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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. 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).
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).
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…[continue]
"Geology Of Little Killary Killary" (2011, December 04) Retrieved December 8, 2016, from http://www.paperdue.com/essay/geology-of-little-killary-48181
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