Origin of the clay mineral assemblages in the Germanic facies of the English Trias : application of the spore colour index method

The origin of the regional and stratigraphical variation in the Triassic authigenic clay assemblages of England is discussed in relation to new estimates of the palaeotemperatures experienced by their host sediments and a preliminary study by transmission electron microscopy of their microtextural features. Spore colour index measurements, based on the spore type Deltoidospora s.l. occurring in the sediments (Penarth Group) at the very top of the Triassic sequence, give estimated palaeotemperatures ranging from 60–74°C (south Devon) to 89–97°C (northeast Yorkshire). Calculated palaeotemperatures, based on a gradient of 25°C/km, for the main one of authigenic clay minerals range from 63–77°C to 89–97°C for the top to 71–85°C to 94–104°C for the base. Irregular mixed-layer smectite-chlorite, corrensite and Mg-rich chlorite are associated with calculated palaeotemperatures of 66–86°C, 66–104°C and 75–104°C respectively. The suggestion that elsewhere in the UK corrensite and Mg-rich chlorite were formed at temperatures in excess of 100°C finds no support. Geothermal gradients would have to have been of the order of at least 100–300°C/km to obtain these temperatures within the Triassic sediments; such values are associated typically with high-level magmatic intrusions or geothermal systems of which there is no geological evidence. The balance of evidence suggests that the Triassic authigenic clay assemblages formed by neoformation during the early stages of sediment diagenesis under the influence of variation in the alkalinity of the depositional environments

Clay- and zeolite-bearing Triassic sediments at Kaka Point, New Zealand; evidence of microbially influenced mineral formation from earliest diagenesis into the lowest grade of metamorphism

The distribution, mineralogy, petrology and bulk and stable isotope chemistry of altered volcanic ash beds in the marine sediments of Mid-Triassic age (Etalian) at Kaka Point, New Zealand, are described and related to lithofacies and the geological processes controlling their development. Three varieties of altered ash occur in the Kaka Point sediments--porcellanite, claystone (bentonite) and albite-rich. Porcellanites are quartz-rich and may contain analcime and heulandite: they are restricted mainly to the on-shore facies. Claystones are rich in smectitic clay minerals and occur in both the on-shore and off-shore facies. They often contain diagenetic nodules of analcime, quartz, apatite and carbonates. The authigenic carbonates of the on-shore facies are variable in composition (sideritic, rhodochrositic, calcitic), whereas in the off-shore facies they consist only of calcite. The albite-rich lithology is very rare and is known only from the off-shore facies. The development of the porcellanite and albite-rich lithologies was restricted to slowly deposited, relatively coarse-grained ash sediments in which extensive interchange took place between the sediment's pore-waters and ambient seawater, resulting in enhanced microbial activity and high pH throughout the pore-waters of the suboxic zone beneath the water-sediment interface. The high pH increased the rate of volcanic ash hydrolysis and provided the conditions necessary for the precipitation of zeolite, feldspar and quartz. The development of smectitic claystones was associated with more rapid deposition and limited interchange between the pore-waters of the parent ash and ambient seawater. The pore-water alkalinity was generally lower and enhanced microbial activity and high pHs were restricted to patches of sediment at which quartz, analcime, apatite and carbonates formed diagenetic nodules. Modelling of the stable isotopes of the smectitic clays (delta 18 O, delta D) and diagenetic carbonates (delta 18 O, delta 13 C) suggest that: (1) ash argillization in the on-shore facies took place in brackish water ( approximately 25% meteoric water) at an average temperature of approximately 50 degrees C and in the off-shore facies in marine pore-waters ( approximately 1O% meteoric water) at approximately 40 degrees C: and (2) diagenetic carbonate precipitation in the near-shore facies took place at approximately 30 degrees C and in the off-shore facies at 60-80 degrees C. The pattern of ash alteration in the marine Triassic sediments at Kaka Point is considered to represent an early stage in the development of the zeolite pattern associated with the classic area of zeolite facies metamorphism in the Taringatura and Hokonui Hills

Triassic sediments of the Kaka Point Structural Belt, South Island, New Zealand, and their relationship to the Murihiku Terrance

The stratigraphy, palynofacies, palaeotemperatures (spore coloration, vitrinite reflectance), illite crystallinity, and carbonate stable isotope patterns (δ18O, δ13C) of the Triassic sediments exposed on the south-east Otago coast between Pilot Point and Roaring Bay are discussed in relation to the Hillfoot Fault and the differentiation of the Dun Mountain-Maitai and Murihiku Terranes. Detailed sections, spore-pollen biostratigraphy, and palaeotemperatures are described from the Etalian Tilson Siltstone and Bates Siltstone at Kaka Point and from an undescribed upper Etalian-lower Kaihikuan succession at Campbell Point. New stable isotope analysis of diagenetic carbonates from the Triassic sediments of Campbell Point and Roaring Bay are included. Differences in the Triassic sediments to the north and south of the Hillfoot Fault are related to minor variations in their tectonic history and not to fundamental differences in depositional setting, lithofacies, and metamorphism. Higher palaeotemperatures and greater mineral alteration to the south of the Hillfoot Fault are interpreted as the effects of localised fluid flow along this structure when it acted as a zone of décollement in a fore-arc basinal setting under low or very low geothermal gradients