Post contributed by Abdallah S. Zaki, Department of Geography, Ain Shams University.
The evolution of inverted topography on Earth and Mars can result from surface armouring of the channel, infilling of channels/valleys by lava flows, and cementation of valley floor by secondary minerals (such as, calcium carbonate, gypcrete, ferricrete, calcrete) – see post by Rebecca Williams. This post specifically concerns inverted wadis, which have been identified in a number of localities on Earth, including multiple localities in the Sahara and Arabia, Australia, the Ebro Basin of Spain, Utah, and New Mexico and west Texas (e.g., Miller, 1937; Maizels, 1987; 1990). Inversion of relief is observed commonly on Mars, for example, Eberswalde Crater, Arabia Terra, Juventae Chasma, Olympus Mons, and Antoniadi Crater (e.g., Pain et al., 2007; Williams et al., 2007).
Image 1 shows an inverted dendritic pattern which covers an area of about 20,000 square kilometres in Nejd, eastern Saudi Arabia. Wind, assisted by percolating action of underground waters, led to the development of inverted topography in this area and the inversion is a result of the valley floor being cemented by calcium carbonate. The exhumed ridges in this image are up to 8 km in length, stand approximately 21 m in height, and are up to 210 m wide. Ridge cross sections range from trapezoidal to convex in shape and have steep side slopes between 8° and 26°.
The inverted wadis in SE Egypt shown in Image 2 are rectilinear in planview, with an average length of about 10 km, widths not exceeding 300 m, and heights of up to 20 m. Inverted wadis in this area developed via mineral cementation and by surface armouring in some localities. Giegengack (1968) found that the cementing minerals are calcium carbonate and iron oxide, and that grainsize of the inverted forms can range from meter-sized boulders to fine sand.
Here I show an alluvial fan on the floor of a 60 km diameter equatorial crater on Mars (Image 3). In the fan, there are well-delineated channels which appear as dissected ridges. These channels could have become inverted because the floor of channels cemented by minerals, covered by large boulders, or filled by lava. Therefore a better understanding of the mechanism leading to inversion in this example could be extremely valuable to better understand paleo-environmental conditions, including a possible warmer, wetter past climate on Mars.
The second martian example is in Anotoniadi Crater (Image 4). These are short dendritic tributaries that connect southward to a large trunk. These dendritic features are up to several kilometres in size. This zone may have had a lake that dried, similar to inverted wadis in SE Egypt, which had many small lakes.
Study of terrestrial analogues is important in order to understand the range of fluvial environments which could be preserved as inverted topography on Martian surface, the nature/source of minerals which could have led to the inverted topography, and the range of geomorphic expressions for this landform. Sites of inverted relief are important to identify as candidate landing sites for future missions, because of their potential to preserve signs of ancient life.
Miller, R. P. (1937), Drainage lines in bas-relief, Journal of Geology, 45, 432-438.
Maizels, J. (1987), Plio-Pleistocene raised channel systems of the western Sharqiya (Wahiba), Oman. In: Frostick, L., and Reid, I. (Eds), Desert sediments-ancient and modern. British Geological Society, Special Publication, 35, 31-50.
Maizels, J. (1990), Raised channels as indicators of palaeohydrologic change-a case study from Oman, Palaeogeography, Palaeoclimatology, Palaeoecology, 76, 241-277.
Pain, C. F., J. D. A. Clarke, and M. Thomas. (2007), Inversion of relief on Mars, Icarus, 190, 478-491.
Williams, R. M. E., Chidsey, T. C., and D. E. Eby. (2007), Exhumed paleochannels in central Utah-analogs for raised curvilinear features on Mars, In: Willis, G. C., Hylland, M. D., and Chidesy, T. C. (Eds), Central Utah-Diverse Geology of a Dynamic Landscape. Utah Geological Association, Salt Lake City, Publication, 36, 221-235.