Tonian-Cryogenian boundary sections of Argyll, Scotland

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, >4 km-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, 70 m 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 1700-2700 ppm Sr. 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...

Stable isotopes of oxygen and hydrogen in meteoric water during the Cryogenian Period

“…The section from Bolsa to Ruvaal crosses the fold hinge of the Islay Anticline. Determination of stratigraphic and structural positions was based on previous work (Spencer, 1971;Skelton et al, 1995;Fairchild et al, 2018;Ali et al, 2018). Samples were used to make thin sections and powders for petrographic and isotopic analysis.…”

“…The en echelon segments of the Islay Anticline are shown. (Spencer, 1971;Fairchild et al, 2018;Ali et al, 2018) (a-c) and on structural plots (d-f) as a function of distance from the Islay Anticline (red line) for the Bolsa to Ruvaal section (B-R on Fig. 1).…”

“…A negative carbon isotope ( 13 C) anomaly is preserved near the top of the Garbh Eileach Formation . This formation is overlain by the 1-km-thick Port Askaig Tillite Formation which comprises sections of inferred glacial origin (Thomson 1871;Spencer, 1971;Fairchild et al, 2018;Ali et al, 2018). Fortyseven diamictite horizons are interlayered with sandstones, mudstones and (sometimes stromatolitic) dolostones (Spencer, 1971), from which 28 glacial advances and retreats are inferred .…”

“…However, unravelling original  18 O and  2 H values of meteoric water can be hampered by post-depositional exchange with reservoir rocks , metamorphic volatilization (Valley, 1986) and exchange with metamorphic fluids (Bickle and McKenzie, 1987). In this study, we sequentially remove the effect of each of these processes on  18 O and  2 H values measured on muscovite and carbonate separates from rocks exposed on Islay and the Garvellach Islands, western Scotland (Thomson, 1871;Spencer, 1971;Fairchild et al, 2018). These were deposited at an approximate paleolatitude of 30-35S (Li et al, 2013) before, during and after early Cryogenian (Sturtian) glaciation and were subsequently metamorphosed at greenschist-facies conditions.…”

“…Formations: Bonahaven Dolomite Formation, Port Askaig Tillite Formation, Garbh Eileach Formation and Lossit Limestone Formation are abbreviated BDF, PATF, GEF and LLF. The Garbh Eileach Formation was previously considered part of the Lossit Limestone Formation, but was redefined byFairchild et al (2018).The  18 O mu * values were calculated from  18 O carb by subtracting a fractionation factor () of 1.8 ‰ for samples containing calcite and 2.2 ‰ for samples containing dolomite. The value of  cc-mu was calculated by combining values of  cc-H2O and  mu-H2O from O'Neil et al (1969) and O'Neil & Taylor…”