CONODONTS FROM TARGET BEDROCK AND IMPACT BRECCIAS OF THE HAUGHTON IMPACT STRUCTURE, DEVON ISLAND, NUNAVUT, CANADA

Preliminary sampling of carbonate bedrock (n=5) near, and melt breccia clasts (n=8) within the Haughton impact structure (Tertiary), Devon Island, Canada (75.2 deg. N. Lat.; 89.4 deg. W. Long.) yielded conodonts that provide new data on their age and thermal history. Most of the target bedrock samples produced conodonts that are indicative only of Late Ordovician age; for some, a Silurian age cannot be discounted. The age range thus is consistent with the Allen Bay Formation (Upper Ordovician-Lower Silurian) as mapped in the area. Most of the samples of clasts from breccias also fall within this age range; however, some contain faunas of latest Early to earliest Middle Ordovician age, consistent with the ages of some of the underlying units, e.g., the Eleanor River Fm., displaced as ejecta and as parts of the central uplift.

Conodont color alteration index (CAI) values in the regional bedrock are about 1.5, indicating maximum post-depositional, long-term heating in the approximate range of 50-90 degrees C. Some samples contain conodont elements having higher CAI values - up to 4. Some of these samples also contain co-occurring conodonts having several different CAI values, and some of the elements in these samples display surface features characteristic of the effects of contact with hydrothermal conditions. These surface features, and the co-occurrence of multiple within-sample CAI values are another of the many indicators of hydrothermal activity associated with the impact. Conodonts may provide some additional constraints on some of the thermal history parameters of this event. For example, one larger carbonate clast (~25cm x 20cm x 10cm) was split into two samples, the outer edges and the center of the clast, which were processed separately. CAI of conodonts in the outer sample range from 3.5 to 4, whereas most elements from the inner part of the clast range from 2 to 3, demonstrating the insulation effect of the poor heat conductivity of rock. Splits of these two samples processed by one of us (PL) using biological marker maturity parameters (pregnane/sterane and tricyclic terpane/hopane) also showed this insulation effect.