Offshore stratigraphical record, Quaternary, Cainozoic of north-east Scotland

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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.

Introduction[edit]

Summary of events preserved in the marine and onshore records. P915341.

An understanding of Scotland’s evolving landscape requires knowledge of how the position of the coastline has varied throughout the Quaternary in response to changes in climate and relative sea level. The current position of the coastline is only a transient feature. For example, most of the southern North Sea was low-lying land like the Netherlands until the first major glaciation. Moraines are preserved at the shelf-edge north-west of mainland Scotland, more than 60 km west of the nearest coastline (Holmes et al., 1993; Stoker et al., 1993). Conversely, marine deposits are preserved more than 10 km inland from the modern coastline at the head of the Moray Firth and at least 6 km inland near Elgin. For the following account, the modern coastline is taken as the boundary separating onshore and offshore Quaternary sedimentary deposits.

Compared with the fragmented onshore Quaternary sedimentary record, that offshore is considerably more complete, especially towards the shelf-edge where large submarine fans formed during major glacations (P915341).

The offshore sequence is generally thicker, more extensive and can be correlated regionally using seismostratigraphical methods (Stoker et al, 1985; Holmes, 1997). At least five major glacial episodes have been recognised in the North Sea basin, within a sequence that is dominanted by deltaic, low salinity cold-water marine and glaciomarine conditions (Sutherland, 1984a). The thickest and most complete sequence (more than 500 m) is preserved in the Central Graben of the North Sea, which has subsided tectonically throughout the Quaternary. At least ten till units are present within the Norwegian Channel with six or seven extending to the distal shelf break (Sejrup et al., 2000). An unconformity occurs at the base of the Quaternary and the magnitude of the hiatus increases away from the graben towards north-east Scotland (Andrews et al., 1990; Johnson et al., 1993; Gatliff et al., 1994). In the Moray Firth, most Quaternary sediments rest on Paleocene, Cretaceous, and Jurassic strata, with successively older formations cropping out as the modern shoreline is approached (Andrews et al., 1990, fig. 2). This style of basin–tectonic subsidence was initiated in the mid-Miocene, and was coeval with Miocene, Pliocene and Quaternary uplift of the mainland, which resulted in increased sediment supply to offshore areas (Jordt et al., 1998). The truncation of Quaternary, Neogene and older sediments towards the margins of the North Sea basin is consistent with a hypothesis involving cycles of sub-glacial and marine erosion removing terrestrial sediments and contributing to long-term isostatic uplift of the mainland and near-shore areas (Japsen, 1998).

Quaternary stratigraphy of the central North Sea. P915280.
General stratigraphical relationships of Quaternary formations in the central North Sea. P915281.

The following account is based largely on Holmes (1997) and the BGS regional reports covering the Moray Firth (Andrews et al., 1990) and the central North Sea (Gatliff et al., 1994). Details of map coverage are given in Information Sources. Interpretations are based mainly on seismic reflection data calibrated by borehole logs. Regionally mappable seismostratigraphical units have been established as formations and members where there is sufficient lithostratigraphical, biostratigraphical or chronostratigraphical control (P915280; P915281), but difficulties in correlation are common. Moreover, the Moray Firth basin was investigated early in the offshore programme when seismic acquisition technology and many procedures and dating techniques were in their infancy. Hence the available data is not as high quality or detailed as that obtained more recently (P915282). A summary of important events is given in P915341.

Generalised sequence of Quaternary deposits in the Moray Firth. P915282.

The north-west European chronostatigraphical classification has been adopted offshore because it is more complete than the British one, especially in the Lower and Middle Pleistocene (P915280). The oxygen isotope stages (OIS) of Shackleton and Opdyke (1973) provide a link between the two schemes, but there is presently vigorous controversy concerning their correlation. The boundary between the Lower and Middle Pleistocene is taken here at the Brunhes–Matuyama boundary of the palaeomagnetic record of Funnell (1995; P915280).

