[X] My Assistant

[tim53]

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.

[aldo12xu]

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

[Bill Harris]

The current Meridiani landscape surface is "erosional"...an unconformity in the making. The rippled eolian material is just a skiff, relatively, and represents material...

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

[CosmicRocker]

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...)

[Bill Harris]

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

[tdemko]

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.

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.

--Bill

[Bill Harris]

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

[CosmicRocker]

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.

[Bill Harris]

Are you hypothesizing that previously eroded strata from above the Burns fm. contained the dark cobbles?

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

[tdemko]

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...

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?

[tim53]

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.

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

-Tim.

[CosmicRocker]

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.

[Bill Harris]

CR, from the "Annulus" thread:

When I saw that sol 912 L7 during my morning MMB update, I nearly jumped out of my seat...

I was expecting the apron to be totally different. I'll take my geologizing over to the halfpipe topic now.

Current thread:

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.

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

[aldo12xu]

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.

[Bill Harris]

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