A Buried Granite Batholith Beneath the East Midland Shelf of the Southern North Sea Basin

Journal of the Geological Sociely, London, Vol. 147, 1990, pp. 133-140, 6 figs. Printed in Northern Ireland A buried granite batholith beneath the East Midland Shelf of the Southern North Sea Basin JOHN A. DONATO' & JOAN B. MEGSON2 'Goal Petroleum plc, New Bond Street House, 1 New Bond Street, London W1Y 9PE, UK 2Maersk Olie Og Gas AS, 50 Esplanaden, DK-1263 Copenhagen K, Denmark Abstract: Published BGS gravity data of the UK Southern North Sea Basin show the presence of a gravity low trending to the ESE across the northern half of Quadrant 47. Two simplified geological cross-sections have been constructed across this feature with sedimentary structure based upon well information and reflection seismic data. Gravity modelling of these sections demonstrates that the gravityanomaly is unlikely to beproduced by the presence of thick, low densitysediments. Consequently, the presenceof a large, buried granite batholithis postulated. The buoyant influenceof such a granite may have exerted considerable tectonic control during the developmentof the Southern North Sea Basin. In particular, its presence may help to explain the development of the East Midland Shelf, the associated complex fault movements along the FlamboroughHead and DowsingFault Zones, and the area1 distribution and thickness of the Rotliegendes sand. Important zones of faulting occur along the Dowsing Fault In the offshore area, the Carboniferous sequence is not Zone and its onshore extension, the Flamborough Head clearly defined on reflection seismic data and consequently Fault Zone. These zones have been intermittently active the tectonic development during this period is not well throughout a long period of geological time,and clearly understood. The analogy with the onshore area to the west, represent an important lineament which separates the more where the sequence is in part exposed, suggests that stable block of the East Midland Shelf from the tectonically Carboniferous deposition commenced with an early, active areas of the Sole Pit Trough and the Cleveland Basin. fault-controlled,differential subsidence phase, driven by The acquisition of extensive seismicreflection data and crustal stretching. This was followed by a more gentle phase borehole information resulting from the exploration for gas of regional subsidence during which numerous interbedded in the SouthernNorth Sea Basin has revealed, in some coals were deposited. Towards the end of the Car- detail, the major and complex tectonic movements which boniferous, Variscan compression caused widespread uplift have occurred along this important lineament. The and an associatedunconformity eroded deeply intothe integration of these data with gravity field measurements are Carboniferous sequence. A period of subsidence followed, presented hereand have assisted in understanding the causedby faulting dueto tensional or trans-tensional tectonic evolution of this complicated area.In particular, stresses and at this time, the Permian Rotliegendes desert the reason for the division of the sedimentary basin into the sandstone was deposited. This sandstone now forms the stable and less stable areas, either side of this fault zone major reservoir rock for gasin the Southern North Sea lineament, may now be understood in terms of the tectonic Basin sourced by the underlying Carboniferous coals. control of a buried, massive granite batholith located Widespread transgressionsflooded the subsidedbasin beneath the northern part of the East Midland Shelf. during the late Permian, resulting in the deposition of the Zechstein evaporite cycles.Whilst the early Permian subsidence and associated deposition was in response to Geological and structural review crustal stretching, these late Permian evaporites appear to Figure 1 shows a simplified version of the solid geology of have been deposited during a tectonically quiet period with the onshore and offshore area (Sub-Pleistocene Geology of little contemporaneous faulting.Similarly, the overlying the BritishIsles andthe AdjacentContinental Shelf, earlyTriassic sequence, whichconsists of desert lake 1 :2,500,000, BGS, 1979) with the location of the -20 mGal sediments (Bunter shale and sandstone), was deposited with Bouguer Anomaly gravity contours superimposed (BGS little evidence of significant faulting. A period of local Bouger Anomaly Maps 1:25O,OOO Series, Tyne-Tees, faulting and slight uplift of basin margins occurred during California, Humber-Trent andSpurn sheets). As may be the late early Triassic in association with the development of seen, rocks varying in age from Permo-Triassic to Tertiary a minor unconformity. This was followed by further gentle are exposed. To the north of the Flamborough Head and subsidence, during which time