Early Quaternary (2.44 to 0.78 Ma)[edit]

The ‘non-glacial’ Early Quaternary (2.44 to 1.2 Ma)[edit]

This part of the record is contained within the Aberdeen Ground Formation (P915280; P915281), the oldest and thickest formation mapped offshore (Holmes, 1977; Stoker et al., 1985). The greater part of the formation off northeast Scotland comprises prodeltaic silts, clays and fine-grained sands deposited in a shallow sea (less than 50 m deep) that lay adjacent to extensive deltas occupying most of the southern North Sea basin (Stoker and Bent, 1985; The unconformable base of the Aberdeen Ground For-Jeffrey and Long, 1989; Cameron et al., 1992). The deltas mation is represented in the central North Sea by the were formed mainly by precursors of the Rhine and ‘crenulate reflector’ (Holmes, 1977, 1997). Locally it lies Thames and by a major river draining the area now largely 50 m above the Gauss–Matuyama palaeomagnetic occupied by the Baltic Sea. boundary (P915280; P915341). To the north and west of 58°N 01°E, the unconformity is interpreted from 3-D seismic evidence to take the form of fluvial channels. These features merge downslope into furrowed surfaces like those known to have been produced by grounded icebergs or sea-ice keels (Holmes, 1997). This evidence for ice scour within the Aberdeen Ground Formation is reinforced by the occurrence of periodic ice-rafted debris in sequences of equivalent age on the Atlantic margins off north-west Scotland (Stoker et al., 1994). Hence, the Scottish Highlands were probably glaciated to some extent during the colder stages of the Early Quaternary before 1.2 Ma.

Early Quaternary glaciations (1.2 to 0.78 Ma)[edit]

In the northern North Sea, the Shackleton Formation (Johnson et al., 1993) includes an erosion surface that separates sand-rich sediments below from mud-rich sediments above. The incoming of mud-rich sediments is reflected by a change of acoustic facies that is tentatively attributed by Johnson et al. (1993) to the first impact of regional glaciation on the depositional environment of the northern North Sea. The facies change is numerically undated, but on the basis of seismostratigraphical correlation, it predates the 1.07 to 0.99 Ma Jaramillo Subchron (P915280), which has been reported from the overlying Mariner Formation (Stoker et al., 1983; compare with Skinner et al., 1986).

There is more robust evidence in the Troll region of the Norwegian Channel for the ‘Fedje Glaciation’, which postdates the 1.19 Ma Cobb Mountain Event within the Matuyama Reversed Polarity Chron (Funnell, 1995; P915341). On the basis of the micropalaeontology, Sr-isotopes, palaeomagnetism and amino-acid geochronology, this glaciation has been assigned an age close to 1.1 Myr (Sejrup et al., 1995). The base of the Mariner Formation is defined regionally on seismic reflection profiles by an irregular erosion surface, which is locally overlain by a diamicton (Johnson et al., 1993). The presence of the diamicton, the geometry of the reflector and other evidence have been put forward as evidence of shelf glaciation extending from Norway across the northern North Sea to areas west of Shetland (Holmes, 1997). However, there is no secure basis for such wide-reaching correlations and the early shelf glaciations may not even have affected offshore areas as far south as the Moray Firth.

The earliest evidence for glaciation of the sea bed offshore from north-east Scotland occurs in the Fladen area of the central North Sea where a 10 m-thick unit of diamicton lies just above the Jaramillo Subchron within the Aberdeen Ground Formation. As the diamicton is overlain by sediment containing microfossils with an interglacial aspect provisionally correlated with the Leerdam Netherlands Pollen Stage, Sejrup et al. (1987) place the glacial event within the Bavelian Complex, between 800 and 900 ka. It has not been established whether the ice that deposited the diamicton flowed from Scotland or Scandinavia, although Sutherland and Gordon (1993) have argued that the latter is more likely. Nevertheless, the Scottish mountains were most probably extensively glaciated during that event.

