Geology of Little Killary, Killary Harbour, Galway, Ireland

Regional Geology Overview

The purpose of this study was to thoroughly examine the regional geology of Little Killary, Killary Harbour, Galway, Ireland, located in northwest Galway just south of Killary Harbour and situated on the Atlantic coast. The geographic region of interest consists of the townlands of Lettergesh East and West as well as Glassilaun. The site also contains three primary peaks — Benchoona, Altnagaighera, and Garraun — together with the bases of Doughruagh and Currywongaun. The topography of the region was formed glacially (MacGabhann).

The coastline expanse comprising the northwest border of the mapped area stretches from Glassilaun beach (L7590 6470) to just west of Gowlaun (L7270 6270). Good exposure was encountered throughout the region for the most part, particularly at Benchoona (L7660 6164) and a few of the other higher points of elevation. Coastal areas were also especially well exposed; however, substantial boglands were encountered in the region between the area south of the mountains and the mountain peaks (i.e., Lettergesh East), which was characterized by very poor exposure. The total area mapped for this study was approximately 18 km², and all location numbers are provided in Irish grid references.

In sum, the Little Killary Bay area is comprised of various sedimentary, metasedimentary, and intrusive igneous rocks (Farrell, 2003). The southernmost area contains metasediments consisting mostly of mica schists that have been involved in numerous deformation events. Situated above these metasediments is a series of sediments that are not overturned and which dip to the north. The Silurian Lough Mask formation (L7370 6279) is situated at the base on the shore, comprising breccias and coarse red sand amalgamated into conglomerate.

The Lettergesh formation is situated above that level as well as above the metasediments in the majority of the area; it is primarily comprised of a basal cobble conglomerate containing turbiditic sandstones. The Glencraff formation is located above these levels and consists of a thin sequence of thinly bedded mudstones and siltstones with some sand. Above the Glencraff formation is the Lough Muck formation, which consists of siltstones, sandstones, and mudstones. Above the Lough Muck formation is the Salrock formation — which entirely disappears slightly west of Lough Mask (Kinahan, 1878) — consisting of a thick sequence of sandstones, siltstones, and mudstones characterized by red beds; however, this formation is situated outside the area of interest so its exact thickness remains unknown.

A few intrusive igneous rocks cross-cut the area, and there is an intrusion of gabbro in the metasediments. It was determined that the gabbro has also experienced intrusion by quartz-diorite-gneiss, which was subsequently metamorphosed before the intrusion of later mafic and felsic dykes (Farrell, 2003). One large and one smaller body of microgranodiorite were intruded into the Lettergesh formation, with the intrusions being parallel to strike in the majority of cases but not in every case. Several sills also intruded the sediments in the Lettergesh formation, with a dolerite dyke cross-cutting the microgranodiorite and the sandstones being the most recent intrusive event (Farrell, 2003).

A large metamorphic belt containing the metasediments, called the "Connemara metamorphic complex," is located to the south (Farrell, 2003). According to Friedrich, Hodges, Bowring, and Martin (1999), "The Connemara region of the western Irish Caledonides records the evolution of a short-lived mid-Ordovician (c. 475–463 Ma) continental magmatic arc that intruded into Dalradian metasedimentary rocks during regional ductile deformation" (p. 1217).

Northward, there is a fault with Ordovician sediments (the Sheeffrey Group) situated above the Silurian sedimentation (Farrell, 2003). Further north, completely across Killary Harbour, are more Ordovician rocks known as the Glenumera Slate and the Mweelrea Grit. Further to the east, the formations of interest continue approximately 15 kilometres to Lough Mask (Farrell, 2003).

In their seminal work "Notes on the Geology of Killary Harbour," McLaren and Miller (1948) report that "The Rosroe Peninsula, on the south side of Killary Harbour, Co. Galway, is formed of the Rosroe Grits, mapped by the Geological Survey of Ireland (1875) as 'Lower Silurian.' Kilroe (1907) in a rapid revisionary survey of Galway and Mayo, described the succession on Rosroe as inverted. Mr. McHenry informs me that the fossils (graptolites) on the south side are indicative of Upper Llandeilo, while those on the north side are Lower. This being the case, the order of the beds is inverted, as they dip northerly" (p. 217).

