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Inner shelf/shoreface-intertidal transition, upper pre-cambrian, Port Askaig Tillite, Isle of Islay, Argyll, Scotland
Kessler, L.G.; Gollop, I.G. (1988). Inner shelf/shoreface-intertidal transition, upper pre-cambrian, Port Askaig Tillite, Isle of Islay, Argyll, Scotland, in: de Boer, P.L. et al. (Ed.) Tide-influenced sedimentary environments and facies. Extended versions of papers presented at the Symposium on Classic Tidal Deposits, held August 1985 in Utrecht, Netherlands. pp. 341-358
In: de Boer, P.L. et al. (Ed.) (1988). Tide-influenced sedimentary environments and facies. Extended versions of papers presented at the Symposium on Classic Tidal Deposits, held August 1985 in Utrecht, Netherlands. D. Reidel Publishing: Dordrecht. ISBN 90-277-2622-1. ix, 530 pp., meer
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  • Kessler, L.G.
  • Gollop, I.G.
Abstract
    Near the top of the Upper Precambrium Port Askaig Tillite of western Scotland is a series of interbedded fine sandstone units up to 100 m thick which represent periods of shallow marine-intertidal deposition between and following episodic glacial advances. A particularly well-preserved sequence of this sandstone is observed north of Port Askaig on the Isle of Islay. Here there are at least three sequences that shallow upward from below fair weather wave base to an intertidal setting. A typical and well developed 14 m thick sequence begins at the base with silty-fine-grained sandstone units containing thin rippled and planar bedded zones and occasional cross-bedded shallow scour fills. Whilst individual beds thicken upward, low-angle truncations of bedding, apparent scours filled with parallel but thickening upward laminae and convex-upward cross-stratification are observed. These bedforms resemble various types of hummocky crossstratification which have been observed in shallow shelf or shoreface depositional settings. Immediately above these bedforms is a 0.2 m interval of desiccated clastic algal mats indicating periods of subaerial exposure and heralding intertidal depositional conditions. The remaining 6.4 m of this section consists of mostly cross-stratified scour fills and bedforms including cross-bedding indicative of both dominant and subordinate bipolar currents. This 14 m section has many characteristics of a rapid transition from a shallow shelf or shoreface to an intertidal setting. The bedforms observed in the lower 7.6 m of the section are strongly suggestive of deposition by dominantly oscillatory storm generated currents in a shallow shoreface setting. The upper 6.4 m of the measured section shows evidence of intertidal deposition with a range of environments from desiccated subaerially exposed tidal flats to accretionary bar filled flood and ebb channels. The strong bipolarity of foreset bed forms in these channels is suggestive of a depositional site near the boundary between separate flood and ebb channel systems. Possible tidal bundle sequences are observed in apparent neap-springneap tide cycles in some of these bed forms. An average number of 27.5 bundles per cycle fits closely with observation by workers in the modern Oosterschelde.
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