--> --> Revise the Paleozoic North American regional time scales by volcanic ashes in the Midland Basin

AAPG Southwest Section Annual Convention

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Revise the Paleozoic North American regional time scales by volcanic ashes in the Midland Basin

Abstract

The correlation between the Paleozoic North American regional time scales (NARS) and International Geological Time Scales (IGTS) is problematic due to the lack of datable materials in the Paleozoic North American strata. Recent discovery of multiple volcanic ash beds in the Pennsylvanian and lower Permian strata of the Midland Basin provides a great opportunity to constrain the absolute ages and revise the chronostratigraphic correlation between the two scales. In addition to the correlation between the NARS and IGTS time scales, a precise chronostratigraphic framework of the Permian Basin will help improve the depositional model of the basin and better understand the tectonic background of the basin. Fundamental understandings to these aspects can help guide future hydrocarbon and mineral explorations in the basin. Here we present LA-ICPMS zircon U-Pb ages for the Leonardian, Wolfcampian and Atoka stages for the first time. We have total nine volcanic ash samples dated including one from Atokan, two from Leonardian and six from Wolfcampian stages (One in Wolfcamp A and five in Wolfcamp B). Estimated maximum depositional ages for each strata unit are calculated either by TuffZirc ages or Weighted Mean ages from the 3-5 youngest zircons. The results indicate that the best estimated depositional ages for Leonardian (lower Spraberry), Wolfcamp A, Wolfcamp B and Atoka are 273.6-282.1 Ma, 283.1 Ma, 284.8-297.6 Ma and 326.7 Ma respectively. Based on the zircon ages, the Leonardian, Wolfcampian and Atokan stages of the NARS could be correlated to the Kungurian, Artinskian to Asselian stages in the Permian and the Serpukhovian stage in the Upper Mississippian of the IGTS respectively. This chronostratigraphic column will be revised by more ash studies in the basin and more advanced dating tools like Thermal Ionized Mass Spectrometry (TIMS) in the future.