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Abstract

The Dinkum graben system beneath the central to eastern Beaufort shelf of Arctic Alaska comprises a complex of grabens and horsts that records multiple phases of extension and contraction spanning the Mississippian through Early Cretaceous (Neocomian). The graben system extends from about 150°W eastward for more than 200 km, approximately parallel to the Alaska Beaufort Sea coast. The eastern extent of the graben system (east of about 145.5°W) is masked by deep burial beneath Cenozoic strata and by complex Cenozoic structures. The graben system developed above regional basement that includes the pre-Mississippian Franklinian sequence, interpreted as Late Proterozoic-Devonian strata deformed and metamorphosed during the Ellesmerian orogeny. Franklinian rocks display in seismic data a range of variably dipping structural and metamorphic fabrics described in a companion abstract (Connors and Houseknecht).

Previously published interpretations of the Din-kum graben suggest two phases of extension related to rift opening of the Amerasia basin, a Jurassic phase characterized by generally south-dipping normal faults and an Early Cretaceous phase characterized by generally north-dipping normal faults. However, our interpretation of 2D seismic data, tied to well control near the coast and potential fields data across the Beaufort shelf, documents a geologic history commencing with Mississippian extension accommodated by both north- and south-dipping normal faults detached along the variably dipping basement fabrics. Growth strata indicate that pulses of extension in the graben system occurred during the Mississippian, Late Triassic–Early Jurassic, and Neocomian. Further, certain faults accommodate Mid-Late Jurassic growth strata that grade from positive to negative growth along strike, suggesting inversion of older structures, perhaps by stresses oblique to older fault planes. This polyphase deformational history is reflected in a complex graben system that accommodates Mississippian-Neocomian strata at least 5 km thick in places.

The newly recognized presence of pre-Jurassic strata in the Dinkum graben system has significant implications regarding petroleum systems. Upper Triassic growth strata likely include oil-prone source rocks in the Shublik Formation, corroborated along the southern margin of the graben by oil accumulations (e.g., Northstar) with implausible migration pathways from sources to the south, and by chemistry that suggests a Triassic source rock containing more detrital components and less carbonate than typical Shublik of the North Slope. Considering the timing of extensional pulses discussed above, the presence of Lower Jurassic and Neocomian source rocks also is likely. Although all these source rocks likely are thermally overmature in deeper parts of the graben, shallower parts of the graben, horsts within the graben, and southern and northern margins of the graben may be in the oil window, and may have been charged from the graben.

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