The locals call them ‘Kerry Diamonds’; beautifully clear, euhedral quartz crystals found in fault zones at the western end of the Dingle Peninsula in south-west Ireland – but the real gem is the Peninsula itself!
Figure 1: View north from Clogher Head of the low cliffs of the Silurian Dunquin Group and high cliffs of the Devonian Smerwick Group. © Brian Williams.
The Dingle Peninsula is the most northerly of the major peninsulas of south-west Ireland and comprises an area of outstanding natural beauty. It is 50 km long and nowhere more than 24 km in width, and is dominated by a mountainous spine stretching from the Slieve Mish Mountains near Tralee in the east, to the dramatic Brandon Mountain Range in central Dingle and onwards to Mt. Eagle in the south-west and the Blasket Islands (the most westerly islands of Europe).
Figure 2: A ‘Kerry Diamond’. Courtesy Monika Razkova, Rocky Road Minerals.
The stunning mountain landscapes (beloved by various film crews, from Ryan’s Daughter to Star Wars!) reach to 950m and commonly terminate in spectacular sea cliffs interspersed with sandy bays. The unique landscapes of the Peninsula are generated by the very wide diversity of preserved rock sequences (mainly sedimentary and volcanic), together with their large-scale tectonic features that have evolved through 485 million years (Ma) of Earth history. Final landscape modification is attributed to both the erosive and depositional glacial processes of the recent Quaternary Period that affected this part of Ireland over the last 3 Ma.
Three Major Deformation Periods
The Peninsula preserves a unique succession of Ordovician to Carboniferous rocks (485 to 330 Ma in age) which is totally dominated by a thick, continental red-bed Devonian sequence which comprises the most complete Old Red Sandstone magnafacies in Ireland. Together with the underlying shallow marine and volcanic Silurian sequence, this Mid-Paleozoic suite accounts for over 5.8 km of the Peninsula’s rock assemblage.
The Dingle Peninsula is situated to the south of the Iapetus Suture zone (the Early Caledonian plate boundary) and is itself bounded by two long-lived, fundamental east-north-east-trending lineaments (major faults) that provided the foundation for both its tectonic and sedimentary evolution. These two deep fractures are called the North Kerry Lineament (NKL) and the Dingle Bay Lineament (DBL).
Figure 3: Simplified geology map of the Dingle Peninsula. Modified after Richmond and Williams, 2000.
The Paleozoic rocks of the Peninsula have been profoundly affected by three major periods of deformation related to plate tectonics over a 185 Ma period: the Early Caledonian (~470 Ma); the Late Caledonian (Acadian) (around 400 Ma); and the Variscan Orogeny (around 318–297 Ma). These deformational episodes generated folding on various scales and thus repetition of rock sequences are present throughout the Peninsula. Five major fold structures dominate the area (see Figures 3 and 4) and five major faults have played an important role in controlling basin initiation, subsidence and sediment infill. One major fault line, the Fohernamanagh Fault (FF), in particular, is possibly of great significance in that it has been interpreted as a ‘terrane boundary’ bringing together two quite different Lower Devonian basin fills of contrasting sediment provenance and climatic overprint (Figure 5).
Figure 4: Geological cross-section of the Dingle Peninsula. Modified from Richmond and Williams, 2000.
Into the West: The Dunquin Group
Figure 5: Simplified geological map of the south-west end of the Peninsula. Modified after Boyd and Sloan, 2000.
The best way to achieve an overview of the Mid-Paleozoic geology of the Dingle Peninsula is to head to the village of Dunquin at the western extremity of the mainland. Two excellent roadside viewpoints en-route are on the flank of Mt. Eagle, where one can take in the breathtaking views of the Blasket Islands (Figure 6). Then to Clogher Head where the view to the north embraces the reference section of the marine Silurian (Dunquin Group), and the high cliffs of the Sybil Head area where red-bed outcrops of the proposed accreted terrane, comprising the Smerwick Group, can be seen on the skyline (Figures 1 and 5).
Figure 6: View of the Blasket Islands from Mt. Eagle. Comprising Dingle Group rocks, these abandoned islands are known for their Irish literary significance as well as their geology! © Ken Higgs.
