THE DETERMINATION OF BACKGROUND EROSION RATES USING COSMOGENIC ANALYSIS OF 10Be IN THE SHENANDOAH NATIONAL PARK

Appalachian Mountains Appalachian Mountains paradox Determine erosion rates within the Park on a 103 to 106 year timescale Testing erosion as a function of lithology Test Hack’s (1960) model of dynamic equilibrium and steady state erosion Relationship between grain size and 10Be concentration (Matmon et al. 2003) My research will investigate the erosional history of the Blue Ridge Mountains in Virginia. The Appalachians have been very intensively studied for decades because of the paradox that exists in the continued existence of he topography of the Appalachians since the cessation of orogenic events. In order to further understand this complex system, I intend to examine the erosional history of the Blue Ridge via erosion rates obtained from cosmogenic analysis using 10Be. I will test Hack’s model of dynamic equilibrium and steady state erosion, and assess the relationship between grain size and 10Be concentration. Blue Ridge Mountains, VA

Specific Goals for Investigation To determine how rapidly the basins of different lithologies of Blue Ridge in Shenandoah National Park are eroding To assess the relationships between erosion rate, lithology, slope and basin area To compare the relationships between 10Be based erosion rates, slope, lithology and grain size with those reported by Matmon et al. (2003a, 2003b), Reuter (2004), U-Th/He erosion rates of Spotila et al. (2004) and fission track erosion rates of Naeser et al. (2005) I will determine how rapidly the lithologies of the Park are eroding in order to be able to determine the erosional history of the Blue Ridge Mountains within the Park. I will assess the relationship between erosion rate, lithology, slope and basin area in order to test Hack’s model of dynamic equilibrium and steady state erosion. I will compare the relationships between 10Be based erosion rates with those reported by Reuter, Matmon and other researchers such as Naeser and Spotila. More about them later.

Physical Setting Appalachian Mountains formed via three major orogenic events: - Taconic (Ordovician) - Acadian (Devonian) - Alleghenian (Pennsylvanian) Continental rifting and denudation in the Mesozoic Continued relief since cessation of orogenic events ~ 300 Mya The Appalachian Mountain Belt stretches from Newfoundland, Canada in the northeast, southward to Alabama. It is divided up into four geologic Provinces, the Appalachian Plateau, Valley and Range, The Blue Ridge, and Piedmont. The Appalachians resulted from the deformation of Paleozoic and Precambrian basement rocks in a series of orogenic events beginning in the Paleozoic era. The Taconic orogeny represents terrane collisions with North America and is noted for thrusting and faulting that occurred mainly in the northern portion of the Appalachians. The Acadian orogeny occurred as the Iapetus Ocean was closing and the Afro-European and North American plates collided. This event was centered around New England and southern New York and was characterized by thrusting, faulting, metamorphism and granitic intrusions. The Allegheny orogeny occurred in the Pennsylvanian and was characterized by folding, thrusting, metamorphism and intrusions from Pennsylvania south to Alabama.

Shenandoah National Park The Shenandoah National Park is situated within the Blue Ridge Mountains of Virginia between Fort Royal in the north and Waynesboro in the south. It is approximately 110 km in length and between 5 and 10 km wide. The mountain tops and slopes are heavily vegetated and the mean annual rainfall is ~ 100 - 150 cm per year. The Blue Ridge Mountains are the northward plunging western limb of an anticlinorium that was formed as the result of complex folding events associated with the Alleghenian orogeny, when the crystalline rocks of the Blue Ridge were thrust westward over younger sedimentary rocks creating an anticline. The anticline is composed of igneous, metamorphic and sedimentary rocks. The broad regional area of the Blue Ridge experienced uplift and arching, the erosion of which supplied sediment to Mesozoic basins. Virginia Occ – Carbonates Cch – Quartzites and siliciclastics Zc – Basalt and granodiorite Zyg – Granite

Principle Rock Types Granites - Old Rag Granite and Pedlar Formation (Granodiorite) Quartzite - Erwin and Swift Run formations Basalt - Catoctin Formation Siliciclastics - Weverton and Hampton formations As you can see form the strat column the rocks are divided into several formations. For analytical and design purposes I have grouped the formations into four main rock types. Granites - Old Rag Granite and Pedlar Formation, Quartzite, Erwin and Swift Run formations, Basalt, Catoctin Formation, Silicilcastics - Chilhowee group.

Fall 2005 Sample Collection Collected samples from four lithologies - basalt, quartzite, granite, siliciclastics Four grain sizes - 0.25 - 0.85 mm - 0.85 - 2 mm - 2 - 10 mm - > 10 mm

lithologic resistance Experimental Design Erosion rate vs. slope Erosion rate vs. lithology Slope vs. lithologic resistance slope erosion rate lithologic resistance erosion rate slope lithologic resistance Once I have erosion rates I can test Hack’s model of dynamic equilibrium and steady state behavior by looking at the relationship between erosion rate and slope, lithology and between slope and lithology.

Hack’s dynamic equilibrium “It is assumed that within a single erosional system all elements of the topography are mutually adjusted so that they are downwasting at the same rate.” Whole system erodes down at the same rate. Steep slopes are more resistant lithologies, lower slopes have less resistant lithologies, therefore, the system can erode at the same rate.

Grain Size Analysis Grain size Lithologies split into 4 grains sizes: 0.25-0.85 mm, 0.85-2 mm, 2-10 mm, > 10 mm Comparison of my grain size analysis with that of Brown et al. (1995), Clapp et al, (1997, 1998, 2001, 2002) Matmon et al. (2003b) The first erosion rate data: 0.25 - 0.85 mm 7 m/My ± 1.48 0.85 - 2 mm 9 m/My ± 1.89 2 -10 mm 10 m/My ± 2.19 Matmon found a dependence on 10Be concentration with grain size. Cosmogenic nuclide concentrations vary systematically with grain size. Smaller grains have higher 10Be concentrations than larger ones. Clasts that begin their journey on high slope locations disintegrate into their constituent grains before reaching streams. They receive dosing during transport. Larger grains can only survive short transport distances and come from lower slope locations where there is less cosmic ray dosing. Lower dosing at lower elevations. Clapp found no dependence on grain size and Brown attributed the relationship to excavation of large clasts by landslides.

Significance of the Results Erosion rates in the context of other research: Matmon et al. (2003) 25 - 30 m/My Reuter et al. (2004) 4 - 54 m/My Spotila et al. (2004) 10 - 20 m/My Naeser et al. (2005) 20 m/My This study 9 m/My I will analyze my data in the context of other researchers. This will provide me with a framework within which to posit my interpretation, and also to be able to garner a wider picture of erosion rates and erosional history of the Appalachian mountains.