Geological Survey of Ireland

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Index tol:100.000 scale

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Front Cover: Sandstones of the Toe Head Formation viewed from the suspension bridge at Mizen Head. (Photo by A.G. Sleeman).

Back Cover: Photomicrograph of the Black Ball Head Diatreme Pyroxenite (crossed nicols); (Photo by M. Pracht).

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GEOLOGY OF WEST CORK A GEOLOGICAL DESCRiPTION OF WEST CORK AND ADJACENT PARTS OF KEERY TO ACCOMPANY THE BEDROCK GEOLOGY 1:100,000 SCALE MAP

SERIES, SHEET 24, WEST CORK

M. Pracht and A.G. Slèeman

With contributions by G. Wright (Groundwater) and W. Cox (Minemis).

Published under the authonty ofthe Director of the Geological Survey of Ireland.

Geology of West Cork

ACKNOWLEDGEMENTS We would like to thank Ken Higgs, Dave Naylor, Ivor MacCarthy, Tom Reilly, George Sevastopulo and Ed Williams for their substantial assistance over many years in helping us to understand and compile the geology of West Cork. We thank Ken Higgs and George Sevastopulo

particularly for providing numerous palynological and conodont determinations respectively, without which we could not have sorted out the stratigraphy with any confidence. We also thank Ivor MacCarthy for generously making available the results of his mapping in West Cork and Dave Naylor for commenting on an early version of the manuscript. AGS would especially like to thank loin and Derry Reilly for their generous hospitality and friendship over many years in West Cork and Tom for his expert guidance, encouragement and assistance in the field. MP would like to extend his thanks to the Sullivan household of Bere Island. We also thank Aileen O'Brien for assisting in digitising the map and Shane Tyrrell who assisted editing the map, creating the cross-sections and figures.

Bibliographic Reference:Booklet:Pracht, M. and Sleeman, A.G. 2002. A geological Description of West Cork and adjacent parts of Kerry to accompany the Bedrock Geology 1:100,000 Scale Map Series, Sheet 24. West Cork, With contributions by G. Wright (Groundwater) and W. Cox (Minerals). Geological Survey of Ire land, 79pp.

Map:MacCarthy, I.A.J., Pracht, M. and Sleeman, A.G. (compilers) 2001. Sheet 24, Geology ofWest Cork. 1:100,000 Scale Bedrock Map Series. © Geological Survey of Ireland 2002.

All rights reserved. No part of this book may be reproduced by any means without prior permission of the Director, Geological Survey of Ireland, Beggars Bush, Haddington Road, Dublin 4.

ISBN 1-899702-42-3

Diagrams, design and layout by Cartography Unit.

Printed in Ireland by Westprint Ltd., Enniscrone, Co. Sligo Graphic Reproduction by Printcraft Imaging

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Geology of West Cork

CONTENTS ACKNOWLEDGEMENTS..................................................................................................

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INTRODUCTION.................................................................................................................. READING THE GEOLOGICAL MAP ................................................................................. 3 TheRock Units ....................................................................................................................... 3 How rock units come to lie alongside one another ................................................................. 3 Constructing a Three Dimensional Picture from the Map ...................................................... 3 What the map does and does not show ................................................................................... 4 1