Middle Quaternary (0.78 Ma to 130 ka)[edit]

Cromerian glaciations (OIS 14, 16 and 18)[edit]

The oldest known glacial deposits laid down offshore by ice flowing from the Scottish Highlands have been found in boreholes in the Forth Approaches and the Moray Firth (Stoker and Bent, 1985; Bent, 1986). There, towards the top of the Aberdeen Ground Formation, glaciomarine sediments laid down by a grounded ice sheet occur in the west, with the facies becoming increasingly more distal to the east. The deposits lie immediately above the Brunhes–Matuyama palaeomagnetic boundary (P915280; P915341) suggesting that the glaciation occurred in OIS 18, within the Cromerian Complex.

The top of the Aberdeen Ground Formation is cut by a succession of isolated, anastomosing and locally stacked channels, some of which may have been formed as sub-glacial or ice-marginal ‘tunnel’ valleys. The channels, together with the oldest units of sediment contained within them (Ling Bank Formation), have been correlated on the basis of regional seismostratigraphy with the widespread Elsterian glaciation of the north-west European mainland (Stoker et al., 1985; Cameron et al., 1987). However, several lines of evidence indicate that the basal sediments of the Ling Bank Formation at its type locality were laid down during a late-Cromerian interglacial (Penney, 1990; Ansari, 1992 Knudsen and Sejrup, 1993; Sejrup and Knudsen, 1993) and that they are overlain by arctic glaciomarine sediments thought to be Elsterian in age, on the basis of amino-acid dating of shells (Sejrup and Knudsen, 1993). The channelled surfaces at the top of the Aberdeen Ground Formation may well have formed in more than one glacial cycle, including the severe Cromerian glaciation of OIS 16 (Holmes, 1997). It follows that the first major interruption of the growth of deltas across the southern North Sea may have occurred in the Cromerian and not the Elsterian as commonly believed (P915341).

Elsterian glaciation (OIS 12) and Holsteinian interglacial deposits (OIS 11 and 9)[edit]

Sediments near the top of the Ling Bank Formation at its type locality have been correlated with ‘type’ Holsteinian sections in Denmark and Germany (Knudsen and Sejrup, 1993). However, it is still widely believed that marine Holsteinian deposits, in general, directly overlie the major erosion surface at the top of the Aberdeen Ground Formation (P915281). There was, therefore, a widespread glaciation during the Elsterian stage, which involved ice flowing from Scandinavia into the southern North Sea (Cameron et al., 1992) and extending to the continental shelf edge north-west of Scotland (Stoker et al., 1994). The timing of the glaciation is significant because the palaeogeography of the North Sea basin and hence the climate of the surrounding lands subsequently changed profoundly. Furthermore, Scotland and the adjoining continental shelves would have been glaciated during that event, which is generally correlated with the Anglian stage of the conventional British chronostratigraphy (P915341).

Saalian glaciation(s) (OIS 6 to 8)[edit]

Evidence for Saalian glaciations off north-east Scotland occurs in the Fisher and Coal Pit formations. The predominantly arctic glaciomarine Fisher Formation rests on a major, gently undulating unconformity that cuts across (onlaps) the Ling Bank and Aberdeen Ground formations (P915280, P915281). The unconformity results from a marine transgression and the overlying Fisher Formation is thought to be no older than OIS 7 (Jensen and Knudsen, 1988; Holmes, 1997). A till has been identified within the Fisher Formation (Sejrup et al., 1987) and contemporaneous subglacial erosion is thought to have occurred to the north of 58.6°N in the Moray Firth (Bent, 1986). The top of the Fisher Formation is defined by another regional unconformity, but unlike the one at its base, this one is typically crenulate (P915281) and is believed to be the result of the Saalian glaciation (Gatliff et al., 1994). The channels are mainly infilled with glaciomarine deposits belonging to the overlying Coal Pit Formation (Sejrup et al., 1987), although some fragmentary beds deposited in warmer waters have been identified within the formation in the northern and central North Sea (Gatliff et al., 1994).

