You're right Nick

This rock formed in a mixed eolean / shallow laccustrine environment. I lean towards probable ground water recharge, but we don't have much evidence for that yet.* We've been seeing evidence of both eolean and water depositional structures since the first outcrop. And in that outcrop, the water cross-beds were created by a water column that had a minimal depth of a few inches. (Sorry, I don't have the link handy).

I tend to associate sabkhas with coastal environments. I know that this is not strictly true, but it is a connotation, at least in my own perception. Given that, I tend towards not invoking an associated large body of water in this setting until a rover finds some bona fide marine sediments somewhere else in the geologic column.

While the sabkha favors evaporites and the playa favors salts, my prefered earth analog is the playa setting of the evaporite dominated dunes and seasonal lake of the White Sands in New Mexico. (I know, you've all heard that from me before) I guess could be argued that White Sands is actually a sabkha, but there seems to be quite a bit of overlap of the two terms. I'm sure there are hydrological studies on these systems to determine rate of evaporite formation and amount of groundwater involved. Not much time in my life for anything more than a quick look and half-assed speculation, but I'll see what I can turn up.

But as many here like to remind us, this is Mars, and we'll probably have to give it its own geographic term. There is a slight wrinkle on this thought. The terms playa and sabkha originally refer to modern, active environments. They have then been used to model ancient, non-active systems in the geologic record with similar deposits and structures. On Mars, we don't have that modern system to compare these ancient deposits to. I have no doubt that the processes on Mars are similar in general but different in detail to those on earth.

Another thought that keeps popping up in my mind is we don't know how this fits into the broader geologic framework yet. We don't know what is up-section, down-section, or even much about the laterial variation of this unit. Much to learn yet.

I'm still waiting for an official formational name for this rock unit. They've named everything else, why not the formation?

~jms~

*did you notice the recemented joint surfaces in the "dark unit" of Tinpuna? At the bottom right. Was that recemented by groundwater?