A Triassic upwelling zone: The Shublik Formation, Arctic Alaska, USA

The Shublik Formation (Triassic, North Slope, Alaska) is an organic-, phosp hate-, and glauconite-rich unit with abundant fossils of marine vertebrates and mollusks, Four lithofacies are identified in the Shublik Formation: nonglauconitic sandstone--thin- to medium-bedded, fine, quartzose, calcareo us to noncalcareous sandstone or silty to muddy sandstone, fossiliferous in places; glauconitic--thin- to medium-bedded, fine, quartzose sandstone, mu ddy sandstone, or siltstone containing 10% to > 50% glauconite grains phosp hatic--thin- to medium-bedded siltstone or sandstone or laminated, black si lty limestone or limestone containing phosphate nodules; and organicrich--l aminated, black limestone, marl, and mudstone, The organic carbon content of the organic-rich facies is as high as 5.45 wt %, despite the fact that the rocks are overmature and have generated oil. The mean P content of the phosphatic facies is as high as 14 wt % elemental P, Ichnofabrics are related to lithofacies and consistent with interpreted oxygen levels. Ichnofabrics also provide evidence of fluctuating oxygen le vels within the facies, especially the nonglauconitic sandstone and glaucon itic facies, The organic-rich facies and, to a lesser extent, the phosphati c facies contain abundant, pristine, disarticulated shells of the clam Halo bia, The facies, geochemistry, ichnofabrics, and taphonomy are interpreted to be related to onshore-offshore gradients in biologic productivity and re dox conditions. The facies array in the Shublik Formation is more similar t o that in modern upwelling zones than facies arrays in other well-studied u pwelling deposits. The Shublik Formation is interpreted as an upwelling-zon e deposit and can serve as an archetype of such deposits.