Structure of channel-belt deposits within sinuous ridges on Earth and Mars

Introduction

Inverted channel deposits or sinuous ridges are common on the Late-Noachian Early-Hesperian terrains on Mars^[1,2]. These exhumed ribbon-like, sinuous sedimentary ridges have been interpreted to be evidence of existence of liquid water on early warm and wet Mars. In addition, these ridges have been interpreted to form as either: 1) short duration single-thread fluvial systems, likely from a single flow event^[3] or 2) longer duration multi-thread systems which form channel-belt structure^[4]. While single-thread systems more closely preserve the actual geomorphology of the fluvial system, the longer period of duration for the channel-belt systems makes it more conducive for ancient habitability. However, the channel-belt structure is inherently complex, preserving an interplay of contrasting lithologies and flow conditions over relatively longer periods of time. Considering these sinuous ridges are also sampling targets for rovers such as NASA’s Perseverance mission^[5], a detailed sedimentary analysis of preserved stratigraphy is important to understand the spatio-temporal distribution of architectural elements. Since in-situ observations of these ridges from Mars is still limited, sedimentology of Earth-based exhumed channel-belt deposits can be an important tool for reconstruction of long-lived fluvial processes on Martian surface.

 

Data and Methods

For this study, we rely on a combination of field-based observations from Earth and satellite image and topographic datasets from Mars. One of the best preserved sinuous ridge structures on Earth in the Oligo-Miocene Caspe Formation, Spain was utilized as a Terrestrial analogue. The sinuous ridges here formed a part of Guadalope-Matarranya fan that developed in an endorheic Ebro basin^[6]. For the purpose of sedimentological analysis, detailed photopanel interpretation (recording detailed facies and architectural observations) was carried out at different roadcut outcrops, which provided a view nearly orthogonal to the axes of the ridges. This was supplemented with secondary observations in longitudinal view, parallel to the axes of the ridge. To compare this with Martian examples, 25–50 cm/pixel HiRISE^[7] orthomosaics and derived DEMs were utilized to construct 3D models and conceptual architectural models were prepared for similar sinuous ridges on Mars at Hypanis fan, Eberswalde fan and Aram Dorsum.

 

Results and Discussions

In our example from Spain, we record that the extensive three-dimensional exposures exhibit facies relationships and sedimentary structures indicative of an amalgamated multistory sand bodies formed due to stacked sandstone bodies separated by erosional surfaces. Each individual story was interpreted to represent singular channel unit. These could be divided into thicker sandstone comprised channel-axis complexes and either associated or truncated thinner channel-wing complexes which extend into the floodplain horizon and preserve abundant floodplain mudstone between two such channel-wings. In addition, on each ridge face, three hierarchal surfaces were identified - Channel-bounding Surfaces, Bar-bounding Surfaces and Accretion surfaces. The nearly orthogonal ridge view also preserved the saw-tooth edges, which are commonly associated with reoccupational stacking where channels avulse and occupy previous channel locations^[8].

Individual elements of this observed stratigraphy in the field are similar to those reported on Mars ^{[e.g. 9]}. These field based observations were used to prepare the conceptual models in form of synthetic cross-sections for Martian sinuous ridges and fluvial channel systems. With HiRISE mosaics, we identified key sedimentary features, such as point bars, recessive and protruding layering on the side of the ridges, etc and associated them with elevation data to identify possible distribution of architectural elements within the Martian sinuous ridges. This is helpful in understanding and predicting the distribution structures and lithologies, especially the finer-grained floodplain lithologies, which are important astrobiological targets. The structure of stacking within the Martian ridges also has important implications for usage of ridge elements in paleohydraulic reconstructions.

 

References

[1] Tanaka et al.,(2014), Geologic Map of Mars: USGS Scientific Investigations Series Map 3292, scale 1:20,000,000, pamphlet 43 p. [2] Carr et al., (2010), Earth and Planetary Science Letters 294(3-4), pp.185-203. [3] Zaki et al., (2022), Journal of Geophysical Research: Planets, 127, e2021JE007087. [3] Hayden et al., (2019), Icarus, 332, pp.92-110. [5] Farley et al., (2020) Space Science Reviews, 216(8), pp.1-41. [6] Cuevas Martinez et al., (2010), Sedimentology, 57(1), pp.162-189. [7] McEwen et al., (2007) Journal of Geophysical Research: Planets, 112(E5). [8] Chamberlin and Hajek, (2015) Journal of Sedimentary Research, 85(2), pp.82-94. [9] Skinner Jr et al., (2021), Icarus, 354, p.114071.