EVIDENCE OF THE ENLARGED FORMER EXTENT OF THE MEDUSAE FOSSAE FORMATION ON MARS, AND IMPLICATIONS FOR AN IGNIMBRITE ORIGIN

The Medusae Fossae Formation (MFF) on Mars is an enigmatic layered deposit that extends over more than 100 degrees of longitude along the equator between the Tharsis and Elysium volcanic regions. Several hypotheses of origin have been proposed for MFF, but most recent results tend to indicate an ignimbrite origin as the most likely process of emplacement of the deposit. On-going geologic mapping in the western portions of MFF have identified several discreet patches of mantling material south and west of the traditional margin of the deposit identified from global mapping, some of which show internal layering that is identical in both scale and morphology to layering evident within the nearby main portion of MFF. The isolated mantling deposits are located between the margin of the main MFF deposit and Gale crater (6S, 138E), where layered materials superposed on the central peak can be hypothesized to be a possible remnant of a previously much broader MFF deposit. 676 pedestal craters around the margin of MFF were used as indicators of extensive erosion of a previously more extensive MFF deposit, which suggest that the earlier volume of MFF was 1.2 to 1.9 times the present volume of MFF (which is about 2.3 million cubic km). The potential for an even larger MFF deposit in the past means that any ignimbrite source(s) must have erupted even greater amounts of material than would be required to explain the current massive deposit. Age estimates for MFF are difficult to obtain from crater densities due to the extensively eroded nature of the surface of MFF, but mass eruption rates required to emplace 1.9 times the present MFF volume in 2.9 Ga (the upper limit for its likely age) are broadly consistent with estimates of terrestrial ignimbrite eruptions. However, if MFF resulted from discreet pulses of ignimbrite emplacement, separated by periods of quiescence, then considerably larger mass eruption rates were required for the eruptive events.