Abstract

Thick Tertiary alluvial fans on the west flank of the Beaver Dam Mountains have been dissected by headward erosion and entrenchment of tributaries of Beaver Dam Wash. This dissection oversteepened slopes and removed downslope footing from many segments of the partially consolidated fan material creating an unstable condition. In the spring of 1980 a small seep lubricated a 120 m by 70 m block of partially consolidated alluvial fan material causing it to detach along a shallow cuspate surface. Many resulting structures within the block and along the detachment surface are well exposed due to three dimensional relief provided by adjacent washes. Visible within the structure are a vertical crevasse at the head of the block, a listric detachment surface, a protruding “toe”, tear faults along the front edge of the block, listric faults that merge tangentially with the primary detachment plane both uphill and downhill, and extensional fracturing in which most fractures are oriented across the block perpendicular to the direction of movement. The extensional fractures generally have maximum horizontal and vertical offset near the middle of the block. The offset decreases toward the sides. The extensional fractures are of four types: a-horst and graben structures with associated keystone blocks, b-stair-step, reverse rotated toreva blocks, c-forward rotated blocks, and d-simple fractures throughout the block.

The slump block is composed of poorly sorted, angular to subangular clasts averaging 10 to 20 cm in diameter supported in a fine grained muddy matrix. The matrix contains a high percentage of clay which may have been derived from the Muddy Creek Formation.

Three factors, clay content, oversteepened slopes and groundwater lubrication, combined to cause the slump failure.