Yorkshire OUGS - Blencathra Field Weekend 8-10.11.13

Yorkshire OUGS - Blencathra field weekend 8-10.11.13

Day 1 9.11.13 – Borrowdale Volcanics tour

With dark rain-bearing clouds overhead, we took a tour of Borrowdale with the resident field geologist, Angela Leng, from Blencathra Field Study Centre, visiting four diverse locations within the valley, looking at rocks from the Borrowdale Volcanic Group (BVG) and ending the day at a quarry to the north-west of Borrowdale, at Mungrisdale, investigating further field relationships of related rocks.

A map of the Lake District geology is provided in Fig. 1.

Fig. 1 Generalised geology of the Lake District. Derwent Water lies to south of Keswick. Locality 1 – Castle Head Viewpoint, Borrowdale - GR NY269227

From this viewpoint to the south of Keswick, we observed the distinctive craggy elevated topography of the central Lake District mountains, made of hard resistant volcanic rocks (BVG), which erupted c450Mya, in contrast to the fells to the south west, including Catbells together with the northern Lakeland fells of Blencathra and Skiddaw, which are more rounded and composed of older Skiddaw Slate Group rocks formed c500Mya (Fig 2).

Fig. 2 From Castle Head Viewpoint, looking south. Note elongate shaped islands within Derwent Water. Photo Maggie Deytrikh

We noted recent quaternary glacial features, such as the prominent U shaped valleys, ribbon lakes and drumlins, which manifest as the elongate shaped islands in Derwent water, giving a ‘basket of eggs’ appearance in the valley floor.

Looking and the rocky outcrop at our feet, we analysed it according to Andrea’s system of 3 ‘S’s and 3 ‘C’s, namely Size, Shape, Sorting and Colour, Composition and Cement. We were standing on a medium grained (1-2mm), dark grey, igneous rock, with crystals of augite pyroxene and calcium plagioclase feldspars in the matrix of the rock, together with larger (up to 4-5mm) phenocrysts of dark pyroxene.

This was interpreted as a volcanic plug of porphyritic micro-diorite or dolerite composition, representing the eroded remnants of a volcano which erupted 460Mya. As its composition is relatively mafic, it represents an early phase from the BVG, and as the cooling joints were more widely spaced and the grain size large, it is interpreted as coming from the core of the volcano rather than a lava flow. We speculated on the direction of ice flow, which would have been in a northwards direction, causing the evident ‘crag and tail’ appearance, with the upstanding resistant volcanic core to the south and the deposited softer rocks, leaving a tail to the north, as the ice flowed northwards, depositing some of its load. There was an interesting ‘exotic’ bench at the viewpoint, made of polished Skiddaw Slate Group material (Fig. 3). It showed graded bedding of course greywacke sandstone through fine grained grey siltstone to mudstone, in repetitive sequences with an erosional base, enabling the ‘way-up’ of the depositional sequence to be determined. We discussed its likely formation in a deep water marine environment, as turbidite sequences, resulting from underwater density flow of sediment down the continental shelf, with courser material sliding down more rapidly and finer materials, progressively deposited as they settled out of the water column. Three Bouma cycles could be identified.

Fig. 3 Part of bench, made of Skiddaw Slate, showing graded bedding and erosive base to a Bouma sequence. Photo Maggie Deytrikh

Locality 2 – Beneath Falcon Crag, Borrowdale - GR NY269205

From the shoreline of Derwent Water, we examined the mainly red coloured rocks. The red coloured matrix appeared to be fine muddy sedimentary material, with various igneous clasts, ranging from mm scale to much larger, 10-20cm diameter ones. The matrix was red coloured and fizzed with dilute acid, and was interpreted that its cement was composed of both haematite, (iron oxide, Fe2O3, reflecting an oxidizing environment) and calcite. The clasts were noted to be angular, and principally of 3 types of igneous material; namely grey andesitic crystalline clasts, vesicular pumice and lighter coloured, felsic, rhyolitic clasts, which were not fully crystalline, but had a streaked, deformed appearance, often twisted and intermingled with the matrix. The rhyolitic magma material had been viscous and plastic when it was incorporated into the muddy matrix.

