Quaternary Addendum
Series Title:- The Orkney Riddle
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Blog Title:- Quaternary Addendum
"Archaeology in the North Sea" is intended to demonstrate that a spine of land connected Dogger Bank to a location in the northern North Sea that was accessible to Mesolithic people 10,000 years ago when sea-level was 50m below present.
"Walkable Land in the North Sea" defines an area of land that would have been available for hunting, foraging, and transit for Neolithic people.
Here, I relate the geology of the North Sea to demonstrate that in a period between 60,000BP and 30,000BP, a time in which an ice age is thought to have weighed heavily upon Britain, in fact , although it was a bit chilly there were significant numbers of animals, plants and people roaming the territory.
The sequence of events that created the undersea landscape of the North Sea is roughly as follows :-
Initially, in the last interglacial, 140,000 BP to 120,000BP, the North Sea may have been a reasonably quiet marine lagoon whose only oceanic front was at the North between Shetland and Norway. In spite of raised sea levels at that time it is possible that the English Channel was not cut through deeply enough to permit the overflow of North Sea marine water, southwest from the Atlantic Ocean.
Heightened sea-level in that extended interglacial period cropped over a million years of sediments in the North Sea. This particularly evident in seabed sections along the English and Scottish coasts.
At the start of the ice age, after 120,000BP, ice sheets developed over Britain, Ireland, and Norway. The ice-sheets were thick on the high ground, the mountains of Wales, northern England, Northern Ireland, and Scotland. Lowland ice sheets were more variable, depending on location and altitude.
After 70,000BP climate warmed, and lowland ice-sheets melted. As the retreating ice sheet drew back to the foothills of mountain ranges, huge amounts of meltwater and sludge were propelled across from northern England and Scotland, southeasterly to the southern North Sea where the sludge contributed to the creation of Dogger Bank.
There followed a period in which deep ice-sheets remained on the mountains of Britain, but where the lowlands, though cold, were mostly free of snow. As most of the ice sheet snow remained in place on the northern hemisphere mountains, sea level remained low. Southern England, southern Ireland, and the land on much of the current North Sea were fertile territories for flora, fauna, and prehistoric humans.
An instance of how the climate was warmed by 43,000BP, is described by Peter Kent :-
“Fluvial sand and gravel exposed in quarries near Tattershall contain evidence of a short but marked mid-Devensian climatic amelioration (Girling, 1974). An organic silt bed within involuted gravel overlying the Ipswichian peat has yielded insects indicative of a high-latitude continental tundra environment, but a slightly higher bed contains a thermophilous insect fauna—some taxa of which are now confined to southern Europe—implying summer temperatures at least as warm as those of southern England today. Radiocarbon dates from these deposits and correlatives elsewhere in England show that this warm interval, the initial phase of the Upton Warren Interstadial Complex, occurred around 43 000 years ago. It was too brief for the climatic improvement to be reflected in the flora. The overlying sand and gravel in the Tattershall area are appreciably cryoturbated and contain numerous remains of mammoth (Mammuthus primigenius), woolly rhinoceros (Coelodonta antiquatis), bison, reindeer and other tundra-favouring mammals.”
As a result of the continuing deglaciation after 60,000BP a series of features developed along the east coast of England and Scotland. At the same time the Norwegian Channel, along the southern Norwegian coastline remained as a free flowing drain from Europe and Scandinavia. Between the features along the east coast of Britain and the Norwegian Channel a ridge of higher ground remained. This would develop in time to be Doggerland, extending from the southern North Sea up to beyond Shetland in the northern North Sea.
Around Shetland a deposit called the Capeshore Formation was laid at this time. The Capeshore Formation material is described as "Soft to firm, highly to moderately silty clay with sporadic shell fragments and pebbles." (BGS)
Diatoms in the sediments are identified as:- "The foraminiferal assemblage is dominated by E. clavatum, Cassidulina reniforme Norvang and Protelphidium orbiculare . " (Johnson) , and the the conditions in which this organism developed are:- "The Cassidulina reniforme–Elphidium clavatum, Textularia spp. assemblage dominates the foraminifera fauna near the glacier proximal that indicates increased sedimentation and meltwater runoff." (Chatterjee)
The presence of this species suggests that meltwater was running away from Shetland probably through MIS 5, 4, and 3.
Moving south from the Capeshore Formation the next deposit, covering an area east of the Moray Firth and Orkney was the Swatchway Formation. This deposit was cut into by a deep series of impact valleys, currently known as the Witch Ground, and filled by the Witch Ground Formation.
In “Late Pleistocene and Holocene deposits in the Witch and Fladen Ground area, northern North Sea” by Fred Jansen, He divides each of these layers, Swatchway and Witch Ground Formations, into Upper and Lower strata. The earliest deposits are the Swatchway Formation, of which the earliest strata is the Lower SwatchWay Beds.
