APATITE FISSION-TRACK DATING OF 2 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 common ly thought to be astroblemes. Apatite fission-track (FT) ages from bas ement rocks in the two structures are markedly younger than those prev iously derived in the region. Constraints from regional geohistory com bined with forward modelling of apatite PT data indicate the following : 1. At High Rock Lake, apatites in a weakly foliated granite and a br ecciated and metasomatised granite from the uplifted southwestern crat er rim, were totally, or nearly totally, annealed in the range of simi lar to 435 +/- 10 Ma. This range is interpreted as dating the time of cratering and is in excellent agreement with stratigraphic evidence wh ich constrains the event as Late Ordovician to Mid-Silurian; 2. At Lak e St. Martin, apatite from the central basement uplift was totally ann ealed in Late Triassic-Early Jurassic time in the range similar to 208 +/- 14 Ma. This range is concordant with a previous Rb/Sr isotope est imate 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 pa rtial annealing effect (temperatures of similar to 125-130 degrees 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 his tory of the region. At High Rock Lake and Lake St. Martin samples achi eved maximum paleotemperatures (similar to 60-70 degrees C) during the Eocene. This temperature range is in good agreement with data indepen dently attained from organic maturity indicators elsewhere in the nort heastern Williston Basin area.