Inverted River Channels and Volcanic Channels

Dr. Ross Irwin

When rivers deposit sediment in their channels and on the surrounding floodplains, the channel sediments usually have a larger grain size than the floodplain sediments. The channel sediments may also become strongly cemented after the area is buried. When these deposits are later eroded, the old floodplain sedimentary rocks are weaker and more easily eroded than the channel-fill rocks, which are left behind as a sinuous ridge, often called an inverted paleochannel. These features are common on Mars, where wind has blown away the fine-grained floodplain sediments. Understanding how these features form and are preserved on Earth helps to constrain the size of the rivers that formed them on Mars1.  This project is a collaboration between the Planetary Science Institute and the Smithsonian.

Inverted paleochannels have been identified in many areas around the globe, but the ones in central Utah are outstanding examples. These paleochannels formed in the Late Jurassic and were buried by ~900 m of marine sediment during the Cretaceous, which experienced a global increase in sea level1. Only after ~75 million years of burial were the paleochannels exhumed during regional uplift in the middle to late Cenozoic1. The carbonate cementation of the channel sediments hardened them and enhanced their resistance to erosion. The less resistant surrounding valleys were eroded away, mostly by water rather than wind, leading to the current inverted state of the Utah paleochannels1.

Whereas most Martian river valleys are networks of tributaries, some emerge from a single pit or fault and appear to have an underground water source.  Some of these valleys may have a volcanic origin, and we need a list of diagnostic features to tell lava and river channels apart.  There are few large lava channels on Earth to compare with those on Mars.  Our field sites include Iceland and Hawai'i. This project is a collaboration between NASA Goddard Space Flight Center and the Smithsonian.

Ross Irwin

Figure: Inverted channels near Green River, Utah, seen from the air.

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1. Williams et al. (2007), Exhumed Paleochannels in Central Utah — Analogues for Raised Curvilinear Features on Mars.