Alaska’s ‘Sleeping Lady’
Every year countless Alaskans and visitors gaze across Cook Inlet from Anchorage at the “Sleeping Lady” silhouette of Mount Susitna. This scenic mountain and the more rounded Beluga Mountain to the northwest rise up steeply from the adjacent lowlands of the Susitna basin where the Yentna and Susitna rivers meander on their way to the Inlet.
The geologic story to explain this low-lying basin in the midst of an otherwise mountainous terrain (the high Talkeetna Mountains to the north and portions of the Western Alaska Range to the west and south) has been difficult to tell. Conventional wisdom assumes a northeast-dipping fault (a “normal” fault) between the Mount Susitna/Beluga Mountain front and the Susitna basin.
This study by R.W. Saltus and colleagues of gravity, seismic, and magnetic data overturns this conventional interpretation and shows that the bounding fault instead dips to the southwest (beneath the mountains); it is a thrust fault. The existence of the Beluga Mountain thrust fault was previously postulated by an Alaskan geophysicist named Steve Hackett in the late 1970s. This work by Saltus and colleagues proves that he was correct.
This changes the current understanding of this structure, and of the recent geologic evolution of the mountains and adjacent basin that are such familiar parts of the local landscape.