QUOTE (Shaka @ Feb 14 2006, 11:50 PM)
From the mouths of
our beloved PIs, neb, "olivine-rich basaltic sands" seem to be the story for this here basin. To the
uninitiated (like me) this would seem to present one of them
quandaries, since water is deemed necessary to mobilise the sulfates, but also is deemed
auntie-thetical to the persistance of olivine. I'm guessin' that the occasional
splash[color=#33CCFF] of water might spread sulfates, while leaving the olivine still intact (though maybe it would
take the edges off, if you know what I mean. I
would dearly like to confirm the sulfates analytically, though. Patience is a virtue, I guess, though not a whole lot of fun.
Shoot, I didn't ask for these pretty-colored fonts, but long as I got 'em...[color=#009900]
The MI's of the "float" block from the upper unit sure make it look like a tightly cemented sandstone. The grains are darker than the cement, and fairly well rounded. The sorting (range of grain sizes) looks to be moderate to well sorted (anyone help me out here with a scale for the MI's to get an idea of grain sizes?). These features, plus the very nice low-angle inclined cross lamination already seen, seem to confirm an eolian interpretation.
However, the MI's of lower interbedded units are much different. There are several different grain types, the sorting is much poorer, and some of the larger grains look angular. There is also some indication of some fining-upward grain size trends in these thinner beds, too. I'll stick with my sediment gravity flow interpretation of these beds, for now, but there is more to be done here, too.
I guess that the composition of the cement is still the big unknown to us outside JPL looking in. Hopefully this will be forthcoming, but there are still a few things we can say about it:
In both units imaged by the MI, the grains do not look like they were compacted much before cementation. There is something called "minus cement porosity" that is measure of how much space there was between grains before the sandstone got lithified. You can get a rough idea of this by looking at how many grain-to-grain contacts you can see. In both the upper and lower units, there seem to be more than a few "floating" grains completely surrounded by the cement, suggesting that there was a lot of pore space between grains when the cement was precipitated. This makes me lean towards an aqueous-mediated cementation event under shallow subsurface conditions (i.e. groundwater) rather than burial diagenesis, pressure solution, or welding of tuff particles in a pyroclastic flow.
(The first one was... 
) the whole shootin' match. Then wind erosion through the Amazonian strips away the surrounding impact breccia and crater rim leaving..
[color=#33FF33]Gotta figure out all these formatting buttons. Fun! Do you have to type in BB codes or can you just click on the icons. Sometimes they work or not.
Damn!
