Petrography, geochemistry, and argon-40/argon-39 ages of impact-melt rocks and breccias from the Ames impact structure, Oklahoma: The Nicor Chesnut 18-4 drill core

The 15 km diameter Ames structure in northwestern Oklahoma is located 2.75 km below surface in Cambro-Ordovician Arbuckle dolomite, which is overlain by Middle Ordovician Oil Creek Formation shale. The feature is marked by two concentric ring structures, with the inner ring of about 5 km diameter probably representing the collapsed remnant of a structural uplift composed of brecciated Precambrian granite and Arbuckle dolomite. Wells from both the crater rim and the central uplift are oil-and gas-producing, making Ames one of the economically important impact structures. Petrographic, geochemical, and age data were obtained on samples from the Nicor Chestnut 18-4 drill core, off the northwest flank of the central uplift. These samples represent the largest and best examples of impact-melt breccia obtained so far from the Ames structure. They contain carbonate rocks, which are derived from the target sequence. The chemical composition of the impact-melt breccias is similar to that of target granite, with variable carbonate admixture. Some impact-melt rocks are enriched in siderophile elements indicating the possible presence of a meteoritic component. Based on stratigraphic arguments, the age of the crater was estimated at 470 Ma. Previous 40Ar-39Ar dating attempts of impact-melt breccias from the Dorothy 1-19 core yielded plateau ages of about 285 Ma, which is in conflict with the stratigraphic age. The new 40Ar-39Ar age data obtained on the melt breccias from the Nicor Chestnut core by ultraviolet (UV) laser spot analysis resulted in a range of ages with maxima around 300 Ma. These data could reflect processes related either the regional Nemaha Uplift or resetting due to hot brines active on a midcontinent-wide scale, perhaps related to the Alleghenian and Ouachita orogenies. The age data indicate an extended burial phase associated with thermal overprint during Late Pennsylvanian-Permian.