Peter Ness had some comments:

It is a very safe bet that the rounded spherical rocks we see scattered over the plain are the same as the pebbles in the bedrock. They are almost certainly just pyrite nodules that have fallen out of the bedded rock and have since been replaced by hematite.

This makes sense because the chemistry should be strongly elevated in sulfur and iron - the components of pyrite. Both Viking landers had similar chemistry and Viking 2 landed in very similar terrain geologically.

Therefore the hematite should be normal Fe2O3 most likely a goethite or a maghemite form with some fine red tugite staining of the rocks. The iron may even be slightly magnetic as it should reduce in the thin Martian atmosphere. This means that the original rock has to be a sediment of some kind - we see the same thing in the Mount McRae shale and a lot of siltstones that are of Archaen age in NW Western Australia. That does not mean the bedded rock is a shale or siltstone - lets wait on the chemistry.

The alternatives: (1) that they are fossils that have been replaced by pyrite, then by iron or (2) that they are some sort of degassing agglomerate of iron that formed when fine moist fragments formed a spherical shape clumping together during degassing expulsion . "

"Since the spherical objects are almost certainly just iron after pyrite nodules that have fallen out of the bedding of the bedded rock, then we can be fairly certain that the bedded rock is either the trough cross bedded material (TCB) or the layered mudstones we see all over Mars in the polar regions (more likely) - thus rather than lacustrine/oceanic deposits we are most probably looking at air fall deposits from wide-scale degassing. Both landers were right on a pole at one stage....as was Viking 2. Of course the water theory is not dead because both parent images show that there was a lot of water in the areas after the poles started to migrate, so the rock could be a younger sediment perhaps (but I doubt it).

The indentations in many of the nodules ones we see on Mars could be caused by two growing side by side, except of course if they were actually ejected as sulfide nodules rather than forming in-situ afterwards, then we would be seeing holes in the sulfide modules caused by other smaller pellets. I think we can see both effects in the microscopic images but it looks like the nodules may have been ejected from degassing vents (a bit hard to believe).

At Tom Price I used to collect these nodules from both the Roy Hill shale and from the Mount McRae shale. Those from the Mt McRae shale were nearly always pyrite and often flattened (like golf balls) or shattered from the mine blasts (as I got them in the mine face), but those from 5km away in the Roy Hill shale were typically spherical with pyrite crystal ribs but were 100% iron hematite as were replaced). There were hundreds scattered over the ground - typically the size of golf balls or large marbles (some the size of ball bearing others larger) - so these images are very similar (just more of the nodules).

This is my take - the nodules could be organic but that is unlikely given this was a pole at the time the bedded rock was formed - so the rock should be equivalent to the mud-like material we see near the current south pole or the TCBs we see at 60-75 latitudes in the valleys north of Chasma Australe.

It could of course be lacustrine or river related sediment because both lander sites show a lot bursts in the craters that suggests there was a lot of water in these regions after the poles started to move - I guess we will see before too long.."