Discussion on Triassic sediments of the Inner Moray Firth, Scotland: early rift deposits

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Abstract
A. M. Roberts , J. D. Price & M. E. Badley write: We have recently completed a structurally-based evaluation of the Inner Moray Firth basin, shortly to be submitted for publication, and read with interest the account of Frostick et al . (1988) describing its Triassic history. While we have no grounds on which to question their detailed sedimentological interpretation, we would challenge the way they have used sedimentological, seismic and other data to conclude that, during the Triassic, the entire Inner Moray Firth basin was a single ‘simple half-graben’, bounded to the NW by the ‘dip-slip’ Great Glen Fault. Of the points which could be raised in discussion we believe the following are the most pertinent. (1) Much of their half-graben interpretation is summarized in fig. 2, which shows successive restoration to a late Triassic configuration. Disregarding the interpreted ‘detachment’ seen on their present-day section, which is not shown on the other sections, we would question two aspects of their interpretation. (a) The Great Glen Fault is illustrated as an east-dipping, listric fault. This contradicts the evidence of all seismic data across this structure (cf. McQuillin et al. 1982, fig. 4) which show it to comprise two near-vertical fault strands. It is the very nature of this vertical, strike-slip boundary which distinguishes the western margin of the Inner Moray Firth basin from the western margin of the Viking and Central Graben. (b) Frostick et al.’s end-Triassic reconstruction shows the ‘top-Trias marker’ defining a roll-over into the Great Glen Fault. As they