|
Timeline for Nanushuk-Torok deposition
Radiometric dating of zircon grains provides insights into the timing of sediment deposition and the origin of the sediments
Alan Bailey for Petroleum News
A recently published Geological Society of America research paper by geologists from the U.S. Geological Survey and the University of California describes the results of the radiometric dating of sediments within the Nanushuk and Torok formations on Alaska’s North Slope. The formations have been the focus of major North Slope oil discoveries in recent years.
Essentially the technique used involved the radiometric dating of grains of the mineral zircon within the sediments. The zircon grains would have originated in the rocks that were eroded to form the sedimentary materials in the Nanushuk and Torok formations. And, because those source rocks have distinctive age patterns, the ages of the zircons provide evidence for the origin of the Nanushuk and Torok sediments, as well as the timing of the sediment deposition.
Ancient marine basin The Nanushuk and Torok formations were deposited in a huge ancient marine basin, the Colville Basin, under what is now the North Slope. Much of the sediment appears to have originated in what is now Russia, and been deposited from west to east, across the western margin of the basin. As the basin filled, the western margin migrated eastward, with the basin shelf margin ultimately coming to a halt near the current Colville River delta. To the south, under what is now the Brooks Range foothills, the ultimate shelf margin swings to the east, while to the north, under what is now the Beaufort Sea, the margin swings west. Although the dominant sedimentation came from the west, some sediment came from the south, from the emerging Brooks Range.
The sediments that formed the Nanushuk were deposited along the upper edge of the basin from a system of river deltas. The Torok was deposited deeper in the basin.
Deposition model confirmed The zircon radiometric dating confirms this model for the Nanushuk-Torok deposition. The age ranges of zircon samples from the shelf margin have values that are characteristic of rocks found in eastern Russia, while basin floor sediments tend to hold older zircons matching the ages of rocks found in the Brooks Range.
Sediments deposited on the slope of the basin margin hold rare younger zircons that appear to originate from volcanic activity to the west at around the time of the sediment deposition. These zircons provide evidence for the ages at which the sediments were deposited. The ages are consistent with the concept of the sediments filling the basin from west to east: The oldest sediments, in the western part of the ancient basin, appear to have been deposited about 114 million years ago, while the youngest rocks, close to the ultimate basin shelf margin to the east, were deposited about 97.6 million years ago.
Moreover, the zircon dating evidence indicates that the rate of migration of the shelf margin slowed significantly at around 107 million years ago, along a zone about four-fifths of the distance from the western margin of the basin to the ultimate shelf margin to the east. The region of this slower basin margin migration is marked by distinct differences in the nature of the sedimentation, with, for example, steeply dipping shelf edges and evidence of the buildup of deltaic sediment on land.
|
