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In sight of the suture: the early Palaeozoic geological history of the Isle of Man

N. H. WOODCOCK, 1 D. G. QUIRK, 2 W. R. FITCHES, 3 & R. E BARNES 4 1Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK 2Department of , Oxford Brookes University, Gipsy Lane, Oxford OX3 0BP, UK Present address: Burlington Resources (Irish Sea) Ltd, 1 Canada Square, Canary Wharf London El4 5AA, UK 3Robertson Research International, Llanrhos, Llandudno, North Wales, LL30 1SA, UK 4British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK

Abstract: The pre- and syn-Caledonian rocks of the Isle of Man are now known to comprise three distinct units: the early Manx Group, the mid- Dalby Group and the ?late Silurian-early Peel Sandstones. The Manx Group is dominated by Arenig deep-marine turbidites and debrites deposited in oxygenated basins on the northwest-facing margin of . Its organization into a sand-rich lower part and a mud-rich upper part invites comparison with the () and Ribband Group (Leinster) and points to control by margin-wide events, in part eustatic sea- level changes. Episodes of mass-wasting and Fe-Mn fluid exhalation also correlate along the margin. A mid-late Ordovician volcanic arc is missing above the Manx Group, although parts of its intrusive substructure may be preserved. The Dalby Group comprises northwest-derived turbidites, sedimented into an anoxic basin during Wenlock (mid-Silurian) time. These turbidites were deposited in a successor basin above the Iapetus suture zone. The Dalby Group sits with a tectonic contact on the Manx Group. No evidence has been found of a pre-Silurian . The main Caledonian D1 and D2 shortening phases are post-Wenlock, comparable in age with those further along the margin in the Lake District and Leinster. The Peel Sandstones preserve a Lower '' sequence, mostly removed by post-Caledonian erosion elsewhere along this outboard part of the Avalonian margin. The unit does not host a definite Caledonian cleavage, and it must have been deposited late in the deformation history. The granitic intrusions into the Manx Group range from early in D 1 to late in D2. The intrusions generate only local aureoles, and the high metamorphic grade in parts of the Manx Group may be enhanced by favourable protolith compositions.

The Isle of Man enjoys a unique geographical & Murphy 1989; Todd et al. 1991; Owen et al. position, lying as it does in the Irish Sea within 1992, Vaughan & Johnston 1992) highlights the sight of Wales, England, Scotland and Ireland (Fig. need for more information from the Isle of Man. 1). However, its geological setting is no less The results from Lower Palaeozoic rocks reported special. Although now part of a block in this volume promise to augment substantially our surrounded by Mesozoic basins, it lies tantalizingly knowledge of the geology of the Iapetus Suture close to the surface trace of that most important of Zone and of the outboard edge of the Avalonian regional Palaeozoic structures, the Iapetus Suture. margin. The Upper Palaeozoic and Mesozoic Geophysical evidence (Soper et al. 1992) suggests geology of the surrounding Irish Sea has been that this boundary, between the former Avalonian summarized recently in the volumes edited by microcontinent to the south and the Laurentian Meadows et al. (1997) and a thematic issue of flae to the north, skirts the northwestern edge Journal of Petroleum Geology (1999) edited by D. of the island (Fig. 1). Over most of the British Isles, G. Quirk. the surface trace of the suture is hidden by Upper Palaeozoic rocks. Only in eastern Ireland and the Research past and present Isle of Man do Lower Palaeozoic rocks crop out at, or close to, the suture. The difficulty in deciphering The Lower Palaeozoic rocks of the Isle of Man, the eastern Irish evidence across the suture (Harper until recently all assigned to the Manx Group, have

From: WOODCOCK,N. H., Qt;rRK, D. G., FITCHES, W. R. & BARNES, R. P. (eds) 1999. In Sight of the Suture: the Palaeozoic geology of the Isle of Man in its' context. Geological Society, London, Special Publications, 160, 1-10. 1-86239-046-0/99/$15.00 ©The Geological Society of London 1999. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021


