TORRIDON Gikoup /~ L Purnpellgik~, E.Pidol'e R. I
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Downloaded from http://mem.lyellcollection.org/ by guest on September 24, 2021 Chapter 5 Overview General aspects of the Torridonian briefly reviewed in the following instability and fault activity is evident from the time the earliest paragraphs include the significance of the similarity in depositional Stoer and Torridon Group sediments were deposited (pp. 20 & 44), style shown by the three component groups, their burial history, even though the soft-sediment contortions which formed during palaeocontinental setting and regional correlation. sedimentation and thought to be seismically induced, are noticeably absent from the lowest units (Clachtoll and Diabaig Formations). Contortions are also absent from the basal Sleat Group (Rubha Depositional style Guail and Loch na Dal Formations). The reason for the lack of contorted bedding in the lowermost formations and their abundance The Stoer, Sleat and Torridon Groups are thick fluvial succes- at other stratigraphic levels remains a mystery. sions, each of which tends to become finer upwards. The last two The tectonic instability recorded in the sediments, taken together groups both show an upward progression from locally derived basal with the palaeocurrent directions and evidence for decelerating breccias into lake deposits, followed by a thick fluvial sequence, subsidence, suggests rifting. The rift-bounding faults are surmised to as if they were deposited in a regime of decelerating subsidence. have dipped mainly eastwards, so that some of them were trans- In addition, both were derived from progressively more acid source formed into thrusts by Caledonian compression (Brewer & Smythe rocks, as the source terrain expanded to embrace not just local basic 1984; Blundell et al. 1985). A similar conversion of normal faults gneisses, but a wider range of rock types, including sediments. These into thrusts occurred when the Mid-continent rift was compressed features are consistent with deposition in an extensional basin sub- during the Grenville orogeny in the Lake Superior region (Cannon jected to two distinct stretching events. The Stoer Group must 1994). The rift faults in NW Scotland may, indeed, have controlled represent a much earlier stretching event, for it was lithified and the orientation of the much later Caledonian orogenic margin, as deeply eroded before the Torridon Group was deposited upon it. suggested by Stein (1988). Similar control is seen in east Greenland The upward facies progression in the Stoer Group is like that in the (Higgins et al. 2001). other groups, except that the lacustrine phase (facies Ct3, see Fig. 5 Later extension of the Torridonian basin reactivated some of & p. 9) is underdeveloped. the thrusts as normal faults (Brewer & Smythe 1984) so that the The orientation of the basin is suggested by the palaeocurrent western part again subsided and received some 4km of mainly directions. Stoer Group directions are bimodal; 77% of the currents continental sediment during the Triassic period (Steel & Wilson flowed westwards (0 = 270 ~ n = 282) and the remainder eastwards 1975; Stein 1992; Hitchen et al. 1995). (0 = 069 ~ n = 84). In the Sleat Group 65% flowed eastwards. In the Torridon Group virtually all the currents flowed ESE (0= 123~ almost exactly perpendicular to the Minch fault and the Moine Burial history thrust which both strike NNE (030~ The palaeocurrent directions suggest a single fault-bounded sedimentary basin striking roughly The total thickness of the Stoer Group exposed today is only 2 km, NNE and receiving sediment from the flanks. Evidence of tectonic but albitization of the entire sequence indicates that the highest STO~Ik TORRIDON SI~-ISOTOPE MOIN[, TPdASSIC GP~Ot.JP GIkOUP IIOMOOENIZKf~ NA/'PE IklI=TINe ,I, iooo~ ,i 500 ~ $ i / ~ ~ i albitiaarion p !00- /~ L~PurnPellgik~, 'C e.pidol'e pumpeJl~if~ Fig. 37. Torridonian thermal history. The -illife graph attempts to summarize the main depositional episodes, thermal events and mineral crystallization phases recorded by a sample of sediment at the base of the r._I<," feldspar Stoer Group at Stoer. Downloaded from http://mem.lyellcollection.org/ by guest on September 24, 2021 48 OVERVIEW i I I ! lI " : 9 i. ,o.,-~ ~P'' ,..o .-'~176 iUos 1141 1169 oso. It !0 50 "~ I Ig0 : ];.'" I o- STdf.R,...& 95 OKOUP.-- logG ,,. gSO ' :•0 oO __ .., " 1087 .o 9 ; ~, 780 E47 1~.67 o. I010 ,~ Fig. 38. Selected palaeomagnetic poles for go 994 Laurentia, detailed in Table 18. Mercator : I .. .o projection. The ages are in millions of years : .~ zoos- (Ma) while the bars estimate the uncertainty in o" TOKP,IDON the pole position at the 95% probability level ", o .."" .o .o o. I OL/P (A95). The Gardar and TugtutSq poles from "" .... 