Chapter 10: Structural features and emplacement of surficial gravity-slide sheets, northern Idaho-Wyoming thrust belt

Abstract

Gravity-slide sheets are common in areas of sufficient topographic relief provided by extensional and compressional tectonism. They occur throughout the Basin and Range Province of Nevada and Utah but are perhaps best exposed in the thrust belt of southeast Idaho and northwest Wyoming. The Idaho-Wyoming slide sheets developed adjacent to extensional basin-bounding faults, which in turn were initiated at a ramp in the Absaroka thrust. Initial coherent sheets may have been up to 12 km 2 in aerial extent, but as a result of differential movement rates, these larger sheets were broken into smaller blocks by strike-parallel extension during emplacement. Folds on the downslope edges of the sheets suggest movement analogous to a tank tread: Through interbed shear the limestone beds rolled forward and were dragged beneath the leading edge of each sheet. It is not known whether initial emplacement was catastrophic or by creep, but undisrupted cleavages within footwall Tertiary mudstones and coherent bedding within the sheets suggest creep for at least the later phases of movement.

Catastrophic emplacement of the Heart Mountain block slide, Wyoming and Montana, USA

“…Anders et al (Fig. 1C in Anders et al, 2000) described lamination of this character from material at the bases of the Heart Mountain allochthon and two other large rockslides, the Palisades slide of Wyoming (Boyer and Hossak, 1992) and Horse Creek slide of Idaho (Beutner, 1972). We have also observed lamination at the base of a Miocene rockslide in the Halloran/Silurian Hills area of southeastern California (Bishop, 1997).…”

Sedimentology and stratigraphy of the Shadow Valley basin, eastern Mojave Desert, California

“…Many other features seen in the Shadow Valley glide blocks are similar to those of other case studies (Pierce, 1973;Braddock, 1978;Hauge, 1990;Boyer and Hossack, 1992). These include a lack of internal brecciation, no mixing, and foliated muds along the basal surface.…”

Timing and development of the Heise volcanic field, Snake River Plain, Idaho, western USA

“…Some of these blocks are kilometers in length and several tens to a few hundred of meters in thickness. Explanations for the duration and causal mechanism of the massive slide block movement are controversial (Moore et al, 1987;Anders, 1990;McCalpin et al, 1990;Boyer and Hossack, 1992). Whereas Oriel and Moore (1985) inferred that these slide masses moved from the crest and southwest fl ank of the Snake Range, they did not propose a triggering mechanism for such large detachments.…”

“…Whereas Oriel and Moore (1985) inferred that these slide masses moved from the crest and southwest fl ank of the Snake Range, they did not propose a triggering mechanism for such large detachments. Boyer and Hossack (1992) inferred that these slide blocks were emplaced over a relatively long time and "crept" into place. They interpreted this from lithologic characteristics within various slide blocks mostly along the Grand Valley Fault and the contact between them and the underlying blocks.…”