Stratigraphic Nomenclature and Dating

Based on the correlation of rock units with established Dalradian stratigraphy in Scotland, the metasediments in the southern part of the area of interest have been assigned to the Dalradian Supergroup (Leake and Tanner, 1994). Scientists have been constrained in their ability to accurately date the Dalradian Supergroup because there are no corresponding datable fossils (Farrell, 2003). By using isotopic dating of metamorphic clasts and igneous intrusions together with field relationship analyses, researchers have adopted a Precambrian to possible early Cambrian age for the Dalradian (Farrell, 2003). Farrell also notes an additional constraint: "The Currywongaun intrusion has been dated using U-Pb geochronology. Its U-Pb zircon age is 474.5 ± 1.0 Ma. This, along with the dating of other intrusions, constrains the time of deformation within the Dalradian, as well as the age of the Dalradian itself" (p. 5).

Mapping of the sediments was conducted as early as 1838 by Griffith (McKerrow & Campbell, 1960), and scientific interest in the region continued throughout the remainder of the 19th century, with the most significant work being conducted by Kinahan's research on the Geological Survey during the period 1870 to 1878 (Farrell, 2003). The major stratigraphical divisions of the Silurian period were defined in Kinahan's work, and subsequent researchers built on these findings throughout the early part of the 20th century (Farrell, 2003). The results of mapping during the period from 1949 to 1957 were published in 1960 by McKerrow and Campbell; the stratigraphic divisions were subsequently revised by Piper in 1967 and then revised once again by Laird and McKerrow (1970) using dating of brachiopods and graptolites for the formations (Farrell, 2003).

The nomenclature defined by Laird and McKerrow (1970) was used in this study and is set forth in Table 1 below.

Table 1: Stratigraphic divisions of the Silurian sediments (Culfin Supergroup)

Killary Harbour Group — Salrock Formation: Middle Wenlock or later. Lough Muck Formation: Wenlock (Eocoelia sulcata) / Middle Wenlock (Monoclimacis flumendosae). Upper Owenduff Group — Glencraff Formation. Lettergesh Formation (Gowlaun Member): Middle Wenlock (M. flumendosae). Lower Owenduff Group — Tonalee Formation. Kilbride Formation: Upper Llandovery C5 (Eocelia curtis). Lough Mask Formation. Source: Adapted from Farrell, 2003.

Silurian Formations

The Silurian sediments in the geographic region of interest are present in five main formations: Lough Mask, Lettergesh, Glencraff, Lough Muck, and Salrock.

There is a unique formation found directly above the metasediments on the shore (L7310 6280). According to Farrell, "The contact is unconformable close to the shore, where unmetamorphosed breccia and sediments are next to highly metamorphosed schists. The contact is faulted near the sea. The base of this formation consists of a red breccia, with clasts of quartz" (p. 13). A coarse red sandstone interbedded with a pebble conglomerate overlies the base. Farrell adds: "The outcrop has been heavily weathered and faulted so the relationships are difficult to see. However, it seems that this formation unconformably overlies the metasediments and in turn is unconformably overlain by the next formation, the Lettergesh formation. No fossils were found in this formation. The weathered surface of the sandstone is brown or black in hand sample. A fresh surface is orangey-red and grey with a maximum grain size of 1 mm and a microscopic modal grain size. Some of the grains weather to a pale green colour. In thin section, quartz is the major grain type (~50%) with some microcline (~5%) and some rock fragments (~1%) all in a fine matrix which consists of quartz and/or feldspar, mica, and some calcite cement. The majority of the grains have a red-brown altered rim. The rock fragments are possibly reworked sediments" (p. 13).

Analysis by Farrell indicates that this formation is primarily comprised of quartz-rich sandstones, mudstones, and siltstones. Farrell notes that "The sandstone should more accurately be called a greywacke and it is the most common rock type. The formation has a total thickness of about 1,096 m including the thickness of the two microgranodiorite bodies which intrude it. At the base of the formation (L754 610 and surrounds) there is a body of conglomerate which is part of the Gowlaun member defined by Laird and McKerrow (1970). This cobble conglomerate is found at the top of Altnagaighera (L7540 6100) and across the valley on the slopes south of Lough Fee (L7760 6117)" (p. 13).

According to Farrell, the area is cut by a single major fault known as the Lough Fee fault, which has an ENE trend and offsets almost all of the other major lithologies of the area — the metagabbro and Dalradian, the Lettergesh, and the diorite. Farrell adds: "It can be seen best in the valley just on the SW side of Lough Fee (L7750 6150 and surrounds). The fault runs through this steep-sided valley and near the top of Garraun (L7685 6140) there is a small narrow vertical dip in topography that marks the fault" (p. 18). Farrell interprets these observations as suggesting a vertical fault; although there is an average trend of 040°, the trend curves. The apparent offset is about 1.4 km, but there is probably some dip-slip movement given the different thicknesses of the formations on either side. The Lough Fee fault is most likely associated with the faults in the Silurian sediments, given that many of them run parallel in a leftward offset. Farrell concludes that "The smaller faults are mostly seen offsetting the diorite by small amounts (usually only a few metres)" (2003, p. 18).

