My question to the geologists on cogitator's ping list is, given that some of those grains are angular, sub-angular, and sub-rounded, are the fully rounded grained due to abrasion (rolling, etc.) or do you think they are concretionary (i.e. chemical)?
The fact that we have the whole range of angularity makes me suspect that the rounded grains are abraded. I still don't have a handle on scale, however.
And why do you think that there is no evidence of fluid flow around the larger grains in several of the photos, especially the one in the upper right corner? I pulled out my old copies of Reineck & Singh and Blatt, Middleton & Murray and found noting even remotely similar. Are we not looking at sediments? If so, is it so quiescent that there is no apparent turbulence wake or wake vortices?
Though I have a strong interest in geology, my training is more geochemical, so things like petrography and sedimentary stratigraphy, both of which come into play here, are not my strong "suits".
But looking at the pictures, it appeared that the blueberries were falling out of the rocks, which would definitely point to the concretion identity. First thing that came to mind as a terrestrial analogue is oolites (click on this to see the full-size pic):
This next picture is of oolitic sand from Great Salt Lake, which is a decent analogue to the environment described at the Opportunity site:
From this Web site:
Oolitic sand at Stansbury Island, Tooele County
This page has some ugly-looking hematite concretions:
concretions
And this is a good article about the identity of the blueberries:
'Blueberries' Reveal Only Part of the Mars Water Story
This was posted before the press release that showed the stratigraphic layers interpreted as formed in shallow water.
Regarding your last question, and drawing on experience from when I visited Great Salt Lake one time: what they're talking about is a surface brine region. The water is going to be much denser (more saline), and the sediments are initially going to be "goopy" thick oozes that only over time will turn into rock. So though you can get layering as successive wet/dry cycles, this isn't a very active flow environment where you'd see evidence of water movement.
One other comment; the fracturing and angularity may be due to post-exposure weathering. I.e., the concretions all start out fairly rounded and spherical, but as they are exposed and dry out or simply crack, you'll get more angular grains.