What gravel size may tell us about the rivers draining from the north wall of Gale Crater

Abstract:

The exposures of interbeds of mostly fine gravels observed en route to Pahrump Valley in Gale Crate appear to provide no direct indication of channel scale or slope, but they for the first time on Mars reveal the bedload grain size distribution. This is in contrast to remote sensing analysis of Martian channel landforms where width and slope can be estimated and possibly flow depth inferred, but grain size of the channel bed is unknown. Strong terrestrial correlations of discharge and channel width (once corrected for reduced velocity due to lower gravity) likely gives the best estimates of channel flows from remote sensing observations. Calculations based on roughness arguments or estimated grain size and critical shear stresses have much larger error. What can be done, however, if we only know the grain size- the case for the gravel exposures in Gale? We show that crude positive correlations of channel slope with median grain size, can then be used to estimate channel depth and width, each of which vary inversely with channel slope. These empiricisms suggest that the fluvial conglomerates in Gale with grain sizes 4.5 to 9.5 mm may have had been deposited by a channel of a slope between 0.01 and 0.0001, about 0.62 to 2.1 m deep, and 10 to 50 m wide. Using the median bankfull stream velocity from terrestrial studies of 0.9 m/s (corrected for Martian gravity) the predicted discharge is about 5.6 to 94.5 m³/s. The estimated widths alone predict 6 to 85.6 m³/s for the 10 to 50 m channels widths, respectively. This analysis suggests that the gravel bedded rivers that delivered the sand and finer size delta building sediment towards the base of the present Mt. Sharp were of modest scale and slope, placing constraints on the eventual stratigraphic reconstruction of the deposits here.