Unique Chronostratigraphic Marker in Depositional Fan Stratigraphy on Mars: Evidence for ~1.25 Ma Old Gully Activity and Surficial Meltwater Origin

Introduction [1]: Since their discovery, a variety of formation hypotheses have been proposed to explain the diversity of gully observations. These hypotheses can be divided into three broad categories: entirely dry mechanisms [2,3], wet mechanisms invoking groundwater release [4,5], and wet mechanisms invoking surficial meltwater [6,7,8]. It has been difficult to differentiate between these hypotheses and test their validity using past observations [9]. Also uncertain is the age of Mars gullies and thus their specific link to recent climate history. Although they appear to have formed contemporaneously with latitude-dependent mantling deposits thought to have been emplaced during recent “ice ages” [10,11,12,13,14], the area of individual gullies themselves is too small to obtain reliable ages using crater size frequency distributions. We document here a unique geological setting that provides new insight into these relationships. Gully-Fan Stratigraphy: In eastern Promethei Terra (~35°S, 131°E), an ~5 km-diameter crater is observed with a single well-developed gully system and several smaller gullies in its north-northeast wall. This gully system (composed of a small western gully and larger eastern gully) shows evidence for incision into the crater wall and has multiple contributory subalcoves and channels. The low-slope depositional fan (Fig. 1) associated with this gully system is significantly larger than the others. The gully fan is composed of multiple lobes with distinct lobe contacts, incised channels, and channel fill deposits – all features similar to those observed in terrestrial alluvial fans: cone-shaped deposits of fluvially transported sediments that accumulate at distinct breaks in slope [15,16]. Secondary craters (~1–25 m-diameter) are pervasive in the vicinity of the gully, but only a portion of the fan itself has superposed secondaries, implying that at least some portions of the depositional fan were deposited both before and after the emplacement of the secondaries. The individual depositional lobes of the fan can be divided into two groups (Fig. 1): a lobe that predates the secondary crater population (1) and younger lobes (2-4), distinguished by stratigraphic contacts and cross-cutting relationships, that are superposed on the lobe with secondary craters. These multiple lobes that post-date the secondary crater population make the emplacement date of the secondary craters a robust maximum age for the youngest lobes of this fan, and therefore the most recent activity of the gully system. Figure 1: The depositional fan is composed of six visible lobes. Lobe 1 is the oldest visible lobe and retains a dense population of secondary craters. The superposing un-cratered lobes (2–4) post-date emplacement of the secondary craters and require episodes of more recent gully activity [HiRISE: PSP_002293_1450].