Synopsis Borehole 80/14 in the Little Minch recovered grey, carbonaceous clays with sandy horizons and abundant plant remains from a small basin adjacent to the Minch Fault. Pollen from the sediment is characterised by a verus-vestibulum association that demonstrates a Late Oligocene (Chattian) age and indicates a terrestrial floodplain environment with arborescent swamps and fens. Both the age and lithology of the deposit are therefore similar to other terrestrial Oligocene basins found in the western British Isles. Petrological studies confirm the environment of deposition, and suggest that the sediment was predominantly derived from metamorphic basement, although there was also reworking of Jurassic sediments. The source was probably the Lewisian to the west of the Minch Fault; the progression in clay and heavy mineral assemblages suggests, unroofing of less deeply weathered material with time.
Upper Jurassic sandstones deposited in a shallow-marine deltaic setting in the Piper Field of the Outer Moray Firth area, North Sea, show high-frequency fluctuations in apatite:tourmaline ratios that appear to be related to sea-level change. Because apatite and tourmaline are both stable during burial diagenesis and have similar hydraulic behavior, variations in the apatite:tourmaline ratio indicate either differences in sediment provenance or in the extent of floodplain weathering, apatite being unstable during weathering. Other provenance-sensitive heavy mineral ratios (rutile:zircon, monazite:zircon, chrome spinel:zircon) and mineral-chemical data from detrital garnet assemblages show that sandstones with high apatite:tourmaline have the same provenance as...
The drilling of hydrocarbon exploration wells in the Faroe–Shetland Basin has provided an expanding sample resource that provides material for testing recently developed palynology-based sediment transport analysis. This technique has been verified by comparison with heavy mineral analysis; both approaches have been used to identify sediment sources and input points along the strike of the Palaeocene West Shetland Platform. Integration of heavy mineral and palynological data has provided a basis for understanding arenaceous and argillaceous sediment distribution and sourcing. In addition to a source from the western, Greenland side of the basin, four argillaceous and four arenaceous sedimentary sources have been identified along the strike of the West Shetland Platform. These vary in temporal and spatial distribution, and thus provide a history of sediment source evolution. This analysis supports a persistent difference in source between the Corona Basin and the Flett and Judd Sub-basins. Although source variation and overlap between basins is evident, transfer zones represent both conduits for and barriers to effective sediment transport. Both palynological and heavy mineral evidence identifies the former presence of Late Namurian–Westphalian strata on the West Shetland Platform, which were removed by subsequent erosion.
Abstract Heavy mineral assemblages in deepwater Paleocene sandstones of the Foinaven Sub-basin (west of Shetland) reflect the influence of three main provenance and dispersal systems. The Schiehallion system has relatively uniform characteristics and persisted through much of the early-mid Paleocene. Its sphere of influence was centred on the Schiehallion Field, but it progressively encroached into the Foinaven area with time. Its main source was the Triassic Foula Formation, with minor supply from Lewisian and Moine basement rocks. The Foinaven system shows marked changes in character, related to evolution of the source area. The earliest sands were derived from a heavily weathered late Cretaceous regolith, and represent the onset of uplift and exhumation of the Shetland Platform. Progressive unroofing led to the incorporation of increasing amounts of Lewisian- and Moine-sourced detritus. Following deposition of the main reservoir sandstones in Foinaven and Schiehallion, there was a progressive change in provenance, with a gradual increase in the amount of sediment shed directly from metamorphic basement. Pulsing of the proto-Icelandic plume has been proposed as the mechanism for repeated influx of sand to the basins around Scotland. However, on the basis of available geochronological data, there does not appear to be a direct link between events in the British Tertiary Igneous Province (BTIP) and changes in provenance in the Foinaven Sub-basin. The initial influx of sediment from a weathered land surface may have been coeval with the onset of magmatism in the BTIP, but the cessation of supply through the Foinaven system at c. 59 Ma does not appear to be related to magmatic events in the BTIP. The waning of magmatism in the BTIP around 58 Ma is broadly coincident with a gradual increase of first-cycle basement detritus.
Synopsis The British Geological Survey’s offshore borehole 78/4 has provided one of the most complete late Devensian to Flandrian Quaternary successions observed in Scottish waters. Molluscs from this sequence have provided radiocarbon dates ranging from 12,785 to 8,240 years BP. These dates are discussed in the context of the palaeoenvironmental history of the succession as indicated by its floras and faunas. Concentrations of volcanic glass shards indicate an ash fall, apparently equivalent to the Vedde Ash of western Norway.