Lower Palaeozoic outcrops L~ ~ sedimentary rocks +! j"i Southern ' . igneous rocks J...:• UP!ands.: I N S major faults 50 km /

~ .., ...... • .. ,

s '~ ..i. -~ • ...,..: - • ,~ . J , " . "~/Isle " ".. Ongfo .a. " • ~ Man

i.-Down• " : .~-..-..- .[.. Irish Sea L Q Aval /

/ ~ .::.: ./ /y-~~ .. :. -. ~--

Fig. 1. Location of the Isle of Man in relation to other Lower Palaeozoic regions around the Irish Sea.

a long history of investigation. This history is Caledonian in age, and generally due to the detailed in the present volume by Ford et al. and Silurian-Early Devonian impingement of Avalonia listed in a comprehensive bibliography of the island with the Laurentian continent (Soper et al. 1987, by Wilson. Despite this diversity of past research, 1992). our present view of the geology of the Manx Group Recent interest in the Manx Group was rekindled has been predominantly formed by the work of two through biostratigraphic work by Molyneux (1979), people: G. W. Lamplugh, who mapped the island metamorphic studies by Roberts et al. (1990) and a for the British Geological Survey at the end of the field guidebook by Ford (1993). New research (e.g. last century, and A. Simpson, who studied the Rushton 1993; Quirk & Kimbell 1997; Stone & Manx slates in the 1960s. The work of both Evans 1997) was eventually focused into a geologists pre-dates, of course, ideas about the multidiscipinary field-based project ca~Tied out on crucial plate tectonic setting of the island. The the island between 1995 and 1998. This volume regional context of the Manx Group has been built reports many of the results of this new wave of up instead from work in related areas, particularly research. The papers are organized into sections the Lake District of England, the Welsh Basin and covering the main themes in the deposition of the the Leinster Basin of Ireland. On this evidence, the Lower Palaeozoic sedimentary rocks of the Isle of Manx Group is seen as part of the early Ordovician Man and their subsequent deformation, meta- sediment prism on the outboard edge of the morphism, intrusion and mineralization. The Avalonian segment of the continent, stratigraphical focus of each of the papers is shown continuous with the Skiddaw Group of the Lake on Fig. 2. This introductory review sets these District and the Ribband Group of Leinster (Cooper papers (denoted by bold type) within an inter- et al. 1995). The polyphase deformation history pretative summary of Palaeozoic geological history established by Simpson (1963) for the Manx of the island, highlighting current debates and the Group is assumed to be predominantly late scope for future work. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021


Ma Strat Geological record Relevant papers Tectonic setting

250 site of future Isle of Man

~ ~ Laurussian margin rifted then shortened ~n~u~j Piper & Crowley NW SE

390 ~ Caledonian orogen uplifted and eroded

>, Caledonian Orogen Kimbell & Quirk collision zone Quirk et al. shortened, 410 Fitches et al. metamorphosed Power & Barnes and intruded

Prd Piper & Crowley 420 Lud _~_ Dhoon Avalonia over- :i~iiiiiiiiiiiiiiiiiiiiiiiiiiii~iiiiiiiiiiiiiiiiiiiiiiiiiiiiii Ater-j-'Dai~i-i~i~iP'-i- 1 HoweM°rriset al. thrusts Avalonia i~,: ~

450 arc shuts down O© Rheic Crd volcanic units? arc volcanism now eroded (Lake District and Leinster)


I~n~h i~ 470 Piper et aL Power & Crowley Avalonia Woodcock & Morris from Gondwana Kennan & Morris Woodcock et al. 480 Orr & Howe lapetus crust lapetus ~ . © Molyneux subducts under Quirk & Burnett Gondwana Woodcock & Barnes Barnes et aL volcanic arc 490 initiated (Wales and Leinster)

Fig. 2. Stratigraphical location of the studies reported in this volume, plotted on a geochronological diagram of the Palaeozoic geological history of the Isle of Man. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021


