Ellesmerian Orogenic Structures Beneath Syn-Rift and Post-Rift Strata, Beaufort Shelf and Barrow Arch of Alaska

Abstract

The Beaufort Sea margin of Alaska, formed by Mississippian – Mesozoic crustal extension and Mesozoic – Cenozoic subsidence, displays a thick sediment prism that obscures the stratigraphy and structure of underlying basement rocks. 100 well penetrations spanning the Barrow arch, a long-lived regional structural high located in the coastal plain and innermost Beaufort shelf, along with exposures in the northeast Brooks Range document a pre-Mississippian basement comprising sedimentary and metasedimentary rocks of the Franklinian sequence, deformed by a Late Devonian – Early Mississippian phase of the Ellesmerian orogeny. Reprocessed industry seismic data reveal that Mississippian – Mesozoic strata beneath the outer Beaufort shelf are underlain by well-stratified, mainly concordant, south-dipping reflectors that flatten at depth and are cut by low-angle faults. We interpret this seismic character and geometry as Franklinian strata in a north-vergent fold-and-thrust belt, which appears to have detached in the upper crust during the Ellesmerian orogeny. Franklinian strata in the fold-and-thrust belt are overlain in angular unconformity by Mesozoic syn-rift strata of the Dinkum graben, or by relict Ellesmerian strata beneath syn-rift strata. We interpret this Franklin reflective package as Lower – Middle Devonian clastics based on seismic similarities with strata in the M'Clure Strait of the Canadian Arctic Islands (CAI). Tectonic vergence and structural style are consistent with the notion of the Beaufort Sea margin of Alaska being adjacent to the CAI in Late Devonian time. More speculatively, we propose that the Barrow arch embodies the vestige of an Ellesmerian anticlinorium or imbricate significantly modified by later tectonism. The Franklinian fold-and-thrust belt beneath the outer Beaufort shelf may have formed a significant highland with substantial irregular relief, as a relict valley-and-ridge topography buried by younger strata. The belt was a possible sediment source for Mississippian clastic strata across Arctic Alaska. Relief on the fold-and-thrust belt appears to have been rejuvenated during the Permian or Early Triassic, likely providing a proximal provenance area for reservoirs associated with the Prudhoe Bay depocenter just 50 km to the south, where the Lower Triassic Ivishak Sandstone is thicker and more coarser-grained than anywhere in Arctic Alaska.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016