Inverted stream channels in the Western Desert of Egypt: Synergistic remote, field observations and laboratory analysis on Earth with applications to Mars

Highlights

• We use satellite images to identify seven sites at which inverted fluvial channels of various scales and generations occur in Egypt that previously were relatively unknown to the Earth and Mars sciences.

• Field access is presently limited; for the two sites we could visit, our geochemical and grain size analyses provide new data regarding sedimentology, mineralogy, and cement composition.

• We identify inverted channels that developed via sediment induration, surface armouring and a potential example of sediment lithification.

• The inverted channels in the Western Desert of Egypt record incision and inversion episodes that occurred during the last 0.1 to 94 Ma.

Abstract

Inverted relief landforms occur in numerous regions on Mars, ranging in age from Noachian to more recent Amazonian periods (<3.0 Ga). A better understanding of the conditions in which inverted fluvial channel features on Earth form, and the geologic records they preserve in arid settings, can yield insights into the development of inverted landforms on Mars. Inverted channel landforms in the Western Desert of Egypt are well represented across an area of ∼27,000 km². We investigated inverted channel features at seven sites using remotely-sensed data, field observations, and lab analysis. Inverted channel features in the Western Desert record fluvial environments of differing scales and ages. They developed mainly via inversion of cemented valley floor sediment, but there is a possibility that inverted fluvial landforms in the Dakhla Depression might have been buried, lithified, and exhumed. A few examples, in the southeastern part of the Western Desert, record, instead, a resistance to erosion caused by surface armouring of uncemented valley floor sediment. We show that the grain-size distribution for investigated and reviewed inverted channels is highly variable, with boulders that are commonly 0.35 – 1 m in size; large particles provide high porosity that influences the cementation mechanism. The studied inverted channel sediments are mainly cemented with ferricrete, calcrete, gypcrete, and silcrete. Inverted channels are valuable for the reconstruction of paleoclimate cycles or episodes on Earth and Mars; observations from the Western Desert, when offered as analogs, add to the growing list of Earth examples that provide suites of observables relevant to reconstruction of paleoenvironmental conditions on Mars.

Introduction

Geomorphic inversion of topography can occur when geological materials that fill or partly fill a negative-relief landform, or the floor-forming materials of a negative-relief landform, offer greater resistance to erosion than the material into which the negative-relief landform was inset, and are subsequently exposed to erosive events that remove the material surrounding the nascent negative-relief landform. In many cases this occurs at the land surface. In others, the inversion of relief occurs long after the negative relief landform was filled and buried, and its fill, floor, and host material became lithified, and are later returned to the surface by some combination of tectonic and erosional exhumation. DiBiase et al. (2013) describe the variants on this theme as “landscape inversion” versus “deposit inversion”. In landscape inversion, denudation leaves a former depression-filling, erosion-resistant material standing higher than the surrounding terrain, whereas in deposit inversion, sediments less resistant to erosion are removed to leave more resistant materials standing higher without necessarily impacting the entire landscape.

Analyses of planimetric patterns have identified hundreds of definitive and candidate sites of inverted channels on the surface of Mars (e.g., Malin and Edgett, 2003, Williams et al., 2007, Williams et al., 2011, DiBiase et al., 2013, Davis et al., 2016). These range from a few hundred meters to several tens of kilometers in length, between 10 m and a few kilometers in width, and they rise as much as 50 m above the surrounding terrain (Burr et al., 2009, Williams et al., 2007). The various geomorphic characteristics of inverted drainage networks on Mars may be indicators of a range of paleoenvironments that provide clues to the complex fluvial and climate history of Mars (Williams et al., 2007, Williams et al., 2009, Pain et al., 2007, Burr et al., 2010, Davis et al., 2016). Williams (2007) concluded that inverted channels on Mars are developed on terrain that ranges in age from Noachian to more recent Amazonian periods (<3.0 Ga).

Inverted stream channels occur at several localities on Earth, including parts of Africa (King, 1942, Giegengack, 1968, Butzer and Hansen, 1968, Haynes, 1980, Aref, 2003, Embabi, 2004, Zaki, 2016, Zaki and Giegengack, 2016, Giegengack and Zaki, 2017); the Arabian Peninsula (Miller, 1937, Holm, 1960, Maizels, 1983, Maizels, 1987, Maizels, 1990, Maizels and McBean, 1990); in South America in the Cañadón Asfalto Basin of Argentina (Foix et al., 2012); in western North America in Utah (Derr, 1974, Harris, 1980, Williams et al., 2007), New Mexico and west Texas (Reeves, 1983), Oregon (Niem, 1974, Orr and Orr, 2000); in Europe in the Ebro Basin of Spain (Friend et al., 1981, Cuevas et al., 2010); at several locations in Australia (Mann and Horowitz, 1979, Pain and Ollier, 1995); and in Asia in the Kumtagh Desert of China (Wang et al., 2015).

We know from Earth that at least five pathways lead to production of the type of landforms interpreted as inverted stream channels (or inverted forms of the valleys in which they occur). Each involves differential erosion that removes channel- or valley-host material and leaves channel- or valley-fill material, or original channel sediments, standing in relief relative to the surrounding terrain:

• Pathway 1–Volcanism

  A channel, valley, or depression becomes filled or the fill is capped with lava or welded tuff which, when solidified, is more resistant to erosion than the material (e.g. bedrock) in which the depression formed or into which the stream was cut. This process was described as early as the late 19th century by Le Conte (1880). Examples on Earth include the Stanislaus Table Mountain in California, USA (Rhodes, 1980, Rhodes, 1987), Wrights Point ridge in Oregon, USA (Niem, 1974), and the Black Ridges near St. George, Utah, USA (Williams et al., 2009, Williams et al., 2011). Many examples also occur in southeastern Australia (Ollier 1967).

