LREE-enriched komatiitic basalts are commonly found in association with LREE-depleted komatiites. This association is found in a sequence of late Archaean lavas from Newton Township, Ontario. The komatiitic lavas at Newton Township differ from most late Archaean examples in that both the komatiites and the komatiitic basalts are depleted in Al and in the HREE. The close association and distinctive Al and HREE depletions of the two lava types strongly suggest a common origin, despite their contrasting LREE patterns.

A model is proposed whereby the LREE-depleted komatiites represent the parental magma to the LREE-enriched komatiitic basalts, the two being linked by a combination of crystal fractionation and crustal assimilation. The composition of the contaminant is estimated by comparing the LREE-enriched komatiitic basalts with the evolved part of a thick layered komatiite flow that has similar major element chemistry. The contaminant composition coincides closely with Taylor & McLennan's estimate of the composition of the Archaean upper crust. It is concluded that LREE-enriched komatiitic basalts can be produced from LREE-depleted komatiite parent magmas by combined assimilation and fractionation, and that such a process best explains the geochemistry and Nd isotopic features of most komatiitic basalts.

Studies of melanges in the Shimanto Belt of southwest Japan have shown that many of the included blocks exhibit asymmetric geometries, similar to boudins and inclusions in medium grade metamorphic rocks which have been subjected to markedly non-coaxial strains. Deformation is accomplished by mesoscopically ductile processes such as pore-pressure-controlled or diffusionally-controlled grain boundary sliding, although localized fracture and cataclasis also occurs. It is suggested that the melanges developed by the propagation of extensional displacement zones, analogous to shear bands, through sandstone beds, so progressively dismembering them. Studies of the detailed internal geometries of melanges along with the deformation mechanisms active during their formation may help resolve the mode of formation of these problematic units.

Arguments on the periodicity of mass extinction have been marred by incorrect statistical arguments. Mass extinction patterns can be best interpreted as chance fluctuations over time.

An assemblage of problematical microfossils of Precambrian–Cambrian boundary age is redescribed from the Chert–Phosphorite Member, at the base of the Lower Tal Formation of Maldeota in the Lesser Himalaya of India. This assemblage has previously been ascribed to various ages, from Precambrian to Cretaceous, but is held by us to contain: Maldeotaia bandalica, Protohertzina anabarica group, trumpet-shaped elements, acicular elements A & B, ?Conotheca sp., Ovalitheca cf. multicostata, allathecid sp. A, Barbitositheca ansata, Hexangulaconularia cf. formosa, Coleoloides aff. typicalis, Hyolithellus aff. insolitus, H. cf. isiticus, H. vladimirovae, Spirellus shankari and Olivooides multisulcatus. These compare closely with assemblages found above the base of the first, Anabarites trisulcatus–Protohertzina anabarica Zone in China and in the second, Pseudorthotheca costata Zone of southern Kazakhstan. The stratigraphic setting of the Krol–Tal succession is reviewed and several similarities are noted between the Precambrian–Cambrian boundary successions of Lesser Himalaya in India and of Yunnan and Sichuan in Southwest China, indicating that correlation between them is possible at several levels.

The Armorican Quartzite (Lower Ordovician) is a very extensive sandstone body found throughout a large part of Iberia and Brittany; similar quartzites are present in north Africa and elsewhere. In Iberia it generally lies unconformably on a thick, folded, late Precambrian to Cambrian turbidite sequence (the Complexo Xisto-Grauváquico (CXG)), but there are some places where the quartzite has a conformable relationship with the underlying CXG. Where conformable the whole succession shows varied facies sequences including: (i) a regressive mud–sand sequence as a precursor to the main quartzite development, (ii) submarine fan sediments of the CXG passing up into slope and then shallow marine facies, or (iii) fluvially influenced breccio-conglomerates of a marginal fan delta passing up into shallow marine sediments.

The inferred development of alluvial fans associated with fan deltas prograding into shallow marine environments implies steep slopes and the likelihood that faulting controlled some of the uplift. The mozaic pattern of differential uplift and subsidence throughout the region suggests that the CXG was affected by local block movements rather than by regional folding.

The presence of upstanding blocks during the initial sedimentation of the Armorican Quartzite suggests that the quartz sand might have been derived from multiple sources dispersed throughout the area rather than from sources along a single shoreline at the margin of the depositional area.

Large-scale sedimentary structures are described from the Lough Muck Formation (Wenlock) of north Galway, Ireland. They consist of bed packages separated by truncation surfaces of varying orientations. They had previously been interpreted as rotational slumps generated on a shelf–slope interface. It is shown that the structures formed in a tidally influenced environment far removed from a slope. They bear strong similarities to structures described from other tidal channel environments and it is proposed that they represent deposition on aggradational margins of such channels. The supposed slope–shelf transition is therefore not present at this level in the Silurian succession and much of the Lough Muck Formation may be of a dominantly shallow-marine origin.

A specimen of the trepostome bryozoan Orbipora from the Lower Arenig of south Wales is described and figured. This is currently the oldest unequivocal bryozoan recorded in the literature. Although mostly decalcified, the colony preserves clear remnants of the large styles which characteristically indent the zooecial apertures in Orbipora. The wide palaeogeographical distribution of Orbipora in the early Ordovician is consistent with the hypothesized planktotrophic larval phase. Theoretical considerations and evidence from the occurrence of Orbipora in south Wales suggest that nearshore clastic environments would be fruitful settings in which to search for further early bryozoans, although quality of preservation may be poorer than in offshore carbonates.