Halfpipe Formation Options

[aldo12xu]

A brief discussion on this began on the Route thread after Pando posted an interesting route map. Pando's image is attached to a lower resolution version below that extends the coverage up to Erebus Crater. What caught the eye of a few people were the areas labeled as the "Halfpipe Formation", coloured yellow below. This unit would seem to represent a new statigraphic horizon that overlies the Burns Formation.


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I went back to images of the Payson outcrop from sol 748-50, where the name Halfpipe first appeared. There was mention of a Mogollon Contact. I wonder if the contact could correspond to the rough textured, darker zone running diagonally in the central part of this photo. It seems similar to the Whatanga Contact between the Middle and Upper units of the Burns Formation. The Whatanga contact is interpreted to be a zone of recrystallization created above the water table.

And I don't know if this was mentioned before, but there seems to be a lot of evidence for cross-bedding (festoon geometry?) and possible soft-sediment deformation at Payson lying below the possible contact. Could this be the equivalent of the Burns Upper Unit?

The other location where the Halfpipe Formation was observed was on sol 818 at our "Sinkhole Crater".

http://marsrovers.jpl.nasa.gov/gallery/all...VMP2410L5M1.JPG

But what about the other Halfpipe location on Pando's image, off the route map to the east? On what basis would NASA have those outcrops to be similar to those at Sinkhole and Payson?

[Bill Harris]

We're thinking along the same lines. If we're looking for "marker bed" it may very well be the "Whatanga Contact " since it is chemically and physically distinct and appears to be widespread. This unit may be present at Beaver on the rim and scattered about. We need a closer look before moving on. Oppy may pick up on this rock again at Victoria but I don'y know how rough and jumbled it will be and it is neatly laid out here.

Attached is the Grotzinger strat column for quick reference.

I have no idea about those Halfpipe Formation spot off Oppy's route someone on Mount Ithaca spotted something and I'll take it on faith for the time being. But keep looking for the connection...

--Bill

[CosmicRocker]

aldo12xu: Thanks for starting a new topic, since there are apparently only a few of us interested in this subject. To the rest of the forum, thank you for not complaining about our arcane geologizing.

Bill: Thanks for the column. It is more detailed than the one I previously had.

...Going back to my LPSC notes, Groetzinger said that the Mogollon contact was above Whatanga, in the uppermost Burns section. The observed berry distribution was not organized along bedding planes or cross-strata, as would be expected in the basal surge model which was an earlier threat to his model. Monte Carlo simulations supporting a diffusion model were a better fit with what was observed. I thought that was a good argument, and probably why the basal surge concept has ebbed. (As an aside, my notes from the time included a note to myself that John "devastated the basal surge hypothesis" in his presentation. It was one of the highlights of the meeting in my humble opinion.) I don't want to belittle the surge hypothesis. It was worthy of investigation, but I think it is history now.

I may be wrong, but I don't think the appearance of dark cobbles at some of the Half Pipe sites is significant, since they appear in many random locations. McLennon also mentioned the Mogollon contact, and his description sounded like a diagenetic contact rather than a stratigraphic one. Unfortunately, I was probably fascinated by his presentation, as my notes become somewhat undecipherable at that point. There was something about the berries becoming fewer, as we noted around Erebus, and also something about multiple water table cycling. He counts 4 of them.

MMB is downloading some sol 907 drive images right now. It looks like a short drive south. I'll see you tomorrow...

[Bill Harris]

Tom--

As much as I'm hoping that Whatanga is our holy grail of a marker horizon, I'm thinking that it may be my windmill to tilt at. A stratigraphic contact would be perfect, but it ain't: it is diagenetic, caused by a fluctuating water table. It would be if we assume that the water table surface was widespread and consistent, but I doubt that it was. It seems that _nothing_ on Mars is simply explainable.

This is why I've jumped up and down and flailed my arms so much at the Payson exposure and at the "Sinkhole" and ranted about doing more IDD work. That would be the first step in finding a regional marker horizon. On Mars that will prove to be difficult to find a chemically and physically distinct zone that can be dated. On Earth bio-markers are used because conditions favor that, even if we didn't understand what the fossils were-- remember conodonts?-- but I doubt that Martian life was sufficiently widespread or well-preserved for use as a marker.

But I think that the Suits in Ithaca know more than they have let out, and this Halfpipe Formation may be the cat out of the bag and running amok...

--Bill

[Bill Harris]

Oppy appears to be moving Southeast, away from Beagle and toward the Victoria ejecta apron. She'll likely spend a couple of Sols on this transitional zone between the rippled zone and the sand sheet.

--Bill

[aldo12xu]

I was also surprised that they didn't RAT a portion of the Payson exposure. And even at Beagle, I was hoping they would've had a closer look at some of the darker toned ejecta blocks lying on the surface. Like the large ejecta block seen in the photo below the darker rocks could represent the equivalent of the Whatanga/Mogollon horizon.


http://www.marsgeo.com/Photos/Opportunity/...agleDark_ze.jpg

Tom, here's a paper that discusses the micro-spherules and their terrestrial analoques. They are found in Jurassic Navajo Sandstone which extends from southern Utah to parts of Northern Arizon. Within such a large area, they found a wide variation in the types of spherules which they attributed to a number of different chemical reaction regimes at different scales. More than likely throughout the Meridiani area there has been a similar variation in the extent of the water table, the degree of porosity, the rate of diffusion through the rocks, the amount of iron and other elements within the groundwater, etc.

http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1377.pdf

When is the next big Mars conference? We need more clues!!

