GEOLOGIC MAPPING TO CONSTRAIN THE SOURCES AND TIMING OF FLUVIAL ACTIVITY IN WESTERN LADON BASIN, MARS

We are mapping two quadrangles in Margaritifer Terra (–15032 and –20032) to define the evolution of the western Ladon basin region. The primary map base is the controlled daytime THEMIS IR mosaic, supplemented with CTX images where available. Mapping is being done at 1:200,000, with an expected map publication scale of 1:1,000,000. As part of this mapping, we are also evaluating the morphology, stratigraphy, mineralogy, and distribution of newly identified sedimentary deposits in small inter-ring basins in the highlands west of Ladon basin. We hope to determine how they may relate to either a past discharge out of Argyre basin along the Uzboi-Ladon-Morava mesoscale outflow system, a possible lake in Ladon basin, deposition in Holden crater and(or) Ladon and Uzboi Valles to the south, or relatively late-occurring alluvial-fan-forming events recognized in the region.

There are fifteen geologic units that we have identified and mapped: Middle Noachian Mountainous Unit (Nm), Late Noachian to Early Hesperian Terra Unit (HNt), Late Noachian to Early Hesperian Channel Units (HNch1 and HNch2), Late Noachian through Hesperian Basin Fill Units (HNb1, HNb2, Hb3), Late Hesperian to Early Amazonian Alluvial Fans Unit (AHf), Hesperian Light-toned Layered Units (Hl1 and Hl2), Hesperian Light-toned Altered Unit (Ha), Amazonian Volcanic Unit (Av), and Crater Units (C1, C2, and C3). The geologic history includes: (i) Formation of Ladon and Holden impact basins in the middle Noachian, producing mountainous unit Nm; (ii) Landscape degradation and infilling during the Midde/Late Noachian with the terra unit HNt; (iii) Formation of Ladon Valles in the Late Noachian to Early Hesperian by catastrophic flooding to produce channel units HNch1 and HNch2; (iv) In the Late Noachian to Early Hesperian, Ladon basin floor accumulated sediments that resulted in units HNb1 and HNb2; (v) Deposition of light-toned layered deposits (Hl1 and Hl2) during the Hesperian within Ladon Valles, Ladon basin, and other smaller valley networks along the western uplands, as well as aqueous alteration along some crater floors (Ha); (vi) Deposition of unit Hb3 within Ladon basin and Ladon Valles; (vii) Eruption of volcanic unit Av in the Amazonian. Craters formed throughout the geologic history of the mapping region.