Environmental change, trilobite extinction and massive volcanism at the Cambrian Series 2 – Series 3 boundary

The Cambrian Series 2 – Series 3 boundary (~509 Ma) was a dynamic interval marked by a major perturbation to the global carbon cycle; widespread sea level fall (Sauk Supersequences); the eruption of the Kalkarindji large igneous province and the extinction of the dominant trilobites- the olenellids of Laurentia and the redlichiids of Gondwana. This thesis investigates the relative timing of these phenomena from field sites across the Laurentian and Gondwanan palaeocontinents, assessing the severity of environmental change coinciding with extinction. We report that the Redlichiid-Olenellid Extinction Carbon isotope Excursion (ROECE) coincides with the Series 2 – Series 3 boundary in Laurentian sections (NW Scotland, SW USA), though only in the SW USA does ROECE co-occur with the trilobite extinction. Scottish successions of this age are hindered by a lack of fossils, and as such the exact timing of ROECE relative to the extinction is unclear. New evidence presented in this thesis shows a variable record of anoxia at the extinction horizon, the most severe manifestation indicates euxinic conditions during the extinction at Ruin Wash, Pioche Formation, Nevada. Elsewhere in the SW USA and NW Scotland, there is evidence for periodic dysoxia (e.g. basal Ghrudaidh Formation, Durness Group, Scotland and Carrara Formation, SW USA) at the Series 2 – Series 3 boundary and extinction interval. Facies analysis reveals that on Laurentia, ROECE and the extinction horizon occur during the transgressive phase of the Sauk II supersequence. To investigate the temporal relationship between the Series 2 – Series 3 boundary, ROECE, extinction and Kalkarindji eruptions, this thesis presents new Hg data as a proxy for volcanism. On Laurentia the data show an inconsistent relationship between these phenomena, whilst on Gondwana a new preliminary study shows evidence for Hg enrichments at this interval, establishing it as a site to further investigate the sedimentary trace of volcanism

Sequence stratigraphy, chemostratigraphy and facies analysis of Cambrian Series 2 – Series 3 boundary strata in northwestern Scotland

Globally, the Series 2 – Series 3 boundary of the Cambrian System coincides with a major carbon isotope excursion, sea-level changes and trilobite extinctions. Here we examine the sedimentology, sequence stratigraphy and carbon isotope record of this interval in the Cambrian strata (Durness Group) of NW Scotland. Carbonate carbon isotope data from the lower part of the Durness Group (Ghrudaidh Formation) show that the shallow-marine, Laurentian margin carbonates record two linked sea-level and carbon isotopic events. Whilst the carbon isotope excursions are not as pronounced as those expressed elsewhere, correlation with global records (Sauk I – Sauk II boundary and Olenellus biostratigraphic constraint) identifies them as representing the local expression of the ROECE and DICE. The upper part of the ROECE is recorded in the basal Ghrudaidh Formation whilst the DICE is seen around 30m above the base of this unit. Both carbon isotope excursions co-occur with surfaces interpreted to record regressive–transgressive events that produced amalgamated sequence boundaries and ravinement/flooding surfaces overlain by conglomerates of reworked intraclasts. The ROECE has been linked with redlichiid and olenellid trilobite extinctions, but in NW Scotland, Olenellus is found after the negative peak of the carbon isotope excursion but before sequence boundary formation

Sequence Stratigraphy, Chemostratigraphy and Facies Analysis of Cambrian Series 2 - Series 3 Boundary Strata in Northwestern Scotland

Globally, the Series 2 - Series 3 boundary of the Cambrian System coincides with a major carbon isotope excursion, sea-level changes and trilobite extinctions. Here we examine the sedimentology, sequence stratigraphy and carbon isotope record of this interval in the Cambrian strata (Durness Group) of NW Scotland. Carbonate carbon isotope data from the lower part of the Durness Group (Ghrudaidh Formation) show that the shallow-marine, Laurentian margin carbonates record two linked sea-level and carbon isotopic events. Whilst the carbon isotope excursions are not as pronounced as those expressed elsewhere, correlation with global records (Sauk I - Sauk II boundary and Olenellus biostratigraphic constraint) identifies them as representing the local expression of the ROECE and DICE. The upper part of the ROECE is recorded in the basal Ghrudaidh Formation whilst the DICE is seen around 30m above the base of this unit. Both carbon isotope excursions co-occur with surfaces interpreted to record regressive-transgressive events that produced amalgamated sequence boundaries and ravinement/flooding surfaces overlain by conglomerates of reworked intraclasts. The ROECE has been linked with redlichiid and olenellid trilobite extinctions, but in NW Scotland, Olenellus is found after the negative peak of the carbon isotope excursion but before sequence boundary formation