LATE JURASSIC – EARLY CRETACEOUS STRUCTURAL INVERSION AND SYNTECTONIC, PETROLEUM-PROSPECTIVE STRATA ON THE CHUKCHI SHELF, ARCTIC ALASKA

The focus of exploration on the Chukchi Shelf is a petroleum system whose objective reservoir occurs in Oxfordian–Valanginian strata coeval to the upper Kingak, Miluveach, and Kuparuk formations on the Alaska North Slope. Interpretation of 2-D seismic data and sparse well control suggests that the Oxfordian–Valanginian succession across the central and southern Chukchi Shelf is a syntectonic clinothem spawned by structural inversion of older extensional faults. Pertinent structural elements include (1) Chukchi Platform, a long-lived and high-standing block of pre-Mississippian and older rocks beneath the western shelf; (2) Hanna Trough, a Late Paleozoic failed rift that bisects the shelf from north to south; (3) Alaska rift shoulder (Barrow arch), a Late Jurassic–Early Cretaceous uplift parallel to the northeast shelf edge; and (4) Herald arch, a Cretaceous fold-thrust belt that trends northwestward beneath the southern shelf.

Oxfordian–Valanginian strata of the North Slope were deposited during rift-shoulder uplift related to opening of the Amerasia Basin, and the same appears to be true of coeval strata on the northern Chukchi Shelf. This succession displays southwest-dipping clinoforms with relict shelf margins that trend northwestward. In contrast, coeval strata beneath the central and southern Chukchi Shelf display eastward growth across a series of en-echelon, north-south-oriented reverse or transpressional faults, which display up to 1 km of structural relief and root into older normal faults associated with the Hanna Trough. This succession in the high-accommodation area east of the reverse faults displays east-dipping clinoforms that define relict shelf margins that trend north-south, suggesting sediment sources on the Chukchi Platform and inverted structural blocks. The two contrasting shelf-margin trends merge on the east-central Chukchi Shelf to form a southeast-facing embayed shelf-margin.

The en-echelon reverse faults on the central Chukchi Shelf are localized along the western margin of the Hanna Trough and, to the south, step eastward towards the center of the trough. This pattern is approximately parallel to, and 50–70 km in front of, the Herald arch. This relationship may suggest that structural inversion was an early phase of tectonic contraction related to the Herald arch.