The Silurian succession in the Dunquin Inlier is 1,500m thick, ranges in age from Late Llandovery to Ludlow, and comprises fossiliferous shallow marine sediments interspersed with volcanic and volcaniclastic horizons. Low in the succession, upward-coarsening parasequences with shelfal siltstones, storm-induced sandy bedforms and interbedded volcanic ash horizons are indicative of rapid sea level fluctuations controlled by volcano-tectonic events. These give way to major volcanic intervals of andesitic/dacitic lavas and pyroclastic fall and flow deposits, magnificently seen in the immediate vicinity of Clogher Head. The geochemical signature of these volcanics indicate occurrence at a destructive plate margin; thus providing one of the rare examples of Wenlock volcanism south of the Iapetus Suture, probably due to the subduction of a localized remnant slice of Iapetan oceanic crust. The post-subduction thermal subsidence, following the acme of volcanic activity in the Late Silurian, saw a return to shallow marine storm-dominated sedimentation as superbly exposed on Drom Point (Figure 7).
Figure 7: Drom Point Formation, Dunquin Group: (A) Hummocky and swaley-bedded sandstones; (B) Wave-rippled surfaces. © Ken Higgs.
The conformable transition from the shallow marine Silurian to the continental, red-bed facies of the Devonian is only seen on the Blasket island of Inishnabro; everywhere else in the Peninsula this contact is unconformable or faulted, with the youngest Silurian stages (Ludlow and Pridoli) of the Dunquin Group missing (Figure 8).
Figure 8: Unconformable contact on Clogher Head. © Ken Higgs.
Backbone of the Peninsula: The Dingle Group
The Devonian Dingle Group dominates the geology and landscapes of the Peninsula. The Dingle Basin developed as a sinistral pull-apart continental basin. The basin-bounding faults were the NKL and DBL, and accommodation space for the accumulation of 4.3 km of sediment was provided by subsidence centered along the line of the Dunquin Fault (Figures 3 and 4). The Dingle Group sediments are exposed extensively around the south and south-west coasts and form the backbone of the mountains in central and western Dingle.
The Dingle Group rocks are red/purple in color due to the oxidation of iron-rich sediments which accumulated under semi-arid climatic conditions. The rocks are almost exclusively fluvial or lacustrine in origin, and as such are largely devoid of body fossils but locally preserve trace fossils. Microflora (plant spores) help to constrain the age of the Group as Lower Devonian (Lochkovian to Pragian/Emsian – around 415 to 407 Ma).
The Siluro-Devonian boundary probably occurs within the initial red-bed deposit of the Dingle Group, the lacustrine Bulls Head Formation. This unit is beautifully seen on Great Blasket Island and at Dunquin harbor. It is termed ‘Lake Blasket’ and has been mapped to cover over 500 km2. Although essentially a shallow ephemeral lake, contemporaneous subsidence facilitated a preserved 220m thickness. Wind-wave ripples and mud cracks abound in this heterolithic fine-grained facies (Figure 9). The lake was rapidly infilled from the south by ephemeral sheet flood sandstones – the Eask Formation – sourced from a hinterland bounded by the DBL. This 800m-thick, low slope fan apron is the result of a series of superimposed sheet flood events with a lateral input into the Dingle Basin. Between the sheet floods the mudstone interbeds and drapes often host immature calcrete vertisols.
Figure 9: Desiccation cracks in the lacustrine Bulls Head Formation at Dunquin Harbor. © Ken Higgs.
In middle Lower Devonian times there was a dramatic change in the fluvial input into the Dingle Basin. Ephemeral conditions gave way to perennial river deposits, this time along a major axial pathway. This change was probably due to a combination of climatic and tectonic events. The result was the fluvial input of major sand and gravel bedload from a catchment area to the west-south-west with sediment dispersal toward the east-north-east. This part of the Dingle Group is known as the Coumeenoule – Slea Head River facies (Figure 10) and appears to represent a low sinuosity fluvial system comparable in size to the Brahmaputra River of today.
Figure 10: Coumeenoule fluvial lithofacies and log, Coumeenoule Strand. © Ken Higgs; Log after Boyd, 1983.
