Abstract

D. Soutter and J. S. Ritchie write: We have the following reservations regarding the model presented by Badley et al.

1. Whilst agreeing with Badley et al. on the existence of an at least early Triassic, probably Permian, ‘pre-rift’ unconformity, we question their interpretation of tilted fault blocks below this unconformity, on the evidence presented in their fig. 2, on two grounds:

(a) Badley et al. state that supposed sub-unconformity reflectors dip at a steeper angle than their picked unconformity. We question their picking of the unconformity downdip of the crestal structure west of fault B on fig. 2 and believe that they have deviated on to an onlapping reflector, thus making the dip of the unconformity appear less steep.

(b) We believe that the picks below the unconformity reflector, in particular those between shot-points 1400 and 1500, are in fact sea-bed multiples.

On the evidence of other seismic lines to which we have access, we think that the ‘pre-rift’ unconformity may well be faulted crystalline basement in this region (i.e. faulting of Permian age).

2. According to Badley et al.’s model, it is regarded as significant that Hauterivian sediments are found in what they term the Oseberg ‘back basin’, and yet coeval sediments are found west of the 30/6 structure towards the Viking Graben axis at a present-day structural level more than 1400 m lower. Further, it is suggested that migration of fault movement progressed eastward away from the graben axis, and that faults F and G of fig.

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