Here are a series of false-color L257 Pancams of the current Oppy traverse stop. I'm still in the "arch the eyebrows and muttering 'fascinating' " mode so I don't have any words of wisdom right away. We can discuss over the next few day/sols before she moves on to the next miracle...

Cabo Frio: (order is #3, #1, #2 from left):

Image #4 at Cabo Frio:

Cabo Verde (order is #1, #2, #4, #3, from left):

Good grief, and mega<clinks> for the swear jar...thank you, Ed.

My first impression of Cabo Frio is that there seems to be a preferential side of the formation for wind erosion, and it looks like it's the opposite side from where Cabo Verde (did I spell it right, ustrax? Please don't hit me again! ) exhibited the same effect, and THAT seems really strange...almost as if there's an "air channel" in and out of the crater. Does the low atmospheric pressure & diurnal variation make that kind of a difference?

Wind erosion - of course we see wind erosion!

Once I did visit Australia, and the tourguide took us to a site and claimed the rocks and cliffs were as they were from the age of the dinosaurs.
Coming from a location where everything have changed since the last iceage just 10 000 years ago, that idea gave me vertigo.

Yet those cliffs and bedrock in Australia are something to compare with what we have here, then multiply the age times 10, 20 or perhaps even 50 times. Over such timescales we only can be happy that erosion might have been slow on Mars.

Bill, I can see you are finding interesting things to do in your retirement.

I have to go to bed...I must perform paid labor, tomorrow... ...Still dreaming...

...enjoy.

Not only the geology of this locale, but also, and as importantly, the geomorphology, "how did it get to look like it is today?" is open for study here. Getting the ground-truth here will help us in interpreting MRO imagery. Although, by Earth-standards Mars has a thin, almost non-existent atmosphere, we can see that it is currently _the_ major player in erosion, transport and deposition.

One thought: Mars has a weak magnetic field and a thin atmosphere, so the cosmic ray flux is orders of magnitude higher than on Earth. And the erosional process is much slower so a given rock will lay on trhe surface for thousands or millions of years. What is the effect of millions of billions of cosmic ray impacts on the component mineral crystals of a rock? The crystal lattice must be perforated like a zeolite...

On the "R-word". I do look at it as "graduation" and having paid my dues on my day job, with a 27-year investment paying off. I've got a lot to do the next few years. I'll continue in the OT section later today...

--Bill

I do find the stratigraphy of Cabo Frio to be mystifying. There are the long diagonal layers on the side, but out at the tip there are horizontal layers. How the <clink, clink> could that happen?

Cabo Frio in 3D

Click to view attachment

Diane

I'm not sure which view you're looking at, but in general, if a series of parallel planes are inclined (as in some cross-bedding), one cross-section will show the dipping layers, but if you rotate the block ninety degrees, a cross section will show horizontal strata. One of the images about 3/4 down on this page http://www.geologie.uni-stuttgart.de/onlin...en/Seite7_1.htm shows what I'm trying to describe.

Lee

I'm annoyed I can't find it but somebody posted the picture in one of the threads with both the horizontal and diagonal lines highlighted and offering the following suggested interpretation: horizontal are stratigraphic beds, diagonal are fracture planes produced by the shock of impact. Seemed reasonable to me.

I am somewhat concerned that what we see as stratigraphic bedding or even fracture planes might be caused by wind erosion that occurs within only a few centimeters of the regolith surface. Then as the surface recedes, more rock is exposed and a new layer of erosion occurs. This seems consistent with some of the angles of the outcrop strata with respect to the angle of the ground surface of Duck Bay.

That "marker bed" on Cabo Verde might not be true layers, but rather evidence for successive cycles of wind erosion. Each layer is a climate change causing a wind change, causing different erosion.

When you see blocks planed flush to the surface of the ground, wind erosion takes on a rather dreadful importance. We sometimes don't understand it because of our earth geology biases.

Scott

Here is my quick-n-dirty interpretation of the stratal geometry of Cabo Verde on a photo posted yesterday by Nix.

A couple of notes:

1) the lighter-colored upper bedrock unit is in stratal continuity with the lower striped unit...it looks like the upper, steeper beds are foreset laminae and the lower horizontal to gently inclined beds are toesets deposited by the migration of a dune (this is what I call festoon crossbedding!)

