A Mid-Carboniferous (Namurian) Basin in Parts of the Counties of Limerick and Clare, Ireland

Abstract

The goniatite-bearing black marine shales at the base of the Namurian in County Clare and County Limerick, described by Hodson in north Clare and at Foynes Island (1954 a , 1954 b ) are redefined as a litho-stratigraphic unit, the Clare Shales, to exclude an upper sandy division. The outcrop of the Clare Shales has been mapped in detail for a distance of about 50 miles from the Atlantic coast of north-west County Clare southwards to the Limerick-Cork border. Wherever possible the faunal succession and the thickness of the goniatite zones have been determined. It is shown t h a t the junction of the Carboniferous Limestone and the overlying Namurian is, over the greater part of the outcrop, and probably the whole of it, non-sequential. The basal beds of the Clare Shales vary from Lower Eumorphoceras age (E l ) immediately north of the Shannon estuary to Homoceras age (H) at the northern and probably at the southern limits of the area investigated. It is demonstrated that a mid-Carboniferous trough of sedimentation exists in County Clare and County Limerick, with its axis roughly along the line of the present-day Shannon estuary. The regularity of the overlap of the lower zones of the Namurian is disturbed north of the Shannon by thinning due to a ridge of Carboniferous Limestone at Carrowkilla, north of Ballynaeally, and by the presence of beds of basal Namurian age containing Cravenoceras leion in the area around Shallee Castle, west of Ennis. Apart from these iregularities, there is a steady overlap of the lower zones by the upper as one proceeds a w a y from the Shannon. By the correlation of accurately measured sections it is possible to reconstruct the form of trough along the line of section examined.

Facies and sequential organisation of a mudstone-dominated slope and basin floor succession: the Gull Island Formation, Shannon Basin, Western Ireland

“…2). The trend is clearly demonstrated by the distribution of goniatite-bearing marine bands, which possess unique fauna (Hodson, 1954a,b;Hodson & Lewarne, 1961). The trend is inferred to record onlap onto basin margins (Collinson et al, 1991), a view that has been disputed by Wignall and Best (2000), but defended by Martinsen and Collinson (2002).…”

“…2). Throughout the stratigraphy, a number of goniatitebearing, up to 1 m thick black shales or marine bands occur, each with a unique species (Collinson et al, 1991;Hodson & Lewarne, 1961). No such bands have been identified within the Gull Island Formation itself (Fig.…”

Biostratigraphy, microfacies and depositional environments of upper Viséan limestones from the Burren region, County Clare, Ireland

“…Nudds (1979) suggested that not only is the Brigantian succession complete in the Burren, but the upper 6 m of strata in the quarry section and below the shales of the Namurian (Serpukhovian) Clare Shale Formation might represent a condensed Brigantian succession. In the Magowna Formation that directly and apparently conformably succeeds the Slievenaglasha Formation in the Magowna and Shallee areas northwest of Ennis, south County Clare, the basal Pendleian (Serpukhovian) ammonoid Cravenoceras leion (E 1a subzone) is recorded (Hodson and Lewarne 1961;Sleeman and Pracht 1999;Sevastopulo and Wyse Jackson 2001).…”

“…Also, there appears to be evidence of very low sedimentation rates with little siliciclastic input. This is suggested by the presence of bored, rolled and 'algal' encrusted skeletal fragments, as well as the presence of phosphatic coatings to bioclasts on the top limestone surface and pebbly phosphate in the overlying basal shale (Hodson 1954;Hodson and Lewarne 1961). Nevertheless, the predominantly transgressive nature of the Asbian to Brigantian transition in much of southern and western Ireland is probably related to increased platform subsidence during this time (see Ramsbottom 1979) overprinting minor cycle development.…”

Facies change in upper devonian and lower carboniferous rocks of Southern Ireland

“…Throughout south central Ireland there is a non-sequence at the base of the Namurian and varying ages of Namurian strata rest upon Visean limestones (Nevi11 1956; Hodson and Lewarne 1961 ;Shepard-Thorn 1963;Morton 1965;Hudson and Philcox 1965). In the Southern Region the youngest Carboniferous rocks probably belong to the E Zone (Naylor 1966).…”