Farrell also reports that "The conglomerate is mostly clast-supported with a matrix of coarse sandstone. It is interbedded with beds of coarse- or medium-grained sandstone. The boundaries between the conglomerate and sandstones are usually sharp. Occasionally there are a few cobble clasts floating in sandstone beds, and sometimes there are beds of conglomerate which are only one clast thick. The clasts are mostly quartz arenite (white, pink, and white-and-pink), as well as some volcanic clasts, some sandstone, some jasper, some granitoid clasts, and one or two clasts of schist. The clast size ranges from 2 cm up to 50 cm with a modal size of about 10 cm. The clasts are all well rounded. There is some limited evidence of imbrication but most of the time the clasts seem to be quite randomly orientated. On Altnagaighera (L7555 6070) the outcrops are steep-sided as they have broken along joint faces. Below them the steep slopes are littered with clasts. These clasts are found in different proportions to those in the conglomerate — there are very few quartz arenite clasts on the scree slopes, probably due to differences in strength and resistance to weathering" (p. 13).

Farrell further reports that "On the coast (L7310 6280) above the Lough Mask formation is a series of thinly bedded green-grey siltstones and mudstones with some beds of sandstone. The beds are from 1–2 cm to 30 cm thick. Since the conglomerate lenses out along strike this is the base of the Lettergesh formation on the shore. There are flame structures found in the beds which give a way-up to the north and indicate that the beds have not been overturned. The flame structures all point to the east. Up in the mountains (e.g., L7435 6225) above the conglomerate the Lettergesh also consists of thinly bedded siltstones. Above these, on the coast (from L7370 6310) and up in the mountains (e.g., L7660 6120) are thick beds (9 cm up to 65 cm) of greywacke interbedded with thinner beds of mud and silt. The greywackes are commonly graded and are greeny-grey in colour. There are examples of flame structures in some of the thinner beds as well as convoluted bedding" (p. 16).

According to Williams and Harper's (1988) assessment of this formation, "The Silurian successions of the northern part are marked by a diachronous late Llandovery (early-middle Telychian) transgressive episode which is common to many successions. The transgression resulted in the development of comparable sedimentary facies and fossil assemblages" (p. 741). These authors further add that "The clast compositions of Llandovery and Wenlock conglomerates suggest erosion of a common provenance of volcanic rocks founded on a metaquartzite basement. Areas of active volcanism appear to have migrated in a north-westerly direction with time. It is suggested that the common features of the Irish successions may be accommodated in a unified basin model for Silurian sedimentation and volcanism. Such a basin may have formed in an intra-arc environment and been controlled by oblique-slip fault mechanisms, although evidence for the presence of a Silurian arc is equivocal" (Williams & Harper, 1988, p. 741).

The Glencraff formation is situated conformably on top of the Lettergesh with a total thickness of 68 metres. According to Farrell, "The formation consists of thin (on average 5–10 cm) laterally continuous beds of siltstone and mudstone with some, rarer, sandstone beds. The beds are quite highly cleaved in places. The outcrop is not as good as for the other formations, perhaps because it is easier to erode. On the coast there is one large outcrop at L7408 6403, below Islandlyre, but further inland (L7440 6395) there is a grassy low-lying area where most of the Glencraff was expected to be. By the Culfin River (L7560 6325) there is reasonable outcrop for a short while but heading towards Lough Muck the outcrop disappears into a bog" (p. 20).

Nealon (1989) advises: "The Silurian, Glencraff Formation of north Galway has previously been interpreted as a deep-basinal, distal-turbidite sequence, representing the culmination of a marine transgression. It is interpreted here as mid- to outer-shelf storm deposits, most of which accumulated below storm wave base. The formation is divided into five lithofacies (A to E): Facies A is comprised of thinly bedded sandstones that exhibit intermittent hummocky cross-stratification (HCS), and abundant low-angle cross lamination and horizontal lamination. Facies B consists of parallel-laminated, thin-bedded sandstones. Facies C consists of generally massive siltstones. Facies D consists of mudstone. Facies A, B, and C are interpreted as having been deposited by storm activity, mainly below storm wave base. Facies D occurs only at the top of the formation and consists of a series of thinly bedded tuffs which contain abundant large-scale hummocky cross-stratification and are therefore interpreted as having been deposited in slightly shallower water than the rest of the formation. A slight gradual decrease in water depth is recorded throughout most of the formation by an increase in the occurrence of small-scale HCS in the Facies A sandstones" (p. 55). Nealon adds that "The enhanced shallowing of water depth during deposition of the tuffs is interpreted as being partly caused by the rapid influx of the tuffs themselves. Paleocurrent data and facies relationships indicate that the shoreline lay to the northwest with a roughly northeast-southwest orientation" (p. 55).