Stratigraphic and structural framework further refinement of the scheme is suggested by the lithofacies mapping of Quirk & Burnett. The prevailing structural view of the Manx Group Using a detailed facies characterisation of the Manx throughout this century has been of a major Group , they have revealed the internal , involving repetition of sandstone-rich complexity of some of the formations. They have units on the northwest and southeast coasts and also begun to delimit possible formation boundaries cored by mudstone-rich units along the northeast- in the northern segment of the Manx Group left southwest spine of the island. However, the undefined by Woodcock et al. stratigraphic scheme required by this synclinal Poor biostratigraphic zonation of the Manx model, outlined by Lamplugh (1903) and Group continues to hamper its interpretation. Orr embellished by Simpson (1963), has proved to be & Howe report new graptolite finds, but the inconsistent with more recent biostratigraphic data material is indifferently preserved and merely (Molyneux 1979; Cooper et al. 1995). In this serves to confirm the Arenig age of the upper part volume, the correlation of the two sandstone units, of the Lonan Flags, now formally defined as the the Lonan and Niarbyl Flags, across the syncline is Santon Formation. More encouragingly, the trace shown to be untenable on lithological grounds by fossil assemblage described by On" & Howe closely Morris et al., Quirk & Burnett and Woodcock & matches that from the Skiddaw Group, strength- Barnes. More conclusively still, Howe has ening the correlation both with the deep-water identified mid-Silurian (Wenlock) graptolites from setting of the Skiddaw Group and its Tremadoc- the Niarbyl Flags, and Molyneux and Orr & Howe Arenig age. However, it is the acritarch assem- have confirmed the early Ordovician (Arenig) blages that still provide the best zonation of the faunas from the Lonan flags. These conflicting data Manx Group itself. Molyneux reassigns the have provoked Morris et al. to remove the formally available data to the new lithostratigraphic scheme, redefined Niarbyl Formation from the Manx Group confirming that the Manx Group includes definite and to place it in a distinct Dalby Group, separated early, mid and late Arenig assemblages. Further from the underlying Manx Group by the Niarbyl biostratigraphic work on both micro- and macro- Thrust. The considerable regional implications of fossils is essential for future understanding of the this relationship are discussed later. depositional history of the Manx Group. The simplicity of a major D1 syncline in the Manx Group has also been questioned by structural Early Arenig sand-turbidite fan deposition observations summarized by Fitches et al. The lithological repetition across the island is due, in The base of the Manx Group, like its correlative part, to significant, probably reverse, faults that Skiddaw and Ribband Groups, is not seen at separate tracts of continuous stratigraphy. Other outcrop. Comparison with the structural model of tracts have suspect boundaries, which may prove to Hughes et al. (1993) for the Lake District, or that of be stratigraphic or faulted. Only a few of the faults Max et al. (1990) for Leinster, suggests that these are well exposed on the coast. However, the units might, in any case, have been tectonically existence of important faults in the less well- detached from their original basement. The earliest exposed part of the central and northwestern Manx evidence of Manx Group sedimentation comes Group is suggested by northeast-southwest and from poorly preserved Tremadoc or early Arenig east-west oriented aeromagnetic lineaments noted graptolites from the Cronk Sumark Slates, within by Quirk et al. the area mapped by Quirk & Burnett. A volcanic Uncertainty over the large-scale structure has arc had already been initiated in Wales and Leinster encouraged Woodcock et al. to reorganize Manx by this time (Kokelaar 1988; McConnell & Morris Group stratigraphy in terms of a series of seven 1997), and of Iapetus oceanic crust northeast-southwest trending tectonostratigraphic must already have started beneath this segment of tracts. The approach is analogous to the successful the Gondwana margin. Unless dramatic reorgani- tract strategy employed in the Southern Uplands of zation of marginal crustal slivers has since Scotland (e.g. Barnes et al. 1989). Faults or high- occurred, the Manx Group was presumably strain zones form some tract boundaries, but the deposited in the forearc of this active margin (Fig. possibility cannot be excluded that some tract 2). Another glimpse of the early Manx Group may boundaries are essentially stratigraphic. Some of be provided by the lower Arenig 'Peel Volcanics' the 12 component formations may be repeated in (Simpson 1963; briefly discussed by Morris et al.) another tract, but have been separately named until and by widespread early felsitic intrusions. How such correlations can be better founded. Because these rocks relate to the arc volcanism awaits type sections are designated for the first time, the further study. new stratigraphic scheme should provide a firm The lower Arenig components of the Manx basis for future work. That such work will require Group seem to be dominated by sandstone-rich Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

