Alston Formation

Alston Formation (AG), Carboniferous, Northern England Province

Alston Formation is part of the Yoredale Group

Name

The name is derived from the former Alston Group of the Alston Block and Stainmore Trough, which formerly included platform carbonates as well as ‘Yoredale’ facies (George et al., 1976). In south Cumbria the Alston Formation now replaces the Gleaston Formation of Rose and Dunham (1977). In north Cumbria it includes the upper part of the now obsolete Chief Limestone Group.

Lithology

The Alston Formation normally (but see below) extends stratigraphically from the base of the Peghorn Limestone (or correlatives) to the top of the Great Limestone (or correlatives). The formation is distinguished from the Tyne Limestone and Stainmore formations by the common presence of thick, commonly bioclastic limestones. Distinctive facies variations within the formation are recommended to be given member status. Numerous local names exist for the major limestones but their lateral persistence allows a single nomenclature to be applied across most of north and north-east England. This has been standardised in the Stainmore region and includes, in ascending stratigraphical order, the Peghorn Limestone, Smiddy Limestone, Lower Little Limestone, Jew Limestone, Tynebottom Limestone, Single Post Limestone, Scar Limestone, Five Yard Limestone, Three Yard Limestone, Four Fathom Limestone, Iron Post Limestone, and Great Limestone.

In a few areas closely spaced, and individually named, limestone beds may merge to give a differently named unit. For example, the Peghorn and Smiddy limestones of the Edenside and north Cumbria districts, when combined, are known as the Askham Limestone Member. The Great Limestone Member includes the uppermost limestone of the formation. This was formerly known as the Main Limestone on the Askrigg Block. Throughout most of the region it comprises an uncharacteristically thick, bioclastic, locally biostromal, limestone. It has been correlated with the Top Hosie Limestone of the Midland Valley of Scotland, and the First Limestone in Cumbria.

In south Cumbria (see Rose and Dunham, 1977; Johnson et al., 2001) the formation comprises a variable succession of thinly interbedded dark grey to black limestone, shaley mudstone and subordinate (and impersistent) sandstone. The limestones are richly fossiliferous in places with a coral–brachiopod fauna, and some beds have been linked with name equivalents in the Askrigg Block area suggesting depositional continuity. The Girvanella Nodular Band of Garwood (1913) occurs at or near the base of the formation in south Cumbria. Carboniferous rocks in the northern part of the Isle of Man are entirely concealed and poorly known, but a cyclothemic, mixed carbonate (grey argillaceous limestone)–clastic sequence similar to the Alston Formation of south Cumbria has been proved in boreholes (see Chadwick et al., 2001).

Genetic interpretation

The Alston Formation typically represents cycles of marine to fluvio-deltaic sedimentation. East of Morecambe Bay it is poorly exposed, shows considerable structural complexity, and displays characteristics of deposition on a deeper shelf. West of Morecambe Bay the cycles pass laterally into the hemi-pelagic succession of the Bowland Shale Formation.

Stratotype

The type area of the formation is the Alston Block. It is proposed that the original type section in Augill Beck, Cumbria (see Burgess and Holliday, 1979) is replaced by the Rookhope Borehole (BGS Registration Number NY94SW/1) (NY 9375 4278) Weardale, from rockhead in the upper part of the Great Limestone at 6.86 m depth to the base of the Peghorn Limestone at 331.7 m depth (see Johnson and Nudds, 1996; Cozar and Somerville, 2004). In south Cumbria the formation occurs in boreholes at Harbarrow No. 1 (BGS Registration Number SD27SE/37) (SD 2531 7197) from 39.01 to 135.94 m depth and Gleaston Castle Farm (BGS Registration Number SD27SE/51) (SD 2549 7185) from 124.27 to 165.36 m depth (the bottom of the borehole). Natural sections in south Cumbria occur at Gleaston and on the East Shore (SD 3906 7469) of Humphrey Head; the Kirkhead Railway Cutting (SD 3949 7524); and, most extensively, in Holker Park at about (SD 350 774) in a succession, mostly exposed, of limestones and sandstones with sporadic siltstone and coal some 140 m thick (see Johnson et al., 2001; Rose and Dunham, 1977). On the north Isle of Man in the Ballaghenny Borehole of Lamplugh (1903, pp. 286–288), cyclothemic ‘Yoredale’ facies of mixed carbonate–clastic lithologies have been attributed to the Yoredale Group (see Chadwick et al., 2001).

Lower and upper boundaries

In the Northumberland Trough and Solway Basin, the base of the Alston Formation, defined at the base of the Peghorn Limestone (or its correlatives), lies conformably on the Tyne Limestone Formation (Figure 6, Column 7; Figure 8, Columns 11,12). In the Brough-under-Stainmore and Penrith districts the Peghorn Limestone is underlain by siltstone or sandstone of the Wintertarn Sandstone Member, Tyne Limestone Formation (Figure 9, Column 16).

