LONG-TERM MEASUREMENT OF RIDGE-SPREADING MOVEMENTS (SACKUNGEN) AT BALD EAGLE MOUNTAIN, CENTRAL COLORADO

Gravitational spreading of mountain ridges has been a topic of interest to geologists since the 1960s, but measurements of spreading movement are rare. Herein, we report on movements measured over 38 years on the northwest side of a broad ridge that extends southwestward from Bald Eagle Mountain in the Sawatch Range. Geomorphic evidence for movement consists of a near ridge-top graben and three trenches with uphill-facing scarps between 2- and 4-m high. The orientations of features indicate that the jointed Precambrian granitic rock underlying the ridge has spread northwestward toward the formerly glaciated Busk Creek valley. The timing of Pinedale deglaciation in the Busk Creek valley is constrained between 13.9 ka and 12.6 ka by recent Beryllium-10 dating.

Spreading movement was measured between the ridge crest and the upslope-most glacial trim line in Busk Creek valley through repeated surveys of a network of points. This network was initially established by USGS field crews led by David J. Varnes in 1975, with additional survey points added in succeeding years. Field surveys were conducted in the summers of 1975, 1977, 1982, 1984, 1989, 1997, 1999, 2012, and 2013. From 1975 to 1989, measurements were made with a theodolite and an electronic distance measuring device. From 1997 to 2013, measurements were made using dual frequency GPS receivers and static GPS surveying techniques. All measurements were relative to a stable survey point at the crest of the ridge. Stability of this point was confirmed by repeated measurements to a survey point southeast of the ridge crest.

Results from repeat surveys indicate that the entire mountainside, from the graben to the trim line, moved episodically during the monitoring period. The area near the graben moved between 1977 and 1982, and between 1999 and 2012. The area near the trim line moved between 1977 and 1989, and between 1999 and 2013. For the entire monitoring period, the average rate of horizontal movement was 1.0 mm/yr near the graben, and 3.6 mm/yr near the trim line. If we assume that the onset of movement coincided with Pinedale deglaciation, then present-day movement rates and the age range for deglaciation yield cumulative horizontal displacements of 12-14 m near the graben, and 45-50 m near the trim line. These displacements are consistent with the sizes of the graben and trenches.