Abstract

J. R. Underhill writes: In their recent paper, Wignall & Pickering (1993) provide an excellent analysis of the sedimentological and palaeontological aspects of the Upper Jurassic exposures on the East Sutherland coast. Their work clears up many outstanding problems relating to the area and deserves wide recognition. However, there are a few minor points relating to the structural aspects of their study or to offshore correlations which may warrant qualification or further clarification.

Controls on Allt na Cuile Sandstone sediment dispersal. Wignall & Pickering contend that supply of the Allt na Cuile Sandstone during the early Kimmeridgian was via a transfer fault zone between two separate segments of the Helmsdale fault. Although an offset in the trace of the Helmsdale fault is supported by onshore mapping, little evidence exists for a connecting fault between the two strands which would be classified as a transfer fault zone. Instead, field relationships could be taken to suggest that the faults overstep with displacement apparently dying out on the more southerly strand such that a tip line probably exists at its northeastern extremity. Consequently, the structural relationship is more consistent with that of a relay ramp than a transfer fault zone (Fig. 1) as suggested by Thomson & Underhill (1993). That interpretation would also be in accordance with offshore observations in the Inner Moray Firth where there is a singular absence of linking transfer faults and a close association between the source of linear sand-prone mounds and overlapping normal faults (Underhill 1991a, b).This interpretation

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