NEW 36CL PROFILE AGE FOR THE BLACKHAWK LANDSLIDE, LUCERNE VALLEY, SAN BERNARDINO COUNTY, CALIFORNIA

Ancient landslide deposits are exposed throughout the mountainous regions of the world. Some of these are long-runout landslides, which extend many km from their source areas. The Blackhawk landslide in southern California is one of the earliest described long-runout landslides, with an initial description published nearly a century ago. However, constraining the age of these pre-historic events can be challenging. Advances in cosmogenic dating techniques have allowed for more precise age constraint of more recent (Quaternary) landslides. In this study, we evaluated the age of the Blackhawk landslide using two ³⁶Cl profiles.

The Blackhawk landslide deposit is 9 km long, up to 2 km wide, and 30 m thick, with an estimated volume of 300 million m³. The landslide was sourced near Blackhawk peak in the northern San Bernardino Mountains and the deposit plunged ~1200 m to its present location. The deposit is dominantly comprised of Paleozoic carbonate clasts, with minor sandstone and quartzite clasts. Initial age constraint was 17.4 ka based radiocarbon ages from freshwater gastropods and pelecypods from an ephemeral lake atop the landslide deposit. Stratigraphically, that age is younger than the deposit, yet may be anomalously old due to possible carbon inheritance from the limestone in the deposit. An early attempt at ³⁶Cl ages were ambiguous, with apparent ages between 12 and 44 ka. ¹⁰Be and ²⁶Al ages from siliciclastic boulders were similarly inconclusive, with ages of 6-8 ka from one boulder and 24-31 ka from another.

In this study we sampled two 3-m depth profiles from the toe of the Blackhawk landslide. One profile, closer to the landslide toe, generated a 17 ka model age, whereas the second profile had a 21 ka model age. These ages are consistent with the previous radiocarbon age. Additionally, this timing is consistent with the last glacial maximum in the area, during which time the southern San Bernardino Mountains were glaciated and the Mojave desert to the north contained large lakes. The wetter climate at the time may have helped this large landslide fail, though the ultimate trigger remains unknown. Finally, advances in cosmogenic dating techniques can help constrain depositional ages of Quaternary landslides, even well-studied examples like the Blackhawk landslide.