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Transitional Facies of the Triassic Shublik and Otuk Formations at Surprise Creek, Northwest Alaska


The Shublik Formation (Middle and Upper Triassic) sourced oil found in the Prudhoe Bay and other accumulations and is a prospective shale-oil play on Alaska's North Slope. Its distal, deeper water equivalent—the Otuk Formation—is a potential hydrocarbon source rock in the Brooks Range foothills. The transition between these two units is rarely exposed and therefore poorly understood. Strata interpreted as the upper part of this transition were studied at Surprise Creek, near the Chukchi coast, and sampled for petrographic and geochemical analyses. The base of the ~30 m section is the core of an anticline and the top is a Jurassic unconformity, overlain by Oxfordian to Valanginian Kingak Shale. Main lithologies are siliceous to calcareous, variously organic-rich mudstones with 0.3–9.5% TOC (>2% TOC for 22 of 44 samples) and sparse to abundant Carnian-Norian flat clams; other bioclasts include pelmatozoan fragments, foraminifers, and conodonts. Subordinate lithofacies include phosphatic mudstones (typical of the Shublik but rare in the Otuk) and radiolarite (characteristic of the Otuk but absent from the Shublik). Phosphate occurs as cm-sized nodules, smaller peloids, and distinctive ooids, some of which have dissolved centers, are flattened, and are partly siliceous and/or calcareous. Comparable phosphatic ooids were found in the Shublik in the Prudhoe Bay area. Radiolarian tests are filled/replaced with silica, carbonate, or pyrite and occur in muddy or calcareous matrices. Yellow-weathering bentonite? layers (2 to 3 cm thick) are similar to beds in the Otuk of the central Brooks Range. Rare thin lenses of silt to fine sand are mainly quartz with lesser feldspar and mica. Vitrinite reflectance data indicate that Triassic rocks at Surprise Creek are immature. The Surprise Creek section has similarities to the limestone member of the Otuk in the Red Dog district ~100 km to the south, but differs in containing phosphate and more abundant megafossils and lacking chert. It also lacks the glauconitic siliciclastic facies typical of upper Shublik sections in wells of the northwest North Slope (e.g., Peard) and Hanna Trough margin (Diamond). Overall, Surprise Creek strata may be most like the “Shublik equivalent sequence” in the Klondike well (Hanna Trough; TOC 2–8%), although that section did not contain phosphate. We suggest the Surprise Creek section may be an outcrop analog for the source rock in the main Chukchi shelf exploration play.