Trip Report – ‘Pennine Basins and Namurian Deltas’ Walton Hall Weekend Field Trip to the and Southern Yorkshire with Dr Kevin Church (OU) May 7th to 9th, 2010

This field trip followed on from a splendid lecture delivered to Walton Hall Branch by Dr Kevin Church of the Department of Earth and Environmental Sciences, at the Open University. Kevin, our trip leader, had done much of his PhD research on mapping of the Namurian deltaic sediments in the Southern Pennines. His work unfolded a story of how carbonate-rich shallow sea basins had become rapidly in-filled by a sequence of deltas advancing from the North/Northeast during Middle Carboniferous times. An accurate timescale was derived from “marine bands” occurring within the thick pile of delta sediments, recording a history of changing sea levels and a steadily evolving benthic fauna (goniatites) throughout the deposition period.

Our field area extended from the locality of Castleton in the High Peak, northward to Pule Hill near Marsden, South Yorkshire. We were accommodated in the town of Glossop, on the edge of the Kinderscout High Peak area, which provided a convenient point central to our main locations. We were to examine the rocks in age sequence, from the lowermost (oldest) units at outcrop near Castleton (), through to the youngest Namurian rocks (delta-top) at Pule Hill, some 20 miles to the north.

Saturday, 8th May

Saturday morning, we travelled from Glossop southward to . We parked on the sloping road opposite the entrance to Blue John Mine. Kevin took us through the swing gate at the bottom of the sloping road and led us beyond, to where we could see the broad scarp face of Mam Tor. We assembled on the marshy ground at the foot of a steep, dark shale bank.

Location 1 - Mam Tor Shales (SK 129832)

We assembled at the base of the shale bank. Kevin made us do a systematic appraisal of the shale. It was dark, crumbly and fell apart on handling. Looking up-slope, it was clear that the shale body had undergone much slumping down-slope. No fossils were visible, but no extensive searching was done. It also contained no grit/silt (detectable by chewing a small piece between the teeth). Environment of deposition? Possibly marine basin, gentle, with no silt present.

Figure 1 Shale Bank and Scarp Face of Mam Tor

The shale smelled faintly of hydrocarbon volatiles (crude oil). Kevin described the shale in some detail. There were occasional iron nodules that could once have contained organic material. If we had found fossils, they would probably have been goniatites (small ammonoids, nektonic/benthic or free swimming).

We walked nearer to the base of Mam Tor cliff. There were many small fallen blocks of sandstone that showed sole marking and occasionally contained mudstone rip-up clasts. The blocks were sections of turbidite pulses, up to 1/2 m thick with the mudstone rip-up clasts appearing as small discs/lenses of dark shale embedded in the sandstone matrix. A closer look at some of the blocks showed mica flakes in some abundance. At the cliff face, the turbidite sequences were clear to see, with the sole marks on the under surfaces. A study of the sole marks suggested a current flow from the north/northeast.

Figure 2 Sole Marks and Rip-up Clasts

Returning to the shales, Kevin described the relationship of the shales to the underlying limestones and the turbidites above. Until borehole data became available, one could only guess at the thickness of the Mam Tor and Edale Shales. Some 250 m of marine shales were found to exist beneath the turbidites. These contain 12 marine horizons (marine bands) that record 12 events of sea level rise, each of which allowed deposition of distinctly different benthonic faunal remains (goniatites) when the zone of detrital deposition shifted (at each event) higher up the advancing delta slope.

We considered the evidence for a depositional history that began with a marine basin, attracting a sequence of prograding delta lobes. The initial sediment arrival would have been slow and gentle, but sustained over a long period to achieve the present thickness of shale. Later, the arrival of the first delta front would have begun with occasional turbidite flows.

We returned to the shale bank and looked at the old abandoned road. The landslide here is undergoing detailed study. In a recent paper (Rutter, Arkwright, et al, 2003) the rate of landslip is recorded at almost 50 cm per year, in places.

Location 2 Windy Knoll (SK 125828)

Our next location was the old limestone quarry at Windy Knoll, on the area of flat ground above the Winnats Pass. The limestone outcrops at Windy Knoll are the remains of a reef knoll close to the reef top at its northernmost point. These are of Asbian age (Late Visean). A point of interest here are the deep fissures in the reef top limestone, in-filled with limestone blocks. These are commonly called “Neptunian Dykes”, believed to be the in-filled remains of a karst “clints and grykes” landform, created during a period of uplift and emergence during Brigantian times. The reef building organisms (Algal mats) are clearly visible here, as are corals. Figure 3 Residue of Trapped Hydrocarbon Deposit

A further point of interest at Windy Knoll is the residue of a former oil reservoir (or lighter hydrocarbons) left in joints and cavities in the limestone. This would have suggested an impermeable cap once existed over the limestone.