2) my interpretation would be that these are the deposits of a prograding eolian dune with well-developed avalanche-face foresets, and that the toesets are built out of grain-flow tongues separated by grain-fall drapes and wind-ripple translatent strata

3) the ejecta is bloody thick here and most, if not all, of the overlying evaporite beds have been blasted away..only a highly-fragmented zone of evaporite-rich ejecta blocks remain...this is likely to be variable around the crater and I'm sure we will see some in-place evaporite beds on top of the eolian beds...maybe even in the next bay

Click to view attachment

That all makes sense to me. What doesn't make sense is how the evaporite-rich ejecta strata seems to be *more* erosion-resistant than the aeolian deposits. We seem to see this phenomenon all around the crater, a lighter-toned bed of apparently evaporite-rich rock that appears to form a resistant bed, which has stayed in place even while the aeolian deposits below them are undercut.

The evaporite we've seen here, both unshocked (out on the plains) and shocked (within Endurance), seems to be the softest rock around. It's certainly a lot softer than the volcanic rocks over at Gusev. It almost appears, at times, so friable that it would simply crumble if held in the hand and squeezed.

Any idea how such soft rock becomes the most erosion-resistant rock in the walls of such a big crater?

-the other Doug

Could it have anything to do with the depth of the crater? ..wind tunneling down inside and around in the bowl of the crater, leaving the top layers relatively intact? just a guess...

Nico

dvandorn

In this case I think what might be happening is that the aeolian layer *is* the more resistant layer which is responsible for the steep cliffs. The evaporite-rich ejecta layer is protected from the undercutting simply because it's sitting on top of the more resistant bed. The more surficial processes have beveled it back somewhat - which makes the aeolian unit that much more prominent.

That's my take anyway.

Here is what I think:

When you get much more than a couple of centimeters about the local surface, the erosion rate decreases dramatically. Almost all erosion occurs within a few centimeters of the surface. Local topography can modify this somewhat. Tops of cliff faces don't get much erosion at all. The evaporite-rich layer is protected from the undercutting because it is several meters above the surface -- and therefore has been subjected to very little wind erosion.

A way to envision it is to imagine that the layer of air right above horizontal surfaces is corrosive.

Scott

my guess: groundwater rising, movement of cementation materials, ground water leaving, slumping some areas.

One layer might be more prone to fail after the water level dropped.

Not necessarily relevant, but do caves form in evaporite (salt, on earth) in the same way that they do in limestone? In other words, is there an evaporite version of karst terrain to look for on Mars?

I don't know if they form Karsts but a quick google search of "large salt cave" or "halite cave" or "gypsum cave" turns up loads of hits.

I am so blown away by the long baseline anaglyphs of the opposite rim that I am having a difficult time thinking of anything else.

tdemko: Thanks. That looks good. Although they were not apparent on some of the images we saw at first, those dune foresets are there if we look for them, even though the upper part seems massive at first. I really was not expecting the ejecta to be so thick here. That, and the apparent, later collapse features are giving me fits. I think the only way I can deal with this is to retire from my day job.

Another thing that continues to confuse me is the way the term "evaporite" is so frequently used here. I thought that all of the rocks we have seen are really sandstones that have been later cemented by evaporites. What is the "evaporite" layer many keep talking about? Is it the upper part of the Burns formation seen at Endurance?
Algorimacer: Such things exist, but they are very uncommon compared to the karst terrains created in carbonate rocks, afaik. There are a lot of reasons for that, but imho the most important reasons are that salts are simply quite soluble in water, and such rocks have little mechanical strength. Carbonate rocks are only slightly soluble in plain water, but more so in commonly available, acidified water, and carbonate rocks have significant mechanical strength...so cavities formed in them are more durable.

O'Doug: These are probably quite soft rocks, but I would think not quite as friable as you suggest. Regardless of that, the important thing to consider in erosive environments is the relative durability of the layers. These cliffs exist, regardless of how incompetent they may be.

Actually, I believe I was the first one here on UMSF to refer to the light-toned, layered bedrock first seen in Eagle and later seen, well, everywhere at Meridiani as evaporite. It was about a week after I first used the term that I can first remember Squyres using it during a press conference or in one of his updates. I started using it because of the Anatolia-like crack features I could make out in the enhanced DIMES images, which suggested strongly to me the polygonality of dried sea floor beds. It just made sense to me that the surface we were seeing was the result of the dessication and shrinkage of a wet sand or mud unit; when the first-look at the rocks revealed them to be composed largely of sulfur salts, I laid the label of evaporite on them.

