Large-scale sedimentary structures are described from the Lough Muck Formation (Wenlock) of north Galway, Ireland. They consist of bed packages separated by truncation surfaces of varying orientations. They had previously been interpreted as rotational slumps generated on a shelf-slope interface. It is shown that the structures formed in a tidally influenced environment far removed from a slope. They bear strong similarities to structures described from other tidal channel environments and it is proposed that they represent deposition on aggradational margins of such channels. The supposed slope-shelf transition is therefore not present at this level in the Silurian succession and much of the Lough Muck Formation may be of a dominantly shallow-marine origin.
The Lough Muck Formation (Wenlock) is part of the Silurian succession of North Galway, western Ireland. The formatiw had originally been interpreted as a transitional sequence between shelf and basinal facies in the succession. Its three members (1,2, and 3 of Laird and McKerrow 1970) were envisaged as representing deposits of the continental rise, slope, and shelf respectively. A subsequent interpretation suggested that the two upper members were of shallow marine origin and that the proposed major Wenlock regressive event was already extant by the time of deposition of Member 2. In this paper we examine Member I to ascertain its environment of deposition and thus refine the sea level curve for this element of the Silurian.Member 1 may be divided into five lithofacies A to E. Facies A comprises amalgamated sandstones with low-angle cross-stratification. Facies B consists of couplets of erosively based coquinas containing a lowdiversity parautochthonous fauna and graded, laminated sandstones. These couplets are interbedded with thin laminated siltstones. Facies C is characterized by parallel laminated siltstones and discontinuous, erosively based sandstones. Facies D consists of fossiliferous siltstone and Facies E of thin sandstones with low-angle cross-lamination. Facies A and B are interpreted as shoreline deposits based upon comparison with published descriptions of such deposits elsewhere. Facies, C, D, and E, lateral equivalents of the first two facies, are interpreted as deposits which accumulated further offshore. The presence of couplets consisting of a coquina and overlying laminated sandstone, and fossil distribution, the presence of amalgamated beds and low-angle cross-stratification suggest the influence of storms on this shallow-water sequence. A palaeoreconstruction is proposed which envisages a shelf deepening to the southeast off a shoreline which had an approximately northeasterly orientation.
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