THE EARLY PALAEOZOIC GEOLOGICAL HISTORY OF THE ISLE OF MAN 5 turbidite successions. Quirk & Burnett chart the and upper Arenig successions of the Manx Group: facies variation in these successions, ranging from mudstones, quartzose sandstones and pebbly very thin-bedded silt-mud couplets through thin- mudstones. Woodcock & Morris interpret one and medium-bedded sand-mud couplets, to thick- example of such a succession, the Lady Port bedded sandstones. Orr & Howe confirm from Formation. They suggest that slumping and debris trace fossils that these successions were deposited flow on the muddy margin slope created an in a deep-marine environment. Woodcock & irregular topography, which in turn trapped the Barnes show the sedimentological complexity of sandy basal parts of downslope turbidity flows. the lower Arenig sandstones, with flows travelling This phase of down-margin mass wasting might downslope to the northwest but with their con- simply reflect the mud-rich nature of the centrated sandy basal parts tending to be deflected contemporary slope. Alternatively, it might record along-slope to the southwest, possibly by a - seismic triggering during the decisive rifting of structured topography on the margin. Avalonia from Gondwana (Cooper et al. 1995; A feature of these lower Arenig sandstone Woodcock & Morris; Stone et al.; Fig. 2), a turbidites is the compositional variation from lithic timing consistent with regional evidence (Prigmore wackes to arenites, in places highly quartzose. et al. 1997). Woodcock & Barnes find this variation too These syn- Manx Group successions also marked to be produced solely by intrabasinal record evidence for a high rate of exhalation of iron sorting or flow-stripping, and follow Cooper et al. and manganese-rich fluids into the late Arenig (1995) in invoking a distinct supply of clean sand to ocean, as deduced by Kennan & Morris from the the Gondwana shelf at this time. This sand was manganese carbonate rocks in the Creggan Mooar plausibly sourced from the continental sheet sands and Lady Port Formations. They also suggest that preserved in Europe as the Armorican quartzite, such rocks are the protolith to the Mn-garnet- implying a continued connection between Avalonia bearing 'coticule' that characterizes many higher and Gondwana during early Arenig time. grade Ordovician rocks bordering the former Geochemical studies by Barnes et al. confirm the Iapetus Ocean. compositional distinction between a silica-rich and The probability that Iapetus oceanic crust had silica-poor group of lower Arenig sandstones. The started to subduct under the Avalonian margin as silica-poor sandstones resemble the Loweswater early as the Tremadoc (Fig. 2) implies that the Formation of the Skiddaw Group. The silica-rich Manx Group lay on an active margin during its sandstones match only the enigmatic Redmain deposition. Deformation of the Manx Group soon Sandstone of the Lake District (Cooper et al. 1995) after its deposition is a possibility in such a setting rather than any unit in the main Skiddaw Group. and Max et al. (1990) have suggested that the The ordering of the lower Arenig Manx Group analogous Ribband Group formed a thrust- sandstone units is only partially constrained by the dominated accretionary complex. The evidence for available field data. Several possibilities are deformation timing in the Manx Group (Fitches et outlined by Barnes et al. al.) suggests that the penetrative D1 event is post- Wenlock (mid-Silurian) in age. If Arenig folds and fabrics are preserved, they have yet to be Later Arenig mud-prone slope deposition discriminated from the overprinting D1 and D2 The relatively good biostratigraphic control in the events. The evidence therefore matches that from Skiddaw Group, summarized by Cooper et al. the Skiddaw Group, where earlier proposals of a (1995) and by Stone et al., reveals a prominent major mid-Ordovician cleavage-forming defor- mid-Arenig transition from the sandstones of the mation are now discounted (Cooper et al. 1993). Loweswater Formation to the mudstones of the Kirkstile Formation. A similar transition occurs Mid-Ordovician to early Silurian history within both the Manx Group (Woodcock et al.) and the Ribband Group (McConnell et al.), a transition In both the Lake District and Leinster, the that the available biostratigraphy also allows to be predominantly sedimentary early Ordovician of mid-Arenig age. The rapid decrease in the rate of successions are overlain by later Ordovician sand supply to the Avalonian segment of the volcanic arc successions of the Borrowdale and Gondwana margin suggests a relative sea-level rise. Duncannon Groups, respectively (Max et al. 1990; However, the continued availability of limited Cooper et al. 1993). Any such volcanic super- volumes of clean sand implies that the Avalonian structure is missing in the Isle of Man. However, a connection with Gondwana was not yet completely suggestion that it may have accumulated comes severed (Fig. 2; Woodcock et al.). fi'om the Poortown intrusive complex in the north- Quirk & Burnett and Woodcock et aL describe western Manx Group. The magnetic survey of the typical mix of facies within the probable middle Piper et al. shows that this complex comprises Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

