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Week
Excursion to the Isle of Eigg
A report on this excursion has been written for
the Edinburgh Geologist - read
it here.
Weekend Excursion
to the Ballantrae Area
by Ian
Jackson
Britain is blessed with a plethora of compact areas
of great geological significance and one of these, the Ballantrae Ophiolite
Complex in the Girvan-Ballantrae area of SW Ayrshire, was the venue for
the annual weekend excursion (20-22 June 2003) of the Edinburgh Geological
Society when a select party of 12 members and guests attended.
The locality’s renown and variety of sedimentary and unusual
igneous rocks with complex tectonics attracted three farther-flung members
from England. The party was based at Girvan, and the excursion
leader was Dr Phil Stone (British Geological Survey), who has an unrivalled
knowledge of the regional and local geology of the area. On Friday evening,
the leader gave an excellent slide talk on the geology with specific emphasis
on localities to be visited.
The Ballantrae
Ophiolite Complex represents a slice through the top of the earth’s
mantle, and comprises three NE-SW volcano-sedimentary tracts (dominated
by basaltic pillow lavas) separated by a Northern and Southern Serpentinite
Belt, composed of ultramafic rocks from the mantle that are now extensively
serpentinized.
Saturday was devoted
to the early Ordovician Ballantrae ophiolite and its associated volcano-sedimentary
assemblage, whereas Sunday concentrated on the Ordovician and Silurian
stratigraphic succession and associated structure.
In general, the two days represented a single traverse through
the Complex and cover rocks, starting in the south and working steadily
northwards.
Saturday morning began with a visit to the classic locality
of Downan Point where pillow lavas of Caradoc age, south of the Ballantrae
Complex and their slightly younger Tremadoc and Arenig pillow lavas, are
superbly exposed (Photos 1-4). Cream-coloured sandy sediment infills the
inter-pillow space and contrasts sharply with the matt black basalt (Photo
2). The convex upper portions of the pillows preferentially contain gas
bubbles (now seen as zeolite-filled amygdales, Photo 3) and identify the
original orientation of the pillow (in Photo 4, the pillow is now inverted).
The first stop in the Ballantrae Ophiolite Complex was at Bennane
Lea where the Southern Serpentinite Belt has
a faulted contact with the central tract of volcano-sedimentary rocks.
On the beach here, members picked up samples of the distinctive Ailsa
Craig Tertiary microgranite (some 15km away in the Firth of Clyde), attractively
speckled with phenocrysts of blue riebeckite, and from which curling stones
are fashioned.
With such an ironically macabre location nearby,
lunch simply had to be taken at Sawney Bean’s Cave at Balcreuchan
Port, where the eponymous cannibal partook of lunch (human variety) on
numerous occasions in the years preceding his demise and the cave’s
demolition at the hands of King James VI’s troops in1604 (see Phil
Stone’s informative article in the Edinburgh Geologist #30, 1997).
Eighteen months later in London, dissident English Catholics mounted a
similar, but this time unsuccessful, venture against the king himself
in the Gunpowder Plot! At Balcreuchan Port, the party viewed the basalt country rocks of the
central tract (Photo 5) and, adjacent to a major fault bounding an offshore
Permian basin, were able to contrast them with the softer serpentinites
at the southern margin of the Northern Serpentinite Belt. Both fault blocks
are intruded by a Tertiary dolerite dyke with infilled vesicles adjacent
to the upper contact (Photo 6). The day concluded with an examination
of contacts and lithologies (gabbros and ultramafics) from the Northern
Serpentinite Belt at the southern end of Pinbain Beach.
Sunday was devoted largely to the transgressive Ordovician
and Silurian sedimentary cover, north of the Ballantrae Complex. In the
morning, after a visit to reef limestones of Caradoc age poorly exposed
in the inland Craighead Inlier, the party returned to the well exposed
coastal sections where, in the northern volcano-sedimentary tract at the
northern end of Pinbain Beach,
one highlight was an unusual porphyritic basalt with large plagioclase
phenocrysts (Photo 7). Ordovician sediments crop out directly north of
the Pinbain block and, north of Kennedy’s Pass where the siltstone
turbidites of the Ardwell Formation comprise a major part of the Caradoc
succession, a series of spectacular chevron ‘boxfolds’ of
probable early tectonic origin are well displayed (Photo 8).
After lunch, the youngest
beds of the area were examined from north to south in a traverse descending
the stratigraphical succession. At Cow Rock
directly south of Girvan (Photo 9), the channelled contact of the mid-Llandovery
Quartz Conglomerate was seen above the slumped and folded turbidites of
the Woodland Formation (early Llandovery). At the nearby Horse Rock, a
slightly older coarse debris flow conglomerate, the Craigskelly Conglomerate
of basal Llandovery age (Photo 10), rests with slight unconformity on
the siltstone turbidites of the Shalloch Formation of Ashgill age (Photo
11). The final locality (optional) was a roadside
farm visited by some members who purchased the local Ayrshire tatties
howked on the adjacent land.

