The tectonic structure of the Hazara-Kashmir Syntaxis, southern Himalayas, Pakistan

Reduction spheroids from northern Switzerland: Mineralogy, geochemistry and genetic models

Diagenesis in oxidising and locally reducing conditions — an example from the Triassic Skagerrak Formation, Denmark

SUMMARY All available information has been used in the preparation of a series of palaeogeographical maps showing the evolution of north-east England at the end of the early Permian and at intervals throughout the late Permian. Most of the area initially lay near the western fringe of the Southern North Sea tropical desert basin, much of which was flooded at the beginning of the late Permian so as to create the epicontinental Zechstein Sea. With continually oscillating relative sea levels and episodic differential subsidence, contemporary coastlines fluctuated widely whilst successive progradational carbonate shelves were constructed in the first two of four main cyclic sequences; the deposition of thick salt at the end of the second main cycle almost filled the basin, eliminating most of the earlier depositional topography and ensuring that carbonates and evaporites of the third and fourth cycles were formed in a less diverse range of coastal plain to shallow marine environments.

The late Permian palaeogeography of north-east England

The mineralogy and geochemistry of reduction spheroids from continental and marine red beds of Europe, North America and Oman were investigated. Reduction spheroids are spheroidal, isolated reduction sites in hematitic rocks. They consist of a mineralized core (0.1 to 5 cm diameter) and a hematite-dissolution halo (1 to 20 cm diameter). Irrespective of origin and age of host rocks, all reduction spheroid cores show a very similar mineralogy dominated by the vanadian mica roscoelite and a similar pattern of element enrichment relative to their host rocks dominated by V and U. Element enrichments in most reduction spheroids are very similar to those of sandstone-hosted vanadium-uranium deposits except for a lack of a molybdenum enrichment. Isotopically light sulfide of in-situ, low-temperature origin is an indication for the involvement of bacterial sulfate reduction during reduction spheroid formation.

Mineralogy and geochemistry of reduction spheroids in red beds

The Littleham Mudstone Formation of southeast Devon consists predominantly of red lutites of Late Permian or Early Triassic age. Present within the lutites are spherical pale patches that have sharp boundaries against the red lutite and often contain dark, organic centers rich in uranium and vanadium. Chemical and Mossbauer analysis of the oxidation state of iron in this formation shows that the red lutite contains an average total iron content of about 5.5%, with an Fe2+/Fe3+ ratio of 0.16, while the pale material has a total iron content of 2.7%, with an Fe2+/Fe3+ ratio of 1.2. In the pale material, iron is present only within silicate (clay material) lattice sites, but the red lutite contains hematite as well. The hematite has an important superparamagnetic component and is considered to have a bimodal grain size distribution with two modes of occurrence: as interlayer crystals within clay minerals and as external coatings or grains. The Fe2+/Fe3+ ratios and total iron content show that the red lutite is not merely the pale material with hematite added, but that about 50% of the Fe2+ in the clay mineral lattice in the pale material is oxidized to Fe3+ in the red lutite. This change in oxidation state is not coincident with the pale-red boundary but begins within the pale patches. The mechanism by which this occurred is considered to be an in situ oxidation that produced the red lutite, accompanied by the precipitation of ferric hydroxide from solution. This process is inhibited in the pale patches where Fe2+ ions would be more soluble and readily removed. Geotechnical tests on the lutites show that the spherical patches must have formed after a thickness of 1,000 to 1,300 m of overlying strata had been deposited.

This thickness encompasses the whole of the Triassic beds of southeast Devon and implies that the reddening occurred at the end of Triassic or in early Jurassic time. It is suggested that the reddening was aided by a change from arid to humid climate that occurred in the area at this time.

R.R.B. Ballard

Papers

Oxidation state of iron in the Littleham Mudstone Formation of the New Red Sandstone Series (Permian-Triassic) of southeast Devon, England