Claire Hallsworth

Processes controlling the composition of heavy mineral assemblages in sandstones

Abstract Sandstone compositions result from a complex interplay between provenance and factors that operate during the sedimentation cycle. Accurate identification and discrimination of provenance depends on isolating provenance-sensitive features, and avoiding parameters that are influenced by other factors. Heavy mineral analysis offers a high-resolution approach to determination of sandstone provenance, because of the diversity of mineral species found in sandstones and because the factors affecting assemblages have been comprehensively evaluated. This paper presents the current understanding of the effects of processes operative during the sedimentation cycle. The original provenance signal may be overprinted by weathering at source prior to incorporation in the transport system; by mechanical breakdown during transport; by weathering during periods of alluvial storage on the floodplain; by hydraulic processes during transport and final deposition; by diagenesis during deep burial; and by weathering at outcrop. The most influential of these processes are hydraulics, which fractionates the relative abundance of minerals with different hydraulic behaviour, and burial diagenesis, which reduces mineral diversity through progressive dissolution of unstable mineral species. There is also evidence that weathering during alluvial storage plays a significant role. Two alternative, complementary approaches are recommended to identify provenance from heavy mineral data. The relative abundances of minerals with similar hydraulic and diagenetic behaviour are largely unaffected by processes operative during the sedimentation cycle, and utilize information gained from the entire heavy mineral suite. Determination of such ratios can be augmented by acquisition of varietal data, concentrating on the varieties shown by mineral types within the assemblage. A number of different varietal techniques are recommended, including optical differentiation of types based on colour, habit and internal structure, single-grain geochemical analysis, and single-grain geochronology.

Identifying provenance-specific features of detrital heavy mineral assemblages in sandstones

Abstract The composition of heavy mineral assemblages in sandstones may be heavily influenced by processes operating during transport, deposition and diagenesis. As a consequence, conventional heavy mineral data may not be a reliable guide to the nature of sediment source material. Certain features of heavy mineral suites, however, are inherited directly from the source area without significant modification, such as the varietal characteristics of individual mineral species. This paper describes an alternative approach to varietal studies that concentrates on relative abundances of minerals that are stable during diagenesis and have similar hydraulic behaviour. Ratios of apatite to tourmaline, TiO 2 minerals to zircon, monazite to zircon, and chrome spinel to zircon provide a good reflection of the source rock characteristics, because they are comparatively immune to alteration during the sedimentary cycle. These ratios are described as index values (ATi, RZi, MZi and CZi, respectively). This approach avoids some of the practical problems associated with varietal studies, such as the need to make subjective decisions about mineral properties or to use advanced analytical techniques that may not be accessible to the analyst. It also makes use of more components of the heavy mineral suite and thus provides a more balanced view of provenance characteristics. The use of these ratios is illustrated with examples from Upper Jurassic sandstones in the Outer Moray Firth area of the UK continental shelf and Triassic sandstones from onshore and offshore UK. Heavy mineral indices, notably ATi and MZi, show marked variations in Upper Jurassic Piper sandstones of the Outer Moray Firth. Main Piper sandstones have lower ATi and MZi values compared with Supra Piper sandstones, indicating significant stratigraphic evolution of provenance. The UK Triassic shows major regional variations in a number of index values, including ATi, MZi and CZi, demonstrating that sediment was supplied from several distinct source regions. This indicates a need for some modification of existing palaeogeographic models for the UK Triassic.

