Geomorphic and Geochemical Characteristics of Five Alpine Fens in the San Juan Mountains, Colorado



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Fens are abundant in the San Juan Mountains. By exploring the geomorphology and geochemistry of fen wetlands, the functions that fens serve can be better understood. In this research, two main studies were conducted involving the geomorphology and geochemistry of fens.

The first study involved a complex investigation of the geomorphology of five fen sites in the San Juan Mountains near Silverton, Colorado. Geomorphic maps were constructed for each fen site at a scale of ~1:3,000. A geomorphic classification scheme was then made based on fen location, and fens were placed in one of three categories: 1) valley-bottom, 2) valley-side, and 3) terrace. Fen circularity and elongation values were calculated for thirty fens to determine morphometry. A pattern for elongation of fens emerged between the three types of alpine fens with valley-bottom fens having an average elongation value of 1.7, valley-side 2.4, and terrace 1.9. Valley-side fens are more elongated than valley-bottom and terrace fens, which exhibit similar elongation values.

In addition, sediment samples at each site were sectioned along visual breaks in the sediment column and were sieved. Mean phi values were calculated for each section and at each site. The mean phi values at California Gulch, Glacial Lake Ironton, Howardsville, Red Mountain Pass North, and Red Mountain Pass South, are 0.2112, 0.9045, 1.6028, 0.0178, and 1.0516, respectively. Overall, coarse-grained particles are associated with valley-side fens, and medium-grained particles are associated with valley-bottom and terrace fens.

The second part of the study involved investigating the geochemistry of fen sediment. The geochemistry portion of this research focused on concentration and isotopic ratios of Pb and the amount of 137Cs in fen sediment to better understand variations of Pb with depth and calculate approximate sedimentation rates. Based on isotopic ratios of Pb, binary mixing was determined with the presence of ore mineralized Pb and non-ore mineralized. Binary mixing of two types of ore-mineralized Pb is present at the Howardsville fen and both ore-mineralized and non-ore mineralized Pb is present at the Red Mountain Pass North fen. Based on 137Cs in fen sediment at Howardsville, an average rate of deposition of sediment is approximately 0.16 cm/yr, with a visible change in sedimentation rates pre- and post-1960s.