8.Major transgression:shelf flooded,shales deposited.

Regression:Sands fill channel

Transgression:channel floods,crust forms.

Regression:karstificationof limestones

? Minor transgression:carbonatesreworked,rapid lithification

Major regression:channel cut.Shoals lithify

Minor regression:carbonate sandshoals on shelf

Deeper marineshelf. Phosphatenodules in shales

sea-level

sea-level



6. INTERPRETATION OF SEA-LEVEL CHANGES

The facies sequence at Meifod provides a record of successive Late Ordovician environmental changes on the shelf

and by inference a record of the amount of sea-level rise and fall. The sequence and magnitude of the sea-level

changes are summarized below.

(1) The initial deep shelf environment (Figure 14.1): thin sandstones without wave-ripple cross-lamination and

a trilobite–brachiopod fauna of BA outer 4 to BA inner 5, mainly preserved in phosphate nodules within

the mudstones, indicate a mid-to deep-shelf environment below storm wave-base with an estimated depth

of c. 60 m.

(2) The initial regression (Figure 14.2): a small latest Rawtheyan sea-level fall led to the deposition and

subsequent lithification of localized patches of bioclastic carbonate sediment on the inner shelf (Figure 14.2).

A similar, latest Rawtheyan regression is recognized in the Oslo region, Norway (Brenchley and Marshall

1999; Nielsen and Harper 2003) where the lithofacies change that marks the regression is not accompanied by

a substantial change in biofacies, suggesting the sea-level change was relatively small.

(3) Major regression: a major phase of sea-level fall followed in the early Hirnantian, when the shelf became

exposed and a widespread erosion surface was formed across the shelf (Figure 14.3), which was incised by

some deep, probably fluvial channels (< 27 mþ deep at Meifod). Large volumes of mud were transported

across the shelf and deposited as a thick apron on the slope (Figure 2b). Coarser material, including

Rawtheyan brachiopods, abundant bioclasts, ooids and carbonate lithoclasts, was eroded from patches of

lithified carbonate, together with minor amounts of Hirnantian material. In addition, phosphate nodules

were exhumed from Rawtheyan mudstones. This varied material was resedimented into the channel seen at

Meifod.

(4) Transgression? the carbonate sediments must have lithified within the channel prior to karstification, but it is

not at all clear how and where this took place. Despite their occurrence in, what is interpreted as, a fluvial

channel, there is no evidence for extensive subaerial, vadose diagenesis. Early isopachous fringing cements

within individual bioclasts suggest phreatic cementation, but the presence of both fibrous and blocky crystal

morphologies alternatively suggest marine and non-marine processes. It remains possible that such grains and

their internal cements may have been transported. If the Hirnantian elements of the fauna in the limestones and

the fibrous cements represent in situ occurrences in the channel, then a minor transgression into the channel is

required (Figure 14.4). Alternatively, if the whole assemblage of grains and cements is transported, then

lithification of the limestones may have taken place in a freshwater environment and no change in sea-level is

required.

(5) Regression (Figure 14.5): exposure of the carbonates during a subsequent sea-level fall led to the erosion,

which formed the karst topography that provides the firm evidence for subaerial exposure of the channel. The

estimated magnitude of total sea-level fall is c. 80 mþ (initial seawater depth of 60 m to the sediment surface

on the shelf þ>20 m to the base of channel).

(6) Transgression: rising sea-level flooded the karst within the Meifod channel and then spread more widely across

the shelf. The encrusting fauna on the karst surface suggests the sea-level rose to mid-shelf depths, estimated to

be c. 30 m. During the transgression very little sediment was deposited in the channel, but may have been

carried shorewards and stored along coastlines or in estuaries (phase 5, Figure 14.5).

3—————————————————————————————————————————————Figure 14. Schematic reconstructions of the successive environments developed through eight phases of sea-level change (see text). Sea-level isindicated on each figure—see arrows on stages 1 and 3. (1) The initial mud environment of a deep shelf. (2) Shallower carbonate shoals formduring a minor regression. (3) Carbonates are lithified as sea-level begins to fall, then a deep channel was incised across the shelf and detritalcarbonate from the shoals was deposited in the channel, following a major fall in sea-level. (4) A minor transgression is inferred, if the bioclastswith Hirnantian isotope values represent an in situ fauna and if the carbonates were pervasively lithified with a marine cement. This minortransgression is not required if the fauna and associated cements are entirely transported (see text for details). (5) A regression exposed thelimestone, on which a karstic surface was formed. (6) A transgression floods the channel and the adjacent shelf, driving sediment shorewards. (7)A regression causes coastal progradation and the filling of the channel and burial of the karst. (8) Sea-level rises and the shelf is deeply flooded.



