Torridon Group Research Articles

Neoproterozoic (Torridonian) alluvial fan succession, northwest Scotland, and its tectonic setting and provenance

The presence of alluvial fan deposits in the lower Neoproterozoic Torridon Group in northwest Scotland illuminates Torridonian basin development at the eastern Laurentian margin. The 450 m thick Cape Wrath Member of the Applecross Formation consists of alluvial fan conglomerate and arkose succeeded by more distal, braidplain feldspathic sandstone. Palaeocurrent data comprising >2650 measurements on trough cross-bedding are of low variability and show overall eastward flow. The projection upcurrent of regionally divergent flow directions for the lower part of the member indicates a fan of c. 50 km radius with its apex 30 km to the west near a basement (pre-Caledonian) normal fault with downthrow to the east beneath the north Minch Basin. Extensional tectonics controlled deposition of the Applecross Formation. Regional uplift, causing erosion of a youthful topography on the Lewisian Gneiss, was followed by the development of the Applecross extensional basin in two main stages. Uplift of a western source area by movement on basin-bounding normal faults occurred first in the north and caused pediplanation and alluvial fan deposition in the Cape Wrath area, with subsequent uplift of the source area for the main body of the Applecross Formation occurring further to the west and south along the line of the Minch Fault. The bulk of the Applecross Formation was derived from a weathered terrain of felsic crystalline and related supracrustal rocks reaching from the Outer Hebrides region westward for up to c. 250 km onto what are now the continental margins of the North Atlantic. The tectonic events may mark an early phase in the crustal extension that led ultimately to the opening of the Iapetus ocean.

Detrital zircon geochronology and provenance of the Torridonian, NW Scotland

Between 30 and 50 single detrital zircons from each of four specimens of the Stoer Group and two specimens from the unconformably overlying Torridon Group were analysed on the GSC SHRIMP II ion probe. 207 Pb/ 206 Pb ages of zircons from the Stoer Group range between 3.00 and 1.74 Ga with 95% concentrated between 2.93 and 2.48 Ga. The Bay of Stoer Formation has a small mode c. 2.55 Ga, matching overgrowth ages on some older grains. A few grains between 1.92 and 1.74 Ga occur in each of the Stoer Group specimens. These data are consistent with conventional provenance information and sedimentology, which indicate that the bulk of the Stoer Group probably was derived from local basement of the Lewisian Gneiss Complex and that deposition occurred adjacent to basin-margin growth faults. The two Torridon Group samples have similar detrital zircon age profiles with distinctive modes at 1.80 Ga, 1.66 Ga and 1.10 Ga, interpreted to represent sources of Ketilidian, Labradorian and Grenvillian affinity, respectively. A less well defined cluster c. 2.85–2.55 Ga reflects reworking of the underlying Stoer Group and possibly direct contribution from the Lewisian Gneiss Complex. The youngest concordant detrital zircon yields a maximum age of 1060±18 Ma for the Torridon Group (Applecross Formation). Our data, together with palaeocurrents from the Applecross Formation, suggest that the Torridon Group could have been deposited by a late to post-Grenvillian intermontane or foreland trunk river system flowing northeasterly, parallel to the Grenvillian orogenic belt.

Some aspects of the geochemistry, provenance and palaeoclimatology of the Torridonian of NW Scotland

