Wednesday 25th November 2020
At our latest Zoom meeting ( November 18th) John Mason gave a well-illustrated talk entitled Extraordinary preservation of original textures in some early Ordovician intrusive rocks of the Mawddach Valley, Coed y Brenin, North Wales.
The area around Coed-y-Brenin is geologically complex. The rocks are sedimentary, Middle to Upper Cambrian in age, are intruded by intermediate to basic igneous rocks of late Cambrian to Lower Ordovician and are uncomformably overlain by the Lower Ordovician Rhobell Volcanics.
The area, once grazed lands, was planted with dense conifer forests from 1920 onwards causing rock exposures to become hidden.This makes geological mapping difficult although cuttings alongside forestry roads do allow for some work to be undertaken. Since the mid-1990’s John has worked in the area undertaking mapping and geochemical analysis. Along with Natural Resources Wales he set up the “Volcano Trail” in early 2000’s. In recent years, John has been doing further detailed mapping of the area, both by himself and in conjunction with students working on collaborative projects.
Part of a map dated 1907, by A. R. Andrew, annotated by John Mason. This map was prepared some decades prior to the purchase of the land by the government and its extensive afforestation. Of the extant maps, it is the closest to what is present on the ground, although he did not differentiate between intrusive rock-types. He likely had the best mapping-conditions of anyone who has examined this ground, since much of it was hilly pasture at the time.
The Northern Porphyry, which occurs as a cluster of angular boulders in excavations in glacial drift in the north of the area examined. Very similar to the Magnificent Uralite Porphyry, but perhaps with even more spectacular hornblende phenocrysts, the parent body remains unlocated, in an area with very poor exposure and dense conifers. Yet the angularity suggests the source is not far away.
Using superb photos of samples/ thin sections of the intrusive rocks John took us through the various Rhobell-related intrusive igneous rocks of the area which describes a classic sequence found in fractionating magma chambers of island arcs. Rocks becoming increasingly siliceous over time. This is nicely shown in the discrimination plot below. One outlier to this sequence was the sample CTG 32 and is probably related to the Aran Volcanic Group with an ocean floor basalt geochemical signature.
cut and polished section from Daren Wyddan, showing angular cognate blocks of hornblende-gabbro in a matrix of leucocratic microdiorite. Daren Wyddan is a large composite intrusion. The gabbro was intruded by the microdiorite; the latter contains a remarkable inclusion-population including cognate cumulate blocks consisting largely of hornblende and reaching several centimetres in size - in this specimen they are the smaller jet-black areas. These are the first such blocks to be described from intrusives here.
Many of the igneous intrusions have undergone intense hydrothermal alteration often having a grey-green colour and are known locally as “Greenstones”, but there are also unaltered rocks present. Using samples form both rock types it is possible to look at what happens geochemically to a rock when it becomes altered.
John went on to show us the results of various geochemical analyses on some of the intrusive rocks, both altered and unaltered. On one least altered rock sample the plot had a trend which would be expected in a fractionation sequence but it’s altered form showed no such trend.
A discrimination diagram plotting TiO2 against Zr for the least-altered Rhobell-related intrusive rocks, which show a progressive increase in Zr with ongoing magma-fractionation, a typical island arc progression with island arc tholeiites early in the sequence trending to more calc-alkaline compositions with time. CTG32 is an obvious outlier - in all plots - and examination of data from other studies indicates it to be probably related to the Aran Volcanic Group, with an ocean-floor basalt geochemical signature. There are other outlier intrusives elsewhere in the Harlech Dome, especially among the dykes, some of which are cleaved but others not. A detailed study is required in order to sort this out: that a probable Cenozoic basalt dyke has been found in Coed y Brenin suggests members of this relatively young suite may also be present elsewhere. At least three phases of igneous activity are represented within the greater area
Another example was of the Magnificent Uralite Porphyry which showed that in it’s altered form some elements remained stable but there had been a decrease in calcium, sodium and phosphorus and an increase in potassium. This was also related to changes in the trace elements in that there was an increase in rubidium ( related to potassium) and a decrease in strontium ( related to sodium).It also appeared that the altered rock had the heavy rare earths stripped out.
This was a most interesting talk and took us into a field we rarely delve into.
Tuesday 10th November 2020
Last month's zoom meeting ( October) John Mason had to cancel his talk on "Coed y Brenin" and we fell back on a more participative format for our meeting, which was quite refreshing and more in the spirit of the club.