These loose rocks at the base of the crag illustrate beautifully, how rocks can tell the events of 460Mya. The clasts are of pyroclastic origin, erupting during an explosive event when one of the Lake District super volcano vents was shattered and the resultant pyroclastic debris fell down to the valley floor, where it mixed with water, then flowed as a highly destructive mud slide or Lahar further down the valley. Evidence of the water lain deposits are from the muddy nature of the matrix which contains muscovite mica. The angular nature of the clasts and the muscovite mica, indicate that the debris was sourced close by and did not travel far.

The clasts represented different stages of magma evolution, from darker grey intermediate clasts of andesitic material to more evolved rhyolitic magmas (Fig. 4). We discussed the ‘brecciated’ nature of the rock, with angular volcanic clasts in a sedimentary muddy matrix of volcanic ash which fell into a lacustrine environment and formed a mud flow. The rock is described as an agglomerate as the clasts are igneous in origin.

Fig. 4 Falcon Crag Agglomerate. photo Maggie Deytrikh

Locality 3 – Quayfoot Quarry, Borrowdale - GR NY253167

Our hard hats doubled as umbrellas as we investigated the fine grained greenish coloured rocks of the disused quarry face (Fig. 5). Cleavage was observed in several directions and difficult to distinguish from bedding planes. They represent a shallow water lain tuff of volcaniclastic origin, where ash fell from the surrounding volcanoes and settled into a lacustrine environment. Subsequent continental collision and regional metamorphism produced cleavage, mainly associated with the general NW-SE direction compressive forces of the Caledonian Orogeny, competed approx 450Mya. The green coloration is from low grade metamorphism during the continental collision processes. Some rocks had a distinct ‘wrinkly sheen’, producing almost a phyllite appearance, of increased metamorphic change. A lens of several metres length and a metre high, of darker material was observed in an upper face of the quarry. This is thought to represent a pyroclastic flow, which rapidly channelled through the water lain ash deposits.

Fig. 5 Soggy and hungry geologists at Quayfoot Quarry, Borrowdale. Photo Maggie Deytrikh

Locality 4 – Sour Milk Gill, Seathwaite, Borrowdale - GR NY232122

At last, we saw the spectacular valley of Borrowdale in all its glory as the sun appeared at the end of a busy day of geology (Fig. 6). We noted the spoil heaps of disused graphite mines on the hillside, which were mined from early 16th century, originally for use in the manufacture of cannonballs before their more famous use as pencil ‘lead’, giving rise to the Keswick Pencil industry, and where today you can see the world’s largest pencil in the Keswick Pencil Museum!

Fig. 6 Borrowdale, looking north-eastwards, from Sour Milk Gill. Photo Maggie Deytrikh Climbing up alongside Sour Milk Gill, we saw tilted volcaniclastic sediments with bedding planes dipping 50°S, towards the centre of the Scafell Caldera. Asymmetrical ripple marks were observed, indicating reworked surface processes and direction of current northwards was inferred, confirmed by seeing cross bedding in other beds (Fig. 7).

Fig. 7 Sedimentary structures, including cross bedding, Sour Milk Gill. Photo Maggie Deytrikh

Rip-up clasts and volcanic bombs, with flattened bases were observed (Fig. 8). All these features are interpreted as a result of volcanic and sedimentary processes, during a major explosive phreatoplinean magmatic event, associated with the Scafell caldera collapse, when ash fell into a large volcanic lake, forming the andesitic bedded tuff succession of the Whorneyside Tuff Formation, in the late Ordovician period. When ash falls into water, it can produce bentonite clay materials, which help weld the sediments.

Fig. 8 Volcanic bomb, Sour Milk Gill. Photo Maggie Deytrikh One of the path steps, although not in-site, showed an almost textbook cross section of brittle and ductile fault processes (Fig. 9).

Fig. 9 Step on path leading up Sour Milk Gill, Borrowdale, showing ductile deformation with Z folds in the bottom left of the picture and brittle fracture towards the top right. Photo Maggie Deytrikh

Locality 5 – School House Quarry, Mungrisdale - GR NY363306

Our last locality did not disappoint, as we put much of the day’s work into context. We could see the Eycott hills to the east (Fig. 1). These form a succession of lavas and interbedded volcaniclastic sediments of the Eycott Volcanic Group (EVG) and are thought to be older than the BVG, with more mafic (basaltic- andesitic), tiers of less evolved lava flows, which give rise to a ‘trap’ topography. These volcanoes were formed at a subduction zone, as ocean crust was being ‘consumed’ in an island arc setting, as the Iapetus Ocean closed.

