Global inventory of fluvial ridges on Earth and lessons applicable to Mars

Orbiting and landed spacecrafts have provided vast amounts of data on fluvial and fluvial-related landforms and sediments on Mars. One variant of these landforms consists of ridges interpreted to be remnants of ancient fluvial activity, observed at thousands of Martian locales, suggesting extensive precipitation and runoff on early Mars. The importance of these ridges increased after the selection of the Jezero crater and the Oxia Planum landing sites for NASA?s Perseverance and ESA?s Rosalind Franklin rovers because these locations preserve fluvial sediments in ancient, lithified deltaic landforms. To further understand Martian fluvial ridges, this contribution presents an inventory of 104 sites of fluvial ridges on Earth, offering Upper Ordovician to late Holocene examples that are analogous to their Martian counterparts. The Earth global data set provides environmental contexts for the formation mechanisms and conditions that accompanied channel and valley formation, erosion, and inversion. We describe four types of erosion-resistant capping materials by which channel and valley forms became ridges after materials adjacent to channels were eroded away: (1) twenty-seven cases of volcanic rock-capped ridges; (2) sixty examples of duricrust-capped ridges (eogenesis); (3) four examples of ridge sites formed through armoring by large clasts; and (4) thirteen locations where ridges were formed by erosion after deep burial and tectonic return to the Earth?s surface (mesogenesis). Water erosion, wind erosion, and mass wasting all play a role in removing the surrounding terrain, leaving the channel deposits or valley fills standing as ridges. Such ridges can contain as little as 0% and up to 100% fluvial sediments deposited in paleochannels and paleovalleys. Many of Earth?s fluvial ridges that preserve channel fill sediments were formed as a response to shortduration seasonal or intermittent fluvial events (i.e. a few thousand to tens of thousands of years). Ridges formed from stacked channel-belts mark vertical aggradation and lateral migration occurring over more prolonged periods of fluvial activity (i.e. millions of years). Underlying materials can be older alluvial or fluvial, aeolian and lacustrine deposits, and weathered bedrock. The wide range of fluvial ridge composition and formational pathways on Earth suggests that the accumulation of the Martian fluvial deposits in paleochannels and paleovalleys (now expressed as ridges) also likely developed by both short-lived and long-lived fluvial activity. Cementation of fluvial deposit and potentially channel-adjacent sediments in the surface/near-surface environment could be prevalent on Mars, particularly because tectonism, as compared to Earth, plays a negligible role in returning buried materials to the surface. Fluvial ridges that were capped by volcanic products are also common on Earth, but very few have been identified on Mars.The discrimination of ridges exposed by erosion after deep burial (mesogenesis) versus those never buried nor exhumed (eogenesis) is not feasible based on remotely-sensed data unless cross-cutting relationships are visible in plan view. Our observations on preserved fluvial deposits compared with the underlying materials in terrestrial ridges infer that uncertainties could be significant in the case of paleohydraulic reconstructions made using measurements of the geomorphic parameters of ridges from high-spatial-resolution orbiter images.

Automated summary

Data from orbiting and landed spacecrafts on Mars have provided valuable information on fluvial landforms and sediments. Ridges interpreted as remnants of ancient fluvial activity are observed at numerous Martian locales, indicating extensive precipitation and runoff on early Mars. Different types of erosion-resistant capping materials have been identified as the formation mechanisms for Martian fluvial ridges. Water erosion, wind erosion, and mass wasting play a role in shaping these ridges by removing surrounding terrain. The composition and formational pathways of Earth's fluvial ridges suggest similarities in the accumulation of Martian fluvial deposits, potentially developed by both short-lived and long-lived fluvial activity.