Extrapolating from the outcrop, a minimum 200 km of drainage length and a drainage area in excess of 10,000 km2 has been proposed. Rapid subsidence along the line of the Dunquin Fault was also necessary to accommodate the 1.25 km of preserved sediments in these formations. Tectonic activity in the hinterland intensified with time, as is evidenced by the upward-coarsening nature of the system. At the same time the NKL, FF and DBL were all active, undergoing sinistral, strike-slip movement and generating gravel-rich alluvial fans with lateral input into the Slea Head River from the north (Glashabeg Conglomerate Formation) and south (Trabeg Conglomerate Formation) (Figure 11).
Figure 11: Depositional model, Upper Dingle Group – the axial Slea Head River and the lateral alluvial fan systems. Modified after Todd et al., 1988.
The Trabeg Conglomerate Formation clearly shows the strike-slip control on the fan by the ‘strewing’ of like clasts upward through the conglomerate as the source area and the basin pass each other. The conglomerates also provide evidence of the unroofing and erosion of different terranes within the Iapetus Suture zone.
In early Emsian times the fan systems shut down and the axial river system became sand dominated. All this occurred prior to the onset of the Acadian Orogenic event in the Late Emsian.
The Acadian Unconformity
The Smerwick Group rocks – up to 1 km in thickness – in the north-west Dingle domain (Figure 3) were deposited in an isolated, hydrologically-closed basin, under an arid climatic regime; the basin sequence contains arguably the oldest aeolian deposits in the British and Irish Isles. The Group’s outcrop is constrained between the NKL and the FF. They are comparable in age with the Lower Devonian Dingle Group to the south of the FF. Some authors interpret the Smerwick Group as an allochthonous terrane tectonically emplaced from the north-east by sinistral strike-slip movement along the FF. What is very evident is that the Smerwick and Dingle Groups were both uplifted and deformed by the Acadian Orogeny in Late Emsian times. Thus the Middle Devonian sequence in the Dingle Peninsula rests with marked unconformity, of varying relief, on the Lower Devonian (and older rocks) of the Peninsula (Figure 12).
Figure 12: The Acadian Unconformity on Ballydavid Head and at Sauce Creek between the Lower Devonian Smerwick Group and the Middle Devonian. © Ken Higgs and Lorna Richmond.
Into the Desert: The Caherbla Group
The Acadian orogenic event was followed by a period of crustal extension; thus the Middle Devonian sediments of the Dingle Peninsula were deposited under a very different tectonic regime compared with the Lower Devonian. The Middle Devonian comprises the coeval Caherbla/Pointagare Groups which crop out extensively in the south-east and north-east of the Peninsula, and the extensional Caherbla Basin, which developed across the entire Peninsula as a hot, arid intra-continental rift bounded to the south by the DBL now reactivated as a normal fault down-throwing to the north. South of the DBL was a Precambrian hinterland of high-grade metamorphic rocks, forming an east-north-east – west-south-west oriented, north-facing fault scarp (DBL) that was at least 500m above the Basin floor (Figure 13).
Figure 13: Depositional model for the Middle Devonian Caherbla Group. Modified after Todd, 2000.
The 1,000m-thick Caherbla Group comprises the Kilmurry Sandstone and Inch Conglomerate Formations. The former is a massive aeolian complex deposited in an erg where the wind-generated phenomena include both dune- and draa-scale bedforms (Figure 14), plus interdune and interdraa corridors which were subjected to flash flood modifications. Into this spectacular erg, the very coarse-grained breccio-conglomerate alluvial fans, with their varied metamorphic clast, were shaped by massive stream/sheet flood events and debris flows from the DBL fault scarp; a scenario not unlike the Oman Mountains and Wahiba Sands today. The hot, arid Caherbla landscape was not devoid of life but instead supported a thriving arthropod community, evidenced by the abundant trackways and burrows in this mixed aeolian/fluvial depositional system.
Figure 14: Aeolian dune bedform, Middle Devonian near Sauce Creek. © Brian Williams.
The Variscan Orogeny: Impact on Dingle’s Late Paleozoic Rocks
The Late Carboniferous Variscan Orogeny (~300 Ma) was a result of the collision of the southern continent of Gondwana and the northern continent of Laurussia. In Ireland this collision resulted in the uplift and deformation of Devonian and Early Carboniferous rocks, the products of which are particularly well seen in the Munster Basin of south-west Ireland. In the Dingle Peninsula, Variscan compression resulted in very large (km-scale) open upright folds which trend north-east to south-west in the west, and east to west in the Peninsula’s eastern sector as evidenced by the Slieve Mish Anticline (Figure 3). The Orogeny also tightened some of the earlier Caledonian and Acadian folds and reactivated many of the older major faults.
