Swatchway Formation

 Glacial Prehistory 

Swatchway 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/or before the LGM, before 23,170BP.

The implication here, is that the period in which the Last Glacial Maximum 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)

Sources  

(Johnson) The geology of the northern North Sea. United Kingdom Offshore Regional Report By H Johnson, 

Jeffery Nicholls 

South Ronaldsay 

Orkney 

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