Location 3 (SK 136831)

Our next location was the public show cave, Treak Cliff Cavern. Kevin negotiated a group rate and we entered the cave. Treak Cliff Cavern is a natural cave system formed in the eastern slope of the limestone reef. Here, in the lower part of the reef slope, the shale is banked against the slope and we passed through a section of it before reaching the reef limestones. The reef slope here consists of limestone boulders in a shale/calcareous matrix.

Figure 4 “Blue John” Fluorspar Mineralisation

We paused for a while to check for fossils in the walls. There were abundant crinoid stems, largely undamaged. In fissures and joints, Blue John and other minerals had precipitated. Sometimes in large voids that had been mostly mined out.

Continuing deeper into the cave, the walls soon became covered in flowstone, concealing most of the interesting rock surfaces. Finally, making our way towards the exit, Kevin pointed out an interesting geopetal structure formed in a shell cavity in the cave wall. This had traces of the original carbonate infill that showed a palaeo-horizontal settling surface. This strongly suggested that the reef slope was indeed close to its original gradient of nearly 30 degrees. Figure 5 Geopetal Structure in the Cave Wall

This cave visit was of limited use for studying the inner structure of the reef, due to the flowstone covering the walls. To get a better appreciation of the inner reef structure, we left Treak Cliff Cavern and walked across to the lower entrance to Winnats Pass.

Location 4 Winnats Pass (SK 138827)

Winnats Pass is a steep-sided cut through the reef, widened to take the A6187 road after the Mam Tor loop was closed. At the entrance to the pass you can see a good section through the fore reef slope, the reef itself and the back reef lagoonal limestones. We spend a little while looking up through the pass and studying the different bedding orientations, satisfying ourselves of the different modes of formation.

The Winnats Pass was our final location for the day.

Figure 6 Sketch Section through the Mam Tor Rocks

Sunday, 9th May Sunday morning, we travelled north from Glossop up to an excellent little Millstone Grit quarry just outside Tintwistle, on the A628. There is a back road north of the village. We parked on the sloping road opposite the entrance to Holybank Quarry.

Location 5 – Holybank Quarry (SK 027976)

We walked up the rough path to the quarry. Here, there is impressive cross-bedding in the Kinderscout Grit that sits on top of the deltaic succession that has the Mam Tor deposits at its base. The Kinderscout delta- top exposed at Tintwistle had very large distributary channels, clearly visible in the large-scale cross- bedding. Here and there, mats of coarse vegetation (Lepidodendron) had been left stranded and quickly incorporated in the sand banks of the distributary channel. Figure 7 Holybank Quarry, Tintwistle

From the shales at the base of Mam Tor cliff to the Kinderscout delta top exposed at Holybank Quarry, there is a thickness of deltaic deposits of some 600 metres. This represented the primary (first) major infill of the Gainsborough Trough and Edale Gulf marine basins in Middle Carboniferous times. The different stages of the infill are marked by different lithologies in the sequence. The first, as discussed, was the basinal muds and shales we had seen at Mam Tor. There then followed the distal turbidite pulses (Mam Tor turbidites) and down-slope feeder channels (Grindslow Shales and “Shale Grit”), which marked the prograding Kinderscout delta front. One day we shall visit Grindslow (in Edale) to study the Grindslow Shales.

Figure 8 Lepidodendron stems/roots Trace Fossils

The delta top at Tintwistle is some 40 m thick and shows a continuous sweep of truncated cross-bedding, spanning the length of this small quarry. Curiously, there are periodic, upslope orientated fore-sets visible in the quarry face, testimony to the size and flow rate of the distributary channel that must have existed here. A look at the grains in the bedding showed both sub-rounded and sharp grains, with occasional gravel drapes. The river bed load contained a proportion of wind-blown grains.

We had now seen both delta-base and deltatop lithologies at close hand. Kevin explained that generally, even in this excellent region, the middle parts of the deltaic succession are not well exposed and can be studied at only a few localities. I particularly wished to see good examples of the marine bands that had been so revealing and conclusive in the dating and timing of the sequences. Our next location was Heyden Brook, off the A6024 (the Holmfirth road).