Geochemical data, mostly from mudstones, are used in an attempt to investigate some aspects of the palaeoclimatology and provenance of the Torridonian succession in NW Scotland. The basal part of the Stoer Group, which has been interpreted as glaciogenic is thought to have formed in a warm arid setting. The presence of oscillation ripples and desiccation cracks in mudstones containing isolated clasts is thought to preclude their interpretation as ice-rafted debris. Some aspects of the major element geochemistry of the Stoer Group mudstones suggest deposition in an environment with little weathering, but chemical weathering is inhibited in both frigid and arid settings and sedimentological evidence favours the latter. The Stoer Group mudstones are enriched in Mg, Ca and Na relative to the Torridon Group samples, which are much closer in composition to an average shale estimate (PAAS). The Cr and Ni contents of the Stoer mudstones are significantly higher than those of the Torridon Group samples. There is, however, no evidence that enrichment in these elements was due to contamination with volcanic material like that of the Stac Fada Member. The Scourian basement appears to be an adequate source of the Ni and Cr in the Stoer Group sediments. Th/Sc ratios are higher in the Torridon than in the Stoer mudstones. This difference is interpreted as indicating that the Stoer mudstones were largely derived from local Archaean sources but that the Torridon Group mudstones incorporated materials from a much wider provenance, including younger and recycled materials. Evidence from rare earth elements also provides evidence for different source terrains for the Stoer and Torridon Groups. The Torridon Group mudstones are more weathered, have a higher REE content and display a more prominent Eu anomaly (lower value for Eu/Eu*) than do the Stoer samples. These results support evidence from Th/Sc ratios, that the Torridon Group was derived from a more varied, recycled and granite-rich terrain than the Stoer, much of which appears to have relatively local provenance. The geochemistry of mudstones can shed considerable light on problems of provenance and palaeoclimatology, because many useful trace elements tend to be concentrated in such fine grained rocks and some aspects of their major element chemistry may be related to climatic factors.

A geochemical investigation of palaeosols developed on Lewisian rocks beneath the Torridonian Applecross Formation, NW Scotland

Synopsis Alteration zones are developed on biotite gneiss and amphibolite of the Lewisian Basement Complex beneath Neoproterozoic rocks of the Applecross Formation of the Torridon Group in northwestern Scotland. The age and origin of these altered rock s has been disputed but systematic sampling and chemical analyses reveal patterns in A1 2 O 3 − CaO + Na 2 O − K 2 O space that are identical to weathering trends in palaeosols elsewhere. The Lewisian palaeosols, like other ancient weathering profiles, display significant loss of Ca and Na and exhibit strong K-metasomatism. K/Rb ratios in the profiles exhibit systematic variations that are thought to be related in part to the nature of metasomatizing fluids, rather than reflecting their provenance. Chondrite-normalized REE patterns for the host rocks and palaeosols demonstrate the conservative nature of these elements. There are, however, some differences between REE patterns obtained from the fresh and altered basement rocks and those of mudstones of the Applecross Formation. These differences suggest that the provenance area for the Applecross sediments included rock types other than those of the immediately underlying Lewisian basement.

Palaeomagnetic dating of sub-Torridon Group weathering profiles, NW Scotland: verification of Neoproterozoic palaeosols

A palaeoplain that cuts across Lewisian Gneiss in the Cape Wrath area, northwest Scotland, is weathered to depths of 1–6 m beneath fluvial red conglomerates and sandstones of the Applecross Formation of the early Neoproterozoic Torridon Group. Weathered Lewisian rocks are enriched in K 2 O and Fe 2 O 3 , and contain hematite as an alteration product of biotite and magnetite. Palaeomagnetic analysis of the weathering profiles has been carried out to constrain the age of alteration. Thermal step demagnetization of 87 oriented core specimens of biotite-gneiss and amphibolite spanning four profiles at Poll a’ Mhurain and Sheigra reveal a stable, high-temperature component with a mean direction of remanence of D= 149.1°, I =57.2° (α 95 = 5.0°) (corrected for dip of the overlying Applecross strata) that gives a pole at 9.8°S, 199.2°E (dp = 5.3°, dm=7.3°). This direction differs significantly from that for little-weathered Lewisian at those two localities and also from directions for the late Mesoproterozoic Stoer Group. The remanence direction for the weathered rocks is, however, near the dip-corrected high-temperature remanence direction (D= 122.3°, I =50.5°, α 95 =10.8°) obtained by us from the thermal step demagnetization of 39 sandstone specimens of the Applecross Formation in the Cape Wrath area. Furthermore, the direction for the weathered rocks agrees closely with other dip-corrected results for the Torridon Group, such as the detrital remanent magnetization (DRM) mean direction (D= 148.3°, I =59.9°, α 95 =13.8°) for the Applecross Formation on Rum and that for the main outcrop of the Aultbea Formation ( D =139.7°, I =52.6°, α 95 = 14.5°). The concordance of palaeomagnetic results for the sediments and weathered rocks implies that the weathering profiles in the Cape Wrath area acquired their remanence penecontemporaneously with deposition of the Applecross and Aultbea formations. The palaeomagnetic results therefore argue strongly that the weathering profiles are palaeosols of Applecross age ( c. 980 Ma) that formed at a palaeolatitude of 38° and constitute the oldest known palaeosols in the British Isles—views that the weathering is substantially older or younger than Applecross deposition cannot be sustained. Our findings carry implications for Applecross palaeoclimate and source and strengthen the conclusion that atmospheric oxygen was a substantial fraction of its present abundance by early Neoproterozoic time.

