FLOOD-DEPOSITED RHYTHMITES IN GALE CRATER, MARS

The Striated Unit is one of six lithologic units examined by the Curiosity rover in Gale crater, Mars. It is the only rock unit investigated by the rover whose layers are not horizontal, striking N60ºE and dipping 10-20º SE. It is 10-20 m thick and deposited on a north-sloping surface after the emergence of Mt. Sharp when the crater had acquired its modern morphology. It is one of the youngest rock units in Gale crater. Its meter-thick, rhythmic layers begin with thin-bedded conglomerate (20-60 cm) that grades upward to cross-bedded conglomerate (~20 cm) and capped by a normally graded bed (15 cm) consisting of pebbly sandstone that transitions upward to laminated medium grain sandstone. Southerly dips indicate that its layers advanced up-hill toward Mt. Sharp.

Up-hill (southward) migration of dipping layers indicates deposition as antidunes by north-flowing supercritical flows. The fining upward grain size distribution and the associated changes in sedimentary structures are identical to those seen in turbidites: a common deep-water deposit. However, the absence of any associated deep-water sedimentation negates this origin for the Striated Unit.

Another mechanism to deposit turbidite-like layers and antidunes is by waning outburst floods, best preserved in Pleistocene flood deposits on Earth. They are termed rhythmites because they are not deposited by gravity flows like true turbidites are. As such, we interpret layers of the Striated Unit as flood-deposited rhythmites in Gale crater. Floods originated from the ice-covered Southern Highlands, moving downhill entering the crater through Farah Vallis depositing the Striated Unit and continued northward until stopped by the northern crater wall forming a lake. The conglomeratic interval of each layer represents the flood event. The distinct graded bed interval that caps each layer formed as slack-water sedimentation in that lake.

Most importantly, occurrences of aqueous sedimentation before the emergence of Mt. Sharp (Heydari et al., 2018 GSA) and afterward (this study) suggest intermittent but persistent flow of liquid water during the entire Late Noachian time to possibly Early Hesperian time. That is: Mars was similar to Earth’s northern hemisphere during Pleistocene Epoch with parts covered by ice and parts above freezing for at least 150 million years.