The status of geological dredging techniques

Scientific sea-floor dredging is currently used in marine geology primarily by the hard-rock community interested in the recovery of basement rock samples from the unsedimented deep ocean floor. The technique has generally been eclipsed by ocean drilling for recovery of sedimentary rocks, because of perceived uncertainties in the location of sampling and in the representativeness of recovered material. This contribution reviews dredging equipment currently in use by marine geological institutions and refers to pinger attachments that allow precise information on the behaviour of the dredge to be telemetered back to the ship. We argue that improvements in ship navigation and transponder navigation at the seafloor, when used in conjunction with surface and/or deeply towed sidescan and swathemapping surveys, now allow for considerably less uncertainty on the location of dredge sampling. Refined sorting criteria for dredge hauls are now also available. Recent comparisons of regional sample recovery by ocean drilling and by dredge sampling indicate that the dredge hauls can usefully supplement the drilling data in the construction of sedimentary and tectonic histories of seafloor areas.     

Crustal strain phenomena in the Solomon Islands, constraints from field evidence, and relationship to the India-Pacific plates' boundary

The Solomon Islands lie along the India-Pacific plates' margin and have recorded a history of deformation resulting from the interaction of these two plates. Various kinematic models have been proposed for the Solomons and these have involved a variety of plate tectonic processes. It is pointed out that almost without exception these models have been based on a provincial geological classification of the island group in which it is assumed that two of these provinces—Pacific and Central provinces—commenced their geological development in regions distant from one another. Invariably such models require that Santa Isabel represents part of a collision zone between these two provinces, though field evidence from Santa Isabel for such a collision has in the past been largely lacking.These various kinematic models are examined in the light of more recent field evidence, and a premise on which they have been based—initial separate development for two of the provinces—is questioned. Rather it is here suggested that the Central and Pacific provinces developed in roughly similar positions, one with the other as they occur today, and that they were at least in part separated from Oligocene time onward by a linear peridotite-gabbro ridge, Korighole-Florida high, which acted as a sediment barrier to much of the coarser clastic and volcanogenic sedimentation.The initial development of the Solomon Islands began in an oceanic environment with the extrusion of extensive submarine tholeiitic ‘flood basalts’ and intrusion of associated gabbroic and ultramafic rocks at depth, during the Late Mesozoic to Early Tertiary. This igneous phase occurred with the whole of the island group representing the western margin of the Ontong Java Plateau. Subsequent asymmetric development of the Solomons during the Eocene and Oligocene resulted in uplift, shearing, and the initiation of arc volcanism, plutonism, and arc-related sedimentation in the Central province to the west. In contrast, through much of the Tertiary the Pacific province to the east continued to receive dominantly pelagic sediments before undergoing uplift and renewed deformation in the Pliocene. The recognition that the ophiolite crust in the Solomon Islands represents an autochthonous entity, which has acted as basement to subsequent arc volcanism, has significant implications on geochemical studies of these islands now being undertaken.     

The origin of scapolite in the regionally metamorphosed rocks of Mary Kathleen,Queensland, Australia

Scapolite at Mary Kathleen (North-Western Queensland) occurs in calcareous and non-calcareous metapelites, acid and basic metavolcanics and metadolerites. Graphical treatment of the relationship between scapolite composition (Me%) and the host rock oxide ratios CaO/Na₂O and Al₂O₃/(CaO + Na₂O) reveals the following points:

1. The calcareous metapelites are also very sodic.
2. Scapolite in calcareous metapelites is more marialitic than that in low-calcium equivalents.
3. In graphs of Me% against CaO/Na₂O and Al₂O₃/(CaO + Na₂O) the metasediments and the metaigneous rocks show markedly different trends.

It is concluded that scapolite in the metasediments originated by isochemical metamorphism of shales and marls containing evaporitic halite. The local abundance of halite was the main control on the composition and distribution of the scapolite, but the relative abundance of CaO and Na₂O was a modifying factor. In the metaigneous rocks scapolite formed metasomatically during regional metamorphism by the introduction of volatile-rich fluids derived from the adjacent evaporitic sediments. The relative availability of CO₂ and Cl₂ again appears to have been the primary control on scapolite composition and may in turn have been controlled by bulk rock composition.     

Spectroscopic observations of the eclipsing polar MN Hya (RX J0929–24)

We present low–medium resolution optical spectroscopy of the eclipsing AM Her system MN Hya (RX J0929–24). We determine the magnetic field strength at the primary accretion region of the white dwarf to be 42 MG from the spacing of cyclotron features visible during π ∼ 0.4–0.7. From spectra taken during the eclipse we find that the secondary has an M3–4 spectral type. Combined with the eclipse photometry of Sekiguchi, Nakada &38; Bassett and an estimate of the interstellar extinction we find a distance of ∼300–700 pc. We find unusual line variations at π ∼ 0.9: Hα is seen in absorption and emission. This is at the same point in the orbital phase at which a prominent absorption dip is seen in soft X-rays.