Gravel‐topped offshore bar sequences in the Lower Carboniferous of southern Ireland

Abstract

Within the Kinsale Formation (Lower Carboniferous) of southern Ireland are pebbly sandstones and conglomerates contained in what is known locally as the Garryvoe conglomerate facies. In this facies there are three main groups of lithologies: (a) heterolithic mudrocks and sandstones characterized by a wide variety of wave‐produced structures; (b) sandstones dominated by swaley cross‐stratification (SCS), parallel lamination, and rare hummocky cross‐stratification (HCS); and (c) pebbly sandstones and conglomerates occurring as discrete beds or as gravel clasts dispersed through SCS sets. Successions of the facies comprise units of heterolithic mudrock and rippled sandstone alternating repeatedly with coarsening‐upward units of SCS pebbly sandstone capped by top‐surface granule and pebble lags. The Garryvoe conglomerate facies accumulated in a system of offshore bars on a muddy shallow‐marine shelf that was dominated by waves and currents generated by storms. Sands and gravels were bypassed from a contemporaneous northerly coastal zone to the shelf, where they were moulded by the storm‐generated flow into low, broad, sand ridges (offshore bars). The elongate bars were spaced kilometres apart, oriented obliquely to the coast, and separated by muddy interbar troughs. Their surfaces were largely covered by hummocky and swaley forms. Long‐term, gradual seaward migration of the offshore bars concentrated gravels on landward flanks from the dispersed pebbly sands that were on the crests and seaward flanks. Exceptionally intense storms could form laterally extensive winnowed gravel lags above thinned bar sequences. Such storms could also flush gravel‐bearing turbidity currents into muddy interbar trough areas.

Facies analysis and sequence stratigraphy of neoproterozoic Platform deposits in Adrar of Mauritania, Taoudeni basin, West Africa

“…However, the very high sandstone/siltstone ratio, the coarser and thicker character of sandstone beds, and the common occurrence of trough cross-stratification in facies Wm3 indicate a more energetic and proximal environment than in facies Wm2. The relatively low amplitude and large wavelength of hummocks and the development of synforms (swales) at the expense of hummock antiforms also suggest shallower water depths (Leckie and Walker 1982;Tillman 1985;Cotter 1985;Allen and Underhill 1989). In addition, the fact that the structures associated with HCS beds are wider and less prominent for the gutter casts and of smaller scale for the polygonal interference ripples is also indicative of a proximal trend (Guillocheau 1983).…”

Late Ordovician sand-wave complexes on Anticosti Island, Quebec: a marine tidal embayment?

“…A2 in Appendix I).' The first two cycle types, involving mixed carbonate -siliciclastic facies, can be interpreted as shoaling sequences related to the migration of subtidal sandbanks or sand-wave complexes formed as detached offshore bars (Swift and Field 1981;Brenner et al 1985;Cotter 1985): the third type represents shoaling by progradation of an open carbonate platform.…”

“…Strata interpreted as subtidal sand-wave complexes have been described from siliciclastic (Hobday and Reading 1972;Berg 1975;Anderton 1976;Johnson 1977;Cotter 1985), mixed (Brenner andDavies 1973, 1974;Bradshaw and Fenton 1982;Morton 1983), and carbonate environments (Teyssen 1984). Like the Anticosti examples, these and other deposits cited by Johnson (1978), Stride (1982), and Walker (1984) commonly exhibit a coarsening-upward sequence grading from mud to coarse sands with thicknesses of 2-15 m.…”

Sedimentology and structure of the Malvern Hills Coal Mine, Canterbury, New Zealand

“…The sand-dominated lithotype is typified by flaser bedding (sand with some mud) and cross-stratification (Dalrymple 2010). Lenticular and flaser bedding are common for an intertidal environment such as mixed (sand-mud) tidal flats (Reineck 1975;Reading & Collinson 1996;Bhattacharya 2006;Dalrymple 2010), but also known from estuarine subtidal environments (de Raaf & Boersma 1971;Reading & Collinson 1996;Boyd et al 2006) and troughs between offshore sandbars (Cotter 1985;Johnson & Baldwin 1996;Suter 2006). In conjunction with the extensive development of tidal indicators is evidence of fining-upwards sediment packages, particularly in the lower Broken River Formation.…”