Dinkum Graben System Beneath the Alaska Beaufort Shelf — Evidence for Mississippian to Early Cretaceous Extension, and Petroleum Systems Implications

Abstract

The Dinkum graben system beneath the central to eastern Beaufort shelf of Arctic Alaska comprises numerous grabens and horsts that record multiple phases of extension and contraction spanning the Mississippian to Early Cretaceous (Neocomian). The graben system extends from ∼150°W east for more than 200 km, and its eastern extent is masked by Cenozoic stratigraphic and structural burial. The graben system developed above Franklinian acoustic basement, interpreted as Neoproterozoic to Devonian strata deformed and metamorphosed during the Devonian Ellesmerian orogeny. Franklinian rocks display variably dipping structural and metamorphic fabrics in seismic data. Published interpretations of the Dinkum graben suggest two phases of extension (Jurassic and Neocomian) related to rift opening of the Canada Basin. However, our interpretation of higher resolution 2-D seismic data, tied to well control near the coast and potential fields data across the shelf, documents a more complex geological history. Mississippian extension was accommodated by both north- and south-dipping normal faults detached along basement fabrics. Growth strata indicate that pulses of extension occurred during the Mississippian, Late Triassic–Early Jurassic, and Neocomian, and that reactivation of older faults was common. Further, certain faults accommodate Middle to Upper Jurassic strata that grade from positive to negative growth along strike, suggesting oblique reactivation of older structures. This polyphase deformational history is reflected in a complex graben system that accommodates Mississippian to Neocomian strata at least 5 km thick in places. The newly recognized presence of pre-Jurassic strata in the Dinkum graben has significant petroleum-systems implications. Upper Triassic growth strata likely include oil-prone source intervals within the Shublik Formation, corroborated along the southern margin of the graben by oil accumulations (e.g., Northstar) with implausible migration pathways from sources south of the Barrow arch, and with oil chemistry suggesting a Triassic source rock more clay-rich than typical calcareous Shublik of the North Slope. Source rock intervals also may be present within Jurassic and Neocomian growth strata within the graben. Although all these source rocks likely are thermally over-mature in deeper parts of the graben, during burial they may have charged positive areas that remain in the oil window within and along the margins of the graben.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016