theRot, Muschelkalk and the Dowsing Fault Zones lie the thick sedimentary Keuper halites were deposited. successions within the Cleveland Basin and Sole Pit Trough. During the Jurassic, a marked differentiation occurred To the south lies the more stable platform area of the East between the rapidlysubsiding Sole Pit Trough and Midland Shelf. The geological evolution of thearea is Cleveland Basin, and the slowlysubsiding East Midland complex and is described extensively in the literature (e.g. Shelf. Differing subsidence patterns were accommodated by Glennie & Boegner 1981; Glennie 1986; Kirby & Swallow significant, tensional and/or trans-tensional faulting particu- 1987; Kirby et al. 1987). Only a brief and simplified review is larly along the two major fault zones. The rapid subsidence attempted here. of the basinal areas, initiated during the Jurassic, continued 133 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/147/1/133/4890723/gsjgs.147.1.0133.pdf by guest on 24 September 2021 134 J. A.DONATO & J. B. MEGSON ('W 0' Fig. 1. Simplified solid geology map of the onshore and offshore area under consideration. The-20 mGal contours along a pronounced gravity low trend and the locationsof areas of complex tectonic activity along the Flamborough Head Fault Zone and the Dowsing Fault Zone are shown. The Cleveland Basin and Sole Pit Trough are areasof thick sedimentation affectedby inversion l*E movements. into the early Cretaceous, although a widespread unconfor- contour interval is 2 mGal and the area covered by the map mity was developed at the Jurassic/Cretaceous boundary. A is identical to that of Fig. l.. thick blanket of Chalk was deposited during theUpper Pronounced E-W and ESE-WNWgravity trendsare Cretaceous under essentially tectonically quietconditions. apparent. The most significant feature is a deep, negative This cover was subsequentlyremoved from the Sole Pit anomalyenclosing three minima, thedeepest reaching Troughand Cleveland Basin by amajor inversion phase approximately -30mGal. The western minimum has been which reached a climax towards the end of the Cretaceous. interpreted by Bott et al. (1978) and Bott (1988) as being Onthe western flank of the Sole Pit Trough, however, produced by a buried granite body, the Market Weighton inversion movementshad commenced during the early Granite, although Aveschough (1986) favours an interpreta- Cretaceous(Glennie & Boegner 1981). Total uplift tionin terms of a thick Carboniferous basin. The estimates of 1.5 km (Marie 1975; Cope 1986) and 2 km interpretation of Bott et al. is preferred here. The locations (Kirby et al. 1987; Bulat & Stoker 1987) have been proposed of three profiles across the low trend are also shown in Fig. for the Sole Pit and Cleveland Basins respectively. These 2. Profile XY is the line of section across theMarket movements caused a widespread erosional unconformity Weighton Granite interpreted by Bott et al. (1978). Profiles resulting in the exposureof Jurassic and Triassic strata along AB and CD are the locations of two sections modelled here. the axes of the Sole Pit and Cleveland Basins. Little is To the north of this gravity low, higher values trend in known of the Tertiary section in this area since, as Fig. 1 E-W and ESE-WNW directions and overlie the axes of the shows, only limited outcrops are presently observed in the Cleveland and Sole Pit Inversions. As discussed earlier, north-eastern cornerof map. these inversion axes areareas where thick sequences of It may be possible to summarize, in simple terms, the deeply buried sediments have been uplifted significantly. tectonic development of thearea as consisting of three This processhas resulted in anomalouslyhigh density major fault-controlled phases of sediment deposition. These sediments close tothe surface with correspondingly high occurred during early Carboniferous, late Carboniferous to sonicvelocities (Marie 1975; Cope 1986; Bulat & Stoker early Permian, and Jurassic to early Cretaceous times. An 1987), and this may explain the presence of the coincident additional, more minor phase also occurred during the early gravity high trend (Donato & Tully 1981). On the northeast Triassic. Sediments depositedat other periods, however, end of Profile CDa narrow gravitylow reaches were more epeirogenic innature. There were also three approximately - 18 mGals. This coincides with the location significant phases of uplift which occurred during late of a salt diapir seen on reflection seismic data.The low Carboniferous, early Cretaceous and late Cretaceoustimes. density halite within the diapir possibly explains the presence of this gravity feature. The two profiles, AB and CD, are shown in Fig. 3. The Geophysical data and models simplified structure is based upon an interpretation of A Bougueranomaly

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