Mam Tor & Winnats Pass

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Mam Tor, Windy Knoll, Treak Cliff cavern and Winnats Pass are four localities with accessible exposures of Carboniferous deposits (Viséan & Namurian) and examples of turbidite flows and Apron Reef structures with supporting fossils and minerals.

Background

The purpose of the day was to provide a revision opportunity for S260 students. The meeting point was the car park outside the Blue John Cavern (SK131833 – see location map) and our leader for the day was Dr Kevin Church. Mam Tor and the Winnats are found to the west of Castleton in the Derbyshire Peak District. (OS Sheet 110). The region is square 99 on the Geological Survey 10 mile map (100800) and the localities are sited along the junction of Units 80 and 81. The rocks of the Peak District were deposited in the Carboniferous period. During the late Devonian and early Carboniferous periods the British Isles were located at latitude 5-10oS and were part of continental Eurasia to the north of the Rheic Ocean. Fluctuations in the size of the ice sheets over Gondwanaland led to a rise in global sea level causing flooding in areas of continental Old Red Sandstone.

Localities

Stop 1 (SK131835) Follow the old road 200 yards north from the car park to gain access on the left to the area of landslip fallen from the east facing scarp of the Mam Tor ridge. The deposits immediately visible were noted to be very dark, friable and extremely fine grained (no visible grains), arranged in bands of varying thicknesses. Harder deposits found were concretions (sedimentary rock in which mineral cement fills the spaces between the grains) often formed around a fossil. Breaking the concretions open yielded a brief smell of hydrocarbon confirming the organic content of the sediment. The fossils found in this locality are Goniatites - an extinct group of ammonite related to nautiloids.

From the observations the sediments were recorded as shale and the fossils indicate deposition in a marine setting. The fine grain indicates that the process was slow settling and the bandings suggest that there were fluctuations in sea level changing the conditions from marine to brackish

and fresh water environments. Away from the road and looking up a clearer view of light brown bands of at least 10cm deep alternating with darker bands could be seen high in the scarp face. Following the lower contours on foot brought us to numerous blocks that had clearly fallen from the face above. Close inspection revealed medium to fine grain sizes, angular texture with occasional infill of much fined grained material in planar arrangements. Ridges and grooves, ball and pillow structures, in alignment could be observed on the face of many blocks.

Dr Kevin Church with Mam Tor behind. The sandstone beds are just visible. (Photo Don Cameron)

These sediments were recorded as sandstones interspersed with mudclast deposits. The ridges & grooves, and ball & pillow structures, collectively known as sole marks are the result of erosion of the surface by coarse material carried in suspension and indicate a high energy, environment. Note that what we see today are the negative impressions of the surface recorded in the overlying deposits. The alignment of the sole marks shows the direction of flow and mapping this area reveals that the paleo-current was from north to south and the surface was also sloping in this direction. It was noted that bedding planes were not visible in the sandstones suggesting that these deposits were “pushed” into place by traction currents- high energy over a short period (hours or days) whilst the alternating beds of shale were deposited from above in low energy environments over much longer periods of time. Further up slope the sandstone beds became thicker and more dominant as the shale beds thinned. In the handouts Dr Church had included a sedimentary log prepared from research in the area which showed that the first shale beds were Edale Shale’s containing 12 Goniatites bearing marine bands totalling 250m depth laid down over a period spanning approximately 12

million years (320Ma – 332Ma). The scarp face consisted of Mam Tor sandstones (75m) – sandstones alternating with shale – and, from data gathered in Edale valley and beyond, the deposits above consisted of Shale Grit, Grindslow Shales and Kinderscout Grit. Dr Church demonstrated how by the application of Walther’s Law – Sedimentary rocks of similar appearance, containing similar fossils and structures (facies) which we can see today in vertical succession were originally deposited laterally adjacent to each other – the sedimentary log which represents stratigraphic deposition can be used to reconstruct the original contemporaneous deposition and thereby identify the sedimentary environment. In the mid to late Carboniferous (Silesian) period during which the mudstones and sandstones were deposited, North Derbyshire was a coastal marine environment of prograding deltas.

With experience geologists are able to recognise characteristic successions in sedimentary logs that enable them to interpret the sedimentary environment directly without reconstruction. The log of the Mam Tor area is recognisable as beds A and possibly B of a Bouma sequence. Therefore we concluded that the sandstone sediments were deposited in fast flowing turbidity currents usually associated with a steep gradient such as a sub marine canyon in the continental shelf. The presence of sole structures at the base of the log, coupled with the fining up of the grain size indicative of decreasing energy are key pieces of evidence. Returning to the road the group was able to observe the effect of the landslip on the area. Rotated blocks of the Mam Tor beds dipping back to the scarp have displaced the road surface. Broken sections of the road provided excellent examples of normal faults and even grabens. Looking down to the lower road we could see how sections of the landslide had moved at different rates and directions and the lower road had been folded by compressional faults. We returned to the car park for lunch.