Carboniferous sand provenance in the Pennine Basin, UK: constraints from heavy mineral and detrital zircon age data

Abstract The integration of heavy mineral analysis and detrital zircon age dating has enabled high-resolution differentiation and characterisation of Carboniferous sandstone provenance in the Pennine Basin of the UK. Heavy mineral data have identified a number of distinct mineralogical groups with different provenance histories and source-area compositions. Single-grain zircon dating on each mineralogical type has placed constraints on the geochronology of the various source terrains. This combination of mineralogical and isotopic data has led to the identification of four distinct source terrains and sediment transport pathways. During the Namurian, the majority of sediment was supplied from the north via the ‘Pennine delta’. The source region comprised a high-grade metasedimentary terrain with granitic intrusions. Zircon age data indicate that this lay within the part of Laurentia–Baltica affected by the Caledonian orogeny. Small amounts of sediment were shed northwards from the Wales-Brabant High, on the southern margin of the basin. Most of this was recycled from the Old Red Sandstone but some of it came directly from late Proterozoic igneous basement. Supply via the Pennine delta declined markedly in the Westphalian, with most of the Westphalian A and B being fed from the west. The western source mainly comprised pre-existing sediment, with variable contributions from ultramafic rocks. The precise location of this source remains conjectural: it is unlikely to be within the British Isles given the size and scale of the Westphalian fluvial systems, but the zircon age spectrum cannot be reconciled with derivation from the Appalachians–Newfoundland–Labrador area. Supply from the uplifting Variscan massif to the south became important in late Westphalian B times and continued into Westphalian D. Zircon age data indicate sourcing from Late Carboniferous granites and Cadomian and Icartian basement.

Chapter 7 Stability of Detrital Heavy Minerals During Burial Diagenesis

Abstract Detrital heavy-mineral assemblages respond to increasing burial diagenesis by progressive dissolution of unstable components. Case studies from sedimentary basins worldwide show a uniform pattern of relative stability. The order of stability during burial diagenesis is olivine (least stable) Interpretation of provenance using heavy-mineral data from sandstones likely to have suffered burial diagenesis must carefully consider the possibility that some heavy-mineral species have been eliminated through dissolution. Evaluation of provenance under such circumstances must rely on parameters that are demonstrably unaffected by diagenesis. A combined approach, integrating provenance-sensitive ratio measurements with varietal data, either petrographic, geochemical or isotopic, is recommended.

Zircon age and heavy mineral constraints on provenance of North Sea Carboniferous sandstones

The understanding of sediment provenance and sediment transport routes is a key element in establishing reservoir presence in clastic petroleum systems. Determination of sediment provenance is particularly difficult in structurally complex areas and in sequences that have undergone extensive burial diagenesis. This paper describes a method that overcomes these problems, by combining quantitative heavy mineral analysis with detrital zircon age dating. Quantitative heavy mineral analysis identifies differences in sediment provenance within the sample set, and zircon age data provide diagnostic criteria for the identification of the various source terrains. The high degree of resolution shown by this approach is demonstrated using the North Sea Carboniferous as an example. The Carboniferous of the North Sea has suffered extensive diagenetic modification during its complex burial history, is difficult to image with seismic data, and in some areas, notably the central and northern North Sea, preservation is patchy. The understanding of Carboniferous sand provenance is therefore rudimentary. The Tayport and Firth Coal formations (latest Devonian to Early Carboniferous) of the Outer Moray Firth (central North Sea) were derived from a source area to the north of the British Isles, with sediment transported along the proto-Viking Graben. Some local input is recognised in the Firth Coal Formation. The Westoe Coal Formation (Westphalian B) in the southern North Sea was derived from the southeast, probably from the Saxo-Thuringian Zone of the central European Variscides. The Lower Ketch Member (Westphalian C) in the southern North Sea has a northern provenance, with abundant chrome spinel suggesting derivation from ophiolitic material on the Rinkobing-Fyn High.

Correlation of reservoir sandstones using quantitative heavy mineral analysis

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.

An integrated approach to provenance studies: a case example from the Upper Jurassic of the Central Graben, North Sea

Abstract The provenance of Upper Jurassic sandstones in a small half graben in the southern Central Graben of the North Sea has been investigated by studying a range of parameters: the composition of feldspars, the nature of rock fragments and their cathodoluminescence properties, heavy mineral assemblages, clay mineral assemblages, whole rock geochemistry and the occurrence of reworked palynomorphs. The combined result of these investigations is that rocks of Triassic, Permian and Carboniferous age are believed to have supplied detritus to the Upper Jurassic basin and to have diluted the contemporaneous bioclastic deposits. However, each individual provenance indicator tended to emphasize the importance of one particular source. Only an integrated study provided a complete picture of the interplay of different sources during deposition. Such integrated studies also identify the limitations of individual provenance indicators, in particular the adverse role of burial diagenesis in the removal of diagnostic grains such as volcanic feldspars and certain heavy mineral grains.