The rocks in the quarry were dark grey Ordovician Skiddaw slates, with areas of larger silica grains, producing resistant layers, compared with the finer grained mud and siltstones, with differential resistance of the rock types giving rise to differential weathering patterns (Fig. 10). Bedding planes and cleavage were parallel. Curious wavy deformation of the cleavage across fault planes was observed too. At the eastern end of the quarry, two vertically aligned course basalt to fine dolerite 50cm wide linear structures, were seen, which could only be distinguished from the surrounding weathered and lichen covered vertical sedimentary layers, by their Lack of cleavage. Closer inspection showed 2-3mm vesicles on one side of the igneous intrusion, representing a way-up structure, and hinting at their intrusion as a sill parallel to the bedding planes. Secondary quartz veining picked out the cooling joints, perpendicular to the cooling surface. We discussed the distinguishing features of igneous sills versus lava flows; the latter having only a lower chilled margin against another rock layer and a rubbly top, representing its aerial exposure as it cooled. Sills are intruded between rock layers, parallel with bedding planes.

Fig. 10 School House Quarry, Mungrisdale, Cumbria

Andrea then helped us to pull all of our findings at the quarry together. The Ordovician Skiddaw Slates had formed as turbidites, as the Iapetus Ocean closed to produce some folding. The dolerite sills (although much of the literature refers to these structures as dykes) were then intruded, and subsequently further compressive forces of the Caledonian Orogeny produced isoclinal (nearly vertical) folding in the competent mudstone layers, less so in the dolerite dykes. These forces also produced low grade metamorphism and hardening of the rocks to produce slates. The folds represent ductile deformation. Subsequently, brittle deformation occurred with the thrust folding ‘extruding’ southwards and outwards from the quarry face, producing drag folds in the process (Fig. 11).

Fig . 11 School House Quarry, Mungrisdale, Cumbria, showing the direction of thrust faulting, and the production of drag fold. Photo from Alison Quarterman presentation We discussed how ductile deformation progresses until the elastic limit is reached, when, brittle fracture ensures, producing thrust faulting. Time considerations are important, as materials can withstand greater stresses and strains over a longer time, but may fracture in a brittle manner if the stress is imposed quickly.

Reflecting upon the days geology, I found it interesting to note that most of the distinct topography of the north Lake District results from very recent effects, during the last glacial period from 110- 12kya, acting upon very ancient rocks of Cambrian and Ordovician age of 400-500Mya, with little evidence of the intervening geological record.

References

Branney M J, Geological Conservation Review, Sour Milk Gill, GCR ID: 294, accessed 10.11.13

Millward D, Geological Conservation Review, Eycott Hill, GCR ID: 291, accessed 10.11.13

Fig 9. from http://www.holiday-cottage-in-cumbria.co.uk/pages/Geology.html, accessed 15.11.13

Fig 11. From http://mrsroger.edublogs.org/files/2009/03/rock_deformation-a2-alison_quarterman.pdf, accessed 15.11.13

Evening session at Blencathra Field Study Centre

Andrea led a classroom session demonstrating the Lake District rocks we had seen today, together with the elusive Skiddaw granite, which some of us were keen to observe in the field. She introduced some of Ann’s ‘newbies’, who were new to geology, to field rock samples. We were able to do some petrology in the old fashioned OU manner, with polarising microscopes rather than a computer, after a ‘mercy dash’ the previous evening to locate some OU slides. A happy time was had by all looking at the wonders of peridotite under crossed polars, and seeing the gasps of joy as the bright secondary interference colours of olivine came into view!

Epilogue

On a personal note, I wish to thank the organisers of this trip, Ann Goundry and Rick Savage for a thoroughly enjoyable, friendly trip, which has helped us all to understand and interpret the rocks of my favourite place, the Lake District.

Maggie Deytrikh