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.


Medium size 915K
Large size 1.9Mb

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?

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

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...

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

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

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!!

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...

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

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.

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

--Bill

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.

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.

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

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

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.

tdemko:

Something like that, perhaps. I have my own ideas that I'm working with, of course, as do many on the science team. I need to hold off on interpretations, though, until the papers are out.

-Tim.

Thanks for clarrifying things, Tim. No more headscratching on this issue, at least for now!

That us close to my mental image of Meridiani. The evaporite is an major erosional surface, but I'm not sure that the recent aeolian material is transitory. Surely, there are some areas where the topography and wind move this sediment (??) but it has to be deposited and indurated somewhere. The "sand" seems thicker to the NW and SE of the MER landing site but seems to be thinnest and most rippled in the "etched plain" area. We need to know much more about erosional and depositional processes on Mars.

--Bill

Well, that clarifies that the Halfpipe formation was an informal name and that it referred to the coarse dark material that some of us have long been calling a lag deposit. I knew "halfpipe" had already been used to describe the geomorphic feature they drove through, and I was looking for a new formation in its vicinity.

Now that I understand, I think it is still a useful term. At least from my perspective, there are still some unanswered questions regarding the placement of the coarse material. Is it simply a lag of material deposited on top of the ripples and left behind as the finer clasts are blown elsewhere? Or, is it a residuum from an earlier period of erosion, or maybe something else? It has appeared in random locations and in variously sized areas, but it seems to have systematically increased in abundance along the southward traverse. We have seen some locations where it is quite locally concentrated, such as recently at Chisholm, but there have been others. The stuff is intriguing.

While in the general location of the Erebus pan where many dark and coarse clasts were observed, we also noticed a large and apparently decrepit boulder of the same darkness, and an abundance of smaller dark cobbles all around. Several of us were disappointed it was not more closely investigated. In a number of the areas where the coarse clasts subesquently appeared, there were also abundant, small depressions on the current ripples. I'll again go out on a limb and suggest that chunks of stuff landed randomly and fragments of those chunks left smaller marks, and then the wind blew while the larger pieces disintegrated.

Meanwhile, I still wonder about the Mogollon contact. I can see that the ledges around Erebus seem to be sometimes darker due to shadows and maybe some textural differences, from the images available to me, but I don't yet see a contact like Whatanga.

(edited for a grammatical error, and hopefully the only one...)

Ah, time for long-winded geologizing. I like that, Tom.

My {initial} view of the cobbles is{was that they were as} a lag deposit, much like the residual soil left over when a cherty dolostone formation weathers in a wet climate: you have chert in a matrix of clay, the carbonate has long since dissolved (think Birmingham Alabama, where I grew up).

As either a component of the evaporite unit or as ejecta from countless impacts in the Meridiani region, the basaltic cobbles are concentrated as a lag deposit in the ripple troughs. As cobble- to pebble-sized particles they are too big to be moved by the wind; as basalt, they are more resistant to abrasion and weathering than the evaporite so they tend to last a long, long time. The silt/sand/fine gravel of the "ripple sand" tends to roll, saltate, bounce or blow under the action of the wind, but the basalt cobbles stay put and concentrate in the troughs. Now, why there are so many bare areas of the evaporite without cobbles is a loose end of this explanation, but this can be tidied up later. I think this explanation is essentially correct.

The apparently decrepit boulder we observed at Erebus was a larger boulder of ejecta in the process of weathering into smaller cobbles. That is speculation since we didn't get a closer look. We should have taken a more detailed look at the bluff at Payson, this was THE major outcrop at the midpoint of this traverse. Ditto at the "Sinkhole" a bit further down the road. And with the (apparent) filled solution cavities we've recently seen in the Beagle ejecta.

I can see that Oppy has a finite life, but OTOH, once you drive by a significant feature, it's gone, forever since, reasonably, one operational parameter is "no retracing of steps".

So there.

