Geology of Little Killary, Killary

Geology of Little Killary, Killary Harbour, Galway, Ireland

Regional Geology. The purpose of this study was to thoroughly examine the regional geology of Little Killary, Killary Harbour, Galway, Ireland located in northwest Galway, which is just south of Killary Harbour and situated on the Atlantic coast (see map in Figure 1 below) with the exception of the Kilbride formation. The geographic region of interest consists of the townlands of Lettergesh East and West as well as Glassilaun. The geographic site of interest also contains three primary peaks (i.e., Benchoona, Altnagaighera and Garraun) together with the bases of Doughruagh and Currywongaun. The topography of the region of interest depicted in Figure 1 below was formed glacially (MacGabhann).

Figure 1. Little Killary Bay and Harbour

Source: Archer, 1984

A real-time graphic image of Killary Bay and Harbour, together with the surrounding environs, is depicted in Figure 2 below.

Figure 2. Killary Bay and Habour

Source: http://en.mapatlas.org/Ireland/Bay/Little_Killary_Bay/7953/3D_earth_map

The coastline expanse that comprises the northwest border of the map shown in Figure 2 above stretches from Glassilaun beach (L7590 6470) to just west of Gowlaun (L7270 6270). There was good exposure encountered throughout in the region of interest for the most part, particularly with respect to Benchoona (L7660 6164) as well as a few of the other higher points of elevation in the area. Likewise, 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 that a was mapped for this study (see Figure 3 below) was approximately 18km2 and all location numbers mentioned are provided in Irish grid references.

Figure 3. Area mapped for study

1.2

Regional Geology -- Summary. In sum, the Little Killary Bay area is comprised of various sedimentary, metasediments and intrusive rocks of igneous origin (Farrell, 2003). By contrast, there are metasediments consisting mostly of mica shists that have been involved in numerous deformation events located in the southern-most area of the region of interest. Situated above these metasediments is a series of sediments that are not overturned and which dip to the north. The Lough Mask formation (L7370 g279) is situated at the base on the shore that is comprised of breecias and coarse red said that is amalgamated into the conglomerate found in this region.

The Lettergesh formation is situated above that level as well as the metasediments in the majority of the area; the Lettergesh formation is primarily comprised of a basal cobble conglomerate containing turbiditic sandstones. The Glencraff formation is located above these levels that consists of a thin formation of thinly bedded mudstones and siltstones that contains some sand. Above the Glencraff formation is the Lough Muck formation which consists of siltstones, sandstones and mudstones. Above the Lock Muck formation is the Salrock formation, which entirely disappear slightly west of Lough Mask (Kinahan, 1878) (see Figure 4 below, consisting of a thick formation of sandstones, siltstones and mudstones that are characterized by their red beds; however, this formation is situated outside the area of interest so its exact thickness remains unknown.

Figure 4. Red and green mudstones of the Silurian Salrock formation, showing folding and fractures of late Silurian deformation. Glassilaun, Renvyle, Connemara, Co. Galway.

Source: Gatley, 2008 at http://www.gsi.ie/NR/rdonlyres/BAD98660-6D5E-4AA8-A5AD-A0D5455510FB/0/SilurianSalrockFm_Connemara.jpg

A few intrusive igneous rocks cross cut the area; in addition, 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). It was also determined that one large and one smaller body of microgranodiorite were intruded into the Lettergesh formation in the sediments with the intrusions being parallel to strike in the majority of the cases, but not in every case. In addition, there has been intrusion by several sills in the sediments in the Lettergesh formation, with a dolerite dyke found crosscutting the microgranodiorite and the sandstones being the most recent (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 still, completely across Killary Habour, are more Ordovician rocks which are known as the Glenumera Slate and the Mweelrea Grit. Finally, still further to the east, the formations of interest to this study continue approximately 15 kilometers 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).

