Stones, soils and streams: differential erosion rates and landscape evolution in Nahuelbuta National Park, south-central Chile

Abstract:

Nahuelbuta National Park, situated in the Chilean Coastal Cordillera at a latitude of 38 degrees south, has a humid climate with >1000 mm of rainfall per year and densely vegetated and soil-covered slopes. Uplift rates in Nahuelbuta increased from 30-40 m/Myr to >200 m/Myr around 4 Ma, and knickpoints in streams surrounding the Nahuelbuta block suggest that an increase in incision reflecting uplift has not yet propagated to Nahuelbuta’s ridges. Throughout the area, sporadic exposures of unweathered granitic bedrock exist near deep regolith, and especially on ridges, large corestones are exhumed and areas of bare bedrock are exposed. The cause of the juxtaposition of these different surfaces is unclear.

To understand the controls on these apparent discrepancies in weathering rates, we collected amalgamated samples for in situ-produced ¹⁰Be cosmogenic nuclide dating from unweathered bedrock, corestones, and soils on ridges above the knickpoints, and compared these to catchment-average erosion rates. Preliminary erosion rates show low ¹⁰Be-derived erosion rates in bedrock and corestones, ranging from ~7-10 m/Myr. Soils surrounding the corestones have erosion rates that are 2-3 times higher at ~14-30 m/Myr. Previous studies show that catchment-average erosion rates from streams surrounding these ridges and upstream of knickpoints are also higher, ranging from ~24-40 m/Myr, and soil pit samples indicate similar denudation rates of ~32 m/Myr.

Catchment average erosion rates of ~30 m/Myr reflect pre-4 Ma erosion rates, consistent with long-term exhumation rates. The discrepancy between the slow erosion rate of corestones in comparison to soil and stream sediment suggests that presently exposed corestones were exposed over time as the landscape uplifted. These unweathered localities have become topographic highs as surrounding areas erode at a faster rate. Large boulders (up to one meter in diameter) in streams suggest that corestones, after being exhumed, may roll downhill and into streams, potentially providing a cover effect that may play a role in inhibiting stream incision and slowing down knickpoint propagation.