New isotopic age determinations for the Torridonian, NW Scotland

Isotopic data are used to determine a new chronology for the Torridonian of NW Scotland. The calcite fraction of the Stoer Group Limestone Member yields a Pb–Pb age of 1199 ± 70 (2 σ ) Ma (MSWD = 1.1). Rb–Sr whole-rock regressions from siltstones of the Diabaig and Applecross Formations (both Torridon Group) produce ages of 994 ± 48 (2 σ ) Ma (MSWD = 17) and 977 ± 39 (2 σ ) Ma (MSWD = 10) respectively. These dates, which agree well with palaeomagnetic estimates, are interpreted as the time of early diagenesis which is probably within error of the time of deposition. Previous isotopic age determinations are shown to be in error. Size fractionsfrom shales of the Stoer Group and Applecross Formation have been analysed for Rb–Sr and produce ages of 689 ± 87 (2 σ ) Ma (MSWD = 14) and 667 ± 66 (2 σ ) Ma (MSWD = 55) respectively. They are used, together with Sm-Nd isotopic data, to suggest a thermal event affecting the entire Torridonian at this time. Our new chronology for the Torridonian is consistent with the Proterozoic geology of Scotland and Laurentia. It suggests that deposition of the Stoer Group is contemporaneous with uplift of the Lewisian and the Torridon Group may be associated with the extensional collapse of the Grenville orogen. The c. 700Ma diagenetic event is possibly related to Neoproterozoic rifting of the Laurentian margin.

Anatomy of a water-escape cusp in Upper Proterozoic Torridon Group sandstones, Scotland

Evidence from recently published experiments is applied to the interpretation of a typical soft-sediment deformation structure in coarse fluvial sandstones from the Applecross Formation of the Upper Proterozoic Torridon Group in northwest Scotland. The structure is an interfacial cusp, formed at the contact between two cross-bedded sandstone units in an upward-coarsening sequence. The characteristics of the structure suggest that deformation took place while the sediment was in a fluidized state. During fluidization a water-filled cavity developed near the top of the lower unit, separating fluidized sediment beneath from a static couplet above. The cavity became narrower and taller as fluidization continued, focussing fluidization at a point. Primary stratification was destroyed in the fluidized zone, but there is no evidence that fluidized sediment burst through the overlying zone. A rupture may be out of the plane of the section, or alternatively fluidization could have stopped before this stage was reached, or fluid pressures could have been dissipated upwards. The cavity closed, leaving the lower part of the static couplet as a distinct unit. Estimates of the fluidization velocity and the duration of fluidization suggest the volume of water required for fluidization. This could have been derived from packing changes following liquefaction of underlying sediment, or from groundwater movements within the braided fluvial environment.