[CosmicRocker]

I think I've read about those hematite concretions, if those are the big ones. I've seen pictures of some that are about a foot in diameter, or maybe more. I can only guess that the length of immersion time and Fe+2 concentration control the size.

This Mogollon contact thing is driving me nuts. They took superres images of Wellington and Whatanga, and the pancam superres site has them, but their imagery for Opportunity ends at sol 729! I need to search the appropriate sols for superres pancams from the time they were doing the "shoot and scoot" thing at Mogollon. I am expecting something like the textural change that the Whatanga image shows, but I really don't see anything obviously like that at those sites.

One thing we noticed around Erebus was the diminution in the size and abundance of hematite concretions embedded in the rock. I think there may have even been a rat hole that essentially had none. I am wondering if that observation could mark the definition of a new diagenetic horizon. This is going to take a bit more digging. Surely, this has been published or pre-published somewhere...

[Bill Harris]

I've long felt that the changes we noticed in the hematite concretion size and presence north of Erebus and at Olympia (and Mogollon) was very significant and should have close study. Perhaps there was miniTES and MB that we are unaware of and there may be a paper in the works on this. But clearly, Erebus does represent a new horizon and travelling South could only have taken us higher in the section. "Scoot and shoot" may be valuable, but lost data is without worth.

I'm glad to see there is someone else who has been fidgeting over this.

--Bill

[CosmicRocker]

I'm glad I went back and checked that terrestrial analog pdf you posted. It was something new to me. I had seen the earlier work they mentioned, but they put a more relevant twist on it in that piece. The conclusions were not terribly surprising, but it did make it clear that we really need to visit more of Meridiani before we think we see the big picture.

I think I've pretty much exhausted my search permutations for info on the new contact and the new formation, without success. Now, would be a good time for an insider to enter the discussion, or at least someone with new ideas. If I had to choose a new direction for the Oppster after it visits Victoria, I would opt for a direction toward the facies change where the evaporite eolianite clasts originated. Water should have persisted the longest there.

[Bill Harris]

And where would the location for the source of the evaporite eolianite clasts be?

--Bill

[CosmicRocker]

I wish I knew. I guess I didn't make it clear, but when I wrote that last night I was assuming that the geologists actively researching the Meridiani sediments probably have an idea about which direction it might be. Who knows, perhaps we'll find evidence of it in some deeper layers when we get to the Victoria road cut.

[tim53]

The name "halfpipe" was first used, by John Grotzinger I think, to describe the dark-surfaced "U"-shaped trough at the base of the Payson outcrop, which turned out to be coarse material thinly overlying outcrop. When we began trekking south across the plains and saw patches of the coarse material overlying the outcrop, with a similar, somewhat darker appearance relative to the general ripple-covered surfaces in the MOC image, I applied the tentative name "Halfpipe Formation" - suggesting that they might represent a coarse sandy or gravelly unit overying the outcrop in the region. Jesse Chisholm is one such patch, though it and a few others we've passed on our way south, has some thickness to it. The name hasn't really stuck, and I probably should have removed it from the press-released version of the planning map to avoid confusion.

-Tim.

[tdemko]

Tim:

Do you think the "Halfpipe Formation" is a pre-"modern eolian period" residuum? It seems to be older than the rippled material.

The current Meridiani landscape surface is "erosional"...an unconformity in the making. The rippled eolian material is just a skiff, relatively, and represents material just passin' through, or making some "short" stops along the way. The flat, almost featureless, plain is the product of a long period of eolian deflation...not that the old Meridiani sea bed would have had much topography to begin with!

The only topographic features today are craters, ejecta, and impact-related, partially-filled, fractures. Even they show evidence of various periods of eolian-related erosion and smoothing. I think the smooth surfaced, mosaic-looking outcrops we see all the time on the plains, and especially the intricate "stonework" we see in the crater walls (like Beagle) are a product of this long-term muting and planing due to wind erosion.

It is interesting to speculate (at least for me) what we could deduce about the geomorphic history of a region if we would encounter a suface such as this in a stratigraphic succesion...in fact, most of my research on earth is based on this concept!
--
Tim Demko
http://www.umn.edu/home/tdemko

The name "halfpipe" was first used, by John Grotzinger I think, to describe the dark-surfaced "U"-shaped trough at the base of the Payson outcrop, which turned out to be coarse material thinly overlying outcrop. When we began trekking south across the plains and saw patches of the coarse material overlying the outcrop, with a similar, somewhat darker appearance relative to the general ripple-covered surfaces in the MOC image, I applied the tentative name "Halfpipe Formation" - suggesting that they might represent a coarse sandy or gravelly unit overying the outcrop in the region. Jesse Chisholm is one such patch, though it and a few others we've passed on our way south, has some thickness to it. The name hasn't really stuck, and I probably should have removed it from the press-released version of the planning map to avoid confusion.

-Tim.

[Bill Harris]

Fooey, Tim, you had us going for a while. I'm sure that the "coarse sandy or gravelly unit" has some significance, we just don't know waht it means, yet. No doubt, a lot of folks are looking for a marker bed.

Tom, early this morning I was hoping that you found out something that that had been puzzling me. Oh well...

--Bill

[CosmicRocker]

At this point, I am stepping back to digest the new contributor's comments... I'd comment, but at the moment I am trying not to say something foolish, as I have done in the past.