Discussion on Palaeoecology and Sedimentology Across a Jurassic Fault Scarp, NE Scotland

Journal of the Geological Society, London, Vol. 151, 1994, pp. 729-731, 1 fig. Printed in Northern Ireland Discussion on palaeoecology and sedimentology across a Jurassic fault scarp, NE Scotland Journal, Vol. 150, 193, pp. 323-340 J. R. Underhill writes: Intheir recent paper, Wignall & additional aspect of the recognition of this important Pickering (1993) provide an excellent analysis of the structural feature which can be matched with Allt na Cuile sedimentological and palaeontological aspects of the Upper Sandstone deposition, regardless of whether a relay ramp or Jurassic exposures on the East Sutherland coast. Their work transfer fault zone interpretation is preferred, is thatthe clears up many outstanding problems relating to the area spatial relations between it and the Allt na Cuile outcrop and deserves wide recognition. However, there are a few pattern rules out signifcantstrike-slip motions on the minor points relating to the structural aspects of their study Helmsdale fault after theJurassic (Thomson & Underhill or to offshore correlations which may warrant qualification 1993; Underhill 1993). The only evidencefor limited or further clarification. post-depositional strike-slip iswell illustrated by the folds described by Wignall & Pickering (fig.17), which have been (1) Controls on Allt na Cuile Sandstone sediment interpreted to be the result of minor sinistral movement dkpersal. Wignall & Pickering contend that supply of the (Thomson & Underhill 1993). 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 (2) True sign$cance of the cemented microfault planes in the Helmsdalefault is supported by onshore mapping, little Allt na Cuile Sandstone. Wignall & Pickering rightly high- evidence exists foraconnecting fault between the two light the occurrence of what they term ‘cemented microfault strands which would be classified as a transfer fault zone. planes’ within the Allt na Cuile outcrop at Lothbeg Point Instead, field relationships could be taken to suggest that the (their fig. 10 and p. 330). However, it is worth documenting faults overstep with displacement apparently dying out on that these unusual phenomena represent excellent examples the more southerly strand such that a tip line probably exists of strain-hardened faults which are specific to high-porosity at its northeastern extremity.Consequently, the structural quartz arenites and are commonly referred to as ‘granula- relationship is more consistent with that of a relay ramp tion seams’ previously described by Aydin & Johnson than a transfer fault zone (Fig. 1) as suggested by Thomson (1983), Bevan (1985) and Underhill & Woodcock (1987). & Underhill (1993). Thatinterpretation would also bein An account of the probable genesis of such features has accordance with offshore observations in the Inner Moray been given by Underhill & Woodcock (1987) using examples Firth where there is a singular absence of linking transfer from the Permian of Arran. The structures’ resistence to faults and a close association between the source of linear weathering is a result of the close packing that characterizes sand-prone mounds and overlapping normal faults (Under- the central part of the fault zones and is not a consequence hill 1991a, b). Thisinterpretation for the Allt na Cuile of cementation, as many of the faults remain devoid of a Sandstone is also consistent with other studies which have binding cement. demonstrated the importance of relay ramps in controlling sediment supply and dispersal (Leeder & Gawthrope 1987, (3) The position of Eathie with respect to half-graben fig. 4; Roberts & Jackson 1991; Gawthorpe & Hurst 1993). geometry. Wignall & Pickering make the assertion that ‘all The structural configuration of the fault blocks and inter- previous studies have considered that the benthic environ- mediate relay ramp suggest that provenance for the Allt na ment deepened away from the foot of the fault scarp into Cuile Sandstone can be interpreted to have been along an deep-water black shale facies of the type seen in the arcuate path from an area to theWSW. Such a conclusion is mutabilis zone of Eathie’. Although they imply that previous also consistent with the units petrography, which is domin- workers (including myself) appeared to believe that Eathie ated by highly mature quartz arenite compositions. These was located in a hanging-wall depocentre, this is not so and, could have been derived from the reworking of submarine, like Wignall & Pickering, I also envisaged that the Eathie or exposed, mature Mesozoic and Palaeozoicsandstones area, like many other offshore fault block footwall highs, and quartz-rich Moine basement rocks from both local cul- was characterized by significant condensation and/or sub- minations which may have taken the fault blocks footwall marine (or perhaps in the cases of some fault blocks, oc- crest above the level of predominant shallow-water shelfal casional subaerial) erosion (Underhill 1991a, 1991b, fig. 13) deposition and from the adjacent eroding hinterland (Fig. similar to the