6 N.H. WOODCOCK ETAL. basaltic sheets, probably sills. Power & Crowley small if the Wenlock and Ludlow rocks of the describe a compositional range from tholeiitic Windermere Supergroup are themselves regarded basalt to basaltic andesite, and a chemistry as the diachronous southward advance of Southern consistent with a volcanic-arc setting. The sills Uplands deposits in a successor basin across the, were intruded at a high level into probable upper now essentially closed, Iapetus Ocean (Barnes et al. Arenig rocks. Although the intrusive complex 1989; Kneller 1991; Fig. 2). could therefore be of any subsequent age, the Faulted against both the Manx and Dalby Groups possibility that it represents the substructure to a are the red alluvial Peel Sandstones (Crowley late Ordovician volcanic succession is supported by 1985). These rocks contain derived Ashgill and estimates of its palaeolatitude, derived from the Wenlock shelly fossils, but otherwise their age has palaeomagnetic work of Piper et ai. Common been unconstrained. Now Piper & Crowley have minor intrusions elsewhere in the Manx Group measured a remanent magnetism in these beds that (Lamplugh 1903) may belong to the same suite as includes a pre-folding component interpreted as the Poortown Complex. late Silurian in age. This evidence favours If an Ordovician arc was indeed constructed deposition of the Peel Sandstones in one of the above the Manx Group, comparison with the 'Lower Old Red Sandstone' basins that accom- Borrowdale Group suggests that it would have been panied the Late Caledonian deformation event mostly of Caradoc age and substantially subaerial. (Allen & Crowley 1983) in the culminating The volcanics, and any post-Caradoc sedimentary collision zone between Avalonia and Laurentia, cover analogous to the Windermere Supergroup, However, a late Silurian age begs several important must have been subsequently eroded. This erosion questions, yet to be satisfactorily answered. Were must have occurred at latest before the the continental Peel Sandstones deposited in overthrusting of the Dalby Group on to the Manx conformity with underlying marine Silurian rocks, Group, probably in early Devonian time (Fig. 2). including the Dalby Group, or in a separate unconformable basin fill? Why are the Peel Sandstones only locally folded and cleaved? Did Silurian deposition of the Dalby Group their deposition post-date at least the D1 event, or was the Peel Sandstones basin simply at too high a and Peel Sandstone crustal level to be affected? A new and intriguing factor in Manx geological In any case, uplift of the Iapetus collision zone history arises from the demonstration by Howe and had already begun by the time the Peel Sandstones Morris et al. that the Niarbyl Formation is of mid- were deposited, prior to presumed exhumation and Silurian rather than early Ordovician age. These erosion during an early Devonian peak of orogenic arc-derived turbidites, now assigned to the Dalby activity (Fig. 2). Group, were supplied southeastward into an anoxic marine basin during Wenlock, probably late Late Caledonian orogenic events Wenlock, time. They were subsequently faulted into their present position above the western Manx Simpson's (1963) three-stage deformation history Group. of the Manx Group is essentially verified by The tectonic significance ascribed to the Dalby Fitches et al., although the third phase (D3) is Group depends crucially on the postulated site of regarded as locally developed and of possible their original deposition. Barnes et al. favour Variscan or later age. A steeply dipping penetrative correlation with the mid-Silurian turbidites of the first cleavage and a more gently dipping second Windermere Supergroup (Lake District). This cleavage, both with geometrically hypothesis implies that the Dalby Group was congruent folds, appear in both the Manx and originally deposited on the Manx Group, probably Dalby Groups. This relationship suggests that D1 above an intervening succession of upper and D2 are essentially post-Wenlock in age, Ordovician to lower Silurian rocks, which are now although Morris et al. urge caution in this cut out by a normal-faulted contact. In contrast, geometric correlation, particularly of D 1 structures. Morris et al. favour correlation of the Dalby Group The localized cleavage in the Peel Sandstones with the Riccarton Group of the Scottish Southern cannot be matched with one in the Manx Group, Uplands, a match also allowed by the geochemical and is regarded as Variscan by Fitches et al. and by data of Barnes et al. The second hypothesis implies Quirk & Kimbell. that the Niarbyl Formation represents the toe end of Power & Barnes show that the main the Southern Uplands turbidite prism, either metamorphic mineral growth in the Manx Group onlapping or overthrusted on to the outboard edge coincided with, or just post-dated, D1, whereas of the Avalonian continental margin. The contrast mineral growth during D2 was more limited. The between these two hypotheses is, in fact, rather intrusion of the Foxdale Granite, tentatively dated Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