--Bill

Bill:

The bare patches of evaporite, and the eroded, "banded" ripples, tell us that the current sediment supply balance is negative. As in all eolian deposits, sand supply is one of the important variables (as those of you now discovering the classic works of Bagnold and McKee know!...wind speed and direction are the others) that contribute to the geomorphic expression and strata geometry. Meridiani is either in a state of net erosion, or the sand supply is such that only the thin, patchy veneer of wind-blown dark material is maintained.

I would imagine that sand supply, wind speed, and wind direction, in the recent geologic past of Mars, have been modulated by the high-magnitude Martian obliquity cycles. If that is so, maybe many of the sedimentologic/geomorphic features of the unconsolidated surficial deposits are relict and/or dormant on Martian-Milankovitchian time scales.

Good points. I guess it's the "half-full or half-empty?" paradigm. I'd assumed that the variable was the wind velocity-- the etched terrain is at the leading edge of a topographic high, which changed the wind effect. Or so I speculate.

I'm looking forward for more/better orbital imagry of this locale. We know the ground-truth of a very small strip, but can extrapolate to a much larger area. I weant ot know where the sand is coming from and where the sand is going.

The long-lasting legacy of the Rovers? There is going to be data analysis for years and papers being written for more years...

--Bill

tdemko: Would you mind commenting further regarding this, "The bare patches of evaporite, and the eroded, "banded" ripples, tell us that the current sediment supply balance is negative." In particular, I would like to better understand the "banded" ripples part. I've been assuming that the bands we see are the exposed slip faces of earlier cycles of aeolian activity. If that is the case, does the fact that we can see them exposed indicate a negative sediment balance? Would they normally be covered by a layer of advancing particles if the supply was positive?

Bill: It's nice to find a corner where we can get comfortably long-winded about esoteric things. We should do this more often. I've seen residual soil containing concentrations of left-over clasts from weathered and eroded cherty dolostone and similar rocks in a lot of places. Are you hypothesizing that previously eroded strata from above the Burns fm. contained the dark cobbles? So far, we have seen a lot of residual blueberries that have been similarly derived from the evaporite cemented sandstone, but we haven't seen a rock with other lithologies embedded.

I am still pondering over the whole Halfpipe formation thing. With the help of the wonderful MMB geologizing program, I have looked back through most of the panoramas, going all the way back to Endurance tonight. I may be way off base, but I think this Halfpipe formation may be significant, and way more widespread than I had thought. I noticed that the banding on the ripples was of variable thickness over the traverse, and that the banded layers usually overlayed a more massively bedded and coarser unit. If I have this correctly, that lower unit appears to fit the definition of the Halfpipe formation. From all the sols I viewed, I would have to choose sol 707-717 as the type locality, as in this view.
Click to view attachment

Oh no, that was simply a poor job of proof-reading. I was thinking, and meant to say, that my initial view was that they eroded out of the evaporite, but since have revised that view.

This is an intriguing area, I constantly end up with a pile of puzzle pieces off to the side because they look interesting but can't quite figure where they fit in. Here's hoping that we can map a thick section at Victoria and that this can be correlated with what we've seen earlier. The "boat ramp" entry seems to cross a nice exposure, although the thickness of the section of the visible evaporite is only 2-3 meters.

--Bill

Tom:

I think we are thinking the same here. The bands could be leeward (slip faces), windward (stoss side), or horizontal to gently inclined sand sheet beds and lamina. In any case, that we can see them in cross-section suggests at least reactivation, if not net erosion, is the current (or was the last) state of sediment transport. Radical changes in net wind direction, or a reduction in sand supply, could have produced these features. Bill has also mentioned the regional orographic effects, which would play into either, or all, of these possibilities.

A nice wheel trench down through one of these banded ripples would be nice...not sure if one has been done already. On the other hand, they tend to be taller, steeper, and more rover-trappy...

Indeed, that was what I was referring to when I coined the term back at Erebus.

-Tim.

Thanks for your replies, guys. I was concerned I would be slapped down on both thoughts.

