A marine geological survey off the north-east coast of (western ) 335

SMITHSON,F. 1942. The Middle rocks of Yorkshire: a petrological and palaeogeographical study. Q. Jl. geol. Soe. Lond. 98, 27-59. STamp, A. H. I959. On the origin of the Dogger Bank, in the North Sea. Geol. Mag. 96, 33-44. S~RTON, H. H. & K~tcr, P. E. I949. The of . Lincolnshire Natural History Brochure No. x. Lines. Nat. Hist. Union. VAN DER GRACHT, W. A. J. M. VAN W. x938. A structural outline of the Vari~can front and its foreland from south-central England to eastern Westphalia and Hessen. Compte Rendu, 2nd Congr. Stratigr. Carbonif. Heerlen. 3, I485-I565. Vr.asEY, H. C. I929. The tectonic structure of the Howardian Hills and adjacent areas. Proc. Yorks. geol. Soc. 2x, I97-227. I93I. Saxonian movements in East Yorkshire and Lincolnshire. Proc. Yorks. geol. Soc. 22, 52-8. t937. The Tertiary history of East Yorkshire. Proc. Yorks. geol. Soe. 23, 3o2-x6. I948. The structure of east Yorkshire and . Proc. Yorks. geol. Soc. 27, 173-9 I.

Submitted 14th December i968; revised manuscript received 19th October x97o; read I5th October I969.

Richard Vernon Dingle, PH.D., F.G.S., Marine Geology Section, Institute of Oceanography, University of Cape Town, Rondebosch Cape, South Africa.

DISCUSSION Dr P. E. KENT commented on the extent and importance of the mapping demon- strated by Dr Dingle. In an area roughly the size of Yorkshire the author had produced a good reconnaissance map using a range of methods with very limited financial resources, under physical conditions which were often difficult and frequently most uncomfortable. The paper represents a notable achievement; Dr Dingle, Professor Donovan and the University of Hull deserve warm con- gratulations. He (the speaker) felt that the major features required a comment. In North Yorkshire the Cleveland brachydome is terminated eastwards by the north-south crossfold of Eskdale, but the trend is resumed further east at Robin Hood's Bay (reference map by F. D. S. Richardson in Lees & Cox Q. Jlgeol. Soc. I937, Fig. 2). Offshore the broad Scarborough dome continues the trend, as Dr Dingle had shown; this also is limited towards the east by a north-south complication (the Phillip's 'anticline'), and it would seem--with some knowledge of the deep struc- turewthat the broad gentle Fox-Strangways structure doming the Chalk to the east and south-east should be regarded as part of the same broad swell. It is thus a continuing although possibly complex feature swinging south-eastwards parallel to the Flamborough fault line. The latter feature--the Flamborough line--was marginal to the Author's area and, as he stated, marks the boundary between the East Midland Shelf and the more mobile Cleveland area. His comments can be supplemented from information obtained in the course of hydrocarbon exploration. The line has been found to continue some 40 miles eastwards into the North Sea, and then to swing south- eastwards to join the complex Rhenish fault belt, flanking a relatively stable area continuous with the London-Ardennes platform. To the west in Yorkshire this 3

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fault belt is aligned with the Craven fault belt with its major controlling effect on Carboniferous sedimentation. To the east, in the North Sea, it corresponds with a change in structural pattern in the pre-Permian floor and controls the western edge of the main Zechstein salt basin and the position of the Sole Pit Trough, in which the Jurassic is abnormally thick. The first line of piercement salt structures lies just south-west of the line, being developed by flowage from the belt of over-thickening immediately to the east of it, as Brunstrom & Walmsley have shown (A.A.P.G. I969 p. 876 ). Later movements involved severe crushing in post- times, as the Flamborough cliffs show. Some of the data bearing on this are included in a paper by P. J. Walmsley and the speaker, currently in the press; it is clear that the structural line is one of the fundamental features of the geology of northern England and the North Sea. The corollary from this comment is that East Yorkshire provides an exposed sample of a major North Sea structural belt, and that the investigation of the North Sea hydrocarbon prospects will have its feed-back in our understanding of the land area. Finally, Dr Kent asked the Author whether the northward continuation of the Peak Fault trend from the North Yorkshire coast to flank the Upper Jurassic outcrops of the Mallard Basin east of the Tees was associated with contemporaneous thickening comparable to that in the Lias, or with any unusual facies development.

