The subsurface as the key to surface on Martian gullies

Post by Dr. T. de Haas, Department of Geography, Durham University.

Martian gullies are composite landforms that comprise an alcove, channel and depositional fan. They are very young geological features, some of which have been active over the last million years. Water-free sediment flows, likely triggered by CO2 sublimation, debris flows, and fluvial flows have all been hypothesized to have formed gullies. These processes require very different amounts of liquid water, and therefore their relative contribution to gully-formation is of key importance for climatic inferences. Formative inferences based on surface morphology may be biased however, because of substantial post-depositional modification (Images 1-3).

Image1

Image 1: Morphometry, morphology and stratigraphy of depositional landforms in Galap crater. (a) Overview and digital elevation model of Galap crater. (b) Detail of northwestern slope showing gradients of catchment and depositional fan. (c) Detail of proximal fan surface. (d) Detail of distal fan surface. (e) Detail of fan surface with incised channels; the dashed line indicates the rockfall limit. (f) Example of stratigraphic section. (h) Same stratigraphic section as in f, but with optimized contrast in the section. Arrows denote downslope direction. HiRISE image PSP_003939_1420.

Many authors have used the surface morphology of gully-fans to infer the formative processes of gullies. In particular, the general absence of paired levees, distinct depositional snouts, and boulders >1 m in diameter, all characteristic of sediment-gravity flows such as debris flows and dry granular flows, has often been interpreted as evidence for gully-formation by fluvial flows.

Both comparisons between stratigraphically young and old gully-fan lobes, as well as comparisons between the surface morphology and the sedimentology in vertical walls along incised channels, show that gully-fan surfaces become rapidly modified by post-depositional processes, including weathering, wind erosion, and ice-dust mantling (Images 1-2). For example, meter-sized boulders, randomly suspended in a fine matrix, are abundant in incised sections in Galap crater, suggesting a formation by sediment-gravity flows. In contrast, evidence for a formation by sediment-gravity flows has been erased from the fan surface by post-depositional processes.

Image2

Image 2: Examples of gully-fan surface modification. (a) Wind erosion on a gully-fan surface. Note dunes on distal fan domains and on the crater floor, and depositional lobes erosionally reworked into ventifact- or yardang-like landforms by the wind (HiRISE image: PSP_002066_1425). (b) Panel of A, detailing lobe reworking into dune shape (PSP_002066_1425). (c) Dune formation on fan surface (PSP_006820_1325). (d) Depositional lobes of different age. The youngest lobe hosts more boulders and presents higher topographic relief (PSP_002317_1445), implying significant secondary weathering and erosion on the older surface. (e) Fan surface subject to deflation: former channels hosting coarser deposits are exhumed and become markedly elevated above the surrounding fan surface (PSP_002291_1335). (f) Strongly deflated and exhumed debris-flow lobes forming a residual, elevated topography over a surface dominated by eolian bedforms (PSP_001712_1405). (g) Gullies and fans covered by the ice-dust mantle (PSP_013585_1115). (h) Fan with lobes of different generation, of which the oldest is strongly smoothed and masked by an LDM cover (PSP_001684_1410).

These observations emphasize that morphological analyses on Mars, and perhaps other planetary bodies, should take into account the possibility of substantial modification by post-depositional processes.

Post-depositional modification of alluvial fan surface is common on many fan surfaces on Earth (Image 3). In essence, whether a fan surface is dominated either by primary processes of deposition or by post-depositional processes that modify the original depositional morphology, depends on the balance between the characteristic time scales to (1) cover and build morphology by primary deposition and (2) to modify morphology by post-depositional processes.

Image3

Image 3: Terrestrial examples of fan-surface modification by secondary processes. (a) Debris-flow deposit on a fan delta in Nevada (USA), of which the surface has been heavily modified to form a desert pavement. Scale bar is 15 cm long (image from Blair & McPherson, 2009). (b) Partly disintegrated boulder on inactive fan surface (Atacama Desert, northern Chile; person is 1.85 m tall). (c) Channels or rills with concentration of gravel and winnowing of finer grain sizes by secondary runoff on the surface of a debris-flow-dominated fan in the Atacama Desert, northern Chile; person for scale is 1.85 m tall. (d) Debris-flow fan surface pervasively covered by aeolian sand (Atacama Desert, northern Chile). Note residual relief given by elevated, gravelly debris-flow levees. (e) Debris-flow fan in northern Panamint Valley (USA), of which the surface is covered by aeolian sand sheets and dunes. The surface no longer hosts typical debris-flow morphology whereas the stratigraphic exposure shows clear evidence of debris-flow formation. (f) Different perspective of the sand covered debris-flow fan surface in northern Panamint Valley shown in panel e.

 

Further Reading

De Haas, T., Ventra, D., Hauber, E., Conway, S. J., & Kleinhans, M. G. (2015). Sedimentological analyses of Martian gullies: The subsurface as the key to the surface. Icarus, 258, 92-108.

De Haas, T., Hauber, E., & Kleinhans, M. G. (2013). Local late Amazonian boulder breakdown and denudation rate on Mars. Geophysical Research Letters, 40(14), 3527-3531.

De Haas, T., Ventra, D., Carbonneau, P. E., & Kleinhans, M. G. (2014). Debris-flow dominance of alluvial fans masked by runoff reworking and weathering. Geomorphology, 217, 165-181.

Malin, M. C., & Edgett, K. S. (2000). Evidence for recent groundwater seepage and surface runoff on Mars. Science, 288(5475), 2330-2335.

Dickson, J. L., & Head, J. W. (2009). The formation and evolution of youthful gullies on Mars: Gullies as the late-stage phase of Mars’ most recent ice age. Icarus, 204, 63-86.

Blair, T. C., & McPherson, J. G. (2009). Processes and forms of alluvial fans. In Geomorphology of Desert Environments (pp. 413-467). Springer Netherlands.

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