THE EARLY PALAEOZOIC GEOLOGICAL HISTORY OF THE ISLE OF MAN 7 by Rb-Sr methods at c. 400 Ma (Crowley & Power, is now important to add to the kinematic knowledge pers. comm.), overlapped or postdated D2. In of these zones for comparison with analogous summary then, it is most likely that both the D 1 and settings bordering the suture zone in the Lake D2 phases on the Isle of Man occurred in latest District (Hughes et aL 1993; Stone et al.), eastern Silurian or early Devonian time, between c. 420 Ireland (Vaughan & Johnston 1992; McConnell et and 400 Ma. This is essentially the same age as that al.) and southern Scotland (Barnes & Stone). deduced for the main deformation in the Lake The metamorphic and igneous history associated District (Soper et al. 1987; Hughes et al. 1993) and with the Late Caledonian deformation is also slightly younger than the southernmost defor- proving a particular challenge to unravel. Roberts mation in the Southern Uplands (Barnes et al. et al. (1990) suggested that the high metamorphic 1989). The deformation in the Isle of Man is grade along the central northeast-southwest spine presumed to be driven by the culminating collision of the island records the enhanced thermal gradient of Avalonia with Laurentia (Fig. 2), although Soper due to granite emplacement. However, Power & et al. (1992) have suggested that the impingement Barnes show that the two main granitic bodies of Armorica on the southern edge of Avalonia may have different time relations to the main defor- have been an important factor. mation, with the Foxdale intrusion postdating D2, Whilst the geometry of the minor structures and the Dhoon intrusion pre-dating D2 and possibly produced by the D1 and D2 events is now well overlapping D1. These authors demonstrate a established, the nature and kinematic significance strong lithological control on metamorphic of the related major structures is more debatable. mineralogy and therefore apparent grade. For Fitehes et al. note that the largest D1 folds often instance, the high content of iron and aluminium in verge southeastward, consistent with a southeast the Barrule Formation favours the formation of directed overthrusting, but that some large D1 chloritoid. Nevertheless, the metamorphic con- structures in the southeast of the Manx Group verge ditions indicate that a substantial missing over- northwestward. Some major high-strain zones, such burden has been removed from above the Manx as the Niarbyl Zone, preserve evidence of D1 Group since early Devonian time. sinistral shear (Fitehes et al.) and possible southeast-directed overthrusting (Morris et al.). Post-Caledonian events The sinistral strike-slip component is also suggested by the patterns of shallow aeromagnetic Mid- and Late Devonian events are not represented lineaments identified by Quirk & Kimbell (1997) in the preserved geological record of the Isle of and Quirk & Kimbell. The kinematic pattern of Man. Uplift and erosion of the thickened collision the D2 deformation locally has a southeast directed zone presumably dominated this time interval (Fig. component (Morris et al.), but in most places 2). Hper& Crowley attempt to constrain the end of resolves into a vertical flattening (Fitches et al.). this interval by palaeomagnetic dating of the The southeast directed theme during the Late Langness Conglomerate Formation, which lies Caledonian deformation of the Isle of Man is inconformably on the Manx Group in the southeast consistent with a regional control in which of the island, beneath the sequence of Lower Laurentian outboard attempted to limestones. However, they have overthrust the Avalonian margin during the final recovered only an Early magnetization, closure of the Iapetus Ocean (Fig. 2). The similar to that which pervades many red bed preservation, as the Dalby Group, of a possible successions in Britain and Ireland. The Peel slice of the Laurentian terranes raises the possibility Sandstones also record this component. Late that a strand of the Iapetus Suture itself passes Carboniferous-Permian hydrothermal activity was through the island. The Niarbyl Thrust and the also responsible for the mineralization that is subjacent , below the Dalby Group, concentrated in fractures cutting the Manx Group. comprise a candidate for such a strand (Morris et The Carboniferous and Permo- strata al.). However, a search for the Iapetus Suture is preserved offshore are the remnants of two, once likely to prove inconclusive amongst the plexus of much more extensive, rift-thermal sag basins, each related shear zones at this high level in the crust. partially removed by 1-3 km of uplift in the early For instance, Kimbell & Quirk show that a major Permian and early Tertiary (e.g. Green et al. 1997). contrast in basement character now occurs on a The Peel and Solway Basins, lying west and north northwest dipping zone that comes to surface of the Isle of Man, respectively, have been a recent further southeast, entirely within the Manx Group exploration target, after the successful hydrocarbon at the present erosion level. finds in the southeast Irish Sea Basin. The same Deformation zones within the Isle of Man may Sherwood Sandstone reservoir is capped by indeed have localized displacement in the conti- saliferous mudstones of the Mercia Mudstone nental convergence zone for some of its history. It Group. but the lower Namurian Holywell Shale Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