Viewingthe Rocks .................................................................................................................. 4 AN INTRODUCTION TO THE GEOLOGY AND ROCK TYPES OF WEST CORK ....... 5 Devonian "Old Red Sandstone "Terrestial Rocks ................................................................. 5 Uppermost Devonian and Carboniferous Marine Rocks ........................................................ 7 Igneousrocks .......................................................................................................................... 8 Geological Events in West Cork after the Carboniferous Period ........................................... 8 TheQuaternary Period ........................................................................................................... 8 THE BEDROCK GEOLOGY OF WEST CORK ................................................................ 10 Old Red Sandstone Magnafacies .......................................................................................... 10 WESTCARBERY..................................................................................................... 10 BEARA PENINSULA-NORTHERN SHEHY MOUNTAINS ................................ 14 IVERAGH PENINSULA .......................................................................................... 17 Uppermost Devonian and Early Carbonìferous Clastic Rocks ............................................ 20 WEST CARBERY .................................................................................................... 20 BANTRY AREA AND THE BEARA-IVERAGH PENINSULAS .......................... 23 Dinantian limestones [DIN] ................................................................................................. 28 Namurian Sandstones and Shales ......................................................................................... 28 EAST POINT FORMATION [EP] ........................................................................... 29 MIDDLE BATTERY FORMATION [MB] .............................................................. 29 KILMORE FORMATION [KL] ............................................................................... 29 ENVIRONMENT OF DEPOSITION ....................................................................... 30 Upper Palaeozoic Igneous Rocks ......................................................................................... 30 INTRUSIVE OR SUB-VOLCANIC ROCKS .......................................................... 32 PYROCLASTIC ROCKS ......................................................................................... 32 TIMING OF THE INTRUSION OF IGNEOUS ROCKS IN SOUTHWEST IRELAND .................................................................... 34 BASIN AND SHELF DEVELOPMENT ............................................................................. 35 TheMunster Basin ............................................................................................................... 35 TheSouth Munster Basin ..................................................................................................... 38 THEBANTRY SUB-BASIN .................................................................................... 38 THE GLANDORE HIGH ......................................................................................... 39 THE SHEEP'S HEAD HIGH ................................................................................... 39 STRUCTURAL GEOLOGY ................................................................................................ 41 41

Thrustsand Faults ................................................................................................................. 43 44

Jointing, veining, boudinage and kink bands ....................................................................... 44

TheVariscan Orogeny .......................................................................................................... 45

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Geology of West Cork

TIMING OF THE VARISCAN DEFORMATION IN SOUTHWEST IRELAND ................................................................................... 48 POST VARISCAN HISTORY ............................................................................................. 49

HYDROGEOLOGY ............................................................................................................. 51 MINERAL RESOURCES .................................................................................................... 52

MetallicDeposits .................................................................................................................. 52 COPPER .................................................................................................................... 52 LEAD-SILVER ......................................................................................................... 54 MANGANESE .......................................................................................................... 54 GOLD........................................................................................................................ 54 IRON......................................................................................................................... 54 Industrial Mineral Deposits .................................................................................................. 54 BARYTES................................................................................................................. 54 DIMENSION STONE .............................................................................................. 58 AGGREGATE........................................................................................................... 58 OtherMinerals ...................................................................................................................... 59 QUARTZ................................................................................................................... 59 LITHOTHAMNIUM ................................................................................................ 59 REFERENCES AND ADDITIONAL READING LISTS ................................................... 60 APPENDIX 1-KEY LOCATIONS ....................................................................................... 74 APPENDIX 2- brief lithological descriptions of all units appearing on sheet 24 ................ 76 APPENDIX3-GLOSSARY .................................................................................................78

FIGURES I. 2.

Geological timescale ................................................................................................. vi Topography of West Cork ..........................................................................................

3.

Simplified Geology of West Cork .............................................................................. 6

4.

The pattern of Glaciation in Ireland during the last (Fenitian) glaciation .................. 8

5.

The Distribution of Igneous rocks on the Beara Peninsula ...................................... 31

6.

Geology of Black Ball Head .................................................................................... 33

7.

The basin depositional framework of Southwest Ireland ......................................... 35

8.

Sketch of Early Devonian palaeogeography relating to sheet 24 ............................. 36

9.

Sketch of the Munster Basin palaeogeography in the early Upper Devonian .......... 37

10.

Major Structures in West Cork ................................................................................. 42

11.

Sketch of the main structural units of the European Variscides ............................... 46

12.

The structural zonation of the Irish Variscides ......................................................... 47

13.

Dolerite samples from Firkeel Bay and Garinish strand in the Zr-Yx3 -Ti/I 00 diagram and Zr versus Zr/Y diagram .......................... 49

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Geology of West Cork

TABLES I.

