A new approach to provenance studies: electron microprobe analysis of detrital garnets from Middle Jurassic sandstones of the northern North Sea

Abstract

Reading provenance from heavy‐mineral suites is hampered by the depletion of diagnostic, but unstable, heavy minerals through intrastratal solution, a common phenomenon in deep sedimentary basins such as the North Sea. This paper demonstrates the potential of electron microprobe analysis in overcoming this problem, by concentrating on the compositional variations shown by detrital garnets, which are relatively resistant to intrastratal solution. Studies of the garnets from the Brent Group (Middle Jurassic) of the Murchison and Tern oilfields in the northern North Sea reveal that three distinct areas supplied detritus. Association 1 which characterizes the Broom Formation in both areas and recurs higher in the sequence in the Tern field, is ascribed to an Orkney‐Shetland source. The location of the areas supplying the garnets of Association 2, best represented in Murchison, and Association 3, common to both fields, is less clear. Their ultimate sources clearly include high‐grade metamorphics, and therefore probably lie on the Norwegian landmass, the Orkney‐Shetland Platform, or, conceivably, the Scottish landmass, but the possibility of recycling makes it difficult to judge their immediate provenances. Nevertheless, the presence of three garnet associations indicates that the most widely accepted model of Brent sedimentation, with sands derived from a domal uplift in the outer Moray Firth and channelled northward along the Viking Graben, is untenable.

Timing and depositional environments of a Middle Pleistocene glaciation of northeast England: New evidence from Warren House Gill, County Durham

“…The data were analysed by multivariate statistical methods to identify trends and correlations objectively (following Davis, 1986;Morton andHallsworth, 1994, 2007;Kovach, 1995;Ryan et al, 2007;Thamo´-Bozso´ and Kova´cs, 2007). Standard descriptive statistics, such as mean and skew, were conducted first, and where applicable, data was square rooted to reduce skew.…”

“…Analysis of stable minerals with similar hydraulogical properties can therefore provide additional provenance-specific data (cf. Morton andHallsworth, 1994, 2007). This study used the minerals zircon, monazite, apatite, and tourmaline.…”

“…This study used the minerals zircon, monazite, apatite, and tourmaline. Although apatite can be susceptible to weathering (Morton, 1985), it is relatively stable when compared to the entire mineral suite. The combined use of descriptive statistics, the PCA and cluster analysis, and the use stable heavy-minerals indices, allows a comprehensive comparison of the different lithofacies at Warren House Gill.…”

Detrital goldmanite from the Palaeocene of the North Sea

“…Separation of heavy minerals from the 63-125 ~tm grain size fraction was achieved by gravity-settling in bromoform (s.g. 2.8), following disaggregation with a pestle and mortar and ultrasonic cleaning to remove any adhering clays or other matter. The goldmanite grain, which was subrounded and clearly detrital in origin, was hand-picked from the dry heavy mineral suite under the optical microscope during separation of the garnet suite for a provenance study based on variations in the garnet geochemistry as described by Morton (1985). The grain was picked for analysis because of its isotropic character and because it was considered to have an unusual compositon in view of its grass-green colour under transmitted light.…”

Provenance study of Permian non-marine sandstones and conglomerates of the Krkonoše Piedmont Basin (Czech Republic): exotic marine limestone pebbles, heavy minerals and garnet composition

“…An effective method for discriminating specific source areas is detailed mineral chemistry determination using an electron microprobe (Morton 1985, Owen 1987. Garnet composition was successfully used for the provenance study of clastic material of the Early Carboniferous foreland Culm Basin deposits of the Moravo-Silesian Zone at the eastern margin of the Bohemian Massif (Čopjaková et al 2005).…”