South Mayo Trough Research Articles

Discussion

D r W. S. M c K errow asked the author if they thought the upward decrease in size of the vesicles in the lavas of the Lough Nafooey Group was related to an increase in depth of water up through the sequence. If so, how did this relate to the overlying clastic sediments? D r M c K errow later wrote: Could the authors elaborate on the relation of the Lough Nafooey Group to the Grampian Orogeny? If it was entirely postorogenic, why were the lavas not interbedded with coarse clastic detritus from the adjacent Connemara highlands? The A uthors reply that they believe the decrease in vesicle size is at least partly related to increasing water depth. It is reasonable to suggest that the Bencorragh Formation developed in shallow water, especially as the vesicles penetrate the glassy selvages. A deeper water origin for the pillow lavas in the Finny and Knock Kilbride formations is supported by their association with cherts and black shales. Furthermore, considerable subsidence would have been required for the build up of this submaride lava pile and this was probably associated with activity on the Doon Rock Fault. The coarse clastic sediments of the Derry Bay Formation record the establishment of a nearby source area and are thought to represent the emergence of the Connemara Cordillera. The Lough Nafooey Group can therefore be interpreted as recording the initial subsidence of the South Mayo Trough floor and the emergence of an adjacent source area. In reply to the second part of Dr McKerrow's question we

Evidence for glaciation in the Ordovician rocks of western Ireland

SummaryThe Maumtrasna Formation is a thick succession of coarse clastic rocks situated in the South Mayo Trough of western Ireland. Its upperage limit is not defined but it is post-Llanvirn in age and probably Ordovician. Several aspects of the conglomerates within the succession are compatible with the action of a glacial influence during its deposition. New evidence is presented which tends to confirm the glacial hypothesis. 10.7% of garnets separated from the matrix of the conglomerates exhibit chattermark trails. Published accounts of such trails indicate that they are only found in sediments which are glaciogenic or were derived from pre-existing glaciogenic sediments. It is suggested that the Ordovician glacial activity in the South Mayo Trough took the form of a highland glaciation which was maintained, possibly at sub-equatorial latitudes, by a continuous uplift of the source area.

The South Mayo Trough: a possible Ordovician Gulf of California-type marginal basin in the west of Ireland

The South Mayo Trough, an early Ordovician sedimentary basin, was developed at the southern margin of the Laurentian plate. It controlled deposition of 12.8 km of sediment. Basic vulcanism accompanied the opening of the trough. This was followed by the deposition of turbidites and finally of fluvio-deltaic sediments. Initial island arc vulcanism was replaced by a bimodal basalt–rhyolite suite during sedimentation. The trough was bounded to the north and south by metamorphic source areas, of which the southerly, the Connemara Cordillera, was the more important. The Connemara Cordillera, comprised of Dalradian continental margin sediments, was deformed and metamorphosed before the trough's formation. Uplift and erosion of Connemara, opening of the trough, and change in vulcanism all occurred during late Tremadoc – early Arenig times. These events are related to the collision of the Iapetus ridge with a trench to the south of Connemara, which initiated a Gulf of California-type marginal basin, the South Mayo Trough. Comparisons between the morphology and sedimentary history of the trough with that of the Gulf of California and the geology of their adjacent source areas support this hypothesis.

Llanvirn stratigraphy of the Galway‐Mayo border area, western Ireland

AbstractThe Rosroe Formation is established as the lowest part of the Llanvirn succession exposed on the southern limb of the Mweelrea‐Partry Syncline. Lithological description is followed by interpretation in terms of two structurally controlled submarine fans, developed at the southern margin of the South Mayo Trough. A review of stratigraphic relations between succeeding parts of the Llanvirn succession leads to reinterpretation of the closing marine phases of the South Mayo Trough: northward slope progradation is inferred to have resulted in graduai northward spread of shallow water and emergent conditions.

Paleomagnetic results from the Lower Paleozoic of Ireland

Paleomagnetic data are presented for a variety of Ordovician rocks from two regions in Ireland: the South Mayo Trough and some Ordovician inliers of east-central Ireland. Some of these data update previous preliminary results, while others provide new information. In some instances the original remanence direction has been obscured by subsequent remagnetization.Although magnetic inclinations of the Ordovician results are mostly shallower than contemporaneous British results, poor precision precludes any reliable estimate of closure across this sector of the Proto-Atlantic ocean. Declination differences between the South Mayo Trough and British data indicate a 30° clockwise rotation of this tectonic block during the Middle to Upper Devonian.

The relationship between isotopic ages, uplift and sedimentation during Ordovician times in western Ireland

Synopsis During Ordovician times, at least 36 000 feet of sediments accumulated in the South Mayo Trough, which lay to the north of a rising cordillera of Dalradian metamorphic rocks in Connemara, western Ireland. A complex faulted relationship existed between trough and cordillera. K-Ar isotopic ages, showing a spread within the range 475 to 420 m.y., are interpreted as representing the time during which the cordillera was progressively uplifted, eroded, and cooled.

The Lower Palaeozoic stratigraphy of central Murrisk, County Mayo, Ireland, and the evolution of the south Mayo trough

The Ordovician and Silurian rocks of central Murrisk, County Mayo, Ireland, are described with reference to their stratigraphy, provenance, and conditions of deposition; 36 000 ft of Ordovician rocks are overlain, unconformably, by 5600 ft of Silurian strata. The Arenig rocks are divided into two series: the Owenmore Series (2000 ft) below and the Murrisk Series (15 000 ft) above. The Owenmore Series (boulder-slides, turbidites, and black slates) was derived entirely from the north; the Murrisk Series (tuffs and turbidites) was derived from the east and south. The Glenummera Series (4000 ft of green slates with thin psammitic turbidites) follows and is overlain by the Partry Series (15 000 ft). The Partry Series is divided into two groups; the Mweelrea Group below, and the Maumtrasna Group above. The Mweelrea Group (7000 ft of current-bedded deltaic grits) contains six beds of ignimbrite, deposited by nuées ardentes with a south-easterly provenance. Periodic westward tilting caused the eastward recession of the delta-front with the concurrent development of boulder-slides and turbidites. The Maumtrasna Group is similar to the Mweelrea Group but contains no ignimbrites, and the grits are coarser, with many interbedded conglomerates. The Silurian rocks are divided into the Lower Owenduff Group (Upper Llandovery, 0 to 20 ft) below, and the Croagh Patrick Series (Wenlock, 5600 ft) above. The Lower Owenduff Group was deposited in a complex littoral environment and contains a large amount of pyroclastic material. The Croagh Patrick Series is divided into six groups (Cregganbaun Group, 500 ft; Pollanoughty Group, 450 ft; Oughty Group, 360 ft; Fiddaunarinnia Group, 600 ft; Lough Nacorra Group, 1500 ft; Owenwee Group, 2000 ft), the Oughty Group containing five tuff bands. The Croagh Patrick Series was deposited in a northwardly transgressive littoral and sub-littoral belt and was derived exclusively from the north and east. The Silurian rocks rest unconformably upon the Owenmore Series as the result of strong pre-Upper Llandovery thrusting, concentric folding, and uplift. During most of Ordovician times, the northern margin of an e-w trough (the South Mayo trough) was denned by a normal fault. The southern margin was defined throughout most of Ordovician times and the whole of the Silurian by a rising Cordillera in Conne-mara. Dal radian rocks provided the source for nearly all the clastic material.