Alaska714

Alaska Offshore

Kaktovik-Yukon Offshore

It is admitted by most geoscientists who have worked in this area that the rifting (formation of rift-type basin filled by pre-Brookian sediments) and the onset of the Brookian orogeny formed three major tectonic features of the northern Alaska margin : the Hinge Line, the Barrow Arch, and the Colville geographic basin. The hinge line, which, often, appears on seismic data as a zone of down-to-the-north normal faulting system, marks the point at which the seaward slope of the basement surface increases markedly into the Canada geographic basin Basin (Grantz and May, 1983). The divergent Atlantic-type margin (Brookian strata) developed northward of the rift-shoulder over a basement and/or the oceanic crust, since the breakup of the Laurentia continental crust, which seems to have occurred at Lower Cretaceous. Rift-type basins were created during the lengthening of the Franklinian sequence and filled by pre-Brookian sediments. The transgressive phase of the Atlantic-type margin is mainly of Cretaceous age, while the regressive phase last since Paleocene.

On this Canvas autotrace of a seismic line of the northern Alaska margin, not only the Atlantic-type divergent margin (Brookian strata) is, easily recognized, but the breakup unconformity as well. Below the breakup unconformity, a rift-type basin, filled by pre-Brookian strata, and the Franklinian basement are visible on the lower left corner of the autotrace. The Brookian clastic wedge prograded northward and eastward from Late Cretaceous through Tertiary time, filling the basins formed by post-rifting subsidence on the north side of the Barrow Arch (Scheer J. et al., 1953).

On this Canvas autotrace, the Jurassic-Cretaceous Kaktovik geographic basin is well characterized by an intense shalokinesis (a form of shale tectonics, in which shales, generally undercompated, flow by gravity, i.e., by release of gravity potential energy alone, in absence of significant tectonic stress, synonym of argilokinesis). The Kaktovik geographic basin corresponds to a widened continental shelf southward of the oceanic crust realm. Its sedimentary column can reach more than 10 km and local geographic sub-basin, as Demarcation geographic sub-basin, are filled with Tertiary strata, which thickness can reach 7 kilometers.