(7) Regression: large-scale cross-stratification suggest sea-level fell from c. 30 m to depths typical of the middle

shoreface (c. 10 m), a fall of about 20 m. The regression mobilized fine sand in the coastal region and promoted

its transport offshore so that it buried the karst surface, filled this and other channels and then spread more

widely across the shelf, during coastal advance (phase 6, Figure 14.7).

(8) Transgression: mudstones lacking a benthic fauna and containing rare graptolites mark the late Hirnantian

transgression, during which the rising sea-level flooded the shelf to depths of c. 80 m. This represents a final

sea-level rise of about 70 m, and a total sea-level rise of c. 95 mþ, above the level of its maximum fall, marked

by the base of the Meifod channel.

7. CONCLUSIONS

The karstified limestones at Meifod provide crucial evidence of subaerial exposure, allowing the overall sea-level

change associated with Gondwanan glaciation to be calculated. A fall from mid/deep-shelf depths to an exposed

surface at a depth of 20 mþ within the Meifod channel gives an estimated overall sea-level change of c. 80 m. The

subsequent rise from the lowest part of the karst surface to marine depths of c. 60–70 m similarly indicates a post-

glacial sea-level rise of c. 80–100 m. Previous estimates of the magnitude of end-Ordovician sea-level fall have

varied between 30 and 100 m. The evidence from the Meifod channel thus supports the higher estimates. An esti-

mate of the Gondwanan ice-sheet area as 11.8� 106 km2 was used to model a change in sea-level of c. 60 m (sea-

level equivalent) or 45 m (with isostatic adjustment) (Crowley and Baum 1991). Both these estimates are lower

than that made on the evidence from the Meifod channel, which therefore suggests that the Gondwanan ice area

and volume may have been larger than that suggested by Crowley and Baum (1991).

The detailed stratigraphy of the Meifod channel provides additional evidence of smaller-scale fluctuations in

sea-level superimposed on the overall pattern of regression and transgression: these may be eustatic or of local

significance. The Late Rawtheyan sea-level fall (1–2 in Figure 15), which forms a precursor to the main regression,

is, as discussed above, likely to be eustatic. The minor transgression and subsequent regression (3–4–5 in

Figure 15) at the lowest part of the sea-level curve may reflect the bipartite subdivision of the glaciation. A similar

record has been described from the Prague Basin (Brenchley and Storch 1998) and from Estonia (Brenchley et al.

Figure 15. End-Ordovician sea-level change as inferred from the succession at Meifod. Numbers refer to the stages outlined in Figure 14 anddiscussed in the text. The sea-level estimates are relative to the shelf surface. End-Rawtheyan water depths are estimated to be 50–60 m; themaximum incision of the channel is some 25 m below this surface suggesting that the overall glacio-eustatic sea-level fall was around 80 m.



2003). This sea-level history is consistent with records from late Ordovician glacial deposits in North Africa,

which may show the two phases of glacial advance and retreat that are assumed to be eccentricity-influenced cycles

of 0.1 Ma (Sutcliffe et al. 2000). In contrast to the widely recognized sea-level fluctuations, the minor regression on

the rising limb of the overall sea-level curve (6–8 in Figure 15) has not been recognized elsewhere; therefore, it

may reflect local changes or may prove to be eustatic. Further detailed work of this nature is required if we are to

fully understand the precise architecture of the end-Ordovician glaciation.

ACKNOWLEDGEMENTS

We would like to thank Alan Owen who identified the trilobites in this study. P. J. Brenchley would like to thank the

British Geological Survey for the Research Agreement Grant GA/95E/23, towards this work. Kay Lancaster

drafted Figure 7. NERC and the University fund the stable isotope laboratory at Liverpool. Dawid Harper thanks

the Danish National Science Research Council for its support. Jim Ball is thanked for the analysis of the carbonate

samples. We thank Alan Owen, Jerry Davies and Ian Somerville for constructive comments on the manuscript.

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