ANALYSIS OF MECHANICAL STRATIGRAPHY AND STRUCTURAL ARCHITECTURE OF THE GILEAD CREEK AREA, NORTHEASTERN BROOKS RANGE, ALASKA

The Gilead Creek along the northeastern Brooks Range front exposes the transition from the frontal Brooks Range to its foothills and reveals structures formed by shortening in two main directions (NNE-SSW and NE-SW). Different structures formed at different levels depending on the mechanical stratigraphy. The youngest structure in the study area is probably the WSW-plunging Echooka anticlinorium, which is the largest (first order, ((abbreviated 1°))) structure in the area. The Echooka anticlinorium is interpreted to be a fault-bend fold in pre-Mississippian basement that formed the NE-trending local range front. A detachment in Mississippian Kayak Shale separates basement from overlying 2° detachment folds in the competent Mississippian-Pennsylvanian Lisburne Group and Permian Echooka Formation. The Echooka Formation is overlain by a detachment in the Kavik (shale) Member of the Triassic Ivishak Formation. The base of the Echooka Formation is shale and can serve as a minor detachment surface. ENE-trending 3° folds to the north and NNW-dipping back-thrusts to the south formed in the Triassic Ledge Sandstone and the Shublik Formation. Up-section is the Jurassic to Lower Cretaceous Kingak Shale, which serves as a detachment interval between the Shublik and mid-Cretaceous Gilead sandstone. ENE-trending 3° folds in the Triassic Ledge Sandstone and Shublik Formation directly underlie the north limb of the 2° Gilead syncline and N and NNW-dipping back-thrusts underlie the south limb of the 2° Gilead syncline. To the northwest, NE-trending 3° and 4° folds and local thrust faults are superposed on the north limb of the Gilead syncline and continue to the NW in Gilead sandstone and overlying upper Cretaceous units (mainly Seabee Formation) of alternating sandstone and shale. The Gilead sandstone is juxtaposed against the Turonian Seabee Formation via a relatively large displacement SE dipping thrust fault. The Hue Shale provides a detachment interval between the Gilead and lower Seabee Formation, which allows folding of shorter wavelength and higher frequency (higher °) in the lower Seabee Formation. 4° folds upper Seabee Formation are separated from 3° folds in the lower Seabee Formation by an inferred detachment interval that allows the upper and lower Seabee Formations to fold independently of each other.