Devonian Extensional Tectonics Versus Carboniferous Inversion in the Northern Orcadian Basin

Journal of the Geological Society, London, Vol. 149, 1992, pp. 21-31, 14 figs, Printed in Northern Ireland Devonian extensional tectonics versus Carboniferous inversion in the northern Orcadian basin M. SERANNE Laboratoire de Gdologie des Bassins, CNRS u.a.1371, 34095 Montpellier cedex 05, France Abstract: The Old Red Sandstone (Middle Devonian) Orcadian basin was formed as a consequence of extensional collapse of the Caledonian orogen. Onshore study of these collapse-basins in Orkney and Shetland provides directions of extension during basin development. The origin of folding of Old Red Sandstone sediments, that has generally been related to a Carboniferous inversion phase, is discussed: syndepositional deformation supports a Devonian age and consequently some of the folds are related to basin formation. Large-scale folding of Devonian strata results from extensional and left-lateral transcurrent faulting of the underlying basement. Spatial variation of extension direction and distribution of extensional and transcurrent tectonics fit with a model of regional releasing overstep within a left- lateral megashear in NW Europe during late-Caledonian extensional collapse. Later inversion (probably during the Upper Carboniferous)is characterized by E-W to NE-SW contraction. It induced reactivation of extensional faults as thrusts,development of small-scale folds and thrusts, and right lateral transcurrent movement of the major faults such as the Great Glen and Walls Boundary faults The Old Red Sandstone of the Orcadian and northern Scot- Coward 1987). These basins form a group of collapse-basins land basins (Fig. 1) was deposited in an extensional setting (Seguret et al. 1989) that developed in NW Europe during late- duringthe Middle Devonian (e.g. Mykura 1976; Enfield & orogenic extensional collapse of the Caledonian orogenic belt (McClay et al. 1986). Later inversion of these Scottish basins resulted in theformation of folds within the Old Red Sandstone and reactivation of basement faults (e.g. Coward et al. 1989). A consensus relates theLate Palaeozoic tectonic SHETLAND 100 km In evolution of that area to the kinematics of the Great Glen fault which is amajor tectonic boundary (Flinn 1977). Attempts to decipher directions of extension and subsequent compression have been based on fold geometry (Mykura 1976), orientation of faults and associated lateral ramps (Coward & Enfield 1987), or by the kinematics of the Great Glen fault (Norton & Way 1991). However, the history of movement and amount of offset of this trancurrent fault remain controversial (see reviewin Rogers et al. 1989). In particular, folds and faults may develop oblique to deformation axes, and changes in orientation of the stress field close to major discontinuities may prevent simple approximations of directions of extension or contraction. Field observation and mapping of repetitive small scale structures within the Old RedSandstone of Caithness,the Orkney and the Shetland islands, and along its contacts with basement, together with analyses of published structural and solid geology maps, has led to a re-interpretation of the structures in this area. This paper aims to show that some of the structures previously attributed to compressional tectonics in the northern Orcadian basin were probably formed earlier, during extension. Reappraisal of these structures will eluci- date the kinematics of both the Middle Devonian extensional event and the Carboniferous inversion phase. Devonian extensional structures Orkney and Caithness Fig. 1. Map of the main structural features of Northern Scotland. Enfield & Coward (1987) and Enfield (1988) have presented Data from Coward et al. (1989) and BGS 1:250000 Solid Geology evidence for Middle Devonian extensional tectonics in the off- Maps of Shetland, Orkney, Caithness. WOB: West Orkney Basin. shore West Orkney Basin (Fig.1). Sediments of Middle 27 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/149/1/27/4891701/gsjgs.149.1.0027.pdf by guest on 24 September 2021 28 M. SERANNE Devonian age were deposited in the hanging-wall of NNE- trending normal faults associated with ESE-trending transfer faults. Thickness variation across the faults and growth structures within the sediments are good evidence for syn-depositional extensional tectonics. Such a simple half-graben geometry may be observed onshore in the Turiff basin(Norton et al. 1987) and in west Caithness (Enfield 1988) but it is not observed in Orkney, probably due to a lack of continuous exposure. Enfield (1988) analysed outcrop-scale examples of extensionalfault systems that he related to gravitational sliding on top of tilted hanging-walls in half-graben. These slickensided faults do not allow