REANALYSIS OF GRAVITY DATA FOR THE ARCTIC NATIONAL WILDLIFE REFUGE, NORTHEASTERN ALASKA

PHILLIPS, Jeffrey D.¹, POTTER, Christopher J.², and SALTUS, Richard W.¹, (1) U.S. Geol Survey, Mail Stop 964 - Box 25046, Denver Federal Center, Denver, CO 80225-0046, [email protected], (2) U.S. Geological Survey, Mail Stop 939, Denver Federal Center, Denver, CO 80225-0046

A reexamination of the gravity field for the coastal plain of the Arctic National Wildlife Refuge, Alaska, using a complete, proprietary data set and trend-enhanced gridding, will contribute to a new structural interpretation. Previous interpretations based on incomplete gravity data, aeromagnetic data, and seismic reflection profiles were used to estimate the size and volume of possible oil-bearing structures and the extent of shallow faulting.

Proprietary gravity data located at approximately 250 m intervals along seismic lines spaced approximately 5 km apart were gridded using a bi-directional algorithm with enhancement along the N65°E trend of the largest known shallow structure, the Marsh Creek anticline. The trend-enhanced gridding improves resolution of short-wavelength features parallel to the trend direction. The gridded data were bandpass-filtered based on a power-spectral model consisting of three equivalent density layers – a shallow thin layer, an intermediate-depth thin layer, and a deeper half-space. The short-wavelength anomalies produced by the shallow layer most likely represent the densities of the folded and faulted sedimentary beds truncated at the Pleistocene erosion surface.

The intermediate-wavelength anomalies produced by the middle layer most likely represent the density variations produced by the shallow anticlines, synclines, and thrust faults. Due to compaction, density should generally increase with depth within the sedimentary section. This implies that the cores of anticlines should be of higher density than the cores of synclines, and that anticlines should produce gravity highs and synclines should produce gravity lows. In the intermediate-bandpass data, this relationship holds for the Marsh Creek anticline, but the opposite relationship is found for one anticline, the Aichilik high, which likely has a mudstone-rich core. Increasing density with depth also implies that thrust faults should exhibit gravity highs over their hanging walls.

The long-wavelength anomalies produced by the deep equivalent half-space are interpreted to be the combined effects of the basin thickness and the basement density variations. Separating these two effects could lead to identification of basement structures representing deeper exploration targets.