Ellen, R. and Callaghan, E. 2015. Geodiversity Audit of Spireslack and Mainshill Wood Surface Coal Mines. British Geological Survey Commerical Report CR/15/126. 70pp.
Spireslack Locality 7 (and 7b): Complex fault zone in McDonald Limestone
NGR: [274845 630590]–[274895 630623]
Key categories of interest Rarity Quality
| Key category of interest | Rarity | Quality |
|---|---|---|
| 1. Structural geology | 5 | 5 |
| 2. 3D visualization | 3 | 4 |
Access: Good access to base of exposure, easily accessible from roadway.
Current safety: Potential for falling blocks/scree from limestone pavement, sharp drop behind access
Measures to enhance site: Viewing platform away from steep drop and large rocks; barrier between platform and steep drops.
Key categories in order of interest (1 = primary interest); Rarity, 5 = only example in Spireslack, 1 = many examples in Spireslack; Quality 5 = exceptional preservation in Spireslack, easy access/viewing potential 1 = average preservation in Spireslack, difficult access/viewing potential
Photograph overview with polygon boundary
© Getmapping: Licence Number UKP2006/01
Site description
Geology
Whilst there are many small faults intersecting the McDonald Limestone pavement, none are quite as spectacular as the structures associated with the > 5 m displacement fault at this locality. The west-most wall of the fault in the limestone is heavily fractured and faulted, containing a complex network of conjugate and linked small- displacement faults. Relay ramp structures are also well developed. As in Locality 3, structures like these are rare but are critical to study in order to further the understanding fault growth and evolution. Fault rocks (primarily limestone fault-breccias) are observed along the planes of some of the larger displacement faults. Thin dykes also
cut the limestone pavement. The main fault also displaces the mechanically weaker seatearth in the lower half of the void, where the west most wall of the fault has a markedly different faulted signature than that of the mechanically stronger limestone. This is important to highlight how faulting affects rocks with differing mechanical strengths.
The fault is visible in section view in the main scarp, where it also displaces one of the Palaeogene dykes. Together the localities on the north and south of the void contribute to a 3D understanding of fault networks cutting coal- bearing sequences.
Access and enhancement suggestions
This locality is accessible from existing roadways and there is sufficient space at the base of the pavement to stand safely to view the structures. The seatearth in the lower part of the void is also accessible, although the pond in the lower void is liable to flood and may impede access at times. A flood barrier could be constructed to avoid this. Barriers at the top of the seatearth horizon would improve safety for viewing the structures in the limestone pavement.