AN ALTERNATIVE HYPOTHESIS FOR THE DEVELOPMENT OF ORGANIC-RICH STRATA IN THE TRIASSIC ARCTIC ALASKA BASIN

The Shublik Formation is an organic-rich, phosphatic carbonate mudrock unit that was deposited during the Middle and Late Triassic on a low-angle shelf in the Arctic Alaska Basin (AAB). The age-equivalent, organic-rich chert and limestone members of the Otuk Formation were deposited in distal parts of the AAB. Today these units are found across the North Slope and in the Brooks Range of Alaska. Although traditional models for the development of the Shublik and Otuk Formations have invoked upwelling along a west-facing (Triassic coordinates) continental margin, two lines of evidence suggest an alternative to upwelling to explain all of the widespread lateral and vertical occurrences of organic-rich intervals in these rocks. First, the Triassic climate was in a greenhouse state, such that meridional temperature gradients were low and oceanic circulation was sluggish. This would have led to a lack of cold, nutrient-rich water and weak upwelling gyres. Second, the distribution of distinct zones rich in glauconite, phosphate, organic material and silica that are commonly found in upwelling settings are not well-defined in the Shublik and Otuk Formations. Rather, glauconite is mainly found in localized nearshore sandstone units, and within hardgrounds. Phosphate occurs in nearshore to middle shelf deposits, while chert occurs in middle shelf to slope deposits. Organic-rich intervals occur widely in nearshore, shelf and slope deposits, and are often interbedded at the centimeter scale with phosphatic and cherty rocks.

An alternative model is proposed wherein nutrients necessary for high primary productivity were provided, at least in part, by volcanic ash, likely from an outboard volcanic arc. Triassic-age detrital zircons in Triassic strata in northern Alaska, as well as possible thin bentonite beds in the Otuk, attest to volcanic input to the basin. Storm or weak upwelling currents may have mixed ash fall and distributed nutrients, particularly iron, around the basin for widespread productivity. This model for the development of organic-rich strata has implications for other source rocks, including those without preserved ash layers.