Hartley & Seaton Sluice Dykes

Location of the Hartley South, Hartley North and Seaton Sluice dykes

A map of the Collywell Bay and Seaton Sluice area showing the roads, the bedrock and dyke locations

Map showing the locations of the Hartley and Seaton Sluice dykes

Key

Key to the Hartley and Seaton Sluice dyke maps

These three tholeiite basalt dykes share a very similar petrology and petrography and are all exposed in the vicinity of Collywell Bay.
They are said to be of the Talaidh typeand not the Brunton type like the Tynemouth dyke to the south.
The Talaidh type is characterised by: undulatory extinguishing labradorite plagioclase plates set in a fine lattice of smaller, more slender plagioclase crystals, allotriomorphic augite, patches of greeny-brown to black iron-rich mesostasis and vesicles filled with oxides, calcium carbonates, chalcedony and chlorite.

Although being of the same type, the fabric of the rock varies considerably between samples depending on the positionsthey occupied in the dyke and the crystallisation andalteration processes in operation there.
Basalt from each of the three dykes is shown below viewed in plane polarised light at x5.

The Hartley South Dyke tholeiite basalt viewed in plane polarised light at  x5

The Hartley South dyke tholeiite basalt

The Hartley North dyke tholeiite basalt viewed in plane polarised light at  x5

The Hartley North dyke tholeiite basalt

The Seaton Sluice  dyke tholeiite basalt viewed in plane polarised light at  x5

The Seaton Sluice dyke tholeiite basalt

The Hartley South dyke

The most easterly exposure of the Hartley South Dyke lies along the western extension of Holywell Dene at Seaton Sluice. The dyke’s continuation to the north-east has been proven through mine workings. D.H. Land writes that the dyke here is intruded into mudstones in ‘two branches, 20ft apart., with a ‘narrow stringer of dolerite ending upwards in the mudstone.’ He reports it to be have been subject to small-scale quarrying for road metal.

Hartley South dyke in the bed of the burn in Holywell Dene, Seaton Sluice, NZ33197616

Hartley South dyke in the bed of the burn in Holywell Dene at Seaton Sluice NZ33197616

Prepared hand specimen from Hartley South Dyke at NZ33197616 viewed in reflected light

Prepared hand specimen from Hartley South Dyke at NZ33197616 viewed in reflected light

Thin section of tholeiitic basalt from the Hartley South Dyke viewed in plane polarised light. Sample measures 45mm across.

Thin section from the same sample viewed in plane polarised light

Thin section of tholeiitic basalt from the Hartley South Dyke viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Petrography and Petrology

Previously, this dyke and the Hartley North dyke that is exposed in Collywell Bay were considered to be one and the same dyke; the Collywell dyke.

J.J. Teall described fresh samples of the rock as, ‘dark, in somecases almost black in colour and finely crystalline in texture. Porphyritic elements are very rare, but now and then crystals of felspar,similar to those of the Tynemouth rock and doubtless belonging to the same species, may be recognised.In some specimens the small spherical amygdaloids are not uncommon.’ He goes on to describe, ‘long narrow sections of plagioclase, irregular grains and plates of a colourless pyroxene and a certain amount of interstitial matter rendered more or less opaque by various devitrification-products, and especially by small crystals, rods, and granules of magnetite. In some cases the ultimate base is seen to be a rich brown isotropic glass. The secondary products comprise, as usual, calcite, quartz, pyrites, and a yellowish green substance giving aggregate polarisation’ adding that in altered specimens, ‘the pyroxene (is) replaced by calcite.’

M.K. Heslop gave a typically full description of the Hartley South dyke rock in a paper that argued that the dyke that is exposed 24 miles to the west in the bed and banks of the River Wansbeck at Crookdene near Morpeth is a continuation of the Collywell dyke.
She described the rock’s long, slender plagioclase crystals as the first to have crystallised out of the melt, around which augite then crystalised in continuous curved strands that form acanthus-like arrangements frequently fringed with iron oxide grains. She recognised these ‘sheaves’ of pyroxene to be a particular feature of this dyke adding that the augite crystals are poorly individualised, one strand merging with another so that when viewed under the microscope with crossed polarising filters, the extinction ‘creeps over the field in a most erratic way’. Iron oxide particles were also seen in the interstitial glass along with ‘elementary growths of feldspar and augite.’
These aspects of the rock’s fabric are all visible in the Hartley South tholeiite basalt although the pyroxene sheaves are rarein our sample.

