THE POINT LAKE OUTCROP, GALE CRATER, MARS: SANDSTONE OR (LESS LIKELY) LAVA FLOW?

The Point Lake outcrop within the Glenelg member of the Yellowknife Bay Formation (YKB; Gale Crater, Mars) was visited by the Curiosity Rover in November 2012 and June 2013. Point Lake is interesting because despite being examined from a few cm to a few m to a few hundred km (i.e., in HiRISE satellite data), its exact lithology was (and perhaps still is) uncertain. Point Lake is dark-toned and forms an erosionally-resistant surface and ~0.5 m-high scarp. Its base is conformable and possibly gradational to the underlying Gillespie sandstone. Exposed upper surfaces have been eroded by the wind into linearly-sculpted ridges and grooves. Point Lake is notable for a high proportion of circular to oval voids 2-3 cm across that vary from simple and rounded to ragged and convoluted. The larger voids have raised margins and some also contain dark grey opalescent particles that occupy 20-50% of the voids. Many of these particles have subhedral shapes and contain voids themselves. Between the voids the matrix surface has rounded bumps and indentations that are 0.05-0.1 mm across, as well as rare subangular particles 0.2-0.4 mm across. Only a few particles were resolvable in MAHLI images, ranging from 0.7-1.6 mm across. A few of these larger particles also have a dark grey opalescent texture, but in general there is little color contrast between them and the finer matrix. ChemCam data indicate that compared to other rocks in the YKB formation, Point Lake contains higher K, Na, Si, and Al, and lower Mg. Except for the K and Mg values, the composition of Point Lake lies on a compositional trend that includes the mudstone at the base of the formation. APXS data are similar and show that Fe, Ni, and Zn are also lower than other YKB rocks. The most likely interpretation is that Point Lake is a void-rich coarse-grained sandstone; its composition differs little from overlying and underlying sedimentary rocks, and the voids may or may not be unusual, depending on how they formed. Since leaving Point Lake, Curiosity encountered a few outcrops elsewhere that are similarly dark-colored with little color or erosional contrast between grains and matrix, but which can more definitively be identified as sandstones. After (but only after) discovering these other outcrops, the interpretation of Point Lake as a vesicular porphyritic lava flow, became much less likely.