model originally proposed for Greenland by 1). That clastic supply occurredduring the early Kim- Surlyk (1978a, b). meridgian (cymodoce-mutubilis) interval at all contrasts markedly with deposition in other areas of the North Sea (4) Thenature of the Beatrice Oilfield reservoir and probably reflects the importance of increased or re- section. Finally, in the last paragraph of their paper, Wig- newed tectonic activity on the Helmsdale fault in deriving nall & Pickering (p. 339) give the impression that ‘small sediment. radius submarine fans’ are developed in the Beatrice oil- In the light of recent debates in this Journal (Flinn 1992, field. Perhaps inadvertantly, they imply that this is the main 1993; Underhill 1993), itneeds to bestressed that an reservoir facies forthe field. It is not. As Linsley et al. 729 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/151/4/729/4890022/gsjgs.151.4.0729.pdf by guest on 02 October 2021 730 DISCUSSION Allt na Culpower Sron Rubha KintradwellAlltPt Choll LothbepCuileBridge Lothbep Pt Burn na Gaoith SW 4 J. J. I EROSION OF SEDIMENT AREA OF SHALLOW MARINE FAULT-SCARP QUARTZ-RICH CONDUIT DOWN SHELFAL DEPOSITION SLOPE APRON DEPOSITS STRATIGRAPHIES RELAY I. (KINTRADWELL DUE TO n CONTOURS OF BOULDER BEDS) FOOTWALL UPLIFT RELATIVE UPLIFT ON \ / FAULT BLOCK ’HIES t+ ++ + GRANITE + + +I PATHWAYS +++++ , \.I I / RAPID SHALE OUT OF BOULDER BEDS Fig. 1. Cartoon depicting the approximate palaeogeographic setting envisaged for Early Kimmeridgian(cymodoce-mutabilk ammonite zone) deposition along the Helmsdale Fault between Brora and Helmsdale. Although partsof the footwall to the Helmsdale Fault is depicted as being subaerially exposed, the predominant clastic supply was derived from adjacent eroding hinterland and sheda across narrow submarine shelf. Only limited clastic supply resulted from subaerial,or the more probable submarine, erosion of local culminations which lay above the level of predominant shallow-water shelfal deposition along the fault block’s footwall crest. (1980) and Stephen et al. (1993) have demonstrated,the developmentshould have been at a minimum. Onthe facies making up the main reservoir are older (Early-Mid- contrary, the Allt Choll Breccia is clear evidence of the Jurassic) and consist of various non-marine to shallow ma- close proximity of a major fault scarp; we therefore prefer rine environments. A submarine fan setting is totally absent to suggest the presence of a transfer fault zone. in the reservoir section. Underhill notes that the sediments of the Allt na Cuile 19 April 1993 Sandstone were ‘focused along and down the relay ramp’. However this cannot be held as evidence for a relay ramp, as the sediments could equally well have been supplied P. B. WignaN & K. T. Pickering reply: We thank Underhill across a transferfault zone, possibly when longshore for his interestand comments on our paper and address sediment transport pathsintersected this step in the them in the order he discussed them. Helmsdale Fault scarp (e.g. Wignall & Pickering 1993, fig. 21). (1) Transfer fault versus relay ramp. Field mapping reveals a 500 displacementm in the line of the Helmsdale (2) We thank Underhill for the discussion and clarification Faultbetween Allt Choll and Allt na Cuile (Wignall & of the origin of the granulationseams which we only Pickering 1993, fig. 10). Asthere is nooutcrop in the mentioned in passing in our paper. intervening ground, circumstantial evidence must be used to deducethe nature of this offset. Underhill’s preferred (3) One of the conclusions of our palaeoecological study at alternative of azone of flexural accommodation (a relay Eathie was that benthic oxygen levels were slightly higher ramp) implies that the fault scarps of the Helmsdale Fault herethan at contemporaneous sites around Helmsdale, gradually died out into this zone. Our preferred alternative presumably because water depths were shallower. We are has a fault linking the two segments, implying the presence not aware that this information has been documented of a continuous but stepped fault scarp. before. Theoutcrops on the foreshore at Kintradwell contain numerous boulders and they were clearly deposited close to (4) We statedthat ‘small radiussubmarine fans’ are an exposed footwall. There is no evidence thatthe fault developed in the Beatrice oilfield but we did not state that scarp was dying out towards a tip line a mere kilometre to these fans were therefore the reservoir facies of this field. the northeast, as Underhill suggests. More importantly for the competingmodels are the outcrops of the Allt Choll We would also like to comment on Underhill’s alternative Breccia at Allt Choll. This is the most spectacular boulder diagram for the Kimmeridgian of the Helmsdale region. bed in the entire coastal section with angular clasts of quartz Underhill’s block diagram is drawnfor ageneral time arenite up to 3 m across. Lithologically these boulders are interval whilst our diagram is a temporal as well as spatial indistinguishable from the mid-Jurassic sandstones seen in depositional model.

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