Universality of delta bifurcations

Post by Dr. Robert C. Mahon, Department of Earth and Environmental Science, University of New Orleans

Which morphologic features of sedimentary systems persist into the stratigraphic record? Ancient river deltas preserved as stratigraphic deposits on both Earth and Mars exhibit remarkable morphologic similarities to modern deltas on Earth (Images 1-3). While channel dynamics may be expected to alter the geomorphic expressions of past channel networks, in many cases channel bodies appear to be preserved in their original configurations. Fully understanding the ways in which geomorphic features become preserved as stratigraphy can provide tools for us to both infer past processes from the ancient deposits with greater confidence, as well as to predict the geometries of ancient deposits in the subsurface (i.e. for resource exploration).

Aeolis

Image 1: CTX image mosaic of a delta in the Aeolis region of Mars, showing distributary channel networks. CTX imagery courtesy of NASA-MRO/JPL/UA.

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The Largest Delta on Mars?

Post by Jacob Adler, School of Earth and Space Exploration, Arizona State University.

Ancient river deltas are found in many locations on Mars [see Di Achile & Hynek, 2010 and references therein], and are formed as sediment drops out of suspension in water as it approaches a wider shoreline of a lake, sea, or (debatably) an ocean. Some proposed deltas on Mars are found in closed basins (e.g. an impact crater) away from the Martian dichotomy boundary, implying an ancient climate during which the crater ponded with water [e.g. Eberswalde or Jezero]. Occasionally, inlet and outlet river valleys are seen at different elevations along the crater rim, lending further evidence to the hypothesis that the crater filled with liquid water at least up to the outlet elevation. Deltas found in open basins, on the other hand, imply a larger body of standing water, and Mars scientists look for other clues to support the deltaic rather than alluvial fan formation mechanism. In our recent papers, we tested whether the Hypanis fan-shaped deposit (Image 1) could be a delta, and discussed whether this supports the hypothesis that there was once a large sea or ocean in the Northern plains of Mars [Adler et al., 2018; Fawdon et al., 2018].

Image1

Image 1: a) The Hypanis deposit stands out as light-toned in the center of this Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) mosaic of our study region. Also marked are Lederberg crater, and the Sabrina deposit in the closed basin of Magong crater. (b) Hypanis and Sabrina have a low nighttime temperature (dark) as recorded by the THEMIS instrument on Mars Odyssey, suggesting it is mostly composed of small grain size material. Image from the Nighttime IR 100m Global Mosaic v14.0 [Hill et al. (2014); Edwards (2011)] and from the Northern Hypanis Valles Night IR Mosaic [Fergason (2009)]. NASA/JPL/ASU. (c) Our proposed fluvial sequence discussed in the paper. Main lobe (A) could once have had continuous layered beds spanning to the distal island deposits (E). The cross-cutting relationships we observed are consistent with hypothesized shoreline regression to the north. Flow migrated to the northern lobe (B), then to braided inverted channels (C and D) as water retreated. NASA/MSSS/USGS. d) Our digital elevation mosaic shows the topography of Hypanis and surrounding features. Elevations are colored from white (-2500 m) to light green (-2800 m).

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Stepped Fans and Phyllosilicates on Mars

Post by Peter Grindrod, Natural History Museum, London, UK.

A number of different studies have catalogued features on Mars that could be given the general heading of sedimentary fans [e.g. Irwin et al., 2005; Kraal et al. 2008]. These features occur whenever the velocity of a river or stream decreases, and the water no longer has enough energy to carry its sediment, and thus begins to deposit its load. This drop in energy often occurs when the water flows into flatter and wider regions. The distribution of these fans on Mars is important because it shows the location of past water flows, and the amount of material that has been transported (which can be used as a proxy for flow duration).

However, one of the fundamental problems when looking at these features with orbital data alone, is that it is difficult to determine whether the river flowed into a standing body of water (for example a lake) or just an empty canyon or crater. Of course, the implications of this problem are important if we want to understand the volume and distribution of past water on Mars, which in themselves feed into understanding the past climate and even habitability of Mars.

Figure 1

Image 1: Location of the two fans in Coprates Catena, SE Valles Marineris. MOLA elevation overlain on THEMIS daytime image.

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Valleys, Deltas, and Lacustrine Sediment in the South-western Melas Basin, Valles Marineris, Mars

Post contributed by Joel Davis, Department of Earth Sciences, University College London, UK.

During the last few decades, dry river valley networks and delta fan structures have been found to be increasingly common on ancient terrains on the martian surface (e.g. Goldspiel and Squyres, 1991; Hynek et al., 2010). They are considered to be one of the main lines of evidence that Mars once had Earth-like precipitation and surface runoff (e.g. Hynek and Phillips, 2003). One such location is the south-western Melas basin, part of a collapsed graben structure on the southern wall of Melas Chasma, Valles Marineris – Mars’ equatorial canyon system (Images 1 & 2). The basin likely formed in the early Hesperian period (~ 3.7 – 3.5 Ga), soon after Melas Chasma opened.

Image 1

Image 1: Context Camera image-mosaic of western portion of palaeolake sequence in the south-western Melas basin. In the left of the image, valley networks can be seen converging on a delta-like structure at the centre of the image. Layered lacustrine deposits are well exposed in the right of the image; about 40-50 packages are visible at this resolution. [Image numbers: G22_026866_1710_XN_09S077W & P07_003685_1711_XI_08S076W]

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Experimental Delta Formation in Crater Lakes

Post by G. de Villiers, Faculty of Geoscience, Utrecht University.

Fan-shaped deposits have been identified on the surface of Mars (Image 1). These sediment bodies often occur within impact craters and, specifically in the cases of fan deltas, suggests that these craters were once lakes early in Martian history. Fan delta morphologies are indicative of upstream (e.g. flow discharge and sediment properties) and downstream (e.g. basin characteristics) parameters, from which the hydrological conditions at the time of formation can be inferred (e.g. Kleinhans et al. 2010).

IAGFigure1

Image 1: Examples of fan delta deposits on Mars, formed in enclosed impact crater or rift basins. A) Single-scarped, branched prograding delta (PSP_006954); B) Single-scarped, smooth prograding delta (I10805012); and C) Multiple-scarped, stepped retrograding delta (V17040003). White line is approximately 5 km.

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