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Tectonic Evolution of Devonian Basins in Northern Scotland and Southern Norway

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Tectonic Evolution of Devonian Basins in Northern Scotland and Southern Norway Tectonic evolution of Devonian basins in northern Scotland and southern Norway MIKE G. NORTON, KEN R. McCLAY & NICK A. WAY Norton, M. G., McClay, K. R. & Way, N. A.: Tectonic evolution of Devonian basins in northern Scotland and southern Norway. Norsk Geologisk Tidsskrift, Vol. 67, pp. 323-338. Oslo 1987. ISSN 0029-196X. The geometries of the Devonian basins of northern Scotland and southern Norway are analysed to determine the regional tectonic control on their formation. The Orcadian Basin in northern Scotland consists of distinct half-graben sub-basins controlled by separate, closely spaced, sub-parallel arcuate extensional fault elements which are parti y con tro lied by the Caledonian crustal fabric. The unconformity at the base of the middle Devonian in this area is interpreted as being formed by a sudden increase in the rate of extension. In southern Norway, in con trast, extensional structures are more widely spaced and larger in scale. Very deep (>8 km) half-graben basins were developed of which only the bottom parts are now preserved. In the Devonian of Norway the Jack of a major stratigraphic break suggests a constant extension rate. The observed differencesin style are thought to be due to different initial crustal thicknesses. M. G. Norton, K. R. McCiay and N. A. Way, Department of Geology, Royal Hol/oway and Bedford New College, Egham, Egham Hill, Surrey, TW20 OEX, U.K. The broad outlines of the Devonian 'Old Red main aspects have been studied; the pattern of Sandstone' basins of northern Scotland, western basin-forming faults and the significance of the Norway and the northern North Sea have been unconformity between lower and middle known for some time (e.g. Ziegler 1982) (Fig. Devonian. In definingthe basin-forming fault net­ la). The tectonic setting of the Devonian sedi­ work, use has been made of the work of Enfield ments has been variously interpreted as related & Coward (1987) in the West Orkney Basin. to megashears (Ziegler 1982; Bukovics et al. 1984) Fieldwork has been carried out in the region or regional extension (Dewey 1982; McClay et al. bordering the Moray Firth and in Caithness, 1986) (Fig. 1). Large basins like the Orcadian Orkney and Shetland. lnterpretation in the Basin of northern Scotland consist of many similar Mora y Firth itself has been carried out using data sub-basins each exhibiting separate tectonic con­ provided by Fina (UK). In the offshore area tro! and distinct sedimentation and subsidence around Shetland the interpretation is based on patterns. This paper provides a detailed analysis the BIRPS SHET survey (McGeary 1987). of some of the individual Devonian sub-basins The identification of faults active during and uses the results to constrain a general model Devonian sedimentation is based on observed or for Devonian tectonics in the north west European inferred faults which correlate well with major continental margin. thickness and facies variations, and published The analysis has been carried out by: the sediment dispersal patterns (Mykura 1983). On reinterpretation of published onshore data, some of the published geological maps faulted fieldwork at various critical localities, the inter­ contacts between Devonian sediments and pretation of offshore geophysical and well data basement are shown to pass laterally into uncon­ together with available structural and sedi­ formable contacts; this is particularly so along the mentological data. north western boundary of the Devonian between Inverness and Helmsdale and in east Shetland. In Northern Scotland most cases unconformity has been inferred along unexposed contacts in which the interface is seen North of the Highland Boundary Fault, Devonian to dip at an angle of less than 45°. Now that sediments define the approximate extent of the the existence of low-angle extensional faults has 'Orcadian Basin' (Fig. 2). Within this region two become recognized such contacts can be con- Ul � HBF - Hlghland Boundary Faun � GGF - Great Glan Faun MF - Mlnch Faull WBF - Wa11a Boundary Faull � MTFZ - More-Trøndelag Fault Zone BF - Blllel)ordafl Fault � � � � � i � () . � ".. X - Wldeapread h.alf-grabena � OEVONIAN - Conaittant eKtenslon direcllonl OISTRIBUTION OF SINISTRAL MEGASHEARS. � . � � SEDIMENT$. - Oreateat eJttenaion in the zone "z o o of greatest cruatal thickening .. ;;; · - Localised øun-aøart basins !:3 · · · �lA-._�. · - Varyitlg extenaion directions �8 .._._.·. · ·J···• • .. ·,· · . aub-parallet to taults .. · · . � ��. �0"�.,o� � . æ er "� ..�� ;:: !3 zz o o ,: -�· .{" _)'J ,&.: �j·/ .