Soft-Sediment Deformation in Upper Proterozoic Torridonian Sandstones (Applecross Formation) at Torridon, Northwest Scotland

ABSTRACT The Applecross Formation (upper Proterozoic Torridon Group, northwest Scotland) was deposited as planar-laminated and cross-bedded coarse sands in braided streams. Soft-sediment deformation is described from a 97 m thick study section, most of which comprises contorted stratification. The main structures are cusps and complex recumbent folded cross-bedding, formed by water escape during fluidization. Four assemblages of structures are recognized. Deformation as isolated structures in an otherwise undeformed unit may have been triggered by fluid turbulence. Discrete and chaotic units record widespread liquefaction and fluidization, respectively, of the most recently deposited sediment. Deformation may have been triggered by upwelling groundwater. Multilayer complexes formed when water escape from a buried layer generated fluidization in the overlying sediment. This may have been triggered by seismically induced liquefaction. Some parts of the succession were disturbed on separate occasions by different processes. Sediment-laden currents are unlikely to have been a major trigger for deformation. It is concluded that specific soft-sediment deformation structures result from the combination of driving forces, deformation mechanism, and initial geometry. Assemblages of structures relate to the timing of deformation and are of more value in identifying the trigger for deformation.

Possible sources of clastic sediment in the Late Proterozoic Torridon Group, from geochemical mass balance

AbstractEquations are derived which link the content of relatively insoluble components Si, Al, Fe and K in source rock and derived alluvial sediment. The equations also involve the ratio of specific suspended load to dissolved loadand specific bed load to suspended load (Tb/Ts) for the rivers which deposited the sediments. These ratios are generally unknown, but by analogy with modern rivers are taken to occupy the rangesand 0 ≦Tb/Ts≦1. This is sufficient to put close limits on the possible composition of the source rock. The equations suggest that clastic sediment forming the Late Proterozoic Torridon Group of Scotland was derived from sediments or metasediments, rather than fromaverageupper continental crust, granitoids or gneisses like those presently exposed nearby.

Late Proterozoic and Late Palaeozoic movement on the Coigach fault in NW Scotland

Synopsis Movement on the NNE-trending Coigach fault in NW Scotland can be dated by reference to the late Proterozoic Stoer and Torridon Groups which it cuts. It behaved as a normal fault in late Stoer Group time, with downthrow to the east. The fault was reactivated after the Cambrian as a listric fault with a downthrow to the west, displacing Torridon Group sediments about 5 km. Roll-over near the fault suggests that it soles out at around 8 km below present sea level. Most of the post-Cambrian movement was probably Devonian or Carboniferous. Mesozoic and later movement was negligible. Other NNE-trending faults on the mainland resemble the Coigach fault in showing large pre-Triassic displacements of the Torridon Group.

Geochemistry and provenance of red sandstones in the Upper Proterozoic Torridon Group in Scotland

Synopsis The Torridon Group consists mainly of alluvial red arkoses (Applecross and Aultbea Formations), over 4 k m thick. The arkoses have average whole rock ratios K/Rb ≈ 320 and La/Th ≈ 3, suggesting source areas on upper continental crust. Palacocurrents and geochemistry point to the Outer Hebrides as the provenance. Scourian amphibolite facies gneisses and Laxfordian granites, like those in the Outer Hebrides today, could have provided most of the arkoses. A small contribution, represented by pebbles, stems from supracrustal metasediments and volcanics not now exposed in Scotland. The Rb and Sr contents of the feldspars, modelled from double ratio plots of whole rock analytical data, show major changes in source terrain at the Applecross—Aultbea boundary, and elsewhere. The oldest Applecross sediment in Coigach was little weathered prior to deposition, but plagioclase becomes less abundant upward through the formation and is almost absent 800 m above the base. From this, the initial source area relief is inferred to have been 2—3 km, subsequently decreasing by an order of magnitude. On average all the amphibole and about three-quarters of the plagioclase was removed from the source rocks of the Applecross and Aultbea Formations by weathering before sedimentation.

