Using Detrital Zircon Geochronology to Solve Complex Structural Problems<subtitle>An Application with Pitfalls in the Helena Salient of the Montana Disturbed Belt, West Central Montana</subtitle>

Abstract

Dating of detrital zircons from well cuttings is a useful technique to constrain stratigraphic ages and structural interpretations in complexly deformed terranes. This technique was applied in the Helena Salient of the Montana Disturbed Belt to determine whether the Norcen, Kimpton Ranch 1-11 and Buckhorn, Federal 2-24 wells penetrated Phanerozoic strata beneath the allochthonous Precambrian rocks carried on the Lombard thrust. Since some Phanerozoic strata have reservoir potential, their presence below the Lombard thrust has important implications for the oil and gas potential of this structural province. Samples from the upper part of the Kimpton Ranch 1-11 well lack zircons younger than 1450 Ma and were derived from the Belt Supergroup. The remaining samples contain variable amounts of zircons younger than 1450 Ma, with no obvious pattern with depth. They include few sill-derived Cretaceous zircons and detrital Paleozoic and young Precambrian zircons. There are no Jurassic zircons. This distribution of zircon ages, and lack of volcaniclastic lithologies eliminates a Mesozoic age for these rocks. A Paleozoic age is permissible, but the 5000 ft (>1600 m) of black fine-grained rocks with organic-poor compositions matches no Paleozoic strata in the area. The authors conclude that these wells penetrated the lower Belt Supergroup that was structurally thickened in a shear zone associated with the basal Lombard thrust. Contamination of cuttings by lost circulation material (LCM) added during drilling explains the anomalously young Paleozoic and Precambrian zircons. A sample dominated by LCM has only Mesoproterozoic and Paleozoic grains that were identified inside coarse muscovite flakes that were used as drilling mud additives. Drilling contamination can be a major issue when applying detrital zircon age dating to samples obtained from oil and gas wells.