HIGH RESOLUTION SEQUENCE STRATIGRAPHY, GEOCHEMISTRY, AND PALEOENVIRONMENTAL INTERPRETATION OF TRIASSIC UPWELLING ZONE DEPOSITS, NORTHERN, ALASKA

Sequence stratigraphic, geochemical, gamma-ray, and ichnofabric analyses of Triassic rocks in Arctic Alaska provide insight into the sea level history and architecture of facies deposited under fluctuating bottom water oxygenation and marine upwelling. Heterogeneous phosphatic, cherty, and organic-rich facies of the Shublik, Otuk, and associated formations record three genetic depositional sequences deposited during the Middle-Upper Triassic. Major transgressive surfaces, facies stacking patterns, and gamma ray data permit high resolution sequence stratigraphic correlation between Shublik and Otuk outcrops in the Brooks Range and with coeval rocks in the Prudhoe Bay subsurface. All outcrop sections record distal depositional settings with Shublik facies indicating more proximal environments compared to the Otuk. Parasequences and parasequence sets generally record shoaling upward deposition from environments that were below storm wave base to subtidal environments near fairweather wave base. Geochemical and ichnofabric data provide insight into fluctuating oxygen levels during deposition of individual parasequences and parasequence sets. Geochemical parameters used to infer bottom water oxygenation include the Ce anomaly (Ce_anom), V, P and TOC levels, and the Ca/Mn ratio of calcic black shales. Facies deposited under low oxygen conditions display a positive Ce_anom, higher V compared to P, high TOC (Otuk up to 10%, Shublik up to 6%), Ca/Mn Group I/II ratios, and a low Ichnofabric Index. More oxygenated environments record a negative Ce_anom, higher P relative to V, low TOC, Ca/Mn Group III/IV ratios, and higher Ichnofabric Indices. Diagenetic processes could have altered some of the geochemical signatures but the concordance of geochemical, ichnofabric, and lithofacies indicators of paleoxygentation support our interpretations. The variations in paleoxygenation recorded here likely resulted from transit of the upwelling-related oxygen minimum zone during relative sea level fluctuations. Additional biostratigraphic data will be needed to constrain the timing of these events but variations within parasequence and parasequence sets (4th-5th order cycles) imply relatively high frequency fluctuations.