Geochemistry, sedimentology, and stratigraphy of the Lower Cretaceous pebble shale unit, northeastern Alaska

Abstract

"Organic-rich, marine mudstone known informally as the Lower Cretaceous (Barremian-Aptian) pebble shale unit (PSU) is exposed in the northeastern Brooks Range. Outcrops were examined along the Canning River, an unnamed tributary of the Katakturuk River, and Marsh Creek. Gamma-ray data was measured for each stratigraphic section in order to correlate outcrop with wire-line well logs. Samples of the PSU were collected for thin section petrography, lithostratigraphy, biostratigraphy, total organic carbon (TOC), Rock-Eval II, vitrinite reflectance (R₀), and X-ray Fluorescence. The PSU contains bioturbated and laminated mudstones with thin-bedded siltstones interpreted as interbedded hemipelagites and turbidites. PSU foraminifers were deposited on the outer shelf and upper slope. Intervals with high TOC (2 to 6 wt %) indicate good source-rock quantity in the PSU; however, Rock Eval results suggest poor source-rock quality related to elevated thermal maturities (1.3 to 1.8 %R₀). The PSU is depleted in metals relative to average black shale"--Leaf iii