The glaciomarine basal deposits of the Coal Pit Formation are succeeded by marine sediments containing foraminiferal assemblages typical of the Eemian–Ipswichian Interglacial (Cameron et al., 1987). These in turn underlie a horizon that has been correlated with the palaeomagnetic Blake Event at the OIS 5e/5d boundary (Stoker et al., 1985).

The evidence cited above suggests that there was a regional glaciation during OIS 6 of the Saalian and possibly an earlier, more limited Saalian glaciation in the north. The glaciation of OIS 6 is thought to have affected most of Scotland, because ice flowing from the mainland affected the northern North Sea at least as far south as 56°N (Sutherland and Gordon, 1993) and it reached the shelf break to the north-west of Scotland (Skinner et al., 1986; Stevenson, 1991; Holmes, 1997). Sejrup et al. (2000) conclude that ice streams occupied the Norwegian Channel during every glacial stage between OIS 12 and 6, each one representing a regional glaciation.

Late Quaternary (130 ka to present day)[edit]

Eemian Interglacial (OIS 5e)[edit]

A record of the Eemian (Ipswichian) is contained within the central part of the Coal Pit Formation, which fills channels that were probably eroded during, or immediately after, the Saalian glaciation in the Moray Firth and central North Sea (Andrews et al., 1990; Gatliff et al., 1994; P915281). Dinoflagellate cyst assemblages, like those found at present in the North Sea, have been identified in interbedded bioturbated sand and stiff, dark grey, shelly, pebbly clay with wood fragments in the type borehole (Stoker et al., 1985). The final occurrence of Elphidium ustulatum (Todd) has been noted; this is a foraminiferid that became extinct in the North Sea by the early Weichselian (Gregory and Bridge, 1979). In southern Norway, the Eemian is now thought to have been succeeded by a prolonged period of interstadial conditions with restricted mountain glaciation (Sejrup et al., 2000).

Early Weichselian glaciation (OIS 4)[edit]

Devensian–Weichselian events in Britain and south-west Fennoscandia. P915290.

No secure evidence of early Weichselian–Devensian glacial deposits has been reported off north-east Scotland, but an ice sheet is believed to have reached the shelf break to the north-west of the Scottish mainland where it formed submarine end-moraines (Stoker, 1988, 1990; Stewart and Stoker, 1990). Sejrup et al. (2000) conclude that an ice stream occupied the Norwegian Channel after 80 ka, suggesting that a regional glaciation occurred equivalent to the Karmoy Glaciation established in south-west Fennoscandia (P915290). Micromorphological studies of sediments from several boreholes in the central North Sea also suggest regional glaciation at that time involving coalescing Scottish and Scandinavian ice sheets (Carr, 1998).

Middle Weichselian (OIS 3)[edit]

The Coal Pit Formation in the central North Sea includes a sequence of shelly glaciomarine clays that have been placed tentatively in OIS 3 on palaeomagnetic evidence (Stoker et al., 1985) indicating that this area was free of glacier ice during this stage. In the northern North Sea and on the West Shetland Shelf a widespread surface of marine erosion has been correlated with the OIS 4/3 boundary. It is overlain by the mainly arctic marine Cape Shore Formation, which is securely placed in the Middle Weichselian on several lines of evidence (Johnson et al., 1993; Skinner et al., 1986; Sejrup et al., 1994; Holmes, 1997). Thus the seas to the north and west of Scotland were also free of glacier ice during this stage and perhaps much of the Scottish mainland.

Indirect evidence of former near-shore environments along the southern coast of the Moray Firth is provided onshore by glacial rafts that were transported by ice during the Late Devensian. At Clava, near Inverness, rafts of highboreal to low-arctic shallow marine mud originally deposited in Loch Ness, then a fjord, are probably early to middle Devensian in age (Merritt, 1992). The deposits correlate with those of the Bø Interstadial in Norway on amino-acid dating evidence (P915290). Rafts of broadly similar age have been located at the Boyne Limestone Quarry, King Edward and Gardenstown sites.

