A case study of the effects of zircon fertility bias in sedimentary provenance studies from the Barents Shelf

Introduction

Sedimentary provenance analysis is an important tool in hydrocarbon exploration. As well as providing information on key issues such as correlation, sand dispersal, regional geology and tectonics, provenance has the fundamental control on the composition of siliciclastic reservoir units. It is the keystone for source-to-sink studies, and plays an important role in subsequent burial diagenesis. As such, it can be used to help predict the quality and distribution of potential reservoir units.

Geochemical and geochronological techniques, such as detrital U-Pb geochronology are commonly used in provenance analysis. However, natural fertility biases between different detrital minerals can sometimes obscure the correct provenance interpretation, particularly when techniques are used in isolation. Recent technological improvements now permit an increasingly diverse assemblage of minerals to be analysed. The contrasting conditions of isotopic closure and different preservation characteristics during source to sink and burial diagenesis can both introduce bias but can also be used to identify them. In this contribution, an example is provided from the Triassic of the Barents shelf, where a multi proxy approach involving U-Pb geochronology on zircon, and combined U-Pb geochronology and geochemical analysis of detrital rutile has identified mineral fertility and bias and enabled improved provenance interpretations to be made.

Geological Background

Thick accumulations of sediment were deposited on the southwest Barents Shelf during the Triassic ¹ (Fig. 1). Various potential source and reservoir rocks were deposited in a series of transgressive-regressive cycles ^2,3. As such, the Triassic in particular has been a major focus for hydrocarbon exploration in recent years.Deposition during the Triassic was characterised by the progradation of large-scale delta complexes ^3-5. These complexes delivered large volumes of sand with compositions and provenance indicators that are consistent with a source from the evolving Uralian Orogen or Arctic Uralides ^6,7. Following an Early Norian maximum flooding surface ^3-5, the compositional maturity of sandstone increases and trends to quartz arenite ^6-8. Both climatic perturbations e.g. ⁹ and tectonism e.g. ⁶ have been proposed to account for these changes.

Methods and results

In this study, sandstone samples were collected from the Fruholmen, Tubaen and Nordmella formations on the SW Barents Shelf above the Early Norian maximum flooding surface (Fig. 1). Rutile and zircon were separated using standard heavy mineral techniques, picked from concentrates and mounted in epoxy, then polished to expose their interiors and imaged prior to analysis. Analysis was carried out at the Central Analytical Facility, Stellenbosch University, South Africa and followed closely the method of ¹⁰.

The data have been combined into groups with regionally distinct signatures representing the west and east of the study area. The zircon age spectra for the east and west groups are remarkably similar (Fig. 2). The most abundant zircons yielded ages between 1450 Ma and 1800 Ma and contain a prominent age peak at c. 1650 Ma. Zircons with ages between c. 950 and 1250 Ma are also recorded in large quantities. However, Phanerozoic zircons and grains with ages greater than c. 1850 Ma are subordinate. In contrast, the combined rutile age spectra for the two groups are strikingly different. The west group comprises of grains that yield ages between 400 Ma and 600 Ma and have compositions which suggest the majority were sourced from metapelitic rocks and crystallised at temperatures between 600 and 700 °C. The east group contains a small population of grains which are identical to those in the west group, however, by far the most abundant grains yield ages between 1800 Ma and 2000 Ma and have compositions which suggest they were sourced from metapelitic rocks and crystallised between 800 and 950 °C.

Interpretations and implications

The rutile data in the east group are consistent with an origin from the northern Fennoscandian Shield. Rutile growth at c. 1900 Ma under unusually high metamorphic temperatures are features that correspond with elements of the Lapland-Kola Orogeny in northern Fennoscandia, most notably the Lapland Granulite Terrane ^12-14. The east group may be inferred to have its rutile sourced from the Lapland Granulite Terrane, or from sedimentary units that may have been derived from it.

The Palaeozoic rutile U-Pb ages of the west group may reflect growth or isotopic resetting and recrystallization during the Caledonian Orogeny. The more modest temperatures calculated from the compositions in the west group would seem to correspond with the age and metamorphic conditions recorded in the northern Norwegian Caledonides ^15,16. It is thus reasonable to assume that the sandstones that form the west group were sourced from the Caledonian Orogeny, or by sedimentary units that were derived from the orogen.

The zircon spectra from the east and west groups closely match each other and have not yielded the provenance information that the rutile data have. The most likely explanation is that zircon recycling from highly fertile metasedimentary rocks from either the Caledonian allochthons or the Fennoscandian Autochthons have non-unique patterns ^17-19 and have obscured this information. In contrast, although it is possible that some rutile was also recycled from these units, due to the lower closure temperatures of rutile, grains had either grown or were reset during metamorphism in the case of the west group, and therefore provide important provenance information that could otherwise be missed by zircon analysis alone. In the case of the east group, the fact that the rutile ages are older than the bulk of the zircon ages demonstrates that the units from which the zircons were recycled were highly fertile in zircon but not in rutile. Conversely, the Lapland Granulite Terrane or other sedimentary successions derived from it which delivered the rutiles to the east group must have had high rutile but low zircon fertilities in order to explain this pattern.

Conclusions

This case study has illustrated that quartz-rich compositionally mature sandstones were recycled from sedimentary rocks. This sedimentary reworking introduced bias such that detrital zircon data were unable to differentiate provenance from the Fennoscandian Shield and the Caledonides, whereas the data obtained from detrital rutile could.

References

1 Smelror, M., Petrov, O. V., Larssen, G. B. & Werner, S. C. Atlas: Geological History of the Barents Sea. (Geological Survey of Norway, 2009).

2 Leith, T. L. et al. in Norwegian Petroleum Society Special Publications Vol. 2 1-25 (1993).

3 Glørstad-Clark, E., Faleide, J. I., Lundschien, B. A. & Nystuen, J. P. Triassic seismic sequence stratigraphy and paleogeography of the western Barents Sea area. Mar Petrol Geol 27, 1448-1475 (2010).

4 Riis, F., Lundschien, B. A., Høy, T., Mørk, A. & Mørk, M. B. E. Evolution of the Triassic shelf in the northern Barents Sea region. Polar Research 27, 318-338, doi:10.1111/j.1751-8369.2008.00086.x (2008).

5 Klausen, T. G., Ryseth, A. E., Helland-Hansen, W., Gawthorpe, R. & Laursen, I. Regional development and sequence stratigraphy of the Middle to Late Triassic Snadd Formation, Norwegian Barents Sea. Mar Petrol Geol 62, 102-122, doi:http://dx.doi.org/10.1016/j.marpetgeo.2015.02.004 (2015).

6 Fleming, E. J. et al. Provenance of Triassic sandstones on the southwest Barents Shelf and the implication for sediment dispersal patterns in northwest Pangaea. Marine and Petroleum Geology 78, 516-535, doi:http://dx.doi.org/10.1016/j.marpetgeo.2016.10.005 (2016).

7 Mørk, M. B. E. Compositional variations and provenance of Triassic sandstones from the Barents Shelf. Journal of Sedimentary Research 69, 690-710, doi:10.2110/jsr.69.690 (1999).

8 Bergan, M. & Knarud, R. in Arctic Geology and Petroleum Potential (eds T. O. Vorren et al.) 481-493 (Elsevier, Amsterdam, 1992).

9 Ryseth, A. in From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin 187-214 (2014).

10 Frei, D. & Gerdes, A. Precise and accurate in situ U-Pb dating of zircon with high sample throughput by automated LA-SF-ICP-MS. Chem Geol 261, 261-270, doi:10.1016/j.chemgeo.2008.07.025 (2009).