Stratigraphic and structural investigations in the Ivishak River and Gilead Creek areas: Progress during 2007

Abstract

Contractional deformation of foreland basin deposits adjacent to the northeast-southwest-trending mountain front in the east-central Brooks Range foothills exposes rock units of Albian to Late Cretaceous age that are largely concealed in the subsurface to the west and removed by erosion to the east. The informally named Gilead sandstone and overlying units in this area provide valuable insights into depositional systems active during the early and middle phases of foreland basin evolution, as well as an opportunity to study their responses to subsequent Brookian deformation. Systematic traverses of the Ivishak River, Gilead syncline, and Gilead Creek yield ubiquitous evidence of sediment gravity flow depositional processes in both the highly amalgamated sandstones and the intervening, finer-grained and thinner-bedded intervals throughout the Gilead sandstone, Seabee-equivalent strata, and the overlying Canning Formation. The amalgamated sandstone facies association of the Gilead succession contains many characteristics of hyperpycnal flow deposits, and submarine channels are locally preserved. Much of the unit may represent east-or northeast-flowing axial foredeep channel belt deposits. Sediment gravity flow sandstones in the upper parts of the Gilead unit that contain possible wave-influenced sedimentary structures may have been deposited south of the foredeep in shallower orogenic wedge-top, upper slope, or ramp settings. The Gilead sandstone has previously been interpreted to belong to a far-traveled allochthon, but we have found no compelling structural evidence to support this interpretation. Instead, observations appear consistent with lower Brookian strata at Gilead syncline forming the passive roof of a triangle zone duplex containing structurally thickened Valanginian and older strata. The Gilead sandstone itself is more strongly deformed on strike to the west at the Ivishak River. Transverse tear and rotational faults may account for the alongstrike compartmentalization of deformation. These preliminary interpretations require further scrutiny and the incorporation of pending analytical results from samples collected during these investigations.