Late Weichselian/Late Devensian glaciation (OIS 2)[edit]

Models of the British Main Late Devensian ice sheet at its maximum extent. P915250.
Reconstruction of the maximum extent of the Main Late Devensian–Weichselian ice sheets. P915288.

The offshore record of Upper Weichselian deposits is of particular importance in establishing models of the main Late Devensian ice sheet in northern Britain, but it is complicated and, as explained below, controversial (P915250). BGS mapping suggests that an ice sheet extended to the continental shelf break, and beyond, to the north and west of Scotland (Holmes, 1991; Stoker and Holmes, 1991; Stoker et al., 1993; P915288). The resultant sediments are correlated on the basis of regional seismostratigraphy with Late Weichselian (OIS 2) deposits in the northern North Sea (Johnson et al., 1993). A radiocarbon date of about 22.5 ka BP from glaciomarine deposits within the limit of glaciation on the outer shelf to the west of St Kilda (Selby, 1989) appears to be consistent with the Late Devensian glacial maximum predating 18 ka BP (P915288).

Second major expansion of the Main Late Devensian ice sheet. P915291.

The important units off north-east Scotland are, from west to east, the Wee Bankie, Marr Bank, Swatchway and Coal Pit formations (P915280; P915281). The Wee Bankie Formation (Stoker et al., 1985) lies directly off the east coast of Scotland. It has a sheet-like geometry with an uneven, ridged top and comprises up to 40 m of stiff, matrix-dominated diamicton with some interbeds of sand, pebbly sand and silty clay. The formation is generally thought to have been laid down beneath the main Late Devensian ice sheet, and its mapped eastern boundary (P915291) has been used in many reconstructions of that ice sheet (e.g. Sutherland, 1984a; Boulton et al., 1985; Boulton et al., 1991), although some have envisaged more extensive glaciation at this time (e.g. Ehlers and Wingfield, 1991).

The Wee Bankie Formation is replaced eastwards by the Marr Bank Formation, commonly at a low, eastward-facing scarp interpreted as a former ice-contact slope, but the two formations probably interdigitate locally (Stoker et al., 1985). The Marr Bank Formation consists mostly of sands and muddy sands of Scottish provenance with a sparse microfauna indicative of shallow, high boreal to arctic waters. It forms a sheet-like deposit up to 25 m thick resting on an extensive surface of marine planation dipping north-eastwards (Holmes, 1977). As it is traced eastwards, the basal reflector of the Marr Bank Formation becomes acoustically indistinguishable from the upper part of the adjacent Coal Pit Formation, and the two formations probably pass laterally into one another locally (Gatliff et al., 1994).

In the central North Sea, the lower part of the Swatchway Formation, at Borehole 77/2 (P915291), is formed mainly of glaciomarine sediments from which AMS radiocarbon dates of 22.7, 20.9 and 19.7 ka BP have been obtained on in situ mollusc and benthic foraminiferids (Sejrup et al., 1994). This evidence suggests that the area was free of grounded ice during that period. However, a diamicton underlying the glaciomarine deposits in that borehole is interpreted as a till (Sejrup et al., 1994). It contains reworked arctic benthic foraminiferids that have provided a maximum AMS radiocarbon age of 42.3 ka BP. The diamicton rests on cold marine deposits assigned to the Ålesund Interstadial of north-west Fennoscandia and it is concluded by Sejrup et al. (1994) that the till was laid down between 28 ka BP and 22 ka BP during an initial, maximal stage of the Late Devensian glaciation. This conclusion hinges on the identification of the diamicton as a till and that the foraminiferids are unlikely to be in situ. Although Sejrup et al. (1994) describe deformation structures in the diamicton, they do not discriminate between subglacial deformation and disturbance by sea-ice. However, recent micromorphological evidence has revealed brittle shear at the base of the diamicton suggesting that it is indeed a subglacial, deforming bed till (Carr, 1998). Furthermore, Sejrup et al. (2000) and Carr (1999) both conclude that the central North Sea was also glaciated in the previous Skjonghelleren Glaciation of Fennoscandia, between about 50 and 40 ka BP (P915290).