Jansen writes, “The Lower Swatchway Beds are heavily overconsolidated, shallow Marine sediments which form pronounced ridges, interpreted as ice Pushed ridges." (The pronounced ridges would be the peaks of ridges between valleys caused by the collapse of ice sheets as they retreated, away from warm water in the Norwegian Channel, towards Scotland.)
“A core in the Swatchway Beds nearby Old Devil’s Hole (74-J62-Z, Fig. I) contains still unidentified peaty material as an admixture in Fine sands, which was 14C dated. The section between 130 and 166 cm Below the seafloor yielded a conventional radiocarbon age of 33 800 (+2600, --2000) y BP and the sample from 248 to 251 cm of 37 500 (+ 1800, --1500) y BP.”
“The Upper Swatchway Beds, filling in the hollows between the Ridges, are composed of usually normally consolidated glaciomarine Deposits, with locally some probably subglacially folded structures. The Swatchway Beds Are roughly 50 m thick.”
The Upper Swatchway Beds would be the residue of materials left behind after impact erosion formed the ridges.
"The Swatchway Formation has an amorphous, or structure-less, seismic signature, with sporadic subhorizontal reflectors. It has a lithofacies of mud and sand with scattered lithic and shell fragments, The sparse, benthonic foraminiferal assemblage is dominated by the cold-water species E excavatum var. clavatum, which suggests deposition in boreal to arctic seas (Stoker et al., 1985)." (Johnson)
Elphidium excavatum var clavatum (A most abundant calcareous foraminifer in cold-water areas (temperatures below 2°C) on the Arctic shelves; inhabits all sediment types; has a complex pattern of distribution; often dominates foraminiferal assemblages in extreme environments, such as low salinities, turbid waters, and low food supply, including sites proximal to tidewater glaciers.) ( Foraminiferal Research at Byrd Polar Research Center)
"Perhaps more significantly, the foraminifera are accompanied by moderately abundant populations of dinoflagellate cysts; a limited spectrum is co-dominated by O. centrocarpum and Bitectatodinium tepikiense Wilson, indicating northern cool-temperate conditions without significant sea-ice cover (Harland, 1988a). " (Johnson)
Lower Swatchway Beds laid down in boreal to arctic seas without significant sea-ice cover after 37500BP. They are heavily overconsolidated, meaning that it was compressed under a thick ice-sheet. The Upper Swatchway Beds are the fill of erosive valleys cut into the Lower Swatchway Beds and they are generally accepted to be dated to between 18,000BP and 15,000BP.
"The principal 14C samples used by HOLMES were recovered from two borings, a commercial boring at Josephine Field and the IGS boring 74/7 at roughly 60 km off Aberdeen (56058 ' N, 1°07 ' W, INSTITtrrE OF GEOLOOICAL SCIENCES, 1975). From the upper sample of the Josephine Field boring a.o. large fragments of Taxus were dated which is a thermophilic tree. An early glacial age of 23 170 y BP, as was measured, is therefore impossible for biological reasons, and the material most probably was contaminated with younger carbon. If this is the case, the older dates of the boring are also suspect." (Jansen)
The finding of Ewe Tree wood at a location near Aberdeen and off the Scottish coast requires some explanation. Initially, if the radiocarbon date is ignored the existence of the tree growing there at any date from 60,000BP to say 12,000BP, changes completely how we view the climate of northern Europe, and specifically the North Sea.
The tree may have grown in that location , or it may have floated there, but "The lower 15 m of the Shell-Fugro boring are correlated with the Upper Swatchway Beds. The pollen spectra of this section has a considerable contribution of tree pollen." So this makes it likely that other trees were growing in the area of the Swatchway Beds both after the LGM, perhaps 18,000BP, and before the LGM, before 23,170BP.
The implication here, is that period in which LGM gripped Britain was perhaps not more than 5,000 years long, enough time to add hundreds of metres of ice to the mountain ice sheets of Britain and Norway, and to dump a varying thickness of snow on southern England, southern Ireland, and much of the North Sea land area.
The cores 72-J1-Z and 72-Jg-Z, located further south (Fig. 1), were taken from the Lower Channel Fill which, at least partly, developed simultaneously with the Upper Swatchway Beds. The pollen spectrum in the lower section of 72-J1-Z also belongs to group III, a high Hystrichosphaeridae percentage pointing to a marine environment (DE JONG, 1976). The mollusc fauna lacks arctic species, suggesting that the sea was even warmer than today (SPAINK, 1973). The mollusc assemblage of core 72-J9-Z resembles the present fauna, but indicates a somewhat shallower sea level (SP.~aNK, 1976a). (Jansen)
Marr Bank Formation
The Marr Bank Formation is described as being deposited in a shallow estuarine environment in "The geology of the central North Sea. United Kingdom offshore regional report by R W Gatliff".