One of the few calcium-rich plagioclase phenocrysts in the Hartley South Dyke tholeiite basalt. Sample viewed in plane polarised light at  x25

One of the few calcium-rich plagioclase phenocrysts in the Hartley South Dyke tholeiite basalt
Sample viewed in plane polarised light at x25

One of the few calcium-rich plagioclase phenocrysts in the Hartley South Dyke tholeiite basalt. Sample viewed with crossed polarising filters at x25

The same sample viewed with crossed polarising filters

The ophitic texture of the pyroxene and feldspars in the Hartley South dyke. Sample viewed with crossed polarising filters at x25

The ophitic texture of the pyroxene and feldspars in the Hartley South dyke.
Sample viewed with crossed polarising filters at x25

A

A ‘sheaf’ of pyroxene in the Hartley South Dyke tholeiite basalt. Sample viewed in plane polarised light at x25

A

The same sample viewed with crossed polarising filters

Inclusions in the mesostasis of the Hartley South Dyke tholeiite basalt PPL x100

Inclusions in the brown mesostasis of the Hartley South Dyke tholeiite basalt
Sample viewed in plane polarised light at x100

An amygdaloid filled with clear, calcium-rich material along with  dark and opaque material in the Hartley South Dyke tholeiite basalt. Sample viewed in plane polarised light at  x25.

An amygdaloid filled with clear, calcium-rich material along with dark and opaque material in the Hartley South Dyke tholeiite basalt
Sample viewed in plane polarised light at x25.

An amygdaloid filled with clear, calcium-rich material along with  dark and opaque material in the Hartley South Dyke tholeiite basalt. Sample viewed with crossed polarising filters  at  x25.

The same sampleviewed with crossed polarising filters

The Hartley North dyke

This dyke has been known in the past as the Collywell dyke.

In 1878, G.A. Lebour wrote, “In some cases the actual natural top or vertical dying-out of a dyke may be seen, as on the coast a little to the south of Seaton Sluice. Here, neara spring on the beach, a dyke, at least 12 feet in width where it rises at the foot of the cliff, is finely shown ending off in two tongues of basalt, the longest of which, after curving amongst disturbed and contorted shale, dies out about 11 feet from the ground beneath a bed of sandstone, which it had not the force to break through. The fault along which this dyke was injected is well seen continuing its upward course. A more instructive section it would be difficult to find.”
Sadly this is no longer visible; the erosion prevention wall has been built in front of the cliff and the dyke is seen here only at the base of the wall where it is a couple of metres wide.
Land writes that it is also visible on the foreshore where its width varies between 4ft and 9ft.

Hartley North dyke in Collywell Bay from NZ 33947637

Hartley North dyke in Collywell Bay from NZ 33947637

Prepared hand specimen from Hartley North Dyke at NZ 33947637 viewed in reflected light.

Prepared hand specimen from Hartley North Dyke at NZ 33947637 viewed in reflected light

Thin section of tholeiitic basalt from the Hartley North Dyke viewed in plane polarised light. Sample measures 45mm across

Thin section from the same sample viewed in plane polarised light
Sample measures 45mm across.

Thin section of tholeiitic basalt from the Hartley North dyke viewed with crossed polarising filters.

The same thin section viewed with crossed polarising filters

Petrography and Petrology

This dyke is said to be another of theTalaidh type but our sample from the foreshore is so altered that the pyroxene and the mesostasis are now carbonatised and the opaque oxide material has come out of the mesostasis and is scattered throughout the rock in small particles and in skeletal masses.

Land’s modal analysis of a fresh sample of the the rock provided percentage by volume figures of:
Plagioclase 35%
Augite 36%
Mesostasis 19%
Chlorite 7%
Iron-oxide 1%
Carbonate 2%
with traces of pyrite.