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Photo 1. Cliff (4m) in basalt pillow lavas of Caradoc age
at Downan Point (scale from tufts of sea-pink at top right). Face
represents a cross-section cutting through the base of the pillows
and dip is steeply away from the observer. |

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Photo 2. Matt black basaltic pillow lavas (Caradoc age)
of pillows of various size with the inter-pillow space infilled by
creamy to ochreous sandy sediment. Downan Point. |

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Photo 3. Clean, wave-washed surface of pillow lavas showing
concentric trains of zeolite-filled vesicles in the upper zones of
individual pillows. Creamy white sandy sediment, infilling the
space between the pillows, is prominent at the bottom right of the
photo, though the actual margin of some of the pillows is rimmed by
a narrow zone of greyish white-weathering crystalline calcite. Downan
Point. |

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Photo 4.
Close up of the basal contact of an individual pillow that, soon after
extrusion, inverted and toppled onto its convex upper surface, as
confirmed by the underlying trains of concentric vesicles in the original
uppermost zone of the pillow. Downan Point. |

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Photo 5.
The dynamited remains of Sawney Bean’s Cave at Balcreuchan
Port, developed along a
minor NE-SW fault in basalt pillow lavas of Arenig age at the northern
margin of the central volcano-sedimentary tract. |

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Photo 6.
Sub-vertical (85º westward seaward dip) Tertiary dolerite dyke
(with hammer) from the Arran swarm intruding
piebald, serpentinised ultramafic rocks, Balcreuchan
Port. Trains of zeolite-filled
vesicles are slightly oblique to the chilled upper contact of the
dyke (left hand margin). Sawney Bean’s Cave lies just to the
right of the photo across a major NNW-SSE Permian fault separating
the sheared and softer serpentinites from the more resistant basalts
forming the cliff at top right. |

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Photo 7.
Porphyritic basalt lava with large plagioclase laths displaying
swirling texture and local flow-banding. Slockenray headland near
Pinbain Beach
in the northern volcano-sedimentary tract (Pinbain block). Nearby,
this lava is mixed with a fine-grained aphyric basalt. |
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Photo 8. Chevron ‘box-fold’ pair (anticline
on the left), north of Kennedy’s Pass in Ardwell Formation turbidites
of Caradoc age.
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Photo 9.
Very steep-sided channel infilled with pebbly Quartz Conglomerate
(a mid-Llandovery matrix-supported debris flow) at Cow Rock south
of Girvan. The conglomerate truncates the slumped and locally strongly
folded, orange-weathering siltstone turbidites of the Woodland Formation
(early Llandovery), left of the hammer. |
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Photo 10.
Thicker beds of coarse cobble conglomerate containing prominent
pink granite clasts interbedded with, and generally softer weathering
than, thinner, slightly more resistant turbiditic sandstones dipping
steeply northwards at 80º (three sandstone beds visible). Craigskelly Conglomerate, a basal Llandovery proximal
debris flow; Horse Rock, south of Girvan. |

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Photo 11. Very tight synform in thin-bedded siltstone
turbidites (Shalloch Formation) of Lower Ashgill age, at Horse Rock. |
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