"Sand, fine-grained, poor- to well-sorted, soft to firm, grey to red brown with abundant lithic granules and pebbles. Locally silty."
The Marr Bank Formation includes "muddy sediments, especially towards the north-west but it is generally formed of sands of varying grain sizes and degrees of sorting (Stoker et al., 1985). Gravelly layers contain a variety of lithic clasts of Scottish provenance, and wood fragments and clay balls occur sporadically. No dinoflagellate cysts have been recovered from the formation."
"A poor ostracod fauna and a moderately abundant population of foraminifera (Thomson, 1978) indicate that the shallow (marine) waters in which the formation was deposited were high-boreal to arctic in temperature, and inner shelf to estuarine in character." (Gatliff)
The marr bank was a shallow lagoon of unfrozen water along the British coast. It was separated from the Norwegian channel by a ridge of land that reached from dogger bank to the Atlantic ocean north of Norway
Wee Bankie Formation
"Overconsolidated sandy and gravelly till with interbeds of fluvial sand and pebbly sand, and, less commonly, accumulations of coarse sand and gravel."
The Wee Bankie Formation rests along the east coast of northern England and southern Scotland. It also follows the southern edge of the Marr Bank Formation, and as an overconsolidated material is likely to have been overlaid by ice-sheets until deglaciation, in spite of the Marr Bank being essentially ice- free.
Bolders Bank Formation
"The Bolders Bank Formation characteristically consists of reddish to greyish brown, stiff diamictons that are generally massive but in places possess distinct, commonly arenaceous layering, and deformational structures. The majority of its pebbles, of which chalk is the most conspicuous component, are derived from the sedimentary rocks of eastern England. The pebble content tends to diminish eastwards. In general, the formation is less than 5 m thick, and commonly less than 1 m is preserved in deep-water areas west of the Dogger Bank, although to the east of Lincolnshire it may be 15 to 20 m thick (BGS Spurn Quaternary Geology sheet). " (Cameron)
Plan of geological deposits on the floor of the North Sea adjacent to Shetland and Orkney.
Section of the geological deposits north-south between the Norwegian Channel and the Witch Ground.
Plan and section of geological deposits in the northern North Sea east of Orkney and northeastern Scotland. Includes the Coal Pit, Swatchway, and Witch Ground Formations.
Plan of geologically deposits in the floor of the North Sea, adjacent to the Scottish and northern English coast. Location of sections across the area are shown.
The Wee Bankie Formation is off the coast of Scotland and England, is overconsolidated, suggesting that it was under ice at the LGM. The Marr Bank Formation is not consolidated, and may have been lacustrine or marine, but unfrozen at the LGM.
Sections across the Marr Bank, and other deposits, located above.
Detailed section across Marr Bank and Wee Bankie Formations
Plan of geological deposits on the floorbof the central North Sea, showing that the Coal Pit Formation which in Sections is clearly derived from England or Scotland, has clearly reached across to the mid-line of the seabed infilling pre-existing channels. Any sediments that connected these Coal Pit Formation filled gullies, to each other , across older Formations has been cropped, and that made-up seabed here has been lost to erosion.
The Bolders Bank Formation, originating from the Wee Bankie Formation, further north, follows the south coast of the Dogger Bank, around towards the coasts of Belgium, Holland, and Denmark. It is cut by the Outer Silver Pit, and pre-dates the cutting of that feature.
The outflow from the Marr Bank, Wee Bankie, and Bolders Bank formations flowed out from the Marr Bank shallow inland sea , between 60,000BP and 30,000BP, along the south coast of Dogger Bank where the seabed depth is now 50 metres.
After the LGM, the Outer Silver Pit was cut.
In the North, the Witch Ground gullies were cut, and these may have enabled marine waters to access the Marr Bank waters.
The survival of an area of water, trapped between Dogger Bank at its east, Bolders Bank at the south, and the Swatchway Formation at the north created the possibility of an ice-free environment through the coldest years of the LGM.
Back to the beginning of the Orkney Riddle
Sources
(Gatliff) The geology of the central North Sea. United Kingdom offshore regional report by R W Gatliff
(Johnson) The geology of the northern North Sea. United Kingdom Offshore Regional Report By H Johnson,
(Cameron) The geology of the southern North Sea. United Kingdom offshore regional report By T D J Cameron,
(All views and opinions expressed in this document are my own)
Jeffery Nicholls
South Ronaldsay
Orkney
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