Sequence Stratigraphic and Geochemical Insights Into Paleoceanography and Source Rock Development in the Shublik Formation and Adjacent Units, Northern Alaska

Abstract

The Fire Creek Siltstone, Shublik Formation, and Karen Creek Sandstone in northern Alaska record four third-fourth order depositional sequences. The Shublik, a proven Prudhoe Bay source rock, records geochemical, ichnologic, and facies evidence of significant variation in bottom water redox conditions, detrital input, and biological productivity. Low oxygen conditions facilitated both the accumulation of organic matter and development of abundant phosphatic facies that include both granular and nodular phosphorites. The Shublik is extremely heterogeneous containing claystone, organic-rich shale, bioclastic wackestone and packstone, sandstone, nodular and pebbly phosphorite, and phosphatic and glauconitic silt/sandstone. Facies are commonly stacked in shoaling upward cycles that evolve from organic-rich shales through phosphatic silt/sandstones to more carbonate rich facies. Total Organic Carbon (TOC) data in conjunction with redox sensitive trace metal concentrations, geochemical indicators of detrital input and bioproductivity, and ichnofabric analysis provide insight into the paleoceanographic changes concomitant with the development of different systems tracts during relative sea level change. The most organic rich facies (up to 4%) are usually contained within the Transgressive Systems Tract (TST) that commonly display the lowest ichnofabric indices. Redox sensitive geochemical indicators of low oxygen environments (Mo, V, U, Ni) are also concentrated over background levels within transgressive facies. Some HST facies, however, record relatively high TOC contents (1–2%) even when fairly coarse grained. Also, while these facies usually display much lower concentrations of redox sensitive trace metals they are commonly still elevated over background levels. This points to at least intermittent development of low oxygen conditions even during sea level highstand. The Shublik is commonly interpreted to represent deposition under upwelling conditions. We, however, also observe that some TOC-rich intervals display concentration of geochemical indicators of detrital input (Al, Si, Ti) and bioproductivity (Cu, Ni, Zn) above background levels. These relationships could point toward productivity driven by detrital input rather than upwelling of nutrients from deeper water.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015