The presence of hydrocarbon-bearing sandstones within the Eocene of the Forties area was first documented in 1985, when a Forties field (Paleocene) development well discovered the Brimmond field. Further hydrocarbons in the Eocene were discovered in the adjacent Maule field in 2009. Reservoir geometry derived from three-dimensional seismic data has provided evidence for both a depositional and a sand injectite origin for the Eocene sandstones. The Brimmond field is located in a deep-water channel complex that extends to the southeast, whereas the Maule field sandstones have the geometry of an injection sheet on the updip margin of the Brimmond channel system with a cone-shape feature emanating from the top of the Forties Sandstone Member (Paleocene). The geometry of the Eocene sandstones in the Maule field indicates that they are intrusive and originated by the fluidization and injection of sand during burial. From seismic and borehole data, it is unclear whether the sand that was injected to form the Maule reservoir was derived from depositional Eocene sandstones or from the underlying Forties Sandstone Member. These two alternatives are tested by comparing the heavy mineral and garnet geochemical characteristics of the injectite sandstones in the Maule field with the depositional sandstones of the Brimmond field and the Forties sandstones of the Forties field. The study revealed significant differences between the sandstones in the Forties field and those of the Maule and Brimmond fields), both in terms of heavy mineral and garnet geochemical data. The Brimmond-Maule and Forties sandstones therefore have different provenances and are genetically unrelated, indicating that the sandstones in the Maule field did not originate by the fluidization of Forties sandstones. By contrast, the provenance characteristics of the depositional Brimmond sandstones are closely comparable with sandstone intrusions in the Maule field. We conclude that the injectites in the Maule field formed by the fluidization of depositional Brimmond sandstones but do not exclude the important function of water from the huge underlying Forties Sandstone Member aquifer as the agent for developing the fluid supply and elevating pore pressure to fluidize and inject the Eocene sand. The study has demonstrated that heavy mineral provenance studies are an effective method of tracing the origin of injected sandstones, which are increasingly being recognized as an important hydrocarbon play.
Heavy mineral analysis is one of a group of provenance-based methods that complement traditional biostratigraphic correlation of clastic reservoirs. A variety of processes give rise to stratigraphic changes in sediment composition, including source area uplift, unroofing, changes in climatic conditions, extent of alluvial storage on the floodplain and the interplay between different depositional systems. Heavy mineral analysis is a reliable and proven technique for the correlation of clastic successions because prolonged and extensive research has provided detailed understanding of the effects of processes that alter the original provenance signal during the sedimentary cycle, such as hydrodynamics and diagenesis. The technique has been successfully applied to a wide range of clastic reservoirs, from fluvial to deep marine and from Devonian to Tertiary, using a combination of different types of parameters (provenance-sensitive mineral ratios, mineral chemistry and grain morphology). The application of heavy mineral analysis as a non-biostratigraphic correlation tool has two limitations. The first is that valid correlations cannot be made in sequences with uniform provenance and sediment transport history, but this is a problem inherent with all provenance-based methods. The other is that the technique can be applied only to coarse clastic lithologies and is not suitable for fine-grained sediments or carbonates.
Heavy mineral assemblages in rivers in the Apure River drainage basin of Venezuela and Colombia closely reflect the nature of the source regions, which lie in the Andean orogenic terranes to the west and northwest. The Caribbean Mountains, largely composed of greenschist-facies pelites, phyllites, carbonates, and metavolcanics, supply assemblages dominated by epidote and calcic amphibole. Minor amounts of the high-pressure index minerals glaucophane and lawsonite indicate the presence of blueschistfacies rocks, reflecting the origin of the Caribbean Mountains by subduction-related tectonism. The northern Mérida Andes, which comprise basement gneisses and granites overlain by unmetamorphosed to low-grade metamorphosed clastics, supply two types of assemblage reflecting these two lithological types: garnet-sillimanite-staurolite-amphibole suites from the basement rocks, and epidote-amphibole suites from the overlying cover sequence. The southern Mérida Andes supply stable heavy mineral suites reflecting recycling from the extensive unmetamorphosed sandstones that occur at outcrop. By considering suites from different physiographical provinces, the effects of short-term alluvial storage in the Llanos on heavy mineral assemblages have been evaluated. Weathering during alluvial storage appears to be effective in modifying the apatite-tourmaline ratio, which shows a steady, marked decline with distance from the mountain front, resulting from the removal of apatite during weathering. Clinopyroxene and garnet may also show evidence of loss through weathering, although the trends are poorly constrained statistically. Epidote and amphibole proportions remain essentially constant, possibly through a balance between loss through weathering and continual resupply from the breakdown of rock fragments. In general, the heavy mineral assemblages are less affected than the bulk mineralogy by alluvial storage on the Llanos.
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