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Fluvial to marine transition in the Ordovician of Ireland–A humid‐region fan‐delta?
Author(s) -
Pudsey C. J.
Publication year - 1984
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.3350190205
Subject(s) - slates , geology , cobble , imbrication , alluvial fan , fluvial , clastic rock , molasse , ordovician , delta , conglomerate , sedimentary rock , geomorphology , paleontology , structural basin , geochemistry , tectonics , ecology , aerospace engineering , foreland basin , habitat , engineering , biology
The Mweelrea Group (Llanvirnian) in the South Mayo Inlier, western Ireland, consists mainly of coarse shallow‐water sandstones with minor slates, cobble conglomerates, and ignimbrite tuffs. The Glenummera Formation at the base (up to 600 m), dominantly well‐cleaved slates with some slump features, was deposited in a slope to deep marine shelf environment. A coarsening upward sequence at its top reflects the interaction of fluvial and marine processes, coarse clastic sediments having prograded from the southeast. The rest of the Mweelrea Group (2100 m in the northwest) is dominated by coarse trough cross‐bedded sandstone with little evidence for channelling. Three lenticular marine slates (Glendavock, Uggool, and Glenconnelly Formations), up to 200 m thick, wedge out to the southeast. A humid alluvial fan‐fan‐delta model can explain many features of the Mweelrea Group. Fans built out to the northwest into a relatively small basin with negligible tides and moderate wave energy. Marginal sediments were reworked by waves, occasional storms, and a burrowing fauna.