Sackungen at the Aspen Highlands Ski Area, Pitkin County, Colorado

Abstract
Numerous linear geomorphic features of suspected gravitational spreading origin, collectively termed sackungen, occur at elevations of 3,475–3,775 m at the head of the Aspen Highlands Ski Area near Aspen, Colorado. Features include double-crested ridges, upslope-facing scarps, linear troughs, and downslope-facing scarps. Most features are aligned with the prominent joint set (strike N25°–45°E, dip 80°–90° NW) that parallels the flanking, 1,150-m deep U-shaped glacial valleys of Castle and Maroon creeks. A backhoe trench 6 m long and 2 m deep was excavated across an upslope-facing scarp and adjacent trough to reveal the style and age of surface deformation. Sandstone bedrock of the Maroon Formation was extensively fractured under the scarp face, with abundant dilational voids. Hillslope colluvium was truncated at the main sackung fault plane, and was overlain by up to 1.2 m of fine-grained sag pond sediments. Seven buried soils were present in the trough fill, composed of A, and E or Bw horizons; A horizons at 35 cm, 84 cm, and 98 cm below the surface yield uncorrected 14C ages of 3060+150, 6290+70, and 7910+80 14C yrs B.P., respectively. Extrapolation of deposition rates to the base of the fill indicates that the trough formed ca. 11–11.5 ka, several thousand years after glacial ice had melted from the flanking glacial valleys. Trench stratigraphy suggests that slow vertical offset across the trough has continued since 11 ka, at rates of 0.14–0.75 mm/yr, perhaps accompanied by horizontal spreading at 0.43 mm/yr in the past 3 ka. This rate of spreading is slower than the 1–5 mm/yr measured by Varnes and others (1990) across sackungen in the Sawatch Range to the east, but still constitutes a constraint on foundation design for buildings and lift towers.

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