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MURRAY, JOHN P., Trinity College Dublin, Dept. of Geology, Ireland

ABSTRACT: Mid to Late Mississippian (Visean) Rocks Around the Shannon and Fergus Estuaries, Southwest Ireland, and Their Bearing on the Evolution of the Shannon Basin

The Carboniferous Shannon Basin is located in southwest Ireland with its axis coincident with the River Shannon. During the mid to late Mississippian (Visean) clear differences in facies between the basin and the surrounding shelf began to develop. Reconnaissance mapping has identified a passage from cyclic platform carbonates through carbonate mudmounds into basinal limestones, which include debris flow breccias and a widespread unit of enigmatic striped limestones

Striped limestone (Parsonage Formation, late Mississippian) occurs over a wide area within the basin and is an important marker horizon, yet its origin remains unclear. The `stripes' consist of alternating light and dark millimeter scale carbonate laminations, which vary from planar to crinkly; localized folding and brecciation is common. The paler bands tend to be thicker and are composed of micro-spar. The thinner, darker interlayers are finer-grained and contain concentrations of dispersed organic material, clay minerals, framboidal pyrite and fine-grained silica. `Grumuleuse' textures, probably microbial in origin, are common in the dark bands. The striped limestones appear to be unfossiliferous, except for the rare occurrence of ostracods. It is possible that they were once evaporites, which have since been replaced by carbonate. The problem with this hypothesis is that their depositional setting was a deep-water basin with localized slopes, with little or no evidence for restricted water circulation.

AAPG Search and Discovery Article #90909©2000 AAPG Foundation Grants-in-Aid