OR/16/053 Geological setting

Ellen, R, Callaghan, E, Leslie, A G, and Browne, M A E. 2016. The rocks of Spireslack surface coal mine and its subsurface data: an introduction. Nottingham, UK, British geological Survey. (OR/16/053).

In the following review of the bedrock and superficial (Quaternary) geology of Spireslack, information was derived from the published geological maps of the area; BGS 1:50 000 scale map sheets 23W (Hamilton), and the geological memoir for Hamilton (Patterson et al., 1998).

Geological overview

Spireslack lies in the Midland Valley of Scotland, bounded by the Highland Boundary Fault to the north, and the Southern Upland Fault to the south (Figure 1). The down-faulted region of the Midland Valley (essentially occupying the same area as the Central Belt of Scotland) between these two major faults consists mainly of Devonian and Carboniferous rocks (Figure 1) overlying Lower Palaeozoic rocks. Carboniferous sedimentary rocks in the Midland Valley of Scotland have been central in industrial development of Scotland, providing key resources such as coal, ironstone, oil shale, fireclay, sandstone and limestone.

File:OR16053fig1.jpg

Figure 1    Generalised Carboniferous geology of the Midland Valley of Scotland, with major bounding faults — the Highland Boundary Fault to the north, and the Southern Upland Fault to the south. The map shows the locations of prospected and active surface coal mines in Carboniferous strata. Sites owned by the Scottish Mines Restoration Trust (SMRT) are indicated, including Spireslack (S) and Mainshill Wood (M).

The Carboniferous rocks exposed at the Spireslack surface coal mine are Viséan to Namurian in age (around 330 to 325 million years old). The strata (Figure 2) are assigned to the Lawmuir Formation of the Strathclyde Group and the Lower Limestone, Limestone Coal, and Upper Limestone formations of the Clackmannan Group (Browne et al., 1999). The strata at Spireslack are of typical Carboniferous rocks of this age: marine limestones and mudstones, shallow deltaic to fluvial sandstones and mudstones, and coals and related palaeosols (seatearths). These strata were deposited as upward coarsening, and sometimes upward fining cyclic packages, partly recording a fluctuating sea level.

File:OR16053fig2.jpg

Figure 2    Stratigraphical framework for coal-bearing strata across the Muirkirk and Douglas area, based on exposures at Spireslack and Mainshill Wood SCMs. The key coal-bearing units and lithostratigraphical markers referred to in this report are highlighted in the more detailed columns.

The strata in the main void at Spireslack lie in the north-western limb of the broad upright north-east trending Muirkirk Syncline (Figure 3), and dip at 30° to 40° towards the south-east. The rocks are also displaced by mostly left-lateral (sinistral) oblique-slip faults. Narrow, up to 1 m thick, Palaeogene basaltic dykes intruded the Carboniferous strata around 60 million years ago.

File:OR16053fig3.jpg

Figure 3    1:50 000-scale geological map of the Spireslack site. The oldest rocks are exposed at the far eastern side of the site. They include sandstones belonging to the Lower Devonian Swanshaw Sandstone Formation. These are separated from Carboniferous rocks by a major north-south aligned fault. The Carboniferous strata have been folded into a broad south-westerly plunging syncline. Within the syncline, strata are offset by many minor faults with a dominant north to north-north-easterly alignment. The main void and Area B1 are indicated.

Late Devensian grey-brown till (approximately 2–3 m thick) and dark brown-black Holocene peat (<2 m thick), overlie bedrock. The till was deposited by ice sheets during the last Ice Age, the last main phase of which ended approximately over 13 thousand years ago. The colour of the till commonly reflects underlying bedrock.

Man-made deposits overlie both the bedrock and superficial deposits. Bell-pits dating from the 18th century are visible on the surface at the south of Spireslack by the site of Glenbuck village, whilst large rock-spoil heaps associated with the late 20th to early 21st century surface mine operations cover the pre-existing ground surface.