Apatite fission-track dating of two crater structures in the Canadian Williston Basin

The High Rock Lake and Lake St. Martin structures, on the northeastern flank of the Williston Basin in Manitoba, are circular craters commonly thought to be astroblemes. Apatite fission-track (FT) ages from basement rocks in the two structures are markedly younger than those previously derived in the region. Constraints from regional geohistory combined with forward modelling of apatite FT data indicate the following: 1. At High Rock Lake, apatites in a weakly foliated granite and a brecciated and metasomatised granite from the uplifted southwestern crater rim, were totally, or nearly totally, annealed in the range of ∼435 ± 10 Ma. This range is interpreted as dating the time of cratering and is in excellent agreement with stratigraphic evidence which constrains the event as Late Ordovician to Mid-Silurian; 2. At Lake St. Martin, apatite from the central basement uplift was totally annealed in Late Triassic-Early Jurassic time in the range ∼208 ± 14 Ma. This range is concordant with a previous Rb/Sr isotope estimate of 219 ± 32 Ma for the impact event. The crater rim at Lake St. Martin records an older apatite FT age which is attributed to the partial annealing effect (temperatures of ∼125-130 deg C) from the same impact event; and 3. The resetting of apatite FT clocks within the basement rocks by two discrete Phanerozoic cratering events provides a unique opportunity to study the post cratering thermal history of the region. At High Rock Lake and Lake St. Martin samples achieved maximum paleotemperatures (∼60-70 deg C) during the Eocene. This temperature range is in good agreement with data independently attained from organic maturity indicators elsewhere in the northeastern Williston Basin area. (author). 44 refs., 2 tabs., 6 figs