Farrell (2003) further reports that "The Glencraff formation was laterally continuous and thinly bedded, suggesting deep-water deposition. No ripples, cross-beds, or any other structures were observed. The grain size is, on average, smaller than the Lettergesh sandstones below. The different grain size suggests either more distal deposition or a different sediment source" (p. 13). It is possible that the Glencraff formation represents the highest point in the transgression; while no evidence of this was found in the field, previous research by Laird and McKerrow (1970) identified graded bedding that interpreted the Glencraff as turbidites (Farrell, 2003).

Found conformably above the Glencraff formation, the Lough Muck formation is 249 metres thick. According to Farrell, "The boundary between the two was difficult to pinpoint and may be gradational. The Lough Muck consists of grey and green mudstones, siltstones, and sandstones. The beds are mostly laterally continuous and are from 1 cm to 20 cm thick (on average about 10–15 cm). The main factor used to differentiate these from the Glencraff was the presence of cross-bedding in this formation and the increased number of sandstone beds. There were a few slump structures found in outcrops on the coast (L7410 6428) and also inland just above the Culfin River (L7590 6331). Flame structures were seen on the shore (L7410 6428). Near the top of the formation (L7427 6435) some trace fossils were discovered. The trace fossils are burrows filled with coarser sand than the surrounding sediment. The outcrop for this formation was good both on the coast and inland" (p. 20).

MacGabhann reports that "The base of the Lough Muck member is identified on the first occurrence of cross-bedding, which contrasts strongly with the underlying units, in which cross-bedding is absent, suggesting an entirely different environment congruent with a constant flow regime" (p. 18). Farrell's analysis found that "The Salrock formation conformably overlies the Lough Muck formation. It is the uppermost formation mapped in the area and has a minimum thickness of 971 m. The formation consists of green, red, grey, and purple siltstones, sandstones, and mudstones. Siltstone is the most common rock type. The red colour is the characteristic feature of this formation, and the boundary between the Salrock formation and the Lough Muck formation was identified by the first appearance of the red beds. The beds are on average 10–15 cm thick. In some areas the beds are slaty and have been quarried. The red of the red-beds is not a surface colouration, and in some beds there is a mixture of colours within a single bed" (p. 22).

MacGabhann similarly notes: "The base of the Fothair member is defined on the basis of the red colour, and is once again conformable. This is seen to be gradational across single bedding planes, though it appears to be roughly continuous over the area. Herringbone cross-bedding indicates bidirectional flow, probably tidal conditions" (p. 18).

Farrell also reports small-scale folding within the Salrock formation: "There is a set of open folds with a late cleavage associated with kinks at the fold hinges. These folds fold the original slaty cleavage and are therefore later structures. They have amplitudes of one or two metres. The folds are tighter in the north than further south. Further south the folds have a lower amplitude and there are no kinks at the fold hinges" (Farrell, 2003, p. 18). Farrell attributes this folding to a large-scale thrust fault to the north: "The fact that the Salrock formation is the only formation of the Silurian to have such extensive folding suggests that this formation acted as the incompetent layer, taking up all the strain from the thrust fault. Another set of folds seen in the Salrock formation are intraformational and found only within single beds. These are most likely due to soft-sediment deformation" (p. 18).

References

Archer, J.B. (1984, December). Clastic intrusions in deep-sea fan deposits of the Rosroe Formation, Lower Ordovician, Western Ireland. Journal of Sedimentary Petrology, 54(4), 1197–1205.

Archer, J.B. (2007). Llanvirn stratigraphy of the Galway-Mayo border area, western Ireland. Geological Journal, 12(1), 77–98.

Chew, D.M. (2001). Basement protrusion origin of serpentinite in the Dalradian. Irish Journal of Earth Sciences, 19, 23–35.

Clift, P.D., & Ryan, P.D. (1994). Geochemical evolution of an Ordovician island arc, South Mayo, Ireland. Journal of the Geological Society, 151, 329–343.

Farrell, U. (2003). Regional geology of Little Killary (Unpublished dissertation).

