Our third visit to the islands and 16 of us, plus organiser Jan and myself, met up in Caleta de Fuste on the 19th. We stayed at
Castillo Playa once again: the 2-person bungalows really suit our groups plus the management is so helpful and friendly. My own aim this time, as well as giving the group a good understanding of the islands' varied volcanic geology, was to explore around 10 new potential field locations and 'road test' them.
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The 2013 Shoestringers at Degollada de Agua Ovejas
with the lateral collapse scar of the Southern Shield Volcano in the background |
Sunday 19th October
An introductory day when we had a look at some aspects of Fuerteventura's "basal complex" and gave the group a chance to settle down. We started off with a quarry face exposure of olivine pyroxene phyric basalt flows near the village of La Matilla: this is an opportunity to introduce the island's shield volcano origins and their current 'post erosional' state where only remnants remain of the original 2000 m high mountains. These remnants take the form of narrow, sharp crested ridges "cuchillos" separated by wide, U-shaped valleys typical of erosion of the near-horizontal basalt flows.
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| Central Shield Volcano flows in the quarry at La Matilla |
From here we moved on towards Puertito de los Molinos, stopping above a barranco east of the village to examine the basal complex exposures of seamount series rocks cut by dykes, which have been eroded and subsequently overrun by an Upper Pliocene olivine basalt flow.
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| Los Molinos barranco - dyke cutting through Basal Complex |
We enjoyed looking at more stunning exposures of the dykes at the mouth of the barranco down in Los Molinos itself and established that they show a strong NNE-SSW alignment. The literature suggests that there is evidence here of the emergent stage of the original Fuerteventuran seamount some 22 Ma years ago but we failed to find the reef material and subaerial weathering evidence that is referred to. The unconformity showed up really clearly in the cliffs however, with young flows and littoral deposits interbedded.
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| Unconformity in the cliffs north of Puertito de los Molinos |
The day finished with a visit to Tindaya, a strikingly pale coloured, conical peak formed of quartz-trqachyte some 18.7 Ma ago. This felsic dome was formed early in the history of the Northern Shield Volcano and is more resistant to erosion than its surroundings. It has been quarried for ornamental stone and on our last day we saw some beautiful examples of the liesegang rings that make it so attractive when we visited Gran Tarajal. Surrounding Tindaya (which was a Majo sacred mountain) we found plenty of shelly material and had fun deciding what could be evidence of Pliocene raised beach and what was a recent 'paella deposit' of limpets and mussels! The problem about the raised beach is that it is much higher than average so various hypotheses were considered with a landslip generated tsunami the winner.
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Tindaya
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| Liesegang rings in decorative slabs, Gran Tarajal. These may have come from Tindaya quarries |
Monday 21st October
After stopping in Tiscamanita to look across to Gairia, one of the early post-erosional scoria cones, we visited Pajara to see the fascinating western facade of the church of Nuestra Senora de Regla which was built in 1687 - 1711 and incorporated many Aztec style motifs - suns, jaguars, birds and the "orobus" - the snake eating its own tail which symbolises the circle of life and death.
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| Nuestra Senora de la Regla, Pajara, |
Along the road to Ajuy we stopped to take a look at an exposure of Jurassic deep sea sediments, 180 Ma old, which were deposited on the Mid-Atlantic Ridge during the earliest stages of Atlantic rifting. Although metamorphosed, hydrothermally altered and cut by numerous dykes the light and dark striped strata are completely recognisable and a very unusual exposure of this type and age of sediment.
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Early Jurassic ocean crust - some of the oldest in existence - cut by Basal Complex dykes
on the slopes of Morro del Moral |
On Ajuy beach there are some stunning exposures of similar rocks which have been worn smooth by wave action so we had a look at these before enjoying some garlic prawns at one of the restaurants at the back of the beach.
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| Jurassic ocean crust sediments cut by basal complex dykes at the north end of Ajuy beach |
There's a convenient path from the beach up onto the north cliffs where it took us along the marine erosion platform to a viewpoint across Caleta Negra. There are 5 Ma Pliocene limestones deposited on the platform and these are overlain by younger lava flows from Morro Valdes.
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| At Caleta Negra the Jurassic marine sediments and dykes of the Basal Complex are unconformably overlain by coastal sediments over which the Morro Valdes lava flowed into the sea forming pillow lava |
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| Morro Valdes lava has deposited blocks on the caliche that coats the marine platform |