Geochemistry, weathering and diagenesis of the Diabaig Formation (Torridon Group) in NW Scotland

Synopsis The late Proterozoic Diabaig Formation consists of alluvial red breccias and sandstones, and lacustrine grey shales, which accumulated in the lower parts of palaeovalleys eroded in gneiss. The sediments are significantly depleted in Ca and Na with respect to the local gneisses which provided 80% of the sediment. Geochemical mass balance calculations, taken together with the mineralogy, show that these losses occurred mainly during breakdown of plagioclase and amphibole during sediment transport and reworking. Mobilisation of Ca and Na during diagenesis was quantitatively insignificant, despite clear evidence for albitization of detrital plagioclasc. There is no evidence of primary carbonates or evaporites in the shales. The Ca and Na removed from the sediments was lost to a hydraulically open, regionally extensive lake.

Precambrian

Abstract For much of Precambrian time, it is not possible to reconstruct any sort of meaningful palaeogeographies. For this Atlas, therefore, the earliest reconstructions are for Proterozoic intervals and are, of necessity, limited geographically. The term 'Torridonian' has long since been used to refer to the entire Upper Proterozoic succession of predominantly fluvial clastic sediments situated along the northwest coast of Scotland. These rocks rest unconformably on Archaean to Lower Proterozoic Lewisian Gneiss, are overlain unconformably by Cambro-Ordovician marine sediments, and constitute the best sedimentary exposures in the British Isles. Stratigraphically, 'Torridonian' deposits are subdivided into the Stoer, Sleat and Torridon groups. The Stoer and Torridon groups are separated by an angular unconformity (Lawson 1965; Stewart 1969) and have been dated at 968 and 777 Ma respectively (Moorbath 1969; Stewart 1982), although these ages may be up to 100 Ma too young (see discussion in Stewart 1988 a , p. 98). Sedimentation dates of c . 1050 (Stoer Group) and c . 850 (Torridon Group) are, therefore, likely to be more realistic. The lowest of the three groups consists of a diverse suite of red bed deposits up to 2 km thick representing fluvial, aeolian and ephemeral lacustrine environments, suggesting a semi-arid climate (Stewart 1988 a ). Deposition occurred at an approximate palaeolatitude of 15°N (Torsvik & Sturt 1987). Stoer Group sediments have been divided by Stewart (1988 a ) into seven constituent facies. The illustrated palaeogeographical time interval encompasses the end of Bay of Stoer facies deposition, the The Bay .of Stoer facies consists of roughly 200 m

Geochemistry, provenance and palaeoclimate of the Sleat and Torridon groups in Skye

Synopsis Arkoses of the Sleat and Torridon groups in south-east Skye form a continuous succession 4500 m thick. Palaeocurrents came from the west. The source rocks were hornblende-biotite-gneisses, granite or granite-gneisses, quartzose sediments and acid volcanics, all with upper crustal geochemical signatures. Detritus from the basic gneisses predominates in the lowest sediments, but is gradually replaced by granitic material stratigraphically up the succession. The lowest sediments accumulated in palaeovalleys with a relief of <200 m. They retain the geochemistry of the parental basic gneisses which formed the local basement. Subsequent rifting brought sediments tens of kilometres from the west, some from as far as the flanking highlands, where acid rocks were common. The climate was initially arid. But the development of rift highlands attracted rainstorms, and the higher runoff leached Ca from hornblende as it weathered to chlorite. Small amounts of Na may also have been lost from plagioclase during weathering. The ratio of material transported in solid form to that in solution is estimated to lie in the range 15–20 for the lowest formation, but to have been <10 higher in the succession. By analogy with very large modern catchments, this suggests latitudes of 22–40 deg. Palaeolatitudes of 30–45 deg. have been deduced for the Torridon Group on the mainland of Scotland, using palaeomagnetism.

