Magnetostratigraphy of the Chinle and Moenkopi Strata Cored at Colorado Plateau Coring Project Site 1A, Northern Petrified Forest National Park
Buhedma, Hesham M. A.
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The Triassic Period (ca. 251.9-201.3 Ma) is bounded by two of Earth’s largest mass extinctions, suffered several giant bolide impacts and eruption of three large igneous provinces, and witnessed evolution of the main components of modern tetrapod communities, and yet has sparse geochronologic calibration. To remedy this problem, the US NSF- and ICDP-funded coring of Phase 1 of the CPCP was completed in late 2013, with the recovery of two major cores (6.35 cm diameter: 1A, 518m length and 2B, 253m; 31km apart) from the north and south ends of Petrified Forest National Park, spanning nearly the entire Triassic section (Chinle and Moenkopi formations.). Typically, multiple specimens were collected from the same core segments, to test for internal consistency of paleomagnetic and rock magnetic results, then subjected to progressive thermal demagnetization or a combination of alternating field (AF) followed by thermal treatment. For the mudstones, siltstones and sandstones of the Chinle Formation, NRM intensities are highly variable and range from 128.7 to 0.07 mA/m and bulk susceptibilities range from ~ 2 x 10-2 to ~ 5 x 10-5 SI volume units. For indurated hematitic siltstones to medium grained sandstones of the Moenkopi Formation, NRM intensities are less variable and range from 0.05 to 53.01 mA/m and bulk susceptibilities are far less variable, with values between ~3.0 X 10-4 and ~0.5 x 10-5 SI volume units. Progressive demagnetization of accepted specimens typically isolates magnetizations of north declination and shallow to moderate positive inclination (interpreted as normal polarity) and antipodes (reverse polarity). IRM acquisition and backfield demagnetization experiments demonstrate both hematite and magnetite as magnetic phases. An attempt was made to correlate current version of the CPCP 1A magnetostratigraphy with the astronomically tuned Newark Basin record.