Logie-Buchan Drift Group, Quaternary lithostratigraphy, Cainozoic of north-east Scotland
|Merritt, J W, Auton, C A, Connell, E R, Hall, A M, and Peacock, J D. 2003. Cainozoic geology and landscape evolution of north-east Scotland. Memoir of the British Geological Survey, sheets 66E, 67, 76E, 77, 86E, 87W, 87E, 95, 96W, 96E and 97 (Scotland).
Contributors: J F Aitken, D F Ball, D Gould, J D Hansom, R Holmes, R M W Musson and M A Paul.
Logie-Buchan Drift Group
It has been widely believed for over a century that during the last (Main Late Devensian) glaciation ice flowing from Strathmore turned northwards between Stonehaven and Aberdeen and then north-westwards on to the coast of Logie-Buchan, between Aberdeen and Peterhead (P915251; Jamieson, 1906; Bremner, 1916; Synge, 1956; Clapperton and Sugden, 1977; Merritt, 1981; Hall, 1984; Munro, 1986; Hall and Connell, 1991). Glacial striae provide evidence of the northerly ice flow (Jamieson, 1882a, 1906; Bremner, 1916; P915377). The onshore movement is confirmed by the clay mineralogy of the red deposits (Glentworth et al., 1964) together with the unique suite of glacial erratics derived from the floor of the North Sea basin immediately off the coast of Aberdeenshire. The cause of the onshore movement is controversial, but the simplest explanation is that it was constrained to do so by Scandinavian–North Sea ice lying offshore. The diversion led to the deposition of a distinctive suite of typically vivid reddish brown, interbedded materials that form the hummocky topography and prime agricultural land to the north of Aberdeen on Sheet 77 Aberdeen, Sheet 87W Ellon and Sheet 87E Peterhead. The Logie-Buchan Drift Group has been established here to include these glacigenic deposits. It correlates in part with the seismostratigraphical Wee Bankie Formation (Stoker et al., 1985; Gatliff et al., 1994), which extends for some 25 to 40 km offshore, where it is bounded to the south of Stonehaven by the prominent Wee Bankie Moraine (P915291).
The Logie-Buchan Drift Group (P915297) is composed mainly of a complex, interbedded sequence of clayey diamicton, clay, silt, mud, sand and gravel in which individual units can rarely be traced laterally for more than a few tens of metres. Most deposits are vivid reddish brown in colour and calcareous. The sands are typically fine to medium grained, silty and micaceous. Laminae and thin beds of yellowish brown, medium-grained sand and fine gravel commonly contain shell fragments. In addition to locally occurring rock types such as amphibolite, feldspathic psammite, quartzite and meta-greywacke, the deposits contain appreciable proportions of rocks derived from the sea bed to the east, including limestone, dolomite, calcareous siltstone, white and red sandstone of Devonian, Permo–Triassic, possible Jurassic, Cretaceous and Tertiary age (P915249). The group underlies the coastal lowlands to the north of Aberdeen, east of Ellon and both south and north of Peterhead, where it forms distinctive, fresh-looking, hummocky topography comprising kettleholes, mounds, plateaux, esker ridges and narrow, winding, steep-sided valleys.
The deposits of the Logie-Buchan Drift Group were first described by Jamieson (1858, 1882a, 1906) as part of his Red Clay Series. They have been described as ‘red drift’ by Merritt (1981) and the Red Series by Hall (1984) and Hall and Connell (1991). The type area is the parish of Slains, between Collieston and Cruden Bay, on Sheet 87E (P915377). Typical sediments include silty and sandy diamictons that appear to have formed as flow tills and subaqueous debris flows, and muds formed in a glaciolacustrine, or possibly glacio-estuarine environment. These deposits, interbedded with glaciofluvial sands, fill hollows in the bedrock surface and collectively reach over 25 m in thickness (Merritt, 1981). Topography bears little relationship to the subdrift surface. The sequence is locally dominated by stiff, stony clayey diamicton, especially over bedrock ‘highs’. The uppermost metre or so generally consists of firm to stiff, pebbly, silty, clayey diamicton that has probably been formed by solifluction and cryogenic mixing. Thick diamicton-dominated sequences probably include deformation tills, as at Sandford Bay and Errollston. Laminated silts and clays overlying buff-coloured, pebbly sands with shell fragments commonly occur at the base of the sequence within hollows. Mappable units of glaciofluvial sand and gravel, till and glaciolacustrine deposits are assigned to the Hatton Till, Kippet Hills Sand and Gravel and Ugie Clay formations, respectively.