In east Cumbria and the Stainmore Trough, the lower boundary occurs sharply at the base of the Askham (Peghorn) Limestone on the mainly sandstones of the Wintertarn Sandstone Member (Figure 9, Column 16; Figure 14, Column 3). On the northern Askrigg Block, the Hawes (Peghorn) Limestone occurs above the cross-bedded, regressive sandstones of the Wintertarn Sandstone Member, Tyne Limestone Formation (Figure 15, Column 3).

In north Cumbria, east of the Bothel Fault, the lower boundary of the Alston Formation occurs at the top of the White Limestone unit at the base of the Fourth Limestone, Eskett Limestone Formation (Great Scar Limestone Group) (Figure 14, Column 2), which comprises pale to dark grey limestones. Immediately west of the Bothel Fault, the lower boundary of the Alston Formation is at the top of the limestones with palaeokarst surfaces and distinctive fossils, the Fourth Limestone (Great Scar Limestone Group). It must be stressed that this division at the Bothel Fault is purely for ease of description. The fault is not implied to have exerted any penecontemporaneous effects on deposition.

In south Cumbria the lower boundary is at the point where the thickly bedded pale grey limestones of the Urswick Limestone Formation, Great Scar Limestone Group, pass up into the predominantly dark grey, thinner-bedded limestone and mudstone of the Alston Formation (Figure 9, Column 14). The Girvanella Nodular Band of Garwood (1913) may mark the base of the formation.

In districts where the formally defined lower boundary to the Alston Formation has not or cannot be mapped at the base of the limestone that marks the base of the Brigantian Stage, that boundary is taken at the top of the next, widely mapped limestone below it. This means that in the Rothbury district, Northumberland Trough, for example, the base of the Alston Formation is taken at the base of the Asbian strata that overlie the Dun Limestone, Tyne Limestone Formation (Figure 13, Column 3).

On the north Isle of Man, the platform carbonates of the Balladoole Formation, Great Scar Limestone Group, are thought to be overlain by a cyclothemic mixed carbonate–clastic sequence similar to the Alston Formation of south Cumbria.

The top of the formation is generally defined by the base of the Stainmore Formation with its cyclical succession of mudstones, siltstones, sandstones and thin limestones and thin coals. However, in south Cumbria the upper boundary is taken at the base of the Cravenoceras leion Marine Band where the thinly interbedded limestones, mudstones and subordinate sandstones of the Alston Formation pass upward into the thick mudstones and fine-grained siltstones of the overlying Bowland Shale Formation, Craven Group (Figure 9, Column 14).

Thickness

The formation tends to thicken into troughs and half-grabens. It is 80–180 m thick in south Cumbria, 200–250 m thick in the Appleby district, up to 300 m thick in Edenside and about 337 m thick in the Rookhope Borehole (see above) on the Alston Block, and 600 m thick in the Stainmore Trough. Offshore, in the Keys Embayment, between the Isle of Man and the west Cumbria coast, a well proved 108 m of strata equated with the probable onshore Alston Formation.

Distribution and regional correlation

South Cumbria and north Lancashire from the Duddon Estuary to the Carnforth area, the Askrigg and Alston blocks, the Stainmore and Northumberland troughs, and the Solway Basin. The formation also occurs in north Cumbria, east and immediately west of the Bothel Fault (though it is not implied that the fault exerted any penecontemporaneous effects on deposition), and apparently, on borehole evidence, on the north Isle of Man.

Age and biostratigraphical characterisation

Asbian in places, but generally Brigantian to Pendleian. In the River Eden (NY 7832 0375) at Janny Wood on the Alston Block, the Peghorn (Lower Smiddy) Limestone is defined as the Brigantian Stage Basal Stratotype (George et al., 1976). Microfaunal dating in the Rookhope Borehole suggests mostly Brigantian (Cf6ð¨ Subzone age), but early Serpukhovian (Pendleian) for the Great Limestone Member (Cozar and Sommerville, 2004). The base of the Namurian, defined as the base of the Cravenoceras leion Marine Band, is taken as near the base of the Great Limestone. In south Cumbria the richly fossiliferous limestones commonly have various species of Lithostrotion and Siphonodendron, and the following corals and brachiopods are, according to Rose and Dunham (1977), restricted to the formation here: Actinocyathus floriformis, Aulophyllum pachyendothecum, Dibunophyllum bipartitum, Diphyphyllum lateseptatum, Eomarginifera cambriensis, Productus hispidus, and Pugilis pugilis. The unit in south Cumbria equates with the ‘Upper Dibunophyllum Subzone’ of Garwood (1913) and is Visean (Brigantian) in age. Onshore, on the north Isle of Man the Yoredale facies attributed to the Alston Formation in the Ballaghenny Borehole may be Namurian (or Visean) in age (see Chadwick et al., 2001).