Geochemical, Geochronologic, Rock-Eval, and Spectral Gamma Ray Data for Selected Triassic Rocks in Northwestern Alaska

The detrital zircon geochronology analyses were performed by GeoSep Services (Moscow, ID). Mineral separation procedures for zircons used lithium polytungstate and a centrifuge. Zircon grains were mounted in epoxy wafers and then ground down to expose the internal grain surfaces, and then polished. Isotopic analyses were performed with either a Wave UP-213 or New Wave YAG 213 laser ablation system in line with a Finnigan Element2 magnetic sector, inductively coupled plasma, mass spectrometer (ICP-MS) at Washington State University GeoAnalytical Laboratory in Pullman, Washington.

LA-ICPMS data from UCSB - Zircon mineral separation was performed using standard crushing, magnetic, and heavy liquid procedures. Zircon grains were mounted in epoxy and sanded and polished for analysis by laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS). These analyses were performed at the University of California, Santa Barbara using a Photon Machines Analyte 193-nm Excimer laser and Nu Instruments Plasma HR MC-ICPMS using standard methodology (Kylander-Clark and others, 2013). Analyses targeted the cores of zircon grains that were free of cracks and inclusions using a spot size of 15 um and repetition rate of 4 Hz. The 91500 reference zircon was used as the primary standard and GJ1, Plesovice, and Peixe were used as secondary standards to monitor data accuracy. Data reduction was performed using the Iolite software (Paton and others, 2010) and age calculations were performed using Isoplot (Ludwig, 2012).

Major, minor, and trace elements were determined using ICP-OES and ICP-MS after sodium peroxide fusion. Data deemed acceptable if recovery of each element is +/-15% at five times the lower limit of detection and the calculated relative standard deviation of duplicate samples is no greater than 15%. Samples were processed from 2018 to 2019 through AGAT Labs, a USGS contract laboratory.

Spectral gamma ray (SGR) data was measured using an RS-230 BGO (Radiation Solutions, Inc., Canada) handheld gamma ray spectrometer to collect potassium (40K), thorium (208Tl), and uranium (238U and 235U) values from exposed outcrop at sample sites. The stratigraphic section was measured perpendicular to the bedding plane in 0.5 meter increments. SGR measurements were taken by placing the spectrometer face directly on outcrop surfaces and were recorded at every 1 foot with a 180 second assay length at the Surprise Creek site (SC1), and every 0.5 meters with a 90 second assay length at the Cape Lisburne sites (CL1 and CL3). If sediment bedding was not apparent, pints were created to reveal minimally weathered bedding. Where vegetation covered the outcrop, the covered area was measured but no SGR data was recorded. The field measurements were taken on at site SC1 on 8/2/2013; site CL1 on 7/7/2016; and site CL3 on 7/9/2016.

Evaluation of organic matter was performed by the USGS Petroleum Geochemistry Research Laboratory and outside contract lab Weatherford. Standard analyses were run for total organic carbon (TOC), pyrolysis indices, and Tmax.

X-ray diffraction (XRD) investigations were performed by the USGS Petroleum Geochemistry Research Laboratory to determine mineral phases based their crystal structure and atomic spacing. Samples were scanned at different times: the Brontosaurus samples were scanned in 2017, Surprise Creek samples in 2015, and the Cape Lisburne samples in 2019.