Friedrich, A.M., Hodges, K.V., Bowring, S.A., & Martin, M.W. (1999). Geochronological constraints on the magmatic, metamorphic and thermal evolution of the Connemara Caledonides, western Ireland. Journal of the Geological Society, 156, 1217–1230.

Jeppsson, L., & Aldridge, R.J. (2000). Ludlow (late Silurian) oceanic episodes and events. Journal of the Geological Society, 157, 1137–1148.

Jolly, R.J., & Lonergan, L. (2002). Mechanisms and controls on the formation of sand intrusions. Journal of the Geological Society, 159, 605–617.

Keegan, B.F., & Mercer, J.P. (1986). An oceanographical survey of Killary Harbour on the west coast of Ireland. Proceedings of the Royal Irish Academy. Section B: Biological, Geological, and Chemical Science, 86B, 1–70.

Kilroe, J.R. (1906/1907). The Silurian and metamorphic rocks of Mayo and North Galway. Proceedings of the Royal Irish Academy. Section B: Biological, Geological, and Chemical Science, 26, 129–160.

Kinahan, G.H. (1878). Manual of the geology of Ireland. C.K. Paul & Co.

Laird, M.G., & McKerrow, W.S. (1970). The Wenlock sediments of north-west Galway, Ireland. Geological Magazine, 107, 297–317.

Leake, B.E., & Tanner, P.W.G. (1994). The geology of the Dalradian and associated rocks of Connemara.

MacGabhann, B. (2003). The geology of Little Killary Bay and Lough Fee, County Galway, Ireland.

McLaren, D.J., & Miller, T.G. (1948). Notes on the geology of Killary Harbour. Geological Magazine, 85, 217–221.

McKerrow, W.S. (1959). The Southern Upland Fault in Ireland. Geological Magazine, 96, 347.

McKerrow, W.S. (1971). Palaeontological prospects: the use of fossils in stratigraphy. Journal of the Geological Society, 127, 455–464.

McKerrow, W.S. (1979). Ordovician and Silurian changes in sea level. Journal of the Geological Society, 136, 137–145.

McKerrow, W.S., & Campbell, C.J. (1959). The stratigraphy and structure of the Lower Palaeozoic rocks of north-west Galway. Scientific Proceedings of the Royal Dublin Society, A, 1, 27–52.

Menuge, J.F., Williams, D.M., & O'Connor, P.D. (1995). Silurian turbidites used to reconstruct a volcanic terrain and its Mesoproterozoic basement in the Irish Caledonides. Journal of the Geological Society, 152, 269–278.

Mohr, P. (1990). Late Caledonian dolerite sills from SW Connacht, Ireland. Journal of the Geological Society, 147, 1061–1069.

Nealon, T. (1989). Deep basinal turbidites reinterpreted as distal tempestites: the Silurian Glencraff formation of North Galway. Irish Journal of Earth Sciences, 10(1), 55–59.

Piper, D.J. (1970, September). A Silurian deep sea fan deposit in western Ireland and its bearing on the nature of turbidity. The Journal of Geology, 78(5), 509–522.

Piper, D.J. (1972). Sedimentary environments and palaeogeography of the late Llandovery and earliest Wenlock of North Connemara, Ireland. Journal of the Geological Society, 128, 33–51.

Tanner, P.W. (1990). Structural age of the Connemara gabbros, western Ireland. Journal of the Geological Society, 147, 599–602.

Trench, A., McKerrow, W.S., Torsvik, T.H., & McCracken, S.R. (1991). Ordovician magnetostratigraphy: a correlation of global data. Journal of the Geological Society, 148, 949–951.

Trench, A., McKerrow, W.S., Torsvik, T.H., & McCracken, S.R. (1993). The polarity of the Silurian magnetic field: indications from a global data compilation. Journal of the Geological Society, 150, 823–831.

Wellings, S.A. (1998). Timing of deformation associated with the syntectonic Dawros–Currywongaun–Doughruagh Complex, NW Connemara, western Ireland. Journal of the Geological Society, 155, 25–37.

Williams, D.M., & Harper, A.T. (1988). A basin model for the Silurian of the Midland Valley of Scotland and Ireland. Journal of the Geological Society, 145, 741–748.

Williams, D.M., & Nealon, T. (1986). The significance of large-scale sedimentary structures in the Silurian succession of western Ireland. Geological Magazine, 124(4), 361–366.

Williams, D.M., O'Connor, P.D., & Menuge, J. (1992). Silurian turbidite provenance and the closure of Iapetus. Journal of the Geological Society, 149, 349–357.