⁸⁷ Sr/ ⁸⁶ Sr chemostratigraphy of Neoproterozoic Dalradian limestones of Scotland and Ireland: constraints on depositional ages and time scales

A composite C-isotope profile for the Neoproterozoic Dalradian Supergroup of Scotland and Ireland

“…In addition, given the likelihood of such long time spans, the Dalradian must contain significant unconformities. One such proposed unconformity at the base of the Easdale Subgroup (Pitcher & Berger 1972) is supported by a step-increase in 87 Sr/ 86 Sr on carbonate rocks (Thomas et al 2004). The discovery and documentation of such surfaces should spur the next major research effort to advance understanding of Dalradian geology.…”

“…For many years, the Port Askaig Formation, recognised in discontinuous outcrop across mainland Scotland and Ireland, was the only glacial deposit identified within the Dalradian (Spencer 1971;Eyles & Eyles 1983;Arnaud & Eyles 2006;Benn & Prave 2006 Boulder Beds in SW and NE Scotland, respectively (Sutton & Watson 1954;Stoker et al 1999;Condon et al 2000Condon et al , 2002. Thomas et al (2004) have shown that diagenetic effects on 13 C in most Dalradian limestones were likely minor. Part of the evidence for reaching this conclusion is the relatively high Sr contents for most of these limestones (~1 to 2 x 10 3 ppm; Hickman & Wright 1983;Rock 1986;Thomas 1989;Thomas et al 2004) which has been used to infer that the original carbonate phase was largely aragonite and that diagenetic pathways were rather simple, consistent with transition from aragonite to calcite in the presence of a fluid dominated by broadly contemporaneous seawater away from the influence of meteoric and other exotic fluids.…”

“…Thomas et al (2004) have shown that diagenetic effects on 13 C in most Dalradian limestones were likely minor. Part of the evidence for reaching this conclusion is the relatively high Sr contents for most of these limestones (~1 to 2 x 10 3 ppm; Hickman & Wright 1983;Rock 1986;Thomas 1989;Thomas et al 2004) which has been used to infer that the original carbonate phase was largely aragonite and that diagenetic pathways were rather simple, consistent with transition from aragonite to calcite in the presence of a fluid dominated by broadly contemporaneous seawater away from the influence of meteoric and other exotic fluids. Regarding metamorphism, the metamorphic fluids passing through the Dalradian rocks are known to have been poor in C-bearing phases (e.g.…”

“…However, recent work by Macdonald et al (2009) Bitter Springs anomaly is correct, the onset of Dalradian sedimentation must be older than 800 Ma. However, 87 Sr/ 86 Sr data for the Kincraig Formation (Thomas et al 2004), which is stratigraphically older than the Ballachulish Limestone, are consistent with an age of c. 750 -800 Ma when compared with the latest compilation of Neoproterozoic 87 Sr/ 86 Sr data (Halverson et al 2007b). Thus the age of the base of the Dalradian is still uncertain and further work is required to resolve this issue, including a circumspect consideration of the basement-cover relationships and the 800 -840 Ma age dates for the sheared gneissose rocks in the Central Highlands of Scotland (Highton et al, 1999;Noble et al 1996).…”

Early Neoproterozoic limestones from the Gwna Group, Anglesey

“…As the values for the Gwna Group limestone are considerably lower, this undermines the basis for a correlation between these limestone units. Thomas et al (2004) have recorded a significantly lower 87 Sr/ 86 Sr value of 0.7064 from the Dufftown Limestone (Ballachulish Subgroup, Appin Group), lower in the Dalradian succession. This indicates that it was deposited around 800 Ma, and is supported by low δ 13 C values that appear to equate to the c. 800 Ma Bitter Springs anomaly (Prave et al 2009).…”