Direct dating of a laminated speleothem coating a karst wall in southeastern Gaspé Peninsula, Quebec, Canada: Carboniferous, not Silurian

In the Gaspé Peninsula, karsts associated with subaerial exposure are interpreted to have developed either during the Silurian or the Carboniferous. Determining the timing of karst formation is valuable for economic reasons (including petroleum system characterization) and for the understanding of basin/climate evolution. In this study, a laminated calcite speleothem coating a karst wall in the McInnis cement plant quarry is studied in detail through petrographic observations, stable and clumped isotope analysis, and geochronology. The speleothem is made of laminae of different colours that are geochemically and isotopically distinct. White and red laminae have δ 18 O and δ 13 C values comparable to contemporaneous (within uncertainty) phreatic calcrete hardpans described in southern Gaspé Peninsula and northern New Brunswick, suggesting that both the speleothem and calcrete precipitated from primarily fresh groundwaters that interacted with soils. Changes in laminae colour most likely reflect different kinetics of precipitation and amounts of impurities. LA-ICP-MS in situ dating of calcite from white laminae gives a date of 331.8 ± 9.1 Ma, which is the first direct evidence that the cavity formed during the Visean (Carboniferous). From a regional point of view, subaerial exposure and karst formation during the Visean could have resulted from a glacially induced sea-level lowstand and/or from an uplift episode associated with magmatic underplating/plume-related magmatism in the offshore part of the Maritimes Basin.