CLAY SEDIMENTS DERIVED FROM FLUVIAL ACTIVITY IN AND AROUND LADON BASIN, MARS

We have identified and analyzed clay-bearing layered deposits that occur in and around Ladon basin and Ladon Valles. Numerous valleys dissect the highland terrain at the juncture between northern Holden basin and southwestern Ladon basin. The highland deposits occur within these valleys and basins and were likely formed via processes similar to those responsible for deposition of the delta in Eberswalde crater to the south. Sediments eroded upstream were derived from the altered highlands and were transported and later deposited downstream within small basins or blocked valley systems along the western Ladon basin highlands. Several smaller valleys intersected a N-S trending Holden crater secondary crater chain, depositing sediments within individual craters forming the chain after the Holden impact event and contemporaneous with emplacement of the Eberswalde delta. CRISM spectra from the highland sedimentary deposits are consistent with the presence of Fe/Mg-smectites.

Clay-bearing sediments associated with layered deposits also occur at the northern, lower reaches of Ladon Valles and extend into the southern portion of Ladon basin. These layered sediments appear similar in morphology and composition to those within Holden crater and along the same Uzboi-Ladon-Morava mesoscale outflow system. Holden secondaries disrupt these Ladon sediments so they must predate the crater and were deposited during the Late Hesperian. The thickest deposit (~60 m) occurs at the mouth of Ladon Valles where it enters into Ladon basin, consistent with fluvial sediments deposited by water flowing through Ladon Valles and into Ladon basin. HiRISE images reveal numerous very bright beds interspersed with medium-toned and even darker beds that could reflect heterogeneities in upstream source regions over time or variable discharge orientation into Ladon basin. CRISM analysis of the medium-toned beds show Mg-smectites are present, whereas the brightest beds have spectra with a 2.3 µm Fe/Mg-OH absorption but no hydration band at 1.9 µm. Additional CRISM analysis is underway using images processed with new algorithms that reduce noise and sharpen spectral features from surface outcrops. Most of the sediments are probably detrital in origin and sourced from clay-bearing weathered highland rocks, but some could be the result of alteration and/or dehydration after the deposits were emplaced. Because the clays occur in layered, apparently sedimentary rocks, they may hold clues as to whether past depositional settings in this region were ever habitable.