-/ z o � V> ,/ �\ ::-: o m V o t""' o O 200KM. O 200KM. 0 200KM. '-----' '-----' a '---' b c l V>8 ::-: ::l o V> Fig. l. Northeast Atlantic region in pre-rift configuration showing (a) Distribution of Devonian sediments of northern Scotland, southern Norway, East Greenland and Svalbard, V> ::-: (b) Sinistral Megashear Hypothesis, and (c) Extensional Collapse Hypothesis. (b) and (c) also indicate expected basin characteristics from the two models. � O>� ...... ....._ � NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 325 fidently reinterpreted as tectoni'c. In some cases of a significant strike-slip component in the fracture sets are developed sub-parallel to the regional tectonic setting; the Great GJen Fault contact with individual fractures showing exten­ itself has no clear expression in either the sional displacement providing supporting evi­ observed fault network or in the pattern of sedi­ dence for the interpretation of these boundaries mentation (Rogers 1987). However, the inter­ as extensional faults. pretation of widespread unconformities at the The fault pattem established by this study is base of the middle and upper Devonian suc­ shown in Fig. 3. This has been constructed by cessions in the Orcadian Basin has been used assuming the following overall strike-slip move­ as evidence for rnajor periods of upliftwhich have ments since the lower Devonian: Great GJen in turn been used to imply periods of transpres­ Fault - 20 km dextral (Rogers 1987), Walls sion. Boundary Fault - 75 km dextral (Astin 1982), The unconformity at the base of the middle Nestings Fault - 15 km dextral (Astin 1982); no Devonian is best exposed on the southern shore allowance has been made for movement on either of the Moray Firth in the excellent coastal the Helmsdale or Melby Faults whose movements exposures at Pennan (Trewin 1987) within the are insufficientlyconstrained. The overall pattem Turriffhalf-graben sub-basin (Fig. 2). At Pennan would not be greatly affected by movements on lower Devonian conglomeratic sandstones dip these faults of 50 km or less. Three major faults moderate! y steeply (ca. 40°) towards the west and incorporated into the network were also impor­ are overlain by shallowly dipping middle tant structures during the Mesozoic development Devonian conglomerates with sharp angular of the Inner Moray Firth Basin; the Smith Bank unconformity, this is the on! y clear example of Fault, the Lossiemouth Fault and the Buckie angular unconformity in the Orcadian Basin, else­ Fault. The evidence for an earlier phase of move­ where it is marked only by a facies change (Rogers ment on these faults comes from seismic inter­ 1987). The base of the middle Devonian is irregu­ pretation of Devonian sediments in half-grabens lar and strongly erosive. The lower Devonian on their hanging walls and fromwell data showing is affected by a series of small-scale faults with age and facies variations across the faults. Fig. 4 displacements of a few cms up to a few metres shows a generalized geoseismic section across two which do not cut the unconformity and clearly of these faults taken from commercial seismic predate this erosional event. All of the fault zones data (the position of the section is shown in Fig. are extensional in geometry, most of them dipping 2). towards the east. This is the only deformation, One of the most interesting aspects of the fault for which there is evidence, that can explain the network deduced in the Orcadian Basin is the rotation of the lower Devonian strata. The fault presence of several arcuate fault elements which zones themselves are rather diffuse zones in which define the separate sub-basins. These have a no fault rock is developed and appear to have characteristic aspect ratio and size with a strike been formed in only part-consolidated sediments. extent of ca. 120 km. These arcuate fault elements We interpret this unconformity not as a period of are very similar in scale to those observed in the general uplift but as a period of rejuvenation Lake Tanganyika region of the East African Rift along pre-existing extensional faults probably (Rosendahl et al. 1986). It is not clear, however, with the initiation of some new extensional struc­ how all of these faults link together either along tures. In the Turriff region this rejuvenation led strike or at depth. to the progradation of alluvial fans from the new The observed arcuate fault geometry implies fault scarps eroding down into the previously that along much of the fault lengths there is a deposited and on!y partly consolidatedsediments. significant strike-slip component to the dis­ This fits well with the interpretation by Rogers placement. At the ends of some of these faults (1987) of a major change in the drainage pattern the displacement must be almost pure strike-slip at this time. It is possible that both the small-scale in character. The orientation of these high! y faulting and the fan progradation date from this oblique segments can be used to constrain the same extensional pulse. Fig. 5 shows a schematic overall extension direction as approximately cross-section over the Turriff half-graben at end northwest-southeast trending (see also Enfield & middle Devonian times to illustrate the proposed Coward 1987). interpretation. The recognition of the middle The overall structural frameworkshows no sign Devonian basal unconformity throughout the 326 M. G. Norton et al. NORSK GEOLOGISK TIDSSKRIFT 67 (1987) KEY UNST W � Mesozoic U.
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