Location 6 – Heyden Brook (SE 095025)

To the northeast of Tintwistle, the A6024 climbs up out of Longdendale towards Holmfirth. Where the road climbs northward up onto Heyden Moor, there is a pull-in on the left side that overlooks Heyden Brook from the east. At this locality, the length of Heyden Brook (from Heyden Bridge up to Upper Heyden) exposes the delta succession from the Kinderscout delta top, all the way up to the Huddersfield White Rock. This includes four delta lobes (Scotland Flags, Midgley Grit, Hazel Greave Grit and the Huddersfield White Rock).

Figure 9 Heyden Brook (Open Moorland)

Kevin had spent much time at Heyden Brook while doing his PhD research, logging the exposure of the Midgley Grit beside the brook. As well as graphic logging, he had done extensive gamma ray spectrum surveys on the marine bands exposed here. He explained that gamma ray analysis (using radiogenic U, Th and K isotopes) was particularly useful in locating the marine bands. Access is not easy now that much of the exposure is overgrown. We viewed the location from the road side, then moved to the next location at Digley Quarry, near Digley Reservoir.

Location 7 – Digley Quarry (SE 110072)

Digley Reservoir and Quarry are about 5 miles north of Heyden Brook. In the quarry car park here (now a picnic site), there is a good accessible exposure of the Scotland Flags Grit. We stopped here for a while to look at the “end-on” trough cross bedding in the southfacing quarry face.

Figure 10 Scotland Flags Grit, Digley Quarry

The Scotland Flags Grit was the second delta to reach the region. It crossed the old highstand of the Kinderscoutian and (according to our field notes) brought the paleo-shoreline almost as far south as Heyden Brook. At Digley (some 5 miles to the north), the distributary channel can be clearly seen. The flow direction can be inferred from the trough cross-bedding as (broadly) from the north. This delta was much thinner than the Kinderscoutian and shale fronted, having no deep basin to fill.

The next, much larger delta to arrive was the Midgley Grit. This was represented at Heyden Brook and pushed the shoreline south of Manchester. We would see the Midgley Grit delta top at Pule Hill later. Our next location was Meltham.

Location 8 – Meltham (SE 100108)

At Meltham, in the supermarket car park, there is a good exposure of the Huddersfield White Rock. We stopped here for lunch and to look at the “end-on” trough cross bedding in the north-facing cliff face. This old quarry face was rather overgrown and poorly lit, but nevertheless showed some interesting complexities. Here, the troughs were numerous and much narrower than in the Scotland Flags Grit at Digley.

The final location and exposure of greatest interest of the day was Pule Hill, where Kevin had done some major work on the Midgley Grit delta top. After a short stop in Meltham, we continued to Pule Hill.

Location 9 – Pule Hill – Deltaic Sandstones (SE 032108)

We parked beside the A62 on the west side of Pule Hill and immediately set off up the path leading up to the gritstone edge. On the top, there was the site of an old quarryman’s winch and ramp. We spent a good hour studying the rock, looking for features and indicators of depositional environment. The Midgley Grit (Marsdenian stage) is quite massive and shows relatively little jointing.

Figure 11 Midgley Grit, Pule Hill

Again, there were major fluvial bedforms visible, cross-bedded with a large channel profile traceable in the bedding. One significant feature here is the occasional large (2m-3m diameter) iron nodules. In one location, there is a good horsetail cast weathering out, but generally the sandstone seems to have few discontinuities and bedding features.

Pule Hill is particularly blessed to have a readily accessible shale sequence at outcrop, at its southern end (grid ref SE 032102). Here, I saw what I was very much hoping to see; a goniatite-rich marine band. These shales are just 40 m (or so) beneath the delta top grit and can be reached a short way up Mount Road, just skirting the southern end of Pule Hill. After just a few minutes of picking at the shale bank, the entire group had found at least one small, flattened goniatite each. Figure 12 Goniatite Reticuloceras Bilingue, Pule Hill

At the finding of the marine band at Pule Hill, we reluctantly had to bring the trip to a close. All members of the group were deeply grateful to Dr Kevin Church for leading us. This had been quite the best weekend field trip we’d had, for quite some time.

Thank You Kevin, it was hugely appreciated.

Tom Miller (photos Tom Miller)