Hatton Till Formation
Hall and Jarvis (1995) describe several sites around Bellscamphie where a stiff, reddish brown, relatively stony diamicton that they named the Hatton Till rests on the dark grey Pitlurg Till (of the Banffshire Coast Drift Group). Elsewhere to the north and east of Ellon similar red tills rest on grey and yellowish brown sandy tills of the East Grampian Drift Group. The Hatton Till, which is raised to formational status here, is typical of the more stony red diamictons in the Logie-Buchan Drift Group described above (Merritt, 1981). Its type section is at Bellscamphie.
Thick units of reddish brown deformation till equivalent to the Hatton Till Formation are exposed at Sandford Bay, near Peterhead, where they have been derived from the south or south-east, and locally incorporate rafts of blue-grey diamicton. To the north of Peterhead, red tills interdigitate with the Essie Till Formation of the Banffshire Coast Drift Group (see above). In contrast, red tills are not known to interdigitate with yellowish brown, sandy tills of the East Grampian Drift Group to the north of the River Ythan. Furthermore, at Cross Stone, to the south of Ellon, the Hatton Till Formation abuts two morainic ridges indicating that Logie-Buchan ice was free to expand into the Ythan valley following the retreat of East Grampian ice (see below).
Kippet Hills Sand and Gravel Formation
In addition to the thin, laterally impersistent beds of sand and gravel that form part of the sequence, several larger bodies have been identified on Sheet 87E. For example, a sinuous ridge, the Kippet Hills Esker, extends northwards from Cotehill Loch (NK 028 294), near Collieston, past Meikle Loch to Ladies’s Brig (NK 029 318), where it widens north-eastwards into a flat-topped, fan-shaped mound at Whitehills (P915297; Kippet Hills). At the partial type section near Knapsleask (NK 0327 3206), the gravel is distinctive in that it contains 40 per cent or more of calcareous material including Palaeozoic dolomite, Mesozoic limestone and calcareous siltstone, Pliocene shelly sandstone (Crag) and shell fragments of early Quaternary and younger age derived from the offshore Aberdeen Ground Formation. Hall and Jarvis (1995) referred to this distinctive glaciofluvial deposit as the Kippet Hills Gravels and Sands and it is named here formally as the Kippet Hills Sand and Gravel Formation of the Logie-Buchan Drift Group (P915347). Hall and Jarvis describe similar material in an abandoned railway cutting at Bellscamphie (NK 0184 3369), which is taken here as another partial type section (Ellon).
Other deposits of sand and gravel
A distinct suite of pebbly sand deposits lies along the boundary between the Logie-Buchan and East Grampian drift groups to the south of the Buchan Ridge, on Sheet 87W Ellon and Sheet 87E Peterhead. In the vicinity of the Hill of Auchleuchries (NK 006 365) (P915376), the moundy deposits are capped by yellowish brown till of presumed western derivation, whereas some of the mounds to the north and west of Hatton (P915377) are capped by red till of the Logie-Buchan Drift Group (Merritt, 1981). The sands contain clasts of a variety of lithologies, but include conspicuous amounts of pink (possibly Peterhead) granite. Surprisingly, they contain very little flint and quartzite from the Buchan Gravels Formation, which crops out just 2 km to the north. The sands overlie dark grey till with shell fragments, which is correlated with the Pitlurg Till of the Banffshire Coast Drift Group at Bellscamphie. This till unit appears to have been deposited by ice moving onshore from the east-north-east (Bremner, 1928; Hall and Jarvis, 1995). As the pink granite clasts in the pebbly sands are likely also to have been derived from this direction, these glaciofluvial deposits probably relate to the same body of ice that laid down the Pitlurg Till. This association is supported by the presence in the pebbly sands of seams of fissile, chocolate-brown to olive-grey clay (Merritt, 1981). Hall and Jarvis (1995) conclude that the Pitlurg Till was laid down in the Early or Middle Devensian (OIS 4–3) and it follows that the pebbly sands are probably of similar age. However, if the Pitlurg Till correlates with the Whitehills Glacigenic Formation, the sands would have been laid down early in OIS 2 (P915347).
It is proposed here that the pebbly sands described above be named as the Auchleuchries Sand and Gravel Formation, the type section being BGS Borehole NK03NW1 (Merritt, 1981) on the Hill of Auchleuchries (NK 0057 3649). The Bellscamphie site (P915319) provides a reference section. The deposits of the Auchleuchries Sand and Gravel Formation have been placed in the East Grampian Drift Group on Sheet 87W Ellon, because they do not contain a significant proportion of clasts derived from offshore. However, the distribution of the deposits at, or within, the western boundary of the Logie-Buchan Drift Group and the rich shell fauna that was recovered from the Hill of Auchleuchries by Jamieson (1882b, p.172) both suggest that the unit is better placed in the Logie-Buchan Drift Group.
