Dating the Blackhawk Landslide

 

 

 

The Blackhawk landslide is best known as an example to support a number of hypotheses regarding large volume landslide mechanics. Surprisingly, there is little dating control on the timing of the landslide. This page briefly outlines our approach to determine the timing of landsliding by using surface exposure age dating.

 

 

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.

 

We feel that best locations, geomorphically, to sample boulders for exposure age dating are on the crest of the levees. These locations would have the simplest exposure histories, and thus the best chance to interpret the data to determine an age. We used Sheave's (1968) geological map to locate the areas with quartz rich rocks to date the landslide. We sampled four gneiss boulders on the left levee as noted above. Three of these samples are located less than 2 meters below the broad levee crest. The fourth sample is located on a side slope. We also sampled one sandstone boulder at the toe of the landslide at the levee crest.

 

RESULTS

The three boulders located near the levee crest have average exposure ages of 7,200 ± 1,400 years (using increasingly accepted production rates of ~5.0 atoms/g for 10Be). The maximum age from these boulders is 8,000 ± 1,600 years. These ages represent the average of the 10Be data and the 26Al data. Since both isotopes reproduce each other, we have no reason to discard either data set. Most people use the oldest age date because the younger boulders could have initially been buried after landsliding and have since been exposed; or the younger boulders could have eroded more also giving younger ages.

 

The side slope boulder has an exposure age of 31,000 ± 6,200 years. As you can see there is a great discrepancy between the two locations. Since we do not know the burial history of the samples, this is an inverse problem and there are numerous possible solutions.

 

DISCUSION

To estimate the age of the landslide there are two endmember solutions. At the young endmember, we can assume the levee crest boulders represent the age of the landslide and the side-slope boulder has nuclide inheritance thus, the side-slope boulder would represent an old age. The levee crest boulder scenario suggests an age of 8,000 ± 1,600 years. At the other endmember we can assume that the side slope boulder represents the age of the landslide and the crest boulders were buried and have since been exhumed. The side slope boulder suggest that the landslide is 31,000 ± 6,200 years old. There is also any possible combination where the age could be intermediate between the two endmembers.

MODELING

If we assume that the side-slope boulder represents the "real" age of the landslide, the levee crest boulders would have to have been buried by 8 m of sediment and the sediment would have to erode at an average of 30 cm/ky. By modeling the levee crest boulders for burial followed by exposure, we can calculate a wide range of ages.

 

Model age of Levee Crest boulders

Erosion rate of levee crest 900 cm 800 cm 700 cm 500 cm 300 cm 100 cm

Erosion of 30 cm/ky 33,351 30,018 26,687 20,044 13,537 7,983

Erosion of 40 cm/ky 26,716 24,216 21,718 16,735 11,855 7,690

Erosion of 50 cm/ky 22,735 20,735 18,737 14,750 10,846 7,514

 

We are currently working to refine our parameters to better constrain the date of the landslide. Come back later this Spring to see our "final results" and to see where we are going to publish the data.

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