There are important implications to these findings, discussed below, which indicate that the Scottish and Scandinavian ice sheets reached their maximum extent in the Late Devensian prior to about 22 ka BP and that they very probably coalesced (P915288). At least the uppermost part of the Wee Bankie Formation postdates this early phase. Following retreat to an unknown position at about 20 ka BP, during the Hamnsund Interstadial of Norway (Valen et al., 1996), the Scottish ice sheet probably then re-advanced to the eastern boundaries of the ‘Bosies Bank Moraine’ (Bent, 1986; P915291). This event probably equates with the Tampen Glaciation of Norway, when ice re-advanced onto the shelf and an ice stream reoccupied the Norwegian Channel (Sejrup et al., 2000; P915290).

Deglaciation, the Late-glacial period and Holocene (OIS 2 and 1)[edit]

Much of the northern North Sea north of 58°N had been deglaciated by 16 to 14 ka BP, and was either subaerially exposed or inundated by a very shallow sea (Peacock, 1995). This is compatible with a maximum age of about 14.1 ka BP for the onset of glaciomarine sedimentation following retreat of ice from the Witch Ground area (P915291; Sejrup et al., 1994) and at about 15 ka BP in the Norwegian Channel (Sejrup et al., 1995). The onset of deglaciation on the Hebridean Shelf has been dated to about 15.2 ka BP, predating the onset of warming in the North Atlantic (Peacock et al., 1992; Austin and Kroon, 1996). Shells within glaciomarine sediments occurring onshore near Peterhead have yielded ages of about 14.3 and 14.9 ka BP (St Fergus).

Radiocarbon dates from Portlandia arctica Gray, a high arctic marine bivalve, indicate that polar water, and probably seasonal sea-ice, remained in the northern North Sea until at least 13.2 to 13.1 ka BP when warmer waters arrived. Glaciomarine and estuarine silts and clays of the Errol Formation accumulated along the coasts (Peacock, 1999), while the muddy St Abbs Formation was laid down in this polar sea off the eastern coast of Scotland (Stoker et al., 1985; P915281). Warm North Atlantic waters did not reach the north-east Atlantic and western Scotland until about 13 ka BP, when conditions changed from high arctic to boreal possibly in less than 50 years (Kroon et al., 1987; Peacock and Harkness, 1990).

A record of the Windermere Interstadial is probably contained within the Swatchway Formation, which occurs to the north-east of Buchan (P915281). It comprises shelly muds and sands with a mixed northern temperate to arctic microfauna (Stoker et al., 1985; Harland, 1988). It occurs more certainly in the Largo Bay Member of the Forth Formation, which is more widespread off north-east Scotland (Stoker et al., 1985; P915281). It includes up to 30 m of silty muds that become coarser grained and pebbly upwards with concomitant decreasing faunal diversity. The trends probably reflect lowering sea level and cooling seas towards the onset of the Loch Lomond Stadial. The overlying St Andrews Member was laid down as coastal sand bars in a very shallow sea during the subsequent Loch Lomond Stadial. High arctic marine fauna returned during the stadial, during which nearshore marine summer temperatures were approaching 10° below present levels (Graham et al., 1990; Peacock, 1996).

Most sea-bed sediments of Holocene age have been mapped lithologically rather than lithostratigraphically. The return of warm North Atlantic Drift waters to the Scottish seas occurred within a few decades just prior to 10 100 ka BP (Peacock and Harkness, 1990). At first sea temperatures were 2 to 3° lower than those of the present day, but a warming occurred at about 9600 BP.

References[edit]

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