The Quaternary succession in the central North Sea

The drainage system beneath ice sheets is poorly understood, even though it is a major determinant of their basal boundary condition, with implications for their form, flow and stability, and particularly of the ice streams that are crucial dynamic components. Understanding can be enhanced by evidence from the beds of former ice sheets. Glaciological principles are applied to geological evidence in the Vale of York that demonstrate occupancy by a major ice stream of the British ice sheet, with deduced gravitational shear stresses in the range 4-24 kPa, compared with a typical glacier shear stress of about 80-100 kPa, and horizontal velocities in the terminal zone in the range of 400-950 ma 1 . It is shown that the ice stream was in an almost buoyant state, with effective pressures in the range 50-100 kPa for a range of ice pressures between 2000 and 5000 kPa. Similar deductions are made for other ice streams on the southern flank of the British ice sheet. The esker system in the area of the ice stream represents locations of subglacial drainage tunnels that are suggested to be the key drainage elements in ice sheets where excess surface water penetrates to the bed. A theory of channel-groundwater coupling which self-organises in response to the rate of water recharge and the transmissivity of the subglacial bed is applied to the sub-ice stream esker system. When matched to local geological evidence, the theory predicts the distribution of basal water pressures, the rate of recharge of surface water to the glacier bed, local shear stresses, the areal pattern of sedimentary processes including bed deformation, and their seasonal fluctuations. Predictions of the annual recharge rate of 950 mm are consistent with the modern values of modern mean annual rainfall in the Ouse (Vale of York) catchment of 906mm. The calculated mean annual discharge of the four major subglacial rivers in 55 m 3 s À1 , compared with a mean flow of the modern Ouse of 49 m 3 s 1 . A transient analysis shows the response time of water pressures in the underlying aquifer to changes in surface recharge to be of the order 10-100 days, suggesting the possibility of instabilities at the ice-bed interface. It is not possible, however, to determine whether the Vale of York ice stream was characterized by stable or instable (surging) flow.

Section: Form and Flow Of Pleistocene Ice Sheetssupporting

confidence: 50%

Drainage pathways beneath ice sheets and their implications for ice sheet form and flow: the example of the British Ice Sheet during the Last Glacial Maximum

Boulton¹

2010

“…Combining the boreholeevidence with the published seismic data (cf. Stoker et al, 1985;Cameron et al, 1987) it is apparent that the delta front sequence in this area is characterised predominantly by flat-lying reflectors with only minor indication of prograding style. This suggests that in such a wide, shallow continental shelf setting, with only limited change in slope angle, a prograding sequence may be replaced by flat-lying deposits.…”

Section: Discussionmentioning

confidence: 99%

“…Facies interpretation From regional seismic evidence, the Aberdeen Ground Formation has been interpreted as a shallow, open shelf marine sequence (Stoker et al, 1985;Cameron et al, 1987). Interpretation of the borehole data allows more precise comments to be made about the depositional system, although the lack of a lateral perspective, inherent in any borehole study, inhibits the development of rigorous facies models.…”

Section: Association Of Facies 3 Andmentioning

confidence: 99%

“…BGS offshore boreholes have On seismic profiles the Aberdeen Ground Formation i s characterised by high amplitude, laterally continuous reflectors which give the formation a strongly-bedded appearance. Holmes (1 977) observed that the reflectors diverge slightly down dip towards the centre of the basin, though the basal reflectors appear to overstep the underlying pre-Quaternary strata in the west (Stoker et al, 1985). Second-order, discontinuous, low amplitude reflectors have been observed in some profiles.…”

Section: Introductionmentioning

confidence: 99%

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Lower Pleistocene deltaic and marine sediments in boreholes from the central North Sea

Stoker

Bent

1987

Self Cite

Lower Pleistocene sediments recovered in boreholes from the Aberdeen Ground Formation in the central North Sea indicate that the unit was deposited in a delta front to prodeltaishallow, open shelf marine setting. Possible estuarine and clastic nearshore marine deposits have been identified on the western margin of the basin. The delta front sediments consist of interbedded, structureless to laminated sands and muds with organic debris, ferruginous nodules and common soft sediment deformation structures. Sporadic rippled and graded beds, basal scours to beds and starved ripples suggest periodic wavGcurrent reworking. Prodeltalshelf marine sediments are predominantly argillaceous with only occasional thin sand beds and rare phosphatic bands. One exceptionally thick sand bed may represent a tidal sand body or submarine channel-fill although this remains uncertain. The estuarineklastic nearshore marine sediments include coarse channel-lag deposits and rippled and laminated subtidal sands. A rich microfossil assemblage recovered from the prodeitalshelf marine sequence indicates that deposition occurred under fluctuating climatic conditions.

“…Seismic profiling in the North Sea has revealed at least three systems of major, over 100 m deep incisions (Stoker et a/., 1985;Cameron et a/., 1987;Wingfield, 1989Wingfield, , 1990. With the exception of parts of the youngest generation, the incisions have been completely infilled.…”

Section: Introductionmentioning

confidence: 99%

The extension of the Late Weichselian/Late Devensian ice sheets in the North Sea Basin

Ehlers¹,

Wingfield

1991

The distribution of large channel-like features, comprising Weichselian/Devensian incisions, in the western North Sea provides evidence for a much larger extension of the last ice sheet than currently assumed. Morphological comparison of the incisions with those in North Germany and Poland reveals a striking similarity in shape and distribution. The features on the North Sea floor are interpreted as being formed by meltwater erosion within the margin of the ice sheet, The widespread absence of Weichselian/Devensian till in the area under consideration may be attributed to later erosion. Large-scale reworking and redistribution of sediments is indicated by the complete sediment infill of the majority of the incisions. Journalof Quaternary Science