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Summary of geology and rock types at Opportunity site
aldo12xu
post Mar 20 2006, 07:42 AM
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Well, it's literally been months in the making -- on and off, whenever I've had some spare time -- but I finally finished a compilation on the geology of the different rock types that were observed by Opportunity on it's trek from Eagle to Erebus craters. In a way, it was good that Opportunity hadn't moved very far since last November. This way my update is still up to date wink.gif

So if you have some spare time, have a look. There's a lot of info there, which I hope will serve as a useful guide once we get to see some deeper layers at Victoria.

The link to the write up is on my homepage.

Cheers,
Aldo.



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CosmicRocker
post Mar 20 2006, 07:26 PM
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Nice work again, Aldo. This kind of on line reference for Meridiani rocks is something we have needed for a while now. Thanks for putting in the effort to compile this excellent summary.


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aldo12xu
post Mar 21 2006, 08:05 PM
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Thanks, Tom.

Now that I can, so to speak, step back and look the summary overall, the one thing I find interesting is that the texture of the chlorine rich rocks Diamond Jenness, MacKenzie, Inuvik and Bylot is so different than that seen along the same stratigraphic horizon (Middle Unit) farther to the east at Burns Cliff. At Burns Cliff the Middle Unit rocks are finely laminated whereas the chlorine rich rocks mentioned above have a more coarser texture with nodules and encrusted spherules.

And I'm also wondering if the Payson exposure might correspond to Middle Unit. But, as usual, recent geochem results aren't coming forth quite that readily. So we'll have to wait and see.


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CosmicRocker
post Mar 22 2006, 05:06 AM
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That reminds me of something I wanted to mention from LPSC. We've heard comments previously from the team that Opportunity was moving up-section as it traversed southward. We could even see for ourselves the south dipping beds at Payson An updated stratigraphic section was shown by John Grotzinger and Scott McLennon in their LPSC presentations, but I couldn't sketch it because it was shown so briefly. It appeared to be an updated version of the one in Grotzinger's Power Point presentation, "The 2004 Mars Exploration Rover Mission-Evidence for Water and Prospects for Life." I borrowed the original schematic section from his presentation which is available at http://www.esi.utexas.edu/outreach/prevlectures.html.

What I was able to notice from my brief glimpses of the updated version was that they had added a short section to the top, apparently because they think what was seen at Erebus was at least somewhat higher stratigraphically than the rocks at Endurance and Eagle. I also noticed that it had another, higher contact labelled as the "Mogollon Contact." The Wellington contact has been described as an interdune deflationary surface possibly controlled by a paleo water table. The Whatanga contact was described by McLennon and Grotzinger as a place where the water table may have remained stable for some time, creating secondary porosity and recrystallization. McLennon went on to suggest that he sees as many as four movements of the paleo-water table, but neither he nor Grotzinger actually mentioned the new Mogollon Contact, so I have no idea what they think it is. I meant to ask about that, but I became sidetracked by other things and forgot to do so.

Since then I have gone back to the Mogollon and Payson Panoramas to try to identify a contact of some kind, but it is not apparent to me.
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Bill Harris
post Mar 22 2006, 10:38 AM
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Thanks for the update on the stratigraphy of the Burns formation. This makes perfect sense: the route has been going topographically higher so we have to be going stratigraphically higher, too. Although we have seen localized dips associated with impact disturbance, my feeling is that the regional dip is essentally falt-lying. And we have observed subtle changes in the distribution and nature of the in-place Blueberries. I wish we'd been inclined to look more closely at the exposures in the North Erebus "Fourlane" locale.

And thanks to Aldo, too, for his site and for the Opportunity overview.

--Bill


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aldo12xu
post Mar 22 2006, 03:44 PM
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OK, that makes sense now. The Erebus rocks represent another cycle where......

1. The water table receded
2. Aeolian deposition resumed on top of the Eagle/Endurance Upper Unit, marking the Mogollon contact
3. The groundwater level increased, soaking the Mogollon/Erebus sands
4. Water evaporated, cementing the sand grains in a magnesium-sulphate salt
5. Various diagenetic processes created the spherules, irregular spherule-type forms, small dark grains, etc.
6. Water table recedes and the cycle begins again.


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Guest_AlexBlackwell_*
post Mar 23 2006, 10:46 PM
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QUOTE (aldo12xu @ Mar 20 2006, 07:42 AM) *
Well, it's literally been months in the making -- on and off, whenever I've had some spare time -- but I finally finished a compilation on the geology of the different rock types that were observed by Opportunity on it's trek from Eagle to Erebus craters.

I just looked through your summary, aldo12xu. Nice work. And thanks, this is really useful.

P.S. I can tell you didn't graduate from the Bruce Moomaw School of Self-Promoting Journalism. How? In my quick reading, I didn't see "as I have repeatedly said," "thus confirming what I've been saying all along," "as I metioned to a MER team member," etc. biggrin.gif
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Guest_BruceMoomaw_*
post Mar 23 2006, 10:57 PM
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Actually, my reasoning has always been more along the lines of "If I can figure it out, ANYBODY ought to be able to figure it out." (In my one unquestioned success -- proposing a cheap design for a Pluto probe -- it turned out that lots of other people HAD figured it out; what I didn't know at the time was that Captain Crazy had been silencing them by threatening to cut off their NASA grants.)

And, yeah, Aldo, it's a superb piece.
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dvandorn
post Mar 24 2006, 05:14 AM
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Remember, when speaking of the general slope over distance and how that might affect what we're rolling over stratigraphically, that the surface may not have always reposed at the same slope that we see today.

As Aldo captured so well, this area seems to have seen repeated episodes of deposition, each involving long periods with high water tables and then undergoing evaporation and salt cementation.

When supported by a water table, such a landfill assumes a pretty flat surface. The deposited material fills in the uneven terrain that underlies it -- and on Mars after the LHB, most of the terrain was pretty uneven.

As the water table recedes, the loosely consolidated sand and cemented sandstone tends to contract and slump a bit. This allows subtle surface expressions of underlying terrain. Of course, buried terrain closest to the surface is expressed the most.

So, it's possible that ridges and depressions expressed faintly in the current topography reflect larger and more impressive terrain that lies beneath. In this case, except for minor amounts of deflation and additional deposition that may have occurred over the millennia, the overall surface can be considered a single, relatively flat unit that is draped over expressions of underlying topography.

Thus, when taking into account deflation and deposition that has occurred slowly since the great dry-out, any given stratigraphic level on the surface is likely to be within a few feet of the level at any other given point.

Or, in more basic terms -- take a lasagna and plop it down on top of a big meatball. The lasagna will have a bulge in its top (an expression of the underlying meatball), and as you traverse along the top of the lasagna you may find local variations in the thickness of the cheese and sauce layers atop the highest noodle layer. But you're ultimately traversing the same stratigraphic unit that you would have seen had the lasagna never been dropped onto the meatball.

-the other Doug


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