Editing Highland Border Complex, Grampian Highlands

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This view was accepted and expanded by Anderson (1947) in an extensive examination of the Highland Border. Anderson was impressed by the similarity between the Dalradian rocks and the adjacent arenite sequences of the Highland Border Complex and proposed that the latter (including the limestones) formed the uppermost part of the Dalradian sequence, which was therefore of Cambrian age. These Dalradian rocks he regarded as unconformably overlain by the black shale and lava sequences which he believed to be of Ordovician age.
 
This view was accepted and expanded by Anderson (1947) in an extensive examination of the Highland Border. Anderson was impressed by the similarity between the Dalradian rocks and the adjacent arenite sequences of the Highland Border Complex and proposed that the latter (including the limestones) formed the uppermost part of the Dalradian sequence, which was therefore of Cambrian age. These Dalradian rocks he regarded as unconformably overlain by the black shale and lava sequences which he believed to be of Ordovician age.
  
In recent years, however, intensive palaeontological investigations of the Highland Border rocks have radically revised the stratigraphy. The results of this work are summarised in Curry et al. (1984), who suggest that the Highland Border Complex rocks rest unconformably on the Dalradian succession and range in age from Lower Cambrian to uppermost Ordovician. They divide the Highland Border Complex into four lithostratigraphical groupings [[Media:P915439.png|(P915439)]]. The first group comprises the serpentinites and associated ophiolitic rocks which are believed to be of pre-Arenig age. The black shale and limestone sequences of the Leny area, which are accepted as late Lower Cambrian age, may also be part of this grouping. The second group, of early Arenig age, comprises the serpentinite conglomerate at Balmaha, the Dounans (or Lime Craig) Conglomerate and the Dounans Limestone at Aberfoyle. The third group consists of the black shale and lava sequences which are ascribed a Llanvirn–Llandeilo age. This group is unconformably overlain by the youngest assemblage which comprises the arenite-dominated sequences (including the Margie Limestone). This last group is believed to be of Caradoc–Ashgill age; its base is an unconformity marked by the basal breccia of Jehu and Campbell (1917) at Aberfoyle and possibly the Green Conglomerate in the North Esk. Curry et al., (1984) point out that the upper group contains blackened and deformed fossils of Llanvirn–?Llandeilo age and unblackened fossils of Caradoc age. This is taken as evidence of a pre-Caradoc period of low-grade metamorphism and uplift.
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In recent years, however, intensive palaeontological investigations of the Highland Border rocks have radically revised the stratigraphy. The results of this work are summarised in Curry et al. (1984), who suggest that the Highland Border Complex rocks rest unconformably on the Dalradian succession and range in age from Lower Cambrian to uppermost Ordovician. They divide the Highland Border Complex into four lithostratigraphical groupings ([[Media:P915439.png|(P915439)]]). The first group comprises the serpentinites and associated ophiolitic rocks which are believed to be of pre-Arenig age. The black shale and limestone sequences of the Leny area, which are accepted as late Lower Cambrian age, may also be part of this grouping. The second group, of early Arenig age, comprises the serpentinite conglomerate at Balmaha, the Dounans (or Lime Craig) Conglomerate and the Dounans Limestone at Aberfoyle. The third group consists of the black shale and lava sequences which are ascribed a Llanvirn–Llandeilo age. This group is unconformably overlain by the youngest assemblage which comprises the arenite-dominated sequences (including the Margie Limestone). This last group is believed to be of Caradoc–Ashgill age; its base is an unconformity marked by the basal breccia of Jehu and Campbell (1917) at Aberfoyle and possibly the Green Conglomerate in the North Esk. Curry et al., (1984) point out that the upper group contains blackened and deformed fossils of Llanvirn–?Llandeilo age and unblackened fossils of Caradoc age. This is taken as evidence of a pre-Caradoc period of low-grade metamorphism and uplift.
  
 
The black shale and limestone sequences exposed around the Leny quarries are recognised to be of Cambrian age. However, their status within the Highland Border Complex stratigraphical sequences is complicated by their relationship to similar sequences in the nearby Keltie Water and the relationship between these sequences and the adjacent Dalradian rocks. Two factors are important in considering the problem. Firstly, detailed mapping by Pringle (unpublished manuscript, deposited with BGS) suggests that the Leny Limestone is the correlative of the limestones in the Keltie Water (cf. Harris, in discussion of Rogers et al., 1989); secondly, the black shales and limestones in the Keltie Water can be traced with apparent conformity into the underlying Dalradian grits (Anderson, 1947, p. 495; Harris, 1969). However, Rogers et al. (1989) have published a U-Pb zircon age of 590 Ma as the date of intrusion of the Ben Vuirich Granite; this granite cuts early structures in the Dalradian rocks, implying that the Dalradian sequence and its earliest deformation phase(s) are Precambrian. This apparent contradiction in the available evidence is still unresolved.
 
The black shale and limestone sequences exposed around the Leny quarries are recognised to be of Cambrian age. However, their status within the Highland Border Complex stratigraphical sequences is complicated by their relationship to similar sequences in the nearby Keltie Water and the relationship between these sequences and the adjacent Dalradian rocks. Two factors are important in considering the problem. Firstly, detailed mapping by Pringle (unpublished manuscript, deposited with BGS) suggests that the Leny Limestone is the correlative of the limestones in the Keltie Water (cf. Harris, in discussion of Rogers et al., 1989); secondly, the black shales and limestones in the Keltie Water can be traced with apparent conformity into the underlying Dalradian grits (Anderson, 1947, p. 495; Harris, 1969). However, Rogers et al. (1989) have published a U-Pb zircon age of 590 Ma as the date of intrusion of the Ben Vuirich Granite; this granite cuts early structures in the Dalradian rocks, implying that the Dalradian sequence and its earliest deformation phase(s) are Precambrian. This apparent contradiction in the available evidence is still unresolved.

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