Images and caption contributed by Ross Irwin.
Alluvial fans are sedimentary deposits that accumulate where streams emerge from steep mountain watersheds onto low-gradient plains. Flooding in the upland area transports a wide range of sediment sizes, and when the stream emerges onto the plain, it no longer has the power to transport the same quantity and size of sediment. Only part of an alluvial fan's width is active at any one time, but as sediment accumulates in that area, flows will move to steeper routes elsewhere on the fan surface. Sediment may be delivered through normal stream flows or debris flows, which are concentrated slurries of sediment and water. Alluvial fans that were formed mostly by debris flows are usually steeper than their stream-dominated counterparts.
Image 1: Holden crater (26°S, 34°W) in a Thermal Emission Imaging System (THEMIS) daytime infrared mosaic (230 m/pixel) colored with Mars Orbiter Laser Altimeter topography. Warm colors are high elevations, cool colors are low, and contour lines mark each 50 m interval of elevation. The green-shaded deposits along the western wall of the crater are the Holden bajada. [Full resolution image]
Alluvial fans are less common on Mars but occur in similar settings, mostly in deep impact craters and occasionally at the base of other steep slopes. These deposits formed shortly before the major decline in fluvial activity around the Noachian/Hesperian transition, about 3.7 billion years ago. The fans show little to no sign of segmentation or late-stage erosion by precipitation directly onto the fan surfaces, suggesting that the climate change was fairly abrupt. The largest alluvial fan complex recognized to date on Mars is located in Holden crater (Image 1), which formed in the Late Noachian (3.7-4 Ga). Erosion of the western wall of the crater has supplied sediment to a coalesced set of alluvial fans called a bajada.
Image 2: A large alluvial fan in the southern floor of Holden crater, showing inverted (positive-relief) distributary channels that were preserved while wind stripped away fine-grained sediment between them. THEMIS visible imaging, 17 m/pixel.
A separate alluvial fan (Image 2) is found near the southwestern corner of Holden crater. The wind has stripped away the finer sediment that once covered much of this fan surface, leaving many of the old stream bed deposits standing in positive relief. The long channels, wind erosion, and low gradients of these fans suggest deposition mostly by normal stream flows rather than debris flows.
Large alluvial fans in Martian craters were supplied mostly by erosion of deep valleys in the crater rims, such as the ones that surround Ostrov crater (Image 3). Most of the floor of this crater is covered with large alluvial fans.
Image 3: Alluvial fans cover the floor of Ostrov crater (26.5°S, 28°W). The fan sediment was derived from deep erosion of the crater rim. THEMIS day IR imaging, 100 m/pixel. [Full resolution image]
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