ABSTRACT:

The Boise Valley contains several columnar jointed basalt cliffs, which were deposited approximately 1.4 to 0.5 Ma on terraces formed by downcutting of the Boise River. Three runout talus deposits on Whitney Terrace were characterized using unmanned aerial vehicle visual imagery. Although the runout talus deposits were from different areas and were of varying size, they contained roughly the same dimensions and distributions of blocks. Images of the cliff face indicated that blocks were detached from the base of columns along horizontal discontinuities which lacked support (undercut columns) and by toppling of basalt columns. The mapped block sizes in the cliff face were larger than the blocks in the associated runout, indicating the cliff blocks were fragmented during impacts in the runout.

1. INTRODUCTION

The movement of geologic materials downslope, commonly referred to as landslides, is one of the most well-known geologic hazards. Varnes (1978) developed the most widely used classification framework for landslides. Since the Varnes classification scheme was developed, various modifications have been proposed and adopted. Still, the goal is to be able to describe the movement(s) and the end result(s) of the landslide using well-known terminology which incorporates the focus of the investigators (Hungr et al., 2014).

Our focus is to characterize the runout talus deposits formed from the dislodgement and subsequent downslope movement of rock blocks from columnar basalt cliffs. Columnar basalt, or specifically columnar jointing in basalt, is a type of rock mass that is divided into long prismatic blocks. The formation of the jointing is complex and thought to be a series of events rather than simple cooling of the lava. The vertical discontinuities are continuous and horizontal discontinuities are less prominent and generally end at the edges of the vertical discontinuities (Spry, 1962).

Failures of rock masses with columnar jointing have been studied in several geographical locations, including Australia (Dahlhaus and Miner, 2000), Chile (Holm and Jakob, 2009), Spain (Abellán et al., 2011), and Washington State (Guzek, 2019). The failure mechanism most often reported in these studies has been the somewhat generic term "rockfall", even though the studies mentioned above have shown that two failure (detachment) modes occur, rockfalls and rock topples.

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