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Low Rates of Bedrock Outcrop Erosion in the Central Appalachian Mountains Inferred from in Situ 10Be

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Low Rates of Bedrock Outcrop Erosion in the Central Appalachian Mountains Inferred from in Situ 10Be Low rates of bedrock outcrop erosion in the central Appalachian Mountains inferred from in situ 10Be Eric W. Portenga1,†,§, Paul R. Bierman2, Donna M. Rizzo3, and Dylan H. Rood4,# 1Department of Geology, University of Vermont, Burlington, Vermont 05405, USA 2Department of Geology and Rubenstein School of the Environment and Natural Resources, University of Vermont, Burlington, Vermont 05405, USA 3College of Engineering and Mathematical Sciences, University of Vermont, Burlington, Vermont 05405, USA 4Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550, USA, and Earth Research Institute, University of California, Santa Barbara, California 93106, USA ABSTRACT similarity may refl ect underlying lithological tural weaknesses in the rock and perhaps by and/or structural properties common to each the process of headward stream capture (e.g., Bedrock outcrops are common on central physiographic province. Average 10Be-derived Clark, 1989; Gunnell and Harbor, 2010; Prince Appalachian Mountain ridgelines. Because outcrop erosion rates are similar to denuda- et al., 2010). these ridgelines defi ne watersheds, the rate at tion rates determined by other means (sedi- Around the globe, drainage basins appear to which they erode infl uences the pace of land- ment fl ux, fi ssion-track thermochronology, be eroding at least as quickly as or faster than scape evolution. To estimate ridgeline erosion [U-Th]/He dating), indicating that the pace bedrock outcrops in any region where both rates, we sampled 72 quartz-bearing outcrops of landscape evolution in the central Appala- types of samples have been analyzed (Portenga from the Potomac and Susquehanna River chian Mountains is slow, and has been since and Bierman, 2011). Measured samples col- Basins and measured in situ–produced 10Be. post-Triassic rifting events. lected both from drainage basins and from indi- Ridgeline erosion rates average 9 ± 1 m m.y.–1 vidual bedrock outcrops on ridgelines provide (median = 6 m m.y.–1), similar to 10Be-derived INTRODUCTION important data for understanding landscape evo- rates previously reported for the region. The lution through time, allowing one to contrast the range of erosion rates we calculated refl ects Appalachian landscape evolution has sparked rate of ridgeline lowering with that of basins as a the wide distribution of samples we collected over a century of discussion. An early theory whole. Comparing ridgeline erosion rates to ba- and the likely inclusion of outcrops affected of landscape evolution suggested that after be- sin erosion rates has the potential to determine by episodic loss of thick slabs and periglacial ing uplifted, peneplains were dissected by the whether relief is changing over time—a funda- activity. Outcrops on main ridgelines erode rapid downcutting of streams, which eventually mental descriptor of landscape development. slower than those on mountainside spur achieved a graded or equilibrium profi le (Davis, However, too little is known about bedrock out- ridges because ridgelines are less likely to be 1899). Davis’s model persisted until Hack (1960) crop erosion rates in the Appalachian Mountains covered by soil, which reduces the produc- suggested that Appalachian landscapes were not to make such a comparison. tion rate of 10Be and increases the erosion the dissected remnants of uplifted plains, but Traditional methods of measuring the pace rate of rock. Ridgeline outcrops erode slower rather resulted from the interaction of numerous of landscape change, such as chemical mass than drainage basins in the Susquehanna driving forces including tectonics, erosion, cli- balances, sediment budgeting, and topographic and Potomac River watersheds, suggesting mate, and physical properties of Earth materials. measures of cliff or slope retreat over time, are a landscape in disequilibrium. Erosion rates Contrary to Davis’s idea that landscapes evolved diffi cult to apply or are unrepresentative at both are more similar for outcrops meters to tens directionally over time, Hack proposed that the temporal and spatial scale of outcrop erosion of meters apart than those at greater dis- landscapes only appear to preserve landforms. (Saunders and Young, 1983). In contrast, cos- tances, yet semivariogram analysis suggests In reality, these landforms are continuously be- mogenic methods are well suited for estimating that outcrop erosion rates in the same physio- ing eroded and uplifted in a dynamic equilib- outcrop erosion rates over millennial time scales graphic province are similar even though rium, where landscapes remain similar over the (e.g., Nishiizumi et al., 1986), and the most they are hundreds of kilometers apart. This large scale but individual elements come and go widely used cosmogenic nuclide for erosion rate over time as they are dismembered by erosion. studies is 10Be measured in quartz (Portenga and In the Appalachian Mountains, bedrock out- Bierman, 2011). †E-mail: [email protected] §Present addresses: School of Geographical and crops are often observed along ridgelines that Within the upper few meters of Earth’s sur- Earth Science, University of Glasgow, Glasgow defi ne watershed boundaries. The existence of face, 10Be is created primarily by spallation G12 8QQ, Scotland, UK, and Department of Envi- such ridges indicates that, at least for some time nuclear reactions during which high-energy ron ment and Geography, Macquarie University, in the past, the landscape within the watersheds neutrons interact with oxygen in the mineral North Ryde, NSW 2109, Australia. 