Late Quaternary foraminiferal stratigraphy from western Svalbard

In the lower part of sections at Skilvika and Linneelva, western Svalbard, marine silts and sands characterized by infinite radiocarbon ages (<40,000 BP) on shells are found. These sediments are covered by at least one basal till of Late Weichselian age. The till is overlain by marine sediments from the last deglaciation (12,800-10,000 BP) which contain shallow-water, subarctic foraminiferal assemblages, similar to modern near-glacial faunas from western Svalbard. The most common foraminifera in all zones in the sub-till sediments are Cassidulina reniforme, Astrononion gallowayi and/or Elphidium excavatum . The richest zones at both localities are found in the sub-till units and contain more than 20 foraminiferal species, including some boreal-arctic species. These faunal assemblages are similar to the living faunas on the west coast of Svalbard. Faunas from the postglacial climatic optimum are not yet described. We suggest that the foraminiferal assemblages in the sub-till sediment reflect Early or Middle Weichselian interstadial environments, although an Eemian interglacial cannot be excluded.     

An experimental investigation of sand–mud suspension settling behaviour: implications for bimodal mud contents of submarine flow deposits

The settling behaviour of particulate suspensions and their deposits has been documented using a series of settling tube experiments. Suspensions comprised saline solution and noncohesive glass‐ballotini sand of particle size 35·5 μm < d < 250 μm and volume fractions, φ_s, up to 0·6 and cohesive kaolinite clay of particle size d < 35·5 μm and volume fractions, φ_m, up to 0·15. Five texturally distinct deposits were found, associated with different settling regimes: (I) clean, graded sand beds produced by incremental deposition under unhindered or hindered settling conditions; (II) partially graded, clean sand beds with an ungraded base and a graded top, produced by incremental deposition under hindered settling conditions; (III) graded muddy sands produced by compaction with significant particle sorting by elutriation; (IV) ungraded clean sand produced by compaction and (V) ungraded muddy sand produced by compaction. A transition from particle size segregation (regime I) to suppressed size segregation (regime II or III) to virtually no size segregation (IV or V) occurred as sediment concentration was increased. In noncohesive particulate suspensions, segregation was initially suppressed at φ_s ～ 0·2 and entirely inhibited at φ_s ≥ 0·6. In noncohesive and cohesive mixtures with low sand concentrations (φ_s < 0·2), particle segregation was initially suppressed at φ_m ～ 0·07 and entirely suppressed at φ_m ≥ 0·13. The experimental results have a number of implications for the depositional dynamics of submarine sediment gravity flows and other particulate flows that carry sand and mud; because the influence of moving flow is ignored in these experiments, the results will only be applicable to flows in which settling processes, in the depositional boundary, dominate over shear‐flow processes, as might be the case for rapidly decelerating currents with high suspended load fallout rates. The ‘abrupt’ change in settling regimes between regime I and V, over a relatively small change in mud concentration (<5% by volume), favours the development of either mud‐poor, graded sandy deposits or mud‐rich, ungraded sandy deposits. This may explain the bimodality in sediment texture (clean ‘turbidite’ or muddy ‘debrite’ sand or sandstone) found in some turbidite systems. Furthermore, it supports the notion that distal ‘linked’ debrites could form because of a relatively small increase in the mud concentration of turbidity currents, perhaps associated with erosion of a muddy sea floor. Ungraded, clean sand deposits were formed by noncohesive suspensions with concentrations 0·2 ≤ φ_s ≤ 0·4. Hydrodynamic sorting is interpreted as being suppressed in this case by relatively high bed aggradation rates which could also occur in association with sustained, stratified turbidity currents or noncohesive debris flows with relatively high near‐bed sediment concentrations.     

Channel formation and migration by mass-flow processes in the Lower Carboniferous fluviatile Fell Sandstone Group, northeast England

Small, steep-sided narrow channels, locally developed within the braided fluviatile Lower Carboniferous Fell Sandstone of northeast England, are attributed to mass-flow processes initiated by channel-bank slumping. The channels have sharp undisturbed margins and are filled with moderately well-sorted medium to coarse sandstone identical to the sandstone which they cut. Most channel-fills are structureless, but a few show diffuse concentric marginal laminations. Laminations also occur towards the top of channel fills, but they are less regular and show low-amplitude trough-like undulations. Channel margin inclinations range from 13° to 55°, and channel depths vary between 1·7 m and 2·5 m. The channels, which are only seen in profile, are orientated perpendicular to the local and regional palaeocurrent trend, and cut through structureless sandstone and planar cross-stratified sandstone characterized by compound and compound-complex bar bedforms. Only one channel has both margins preserved; and of the remainder (seven) only one has its northern margin preserved. Undercutting and retrogressive slumping of the sandy channel bank is thought to have initiated a subaqueous sediment gravity flow, which moved across the main channel along a transverse scour at the base of the slip face of a large composite bar bedform. Such scours have strong asymmetric profiles with the steeper slope along the base of the active slip face (northern side) and the lower slope on the downstream (southern) side of the scour. Thus, the northern channel margin was steeper, less stable and more prone to collapse than the southern margin, and is almost never preserved. Flow reconstruction indicates that on entering the channel a hydraulic jump developed at the change in slope, and the flow became more turbulent and erosive. Scoured sand incorporated into the flow increased concentration and shearing along the boundaries. Reverse shear was also exerted by the overlying current, on top of the flow, but the central part remained relatively unaffected. The presence or absence of marginal laminations is probably related to marginal shearing and flow dilution. Deposition occurred in response to deceleration and frictional freezing of the sand flow.     

Late Pleistocene environmental changes indicated by fossil insect faunas of the English Midlands

Twenty-five fossil insect assemblages are described from discrete lenses of or-ganic material in a gravel sequence at Four Ashes. The youngest date of 30,500 years B.P. obtained on the organic materialhas confirmed that the till overlying the gravels is Late Devensian (Weichselian) in age. The analyses of the insect faunas have shown conclusively for the first time the existence of climatic changes in one geographic area during the Early and Middle Devensian in Britain. Some of the earliest insect faunas can be correlated with the Brorup Interstadial, when boreal forests existed in the English Midlands. It is suggested that a cold period prior to 43,000 years ago (but post-Brorup) may have caused the elimination of the trees, because around 40,000 years ago the insects indicate that there was a rapid climatic amelioration when it was warm enough for trees to grow again in that area. Around 36,000 years ago there was another climatic deterioration when the thermophilous insect species were replaced by a large number of arctic stenotherms and a tundra type of environment. This cold period lasted for at least 6,000 years and probably became increasingly severe with the approach of the main Devensian ice advance sometime after 30,500 years B.P.