Thesis: Geology Around Little Killary

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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… [END OF PREVIEW]

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