Mineralogical, Sedimentological, and Geomorphic Distinction of Fan-Delta and Alluvial Fan Deposits in Southeast Coprates Chasma: Observations from CaSSIS and OMEGA

Fan-shaped deposits (FSDs) on Mars are key geomorphic indicators of past surface water activity and provide important constraints on sedimentary processes, hydrology, and paleoenvironments [1]. These landforms have been widely detected across the planet using orbital imagery, particularly at the margins of basins, craters, and valley networks, recording the sediment transport and water availability [2]. Within Coprates Chasma, fan-shaped deposits offer an opportunity to investigate localized depositional processes in Valles Marineris, where fluvial, lacustrine, and mass-wasting processes have been documented [3].

Using high-resolution imagery from the Colour and Stereo Surface Imaging System (CaSSIS) [4] aboard the ExoMars Trace Gas Orbiter, we identify two fan-shaped deposits in Southeast Coprates Chasma. In CTX basemap imagery, both deposits display similar geomorphic characteristics, including (1) branched channel networks in their source regions, (2) evidence for sediment transport along a ~35 km thalweg toward their apices, and (3) radially convex sedimentary bodies with comparable dimensions, approximately ~5 km in width and ~3 km in length at the downstream end of the source areas. Despite these geomorphic similarities, CaSSIS near-infrared, panchromatic, and blue (NPB) composites reveal distinct colour differences between the two FSDs. FSD A exhibits a light purple tone, whereas FSD B appears to be dark blue. Observations from Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) [5] spectra hint that the light purple signature has an absorption band in 0.91 µm, while the dark blue signature in 1.04 µm.

We interpret these combined geomorphic and spectral observations as evidence for differing depositional environments. FSD A is interpreted as a fan-delta [6], formed where sediment-laden flows entered a standing body of water, promoting finer-grained deposition and the relative enrichment of Low-Calcium Pyroxene (LCP) bearing materials. In contrast, FSD B is interpreted as an alluvial fan, deposited under subaerial conditions dominated by episodic fluvial activity and limited aqueous alteration, preserving High-Calcium Pyroxene (HCP) rich compositions. These findings highlight the importance of integrating high-resolution morphology with spectral data to distinguish between superficially similar fan-shaped landforms and to better constrain the hydrological history of Coprates Chasma.

 

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