• Pathway 2–Sediment induration (early diagenesis; eogenesis)

  Eogenesis, or early diagenesis, occurs in the sedimentary depositional setting (Worden and Burley 2003). Via this pathway, stream sediment, or sediment that fills or partially fills a depression, channel, or valley, becomes cemented (e.g. via formation of silcrete, calcrete, ferricrete, etc.) but is not deeply buried. The material becomes more resistant to erosion than the material in which the depression, channel, or valley was formed. Pain and Ollier (1995) described on Earth.

• Pathway 3–Sediment lithification (burial diagenesis; mesogenesis)

  Stream sediment, or sediment that fills or partially fills a depression, channel, or valley, becomes buried and lithified and becomes or is intrinsically (owing to clast content) more resistant to erosion than the material in which the depression, channel, or valley was formed. Subsequent exhumation, on Earth commonly driven by tectonic uplift, returns these materials to the surface. Examples on Earth include the sandstone-channel sediments near Green River and Hanksville, Utah, described by Williams et al., 2007, Williams et al., 2009, Williams et al., 2011, and Clarke and Stoker (2011).

• Pathway 4–Armouring lag formation

  Stream sediment, or sediment that fills or partially fills a depression, channel, or valley, does not become cemented but is more resistant to erosion because of the relative abundance of large clasts that are less easily removed by erosion (e.g. pebbles, cobbles, and boulders). Examples on Earth include landforms in New Mexico, USA (Bryan 1940), in the Kumtagh Desert, China (Wang et al., 2015), and the southeastern part of the Western Desert, Egypt (Zaki and Giegengack, 2016; Giegengack and Zaki, 2017).

• Pathway 5–Lowering after ice removal

  Glacial eskers are sinuous ridges produced by fluvial sedimentation that occurs when water and sediment flow through a stream in ice sheet within a glacier (Pain et al., 2007). After the glacier is gone, the valley sediment stands as a ridge. A stream that transports sediment across the surface of an ice-covered lake can also leave behind a ridge indicative of its former presence (Hall et al., 2006).

• Pathways–Mars

  Understanding which of the above pathways led to the inversion of a fluvial form on Mars is important for understanding the paleoenvironmental circumstances that are recorded by it, both in terms of its formation as well as its inversion. The ability to distinguish among these, on Mars, will require development of observable criteria which can be identified via tools and instruments aboard orbiting, airborne, or mobile landed platforms. Making such distinctions is, at present, challenging. An example case of channel inversion resulting from Pathway 1, volcanism, is presented in Fig. 1. Based on its context within the volcanic Tharsis region of Mars, Fig. 1 shows a former channel which had cut across a crater ejecta blanket, became the course for a lava flow, and later became inverted as the host material (the impact ejecta deposit) was lowered by erosion to leave the occupying lava flow standing higher than the surrounding terrain. Figs. 2 and 3 show inverted channel landforms that appear to have been exposed by removal of overlying rock, as well as removal of the rock that once hosted the channels. Are these examples that followed Pathway 3, in which channel sediments were deeply buried, lithified, then later returned to the surface, or were they examples that followed Pathway 2, via cementation in the surface/near-surface environment, but not sediment compaction and cementation via deep burial? Fig. 4 shows a case of inverted stream networks on Mars that exhibit bouldery surfaces; are these examples of armoring by lags formed from boulder-rich channel sediment (Pathway 4), or are the boulders the erosion-resistant remnants of lithified channel sediment (e.g., sandstones) once deeply buried (Pathway 3)?

  Distinction of pathway and process is important for deciphering not only the history recorded by a given material or landform present at the surface of Mars today, but also to provide certainty regarding features which can be measured and fed into quantitative models that probe paleohydrology and paleoclimate. For example, knowing whether the boulders observed on the surface of a given inverted channel on Mars in high spatial resolution images acquired by cameras on aerial or orbital platforms are clasts produced by the weathering and erosion of lithified, finer-grained channel sediment, or whether they are large clasts that were transported in the stream itself, is extremely important to understand before proceeding with further analysis or interpretation. This can be challenging to do without a depth and breadth of knowledge regarding Earth analogs that represent the range of forms and pathways that led to creation and erosion of inverted stream landforms.

• Investigation in the Western Desert of Egypt

  The Western Desert of Egypt exhibits a wealth of inverted fluvial landforms—also known as inverted wadis (Embabi 2018)—that are mostly unknown to investigators of inverted topographic forms on Earth and Mars. This paper is an initial summary of current knowledge and our initial remote sensing, field, and laboratory observations regarding these inverted channels. The purpose of this paper is to add a brick to the foundation necessary for the interpretation of the pathways by which stream channels and their enclosing valleys might have become inverted on Mars. Ultimately, the goal is to be able to identify diagnostic features at scales observable by instrumentation carried by orbital, aerial, and surface-mobile (e.g., rovers) platforms on Mars. To that end, we present new information about a little-known suite of inverted stream landforms in Egypt. In particular, we present observations regarding the following:

  • Detailed maps of inverted stream channel occurrences at previously unpublished sites on the surface of the Limestone Plateau in the Western Desert (sites B, C, D and E) to document where they occur as a step toward understanding behavior of their paleodrainage systems (paleohydrology and geochronology) in future work;

  • Preliminary geomorphic observations from satellite images and field investigation;

  • Sedimentology and mineralogy of a few of these inverted channel bodies to probe how they became cemented;

  • The time scales over which the inverted channels in the Western Desert may have formed, as this may be relevant to eventual efforts to constrain the timing of incision and inversion events or rates on Mars.