STREAMLINED ISLANDS OF MARS

It has been long debated whether streamlined forms located on the floors of Martian channels are depositional or erosional in origin. Streamlined elevations may form by deposition, erosion or both, where the channel floor is exposed to liquid water, but not generally in channels whose morphology is dominated by sapping processes.

On Earth, bars form where sediment is deposited within the channel and their surface remains submerged. Islands are erosional remnants of the floodplain or bedrock. Terrestrial definitions are difficult to apply to the now-dry and likely ephemeral paleochannels of Mars. Bars may be bedrock-cored, formed by accumulation of fine sediment on top or beside bedrock outcrops, e.g., pendant bars. On Mars, crater-cored islands may be the most common island form, where the tail or downstream side of the island is depositional.

There are few instances of channel belt features interpreted as likely depositional. These sites occur in ancient valley networks, small mid-latitude valleys, and outflow channels. However, most streamlined islands in outflow channels are still thought to be erosional remnants.

In Navua Valles, fluvial sediments were deposited from the Late Noachian to the Late Amazonian. Many bar fields are located within channel bends or where the channel widens. This setting is characteristic of expansion bars in scabland areas. Two other terrestrial channel bar types are also possible analogs: 1) Pendant bars, which are often separated from the channel walls on their landward sides by “fosses” (narrow secondary channels) that prevented deposition; and 2) Free, migrating longitudinal mid-channel bars, which also form after large flood events. The erosional events that carved the widest Navua Valles channels may have had bankfull discharge rates on the order of 100,000 m³/s. These rates are consistent with the order of magnitude of peak discharges (60,000 m³/s ) that formed Box Canyon, ID, and are one and two orders of magnitudes lower than the Missoula or Bonneville floods, respectively.

Fluvial dunes have also been proposed in a few locations on Mars. These include flood-formed dunes, antidunes or transverse ribs, and “flood bars”, all proposed to be catastrophic flood-related. In Navua Valles, no signs of fluvial dunes were found on almost complete CTX and available HiRISE coverage.