Stop 2 Windy Knoll (SK126830)

Leaving the car park and walking up hill to the south until joining the main road at the junction we continued along the road to the footpath on the right to Windy Knoll quarry and cave. The cave was excavated in the 1870’s when animal bones of Mammoth, Rhinoceros, Hyena, Lion, Wolf, Fox and Bear were found. Moving further round we were able to examine an outcropping rock wall of about 10m high. The rock was grey/blue, composed of fine grained material in irregular shapes bounded by joints, and with no obvious bedding. Fossil evidence included brachiopod shells in cross section, crinoids, corals – whole and broken – stromatolitic algal mats and bioclasts. Spots of the mineral galena were noted, and oil residues were also seen in the joints. The rock reacted with hydrochloric acid. From the observations we recorded the rock as limestone, and based on the bioclasts and the micrite matrix it is a biomicrite limestone. The overall structure is an algal reef indicated by the reef building organisms. The presence of galena indicated a local source of heat, and because the fossils were broken and mixed we concluded that they were not in life position but had been brought in by movement.

Stop 3 Treak Cliff Cavern (SK137830)

Access to Treak Cliff cavern is from the lower part of the old road out of Castleton. The cavern is a SSSI and the only place where banded Blue John Stone occurs naturally. This is because of the proximity of the shale to the limestone at the end of the Namurian which appears unique to the Castleton area. Blue John is a species of the mineral fluorite famous for its dark blue and purple colouration. Fluorite and other minerals (galena and barite) are thought to have come from the Edale shale basin to the north where they were dissolved by hydrothermal fluid, transported and deposited in faults and fissures in the limestone. Hydrocarbons contained within the shale deposits provided the unique colouration of the blue john fluorite. The hydrothermal fluids and mineralogy are indicative of igneous activity nearby thought to have been shield volcanoes.

Treak Cliff cavern also provides an excellent showcase of classic cave formations including stalactites, stalagmites and flowstones. Many good examples of water worn channels could be seen. Fossils found included brachiopods and crinoids. Partial infilling of the brachiopods indicate that the sea floor was gently sloping at the time of deposition. Nearly complete crinoids found towards the back of the cave indicate a low energy environment at deposition. Studying the limestone outside the cavern revealed a very fine grained rock with little or no bedding. Fossils found in the area indicate that this exposure is a reef limestone similar to Windy Knoll. Taking all the evidence together the group agreed that the limestone represents an Apron Reef – a marginal facies between a basin, lying east of Treak Cliff and north of Castleton, and a limestone shelf

behind; the Back Reef.

Blue John—Fluorspar crystals in Treak Cavern. (Photo Don Cameron)

Stop 4 Winnats Pass (Cow Low Nick SK136827)

From the entrance to Treak Cliff cavern a footpath (south) takes you to the foot of Winnats Pass. Walking a short way up the grass verge on the right hand to a prominent limestone exposure known as Cow Low Nick we observed beds dipping approximately 30^o east. Lower than the Algal reef at Treak Cliff, these beds form the Fore Reef. Looking at the upper layers evidence could be found showing that reef growth had kept pace with sea level rise with the youngest reef further back representing the top of the Carboniferous period. Further up the pass the beds are horizontal and the fossil population is quite different, providing evidence for the Back Reef. Stepping back from the reef and looking up the pass the group considered various theories of its formation. The favoured view is that Winnats originated as a channel in the reef that has since been exhumed by glaciation.

The group discusses the day’s geology. (Photo Don Cameron)

Discussion & conclusion

By the end of the day we were able to bring together the conclusions from each locality and to put these in the context of the overall setting, and note some of the differences. Putting these in stratigraphic order we know that the limestone was deposited between 332Ma and 350Ma – the Dinantian in the Lower Carboniferous, whilst the sandstone and shale were laid down in the middle Carboniferous (Namurian) 315Ma – 330Ma.

In the late Devonian and early Carboniferous a series of subsiding basins developed in north and midland England. These were formed by crustal extension between buoyant blocks of granite which preserved the surrounding upland. With the closure of the Rheic ocean and a melting phase of the Gondwana ice sheet, sea levels rose creating a shallow lagoon in the bed of the basins, conditions ideal for the formation of reefs and limestone beds. Towards the mid Carboniferous period Grabens and half grabens formed as faults developed along the margin of the blocks deepening the basins. Later uplift and tectonic movement resulted in the erosion of the Caledonian Mountains and local highlands. Mud, silts and sandstones were transported in the rivers of huge deltas spreading from north east to south west and deposited in the basins in alternating series. The Mam Tor sandstones were perhaps the result one such river which when flowing over a steep limestone reef created a turbidite flow. Further examples can be seen in the Millstone grit of Burbage Edge, Higgar Tor and Carl Walk. Volcanic activity associated with the tectonic movements provided a source of basalt lava and hydrothermal fluids, both of which have left deposits of rocks and minerals in amongst the sediments. In the late Carboniferous the Variscan orogeny caused land to rise and sea levels to fall and in the Derbyshire and Yorkshire areas shallow lakes and lacustrine deltas created the right conditions for the formation of coal swamps.

References and acknowledgements

Dr Kevin Church- for a most informative and enjoyable day

Don Cameron – for the photographs

Surface Processes Block 4 S260 – The Open University 1999

The Geological History of the British Isles – SXR260 – The Open University 2001

http://www.thepeakdistrict.info/fast/html/peak_district_geology.html

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