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ABSTRACT

The Jura Quartzite, a formation of probably late Precambrian metasediments over 5 km thick from the Caledonian belt in Southwest Scotland, has been divided into a coarse and three fine facies. The former comprises cross-bedded sands with some laminated sands and silt horizons, interpreted as the deposits of shallow marine tidal dunes and other bedforms together with some beach units. Deposition from suspension of silt and sand formed climbing dunes while largescale erosion produced flat or channelled surfaces. The fine facies comprise laterally persistent, parallel and cross-laminated sand units from millimetres to decimetres thick, interbedded with muds. The coarse and fine facies can be finely interbedded, the former sometimes filling decimetre deep, straight channels, cut in the latter. The fine facies exhibit structures indicative of deposition from decelerating currents and are interpreted as shallow marine storm deposits.

The facies are compared with a model developed from published observations on modern shelf areas. Zones of erosion, large and small dunes, flat bedded sand and mud are considered to be the end product of a wide spectrum of tidal and storm conditions. During severe storms the fair weather tidal dunes may be modified or washed out, new dunes may be initiated downcurrent of the normal dune field while storm-sand layers are deposited in the distal zones. Hence, the nature of the preserved sediment blanket reflects the rare severe storm event rather than normal tidal conditions.

The Jura Quartzite was deposited in a tidal gulf intimately connected with an ocean basin. The north-northeast directed palaeocurrent modes are probably roughly parallel to the coastline.