GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 282-12
Presentation Time: 9:00 AM-6:30 PM


MMASA, Dennis, OEFINGER, Jordan, BOUDREAUX, Asher, SUAREZ, Celina and SHARMAN, Glenn R., Department of Geosciences, University of Arkansas, Fayetteville, AR 72701

The Triassic is a period of abrupt climate change bracketed by mass extinctions including one driven by the eruption of the Central Atlantic Magmatic Province (CAMP) in the Late Triassic. The Moenave Formation (MF) of Utah and Arizona is ideal to investigate effects of the CAMP on continental strata distal to the eruptive center. Thus, the C-isotopic composition of bulk organic and carbonate C was analyzed to determine a chemostratigraphic record to compare to globally established record, relative to the CAMP events. Samples were collected every 25 cm and crushed. For carbonate carbon 80-100 mg was measured and analyzed using a gas bench 2 couple to a Thermo Advantage Plus continuous flow isotope ratio mass spectrometer (IRMS). Bulk organic C powders were, decarbonated using 3M HCl at room temperature until no reaction. 18-20 mg of rinsed and dried powders were analyzed on an elemental analyzer attached to the Thermo Advantage Plus IRMS. All samples are reported relative to Vienna PeeDee Belemnite (VPDB). The ẟ13CCO3 values range between -2.05 and -7.41‰ VPDB and the ẟ18OCO3 values range between -0.27 and -8.28‰ VPDB. For the d18OCO3, there is a short-lived positive C-isotope excursion (CIE) at 250 cm above the base of the Chinle with a decrease to -8.28‰ at 2,386 cm. Values increase to -1.92‰ near the top of the Dinosaur Canyon Member (DCM), decrease and oscillate around -4.8‰ for the remainder of the record into the Whitmore Point Member (WPM). For d13CCO3, there is a short-lived positive CIE at 250 cm, a decrease to ~ -4‰ with a small CIE to -5.27‰ at 2,786 cm and then an increase to ~ 3.3‰ through the upper DCM through the lower 5 m of the WPM and a gradual decrease through the end of the record. The ẟ13Corg samples range between -25.36‰ and -29.77‰. The d13Corg record remains at ~ -28‰ until a small positive CIE at 2,236 cm to a value of -25.49‰. This is followed by a -4.30‰ CIE to -29.79‰ at 2,511 cm. Values then increase over the next 6m to ~27‰ where it remains throughout the upper DCM, has a short-lived positive CIE at the base of the WPM and gradually increases through the WPM to the top of the record. All three records show CIEs between 2336 to 2786 cm above the contact. Based on detrital zircon constraints, we suggest these excursions are a continental representative of the initial CIE documented in other global records.