Bulk composition and mineral parageneses of sapphirine-bearing rocks along a gabbro-lherzolite contact at Finero, Ivrea Zone, N Italy

Sapphirine occurs in a 3-5 m wide zone between amphibole-lherzolite and garnetiferous metagabbro at Finero in the Ivrea Zone, NW Italian Alps. Layers consisting of plag + hb + sa + cpx + opx + sp + gt are interbanded with spinel pyroxenites, which may contain sapphirine replacing spinel. All minerals are very magnesian, with X_Mg between 0.78 and 0.92. Bulk rock analyses suggest that precursors to the sapphirine-bearing rocks were igneous cumulates of plagioclase + olivine + hornblende + spinel. Up to 16wt% CaO does not inhibit sapphirine formation and it is the unusually Mg-rich nature of the host rocks which allows sapphirine development. The early igneous assemblage was replaced by one of cpx + sa + hb +± plag at a pressure of 9 ± 1 kbar and temperatures of 900 ± 50°C. Subsequent rapid uplift caused the instability of gt, gt + hb, hb and sa + cpx to form opx + plag ± sp ± sa symplectites.     

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.     

A THREE-DIMENSIONAL NUMERICAL BOTTOM-HOLE TEMPERATURE STABILIZATION MODEL*

The heat flow equation in cylindrical coordinates is solved numerically for any general distribution of thermal diffusivity. The temperature stabilization of a borehole is considered, and solutions for the case where thermal diffusivity is a function of radial distance from the borehole are obtained and compared to solutions for uniform diffusivity. The results are discussed in terms of thermal diffusivities that are different for the well contents and for the surrounding material. It is found that the approach to formation temperature is affected by differences between well contents and the surrounding region.     

The last Scottish ice-sheet: facts and speculative discussion

Evidence relating to the extent of the last (Late Devensian) Scottish ice-sheet is critically discussed, particular attention being given to the limitations of some radiocarbon dates and incorrect inferences based on radiocarbon dates. It is suggested that the last Scottish and Scandinavian ice-sheets were not confluent and that Orkney and NE Caithness may not have been covered by the last Scottish ice-sheet. Ice-sheet growth and decay are considered in relation to possible positions of the oceanic and atmospheric polar fronts: implications are that much the greater part of ice-sheet decay resulted from inadequate snowfall and that the maximal limits of the last ice-sheet may not have been synchronous. Ice-sheet calving may have resulted in an independent ice mass over the Outer Hebrides. It is suggested that most of the bed of the Central North Sea became land during the Late Devensian and that a large delta existed in the eastern part of this area. It is also suggested that the buried and infilled channels of this eastern area, which are normally interpreted as tunnel valleys, are shallow delta channels whose present depth is due to delta subsidence