Technically, I suppose only the sulfur-salt matrix of the light-toned rocks are actually formed from evaporation of acidic, high-sulfur groundwater, and the resulting rocks are more accurately described as sandstones with evaporitic cementation. But since the high-sulfur-salt-content rocks were formed by multiple epochs of evaporation (and possibly sublimation?) of sulfurous, acidic water, it's just been a good shorthand to refer to them as evaporite rocks. And it differentiates them from the more pure sandstones that seem to underly the evaporite layers (the lower portions of the Burns formation).

I just keep wondering -- if there's a Burns formation, oughtn't there be a Smithers formation?

-the other Doug

I just checked my old posts, and the term I used first on May 7, 2004 was "evaporation layers" to describe the light-toned rock beds evident throughout the Meridiani area. In a post I made ten minutes after that, I referred to individual rocks within the unit as evaporite.

Funny -- I made two posts on the old forum on 2/9/04, and then didn't post again until 5/7/04. Well, my marriage was falling apart at the time, I suppose I was allowed a little distraction... *sigh*...

-the other Doug

OMG!
Where have I seen all this before?!...

Now that we have "drill holes and cores" to look at here, it appears that this will prove to be a very complex area to understand.

Tim Demko has it nailed: the cross-bedded evaporite unit overlain by ejecta rubble. The underlying bedrock has it's own complex history of aeolian activity, playas, groundwater interaction and earlier impacts. And then there was the more recent impact of Victoria which disrupted, fractured and shocked the bedrock; we may be looking at upturned beds here. And then there is the subsequent weathering and erosion of the freshly-exposed evaporite surfaces. Subsequent processes include undercutting, slumping and collapse of the cape blocks. And so on...

"Evaporite". That is a frequently-used misused term. I use that as a catch-all term to refer to the primary class of rock here: the sulfate-rich indurated silicic sedimentary unit that has a light-toned IR appearance and has the salmon-ochre hue in false color. We started using that term when we discovered the magnesium sulfate rich layered rocks exposed in Eagle crater. There may be a proper petrographic term for this rock, but the term of "evaporite" is well-understood here.

Smithers Formation? Ugh.


--Bill

Belated congratulations on your retirement, Bill!! You couldn't have picked a better time

Amazing how many new posts there have been in the past 3-4 days. I hardly know where to start. Amazing images, nice colour renderings and enhancements.

Tim Demko's aeolian unit seems to correspond the unit making up Cabo Frio. I wonder if this is also stratigraphically correlated to the cross-stratified Burns Cliff sandstones (Burns Formation Lower Unit). If that's the case we've only gone down 4 metres in the stratigraphic column over a distance of 5 km:

http://www.unmannedspaceflight.com/index.p...ost&id=6987

Or, since the elevation has been increasing as Opportunity traveled south, could the Cape Verde rocks be overlying the well laminated Upper Unit we saw at the top of Eagle and Endurance Craters? Phil Stooke's enhanced version of Cape Verde shows the unit underlying Tim's aeolian unit to be very finely laminated:

http://www.unmannedspaceflight.com/index.p...ost&id=7817


Also, could the Cape Verde horizon be the source for the dark cobbles seen between Erebus and Victoria?

Yes they do, though it is very rare on Earth because the climate is very rarely dry enough. The main example is Mount Sedom southwest of the Dead Sea in Israel. Monut Sedom is basically a salt dome that has risen to form a modest mountain, since there is not enough rain to melt it (as happens almost everywhere else). There is however enough to form a fairly complex cave system, "halokarst".

tty

I understand that we have all conveniently used the term "evaporite" to describe the light toned bedrock everywhere since early in this mission. My question about the recent use of the term was based on some comments where I thought some were suggesting that there were evaporites overlying sandstones. I only wanted to clarify that. I was afraid that I might have missed an important contact. I can't lay my hands on one at the moment, but the last vertical profile of sulfate and halite content I remember seeng from the Endurance section showed a small decrease in those salts at the bottom, but not a lot.