The Late Devonian, mainly the Slieve Mish Group (largely fluvial sandstones and conglomerates), and the overlying shallow marine shelfal limestones of the Lower Carboniferous Tralee Group dominate the Upper Paleozoic sequences in the Dingle Peninsula (Figure 3). These are best seen at outcrop toward the eastern end of the Peninsula near Tralee, flanking the Sieve Mish Anticline, which plunges eastward and closes near the town of Tralee, with the declining topography of the Slieve Mish Mountains beautifully reflecting this closure.
The Quaternary: Final Glacial Modification Dingle’s Landscapes
The Quaternary ice age in Ireland (2.85 Ma to 11.7 Ka) had a profound effect on the Irish landscape which was molded into its present shape during this period. There were several cold glacial events and warm interglacial episodes, and it is the products of the younger Munsterian and Midlandian glaciations that are best preserved in the Peninsula.
The glaciated valleys of Derrymore Glen and Annascaul illustrate well the glacial modification of the landscapes in the eastern area. More dramatically, the higher mountain ranges of central and western Dingle display magnificent erosive landforms, particularly in the Mt. Eagle area, Brandon Ranges (Figure 15) and the Owenmore Valley. U-shaped valleys; misfit streams; hanging valleys – often with striated margins; glacial lakes in corries such as at Lough Doon, or drowned corrie amphitheaters like Sauce Creek all attest to the power of glacigenic processes in shaping landscapes.
Thus, the final touches were produced to the topography of the beautiful Dingle Peninsula for us to enjoy today – a ‘Kerry Diamond’ indeed!
Figure 15: Glaciated landforms viewed from the vicinity of Mt. Brandon. © Ken Higgs.
Acknowledgments
Much of the illustrative material used in this article is derived from Higgs, K. and Williams, B., 2018, The Geology of the Dingle Peninsula: A Field Guide, Geological Survey of Ireland, Guide Series. I thank Professor Ken Higgs (University College Cork) for his excellent field photography, Geological Survey of Ireland (Dublin) for permission to reproduce the figures. Many of the maps and models are from the Ph.D. Geology research in Dingle over a 20-year period by my splendid students – Drs. Doug Boyd; Simon Todd; Rod Sloan; Lorna Richmond and Lance Morrissey, from both Bristol and Aberdeen Universities.
About the author…
Brian Williams is Emeritus Professor of Geology, University of Aberdeen and Adjunct Professor in Geosciences, University College Dublin.
References:
Boyd, J. D. 1983. Sedimentology of the lower Dingle Group, southern Dingle Peninsula, southwest Ireland. PhD thesis, University of Bristol.
Boyd, J. D. & Sloan, R. J. 2000. Initiation and early development of the Dingle Basin, SW Ireland, in the context of the closure of the Iapetus Ocean. In: Friend, P. F. & Williams, B. P. J. (eds) New Perspectives on the Old Red Sandstone. Geological Society, London, Special Publications, 180, 123-145.
Richmond, L. K. & Williams, B. P. J. 2000. A new terrane in the Old Red Sandstone of the Dingle Peninsula, SW Ireland. In: Friend, P. F. & Williams, B. P. J. (eds) New Perspectives on the Old Red Sandstone. Geological Society, London, Special Publication, 180, 147-183.
Todd, S. P. 2000. Taking the roof off a suture zone: basin setting and provenance of conglomerates in the ORS Dingle Basin of SW Ireland. In: Friend, P. F. & Williams, B. P. J. (eds) New Perspectives on the Old Red Sandstone. Geological Society, London, Special Publication, 180, 185-222.
Todd, S. P., Boyd, J. D. & Dodd, C. D. 1988. Old Red Sandstone sedimentation and basin development in the Dingle Peninsula, southwest Ireland. In: McMillan, N. J., Embry, A. F. & Glass, D. J. (eds) The Devonian of the World. Canadian Society of Petroleum Geologists Memoirs, 14, 251-268.
