MESOZOIC EVOLUTION OF THE DINKUM GRABEN, ALASKA BEAUFORT SHELF, AND PETROLEUM SYSTEMS IMPLICATIONS

The Mesozoic Dinkum graben of the Alaska Beaufort shelf developed above pre-Mississippian Franklinian “basement” rocks, which display in seismic data dipping structural fabrics. The graben extends from about 150°W eastward more than 200 km. The eastern extent of the graben is masked by thick Cenozoic strata and complex Cenozoic structures. The Dinkum plateau north of the graben includes in Franklinian strata a north-vergent thrust belt, whose relief was rejuvenated by Mesozoic extension, and perhaps by contraction and thermal bulging. As a result, the plateau includes exhumed early syn-rift grabens and relict valley-and-ridge topography beneath post-rift strata.

The earliest phase of Mesozoic extension on the Beaufort shelf includes bounding normal faults that display opposing dip into the graben, and that mainly detach in Franklinian fabrics. Ties to well control near the coast reveal that normal faults accommodate Upper Triassic growth strata, indicating earlier extension than literature suggests. Growth strata include petroleum source rocks in the Shublik Formation, an interpretation likely corroborated by oil accumulations (e.g., Northstar and Sandpiper) with implausible migration pathways from North Slope sources and whose chemistry suggests a clay-rich Triassic source rock. Seismic data also indicate the local preservation of sub-Upper Triassic strata, which may include Mississippian through Lower Triassic rocks of the Endicott, Lisburne, and Sadlerochit Groups.

Subsequent phases of extension are expressed mainly as north-dipping normal faults, which accommodate at least two intervals of growth strata, Lower Jurassic and lowermost Cretaceous. These strata are thickest near the graben axis, from which they thin northward in sag-basin geometry. Both intervals on the North Slope include oil-prone source rocks, indicating the potential for similar source rocks in the graben.

The presence of Upper Triassic, and perhaps Jurassic and Cretaceous, source rocks in the Dinkum graben is significant for reconstructing petroleum systems. Although these source rocks likely are thermally overmature in deeper parts of the graben, they may remain in the oil window in shallower parts of the graben system, including subsidiary horsts and graben margins, and may have charged shallower traps in younger strata.