Structural and depositional controls on the distribution of the Upper Jurassic shallow marine sandstones in the Fife and Angus fields area, Quadrants 31 & 39, UK Central North Sea

Abstract An integrated evaluation of the tectonic and depositional history of the Upper Jurassic of the Fife and Angus area of UK Quadrants 31 and 39 has been carried out through 3D seismic interpretation and a range of geological studies. Sedimentological and petrological data indicate that the Upper Jurassic sands in the study area are the lateral equivalents of the Fulmar Formation. They can be divided into three groups of sandstones, deposited by different processes during an interval of almost 10 million years: (i) open shelf, Group 1 sandstones comprise sands dominated by storm/wave activity, which accumulated in fault-bounded mini-basins (‘embayments’); (ii) Group 2 sandstones, possibly deposited by storm-induced flows; and (iii) Group 3 sandstones, which may represent subaqueous dunes deposited in a tidally-influenced, shallow marine environment. Sedimentological and heavy mineral studies suggest that the sands in the study area were sourced from the adjacent Mid North Sea High and were transported along the shelf probably by wave, storm and tide-generated currents. Biostratigraphic dating indicates that the Upper Jurassic sands in the Fife/Angus area were deposited during Late Kimmeridgian to upper Middle Volgian. An earlier depositional period with a possible Callovian age is also inferred. Deposition of Jurassic sediments in the study area occurred during two transgression periods (Callovian and upper Late Jurassic), separated by an erosional/non-depositional phase (Oxfordian to Middle Kimmeridgian). The accumulation of Late Jurassic sands started in the Fife embayment and progressed northwards. It was confined, however, to the eastern side of the embayments, possibly due to the hydrodynamic conditions of the area.

Late Oligocene terrestrial sediments from a small basin in the Little Minch

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.

Contrasting mineralogy of Upper Jurassic sandstones in the Outer Moray Firth, North Sea: implications for the evolution of sediment dispersal patterns

Abstract Upper Jurassic sandstones in and adjacent to UK North Sea Block 15/21 (Outer Moray Firth) contain heavy mineral assemblages that provide evidence for differences in provenance, sediment dispersal patterns and depositional histories between the Ivanhoe/Rob Roy Terrace, North Halibut Graben, South Halibut Graben and Tartan Ridge. Variations are demonstrated using provenance-sensitive parameters, including apatite/tourmaline ratios, monazite/zircon ratios and garnet compositions. Seven different sandstone types have been identified on this basis. Scott and Piper sandstones (late Oxfordian-early Kimmeridgian) have contrasting mineralogies, indicating that they were derived from different provenances. However, the two sandstone packages lack internal stratigraphic variations, indicating that each is relatively homogeneous in terms of sediment supply. The succeeding Claymore sandstones (late Kimmeridgian-Volgian) display much greater variation, both regionally and stratigraphically. Heavy minerals suggest that the Main Claymore depositional system was more laterally extensive than that of the Upper Claymore, which was more locally developed and coalesced to a smaller degree. The most significant change in provenance took place with the onset of Main Claymore deposition, when the first major recycling of older Upper Jurassic sand took place. This indicates shedding of sediment from the Halibut Horst area, due either to uplift through intrabasinal tectonism or to a fall in sea level. Sediments in the South Halibut Graben contain minerals indicating exposure of granitic skarns on the Halibut Horst at this time. The area was compartmentalised in Upper Claymore times, with different dispersal systems operating north and south of the Halibut Horst, indicating that the horst continued to be an important control on sediment distribution.