Ugie Clay Formation
Following the withdrawal of the East Grampian ice sheet, the coastal ice that laid down the Logie-Buchan Drift Group dammed the lower reaches of the valleys of the Ythan and Ugie causing extensive lakes to form (Hall, 1984; Hall and Connell, 1991; P915289). The fine-grained deposits laid down in these lakes are assigned here to the Ugie Clay Formation. Laminated silts and clays of the formation, mostly reddish brown in colour, occur extensively beneath the floodplains and glaciofluvial terraces of the North and South Ugie waters on Sheet 87E (McMillan and Aitken, 1981) and beneath similar features in the valley of the River Ythan on Sheet 87E (Merritt, 1981; Glaciolacustrine deposits). Lakes also formed in the Don valley upstream of the Mill of Dyce, but as the sediments there are dominated by materials derived from the retreating East Grampian ice sheet, they have been assigned to the Glen Dye Silts Formation of the East Grampian Drift Group (see below).
The type section of the Ugie Clay Formation is a stream section (NK 0050 4732) near Baluss Bridge, south of Mintlaw, where over 2 m of reddish brown and dark grey plastic clay is thinly interbedded with yellowish brown sand, gravel and silt. A BGS Borehole NK04NW2 drilled 190 m to the south-south-west of the section proved at least 4 m of the sequence overlying orange-brown, very sandy till, resting in turn on weathered psammitic bedrock. Till exposed nearby has a macrofabric indicating that it was deposited by the East Grampian ice sheet flowing eastwards (Hall and Connell, 1991). The Ugie Clay Formation is capped locally by terraced glaciofluvial sand and gravel. Organic muds sampled from temporary sections in the formation at Baluss Bridge itself, yielded rich Palaeozoic and Mesozoic palynomorph assemblages and unreliable radiocarbon dates (Ugie valley). Similar laminated organic deposits occur at [[Errollston Clay Pit, Cruden Bay - locality, Cainozoic of north-east Scotland|Errollston clay pit (NK 088 368), near Cruden Bay.
Several deposits of red and brown silt and clay in the Aberdeen area are named here as the Tullos Clay Member and assigned tentatively to the Ugie Clay Formation (rather than the Ury Silts Formation). They include those worked in clay pits at Tipperty (NJ 970 268) (Bremner, 1943; Munro, 1986) and Blackdog (NJ 962 139) (Jamieson, 1906; Bremner, 1916; Peacock, 1975). They also occur within the valley between Torry and Tullos Hill in Aberdeen (Simpson, 1948), and have been exposed at the Nigg Bay site. The deposits are locally fossiliferous and those occurring at lower levels may be glaciomarine (up to about +30 m OD) (Raised marine deposits)
Relationships between Logie-Buchan and East Grampian drift groups
Apart from at the Hill of Auchleuchries (see above), deposits of the Logie-Buchan Drift Group generally occur at the top of the glacial sequence to the north of Aberdeen (P915297). This suggests that they were the youngest glacigenic deposits to be laid down in the area. The interdigitation of bluish grey and red units to the north of Peterhead indicates that the Moray Firth ice stream laid down deposits of the Banffshire Coast Drift Group contemporaneously (see below). To the north of Ellon, red tills generally overlie tills of the East Grampian Drift Group.
In the Aberdeen area, the stratigraphical relationship between deposits of the Logie-Buchan Drift Group and the brown and grey tills derived from the East Grampian ice sheet is more complex. In the north-eastern part of Sheet 77, red-brown till is generally about 1 m thick and underlain by grey/brown till resting on bedrock (Munro, 1986). Isolated, ill-defined rafts of red till, up to 1.5 m in diameter, occur within the grey-brown till beyond the western margin of the Logie-Buchan Drift Group near Ardo House (NJ 929 208) (P915379). Near Tipperty (NJ 969 277), isolated patches of grey-brown till are incorporated in red-brown till, and vice versa. Where a discrete unit of red till overlies grey-brown till, as in a pipeline trench in the vicinity of Mill of Ardo (Munro, 1986, fig. 30 A-B), the boundary between the two is generally regular and sharp. Where there are more complex field relationships between the two till units, or where the grey-brown till overlies the red, contacts are commonly ill-defined and gradational. Similar stratigraphical relationships also occur in several BGS boreholes and trial pits north of Aberdeen (Auton and Crofts, 1986). In most instances (e.g. Borehole NJ91NE14, near Blackdog Rifle Ranges), red-brown silts and clays, overlie greyish brown till, but rarely, in Borehole NJ81SW2, near Little Clinterty, brown sandy till overlies red-brown clayey till.
