THE DISTRIBUTION AND ORIGIN OF NODULES AND MINIBOWLS WITHIN THE SHEEPBED MEMBER: IMPLICATIONS FOR EARLY DIAGENESIS IN YELLOWKNIFE BAY, GALE CRATER, MARS

A variety of diagenetic features are observed in the clay-bearing mudstones of the Sheepbed member, the basal member of the 5-meter thick section of the Yellowknife Bay formation examined by the Curiosity rover in Gale Crater. Among these features are nodules expressed as mm-scale protrusions of the outcrop with three-dimensional relief suggesting a spherical geometry more resistant than the surrounding mudstone. The size-distribution of nodules is positively skewed with diameters ranging from 0.4 to 8 mm. Mean nodule diameter is 1.2 mm. Mastcam, MAHLI, and ChemCam RMI data show that nodules are patchily distributed both laterally and vertically throughout the Sheepbed member, do not form their own beds, and show no evidence of grading. Based on these characteristics, the nodules are likely of concretionary origin.

Also present in the Sheepbed member are mm-scale circular rims with hollow, bowl-like depressions in the center called “minibowls.” Whereas nodules and concretions have been found elsewhere on Mars, to-date minibowls have been observed only at Yellowknife Bay. Mastcam, MAHLI, and ChemCam RMI data show that minibowls are similar in size to nodules, ranging from 0.6 to 5.6 mm in diameter, with mean diameter of 1.2 mm. The mean diameter of the minibowl depressions is 0.7 mm. Like the nodules, minibowls are variably distributed throughout the outcrop, and although both sets of features are not always co-located they most often occur together. We consider two hypotheses for minibowl formation. In the first, the void space represents secondary porosity resulting from dissolution of a relatively high solubility mineral during percolation of diagenetic fluids. However, filled minibowls are rarely observed and occur only when connected to hairline fractures filled with sulfate cement thought to represent a later phase of diagenesis. We favor a second hypothesis in which minibowls represent primary voids formed by gas bubbles during or soon after deposition. In this scenario, gas may have been produced by a number of different processes including diagenetic reactions, UV photo-oxidation, or pressure-induced release from sedimentary pore fluids. Most importantly, this hypothesis suggests that minibowl formation occurred prior to sediment lithification and was an early diagenetic event.