Altered mesostasis and pyroxenes in the Hartley North Dyke PPL x25

Altered mesostasis and pyroxenes in the Hartley North Dyke
Sample viewed in plane polarised light atx25

Altered mesostasis and pyroxenes in the Hartley North Dyke viewed with crossed polarising filters at x25

The same sample viewed with crossed polarising filters

Alteration in   the Hartley North dyke tholeiite basalt.  In this sample the pyroxenes and mesostasis have been  altered to  carbonates. Sample viewed with crossed polarising filters at x10

Alteration in the Hartley North dyke tholeiitebasalt.
In this sample the pyroxenes and mesostasis have been altered to carbonates.

Opaque oxides separated out from the altered mesostasis in the Hartley North dyke. Sample viewed in plane polarised light at x25

Opaque oxides separated out from the altered mesostasis in the Hartley North dyke
Sample viewed in plane polarised light at x25

Opaque oxides separated out from the altered mesostasis in the Hartley North dyke. Sample viewed with crossed polarising filters at x25

The same sample viewed with crossed polarising filters

The Seaton Sluice Dyke

The dyke measures approximately 2 metres wide and crosses the foreshore at the northern end of Collywell Bay. It culminates close to the cliff although Land writes that it restarts about 15 m to the south. It would have been visible in the cliff prior to the building of the erosion prevention wall.

The Seaton Sluice dyke at Seaton Sluice, NZ 339766

The Seaton Sluice dyke at Seaton Sluice, NZ 339766
Looking back along the dyke from the foreshore to the point where it terminates near the wall to the left of the cliff.

Prepared hand specimen from Seaton Sluice Dyke at NZ 339766 viewed in reflected light.

Prepared hand specimen from Seaton Sluice dyke at NZ 339766 viewed in reflected light

Thin section of tholeiite basalt from the Seaton Sluice Dyke viewed in plane polarised light. Sample measures 40mm across.

Thin section from the same sample viewed in plane polarised light
Sample measures 40mm across.

Tholeiite basalt from the Seaton j Dyke viewed with crossed polarising filters. Sample measures 40mm across.

The same thin section viewed with crossed polarising filters

Petrography of Seaton Sluice dyke

As will be seen from the images, this is a very altered and weakened rock. In our sample, the fine lattice of plagioclase remains with an occasional microphenocryst of zoned plagioclase.Much of the mesostasis and has been oxidised to carbonates and there are only a few remaining traces of augite. Amygdaloids of calcium rich material are abundent.

An area of the   tholeiite basalt of the Seaton Sluice dyke  that has been altered to carbonates. Sample viewed with crossed polars at  x25

An area of the tholeiite basalt of the Seaton Sluice dyke that has been altered to carbonate material
Sample viewed with crossed polarising filters at x25

An amygdaloid and shadow-zoned plagioclase phenocryst in the Seaton Sluice dyke tholeiite basalt.  Sample viewed in plane polarised light at  x25

An amygdaloid and shadow-zoned plagioclase phenocryst in the Seaton Sluice dyke tholeiite basalt.
Sample viewed in plane polarised light at x25

An amygdaloid and shadow-zoned plagioclase phenocryst in the Seaton Sluice dyke tholeiite basalt.  Sample viewed with crossed polarising filters at  x25

The same sample viewed with crossed polarising filters

References

Lebour, G.A. 1878. Outlines of the Geology of Northumberland, Lambert & Company, Ltd, Newcastle
Teall, J J H. 1884. Peteological Notes On Some North-Of-England Dykes. The Quarterly Journal Of The Geological Society Of London, Vol. 40. Pp. 209-247.
Heslop, M.K. & Smythe, J.A. 1910.TheDyke at Crookdene (Northumberland) and its Relations with the Collywell, Tynemouth and Morpeth Dykes. Quarterly Journal of the Geological Society of London, Vol.66, Part 1.
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.
Land D.H. et al 1974.Geology of the Tynemouth District (Explanation of One-Inch Geological Sheet, 15, New Series) HMSO.
British Geological Society, Sheet 15 Tynemouth, http://www.largeimages.bgs.ac.uk/iip/mapsportal.html?id=1001485

No vestige of a beginning, – no prospect of an end

error: