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The original dysaerobic zone concept defined a unique assemblage of benthic organisms and sedimentary fabrics associated with dissolved oxygen concentrations ranging between 0.3 and 1.0 ml/l. Subsequent studies of the Santa Barbara Basin and oxygen minimum zone (OMZ) off the California coast suggest that this facies is bounded by dissolved oxygen in the range 0.1 to 0.5 ml/l. This range of dissolved oxygen is optimal for the growth of magnetotactic bacteria. We present a hypothesis predicting that the dysaerobic zone should be associated with a high inventory of biogenic magnetite and that this magnetite can be used as a palaeo-oxygen indicator. We have tested this hypothesis with a magnetic analysis of samples from aerobic, anaerobic, and dysaerobic sediments taken off the Peruvian coast. The mean magnetic susceptibility of the dysaerobic zone is 3 to 5 times higher than the aerobic and anaerobic parts of the Peruvian oxygen minimum zone (OMZ). Micrographs (TEM) of these sediment samples indicate that only the dysaerobic zone contains appreciable amounts of fine-grained magnetite.

The interaction of river and marine processes in the fluvial to marine transition zone fundamentally impacts delta plain morphology and sedimentary dynamics. This study aims to improve existing models of the facies distribution, stratigraphic architecture and preservation in the fluvial to marine transition zone of mixed-process deltas, using a comprehensive sedimentological and stratigraphic dataset from the Middle Miocene Lambir Formation, Baram Delta Province, northwest Borneo. Eleven facies associations are identified and interpreted to preserve the interaction of fluvial and marine processes in a mixed-energy delta, where fluvial, wave and tidal processes display spatially and temporally variable interactions. Stratigraphic successions in axial areas associated with active This article is protected by copyright. All rights reserved. distributary channels are sandstone-rich, comprising fluvial-and wave-dominated units. Successions in lateral, or interdistributary, areas, which lack active distributary channels, are mudstone-rich, comprising fluvial-dominated, tide-dominated and wave-dominated units, including mangrove swamps. Widespread mudstone preservation in axial and lateral areas suggests well-developed turbidity maximum zones, a consequence of high suspended-sediment concentrations resulting from tropical weathering of a mudstone-rich hinterland. Within the fluvial to marine transition zone of distributary channels, interpreted proximal-distal sedimentological and stratigraphic trends suggest: (i) a proximal fluvial-dominated, tide-influenced subzone; (ii) a distal fluvial-dominated to wavedominated subzone; and (iii) a conspicuously absent tide-dominated subzone. Lateral areas preserve a more diverse spectrum of facies and stratigraphic elements reflecting combined storm, tidal and subordinate river processes. During coupled storm and river floods, fluvial processes dominated the fluvial to marine transition zone along major and minor distributary channels and channel mouths, causing significant overprinting of preceding interflood deposits. Despite interpreted fluvial-tidal channel units and mangrove influence implying tidal processes, there is a paucity of unequivocal tidal indicators (for example, cyclical heterolithic layering). This suggests that process preservation in the fluvial to marine transition zone preserved in the Lambir Formation primarily records episodic (flashy) river discharge, river flood and storm overprinting of tidal processes, and possible backwater dynamics.

Deposition of Dalradian sediments in a stretching ensialic basin peripheral to Iapetus continued into Cambrian times. Late in the history of Dalradian deposition a southern continental landmass was removed by strike-slip motions and a subduction zone then developed beneath the Dalradian sediments. Deformation and moderately high pressure metamorphism of Dalradian rocks occurred and was followed by thermal relaxation and development of a volcanic arc with magmatic centres in Connemara by Tremadoc times and in NE Scotland by the Arenig. The S Mayo Trough was a back-arc basin that opened shortly after the initiation of the arc, the oldest volcanic rocks seen there being the extrusive equivalents of the basic intrusions of Connemara. Dalradian rocks outcropping in Ordovician times over the Highlands and probably extending southwards are a possible source for high pressure metamorphic clasts at Ballantrae, while clasts of arc volcanics may have come from N of the Highland Boundary Fault. Plate models that involve a separate arc in the Midland Valley are rejected because the tectonic elements are impossibly small by comparison with modern convergent plate margins. The arc rooted in Dalradian rocks ceased to be active at the end of the Llanvirn as a result of a further change in plate motions, and there are considerable spatial problems in locating an arc coupled to the Southern Uplands accretionary prism after Llanvirn times.