Single-thread rivers in barren landscapes: Formation, lateral migration, and deposits of pre-Silurian and ancient Martian rivers

Abstract:

Rivers are the arteries of Earth’s continents, carrying water, sediments, and solutes from headwaters to the oceans. In doing so, they contribute to complex feedbacks between erosion, deposition, climate, and tectonics. Thus, understanding river morphodynamics is a critical aspect to deciphering the geologic history of planets. Today, land plants have colonized most terrestrial environments and are a primary component of fluvial systems: they affect river functioning and have even been proposed to enable the formation of single-thread rivers. However, land plants have only been present for the most recent ~10% of Earth’s history. Unfortunately, Earth’s geodynamic regime strongly biases the terrestrial fluvial record towards that 10% of Earth’s history, and very little is known about how rivers might have functioned prior to the advent of land plants. Conversely, Mars most likely never had land plants, and because of the absence of plate tectonics, the deposits of billion-year-old rivers are readily observable on the Martian surface.

Here, we present a combination of field observations from modern unvegetated rivers, conceptual modeling, and ancient fluvial deposits on Mars that summarizes our recent findings on the formation, lateral migration, and deposits of single-thread rivers in barren landscapes. First, we show that contrary to the classic view, single-thread rivers may readily form in the absence of vegetation when mud is present in the banks. Second, we show that, although the geometry of single-thread rivers appears to be largely unaffected by the presence of vegetation, unvegetated rivers migrate laterally about an order of magnitude faster than their vegetated counterparts. Finally, we discuss the implications of these findings for interpreting the rock record of pre-Silurian and ancient Martian rivers.