I don't know about the rest of you, but I am suffering from a serious information overload since we arrived here. Let's all keep our attention up at this Friday's briefing. I wish they had more than the few speakers who are scheduled, though if I had to pick two, my choices will be speaking. It's been a long time since the last briefing, but they always were pretty enlightening in the past...

Complex, yet unknown, Tom. Let me post again the Grotzinger, et al strat section.

A good review of the known strat is at Aldo's MarsGeo site, Burns Cliff and Rock Types.

I am starting, on my local disk, a directory of images of the strat section arranged by capes/bays so I can keep up with the info as it comes in. Wonder if we can/should start something like that on UMSF?

--Bill

Bill,

I'm assuming that the Upper Unit in that sequence is what we've been calling evaporite. I must admit I've not really been happy with the term evaporite in this setting. I've worked the Permian of the UK Southern North Sea, where similar formations exist. The term interdune/playa is more appropriate, with evaporites referring to the salts that crystalise in amongst the interdune sediments. To me a true evaporite is something like a thick halite or anhydrite section, such as you get in seas/lakes on Earth in hot climates with little sediment input. In those cases the evaporite forms by evaporation of the water under high temperatures. Here on Mars, the process is more likely to be due to loss of atmospheric pressure causing evaporation. It will be interesting to see if we can correlate the Burns Cliff section with Victoria, although any interpretations are likely to be speculative without having continuous exposure of rocks in between.

Aberdeenastro
(previously known as Castor)

It may not be a bad idea to pin a few separate threads for individual capes/bays as you propose Bill.

This place is big after all and Oppy will spend at least a year here.

Nico

Thanks, Bill, for reposting the stratigraphic column by Grotzinger et al. constructed from the efforts at Eagle and Endurance craters. It is a good place for us to start our discussions of the Victoria exposures.

This is also a good place to do a little nomenclatural housekeeping. I tend to be very exacting with my students on these topics, but get a little sloppy myself, especially in an informal situation like internet discussions or email.

When communicating ideas about rocks or sediments, especially layered rocks or sediments, it is always good practice to keep observations and interpretations separate, especially in how we name and classify them. This is expressed in the concept of lithofacies in which we classify and name a sediment or rock based upon the fundamental properties of composition, texture, sedimentary structures, form, association, and fossils (if present). An example of a lithofacies would be a "medium-grained, cross-bedded sandstone". On the other hand, the depositional environment of a package of ancient sediments or sedimentary rocks is an interpretation based on analyses of these fundamental properties and the changes between, and association with, units above, below, and laterally adjacent. We sometimes mix these two concepts into a hybrid "depositional facies" like an "eolian sandstone". The word "evaporite" also fits this situation. The lithofacies is probably something like a "thinly-laminated to ripple cross-laminated, tightly cemented, recrystallized, magnesium sulfate sandstone". I'm not about to type that every time, so I lazily fall back on evaporite.

There is one more way to classify and name layered rocks, and that is formal stratigraphic nomenclature. These would be the formal names of supergroups, groups, formations, and members. There are two publications that deal with the details of naming these units that codify how it is done, the North American Stratigraphic Code and the International Stratigraphic Guide. Inherent in both are the concepts that depositional environment AND age should have no bearing on the delineation and naming of formal stratigraphic terms. The preferred root of the name is typically a geographic location where the unit was first described, or where it is exceptionally exposed.

So back to the figure from Grotzinger et al....they term the units as parts of the "Burns formation". By the fact that they do not capitalize the word "formation", I am guessing that they are not trying to erect a formal stratigraphic nomenclature here. In fact, I'm not sure if the Code or the Guide have any extrerrestrial impact at all (although I do not see why this should matter). However, they do erect several subformational units, the Lower, Middle, and Upper units, which they (confusingly) do capitalize, implying that they are "formal" stratigraphic units. The other parts of the diagram, especially the sections labled "Primary Facies" and the text to the right of the column have the evil mix of observation and interpretation that I was discussing above. The "Primary Facies" are grouped by their interpretation of paleohydrology (Dry, Dry to Damp, and Damp to Wet), but followed by both terms of environmental interpretation and physical sedimentologic description (i.e. eolian sandsheet interdune and ripple cross-stratified). The text to the right of the column groups the units by depositional environment and diagenetic features, and the smaller font supporting text has physical sedimentological lithofacies descriptions, environmental interpretations, and even speculative interpretations. It's a bit of a mess...if this manuscript was sent to me to review (hint, hint to any editors or PI's reading...), I would have recommended that the observations/data be convincingly separate from the interpretations in this diagram (and in the main text, by the way...).

