Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 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 Geology, 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 Ordovician Manx Group, the mid-Silurian Dalby Group and the ?late Silurian-early Devonian Peel Sandstones. The Manx Group is dominated by Arenig deep-marine turbidites and debrites deposited in oxygenated basins on the northwest-facing margin of Avalonia. Its organization into a sand-rich lower part and a mud-rich upper part invites comparison with the Skiddaw Group (Lake District) 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 cleavage. 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 'Old Red Sandstone' 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 horst 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 continent 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' Iapetus Ocean 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 2 N.H. WOODCOCK ETAL. 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 Gondwana 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 THE EARLY PALAEOZOIC GEOLOGICAL HISTORY OF THE ISLE OF MAN 3 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 Laurentia over- :i~iiiiiiiiiiiiiiiiiiiiiiiiiiii~iiiiiiiiiiiiiiiiiiiiiiiiiiiiii Ater-j-'Dai~i-i~i~iP'-i- 1 HoweM°rriset al. thrusts Avalonia i~,: ~<iiiiiiiiiiiiiiiiiiiiii;i ~!iiiiiiiiiiiiiiii~@~iiiiii~iiiiiiiii <--z 430 d .. Avalonia moves Lly north towards Laurentia Rheic 440 sedimentary units? lapetus Ocean now eroded Ash 450 arc shuts down O© Rheic Crd volcanic units? arc volcanism now eroded (Lake District and Leinster) 460 I~n~h i~ 470 Piper et aL Power & Crowley Avalonia rifts 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 4 N. H. WOODCOCK ET AL. 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 lithologies, they have revealed the internal syncline, 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.