REDUCTION PROCEDURES AND IMPLICATIONS OF A HIGH-RESOLUTION GRAVITY TRAVERSE IN THE BROOKS RANGE FOOTHILLS, ALASKA

Understanding the structural transition between the Brooks Range and the North Slope coastal plain of Alaska is critical to hydrocarbon assessment in the region. We are studying this transition in the Chandler Lake Quadrangle just north of Anaktuvuk Pass. Surface exposures along Tiglukpuk Creek and Siksikpuk River provide important constraints on structural models. To the south, Mississippian carbonates to Triassic siliciclastic rocks form the Endicott Mountains Allochthon (EMA). The EMA is overlain by rocks previously assigned to the Ipnavik River Allochothon (IRA), including Cretaceous turbidites of the Okpikruak Formation that commonly contain blocks of diverse Mississippian to Cretaceous rocks. Structurally, these allochthons are in stacked thrust sheets that are folded in surface exposures to the south. Cretaceous shale, sand, and conglomerate of the Fortress Mountain, Torok and Nanushuk Formations are present atop the EMA thrust sheets to the north. Hydrocarbon potential in the area depends upon the thickness and structure of these overlying siliciclastic units and the geometry and northward extent of the underlying EMA and IRA wedge.

We collected high-resolution gravity data to assist with structural modeling along this transect. Since the modest geophysical and petrophysical data available in the area suggest little density contrast between the various geologic units, great care was taken during gravity data acquisition and processing to minimize uncertainty and to allow differentiation of the subtle density variations in the subsurface. Elevation was controlled by a survey-grade Differential Global Positioning System (DGPS) and post-processing of the DGPS data reports less than a 3 cm error in height. Relative gravity measurements with a 500 m station separation were made using three different gravity meters so that the amount of instrument error associated with each station could be determined. Constraints were placed on the gravity modeling based upon surface sample and outcrop data, seismic horizon picks and well log data. Preliminary models suggest that the IRA and EMA extend significantly farther north than they are mapped at the surface. This result has implications for the extent of possible hydrocarbon (gas-prone) reservoir structures within the central North Slope foothills belt.