Bothal Dyke

Location of the Bothal dyke

Very little has been written about the the Bothal dyke.
Like the Morpeth dyke, it appears on geological survey maps published prior to 2001 but it is not marked on current maps.
Holmes and Harwood wrote that the rock, ‘exhibits characters which are intermediate between those of the Salen and Brunton types.,’ indicating that it is ophitic with granular augite and elongated labradorite appearing in roughly equal measure and that a dark brown mesostasis occupies about 20% of its volume.
Later, Laurence and Jackson wrote, “The dyke formally seen at Bothal…. probably represent(s) a continuation of the West Sleekburn dyke’, a dyke visible only in the Yard and Plessey seams of the old mine workings east of Guide Post.
Using pre-2001 maps we have searched the two locations where the dyke is shown to cross water courses; the River Wansbeck at Bothal Mill and a small tributary to the river a short distance to the north-west.

Map based on pre-2001 BGS maps showing approximate location of the Bothal Dyke and our sample points

The location of the Bothal Dyke based on maps published before 2001.
Grid referenced points show the locations of sampled basalt.

Key to Bedrock Types

Key to map

We believe we have found an exposure of the dyke at NZ234861, a location on the south bank of the river between the bridge and the weir opposite Bothal Mill. The location is difficult to get to and the outcrop is often submerged by the Wansbeck.

Bothal Dyke outcropping on the north bank of the Wansbeck at NZ234861

Bothal Dyke outcropping on the south bank of the Wansbeck at NZ234861

The location of the outcrop of the Bothal Dyke when river levels are higher

The south bank location of the outcrop of the Bothal Dyke viewed from the north bank when river levels are higher

On the north bank of the river, there is a lot of basalt in the vicinity of the weir where large basalt boulders have been used to repair and extend a stone platform. Some of the basalt stones and boulders may be left-overs from the construction work, but some of the more weathered and embedded rocks look as if they might be associated with the dyke. Certainly, the texture of the rock in these samples is very similar to the texture of the rock that outcrops on the south bank.

Further to the north-west, a small stream flows down a deep gully into the Wansbeck. Basalt stones and boulders, some of them quite large, can be found in the water course all the way up to the locations where the old maps show the dyke intersecting it.
To date, we have not been able to find an exposure of the dyke here, but the presence of these boulders in this fairly inaccessible place suggests there is, or was, a natural source nearby.

Bothal Dyke, north-west location (NZ231863)

The stream-bed in the locality of the mapped north-west end of the dyke

Petrography

Thin sections of rock from the boulders on the north-bank of the river near the weir and those in the stream bed at the north-west location all reveal quite similar textures to those of the outcropping basalt at location NZ234861.
All the rock associated here with the Bothal dyke is less coarsely crystalline than the sample found near the south-eastern location of the Morpeth dyke. There is also less iron-titanium opaque material, fewer amygdaloids and less green-brown interstitial material.

Sample from the outcrop on the south bank of the Wansbeck at NZ234861. The section is 38mm from side to side and is viewed in plane polarised light.

Sample from the outcrop on the south bank of the Wansbeck near Bothal Mill (NZ234861)
The section is 38mm from side to side and is viewed in plane polarised light.

Sample from the outcrop on the south bank of the Wansbeck near Bothal Mill. The section is 38mm from side to side and is viewed with crossed polarising filters.

The same section viewed with crossed polarising filters

Bothal Dyke SE (NZ234862.) Whole slide measuring 4mm across the top viewed in plane polarised light.

Basalt from a boulder on the north bank of the Wansbeck near the weir at Bothal Mill (NZ234862)
The whole thin-section (40mm across the top portion) viewed in plane polarised light.

 Basalt from  a boulder in the stream-bed at the north-west location  (NZ231863). The whole thin-section (39-mmm across) is shown  in plane polarised light.

Basalt from a boulder in the stream-bed at the north-west location (NZ231863)
The whole thin-section (39-mmm across) is shown in plane polarised light.