We had lots to talk about, in particular, our September trip ( following Corvid guide to six participants) to Tonfanau. Both Bill Bagley and Chris Simpson had taken lots of photographs and Chris had prepared and gave a presentation, with comments and contributions from the other members. For our members who live inland on top of a post glacial landscape, where sections quickly become vegetated, Tonfanau offered a rare chance to see features in an extensive clean vertical section.
The section is accessed through a gate at the southern end of the section, near the site of a borehole which showed some 36m of Quaternary till and gravel, on Tertiary deposits, with Cambro-Ordovician sands and siltstones at 71m. So we were seeing a thin section above a thick pile of glacial deposits.
South section deposits
The cliffs here comprise 2-3m of a massive beige-brown till containing clasts from pebble to boulder size, ranging from angular to rounded, with some being facetted. The spit test shows it to be quite sandy. Provenance of the clasts has shown them to have come from the north, so this is a lodgement till from the Irish Sea Ice stream. (That is debris carried along at the base of the glacier and deposited without being washed out and sorted with the meltwater.) As some of the clasts are well rounded, there must have been water involved in their history at some time, but they have come a long way and could have a complex past.
Channel fill deposits
Walking further north, we found Welsh ice protruding at the foot of the cliffs, below the Irish. It was a darker green-grey and was (spit) more clayey. It is interesting that, although clay particles stay suspended longer than sand, once deposited and indurated, they are more resistant to wave erosion and therefore project further onto the beach. On this coastline, at different times, either the Irish or the Welsh Ice, from the hills to the east, must have become dominant.
Possible cryoturbation structure
As we went further on, the cliffs got higher, up to 4 metres, and more interesting. We were getting gravel beds of pebble to cobble-sized clasts with little matrix. These must have been transported by very fast flowing water as outwash or esker discharges from the melting glacier. Many of these were broken up as the section cut meandering or braiding outwash channels which were also broken up by ice movement. Thinly laminated silts and sands were also appearing with the occasional lone stone which appeared to be a "dropstone" i.e. one released from the bottom of floating ice or an iceberg.
Current waveforms with dropstone
We were left with a vivid dynamic image of the melting snout of a glacier, with braiding outwash channels and proglacial lakes with icebergs.
Saturday 18th July 2020
Geopark terras de Cavaleiros - zoom meeting July 15th 2020
Earlier this year club secretary Dr Chris Simpson went on a tour to the Geopark Terras de Cavaleiros Portugal which was organised by Chris Darmon (Geo-supplies www.geosupplies.co.uk). The talk is based on photographs taken on the tour and information supplied by Chris Darmon. Further details of the Geopark can be found on his website
The Geopark is located in north-eastern Portugal on the border with Spain. The Iberian Peninsula has a lot of interesting geology; but this area is very special because in addition to the background autochthonous ( formed in it’s present position) rocks, there are two separate assemblages of allochthonous (formed away from present position) rocks, i.e. rocks which have been formed elsewhere, and then moved to their present site by large-scale thrusting.
The photograph below shows a portion of a 2.5m map of the Geopark in the main Geopark building. The allochthonous rocks are the roughly circular cluster near the bottom of the photo divided into two halves by a nearly horizontal fault line.
The bottom half is the superior allochthonous group which represent a piece of continental crust. The top half is the inferior allochthonous group which represent a piece of oceanic crust. Both these assemblages were emplaced at differing times during the compression and eventual disappearance of the Iapetus Ocean between about 450Ma and 350Ma.
Apart from the main building in Macedo de Cavaleiros with plentiful rock samples, diagrams of the geology and information sheets, each site in the park has an information board in Portuguese and English. What follows is some examples of the range of geology available in this geopark.
parasitic folding in Ordovian quartzite. Compare the undeformed top stratum in the distance with the folds in the foreground.
Flasergabbro – a distinctive rock type which is a partially metamorphosed gabbro from the oceanic crust allochthen
a remarkable contact. The bottom layer is amphibolite, i.e. metamorphosed basalt from the oceanic crust. The top layer is gneiss, which is metamorphosed continental crust rock. (One post of an information board is seen at top left.)
an exposure of granulite – a rock type which is found below the Conrad Discontinuity and is thus a very rare finding. This site is of international importance. (A discontinuity is a boundary between two rock layers of significantly different density. The Conrad Discontinuity marks the lower boundary of continental crust.)
