Dichotomies in the fluvial and alluvial fan deposits of the Aeolis Dorsa, Mars: Implications for weathered sediment and paleoclimate

The Aeolis Dorsa region of Mars preserves many ancient and topographically inverted fluvial deposits, some of which represent analogs to pre-vegetated meandering fluvial deposits on Earth. The regional stratigraphy of the Aeolis Dorsa preserves a tradition from deposits of meandering fluvial channels to alluvial fans. On Earth, fluvial channel and alluvial fan environments have different hydrologic regimes, sedimentary conditions, and depositional slopes, implying that the Aeolis Dorsa region experienced significant changes in hydrology, sedimentology, and topography. Here, we map deposits and derive stratigraphic columns of four local areas-two in southeast Aeolis Dorsa and two in the northwest-to elucidate the local hydrologic processes and sedimentary conditions coincident with regional change. Formative processes and conditions are inferred from shared morphologic attributes between Martian and terrestrial analog deposits. Results suggest mutually consistent local histories; all areas show a transition from fluvial deposits to alluvial fan deposits. However, the hydrologic processes and sedimentary conditions were non-uniform. Specifically, local deposits form two dichotomies: (1) southeast Aeolis Dorsa preserves meandering fluvial deposits, whereas the northwest preserves only wide channel fills; (2) southeast Aeolis Dorsa includes alluvial fans with debris- flow deposits, but northwest fans include only sheetflood or channelized deposits. A relative abundance of cohesive, weathered sediment in southeast Aeolis Dorsa explains both the fluvial meandering channels and debris flows on alluvial fans in those localities. Greater weathering in southeast Aeolis Dorsa is consistent with previous theories of enhanced snowmelt or orographic precipitation in southeast Aeolis Dorsa.