Editing Pyroclastic rocks of the Skye Central Complex

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What is not clear from the above sequence of events is whether or not the vent structure, itself, was located in the immediate area. It is important to note that the outcrop pattern of the pyroclastic rocks is controlled by ring-faults, not intrusive contacts. It is therefore possible that the vent occupied a position which has subsequently been engulfed by later intrusions (for example, further north), and that the pyroclastic rocks were deposited on the flanks (or even further away) from any vent structure. This possibility would alleviate the problem of having to explain the presence and mode of formation of the bedded tuffs within the sequence. However, the mechanism by which the gabbro slabs were incorporated into these deposits is not easily explained. It is possible that the original vent-forming phase, in (i), above, transported fragments of these rock-types to the surface, where they were subsequently incorporated into subaerial deposits by gravitational slumping on an irregular topography.
 
What is not clear from the above sequence of events is whether or not the vent structure, itself, was located in the immediate area. It is important to note that the outcrop pattern of the pyroclastic rocks is controlled by ring-faults, not intrusive contacts. It is therefore possible that the vent occupied a position which has subsequently been engulfed by later intrusions (for example, further north), and that the pyroclastic rocks were deposited on the flanks (or even further away) from any vent structure. This possibility would alleviate the problem of having to explain the presence and mode of formation of the bedded tuffs within the sequence. However, the mechanism by which the gabbro slabs were incorporated into these deposits is not easily explained. It is possible that the original vent-forming phase, in (i), above, transported fragments of these rock-types to the surface, where they were subsequently incorporated into subaerial deposits by gravitational slumping on an irregular topography.
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== (D) The Creagan Dubh and Kilchrist deposits: description ==
 
== (D) The Creagan Dubh and Kilchrist deposits: description ==
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Amongst the basic blocks, both the transitional Skye Main Lava Series and the high-calcium, low-alkali tholeiite groups defined by Thompson et al. (1972, 1980a) and Esson et al. (1975) are represented (see Section [[Palaeogene extrusive products on the Isle of Skye#(D) Plateau lavas|(3D)]] of Chapter 3), as well as big-feldspar basalts and mixed (acid-basic) lavas. One granite block was analysed and is typical of the Group (1) of Thompson (1982), which are compositionally "primitive", and usually feature early in the evolution of British Tertiary Volcanic Province centres (see Section (12D) of Chapter 12). The field and analytical evidence therefore indicates that all these igneous rock-types pre-date the formation of the agglomerates and were locally available for incorporation into the agglomerates during Lower Tertiary times.
 
Amongst the basic blocks, both the transitional Skye Main Lava Series and the high-calcium, low-alkali tholeiite groups defined by Thompson et al. (1972, 1980a) and Esson et al. (1975) are represented (see Section [[Palaeogene extrusive products on the Isle of Skye#(D) Plateau lavas|(3D)]] of Chapter 3), as well as big-feldspar basalts and mixed (acid-basic) lavas. One granite block was analysed and is typical of the Group (1) of Thompson (1982), which are compositionally "primitive", and usually feature early in the evolution of British Tertiary Volcanic Province centres (see Section (12D) of Chapter 12). The field and analytical evidence therefore indicates that all these igneous rock-types pre-date the formation of the agglomerates and were locally available for incorporation into the agglomerates during Lower Tertiary times.
  
(ii) Intercalated with the agglomerates are numerous horizons of basic tuffaceous material. Commonly, the agglomerates grade through lapillituffs into tuffs, over distances of between a few centimetres and several tens of centimetres. The tuffs are dull grey-green and contain a small proportion of lapilli-size (up to 10mm) lithic clasts. Other features include eroded tops and abrupt upward changes into much coarser agglomerates.
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<div style="margin-left:0cm;margin-right:0cm;">(ii) Intercalated with the agglomerates are numerous horizons of basic tuffaceous material. Commonly, the agglomerates grade through lapillituffs into tuffs, over distances of between a few centimetres and several tens of centimetres. The tuffs are dull grey-green and contain a small proportion of lapilli-size (up to 10mm) lithic clasts. Other features include eroded tops and abrupt upward changes into much coarser agglomerates.</div>
  
(iii) Closely associated with the agglomerates and basic tuffs are thin horizons of acid tuff. Twenty-five distinct exposures have been mapped (B.R. Bell 1985), although some may be lateral equivalents. Typically, they are less than 2.5m thick (and often less than 1m). Except for some of the deposits exposed on the crags south of Beinn Dearg Bheag [[Media:GSG_SKYE_11.jpg|(Figure 11)]], they are observed only where cut by the numerous streams which dissect the area.
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<div style="margin-left:0cm;margin-right:0cm;">(iii) Closely associated with the agglomerates and basic tuffs are thin horizons of acid tuff. Twenty-five distinct exposures have been mapped (B.R. Bell 1985), although some may be lateral equivalents. Typically, they are less than 2.5m thick (and often less than 1m). Except for some of the deposits exposed on the crags south of Beinn Dearg Bheag [[Media:GSG_SKYE_11.jpg|(Figure 11)]], they are observed only where cut by the numerous streams which dissect the area.</div>
  
 
The acid tuffs often have irregular, weathered tops, which in some cases are infilled with fine-grained basic tuff from the overlying formations. Bedding may be discerned in some instances (for example, the two deposits NE of Meall Coire Forsaidh, [[Media:GSG_SKYE_11.jpg|(Figure 11)]]), whilst in others very little vertical variation in grain-size is noted.
 
The acid tuffs often have irregular, weathered tops, which in some cases are infilled with fine-grained basic tuff from the overlying formations. Bedding may be discerned in some instances (for example, the two deposits NE of Meall Coire Forsaidh, [[Media:GSG_SKYE_11.jpg|(Figure 11)]]), whilst in others very little vertical variation in grain-size is noted.

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