CONSTRAINING THE RELATIVE AGES OF COMPLEXLY STRATIFIED DEPOSITS ON ANOTHER PLANET: CASE STUDY OF THE AEOLIS DORSA, MARS

The Aeolis Dorsa region, Mars, includes complexly stratified aeolian/volcaniclastic deposits and thousands of exhumed fluvial deposits, many of which have been topographically inverted. Elucidating the ages of these deposits is important for understanding the history of water on Mars. However, constraining the timing of formation processes is made difficult by the repeated cycles of widespread burial, exhumation, and re-deposition of mobilized sediment that produced the complex stratigraphy. Nonetheless, many useful principles (e.g., superposition, crosscutting, embayment) from terrestrial geologic mapping have been used in mapping the Aeolis Dorsa region. Analyses of crater size-frequency distributions constrain surface ages, but the pervasive burial and erosion in the region limits this technique to estimates of exposure ages (i.e., the length of time a surface has been exposed to impact cratering). Here, we present several key observations that help establish a general history of the Aeolis Dorsa deposits.

The oldest feature in the map region is the highland-lowland boundary, which circumvents most of the planet, and is apparent in the southern part of the map area. South of this boundary are units of the southern highlands, which have been crater-age dated to be of Noachian age (>3.7 Ga). The formation ages of units north of the highland-lowland boundary are less certain as fewer craters are preserved, especially on the western and eastern plateaus of Aeolis and Zephyria plana, respectively. Most fluvial deposits are located in the central depression between the plana. Fluvial features crosscut and embay the plana indicating that the fluvial deposits are younger than the plana. Craters in the central depression may therefore be used to estimate an exposure age for the fluvial deposits and provide a minimum formation age for the plana. Finally, numerous parallel ridges, interpreted to be yardangs, superpose the plana and fluvial deposits. These yardangs suggest a history of widespread deposition, followed by aeolian abrasion. An outstanding question is the undefined relationship between the plana and the southern highlands. Defining this contact is important for constraining the maximum age(s) of the plana. Continued mapping in southern Aeolis Dorsa will focus on elucidating this contact.