The Geology of the Castlebar Syncline, Co. Mayo

Brindle, John E (1958) The Geology of the Castlebar Syncline, Co. Mayo. PhD thesis, University of Glasgow.

Abstract

The Castlebar syncline of Carboniferous rocks lies in the southern half of County Mayo, Eire, and forms a variably pitching downfold between the Ox Mountains anticline and an echeloned extension of the Curlew Mountains anticline. The area mapped extends from the village of Strade in the north to an east-west line running a little south of the town of Mayo, and also includes the Carboniferous rocks of the upper valleys of the Aille and Derrycraff Rivers, west of the Partry Mountains, The area as a whole is low-lying, the greater part being less than 300 feet above sea level. The sandstones and shales of the highest beds form two north-and-south ridges in the east-oentral part of the area, the eastern ridge, which rises to over 800 feet above sea level, being the highest ground of the area. The Carboniferous strata rest unconformably on a basement of Dalradian schists, Silurian schists and schistose grits, and pebbly red sandstones and conglomerates of the Old Red Sandstone period. The following succession of Carboniferous rocks has been established: 14. Sandstone 200 to 0 feet. LOWER 13. Shale 0 to 350 feet. NAMU RIAN UPPER 12. Sandstone 300 to 0 feet. AND SERIES 11. Shale 0 to 350 feet. UPPER 10. Sandstone 240 to 0 feet. BOLLANDIAN 9. Shale 100 to 400 feet. DISCONFORMITY 8. Balla Limestone (including reef limestones) 200 feet. 7. Ballycarra Limestone 550 feet. 6. Ballyhean Oolite 80 feet. CARBONIFEROUS LOVER 5. Barney Limestone 300 feet. LIMESTONE SERIES 4. Aille Limestone 450 feet. (VISEAR) 3. Castlebar River Limestone 160 feet. 2. Lough Akeel Oolite 150 feet. 1. Basal Sandstone 80 to 150 feet. The fossils of the Basal Sandstone are confined to the uppermost beds. Fossils are scarce in the two Oolites and in the fine-grained, shaly Castlebar River Limestone. The Aille, Barney, and Ballycarra Limestones are crinoidal limestones having a rich coral-brachiopod fauna throughout, that of the Aille Limestone indicating a C2S1 age for that part of the succession. Typical S2 zone fossils are present in the upper levels of the Barney Limestone and in the lower beds of the Ballycarra Limestone. The uppermost 150 feet of Ballycarra Limestone, together with the Balla Limestone (a shallow-water deposit of calcite-mudstones, oolites, "mud-pellet" rooks and thin shales, contain a D zone fauna, as do the reef limestones. In the west of their outcrop, with the exception of the shales and mudstones of the lowest group, the whole thickness of the Upper Series is made up of unfossiliferous sandstones divisible into three groups. Each sandstone group thins eastwards, and the sandstones finally divide into three distinct beds with Intervening shales. Each sandstone group shows a gradual eastward diminution in grain-size. In the east of the outcrop the full thickness is made up of black shales, mudstones and rare siltstones. The shales and mudstones have a sparse fauna of goniatites and lamellibranchs. Goniatites collected from the lowermost 200 feet of shales, both in the east and west of the outcrop indicate an Upper Bollandian (P2) age for the containing beds. About 250 feet above the base of the shales low E1 zone goniatites were collected from an exposure in the north of the outcrop. No fossils were found at any higher horizons and the age of the upper beds remains uncertain. The rocks of the area are folded into a series of major and minor folds. The fold axes generally have an ENE. -WSW. or NE. -SW. trend, except in the south of the area, just east of the Partry Mountains, where the fold axes swing round to a N. -S. direction, paralleling the axis of the Partry Mountains. (Abstract shortened by ProQuest.).

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Geology
• Date of Award: 1958
• Depositing User: Enlighten Team
• Unique ID: glathesis:1958-79238
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 31 Mar 2020 09:09
• Last Modified: 31 Mar 2020 09:09
• URI: https://theses.gla.ac.uk/id/eprint/79238