Now, this is not to take anything away from this paper...it's a great summary of some landmark research that has implications far beyond Endurance crater, and in fact, along with other information coming out of the MER program, it has changed some of our basic ideas regarding the history of surfical conditions on Mars.

However, it does show that even the Big Guns/Chosen Ones get a little sloppy sometimes, too...and mea culpa, I will try to strive to set a better example, even in the informal setting of UMSF!

Very good duscussion and guidance, Tim. I confess that I may be the world's worst at naming rocks, I'm 30-odd years out of school and have spent the last quarter-century knocking around the Pottsville Formation (essentially flat-lying beds of sandstone and shale with enough coal to make it economically viable) AND reviewing geologic descriptions written by engineers. I've picked up horrid habits. We do need to work on our descriptions, but we have only so much to work with online. I'll dust off my textbooks.

Grotzinger, et al is the landmark-but-initial paper on the first leg of the traverse at Meridiani. As more data comes in and as subsequent papers come out we'll know mnoe wbut thisis the standard reference so far. The "Burns formation" is more honorary and is shorter to say that "them strata we first saw at Endurance" for the time being. I wish we had been able to get a close look at the bluffs at Payson to get a midpoint between Endurance and Victoria. The jury is still out for me with the so-called Halfpipe formation. I've not quite figured out what it is.

Victoria is exciting. I look at each Cape as a drill hole or "highwall exposure", Twenty-four of 'em arranged in a 700 meter circle. There is an apparent stratigraphic marker near the top and I think I can see continuity as well as change between exposures.

--Bill

Thanks everyone, for the more-than-adequate explanation of the use of the term "evaporite." I guess I understood what it has historically meant here. In some discussion in this forum I thought I remembered it being distinguished from the sandstones. I guess we can carry on. I am comfortable with simply calling this light-toned pile of sediment "evaporite."

I would like to introduce some of the concepts from the Edgett paper (in volume 1, here: http://marsjournal.org/contents/ ). Keep in mind that this stuff is taken somewhat out of context. The original paper covers many concepts. I have been waiting a long time to see Victoria, and to see if his crater exhumation ideas will be upheld. So far, I think they are.

Here are two captioned images from the larger paper. I think they provide a good overview of his crater exhumation ideas. If we can find evidence here that this process is occurring at Victoria, we will need to distinguish between pre-impact stratigraphy and post-impact stratigraphy...and even more recent stratigraphy.

Click to view attachmentClick to view attachment

In order to keep this in context as well as I can without copying the entire paper, here is the description of figures 21 and 22 from the main text:

"MOC images of craters near the MER-B site suggest that the plains-forming unit exhibits a progression of crater expressions, from those that are buried to those that are partially to fully exhumed (Figure 21). Endurance Crater, explored by MER-B, might have once been partially filled like the crater in Figure 21a. Victoria Crater (Figure 21b) illustrates the next stage in the exhumation of a crater in the plains-forming unit. The U-shaped alcoves eroded into rock around the crater’s circumference indicate erosion by undermining and collapse as less-resistant crater-filling material and/or brecciated crater wall material was broken down and removed from the crater, perhaps by wind. The rock into which the U-shaped alcoves formed overlies the original (presently buried) Victoria Crater rim. Endurance Crater (Figure 21c, d) might be showing the next stage in the process. At Endurance, the raised crater rim is topographically expressed, as are some aspects of ejecta blanket, but none of the original rim nor ejecta are fully exhumed. For comparison, Figure 21e shows a fresh crater—one never buried—on Meridiani Planum."

At marsjournal.org you can download the whole paper, and all of the images in their png splendor.

I didn't quite have enough space left to include this last image. I didn't appreciate this burial and exhumation process when we were at Endurance, so I think I missed a lot of important observations when we were there. I did remember a few things though, and went back to find this image.

There were very few observations of something like this. On sol 118 Opportunity captured this image of what I am interpreting as possibly some draping layers of the initial crater fill. Granted, this might also be explained by some kind of secondary weathering or diagenetic process, but I could never forget this picture.
Click to view attachment

Thanks, Tom. Very relevant post for understanding what is happening here at Victoria and at Meridiani. I had been meaning to grab that Edgett paper thru our big pipeline at work but never got around to it, so I'll wait for a good dialup time and download it soon.