Correlation chart of Devonian Stratigraphy ...............................................................

2.

Correlation chart of Carboniferous Stratigraphy ...................................................... 20

3.

Summary of dated tuff bands in the Munster Basin ................................................. 32

4.

Geological Information relating to the Major Mines of West Carbery ..................... 55

5.

Geological Information relating to the Major Mines in the Allihies District ........... 56

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PLATES 1.

View of Bantry Bay and Sheep's Head looking south, taken north of Glengarriff (photo by A.G.Sleeman) ...........................................................................

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2.

Vertical sandstone beds in the Sherkin Formation east of the Beacon at the entrance to Baltimore Harbour (Photo J.M.Pulsford) ...... 12

3.

Entrance to Pulleen Harbour-locally overturned Gun Point and Old Head Sandstone Formations looking east (Photo by M.Pracht) ................. 16

4.

Anticlinal fold in the Caha Mountain Formation at Coomacloghane Mountain (Photo by M.Pracht) ................................................................................................. 16

5.

A sill intruded into cleaved heterolithics of the Ardnamanagh Member on Black Ball Head (Photo by J.M. Puisford) ............................................................... 30

6.

Close up of the intrusion breccia at Black Ball Head (Photo by C. MacDermot).

7.

The man-engine house at Allihies. (Photo by J.H.Morris, Mining Heritage Trustof Ireland) ....................................................................................................... 53

8.

The production of aggregates at the former Leahill quarry, Adrigole (Photo by M.Pracht) ................................................................................................. 58

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Geology of West Cork

EON

ERA

HEyENTS RELATING TO SHEET 24*

PERIOD

Spread of vegetation, arrival of man. Series of ice ages.

Quaternary N

1.6

u 135

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Tertiary

Erosion. Sheet-like intrusions in Kerry and Cork. Tectonic activity.Continuation of opening ofNorth Atlantic Ocean. Volcanoes in NE Ireland.

Cretaceous

Limestone sedimentation with subsequent karstiJication and erosion.

Jurassic

North Atlantic Ocean starts to open. Upliji & erosion. Irish Sea & Celtic Sea basins develop east & south of Ireland.

C N

250

Triassic

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Active faulting. Erosion and deposition under desert conditions. Hypersaline conditions in NE Ireland.

Permian

290

The Black Ball Head diatreme was intruded during the deformation. Intrusion of the Southern Suite. Turbidites were later deposited in the basin before it shallowed with deltaic conditions and filling of the basin before the onset of the Variscan deformation in the SW resulting in mountain building in the late Carboniferous. Continued deposition of marine sand and mud gradually_deepening South Munster Basin.

N

C

OROGENY

U

Carboniferous

VA1IAN

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A 355

A

In the latest Devonian start of a major marine

across the margins of the ORS Intrusion of sills and dykes, Northern Suite. Deposition of volcanic ash. Rapid erosion of mountains to the north leads to the deposition of red sandstones, siltstones and mudstones from ephemeral rivers in arid desert conditions in the Munster Basin.Short marine incursion from the SE in mid Devonian times. transgression continent.

Z

Devonian

CALEDONIAN Onset of red bed sedimentation on Dingle. Deformation causes inversion and uplift of Ding/c Basin. Closure of lapetus Ocean with continental collision and initiation of mountain building. Marine clastics and widespread volcanism during Ding/c Basin develppf Island arc & marginal basin volcanism along SE margin of lapetus as ocean floor is subducted beneath it. Deep marine mudstone deposition. Sedimentation terminated kv uplift of Cambrian MONIAN basin margin tofòrmalocal landmass to SE. Greywacke and quartzite deposition in basin following continental rfiing. Generation of oceanic crust in newly fbrmed lapetus Ocean to NW

iurian .

S.1

N 438

Ordovician 510

Cambrian 544

LATE PROTEROZOIC

Z

Formation of Rosslare Complex gneisses to the SE, forming basement to all subsequent sedimentation in