8 N. H. WOODCOCK ET AL. source rock is apparently absent in the Peel and deformation in parts of the Manx Group has yet to Solway Basins (e.g. Armstrong et al. 1997). Wells be conclusively disproved. have penetrated Dinantian limestone directly below Sedimentological studies of the turbidites and the base Permian unconformity (Newman 1999). debrites in the Manx and Dalby Groups have Recent seismic analysis has indicated that the revealed a number of generally important features. Holywell Shale may be preserved on the undrilled The apparent dual supply of clean and dirty sand to downthrown side of major normal faults (Quirk the Gondwana margin will repay further sedi- 1999) and geochemical work also suggests that mentological and geochemical investigation, Dinantian source rocks may remain (Clayton et al. particularly to test the link with remote sources in 1999; Racey et al. 1999), so prospects may still the Gondwana interior. Trace fossil assemblages exist. clearly hold further information on the deep-water Many of the faults bounding the offshore basins, environmental conditions on the margin. including those defining the present shape of the Refinement of the chronology of the Caledonian Isle of Man, display kilometres of normal offset due and magmatism depends partly on to early Permian, ?late and early Tertiary the acquisition of radiomettic dates, but partly on extensional episodes (Quirk & Kimbell 1997). further studies of textural relationships with respect Some of these, such as the southeast dipping to the main deformation phases. Whether the high- Lagman Fault to the northeast of the Isle of Man, grade 'spine' to the Manx Group is entirely probably represent reactivated Caledonian explained by compositional control will repay structures. Others, particularly north-south faults further work. defining, for example, the western side of the Peel The need for a comprehensive remapping of the Basin, were formed in the Mesozoic. Whether Isle of Man has frequently been voiced (e.g. northeast-southwest trending faults, such as the Roberts et al. 1990; Rushton 1993; Ford 1993). Central Valley Lineament crossing the Isle of Man Invaluable though this would be, the difficulty and and the Keys Fault east of the Isle of Man, are cost of such an exercise should not be Caledonian or younger is as yet uncertain. underestimated. The inaccessibility of many of the coastal exposures and the thick drift cover inland would demand a considerable effort to improve on Scope for further work the century-old results of Lamplugh. The new wave of work on Manx geology reported in this volume has greatly enhanced the knowledge of local relationships and provides a firmer basis Regional conclusions for regional comparisons. However, a number of The results reported in this volume fill a major gap old problems persist and new ones have been in understanding the northern margin of Avalonia, identified for future attention. and its subsequent deformation, and meta- The most pressing need is for better biostrati- morphism. The main conclusions of regional graphic control of the