Burial and exhumation explains a lot on what we are seeing at Victoria. It looks so fresh and new, but only because we are seeing newly exposed and active surfaces in the bluffs. Not only do we have the pre-impact Meridiani units to contend with we also have the intermediate fill or burial units present. This explains the character of the ejecta blanket: we haven't been seeing the ejecta blanket per se but the expression of the ejecta blanket as it has filled in the rough surface of the etched terrain. This explains why we the the everpresent basaltic sand and blueberries where I/we had hoped to see pulverized "victoria guts".

I've had a hard time getting my hands around the exhumation process. Not that I doubt that it exists, I just can't clearly visualize how the infilling sand, silt and (presumably) "re-indurated evaporite material" is removed from the crater by the wind. Some things you take on faith without completely understanding (for the time being).


--Bill

This is a very interesting line of speculation and I can see why it appears to fit with the surface materials and appearance of the apron, but I have the same difficulty in coming to terms with exhumation by wind. Why should a crater steadily fill up with dust and sand after its formation and then systematically empty itself of the same materials? I think we'll have to wait for more pieces of the jigsaw before the real story emerges. Fortunately there's quite a few lying around.

Yes, thanks Tom for reminding us of Edgett's paper. Like Bill, I never had chance to read the paper in detail. The exhumation process certainly seems to explain the appearance of crater rim wallrocks at Endurance and Victoria. But I'm having trouble picturing the step by step process:

1. Pre-existing Meridiani "evaporitic sandstones"
2. Meteorite impact
3. Impact crater gets buried by windblown sand
4. Water table rises, soaking sands, water evaporates, diagenesis, etc., creating 2nd sequence of "evaporitic sandstones"
5. More aeolian deposition, water table interaction, etc., creating additional evaporitic sequences
6. Erosion sets in and removes evaporitic sequences, layer by layer
7. As erosion works its way down to the old crater rim.......???

.....this is where I have the problem: If the rate of erosion over the crater is constant, why is that we don't see one contiuous erosional surface stretching from one side of the crater rim to the other. For example, the cross-stratified sandstone at Cape Verde seems to be an extension of the same unit seen at Cabo Frio, and, more than likely, continues along the other alcoves. I understand how undercutting could've occurred due to the lower unit being less resistant to erosion once the rim of the crater became exposed, but how is it that material seems to have been preferentially removed from the interior "bowl" of the crater first?

Well not to belabor the point, I would distinguish between an evaporite and a sandstone/siltstone in the following way. An evaporite is created from an accumulation of chemical precipitates that had been dissolved in solution. The desert playas of the American Southwest are covered with the stuff. The floor of Death Valley is exceptionally thick. Sandstone on the other hand is created by solid granules, eroded from the mother rock and carried in suspenison, typically water or air, or sometimes simple downslope mass wasting. Those particles then drop out of suspension and go through a cementation process, either a chemical precipitate or they become cemented through heat and pressure.

The fact that exhumation of layers is steadily taking place around the inner rim of Victoria does not conflict with the intuitive expectation that the crater is slowly filling up with (a) liberated erosional products tumbling down from the eroding faces, ( wind blown fines from further afield on Mars, and © occasional ejecta from other impacts. Most of what gets into Victoria never gets out. This would be due to the low "lift" capability of the local winds (speed and atmospheric density), their likely reduced strength inside the bowl vis a vis outside on the open Meridiani Plains, and the simple geometry of a crater - scouring out requires upwardly-corkscrewing winds fast enought to lift solid material. A tall order for anything other than very fine dust, I suggest.

Victoria as a crater is doomed, its bottom already a deep pile of tumbled-in material overlaid by the net of wind-blown dust ripples we see on top. It is slowly on its way to becoming an Erebus.

