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Sill in Fractured Basement, Aberdeen

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There is a very nice set of fractured basement outcrops (Dalradian metamorphic rocks) along the coast just South of Aberdeen (Figure 1). Worth a look if you live in the city (close to Altens industrial estate) - stick to the coastal path though as the outcrops are treacherous in places.

A pink/orange coloured felsite sill intrudes into the Dalradian rocks (over distance of c. 300 m). It can be seen on the sides of inlets 1 - 3 (Figures 1, 2). Inlet 2 is locally known as Cave of the Red Rocks (NJ 965 029). Felsite is a silica rich, intrusive igneous rock which has a very homogenous appearance (hand-specimen) compared to the often highly layered mica- schists. 

Figure 1. Location of felsite sill and fractured basement outcrops, on Aberdeen coast. The red bars are where the sill outcrops on either side of the inlets marked 1 - 3. Dashed line shows approx. extent of the sill. 

Figure 1. Location of felsite sill and fractured basement outcrops, on Aberdeen coast. The red bars are where the sill outcrops on either side of the inlets marked 1 - 3. Dashed line shows approx. extent of the sill. 

The sill is intensively fractured by tectonic fractures which strike in numerous directions (Figure 2). Note the pile of felsite blocks which have fallen onto the Dalradian rocks (right side), whose edges are fracture planes. The sill appears more heavily fractured (than the Dalradian rocks) in places although both rock types are heavily fractured.  A close up view of the baked margin (inlet 1) between the sill and Dalradian rocks is shown in Figure 3 - there are nice examples of quartz boudins (label 1). The sill has some good examples of conchoidal fracturing (Figure 4) - these curved breakage surfaces are common in silica-rich fine grained rocks such as obsidian and felsite. 

Figure 2. Felsite Sill on north side of inlet 1 (tabular, pinkish body at top, up to c. 2 m thick). It is overlain by glacial material (with grass at top). Facing North. 

Figure 2. Felsite Sill on north side of inlet 1 (tabular, pinkish body at top, up to c. 2 m thick). It is overlain by glacial material (with grass at top). Facing North. 

Figure 3. Lower Baked margin of the felsite sill at location 1. The clinometer sits on the contact between the schists and the baked margin. The sill overhang is at the top. 

Figure 3. Lower Baked margin of the felsite sill at location 1. The clinometer sits on the contact between the schists and the baked margin. The sill overhang is at the top. 

Figure 4. Concoidal fracturing in the felsite sill at location 1, on left hand side of the clinometer. 

Figure 4. Concoidal fracturing in the felsite sill at location 1, on left hand side of the clinometer. 

Label 3 on Figure 1 points to the end of the sill, about half way along the inlet in landward direction. Here the sill has a boudin-type shape with a rounded termination to the right (Figure 5). Also, note the much smaller related sill just below this (label 1, Figure 5). The sill is not present on the other (N side) of inlet 3, suggesting that this is also the northward extent of it. 

Figure 5. Felsite sill at inlet 3 (facing south). note the rounded termination to the right and the small related sill in the host rocks just below this at label 1. Here the Dalradian rocks can be seen above and below the sill. 

Figure 5. Felsite sill at inlet 3 (facing south). note the rounded termination to the right and the small related sill in the host rocks just below this at label 1. Here the Dalradian rocks can be seen above and below the sill.