Newsletter - March 2018

Limestone and The Derbyshire Platform: Castleton On Sunday 3rd September 2017, a grey and drizzly day, 10 MGA members were joined by five members of the Leeds Geological Society for a field trip led jointly by the MGA Vice President Cathy Hollis and her PhD student Lucy Manifold. We were to explore the northern margin of the Derbyshire Platform via Cave Dale and Pin Dale. We met in the main car park at Castleton where Lucy gave us a briefing on the trip. She then outlined the regional geology. We were going to 'visit' the Visean (Lower Carboniferous). Sedimentation occurred over the whole of Northern England that was under an extensional regime at the time. The underlying basement (granite or low grade metamorphic rocks) was stable. Tectonic subsidence resulted in half-graben formation and this affected the depositional pattern as areas of crust underlain by basement remained buoyant. Sediment was received from the Caledonides to the north and St George's Land (Wales Brabant Massif) to the south. During the Visean, the hanging walls subsided by around half a kilometre and carbonate platforms were formed on the more slowly subsiding blocks. The whole period was punctuated by relative sea level rises and falls. In highstand/transgressive periods, carbonate sedimentation on the platforms recurred, locally providing limestone turbidites and other gravity flow deposits along faulted basin margins. During periods of sea level lowstand, the platform top was often exposed above sea level. Eventually as time progressed, the fluvio-deltaic sedimentation pro-graded south with carbonate production ceasing except in the south of the area in the East Midlands. By the end of the Visean, as the Namurian commenced, the Yoredale sequences had started and the whole area was subsequently covered by thick layers of feldspathic sandstones. Cathy explained that almost no work had been published on the sedimentology of the Derbyshire Platform since the 1960s and 1970s, but Lucy was undertaking her PhD revisiting (and improving) the information available. We would be walking up slope onto the platform and be able to see how the structure changes. Locality 1 After winding our way through a very busy Castleton, our first stop was at the bottom of Cave Dale, the start of our transect through the Derbyshire Platform. We walked onto the carbonate platform, the Bee Low Limestone formation, via the foreslope of the Apron Reef. The beds were dipping towards Castleton and represented the shedding of material off the platform top into deeper water. The water depth was estimated at 15-20 metres. Lucy explained that the dips are very variable on each platform. The platform margin had cemented easily allowing the steep slope to form. In the past various features, such as cross-bedding and geopetal structures had apparently been seen. However because of increased rainfall, particularly since 2000, there is now so much vegetation that these features can't be seen. We did however spend a little while searching the outcrops and found examples of fragmented bioclasts. Locality 2 Our next stop was about half way through Cave Dale itself. The beds were flattening. We had reached the Margin of the platform where the reefs were growing. There were small mounds representing the reefs. Others in Winnats Pass are up to 20m in height. There were no large corals, but we did see bioclasts, which under the microscope show to be formed of bryozoa, brachiopods, small corals and crinoids set in a micrite cement. We did not see any evidence of the algal reef. Cathy and Lucy explained that it was difficult to tell how the limestone had formed. Carbonate mud had spread across the area and was trapped by whatever was growing there at the time. Whether this mud was a precipitate or ground up material is not known. The composition of the seawater at the time was different from today. There were no icecaps in the Lower Carboniferous although the South Pole Ice cap was starting to form by the end of the Visean; thus the seawater chemistry was changing. This is one instance where we cannot use the modern carbonate-forming environments when assessing palaeo-carbonate systems. As an example, the calcite then was low in magnesium which made it very stable and less reactive. Formation of the Dale itself may well be the result of a cave collapse that had been exploited by ice sheets during the last glaciation. Locality 3 We proceeded up the Dale to just below the Castle ruins. On the opposite side of the valley we could identify a fault. We were still on the platform margin though there was more evidence of bedding rather than reef mounds although the beds were disturbed by fracturing near the fault. Compaction and cementation of the beds had made them brittle. The fault trended E-W and is thought to be part of alarger strike slip fault system, although it may have been in extension during deposition. In the fault zone, we found calcite veins that were formed when hot fluid migrated along the fault. The basin temperatures were high - the area being buried to 3km - so that when faults were reactivated it was like squeezing a sponge. Water trapped by fast burial became heated and mineralised, emerging with fault movement. The temperatures for the calcite formation match those of the basin, up to~200ºC. Locality 4 Our final stop in Cave Dale was very near the top. During the Carboniferous there were volcanic centres basalt. It was almost columnar!. Whilst some of the volcanism may well have been shallow sub-aerial volcanism also occurred because there were a great many changes in relative sea levels the period. Some were due to changes in the icecaps, but others were due to basin subsidence subsequent fill. When the land was first exposed there was ash in the air that became trapped in the soils as they formed. This is a topic which is poorly understood as no research has been undertaken. Cores from boreholes within the Derbyshire Platform have shown amygloidal basalts. The limestones we saw above and below the basalt were more horizontal and much thinner, indicating that we had moved from the margin onto the platform top. We had risen 220 metres since we entered Cave Dale and were now above Castleton with views over the Peak District had it not started to rain. Locality 5 After walking towards Castleton, along the road that runs along the top of Dirtlow Rake, we sat in increasingly wet conditions to eat our lunch. Then we walked down into Dirtlow Rake itself. This 'rake' or mineral vein system trends south west to north east to Pindale Quarry, which we would be visiting towards the end of the day. It is part of the strike-slip system where the faults generally follow a E-W or NW-SE trend. This is in alignment with the regional tectonic trend. Some of the strike-slip movement may have occurred during the growth of the fault system during the Lower Carboniferous. Then during the Variscan Orogeny there was large-scale fault reactivation. This reactivation resulted, as mentioned earlier, in heated water from the sedimentary basin being squeezed out. As a result, coarse calcite crystals which can now be seen in the walls of the fault are intergrown with barytes, galena and fluorite. The mineralised rake that resulted has been exploited by man particularly over the 50 years or so. Since the fluorite mining has ceased the rake has been open for geologists such as us to explore. The fault system has many planes and movement resulted in slickensides which we were able to find on the sides of the rake in several places. The water passing through also brecciated the rock which is then relinked by calcite crystals. Locality 6 We walked further down the road through some woods into Pindale Quarry which is in the Brigantian. The beds are thinner as sea level was shallowing and there are more clastics. We were able to identify shoals by the coarse grainstone texture of the rock (often more than 50% bioclasts and with very little mud). It was explained that the current grassy erosion surfaces represent exposed surfaces during relative sea-level fall clints and grykes formed together with palaeosols and now these erode more easily than the hard limestone. There are Gigantoproductus fossils within some of the beds. We were now about to go over the edge of the platform (up which we 'climbed' earlier). The rocks were fairly horizontally bedded but then we saw mounds and dips, the beds became a bit sandier and there were more slickensides. Corals were also seen. Towards the end of the quarry, the exposure dipped down over a large mound. Unfortunately we were unable to see much due to growth of vegetation. Cathy suggested returning in February or March when everything had died back then the structure together with bryozoa, crinoids and brachiopods can be seen. Work is currently being done on the palaeoshoreline: it is thought there may have been embayments in the platform edge with resulting minicurrents affecting the geomorphology. Leaving Pindale Quarry completed the 'official' part of the trip. However on our walk back to Castleton we came across an exposure of bioclastic limestone filled with brachiopods. Back in the car park, Cathy and Lucy were thanked warmly by everyone for such an interesting trip. Jane Michael