97S ...... Greenland have been rotated -12.2 ~ about an Euler pole at 66.6~ 240.5~ (Roest & Srivistava 1989). The Stoer and Torridon Group poles have been rotated -38 ~ about an Euler pole at 88.5~ 27.7~ (Bullard et al. 1965). The apparent polar wandering path defined by the poles is dotted. The Keweenawan track starts at 1108 Ma (top right) ! I50~E I I IgO" I i zm]E. I and ends at 975 Ma (bottom left). Table 18. Selected Proterozoic palaeomagnetic poles.from Laurentia Rock unit Age Pole lat. Pole long. A95 Reference (Ma) (deg. N) (deg. E) (deg.) Western N. America intrusions 780 O9 136 12 Buchan et al. (2000) Grenville B overprint 850 :k: 50 23 166 l0 Buchan et al. (2000) Grenville A overprint 975 4- 50 -29 149 15 Buchan et al. (2000) Torridon Group 994 + 48 -19 222 23 Buchan et al. (2000) Jacobsville Sandstone 1010 -+- 40 -09 183 6 Weil et al. (1998) Freda Sandstone 1020 + 40 01 180 3 Weil et al. (1998) Nonesuch Shale 1047 + 40 10 177 6 Weil et aI. (1998) Copper Harbor Conglomerate 1060 + 30 35 176 4 Weil et al. (1998) Michipicoten volcanics 1086 + 2 25 175 8 Weil et al. (1998) Lake Shore traps 1087 + 2 22 181 5 Buchan et al. (2000) Mamainse Pt. volcanics 1090 + 7 38 188 1 Weil et al. (1998) Portage Lake volcanics 1095 4- 3 27 181 2 Buchan et al. (2000) Upper Osler lavas 1105 4- 2 43 195 6 Buchan et al. (2000) Logan sills 1108 + 1 49 220 4 Buchan et al. (2000) Abitibi dykes 1141 4- 1 43 208 14 Buchan et al. (2000) Stoer Group 1150 4- 50 35 234 7 This volume Tugtut6q 1163 4- 2 42 226 11 Buchan et al. (2001) Late Gardar dykes 1165 4- 15 37 222 7 Buchan et al. (2000) Sudbury dykes 1235 -4- 5 -03 192 3 Buchan et al. (2000) Middle Gardar dykes 1235 4- 15 O5 202 8 Buchan et al. (2000) Mackenzie dykes 1267 4- 2 O4 190 5 Buchan et al. (2000) Downloaded from http://mem.lyellcollection.org/ by guest on September 24, 2021 CHAPTER 5 49 beds were once buried to a depth of 3-4 km, sufficient to raise the Srivistava 1989). The resulting fit of Scotland NW of the Moine temperature by the required 100~ The growth of pumpellyite thrust zone (the Hebridean terrane) with Laurentia during the early at the base of the Stoer Group indicates temperatures of at least Neoproterozoic can be regarded as well established. The Cambro- 125~ possibly as much as 230~ (Hay et al. 1988), correspond- Ordovician Durness sequence, unconformably overlying the Tor- ing to over 3 km of burial (p. 20). Another estimate of the maxi- ridon Group, contains fossils belonging to the American faunal mum temperature at the base of the group may be given by the province, confirming that the Hebridean terrane was still part of the fluid inclusion homogenization temperatures of 279 + 35~ in the Laurentian shield as late as the Arenig (Salter 1859; Huselbee & K-feldspar + calcite vein suite (S. J. Hay, A. E. Fallick, P. J. Hamil- Thomas 1998). The Hebridean terrane is shown, together with the ton, pers. comm.) described on p. 20. These temperatures would palaeomagnetically derived palaeolatitudes, in Figure 39. On tec- have been quite sufficient to have albitized the entire sequence tonic grounds Baltica is generally believed to have lain just south of shortly after deposition, as shown in Figure 37. Another possibility, Greenland but the lack of reliably dated palaeopoles for Baltica however, is that the Stoer Group was albitized at the same time as over the period 800-1200Ma mean that its precise position and the Torridon Group, perhaps at about 675 Ma when both of them orientation are poorly constrained. experienced strontium isotopic homogenization at temperatures The maps in Figure 39 show that the Stoer and Torridon under 150~ recorded by illite grains <2 #m (Turnbull et al. 1996). Groups formed in quite different geographic positions. The Stoer Turnbull et al. rejected this hypothesis because of the long interval Group was deposited on the passive margin of Laurentia, only a between deposition and albitization, but such intervals are by no few hundred kilometres from an ocean, while the Torridon Group means rare (e.g. Fedo et al. 1997). formed in the heart of the continent Rodinia, close to the Grenville Epidote has been reported from the basal beds of the Torridon orogenic belt. The subtropical steppe palaeoclimate deduced from Group at Upper Loch Torridon and Raasay (q.v.), in gneiss just Stoer Group sediments is consistent with its palaeolatitude and below the sediments near Canisp and Cfil M6r (Peach et al. 1907, marginal continental position, but the Torridon Group palaeocli- p. 306-7), and in hematite-quartz veins cutting hornblende schist mate is problematic. The available data (p. 43) indicate a climate at next to the unconformity at Slattadale and Beinn Lair (Peach et al. least as wet as the present-day Mediterranean. Considering that the 1907, p. 244 & 315).