Thanks to Peter Bridges’ usual excellent organisation, we had a good time clearing sections in Tank and North Quarry (Malvern) on 7th October. Peter even arranged a break in the rain for us too, so the first clearance of the season for the Malvern Hills Trust went very well. Sections in the Malvern Complex around the Tank Quarry car park and behind the Clock Tower were cleaned up to improve the visibility of features like granitic sills, faults and fault breccias. Despite the wet year we have had, the clearances were generally considered to be easier this time round, reflecting the good work done in the prior year at these sites.
In addition to tidying the Malverns Complex outcrops, the section of conglomerate/breccia exposed near the information panel in North Quarry was scrubbed clean of moss by our diligent volunteers. They also exposed a further section on the wall of the quarry about 20m south of the information board. This section was specifically worked on so as to expose the contact between the sediments and the Malverns Complex, the location of which had been (re-)identified during last year’s works.
The contact dips at 80° towards 020° (NNE) and is obvious on the photograph below, which shows the newly cleaned exposure looking towards the west. The notebook is 20cm high. The map shows the location where the photography was taken and reproduces the current BGS 1:50,000 mapping for the area, while adding in grey type the dips and hand written annotations of the 1990 1:10,000 Solid and Drift map (Sheet SO74NE).
Image 1: North Quarry outcrop following clearance
The sediment to the north of the contact is a red-brown coloured, matrix-supported conglomerate (to breccia) with granule to pebble sized clasts. A few cobble grade clasts are present. Tracing the continuous but patchy exposure through to the exposure near the display board, the maximum clast size reaches boulder grade and there is a sense of a general coarsening up, although with much variation. As far as we could see, the large majority of the clasts are Malvernian and there were no obvious clasts of Silurian material, contrasting with the Permo-Triassic breccias that occur in the Knightwick area, for example. There is faint variation in the clast abundance and indistinct lamination in the sandy matrix, both of which appear to be sub-parallel to the contact in the photograph. When the display board outcrop is viewed with the dip/strike of the contact in mind, the sedimentary architecture seen in the exposure can, perhaps, be better understood. The section is provisionally interpreted as a set of debris flow deposits, although whether of marine or sub-aerial origin remains unknown.
What is noticeably absent at the newly cleaned contact is any shearing in the sediments parallel to the contact, nor any evidence of rotation of the clasts. Nor is there any evidence of slickensiding on the contact, although there is abundant slickensiding on joint and possible fault planes in the Malverns Complex face immediately to the south of the contact exposure.
Image 2: North Quarry outcrop location with BGS geology
So what are we looking at here? On the current BGS 1:10,000 scale mapping shown in the figure above, the area of the contact as photographed lies on a fault boundary between two annotations. To the south the annotation suggests an unconformity between a marginal facies of the Bromsgrove (now Helsby) Sandstone and the face of the East Malvern fault. Northwards, the annotation suggests a faulted contact between the two units (note all the boundaries on the map included are interpreted by BGS as faults). Our observations strongly support the presence of an unconformable boundary through the cleared section. However, if the sedimentary features we saw are parallel to the contact, as per our interpretation, the dip is too steep for these to be depositional features in material “banked-up” against an exhumed or active fault scarp: stable slope angles in debris flow deposits are unlikely to be higher than 40°. If the current 80° dip of the contact applies to the sediments, then the section we cleared (Malvernian and sediment) must have been substantially rotated post-deposition, notably without causing any shearing on the boundary.
This is a difficult conundrum to unravel. If the sediments are, indeed, Triassic in age, then substantial deformation of the section including the Malvernian in post-Triassic times is difficult to fathom. Which raises the intriguing possibility that the sediments are not Triassic. As already mentioned, the clasts in the sediments appear not to include any Silurian material after quick examination. Might it be that this sediment is actually much older? What about a Llandovery age, making the sediments represent the first phase of deposition onto a peneplained Malvernian landscape which continued with the Cowleigh Park Formation? The sediments are not dissimilar to the coarser-grained examples of the Cowleigh Park units that outcrop at the Playing Fields site in West Malvern perhaps? Also, there are red conglomerates and sandstone noted to be Llandovery in age immediately over probable Malvernian basement in the Collington-1 borehole north of Bromyard, with a full section of later Silurian above.
If the sediments at North Quarry are Llandovery in age, then deformation during the main “Variscan” phases of deformation of the Malvern area would, perhaps, be a more acceptable explanation for the steep dip inferred for the unconformity and the overlying units. But this has implications for the geometry of the features (faults, fault breccias and intrusions) recorded in the adjacent Malverns Complex exposures: were they too rotated from their original orientations?
Your task, should you choose to accept it (to coin a phrase), is to visit this new cleared exposure and make observations for yourself: can you add to the debate and help explain what might be going on here?
So, to conclude, the clearance work EHT has undertaken for the Malvern Hills Trust over the last few years does preserve and enhance exposures that have significant geological importance. The example discussed above shows how important well-maintained local exposures can be for moving the geological story of an area forward.
Andrew Sims, October 2024.