10 #Present address: Scottish Universities Environ- must have eroded more quickly than the rock structure. The Be subsequently decays radio- mental Research Centre (SUERC), East Kilbride ridges defi ning the watershed boundaries. This actively (t1/2 = 1.39 Ma; Chmeleff et al., 2010; G75 0QF, Scotland, UK. rapid downcutting may be initiated along struc- Korschinek et al., 2010). The production of 10Be GSA Bulletin; January/February 2013; v. 125; no. 1/2; p. 201–215; doi: 10.1130/B30559.1; 10 fi gures; 3 tables; Data Repository item 2013023. For permission to copy, contact [email protected] 201 © 2013 Geological Society of America Portenga et al. decreases exponentially with depth through 8; Hancock and Kirwan, 2007). All three studies deposited in a foreland basin and now underlie Earth’s surface, such that at a depth of ~2 m in report low rates of outcrop erosion with means the Valley and Ridge Province (Roden, 1991). rock, little 10Be is created through spallogenic ranging from 4 to 7 m m.y.–1. In the Valley and Ridge Province, highly de- reactions; muon-induced reactions continue to This study presents 72 new 10Be-based bed- formed, plunging anticlines and synclines trend depths of tens of meters but at a much lower rock outcrop erosion rates from a variety of parallel to the post-Alleghenian rift margin, ex- production rate. Thus, by sampling the upper- locations within the central Appalachian Moun- posing sandstones and arenites along the ridges most portion of an outcrop and assuming steady tains, specifi cally the Susquehanna River Basin and limestones and shales beneath the valleys. and uniform erosion, the concentration of 10Be (n = 26) and the Potomac River Basin (n = 46; Post-Jurassic denudation rates throughout the refl ects the time required for material to pass Fig. 1); 62 of the 72 samples come from main region, as determined by long-term sediment through the uppermost several meters of rock ridgelines. The size of this new central Appala- budgeting as well as (U-Th)/He and fi ssion- and regolith (Lal, 1991); this is the basis of ero- chian bedrock outcrop 10Be erosion rate data set track thermochronologies, have fl uctuated but sion rate calculations. and the spatial distribution of these data allow on average remain low (~21 m m.y.–1; Blackmer During the past decade, hundreds of 10Be us to test for relationships among erosion rates et al., 1994; Boettcher and Milliken, 1994; C.W. measurements have been made on sediment and climatic and topographic parameters. By Naeser et al., 2001, 2005; N.D. Naeser et al., samples collected from various streams and measuring the rate of bedrock outcrop erosion, 2004; Pazzaglia and Brandon, 1996; Reed et al., rivers near and within the central Appalachian we can better assess the processes infl uencing 2005; Sevon, 1989; Spotila et al., 2004). Mountains, including drainage basins of differ- modeled erosion rates (e.g., block removal and Over the Quaternary, continental ice sheets ent sizes in Shenandoah National Park (n = 37; periglacial activity) and discuss the relationship affected the northern Appalachian Mountains, Duxbury, 2009), the Susquehanna River Basin between the ridgeline and basin-averaged ero- advancing and retreating numerous times. At its (n = 79; Reuter, 2005), and the Potomac River sion rates, which is prerequisite to understand- farthest extent, ice covered the northern reaches Basin (n = 62; Trodick, 2011). Paleo-erosion ing large-scale landscape change on millennial of the Susquehanna River Basin (Fig. 1). rates of contributing drainage basins to the New time scales. Though the southern Appalachian Mountains River in West Virginia were inferred from 10Be were not directly affected by glaciation, the and 26Al concentrations in sediments deposited Geographic and Geologic Setting climate was cooler and drier during glaciations in caves (Granger et al., 1997). Basin-averaged than it is today, and at least some of the ridge- erosion rates from these studies are low, with The central Appalachian Mountains are a lines were affected by periglacial activity during means averaging from 10 to 27 m m.y.–1 for dominant physiographic feature inland of east- cold phases (Braun, 1989). sampled drainages in our study area. Incision ern North America’s Atlantic passive margin. rates of the New River (Ward et al., 2005) and of This linear mountain chain extends 2500 km METHODS the Potomac and Susquehanna River (Reusser from the subpolar Canadian Maritime Provinces et al., 2006) were inferred from measurements to humid, subtropical southern Georgia (Fig. 1, Field and Laboratory Methods of 10Be; such incision rates are refl ective of only inset).
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