Petrological and Geochemical Studies of Samples from the Nicor Chestnut 18-4 Drill Core, AMES Impact Structure, Oklahoma

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Ames Impact Crater, Impacts, Iridium: Enrichment, Oklahoma

Scientific paper

The near-circular 15-km-diameter Ames structure is located at 36 degrees 15' N and 98 degrees 12' W in southeastern Major County (NW Oklahoma). The structure, which is set in Cambro-Ordovician Arbuckle dolomite, consists of two concentric rims, an outer rim, which is about 1.5 to 3 km wide, and an inner "rim". The rocks of the outer rim consist mainly of fractured and brecciated Arbuckle dolomite. The inner "ring" (about 5 km in diameter) seems to be the eroded remnant of a central structural uplift, with rocks comprising brecciated Precambrian granite and Arbuckle dolomite. The depression is covered by Middle Ordovician Oil Creek shale. The structure is penetrated by a number of oil- and gas-producing wells in the crater rim and the central uplift. The production from these wells indicate that Ames represents one of the largest - if not the largest - single oil fields in Oklahoma. Currently the structural disturbance is buried beneath almost 3000 m of sedimentary rock. The origin of the structure has been intensely debated since the discovery of the structural anomaly, but geophysical and geological, as well as petrological and geochemical data provide very good evidence that it was formed by impact, and not by volcanism or even more esoteric processes. In the present study, we analyzed 17 samples, including impact melt breccia, from the Nicor Chestnut 18-4 core. These samples represent the largest and best examples of impact melt breccias and melt rock obtained so far from the Ames structure. One important result of the petrographic analyses is the observation that not all carbonate rocks postdate the impact, but some were clearly present among the target rocks. The chemical composition of the impact melt breccias is similar to that of other melt rocks from the Dorothy 1-19 core, as well as to the target granite, with variable carbonate admixtures. Some impact melt rocks are enriched in siderophile elements, indicating a possible meteoritic component.

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