A Triassic Upwelling Zone: The Shublik Formation, Arctic Alaska, U.S.A.--Reply

Abstract

ABSTRACT The Shublik Formation (Triassic, North Slope, Alaska) is an organic-, phosphate-, 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, calcareous to noncalcareous sandstone or silty to muddy sandstone, fossiliferous in places; glauconitic--thin- to medium-bedded, fine, quartzose sandstone, muddy sandstone, or siltstone containing 10% to > 50% glauconite grains phosphatic--thin- to medium-bedded siltstone or sandstone or laminated, black silty limestone or limestone containing phosphate nodules; and organic-rich--laminated, 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 levels within the facies, especially the nonglauconitic sandstone and glauconitic facies. The organic-rich facies and, to a lesser extent, the phosphatic facies contain abundant, pristine, disarticulated shells of the clam Halobia. The facies, geochemistry, ichnofabrics, and taphonomy are interpreted to be related to onshore-offshore gradients in biologic productivity and redox conditions. The facies array in the Shublik Formation is more similar to that in modern upwelling zones than facies arrays in other well-studied upwelling deposits. The Shublik Formation is interpreted as an upwelling-zone deposit and can serve as an archetype of such deposits.