The AUTHOR thanked Dr Kent for his kind and interesting contribution. Dr Kent's comments on the Flamborough fault line were of particular interest. The fact that this feature can be stated to extend about 4 ° miles seawards before swinging to the south-east would seem to preclude the possibility that the NW-SE, NNW--SSE Hudleston and Phillips fault zones themselves extend southwards to form the eastern boundary of the East Midlands Shelf: a hypothesis which the Author had previously fancied. In attempting to reply to Dr Kent's question regarding thickness and facies variations across the northerly extension of the Peak Fault trend, the Author had, unfortunately, little data with which to work; only three samples were collected from the Middle-Upper Jurassic outlier in the Mallard Basin. Certainly, there was a greater thickness of Middle-Upper Jurassic rocks to the east of the fault line in the Mallard Basin (at least 8 7 milliseconds one-way-time in the east, and only about 15 milliseconds in the west), but these differences could be adequately accounted for by postulating post-faulting erosion, and the likelihood of contem- poraneous thickening could not, with the data available, be assessed. With regard to facies variations the evidence was again ambiguous. The two apparently older samples were both collected east of the fault line and were considered to be from the Deltaic Series. They did not appear to represent a facies differing to any marked degree from that found on land, but as the age determinations were based upon this similarity in lithology the possibility of a circular argument existed. The third sample: a soft oolitic limestone, was also collected from east of the fault line, but from an apparently higher horizon. It probably either

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represented an oolitic facies within the Middle Jurassic or a northerly extension of the Corallian facies. In either case it was taken from the most northerly proven outcrop of an oolitic limestone facies within the Middle-Upper Jurassic succession of the Cleveland Basin. Whether its occurrence had any connection with con- temporaneous facies control by the Peak Fault line, however, was a question whose answer must await more detailed sampling of the Mallard Basin area.

Professor W. D. GILL agreed that the paper was an important contribution to our knowledge of the surface geology of the North Sea. The division of this area, however, into discreet tectonic elements depended on the sub-surface development which tended to become more and more complicated in depth and which, of course, could only be studied from seismic records and the results of deep drilling. It seemed unfair that the Author had had no access to at least some of the conclusions which had been arrived at from its now very considerable array of sub-surface data. The speaker had to disagree with the delimitation of the edge of the Midlands shelf as shown on the Author's diagrams as structures palpably due to salt flowage extended much further south-westward than shown. Along the edge of this shelf the salt structures were flat pillow-like bodies with very little surface expression. The Author's attempt to project Jurassic land isopach data into the North Sea was somewhat meaningless in view of the known highly variable development of Jurassic rocks due to the influence of structures like the Market Weighton Uplift.

The AUTHOR concurred with Professor Gill that the non-availability of all, with one or two notable exceptions, of the data from recent deep drilling and seismic operations in the North Sea militated against the presentation of an accurate picture of the geology of this interesting area. In any event it would be no doubt some time before all the complexities were understood, and an even longer time before the data became available to the majority of non-commercial geologists. It was to be hoped that a synthesis attempted with the limited data at the author's disposal would prove to be, however incomplete, of some value to interested geologists not fortunate enough to be in the oil companies' confidence. The question of the delimitation of the eastern edge of the East Midlands Shelf had already been raised by Dr Kent, although the Author was under the impression from Dr Kent's statement and from Brunstrom & Walmsley (I969) that this boundary lies, if anything, further east of the line shown on the Author's Figure 7, and not much further south-westwards as stated by Professor Gill. Salt structures west of this line were, if the Author understood Dr Kent correctly, intruders which had transgressed over the edge of the East Midlands Shelf area from the English Zechstein Basin. As pointed out by Professor Gill, attempts to project Jurassic land isopach data into the North Sea were fraught with difficulties, but, nevertheless the Author felt that Figure 6 and the table setting out thickness variations for the whole of the Jurassic serve some purpose in summarizing the land borehole data and the published offshore results.

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Author's note added in press Additional oil company data on the area studied by the writer have been published in a review by Kent & Walmsley (197o). In particular, the succession in Burmah's 42/23-1 bore was included. This well lies on a subsidiary structure on the eastern flank of the Hudleston Dome, to the south of the Phillips Fault Zone, in an area where the writer has mapped Kimmeridge Clay beneath Quaternary sediments. Kent & Walmsley (op. cit. fig. 3) indicate the youngest pre-Quaternary strata as being Upper/Middle Jurassic in age. They are about 7oo ft thick and overlie about 15oo ft of Lower Jurassic and Rhaetic. The writer has meagre evidence in this area because of correlation difficulties associated with nearby faulting, but Sparker evidence suggested about 2o-4o milliseconds (15o-3oo ft) of Kimmeridge Clay in the vicinity of the Burmah well. The rather thick Jurassic succession proved by 42/23-1 suggests that the location lies in a north-west extension of the Sole Pit Trough. Interesting data were presented on the age of the Tertiary strata. Total's well 44/2 I-I (about 60 miles ESE of 42/23- I) shows about 300 ft of Tertiary sediments overlying the Upper Cretaceous, but no additional age data were given. Further south, off the north-east Norfolk coast, a progressive westward younging of the basal Tertiary is noted (op. cit. fig. I) ; in Gulf53/IO-I the Tertiary is about 2 IOO ft thick with a basal Eocene unconformity, and in Conoco/NCB 49/22-I (about 4 ° miles to the north-west) there are about 3oo ft of Pliocene resting on Upper Cretaceous. This evidence, though far to the south of the area studied by the writer, suggests that the age of the basal Tertiary strata youngs towards the western margin of the basin, and that Neogene sediments form the western feather edge of the main Tertiary unconformity. This has been postulated by the writer for the area off North East England.

REFERENCE KENT, P. E. & WALMSLEY, P. J. i97o. North Sea progress. Bull. Am. Assoc. Geol. 54, x68-Sx.

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