I can't help but speculate further after seeing the sol 912 L7s that came down of Espanola. No time for a pretty poster tonight, just a very quick and dirty side-by-side. The image from near Erebus on sol 712 is one of the rare L1s with the wide bandpass, so it was hard to know which 912 image to compare it to, but it probably doesn't matter that much. I had been expecting the basement of Victoria's apron to be more of a jumble of lithologies, but it looks pretty similar to the basement we've been driving over for a long time. It's time to sleep on it.
Click to view attachment

CR, from the "Annulus" thread:

Current thread:

When I saw that Sol 912 L7 I fell out of my chair, jumped the gun and tasted filet au sole. This area is creating some geo-philosophical distress for me in that it doesn't fit my mental image of what is happening here. OK, pre-conceived notion, I confess.

I , too had expected a jumble at the edge of the apron, but things are too orderly. I was looking at Chisholm and Espanola as outliers of the ejecta blanket, not as pieces of a (apparently persistent) unit at the evaporite contact.

This has been an eye-opener, and indeed it is time to sleep on it. But not before dropping over to Exploratorium to see what awaits...

--Bill

When I first saw those images, I thought Exploritorium was acting up and we were getting duplicate images from sevral sols back! Very unexpected, to say the least.

As for the the Halfpipe formation, now that Tim has pointed it out and we have a full description of it, it sure stands out like a sore thumb. And Tom's analysis makes a lot of sense.

Attached is a Route Map for Sol 742, which covers the Sol 712 timeframe. At Sol 712, Oppy was at the southern end of Olympia preparing to leave for Mogollon/Payson. It was also the time of the IDD arm problem and also the festoon "festival". It seems to me that the type locality for the Halfpipe Formation is the dune-drift area west of Overgaard; we'd been seeing the dark cobble fields for some time. Not only did we see these features to the western edge of Olympia/Mogollon, from the MOC image of the area it appears that these dark cobble areas were also east of the evaporite outcrop and may have been present inside of Erebus. I'm thinking that this Halfpipe Formation may have been the dark unit visible on the nose of the
"Payson Promontory", which we had been observing during the trip north around Erebus.

--Bill

Bill: I don't know if I am way out in left field on this, but I think the Halfpipe formation really is the coarse and dark, mostly massive layer below the lighter colored and layered ripples. We have often seen it on the surface, but there are many fewer observations of it and it's contacts in cross section. After looking at almost all of the panoramas between Endurance and the current location, it seems to me that this stuff is present below the finer ripples almost everywhere.

In the few places I have seen it in section it is mostly massive, with the occasional hint of a layer. I don't know what depositional environment to ascribe to it. I have been looking for a wheel trench that cuts through it, but I don't think there was one. Without much other evidence, one must wonder about an earlier residual layer weathered out of the Meridiani section that has been removed...as you previously suggested. Then, we found that big chunk of dark stuff sitting on the surface in the same area as the cross-sectional view I previously posted. My pile of puzzle pieces has been growing lately, too.

...and then we climb up onto the Victoria apron and find the same stuff being revealed below the diminishing drifts... Maybe I am missing something, but I would think that a wheel trench is long overdue.

I think you've got it. Whereas the current "soil" is a basalt sand-blueberry composition, the Halfpipe Fm is a paleosoil composed of subangular basaltic cobbles and other components. It is widespread and in that earlier era was the equivalent to the drifts we see on Meridiani today. Perhaps not a 'soil', strictly, but a surface unit more weathered than a regolith. It is different from the current soil because the environmental conditions were different in that earlier time. Nothing astounding: in the terrestrial Sahara desert we can find evidence of earlier, wetter, more temperate conditions under all the sand dunes currently present.

The "Stump", that disintegrating boulder we looked at from Overgaard, is likely connected with Halfpipe, and should have been examined while Oppy was there. Along with several other areas along the way to Victoria. This is not so much 20/20 hindsight as there were folks here saying "Stop. Look!". I'm not finger-pointing, but simply relaying observations.

Hopefully, we'll make a stop at the "Epsilon" crater ahead and look at the ejecta unit and the underlying units before heading on to Victoria. And perhaps we'll be able to see the same sequence once we get to Victoria, if there is not too much disruption.

Exciting times, no?

--Bill