STRUCTURE OF HANNA TROUGH AND FACIES OF ELLESMERIAN SEQUENCE, U.S. CHUKCHI SHELF, ALASKA

Hanna trough is a rift basin of probable Devonian to Late Jurassic age that trends north beneath the U.S. Chukchi shelf offshore northwestern Alaska. The basin formed a depocenter for the accumulation of over 38,000 feet (11,600 m) of strata of the Ellesmerian sequence and is a westward extension of the petroleum-rich Arctic Alaska basin that underlies the Alaska North Slope. Hanna trough overlies a seismic basement that appears to consist of Devonian and older terranes that were assembled into a crustal unit with a northerly structural grain in pre-Middle or Late Devonian time. A magnetic high may mark a magmatic arc within basement that apparently contributed granitic sedimentary debris found within Mississippian and Pennsylvanian calcareous sandstones beneath Chukchi shelf.

Hanna trough rift structures also developed along northerly lines that mimic the northerly grain of basement. The fundamental structural units of Paleozoic rifting, and the sites of earliest sedimentation, are half-grabens that pervasively floor Hanna trough. The half-grabens are floored by wedge-shaped bodies of strata that are correlated by seismic inference to the Endicott Group (Upper Devonian to Mississippian). Fault-driven subsidence occurred mostly from Devonian (inferred) to Permian time and controlled deposition of over 36,000 feet (11,000 m) of strata of the Lower Ellesmerian sequence. From Late Permian to Late Jurassic time, a “sag” phase of subsidence controlled the deposition of up to 12,000 feet (3,660 m) of strata of the Upper Ellesmerian sequence in Hanna trough.

The oldest rocks penetrated by wells on Chukchi shelf are Upper Mississippian rocks equivalent to the Lisburne Group. The lithofacies and petrology of clastic rocks suggest the existence of highland sediment sources both east and west of Hanna trough for Mississippian through Permian sequences. The western highland (Chukchi platform) was subdued or absent by the time of deposition of the mostly basinal rocks of Triassic and younger ages in western Hanna trough. The eastern highland sediment source (Arctic platform) remained active through Late Jurassic time. Within the Upper Ellesmerian sequence (Late Permian to Late Jurassic), facies transitions mimic the transitions in time-equivalent strata from north (margin) to south (deep basin) in the Arctic Alaska basin.