Holocene landscape evolution of alpine areas in the southern San Juan Mountains, Colorado

Bradley Johnson
University of North Carolina Charlotte, Geography and Earth Sciences
Author Profile

Shortcut URL: https://serc.carleton.edu/44000


Continent: North America
Country: United States
City/Town: Platoro
UTM coordinates and datum: none


Climate Setting: Semi-Arid
Tectonic setting: Continental Collision Margin
Type: Process, Chronology


Alpine landscapes that were glaciated during the Last Glacial Maximum are filled with landforms which are often confusing and difficult to understand. Yet, many researchers are drawn to researching problems in these areas because of their beauty and mystery (Figure 1). Alpine areas were generally stripped of all previous sediments during the last glaciation and are now filled with newer landforms that were deposited by large retreating glaciers, enormous outburst floods, unstable hillslopes, and river systems attempting to adjust to a post-glacial world.

Often overlooked in this chaotic system are smaller, Holocene (the last 10,000 years) features, which were deposited by systems similar to modern ones. These features are overlooked because 1) they are small compared with large glacial features, 2) they are difficult to date using absolute dating methods, 3) they are caused by small changes in climate that are difficult to measure, and 4) they are difficult to access. Examining these small features may provide evidence as to how landscapes can be affected by modern global climate change.

Before we look at small Holocene features, we should first examine how sediment is deposited during glacial periods. Glaciers are the most erosive force on earth and large quantities of sediment are produced when they are present. This sediment is transported downhill by glacial ice and water and deposited downstream of the glacier's toe. Other erosive mechanisms occur in unglaciated areas, where wetter climates associated with glacial periods increase erosion by streams. Additionally, rapid changes in climate can destabilize the vegetation that holds hillslopes in place which initiates periods of erosion and subsequent deposition.

In comparison, depositional landforms created during the Holocene are not as well understood as those that formed during the Pleistocene. Climate during the Holocene has varied much less than it did during glacial periods (in the Pleistocene). Thus, the cause of erosion and subsequent deposition is not well understood. Erosion may occur during cold periods when precipitation is high, or when climate changes rapidly and destabilizes vegetation. In these ways, erosion and deposition may occur in similar ways during the Holocene and during the Pleistocene.

In the southern San Juan Mountains of Colorado (Figure 1, inset), recent research projects have both mapped Holocene age depositional features in alpine areas, and created a climate record for the Holocene from bog cores. These two records can be examined against one another in order to determine what the climate was doing during periods when deposition was occurring.

Mapping, combined with radiocarbon dating, showed that small terraces and alluvial fans had formed between 1200 and 2100 years ago (Figure 2). These small features are inset into an older and larger series of alluvial fans and terraces (Figure 3). Additionally, a small, discontinuous series of alluvial fans has been dated to around 4500 years ago. Lastly, hillslopes (colluvium) throughout the area have been dated to around 9500 years ago. By examining these periods of deposition, and comparing them to the climate record, we can determine how the landscape evolved during the Holocene.

The climate record shows that the climate was relatively stable throughout the first half of the Holocene with the exception of a cold event around 8200 years ago (Figure 4). The little variability that did occur during this period occurred at a low frequency (climate frequency is the amount of times the climate changes during a set period of time). Beginning 6000 years ago, the frequency of climate increased and cold periods became more evident. This lasted until 3000 years ago when the frequency of climate change increased again.

Combining results from mapping with climate data can allow us to make the following conclusions about landscape evolution in the San Juan Mountains. First, deglaciation from the Last Glacial Maximum ended at about 12000 years ago. After this, it took roughly 2500 years before the hillslopes stabilized (which is why hillslopes date to 9500). Then, the landscape was stable through the Early Holocene because climate was similarly stable. However, the increase in climate change frequency, combined with a short cold period, around 6000 years ago led to the creation of the alluvial fans dated to 5500 years ago. These fans are rare though because they were re-eroded by the additional increase in climate change frequency around 3000 years ago. This re-erosion is called reworking and occurs when water erodes the previous generation of landform and reuses the sediment, leaving little evidence of the original landform. This "reworking" of small alluvial fans created the terraces and additional alluvial fans that have been dated to between 1200 and 2100 years ago.

These conclusions show us that alpine landscapes have evolved significantly during the Holocene in response to small changes in climate. While these events were small, they are still significant in altering hillslopes, valley bottoms, and fluvial systems. Most importantly, this research has shown that periods of rapid climate change can destabilize sedimentary environments and cause erosion. This is important in understanding how future anthropogenic (human caused) climate change may influence natural landscapes in alpine areas.

Associated References

  • Atwood, W. W., and Mather, K. F. (1932). Physiography and Quaternary geology of the San Juan Mountains, Colorado. U.S. Geological Survey Professional Paper 166, 176.
  • Ballantyne, C. K. (2002). Paraglacial geomorphology. Quaternary Science Reviews 21, 1935-2017.
  • Bull, W. B. (1991). "Geomorphic responses to climate change." Oxford University Press, New York, NY.
  • Johnson, B. G., Eppes, M. C., and Diemer, J. A. (in press). Surficial Geologic Map of the Upper Conejos River Drainage, Southeastern San Juan Mountains, Southern Colorado. Journal of Maps.
  • Johnson, B. G., JimĂ©nez-Moreno, G., Eppes, M. C., Diemer, J. A., and Stone, J. R. (in prep). An A 18k high resolution multi-proxy paleoclimate record from a sub-alpine bog in the San Juan Mountains of southern Colorado. Geology.
  • Slaymaker, O., Souch, C., Menounos, B., and Filippelli, G. (2003). Advances in Holocene mountain geomorphology inspired by sediment budget methodology. Geomorphology 55, 305-316.