STRATIGRAPHY OF YELLOWKNIFE BAY, MARS: CHEMCAM OBSERVATIONS OF CHEMICAL VARIABILITY BETWEEN MEMBERS
The 4.75-m-thick assemblage consists of the Sheepbed mudstone and Gillespie Lake sandstone members whose chemistry resembles the bulk upper crust. The uppermost Shaler member is dominated by a more alkali-rich composition and the Point Lake member has an intermediate composition; it is fine-grained with lighttoned veins and nodules – possible sulfates. Oxide ratio plots (K2O/Al2O3, Al2O3/ TiO2, and FeO/MgO) best show trends in the chemistry of the members as they are less affected by the compositional variability of the targets. The Point Lake member has similar K2O/Al2O3 and FeO/MgO to the Shaler member as well as similar Al2O3/TiO2 to the Gillespie Lake member, all within a 95% confidence interval. FeO/MgO mirror K2O/Al2O3, with Sheepbed and Gillespie Lake having similar but lower ratios than Point Lake and Shaler. ChemCam and APXS data agree for K2O/Al2O3 and FeO/MgO, but the ChemCam Al2O3 generally show higher abundances than APXS data. Major oxides show little variation within a 95% confidence interval. The most notable chemical boundary is between Gillespie Lake and Point Lake, seen in the differing SiO2, MgO, CaO, Na2O, and K2O. TiO2 and FeO(T) also differ at the Sheepbed/Gillespie Lake boundary. An increase in alkaline components from the lower to upper two members is clear. This trend is loosely anticorrelated with Al2O3. Trace element abundances of Rb, Sr and Li between these members show that Shaler, Point Lake and Gillespie Lake are more diverse than Sheepbed. Sheepbed is particularly low in Li relative to the other members. These differences in chemistry may reflect differences in provenance of each member.