The local evidence of interdigitation of red tills and grey tills of inland provenance led Clapperton and Sugden (1977) to conclude that the ice responsible for laying down the Logie-Buchan Drift Group was in contact with ice flowing westwards from the East Grampian ice sheet. More commonly, red tills overlie tills of the East Grampian Drift Group (Bremner, 1916; Auton and Crofts, 1986) suggesting that East Grampian ice expanded to the present position off the coast, if not beyond, before retreating sufficiently to allow the encroachment of ice from offshore (Hall and Connell, 1991). Clapperton and Sugden (1977) concluded that the early expansion of East Grampian ice was the result of fluctuating flow strength between ‘Cairngorm–Grampian’ and ‘Strathmore’ ice in the Late Devensian. In contrast, Connell and Hall (1987) concluded that the tills were laid down in separate glaciations, in the Early and Late Devensian, respectively. This was based mainly on evidence from the area north of the River Ythan, where diamictons of the two groups are not seen to interdigitate but locally are observed to be separated by the Ugie Clay Formation. However, as noted above to the north of Ellon, yellowish brown till, derived from inland, caps shelly deposits of the Auchleuchries Sand and Gravel Formation. This indicates the former close proximity of Logie-Buchan Drift Group and East Grampian Drift Group ice at this location, a situation that apparently has not been previously described north of the Ythan (see Jamieson, 1906, p.26–27; Bremner, 1916, p.337).
A pair of crescentic, asymmetrical end moraines have been identified during recent fieldwork on Sheet 87W in the vicinity of Cross Stone (NJ 954 278), near Ellon, at the western boundary of the Logie-Buchan Drift Group (P915297; P915376). These features indicate quite clearly that a lobe of ‘Logie-Buchan’ ice advanced directly onshore up the valley of the River Ythan following retreat of the East Grampian ice sheet. The latter retreated sufficiently to allow a substantial glacial lake to form in the Ythan valley upstream of Ellon (Merritt, 1981; Hall, 1984). The inland margin of the Logie-Buchan Drift Group can be traced north-eastwards towards the Buchan Ridge, where it is quite distinct in the vicinity of the Den of Boddam (NK 102 411). It reaches about 80 m above OD at the Den before descending to about 50 m OD to the west of Peterhead (P915377).
Relationships between Logie-Buchan and Banffshire Coast drift groups
Red tills overlie yellowish brown and grey tills derived from the west between Ellon and Peterhead (Wilson, 1886; Jamieson, 1906). Immediately to the south of the River Ugie, they overlie greenish grey till that was probably also laid down by East Grampian ice (McMillan and Aitken, 1981). West of Peterhead, at Downiehills (NK 089 471) (P915377), Jamieson (1858) recorded the occurrence of large blocks of Peterhead Granite in reddish brown till together with possible shell fragments. Later in the same area, he noted the occurrence of dark blue clay intermingled with deposits of the ‘Red Series’ (Jamieson, 1906). The latter description refers to exposures in the former claypit at Ednie (NK 085 500), where the red deposits were seen to enclose irregular masses described as ‘dark grey till’ and ‘dark blue clay and sand’ (Wilson, 1886; BGS records). These observations have caused confusion. On one hand they have been taken to support the view that the deposits of the ‘Blue Grey’ and ‘Red’ series are intermingled and are of approximately the same age (Jamieson, 1906; Hall and Connell, 1991). The alternative view is that older ‘Blue Grey Series’ (Banffshire Coast Drift Group) sediments have been incorporated into the red glaciotectonically by a later, northward movement of ice (Sutherland, 1984a; see Sandford Bay).
The situation is reversed to the north of the River Ugie, however, where red sediments are locally overlain by the dark grey, shelly Essie Till Formation, which contains erratics of pink granite, quartz and metasedimentary rocks. This diamicton, assigned to the Banffshire Coast Drift Group above, was probably laid down by a late re-advance of the Moray Firth ice stream. Red deposits of the Logie-Buchan Drift Group locally overlie blue-grey tills in the vicinity of Ellon and Aberdeen, but the age of the older tills is uncertain (see Banffshire Coast Drift Group above).
Relationships between the Logie-Buchan and Mearns drift groups and the Benholm Clay Formation
Deposits of the two groups do not appear to abut one another at the surface on land (P915251), and offshore no contact has been recognised within the Wee Bankie Formation. However, there are several notable occurrences of dark bluish grey, shelly diamicton underlying red till along the coast to the south of Aberdeen (Campbell, 1934). The best known locality is at the Burn of Benholm (P915381). These shelly diamictons have been assigned to the Benholm Clay Formation (Auton et al., 2000) and they were almost certainly laid down by ice moving onshore from the North Sea basin during a pre-Devensian glaciation (P915347). Despite its colour, the Benholm Clay Formation is placed tentatively in the Logie-Buchan Drift Group because of its derivation by ice from the North Sea basin (P220762).