Observations of weathering and deep roots exposed by a rockfall from a limestone cut bank on the Pedernales River, TX, USA

Abstract:

Direct observations of deep rooting in carbonate bedrock are rare, and few studies have investigated the association of deep rooting, degree of bedrock weathering, and cliff failure. On January 16, 2019, an approximately 350 m² face was exposed by a rockfall along the Pedernales River in Central Texas, USA. This new exposure in the limestone-dominated Lower Cretaceous Glen Rose Formation revealed soil, variably weathered rock, fractures, and woody and fine tree roots. Woody vegetation species that presently live on the thinly soil-mantled cliff rim are Plateau live oak (Quercus fusiformis), Texas oak (Quercus texana), and Ashe juniper (Juniperus asheii). Using observations of the rockfall face, we explore the role that roots play in weathering limestone cliffs by enhancing both mechanical and chemical weathering processes. We map the degree of weathering and the shape, spatial extent, and density of exposed roots and fractures on a structure-from-motion-derived digital outcrop model generated using a DJI Mavic 2 drone. We document roots within fractures and the bedrock matrix up to 10 m below the cliff rim throughout the exposed face. Root density and the extent of chemical weathering are correlated and spatially variable. Root diameter ranges from millimeter-scale fine root mats to centimeter-scale deformed woody roots that reflect fracture geometry and set a lower limit on pre-rockfall fracture aperture. Roots are frequently surrounded by weathered bedrock and soil. Neither root density nor root diameter decreases systematically with depth, but roots tend to occur more frequently in specific stratigraphic beds. Documenting this deep root distribution is necessary to understand how water travels through the vadose zone in carbonate rocks, how bedrock river cut bank morphology changes over time, and how rockfall hazard is assessed in carbonate landscapes.