Samples from the south east location of the Bothal dyke

The plagioclase phenocrysts in the outcropping rock of the dyke occur individually as well as in glomerocrysts. Most plagioclase phenocrysts are distinctly zoned with high calcium cores that approach bytownite in composition. These cores have a yellow to dark-orange interference colour even when the sample is thinned to less than 30 microns.
One phenocryst in the outcropping rock has a spherical array of opaque material.

Porphyritic plagioclase  concentrated in one area of the sample occurring as glomerocrysts and individual crystals. Sample viewed in plane polarised light  (x5)

Porphyritic plagioclase concentrated in one area of the sample from the dyke’s outcropping rock occurring as glomerocrysts and individual crystals. Sample viewed in plane polarised light (x5)

Porphyritic plagioclase  concentrated in one area of the sample occurring as glomerocrysts and individual crystals. Sample viewed with crossed polarising filters  (x5)

The same group of plagioclase phenocrysts viewed with crossed polarising filters (x5)

Plagioclase phenocryst in the sample from outcropping rock showing the yellow to deep orange-brown interference colour and zoning. Sample viewed with crossed polarising filters at X25

Plagioclase phenocryst in the sample from outcropping rock showing the yellow to deep orange-brown interference colour and zoning
Sample viewed with crossed polarising filters at X25

Plagioclase phenocrysts in the South-east sample with calcium-rich cores that have the composition of Bytownite

Plagioclase phenocrysts in rock from the north bank of the river close to Bothal weir. (NZ234862). Sample viewed with crossed polarising filters
These plagioclase phenocrysts with their zoned high-calcium cores are very similar to those of the outcropping rock downstream on the south bank. The titanium-iron content, however, does occur as more sharply defined crystals rather than being incorporated into the mesostasis.

The same crystals rotated to reveal their zoned extinction

The same group of crystals rotated to reveal their zoned extinction

Plagioclase phenocryst with opaque inclusions. Viewed in plane polarised light at x25

A plagioclase phenocryst in the sample from the outcropping rock that has an unusual spherical array of opaque material scattered across the twins. Sample viewed in plane polarised light.

Stellar groupings of plagioclase laths around pyroxenes in the Bothal dyke basalt. Sample viewed with crossed polarising filters.

Plagioclase laths in the matrix of the outcropping Bothal dyke rock viewed with crossed polarising filters
Some of the plagioclase laths that occupy the matrix of the rock in the outcropping sample exhibit the ‘stellar arrangement’ associated with the Brunton type of tholeiite basalt.

The ophitic texture exhibited by the pyroxenes and plagioclase laths in the outcropping rock of the Bothal dyke. Sample viewed with crossed polarising filters at x25

The sub-ophitic texture exhibited by the pyroxenes and plagioclase laths in the outcropping rock of the Bothal dyke viewed with crossed polarising filters

Amygdaloid filled with mesostasis and calcite surrounded by plagioclase laths and pyroxene grains that are randomly distributed in the dark mesostasis.

Amygdaloid filled with mesostasis and calcite surrounded by plagioclase laths and pyroxene grains. Sample viewed with crossed polarising filters.
In this portion of the sample, plagioclase laths are randomly distributed rather than appearing in stellar clusters.

Samples from the north-west location

Zoned plagioclase phenocryst in a sample from the NW location having the composition of Labradorite . PPL

A plagioclase phenocryst in a sample from the NW location viewed with crossed polars
Plagioclase in this sample tends tothe composition of Labradorite

Calcite in amygdaloid in north-west sample viewed with crossed polars

Calcite in an amygdaloid in a sample from the north-west location

References

Holmes, A and Harwood, H F. 1929. The Tholeiite Dikes Of The North Of England. The Mineralogical Magazine and Journal Of The Mineralogical Society, No. 124. Vol 22.
Lawrence, D J D and Jackson, I. 1990. Morpeth-Bedlington-Ashington Technical Report Wa/90/19, Geology And Land-Use Planning: Morpeth-Bedlington-Ashington Part 2. Geology. Keyworth, Nottingham: British Geological Survey.
British Geological Survey. Sheet 14, Morpeth. http://www.bgs.ac.uk/data/maps/maps

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