Geology Cafe.com
Blackhawk Landslide, Lucerne Valley, California
Blackhawk Landslide

Certainly the largest slide in the Transverse Range province is the Blackhawk, on the north slope of the San Bernardino Mountains. This prehistoric slide is one of the largest known in North America. It was studied in detail by R. L. Shreve, who showed that the slide moved to it's resting place on a cushion of compressed air. (This mechanism has since been recognized as applicable to other slides.) The end of the slide can be seen from State Highway 247, about 10 miles east of Lucerne Valley; the only satisfactory way to see the entire slide is from the air. The Blackhawk slide is 5 miles long, about 2 miles wide, and 30-100 feet thick. It is a tongue like sheet of brecciated Pennsylvanian Furnace Limestone derived from Blackhawk mountain about 4,000 feet above. In the source area, the Furnace Limestone has been thrust northward over uncemented sandstone and weathered gneiss that subsequently were eroded away, leaving a precipitous slope. Once the softer rocks were undermined, presumably during a wet period about 17,000 years ago, a mass of limestone breccia collapsed and slipped rapidly into upper Blackhawk Canyon, forming a stream of rubble about 2,000 feet wide and 300-400 feet deep. As the slide moved down the canyon (at about 170 mph), it passed over a resistant gneissic ridge that crosses the canyon, and was thus launched into the air-a geologic version of a flying carpet. Calculations indicate that sheet of moving breccia was probably as high as 400 feet above the canyon floor immediately after becoming airborne, but that it settled quickly, compressing the air trapped beneath to a frictionless blanket less than a meter thick. While airborne, the slide possibly attained velocities of 270 mph, and the entire distance from launching point to resting place were covered in about 80 seconds. These values are based on a consideration of local geometry and are consistent with the behavior of similar slides observed during formation. As the slide spread over the desert floor, the air cushion became thinner, permitting the slide to settle. A characteristic of such slides is the presence of large blocks that, although badly shattered, have fragments that retain their original orientation to one another-much like a jigsaw puzzle with pieces pulled slightly apart. This feature supports the view that carpet like sheets of rock can be moved almost intact on cushions of compressed air. Shortly after the slide occurred, small ponds developed in depressions on it's surface and one of these has yielded fresh-water mollusk shells that give a radiocarbon age of 17,400 years. Because the pond sediments are composed largely of materials pulverized during the slide and covered with different, probably windblown materials, the ponds are probably only slightly younger that the actual age of the slide.

The Blackhawk landslide is a granular slide that sourced in the San Bernardino Mountains. The runout zone is approximately 3 km wide by 10 km long. The runout is particularly long compared to the drop relief. The landslide debris is over 95% marble and limestone, with localized areas of gneiss and sandstone boulders. Because of the accessibility to the landslide and the debate over the landslide mechanics of large volume slides, the Blackhawk Landslide is a common stop of geology field trips in the Mojave Desert.

Stout (1976) dated the landslide using radiocarbon on freshwater pelecypod and gastropod shells. Stout determined an age of 17,600 ± 600 ybp. There are two possible problems with this date. 1) The shells were from a calcareous mudstone layer in old pond sediments. Thus, the pelecypods and gastropods could have incorporated a significant amount of dead carbon, which would make the estimated age older than the true age. 2) The shells were picked from lake sediments. Therefore, the slide mass has to be older than the shells thus, the shells give a limiting age and there is no sure way to determine the time gap between sliding and pond formation. We can date the age of landsliding directly using cosmogenic isotopes.

Stone and Fifield (1995) used cosmogenic 36Cl to date the Blackhawk Landslide. Their data however, has a range from about 10,000 years to 55,000 years. The discrepancy is due to uncertain exposure history of the slide mass. Samples taken at the surface can have significant inheritance if they were exposed at the surface prior to landsliding.

 
Continue to the next page. Return to the image gallery

http://geologycafe.com/landslides/big/43.html