Examine the aerial photograph of Death Valley, California, part of the Basin and Range Province

Question

Note that the western side of Death Valley, up against the Panamint Range, features a well-developed bajada, while the east side of the valley, up against the Amargosa Range, shows only a few isolated alluvial fans. Suggest an explanation for this asymmetry, and how you would test your hypothesis with an investigation on the ground.
What will be an ideal response?

Answer 1

Answer: A bajada is formed when numerous alluvial fans grow and coalesce. Therefore, the west side of Death Valley must have seen more advanced growth of its alluvial fans than the east side. Why is this? A couple of reasons suggest themselves: It may be that there is simply more sediment being supplied on the west side. This effect, in turn, could be due to a couple different reasons. The drainage basin supplying those western fans/bajada may be larger, or it may have to do with the Panamint Mountains being taller (note the snow) than the Amargosa Range. Or perhaps the drainage basins are the same size, but the Panamints are being weathered more rapidly than the Amargosas. Or perhaps the reason has nothing to do with sediment supply, but with the tectonics of the Basin and Range. If the faults bounding Death Valley are operating at different rates, that could explain the asymmetry to the sedimentary landforms flanking it. For instance, if the western basin-bounding fault was active in the past, but shows no modern movement, then sediment could pile up atop its trace. Meanwhile, if the fault between the Amargosa Range and eastern Death Valley is really active, then perhaps the eastern side is dropping down much faster, like a trap door, and any alluvial fans are getting buried beneath playa deposits. Evidence supporting this interpretation includes (a) the fact that the playa lake at Badwater is noticeably deeper in the east, and (b) the fault scarp that is cutting across the alluvial fan in the inset photograph.