Christmas Valley Dunes

For a Google Earth .kmz file to explore Christmas Valley, scroll to the very bottom of the page.

The Christmas Valley Dunes are an impressive geologic region for several reasons. The geologic activity that led to the formation of active sand dunes started quite a long time ago and has evolved to form the dunes that we see today. The Christmas Valley basin in which the dunes are situated has a long volcanic and tectonic history. However, for this virtual field trip, I am going to briefly discuss how a basin is created and then focus more on the formation of the recently created Pleistocene lakes and Holocene dunes fields in the area.

The area that Christmas Valley is located in is the very northwest portion of what is referred to as the Basin and Range province, specifically located in the Brothers Fault Zone (Bevis, 2013). In the Basin and Range province the crust is thinning due to extensional forces. Basically, the land is being pulled apart and the result is the formation of normal faults. Normal faults are created when the hanging wall of a fault drops relative to the foot wall creating what is called horst and graben topography. A horst is the up-thrown block of the fault or the range, and a graben is the down-thrown block of the fault or the basin (see Figure 1). Since a graben or basin is a natural low spot, given a wet climate, rain water can begin to collect making it a perfect place for a lake. Currently, however, the climate of Eastern Oregon is very arid. How was it then that the Christmas Valley basin housed large lakes in the past? Simple, the global climate used to be much cooler than it is today.

During the Pleistocene, starting approximately 2.6 million years ago and ending 11,700 years ago, several “ice ages” took place causing the advancement of the polar ice caps and the formation of many

alpine glaciers at lower latitudes on the Cascade Volcanoes (http://www.ucmp.berkeley.edu/quaternary/pleistocene.php). It’s not clear at this point what caused these glacial periods exactly, it could have been ocean current changes, tectonic changes, the location of the Earth in relation to the sun, the composition of the atmosphere, or a combination of all of these. What is certain is that glaciers advanced and retreated several times during the Pleistocene due to Earth's cooler and drier conditions and this led to the formation of glacial ice. For Oregon, this meant that the basins discussed previously filled with water from rainfall and ice melt forming shallow lakes that extended over large areas. These lakes are referred to as pluvial lakes, or lakes that are created by rainwater during times of glaciation. The importance of the cooler Pleistocene climate is that these lakes were able stick around longer because there was less evaporation.

In Oregon there were quite a few of these pluvial lakes near Christmas Valley and in fact the Christmas Valley Lake was once part of a much larger lake called Fort Rock Lake. The presence of this lake during volcanic eruptions created some very famous volcanic features in Oregon, including Fort Rock and Hole in the Ground (not to be confused with The Hole in the Ground, another geologic place of interest in Oregon, see Owyhee River Landslides) (Bevis, 2013). There were many other pluvial lakes in the area and some remnant lakes still exist today. Malheur, Harney, Silver, Summer, and Albert lakes are all remnants of larger pluvial lakes that once covered much of south central Oregon (2013).

Volcanism from the High Cascades (otherwise known as the modern Cascades) and the High Lava Plains erupted tephra into the atmosphere that then rained down and came to rest in the bottom of these pluvial lakes. One of the most explosive examples of these eruptions was the eruption of Mt Mazama 7,700 years ago. Mt. Mazama erupted so much tephra that it created an ash plume extending 30 miles into the atmosphere (Klimasauskas et. al., 2002). Although some of the ash travelled to the northeast, a lot of it rained down on the areas closest to the volcano, areas like the Fort Rock Lake. This means much of the sediments that can be found in Oregon's old pluvial lake basins have either fluvio-lacustrinan origin (meaning they came from lakes or rivers) or volcanic airfall origin (Edgett et al., 1995). Once the last "ice age" ended 11,700 years ago, the climate in the area began to warm and Oregon's pluvial lakes began to evaporate. Without water, the loose sediments and volcanic ash that accumulated in the bottom of the lake basins were subject to the wind. This is how the Christmas Valley Sand Dunes began to form.

The Christmas Valley Sand Dunes are situated in an area that is about 18.5 mi wide and 6 mi long (30 km x 10 km) (Edgett et al.,1995). The dune area formed in what was once the eastern portion of Fort Rock Lake and consists of two large northeast-trending parabolic dunes (Edgett et al.,1995). One of these large dunes is not active and contains the Lost Forest Research Natural Area. The Lost Forest is of particular interest since the next closest nearest forest is about 40 mi northwest and these trees subsist on much less water than their counterparts in other parts of the state (http://www.blm.gov/or/resources/recreation/files/brochures/Sand_Dunes.pdf). The forest was initially established in wetter and colder times but seems to remain because of a combination of unique soil and hydrologic properties in the area (see Figure 2). The active parabolic dune has smaller dunes superimposed on top of it and those dunes are mainly barchan and transverse in nature, though there are other dune types found such as linear, dome, and parabolic dunes. The barchan and transverse dunes have linear ridges on them which suggest a bimodal wind pattern (Edgett et al., 1995) meaning the wind blows in at least two directions, forming two different types of dunes that are superimposed on top of each other (see Figure 3). A great animation that shows these changes can be seen if you click here and scroll down to the reversing dunes picture: Linear Ridges.

The two main sand dune types in the area form in very different ways. Parabolic dunes form when the horns of a dune are anchored up wind of the main part of the dune. The dune will move away from those horns down wind. The slip face, or the steepest part of the dune, is on the convex part of the dune (see Figure 4). Barchan dunes are the opposite, they are anchored in the main part of the dune and the horns point upwind. The slip face of this dune type is on the concave portion of the dune (see Figure 4). When barchan dunes come together, they are called transverse dunes. The dunes of Christmas Valley are still very active and we can actually watch this dune field travel and change location over time. This makes the Christmas Valley Sand Dunes a great natural laboratory of geomorphologists.