A coarse tuff, the product of an andesitic volcano, in which there are several pale clasts, all showing the same orientation. This appearance suggests that this tuff was not the usual aerial deposition, but came from a lahar of tuff cascading down the side of a volcanic cone.
an old quarry. This is an isolated limestone deposit showing partial metamorphism to marble. It is poor quality stone – but as it is the only deposit of its type for hundreds of miles around, it was worth quarrying in the 19th century. There are no fossil corals, so this was likely to be an isolated stromatolite reef.
Friday 6th March 2020
At the last meeting Dewi Roberts gave a very interesting talk on fluvial geomorphology. The interest was enhanced by the use of underwater photography within the river which highlighted the processes occurring on the river bed.
Streams begin with water added to the surface which will run off down the steepest slope.Subsurface water and ground water also add water to the system. Erosion commences with the creation of small rill channels which coalesce and deepen into channels. The channels lengthen upslope by headward erosion. Dewi gave a good explanation of the latter by showing a clip of headward movement in beach sand. Over time channels merge and tributaries join a large trunk system. The types of drainage pattern that are observed are dependent on the type and structure of the rock and how easily the rock can be eroded.
He went on to discuss the load that is carried by a stream which can be divided into three types:
suspended load which are those particles which are carried along with the water in the main part of the stream, the size of which is dependant on their density and the velocity of the stream.
bed load which are the coarser and denser particles which move along the bottom by the process of saltation.
dissolved load which are the ions dissolved within the water.
The maximum size of particle that can be carried is called the competence and this varies with the sixth power of velocity. The capacity is the maximum stream load that a stream can move over a given time and varies greatly over time. So that the most work a stream does will occur during floods when stream velocity and discharge are greatly increased.
Pot-hole erosion in the river Marteg
As a stream moves down from it’s head to the mouth of the estuary then it’s discharge increases as does it’s cross sectional area and it’s velocity ( on average). This latter idea of velocity increase had us all wondering but it seems that the broad lowland waters have a much greater discharge and hydraulic radius and the waters flow much more freely. Hence the increase in velocity.
Severn Break-it's-Neck waterfall in the upper reaches of the river Severn
Using photos of local rivers we looked at the evolution of the stream valley from it’s youthful v-shaped valley with little or no flat land next to the stream. As the stream continues downstream it begins to erode more laterally than vertically and the valley widens and the stream begins to meander causing erosion on the outer portion of the meandering bend where velocity is highest and sediment will be deposited on the inner bend where velocity is low. If erosion continues on the outer bend then a meander may eventually be cut off leaving an oxbow lake.
Meanders in the river Severn
Stream deposition occurs whenever there is a change in velocity. Velocity varies along the length and wherever there is a lowering of velocity some sediment will come out from suspension and be deposited.For example if the discharge is suddenly increased, as it might be during a flood, the stream will overtop its banks and flow onto the floodplain where the velocity will then suddenly decrease. This results in deposition of such features as levees and floodplains. If the gradient of the stream suddenly changes by emptying into a flat-floored basin, an ocean basin, or a lake, the velocity of the stream will suddenly decrease resulting in deposition of sediment that can no longer be transported. This can result in deposition of such features as alluvial fans and deltas.
This was a very enjoyable talk.
The next meeting will be on Wednesday 18th march when Prof.Ian Stimpson will give a talk on Geothermal Energy in the UK
Monday 20th January 2020
The meeting on January 15th consisted of the AGM followed by an auction of books and maps kindly donated by Prof. Bill Fitches. Bill Bagley (Chair) then gave a talk on Chalcedony.
Chalcedony is one of the multitude of varieties of quartz (formula
SiO2). It is microcrystalline or cryptocrystalline, often botyroidal and often crystallises in layers in voids in the rock.
chalcedony from Normandy France
Bill introduced the talk with an old slide showing the range of
varieties of quartz which was now outdated as a new variety, moganite,
had been discovered in recent years.
moganite by Bergminer creative commons
Moganite has the same composition as quartz, but its crystal form is monoclinic, whereas quartz is trigonal. It is now known that chalcedony often comprises a proportion of moganite. X ray fluorescence or spectroscopy is required to distinguish between them.
Bill took us on a grand visual tour of the colourful varieties of
chalcedonite, agate, opal, onyx and jasper. A real mineralogical
banded chalcedony from Lincolnshire fens