Kenny

I agree there's a bit of a problem, Aldo, but how about if we rewrite the steps accepting that the VC impact occurred during Mars 'wet period', to whit:

1. Pre-existing Meridiani sandstones - saturated (+ or -) with water/ice
2. VC impact forms crater.
3. Hot crater fills with inflowing water and/or windblown sand and/or ice crystals and/or snow.
4. Water-rich fill freezes and is covered by layers of windblown sand.
5. Water covers/saturates surface layers, indurating sands with sulfates.
6. Later acid-wet episodes leach and deposit hematite concretions in surface layers.
7. Mars enters dry period (> present).
8. Eolian erosion deflates surface layers leaving lag 'blueberries'.
9. Water in crater fill escapes through warming or sublimation, leaving spaces in fill.
10. Surface 'evaporite' layers collapse into crater cavity exposing broken edges to rapid erosion.
11. Victoria cavity "exhumed" by continuing collapse and erosion from strong winds.
12. Original VC rim remains buried, or eroded away between steps 3 and 6.
???

I find the caption on this image to be completely counter-intuitive. But keep in mind that I am a complete layperson when it comes to geology

The caption imples that crater C (Endurance) once looked like A and B.

But to my eye it looks rather obvious that the progression shoud be from C to B to A, as the crater erodes and is filled in by sand. But I could of course be completely wrong.

Your confusion is entirely understandable, but you aren't following the researcher's explanation that his steps a>b>c represent the exhumation of an ancient (3-4 billion year-old) crater which has been formed, eroded, filled and buried by sand BEFORE steps a, b and c take place. These much later stages superficially resemble the earlier steps (in reverse order), but the fundamental differences become evident on closer examination: the ancient Meridiani craters depicted have had their upraised rims and coarse, blocky ejecta fields and ejecta rays removed by erosion or else deeply buried. They are nowhere to be seen (at least from MOC views - we are about to see what we can find from our surface rover). What is especially intriguing is that the hole in the ground we call Victoria Crater may be only a vague shadow of the original impact structure. Unless we find hard evidence like impact breccia in the 'cabo' walls, we may not be seeing any part of the original structure. At least that's the ultimate implication of the 'ancient exhumed crater' hypothesis.

Ready,...Aim,...Open fire!

Dan: I think most of us understand those distinctions. The "side discussion about what an "evaporite" was originated from my earlier comment that these rocks are probably aeolian sandstones that have been cemented by the precipitation of salts in an evaporative environment. I think we are all on the same page here, regardless of how we prefer to label the rocks.

I think Shaka explained it pretty well. We are trying to figure out how the erosion of the later crater fill occurred.

I've been stewing this apparent paradox for a while and waiting for one of our rockhounds to explain it, since similar processes should occur on this planet. Why can't the shift from deposition to erosion/exhumation be a simple function of wind speed? Below a certain threshold speed the wind blows particles into a crater, but lacks the energy to lift the larger ones out on the other side - so the crater fills. If the planet or region later enters an era of higher wind speeds, the particles, large and small, exit on the downwind side, and so the crater empties and erodes - i.e. exhumation. If there is a basic flaw in this idea, can someone explain it to me? Tim Demko?
I wish someone could search out or produce a diagram of the wind patterns and velocities produced in a crater -shaped depression (or for that matter, a circular sports stadium), showing how velocities differ from a basic prevailing surface wind. We really need this to predict the best locations on the Victoria rim to position Oppy in order to get a good cleaning.

I don't see how a crater the size of Victoria can fill up; the increase in the amount of debris at the bottom is probably more than counteracted by the expansion in the crater's size due to erosion. I.e., Victoria might be getting shallower, but it's also getting bigger, and the bigger it is the more room there is for the debris inside to settle into. Over eons, I suppose, it could change from a relatively neat hole to a miles-wide depression, but its overall depth ought to remain more or less the same.

David,
You seem to be making the unconscious assumption that there is fixed relationship between the rates of rock erosion and sediment transport by wind. Certainly this is unlikely given the broad range of rock hardnesses. Imagine two craters swept by identical sediment-laden winds - one crater in "piecrust" rock like the Meridiani 'evaporite' and another in basalt. You wouldn't expect them to erode at the same rate, and, indeed, the basalt crater might fill up, while the piecrust eroded. If you've looked at many MOC images, you know that Mars is covered with craters of all sizes in all stages of filling, burial, exhumation and erosion.

