Molecular organic geochemistry of a proposed stratotype for the Oxfordian/Kimmeridgian boundary (Isle of Skye, Scotland)

Abstract

<jats:title>Abstract</jats:title><jats:p>The composition of the soluble organic matter of the Oxfordian–Kimmeridgian Flodigarry Shale Member (Isle of Skye, Scotland) is presented for the first time. A continuous succession of silty clays and nodular limestone beds is exposed on a rocky shore to the north of Staffin Bay. This succession is proposed as a potential stratotype of the boundary between the Oxfordian and Kimmeridgian stages. This paper points out the exceptional preservation and very low thermal degradation of the organic matter. Indeed, the molecular composition is characterized by the abundance of unsaturated biomarkers (hopenes and diasterenes) as well as undamaged bioterpenoids (ferruginol and sugiol). The abundance of long-chain<jats:italic>n</jats:italic>-alkanes characterized by an odd-over-even predominance reveals a dominant continental contribution. This is also attested to by the relatively high amounts of plant biomarkers (e.g. ferruginol, sugiol, cadalene and retene), which suggest a palaeovegetation largely composed of pinophytes, especially Cupressaceae, Taxodiaceae and Cheirolepidiaceae, on the nearest emerged lands. The water column of the depositional environment was oxic in its upper part and rather dysoxic in its lower part. The composition of the organic matter does not significantly change along the Flodigarry Shale Member. In other words, no evolutionary events or drastic change in palaeoenvironments can be deduced from the molecular content of these sedimentary rocks, and it does not allow us to support a precise location for the Oxfordian/Kimmeridgian boundary in the succession.</jats:p>

Contrasting mechanisms for crustal sulphur contamination of mafic magma: evidence from dyke and sill complexes from the British Palaeogene Igneous Province

“…The thermal effect of the Cuillin Complex was previously recognized as highly localized, using organic geochemical molecular maturity parameters (Thrasher 1992). Similarly, this localized thermal alteration has been found to be the case in limestone beds underlying the Trotternish Sills at Staffin Bay (Lefort et al 2012). Nevertheless, sediments underlying sills at Duntulm show a more developed contact metamorphic sequence, including grossular garnet and pyroxene (contact metamorphism) and low-temperature alteration in microfractures containing clays (Kemp et al 2005).…”

Taking the heat out of British Jurassic septarian concretions

“…The clumped isotope temperatures for all concretion calcite phases are below the maximum burial temperature estimates for their locations (<60°C for the Staffin Shale; Lefort et al, 2012;Pearson et al, 2005;Vickers et al, 2020;≤50°C for Southern England;Green et al, 2001). The highest temperature measured (43 ± 6°C) was for a spar from P1A-B.…”

The Global Stratotype Section and Point (GSSP) for the base of the Kimmeridgian Stage (Jurassic System), at Flodigarry, Staffin Bay, Isle of Skye, Scotland, UK

“…The exceptional preservation and very low thermal degradation of the organic matter in the Flodigarry Shale Member at Flodigarry have been shown by Lefort et al (2012). The soluble organic matter is characterized by exceptional preservation of hopanids and biomolecules.…”

“…The soluble organic matter is characterized by exceptional preservation of hopanids and biomolecules. Preservation of such high quality as this, in deposits of Jurassic age, is very rare worldwide (Lefort et al, 2012). The distribution of nalkanes shows that there is organic matter from both marine and terrestrial sources but that the predominant contribution is continental, consistent with the presence of communicated terrestrial plants.…”

“…Jurassic sediments in the area are intruded by occasional Paleocene dolerite sills and dykes, no metamorphic alteration is observed except within about a decimeter of the margins of the intrusions (Thrasher, 1992;Bishop and Abbot, 1995). An organic geochemical study (Lefort et al, 2012) has shown that the organic matter in the uppermost Oxfordian and lowermost Kimmeridgian deposits has very low thermal degradation and is exceptionally well-preserved.…”