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 beneith 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.