THE LAST RECORDED DELTAIC DEPOSITION IN GALE CRATER BEFORE MARS WENT COLD: EVIDENCE FROM THE RUGGED TERRAIN UNIT IN THE CURIOSITY ROVER'S LANDING ELLIPSE

The Rugged Terrain Unit (RTU) is one of the four rocks units in the landing ellipse of the Curiosity rover in Gale crater, Mars. It is about 5 meters thick and extends from the foothills of Mt. Sharp to the northern edge of the crater. The RTU overlies all previously deposited strata suggesting that it consists of the youngest sedimentary rocks in the Curiosity rover's landing ellipse.

From north to south, the RTU was investigated at the following locations along the rover's traverse: the Cooperstown, Dingo Gap, Junda, Kylie, and Kimberley. The RTU forms cliffs up to 5 m high in many of these localities limiting detailed contact science by the rover except for the Kimberley and Cooperstown exposures. However, all outcrops were imaged by the Curiosity rover's Mast Cameras.

The RTU consists of three members. Upward they are: the Dillinger, the Mt. Remarkable, and the Beagle (Treiman et al., 2016; Rice et al., 2017). Only the bottom two members are fully preserved. They are 1.5 m to 2.5 m thick each and display distinct variations in lithologies from south to north. Near the Kimberly locality both members consist of sandstone. Northward, each member displays a coarsening upward pattern, starting with thin-bedded sandstone to siltstone at the base that grade upward into massive pebbly sandstone or conglomerate at the top. Occasional cross-bedding indicates flow direction from south to north (i.e., from Mt. Sharp toward the northern crater rim).

The coarsening upward stacking patterns are interpreted as prograding river deltas. Sandstone and siltstone layers at the base of coarsening upward successions were deposited in a subaqueous settings corresponding to delta front, prodelta, or basinal environments. The overlying massive, pebbly sandstone or conglomerate lithogies were most likely deposited as distributary mouth bars (DMBs) of streams that entered the lake. Coarsening upward patterns were not observed at the Kimberly area because it was located in the upper delta regions plains dominated by fluvial sedimentation. This interpretation suggested a lake (100-200 m deep) existed in Gale crater during deposition of RTU. This lake was younger than the one previously reported by Grotzinger et al. (2015). No sedimentary rocks overly the RTU, possibly suggesting that Mars went cold and dry afterward and geological processes were halted.