1.3

Naming and dating of the stratigraphical divisions. 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, though, because there are no corresponding datable fossils found (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). In addition, Farrell cites another limitation with respect to establishing an accurate age for the Dalradian: "The Currywongaun intrusion has been dated using U-Pb geochronology. Its U-Pb zircon age is 474.5 ± 1.0. 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 there was continuing scientific interest in the region during 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). These major stratigraphical divisions of the Sulurian period were defined in Kinhahan'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 and the stratigraphic divisions were subsequently revised by Piper in 1967, which were 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

Group

Formation

Stratigraphical Age

Killary Harbour

Salrock

Middle-Wenlock or later

Lough Muck

Wenlock (Eocoelia sulcata)/Middle Wenlock (Monoclimacis flumendosae)

Upper Owenduff

Glencraff

Lettergesh

Gowlaun Member

Middle Wenlock (M. flumendosae)

Lower Owenduff

Tonalee

Kilbride

Upper Llandovery C5 (Eocelia curtis)

Lough Mask

Source: Adapted from Farrell, 2003

2.

Formations

2.1

Silurian formations. The Silurian sediments in the geographic region of interest described above are present in five main formations, Lough Mask, Lettergesh, Glencraff, Lough Muck and Salrock as described further in Table 2 below.

Table 2

Little Killary Formation Descriptions

Formation

Description

Lough Mask (Loch Measca)

This is a limestone lough consisting of 89 square meters (or 22,000 acres) in area (see map below).

Source: http://www.irishfisheries.com/waterways.asp? P=3&W=29

There is a unique formation found directly above the metasediments on the shore (L7310 6280) at this location. 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 sandstones interbedded with a pebble conglomerate overlays the base, and Farrell adds that, "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).

Lettergesh

Analysis by Farrell indicates that this formation is primarily comprised of quartz-rich sandstones, mudstones and siltstones. This authority adds that, "The sandstone should more accurately be called a greywacke and it is the most common rocktype. The formation has a total thickness of about 1096m 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 in the field that has an ENE trend which offsets almost all of the other major lithologies of the area (i.e., the metagabbro and Dalradian, the Lettergesh and the diorite). Farrell adds that, "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. According to Farrell, "The apparent offset is about 1.4km but there is probably some dip-slip movement given the different thicknesses of the formations on either side. Some of the lithologies seem to have a small amount of drag and the diorite nearest the shore is quite sheared and recrystallised, probably due to movement of the fault" (p. 18). 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 such as evinced in the Lough Fee fault (Farrell, 2003). In sum, Farrell concludes that, "The smaller faults are mostly seen offsetting the diorite by small amounts (usually only a few meters)" (2003, p. 18). In addition, Farrell 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 2cm up to 50cm with a modal size of about 10cm. 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 on the scree slopes except the white and pink quartz arenite, probably due to the differences in strength and resistance to weathering" (p. 13). Finally, Farrell adds 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 1-2cm to 30cm 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 (9cm up to 65cm) 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). Other researchers have investigated this area as well. For instance, 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 authorities 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).

Glencraff

The study by Farrell indicates that the Glencraff formation is situated conformably on top of the Lettergesh with a total thickness of 68 meters. According to Farrell, "The formation consists of thin (on average 5-10cm) 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). Besides Farrell (2003), there are few other scholarly sources for this formation but one seminal source, Nealon, 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 ranged 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. Of generally massive siltstones, and Facies D. 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." Finally, Neaton 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). In addition, Farrell (2003) reports that, "The Glencraff formation was laterally continuous and thinly bedded suggesting deep-water deposition. No ripples, crossbeds 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, and while no evidence of this was found in the field, previous researcher by Laird and McKerrow (1970) identified graded bedding that interpreted the Glencraff as turbidites (Farrell, 2003).

Lough Muck

Found conformably above the Glencraff formation, the Lough Muck formation is 249 meters 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, siltsones and sandstones. The beds are mostly laterally continuous and are from 1cm to 20cm thick (on average about 10-15cm). 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, which are filled with [coarser sand than the surrounding sediment being found here]. The outcrop for this formation was good both on the coast and inland" (p. 20).

Salrock

In this formation, MacGabhann reports that, "The base of the Loch 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 that would be congruent with a constant flow regime" (p. 18). Likewise, Farrell's analysis of this formation 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 971m. The formation consists of green, red, grey and purple siltsones, sandstones and mudstones. Siltstone is the most common type of rock. 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 10-15cm 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 in the one bed" (p. 22). Similarly, according to MacGabhann, "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 that there is some small-scale folding within the Salrock formation. According to this researcher, "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 as most likely being the result of a large scale thrust fault to the north. In this regard, Farrell adds that, "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).

