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Timing and style of deposition on humid-temperate fans, Vermont, United States

Karen L. Jennings^,1, Paul R. Bierman^,1 and John Southon^,2

¹ Geology Department and School of Natural Resources, University of Vermont, Burlington, Vermont 05405, USA
² Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550, USA

Fans in the once-glaciated, mountainous landscape of humid-temperate New England preserve a long and unique record of deposition and, thus, of hillslope erosion. By using multiple backhoe trenches and radiocarbon dating of wood and charcoal, we determined the history of five small fans (1900–14,850 m³) that range in age from historic to 13,320 calibrated (cal.) ¹⁴C yr B.P. Three fans located on river terraces have depositional records whose ages are limited by the age of the terrace on which they are situated. Two other fans, located in glacial valleys, preserve records that extend back nearly to deglaciation.

The stratigraphy of all five fans contains evidence suggesting episodic activity, including scoured surfaces and layers of gravel and cobbles. Periods of little or no activity are indicated by development of now-buried soils. Dated sand and gravel strata in several fans suggest correlative periods of increased sediment yield and by inference, runoff, at ca. 9650–9340 and 6900–6020 cal. ¹⁴C yr B.P. Soils preserved within at least two of the five fans suggest lower sediment yield at ca. 12,900, ca. 5500, ca. 4300, and ca. 3200 cal. ¹⁴C yr B.P. At least three of the fans aggraded rapidly during the past several hundred years in response to land clearance and disturbance; however, many aggradation and scour events in the Holocene cannot be correlated definitively between fans because of the discontinuous nature of gravel beds and the lack of radiocarbon-datable material in the coarsest strata.

Drainage-basin sediment yields implied by the fan volumes and integrated over the Holocene are quite low, 4–11 x 10³ kg·km^–2·yr^–1. Sediment yields since settlement by European and other immigrants are several to hundreds of times higher, demonstrating the connections among forest clearance, agriculture, and increased erosion rates of New England hillslopes.

Key Words: erosion • fans • New England • radiocarbon dating • Vermont

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