Layperson or not, you're right to argue the point. I certainly don't see any "buried rim" with Victoria - I think whatever rim there was has collapsed and traces of the collapsed rock neatly covered with dust. However, I think it's too neatly covered. While wind might accomplish the C&B ["Cape & Bay] rim features we see, in this case it's quite clear that wind didn't do the bulk of the work. I'm voting for water. I'm pretty confident in this, and if I was at any conference discussing it, I'd be the last one out of the auditorium, arguing all the way.

My primary reason is simple: Where did the rock from the "Bays" actually go? Small slumping/breakaway and then neatly rolling to the bottom of the crater? At the very least, we should see some decent sized blocks slumped up, perhaps peaking out from the dust at the bottom of the crater. We see some of the collapse of the "Capes" like the "F Cape". None from the "Bays". Why is that? Why no "failed bays"?
I seriously doubt that rubble is going to melt away into the crater floor any time soon... or any time ever, for that matter. Unless there's water involved.

There are probably three scenarios that might explain the complete lack of rock remnants from the various Bays, but the simple one works the best for me, and it's the argument I'd salute at the moment and it involves copious amounts of ground water either during or shortly after the initial impact. Seeing that there's no significant discrete ejecta rays coming out of the crater, I'm leaning to the "ground water intrusion AFTER impact" scenario. This is flawed because I have no good explanation for how the water got there with the current topography, so I'm kind of waiting for help on this one. I'm assuming since we have some ground water alteration seen elsewhere, we'll see some here. (The third scenario would involve late water incursion into the area, "melting" the rubble and that's the easiest to disprove, I think)

That said, water would aid in the bay formation, and a one time slump (rather than some gradual slump and crumble over time) is evidenced by what I think we'll see upon closer examination of the walls of the Capes, starting with what we've already seen with Cape Frio. One scenario here would involve ground water seepage in and perhaps one primary zone being weak enough to collapse. That thinly layered bedding we've seen at the base of Cape Verde will be suspect #1. I sure hope we see plenty of altered basalt in that layer.

However, I'm no expert on this. While I've fondled rocks for pay in the past, I've no experience with craters, terrestrial or extraterrestrial. But I'd be hard pressed to see wind be the culprit in completely eliminating all bulky rocks that slumped, and then covering the whole thing with thick dust. So, I'll go for a wet Mars on this one.

We may be clearing up much of the uncertainties about Vikkie's formation in the coming months. Or we might not.
If all we can find in the walls of the Cabos is the sort of sandstones we've been traversing since landing, and if those sandstones don't show any of the transformations caused by impact - i.e. melt, breccia, shattercones, even large scale disruptions, other than those that can result from simple collapse into a void, as I hypothesized above in post #41 - then I will be increasingly convinced that we aren't seeing the original VC at all. We would then be exploring an excavated pit that merely coincides with the point of the original impact and was formed by its collapsible fill. The real Victoria Crater might be deeply buried and its original rim diameter, if not removed by erosion, may actually be significantly larger than the edge of this pit, and it might take another 5, 10 or 50 meters of exhumation before the two coincide.
We admit that the sinuous, cape-bay-cape edge is not typical of most craters. Have any been seen on the Moon or other bodies. It may be that they can only form by the sort of elaborate sequence of post-impact events such as in #41.
I must say that, apart from the infamous "festoon cross-laminations", I have seen precious little sign of running water effects on the Meridiani Plain. Maybe close inspection of the Cabo walls will reveal some, but I will need someone to point them out to me.

(That ought to bring out the heavy artillery! )

I keep hearing people wondering how a crater the size of Victoria could get filled in. It *is* counter-intuitive to try and imagine a crater this size being filled by windblown sand and later exhumed.

I think the key is in the fact that it may well have been that the crater wasn't filled by windblown sand. The ground that is collapsing into the crater, and thus ought to be of the same composition as anything that filled the crater, is made up of evaporite-cemented sandstone. Which could have been laid down by water, not wind.

It's more intuitive to me to propose the deposition of a thick layer that filled Victoria (and other craters) via aqueous deposition. If you had a shallow acidic sea, for example, which formed *over* a young Victoria and then gathered up tons and tons of air-fallen sulphurous volcanic ash from neighboring volcanic vents, it would create the kind of layering we see, and leave a quite erodable layer of soft sandstone that millions of years of winds could have removed, exhuming the original crater pit.

What do y'all think? Makes more sense to me than trying to fill over a Vickie-sized crater with only aeolian deposition...

-the other Doug