A palaeomagnetic study of recumbently folded and thermally metamorphosed Torridon Group sediments, Eishort anticline, Skye, Scotland

Two geologically significant palaeomagnetic components have been isolated in samples of Torridon Group sandstones from the Kishorn nappe of NW Scotland. The samples were collected from the Eishort anticline, a large recumbent fold of Caledonian age. The softer component (which is demagnetized by low temperatures and weak magnetic fields), when in situ, displays a steep upward direction typical of reversed Tertiary magnetizations. Both polarities of the harder component (which is demagnetized by high temperatures only) are present and, when dip corrected, display either northwesterly upward or southeasterly downward directions characteristic of Torridonian remanences. It is suggested that the Tertiary component is of chemical origin and the result of alteration of magnetite (titanomagnetite) grains. The geographical distribution of these grains is irregular and as a result the distribution of the Tertiary overprint is irregular. The Torridonian palaeomagnetic components from both limbs of the fold can be restored by rotation about the fold axis to internally consistent directions which lie close to previously published data from the Caledonian foreland. No correction is necessary for finite strain which suggests that the Eishort anticline formed by a flexural slip mechanism. However, an anticlockwise rotation of 26° is required to reconcile the Torridonian directions from the Kishorn nappe with those of the Caledonian foreland which suggests that, during emplacement, the thrust sheet rotated on the underlying fault surface.

On the origin and stability of remanence and the magnetic fabric of the Torridonian Red Beds, NW Scotland

Synopsis Primary (compactional) magnetic fabrics and multicomponent remanences are recognized in the Stoer and Torridon Groups. Low temperature (LT) blocking remanences are randomized around 400–600°C and relate to a post-Torridonian magnetic overprint, possibly of early Mesozoic age. In the Stoer Group (and some Stoer boulders in the basal Torridon Group) LT remanences are partly or fully carried by magnetite (titanomagnetite). High temperature (HT) remanences are characterized by discrete unblocking above 600°C, having a specular haematite remanence carrier. Results of a conglomerate test of some Stoer boulders provides a positive stability test for HT remanences in the Torridon Group, and a convergence of evidence suggests that remanence acquisition of both the Stoer and Torridon Groups was facilitated by both detrital and early diagenetic processes. A fold test of the Stoer boulders (inclination data) indicates that the boulders have preserved a primary HT remanence. Secondary fabric is readily observed in the boulders, hence, HT remanences most likely reside in different grain- or mineral fractions than the bulk susceptibility. The overall palaeomagnetic data (HT remanences), indicates that the angular unconformity, separating the Stoer and Torridon Groups, represents a considerable period of time. Palaeomagnetic results from the underlying Lewisian basement have traditionally been interpreted in terms of post-orogenic uplift associated with the Laxfordian event ( c. 1800–1400 Ma). Due to an apparent polar fit between Lewisian and Stoer Group data, we address the question as to the significance of post-Laxfordian magnetic overprinting in the Lewisian Complex.

Comments on some of Britain’s oldest microfossils

Abstract. A critical re-appraisal of some reports of British Precambrian microfossils illustrates potential pitfalls in describing Precambrian microbiotas. The Ceinwen and Llanddwyn Spilitic Formation of the Gwna Group contains convincing filamentous microfossils, but a report of acanthomorph arcitarchs has not yet been confirmed. Although the Diabaig Formation of the Torridon Group is known to be abundantly fossiliferous, a report based on S.E.M. without thin section control is rejected. Chert pebbles from the Applecross Formation of the Torridon Group do not contain convincing microfossils. They are contaminated by fungi and contain abiogenic structures which resemble cells and tissues. Reports of diagnostic Middle Riphean microfossils from the Blackbrook Formation and the Maplewell Group of the Charnian and the Buxton Rock of the Longmyndian await confirmation.

