Geochronology and bulk sediment geochemistry of the Late Cretaceous maar lake sedimentary fill of the Wombat kimberlite pipe, subarctic Canada

• Author / Creator

  Buryak, Serhiy D.

• The Wombat pipe (64.918° N, 110.447° W) is a diamondiferous kimberlite in the Lac de Gras kimberlite field of Northwest Territories, Canada. Two drill cores, CH93-29 and 00-05, intersect the Wombat crater facies and include 195 m of well preserved, undisturbed lake sediment fill. Bulk sediment elemental analysis, C isotope composition, and Rock-Eval pyrolysis together with inferences from microfossils, are used to characterize conditions of sedimentation and paleoenvironmental variability in the maar lake.

  Bulk sediment C/N, hydrogen index (HI), and d13C indicate material derived from C3 land plants dominates the sedimentary organic matter, with a possible minor algal contribution. The d13C values range from -25.3 ‰ to -30.2 ‰ and C/N ratios vary between 14.6 and 38.4, recording the shifts in the proportions of land-derived material and algal organic matter as climate conditions fluctuated. Eighteen samples analyzed by Rock-Eval pyrolysis all plot in the Type III kerogen field for HI vs. Tmax, with average Tmax values ~425 °C indicative of the low thermal maturity of organic matter. Total organic carbon (TOC) averages 3.6 wt% and average total carbonate content is 14.1 wt%, indicating bottom water anoxia and substantial carbonate input from weathering of overlying carbonate cover rocks, respectively. Together with well-preserved freshwater microfossils (e.g. diatoms, chrysophytes, synurophytes), the results indicate deposition in a non-marine setting, likely during a period of regression of the Western Interior Seaway.

  The age of the Wombat maar lake sediments is determined using MC-LA-ICP-MS U-Pb zircon geochronology from two distal rhyolitic tephra beds found in the core 00-05, yielding a date of 82.36 ± 0.44 Ma (n = 18/34, MSWD = 1.51). This minimum age suggests that Wombat kimberlite pipe emplacement occurred during the Late Cretaceous, with sedimentation in the maar beginning shortly thereafter. The major and trace element geochemical analysis of the tephra glasses allows to make inferences about the type and the spread of the Late Cretaceous volcanism in subarctic Canada. The findings from the Wombat pipe post-eruptive maar lake sediment fill provide direct evidence for a non-marine environment in the Lac de Gras area during the Late Cretaceous. Furthermore, microfossils in the Wombat pipe sediment fill likely include the oldest-known occurrence of freshwater diatoms.

• Subjects / Keywords

  • Wombat kimberlite
  • maar
  • paleoenvironment
  • geochronology
  • Lac de Gras

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-9dsr-wh73

• License

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