Black Metals Member

Black Metals Member (BKME), Carboniferous, Midland Valley of Scotland

Black Metals Member is part of the Limestone Coal Formation

Name

Clough et al. (1916) previously named the member the Black Metals. It should not to be confused with the locally named Black Metals (Haughton) Marine Band of the Cumberland Coalfield which is Duckmantian (Westphalian.B) in age.

Lithology

The Black Metals Member is a thick, laterally persistent dark grey mudstone (Read, 1965) distinguished by one or more beds, termed ‘Black Metals Marine Bands’. Read (1965) described a sequence within the member of mudstone with Lingula at the base; overlain by mudstone with nonmarine bivalves or unfossiliferous mudstone, overlain by mudstone with a varied marine fauna that comprises the middle part of the member; which in turn is overlain by mudstone with nonmarine bivalves. The characteristic lithologies of the member vary laterally within the Midland Valley of Scotland. On the western side of the Central Coalfield in the Glasgow district strata that comprise the member are described and illustrated by Hall et al. (1998, fig..11) as black and dark grey, carbonaceous mudstones and silty mudstones with characteristic development of brownish-grey clayband ironstone nodules (California Clayband Ironstone). Sandstones are restricted to thin beds, collectively 2 or 3 m thick in a total thickness interval of about 32 m. Eastwards from the Central Coalfield the member includes siltstone and sandstone beds that increase progressively in number and thickness to become the predominant lithology; seatearth and coal also appear (Read, 1965). In the western coastal Irvine district, the Logan’s Bands, comprising thin beds of ironstone in a laterally persistent mudstone sequence, have been correlated with the Black Metals Member of the Glasgow district (Richey et al., 1930).

Genetic interpretation

Beds containing shelly marine faunas indicate a marine depositional environment (Wilson, 1967). However, beds with shells including Naiadites sp., indicate nonmarine depositional conditions.

Stratotype

The type section is north of Glasgow in the Cardowan No. 2 Bore (BGS Registration Number NS66NE/66) (NS 6706 6752) from 470 to 502.3 m depth. Reference sections are provided in Fife, at the Dora Opencast Coal Site, Area G (BGS Registration Number NT19SE/550) (NT 1830 9100 to 1860 9130), and in Midlothian, in the Monkton House Borehole (BGS Registration Number NT37SW/43) (NT 3325 7044) from about 888 to 907 m depth (see Tulloch and Walton, 1958, plate 3). Both reference sections include interbedded sandstone and coal-bearing strata within the member.

Lower and upper boundaries

In the Central Coalfield the base of the mudstone comprising the member is picked at the top of the Black Metals Basal Coal (Forsyth and Read, 1962). Eastwards the base is picked at the top of a laterally equivalent seatearth and correlated to a thin coal above the Torrance Four-inch Coal farther east in Fife (Read, 1965, fig. 2). Westwards, in the Glasgow district, marine fossils are recorded just above the base of the Black Metals Member, the boundary being picked at the roof of the Upper Garscadden Coal which here may be developed as a blackband ironstone (Upper Garscadden Ironstone) (Hall et al., 1998, fig. 11; Forsyth and Read, 1962).

At its upper boundary in the Central Coalfield, mudstone of the Black Metals Member is interbedded with sandstone of the overlying Limestone Coal Formation (Figure 6, Column 4). The boundary is picked at the base of the lowermost bed of coarse siltstone or sandstone (Read, 1965). In the Bo’ness area of the Livingson district (Read, 1965), the erosive base of the sandstone has cut down into the member, or it is abruptly overlain by lavas of the Bathgate Hills Volcanic Formation. In the Glasgow district the upper boundary of the Black Metals Member is just below (within about 5 m of) the Knott Coal horizon (Hall et al., 1998, fig..11).

Thickness

Read (1965) described the thickness of the member as exceeding that of typical fining-upwards cycles of the Limestone Coal Formation, which are usually about 3–10.m thick. North of Glasgow on the western side of the Central Coalfield in the Cardowan No. 2 Bore (see above) the member is 32.3 m thick (from 470 to 502.3 m depth) (Hall et al., 1998, fig. 11). The thickness is similar in the Kincardine and Fife–Midlothian basins. However, in the Kincardine Basin in west Fife the thickness of the member increases eastwards from 33 m to a maximum of 66 m, associated with the lateral change to an increasingly sandstone-bearing succession (Francis et al., 1970, p. 190, and plate 8).

Distribution and regional correlation

Central Scotland, but not identified in southern and central Ayrshire by Simpson and MacGregor (1932) or Eyles et al. (1949).

Age and biostratigraphical characterisation

Pendleian. The ‘Black Metals Marine Bands’ yield macrofossils in most parts of central Scotland although in some areas only Lingula is present. The annelid Serpuloides carbonarius, the brachiopods Buxtonia spp., Pleuropugnoides cf. pleurodon and Lingula spp. and the bivalve Streblopteria ornata are the main forms. The bivalve Aviculopinna mutica is commonly present in Midlothian and Fife, but has not been found elsewhere (Wilson, 1967). The member also includes beds with a nonmarine shelly fauna including Naiadites sp. The Logans Bands in the western coastal Irvine district contain Lingula and Naiadites or nonmarine bivalves (Monro, 1999).