the reconstruction of the regional palaeostress field as they respond to local conditions. Consequently, the Middle Devonian regional extensiondirec- tion in Caithness and Orkneyis best approximated by the geometry of the normal faults and associatedlateral ramps in the West Orkney Basin: a NW-SE direction of extension is sug- gested by the ‘bow and arrow’ rule (Enfield & Coward 1987; Enfield 1988) (Fig. 1). Shetland The Devonian rocksof Shetland belong to threedistinct basins (Fig. 2): (1) The Melby basin in the west which consists of volcanic rocks (Esha Ness) and sandstones of Middle Devonian age (Foula and Melby) limited by the Melby fault. (2) The Walls basin covers mostof the Walls peninsula.It is limited to the east by the Walls Boundary fault and unconfor- mably overlies Lewisian-type schists and gneisses to the north (Flinn 1985). The basin comprises sandstones and interbedded a’t acid volcanic rocks and it is intruded by the Late Devonian Sandsting granite (BGS 1:63650 W Shetland, Sheet No. 127) Fig. 2. (a). Simplified map of Devonian bedding in the Walls basin (3) The Southeast Shetland basin (including Fair Isle) in (west Shetland, location on Fig. l), modified from Mykura (1976) which Old Red Sandstone sediments are in contact through and from personal observation on the northern margin. The effects unconformity or oblique normal fault with Dalradiangneisses of north trending folding have been removed. Sandsting granite and and a ‘tectonic melange’ (BGS 1:63650 S Shetland, Sheet No. interbedded volcanic rocks are not represented. Old Red Sandstone 126). series display internal unconformities and progressive syntectonic unconformities that demonstrate syn-depositional folding. Inset (l) Melby basin. Westof the Melby fault, the Middle map shows the position of Devonian rocks (stippled) in Shetland. DevonianMelby formation consists ofsandstones and MF, Melby fault; SW, Sulma Water fault; WBF, Walls flagstones that arerelated to the Old Red Sandstoneof Orkney Boundary fault. (b) Section across Walls basin showing the on sedimentological and palaeontological grounds (Mykura relationship between basement faulting (left-lateral strike-slip) and 1976; Marshal1 1988). Onthe island of Foula,structures folding of the sedimentary cover.Vertica1 and horizontal scales are within the Devonian sediments and underlying basement in- the same. dicate a NE-SW to E-W extension (Norton et al. 1987). In the north, sandstone gives way to volcanic lithologies (EshaNess). The exposed Melby basin has been juxtaposed to the Walls basin by dextral strike slip along the Melby fault in post Old strike-slip fault. North of the Sulma Water fault and east of Red Sandstone time (Donovanet al. 1976). No further evidence the Melby fault the Old Red Sandstone formations preserveat of syndepositional tectonic activity has been found. their base ENE-WSW trending synclines which are unconfor- mably overlain by later Old Red Sandstone. Such local un- (2) Walls basin. The Old Red Sandstone of the Walls basin is conformitiesattest to activetectonics during Old Red poorly exposed and complexly folded and faulted. The BGS Sandstonetime. The Sandness formation onlaps eastwards one inch map provides a good density of bedding dip and strikeonto Lewisian-type basement. This reflects an eastwards shift measurements. Mykura (1976) gave a cartographic interpreta- of the source area with respect to the locus of sedimentation tion based on the recognitionof several marker horizonstrace- which, in turn suggests lateral movement of the basement dur- able throughout thewhole basin. Re-mappingof the northern ing Old Red Sandstone deposition. margin of the Old Red Sandstone outcrop and reappraisal of Inthe centre, Walls peninsula is occupied by theWalls the nature of the Sulma Water fault has led to a simplified formation whose sedimentologyis detailed by Mykura (1976) interpretation of Mykura’s map in which the lateN-S trending and Melvin (1985). The formation presently dips towards the folds have been removed for clarity (Fig. 2a). west and appears to have a cumulative stratigraphic thickness The northernmargin of the Walls basinis an unconformity of about12km. It islikely that this largestratigraphic which suffered minor reactivation as a discontinuous sinistral thickness exceeds the final vertical depth of the basin, as a Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/149/1/27/4891701/gsjgs.149.1.0027.pdf by guest on 24 September 2021 TEC TO NICS IN THE NORTHERNORCADIANTHE TECTONICSBASIN IN 29 result either of westward shifting of depocentres duringdeposi- tion, or deposition above an east-dipping low-angle normal fault. South of the Sulma Water fault Devonian strata arefolded in a manner compatible with left lateral motion on the fault and toward the south they swing into

Load more