Palaeomagnetic studies of the Torridonian sediments, NW Scotland

Synopsis This study covers thermal demagnetisation results from 172 localities distributed through the Upper Proterozoic Stoer Group and Torridon Group sediments of NW Scotland. Chemical leaching experiments and results from slump horizons confirm earlier studies showing that the bulk of the total NRM in these rocks resides in specularite grains and is related to deposition and/or initial lithification processes; in the Stoer Group and Aultbea Formation a considerable fraction of the remanence is held by magnetite. Thermal demagnetization partially, but not completely, removes that part of the small secondary remanence residing in the high blocking temperature fraction overlapping with the initial remanence. Demagnetization trends suggest that this secondary remanence is a composite one including both a Caledonian ( c . 400 Ma) and a Tertiary/Recent component. The palaeopole estimates in stratigraphic order are: Stoer Group (main outcrop) 253°E, 46°N (A 95 = 10°), Stoer Group (Enard Bay) 263°E, 37°N (A 95 = 11°), Diabaig Formation 233°E, 17°S (A 95 = 21½°), Cape Wrath sediments 237°E, 37°S (A 95 = 16°), Lower Applecross Formation 230°E, 40°S (A 95 = 16½°), Upper Applecross Formation 212°E, 27°S (A 95 = 14½°), - Applecross Formation (Loch Maree), 231°E, 23°S (A 95 = 17°), Aultbea Formation (Coigach Area) 231°E, 12°S (A 95 = 8½°) and Aultbea Formation (main outcrop) 207°E, 7°S (A 95 = 17°). The sampled sections complement and extend earlier studies to define in outline the magnetostratigraphy of these formations, and show that the contact between the Applecross and Aultbea Formations is strongly diachronous. The time-sequence of poles from the Lower and Upper Torridonian coincide with the Laurentian a.p.w. path at c . 1100 and c . 1040 Ma respectively and imply that deposition and magnetization of these sediments predated isotopic closure by c . 130 Ma (Stoer Group, Rb–Sr isochron 968 Ma) and c . 260 Ma (Torridon Group, Rb–Sr isochron 777 Ma). This supports the views that the Torridonian and Moinian rocks are broadly coeval.

Late Proterozoic rifting in NW Scotland: the genesis of the ‘Torridonian’

The two major “Torridonian” successions, viz . the Stoer Group (~968 Ma) and the Sleat–Torridon Groups (~777 Ma) occupy NNE-trending rifts cutting Archaean and early Proterozoic crust. Palaeocurrents, pebble petrography and age, and the initial 87 Sr/ 86 Sr ratios of clay-rich sediments indicate derivation from flanking areas of this old crust, rather than from late Proterozoic Grenville or Morarian metamorphic rocks. The earlier rift seems to be roughly contemporaneous with Grenville metamorphism but genetically unrelated to it. The rifts perhaps mark initiation of an Iapetus-related ocean in the same way as very similar fault-bounded Triassic and Lower Jurassic sediments mark the start of continental break-up and initiation of the present Atlantic. The western margin of both rifts probably coincides with the Outer Hebrides Thrust and the eastern margin with the Moine Thrust Zone. Reactivation of the old rift-margining faults by Caledonian compression may have generated the lower thrusts of the Moine Thrust Zone, while the Moine nappe itself, by analogy with Lower Devonian events in southern Appalachia, represents a slice of the Grenville microcontinent emplaced after closure of the intervening ocean.

Jasper pebbles in the Dalradian at the Highland Border

Synopsis Southern Highland Group (Upper Dalradian) conglomerates at the Highland Border contain subangular jasper pebbles, probably derived from a Precambrian iron-formation. The angularity of these pebbles suggests iron-formation rocks were formerly present in the source area for the Southern Highland Group. This source area is now concealed beneath the Midland Valley or possibly the Grampian Highlands. Jasper pebbles in the Torridon Group were probably derived from another, though petrographically similar, iron-formation to the NW of Scotland. Rounded jaspers in the Dalradian, therefore, are not distinctive of one particular source area, as has been recently proposed.