Fairchild, IJ;
Spencer, AM;
Ali, DO;
Anderson, RP;
Anderton, R;
Boomer, I;
Dove, D;
... Zhou, Y; + view all
(2017)
Tonian-Cryogenian boundary sections of Argyll, Scotland.
Precambrian Research
10.1016/j.precamres.2017.09.020.
(In press).
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Abstract
The Tonian-Cryogenian System boundary is to be defined at a GSSP (Global Boundary Stratigraphic Section and Point) beneath the first evidence of widespread glaciation. A candidate lies within the Dalradian Supergroup of Scotland and Ireland, which is least deformed and metamorphosed in Argyll, western Scotland. We present new stratigraphic profiles and interpretations from the Isle of Islay and the Garvellach Islands, update the chemostratigraphy of the Appin Group Tonian carbonates underlying the thick (ca. 1. km) glacigenic Port Askaig Formation (PAF) and demonstrate an environmental transition at the contact.The Appin Group forms a regionally extensive, > 4km-thick, succession of limestones, shales and sandstones deposited on a marine shelf. On Islay, the upper part of the lithostratigraphy has been clarified by measuring and correlating two sections containing distinctive stratigraphic levels including molar tooth structure, oolite, stromatolitic dolomite and intraclastic microbial mounds. Significantly deeper erosion at the unconformity at the base of the overlying PAF is demonstrated in the southern section. Carbonate facies show a gradual decline in δ 13 C VPDB from +5 to +2‰ upwards.In NE Garbh Eileach (Garvellach Islands), a continuously exposed section of Appin Group carbonates, 70m thick, here designated the Garbh Eileach Formation (GEF), lies conformably beneath the PAF. The GEF and the GEF-PAF boundary relationships are re-described with new sedimentological logs, petrological and stable isotope data. Interstratified limestone and dolomicrosparite with δ 13 C of -4 to -7‰ (a feature named the Garvellach anomaly, replacing the term Islay anomaly) are overlain by dolomite in which the isotope signature becomes weakly positive (up to +1‰) upwards. Shallow subtidal conditions become peritidal upwards, with evidence of wave and storm activity. Gypsum pseudomorphs and subaerial exposure surfaces are common near the top of the GEF. The basal diamictite (D1) of the PAF is rich in carbonate clasts similar to slightly deeper-water parts of the underlying succession. D1 is typically several metres thick with interstratified sandstone and conglomerate, but dies out laterally. Scattered siliciclastic coarse sandstone to pebble conglomerate with dropstones associated with soft-sediment deformation is interbedded with carbonate below and above D1. Dolomite beds with derived intraclasts and gypsum pseudomorphs are found above D1 (or equivalent position, where D1 is absent).Published and new Sr isotope studies, including successive leach data, demonstrate primary Tonian 87 Sr/ 86 Sr values of 0.7066-0.7069 on Islay, decreasing to 0.7064-0.7066 in the younger GEF limestones on the Garvellachs, with 1700-2700ppmSr. Other typically Tonian characteristics of the carbonates are the Sr-rich nature of limestones, molar tooth structure, and dolomitized peritidal facies with evidence of aridity. Seabed surveys suggesting uniformly-dipping strata and shallow borehole core material illustrate the potential for extending the Tonian record offshore of the Garvellachs.A candidate Tonian-Cryogenian GSSP is proposed on Garbh Eileach within the smooth δ 13 C profile at the cross-over to positive δ 13 C signatures, 4m below the first occurrence of ice-rafted sediment and 9m below the first diamictite. Although lacking radiometric constraints or stratigraphically significant biotas or biomarkers, the Scottish succession has a thick and relatively complete sedimentary record of glaciation, coherent carbon and strontium chemostratigraphy, lateral continuity of outcrops and 100% exposure at the proposed boundary interval.
| Type: | Article |
|---|---|
| Title: | Tonian-Cryogenian boundary sections of Argyll, Scotland |
| Open access status: | An open access version is available from UCL Discovery |
| DOI: | 10.1016/j.precamres.2017.09.020 |
| Publisher version: | https://doi.org/10.1016/j.precamres.2017.09.020 |
| Language: | English |
| Additional information: | This version is the author accepted manuscript. For information on re-use, please refer to the publisher’s terms and conditions. |
| Keywords: | Neoproterozoic, Limestone, Glaciation, Dolomite, Carbon isotopes, Strontium isotopes |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10025140 |
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