An Unusual Lobate Deposit in Coprates Catena, Mars

We have studied a lobate deposit centered at -15.2 S, 299.7 E within one of the troughs of Coprates Catena in order to investigate the role that water may have played in its formation. The deposit, which is fan-like in appearance, is 7.1 km in width and 8.3 km in length, and it emanates from a 45 km long sinuous channel that cuts through the plains before terminating in the trough system. The channel is one of several in the area that flow northeast, roughly perpendicular to the NW-SE trough system of Coprates Catena. The channel increases in depth and width downvalley, with a maximum depth of 1.7 km and width of 4.0 km measured where it enters the trough. The channel has no obvious source visible in the MOC or THEMIS images. Although the main channel terminates at the intersection of the trough, a much smaller channel has incised the uppermost portion of the deposit and is oriented along the same northeast trend. There is no terminal deposit associated with this smaller channel. The lobate deposit is approximately 1.2 km thick based upon a MOLA profile that crosses it, and the top of the deposit lies 1.7 km below the top of the trough's southern end. There are concentric steps along the sides of the deposit, suggesting either multiple periods of deposition with each subsequent deposition episode smaller in size or modification of the unit post-deposition. The clear association between the channel and lobate deposit indicates that material was transported along the channel and deposited on the trough floor once the channel entered into the trough. Although both the channel and lobate deposit are younger than this particular trough, to the south another NW-SE system of troughs has disrupted the channel by cutting though it. There are also several small impact craters on the lobate deposit, suggesting that it is not much younger than the adjacent plains or the trough system where it is located. A light-toned unit in the eastern portion of the trough may represent another water-lain deposit, perhaps an evaporitic unit formed when water from the channel partially filled the trough. Thus far, we have not seen any other similar lobate deposits within Valles Marineris.