Paleoenvironmental analysis of the Late Cretaceous Wombat kimberlite maar sediments, subarctic Canada: implications from geochemical and microfossil data

Abstract

The Wombat pipe (64.92◦N, 110.45◦W) is an Upper Cretaceous kimberlite locality within the Lac de Gras kimberlite field in the Northwest Territories, Canada. Two exploration drill cores, CH93-29 and DDH00-5, intersect the Wombat crater facies and contain ∼195 m of well-preserved, undisturbed lake sediment fill. Bulk sediment elemental analysis, carbon isotope composition, and programmed pyrolysis, combined with micropaleontology, are used to characterize the depositional setting and paleoenvironment within the maar lake. Bulk sediment Corg /Ntot, hydrogen index (HI), and δ13C suggest that the sedimentary organic matter is primarily derived from C3 land plants, with a minor contribution from algae. The δ13C values range from −25.3%0 to −30.2%0, andCorg /Ntot (atomic) varies between 14.6 and 38.4, reflecting shifts in the relative contributions of terrestrial and algal organic matter in response to changing climatic and lacustrine conditions. All 18 samples analyzed by programmed pyrolysis plot within the Type III kerogen field (HI vs. Tmax), with an average Tmax of ∼425◦C, indicating low thermal maturity of the organic matter. Total organic carbon (Corg)averages∼3.6 wt.%, while the average total carbonate content is ∼14.1 wt.%. Kimberlite maar sediments in the Lac de Gras and other Canadian Shield localities provide the only known terrestrial records of Late Cretaceous climate in the Northwest Territories, preserved in settings otherwise devoid of Mesozoic strata. These deposits constitute rare, high-resolution archives of subarctic paleoenvironment and ecosystem responses to changing climate conditions.