ELLESMERIAN OROGEN BENEATH THE ALASKA BEAUFORT SHELF

Reflection seismic data and sparse well control reveal that the central and eastern Beaufort shelf of Arctic Alaska includes a record of the Devonian – Early Mississippian Ellesmerian orogeny. Along the coast, the pre-Mississippian Franklinian sequence, comprising sedimentary and metasedimentary rocks, displays generally north-dipping fabrics and hints of south-vergent thrust faulting. Beneath the mid to outer Beaufort shelf, Franklinian rocks display stratified, mainly concordant, south-dipping seismic reflectors that flatten at depth and are cut by low-angle faults. We interpret this seismic character and geometry as a north-vergent fold-and-thrust belt, which appears to have detached in the upper crust. Locally, this fold-and-thrust belt displays relict valley-and-ridge topography with more than 1,000 meters of relief, buried by Mesozoic and Cenozoic syn-rift and post-rift strata. Structural fabric in the Franklinian rocks displays maximum apparent dips of 25–30° for reflectors in horses within the northern fold-and-thrust belt and 30–45° for reflectors in the coastal domain. Although the relationship between these tectonic domains is poorly defined because the rocks lie near the limit of seismic resolution, we interpret them as part of the “Ellesmerian Mountains,” which shed a large volume of sediment to both Arctic Canada and Alaska during the Devonian and Early Mississippian.

Franklinian strata are overlain in angular unconformity by Mesozoic syn-rift strata (Beaufortian sequence) or relict Mississippian – Permian pre-rift strata (lower Ellesmerian sequence) in the Dinkum graben, and by post-rift Cretaceous or Cenozoic strata (Brookian sequence) outside the graben. The geometry and dimensions of Dinkum grabens and horsts were influenced by the Franklinian fabric, as many of the boundary normal faults detach along the metasedimentary foliation (relict bedding?) or primary stratification. Beneath the eastern shelf, where the Paleogene to recent Brooks Range tectonic front deforms the sediment prism, Ellesmerian tectonic fabrics also may have influenced deformation. There, syn-rift strata are inverted by thrust faults that break through southern margins of grabens, detach near the Franklinian unconformity, and ramp above south-dipping normal faults at northern margins of grabens.