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Stratigraphic Reanalysis of Seismic Data of Northern Brooks Range Alaska: Comparison of Unit Volumes and Slopes


2D seismic lines from the U S Geological Survey (USGS) public database onshore and offshore northern Alaska were used to study the stratigraphic evolution of the Lower Cretaceous Brookian sequence, which is composed of thick prograding clastic deposits that originated from the ancestral Brooks Range thrust. In particular this study focused on the Torok and Nanushuk Formations of Lower Cretaceous age which are well-recognized for large-scale clinoforms with depositional relief on the order of 400 to 600 m. For the onshore National Petroleum Reserve Alaska (NPRA) we utilized approximately 5700 line km of seismic data from the 1974-1981 vintages which was post-stack reprocessed in 2000. The offshore seismic data utilized were mainly from the 1977-1987 Beaufort Sea vintages, with a total seismic line length of approximately 4500 km. Eighteen progradational surfaces were interpreted and mapped in TWT and then converted to depth. The depth surfaces were used to calculate the present-day volumes of individual prograding sedimentary packages, which were then decompacted in order to estimate the original sediment volume. Ongoing efforts to improve the velocity model will allow the calculation of more accurate volumes. The strike orientation of Torok clinoforms is generally N-S but the volume of each clinoform varies; they are smaller in the northern part and increase gradually southward, reflecting changes in accommodation from the rift shoulder into the foredeep part of the basin. Previous workers have reported that average slopes vary from 0.6 to 2.5 degrees with maximum dips of 10 degrees. The mapped clinoform surfaces show an eastward- northeastward progradational direction, and a geometry that suggest that more sediment volume was deposited on the slope than on the shelf. Understanding the relationship between clinoform sediment volume and steepness of slope is imperative for the recognition of basin evolution, asymmetry and tectonic processes. The NPRA area provides a unique setting for the calculation and comparison of sedimentary volumes and depositional slopes and will allow meaningful comparisons to other basins.