3.

Interpretations

Based on his analysis of the Lough Mask sediments field data, Farrell (2003) determined that the red colour, lack of fossils and the pebbly conglomerate indicated these were deposited in a fluviatile setting. According to Farrell, "The fact that the red colour of the sands is not a surface feature but is found throughout the rock shows that the grains were probably oxidised before deposition, suggesting a terrestrial origin" (p. 13). In addition, a braided river system is indicated by the pebble conglomerate and the coarse sand instead of one that meandered (Farrell, 2003). Although there are two other formations overlying the Lough Mask (the Kilbride formation and the Tonalee formation), they are not present in this site, having probably been eroded to the extent that the Lough Mask formation remains the single formation from the Lower Owenduff group in the area (Farrell, 2003).

With respect to the Kilbride and Tonalee formations, Williams and Nealon report that, "They consist of bed packages separated by truncation surfaces of varying orientations. They had previously been interpreted as rotational slumps generated on a shelf -- slope interface. It is shown that the structures formed in a tidally influenced environment far removed from a slope" (1986, p. 361). Both formations are also similar to structures that have been described from other tidal channel environments; as a result, Williams and Nealon suggest that these formations "represent deposition on aggradational margins of such channels. The supposed slope -- shelf transition is therefore not present at this level in the Silurian succession and much of the Lough Muck Formation may be of a dominantly shallow-marine origin" (1986, p. 361).

The results of Farrel's field investigation of this area support this interpretation. For instance, Farrell reports that, "The cobble conglomerate lenses out along strike, which suggests either a fan or channel deposit. The rounded nature of the clasts either suggests a long transport distance or wave action, although the large size of the clasts would help speed up the rounding process" (p. 13). Intermittent imbrication of conglomerate points indicate a northern source, and the interbedded sands are characterized by occasional graded bedding as well (Farrell, 2003). According to Farrell, "The cobble conglomerate may be responsible for the erosion of underlying sediments, in which case it is possibly a channel rather than a fan. It was possibly deposited in shallow water at the front of a river system which was depositing sediment into a basin. Wave action could be responsible for the rounding of the clasts in that case" (2003, p. 13).

Despite some consensus concerning the above issues, there remains some controversy about other aspects of the deposition of the conglomerate (Farrell, 2003). For instance, Farrell cites Piper's (1970) argument that turbiditic currents were responsible for the deposition of the conglomerate; Piper concedes, though, that there is no direct evidence to support this conclusion (Farrell, 2003). In addition, Piper (1970) demonstrated that there was interfingering shallow water formations to the east of the formation and subsequent research by Williams and O'Connor (1987) indicates a fan delta deposition occurring in shallower water than the research to that point indicated (Farrell, 2003). Based on Farrell's (2003) investigation, the conglomerate was interpreted as a channel, primarily attributable to outcrop pattern).

The thickly-bedded sandstones that are interbedded with muds and silts situated above the conglomerates in the Lettergesh formation are interpreted by Farrell (2003) as turbidites. According to Farrell, "The graded bedding, the presence of convoluted bedding and flame structures in the thinner beds, the load casts and groove casts and also the sharp boundaries of the graded beds support this hypothesis" (2003, p. 13). The results of Farrell's (2003) field work also suggest a deposition in relatively deep water. With respect to transgression, Farrell also suggests this has taken place irrespective of whether the conglomerates from the Lough Mask formation to the Lettergesh formation were deposited in the same fashion as the turbiditic sands. Notwithstanding the fact that Farrell did not obtain three-dimensional flame casts, he points out that, "The fact that they consistently point to the east suggests that the sediment source was somewhere either west, north west or south west. The content of the sandstones indicate that the Lettergesh had a volcanic source. The shape and freshness of the volcanic rock fragments indicate a short transport distance and an almost contemporaneous source" (2003, p. 13).

The foregoing interpretation of geological events by Farrell and others is also congruent with the established sequence of events in the Dalradian as summarized by Wellings (1998) in Table 3 below.