sedimentary succession. The importance are: identification of Silurian rocks on the island has been an important breakthrough, but the calibration * the Manx Group mainly comprises Arenig and intertract correlation of the Manx Group (Lower Ordovician) deep-marine turbidites and remains poorly constrained. Equally crucial are debrites, supplied from the southeast into better radiometric ages of igneous and meta- oxygenated basins on the northwest facing morphic events. Palaeomagnetic dating is proving margin of Avalonia; helpful in the absence of other methods. • the gross organization of the Manx Group into a The stratigraphic organization of the Manx sand-rich lower part and mud-rich upper part Group needs further refinement. The tectono- echoes that in the Skiddaw Group (Lake District) stratigraphic tract scheme provides a testable model and Ribband Group (Leinster); this organization for future studies, but removes the predictability of points to control by margin-wide events, in part previous synclinal models. Detailed interpretation eustatic sea-level changes; of each individual tract is now required. Further • episodes of down-margin mass wasting and structural studies are needed, particularly to Fe-Mn fluid exhalation also correlate along the establish the magnitude and kinematic pattern of margin, and may relate to rifting of Avalonia displacement on the tract-bounding structures. The from Gondwana; correlation of deformation phases between Manx • a mid-late Ordovician volcanic arc is missing Group tracts, and between them and the Dalby and above the Manx Group, although remnants of its Peel tracts, becomes a debatable issue, and the intrusive substructure may be preserved; possibility of a pre-Silurian cleavage-forming • a succession of northwest derived turbidites, Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021


sedimented into an anoxic basin during Wenlock not host a definite Caledonian cleavage, and (mid-Silurian) time, sits with a tectonic contact must have been deposited late in the deformation on the Manx Group; these Silurian sediments history; were deposited in a successor basin above the • the granitic intrusions into the Manx Group range Iapetus Suture Zone; from early in DI to late in D2; generating only • no evidence of Ordovician cleavage-forming local aureoles, and the high metamorphic grade deformation has been found; the main D1 and D2 in parts of the Manx Group may be due rather to shortening phases are post-Wenlock, comparable lithological control. in age with those further along the margin in the The authors particularly thank Padhraig Kennan, John Lake District and Leinster; Morris, Mike Howe, Greg Power and Dave Burnett for • the Peel Sandstones preserve a Lower 'Old Red numerous discussions on Manx geology. RPB publishes Sandstone' sequence, mostly removed by post- with permission of the Director, British Geological Caledonian erosion elsewhere along this Survey. This work was funded by NERC research grant outboard part of the Avalonian margin; they do GR9/01834.

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