Geomorphology of Cape York, Examining Opportunity's destination at Endeavour Crater Options

[Gladstoner]

More thoughts on the Cape York strata:



Here, they are shown as hogbacks dipping away from the core rock of Cape York.

Varying resistence to wind erosion of the layers would leave them at different levels on an erosional surface.

Most resistant of all is the old crust (orange) exposed in the hills and mountains that make up the rim of Endeavor.

The light unit (yellow) seems to be more resistant than the other strata. There may even be a thin cap rock (shown here in maroon), which can be seen in places as a thin whitish line on images of the outer edge of the terrace.

The next unit up-strata (gray) is less resistant and forms a shallow trench.

The lower (gray) unit appears to be less resistant as well and would form a very interesting unconformity above the (orange) material. Some of this (gray) unit could be drifts and debris from the hill.

Further up the section, the thin (bronze) unit could be 'softer' still, a stratum that is particularly prone to erosion. In any case, it seems to be a significant bedding plane of some kind. A second, similar feature may lie above the (yellow) unit just above the 'cap rock', as a similar trench seems to occur in places.

Further up (and down-slope), the layers continue, but they tend to be covered by debris. Some of these layers could be similar to the (yellow) layer, but here their dip would be closer to horizontal, leaving a more ambiguous surface expression of the units (as it is over much of Meridiani).

Further away from Cape York, there are likely deeper strata that are not exposed at this location.

(For the sake of simplicity, I didn't include the slope of the surface surrounding Cape York.)

Just for fun, here are the layers as they might appear if they were level and eroding on a steep slope:

[Juramike]

I think I like your very first diagram the best (post 87 here). It explains the similar unit at the margin surrounding the central material of the ancient rim.

I'm not sure how I see that the interior layers could get bent to follow the crater interior. The initial whack should've gouged a big hole, and any layers that formed afterwards should have been dropped in parallel and flat. Any tilting after that would be uniform across the region...and you should see that as a difference in remnant rim heights.

[Gladstoner]

I was thinking along the lines of something like this:



In the previous illustration, I was concentrating on the surface expression of the exposed layers. I exagerated the dip of the layers for illustrative purposes. Perhaps I was too simplistic in showing the subsurface nature of the contact.

[Gladstoner]

After seeing the jumbled mess that Opportunity is now sitting upon, I decided to tweak my guesswork a bit.

The Cape York bench edge appears to be an old erosional surface.

A very simplified geologic history....

Original Endeavor crater surface, at Cape York:



Meridiani sediments laid down. I'm not sure if the original thickness of this stratigraphic sequence has been determined:



Over time, these sedimentary rocks are eroded and covered with ejecta (made up of the same rock):



With more time, wind eroded the ejecta deposits and remaining bedrock exposures, into the flat Meridiani plains:



Opportunity sits upon the ejecta deposit. The Cape-York-terrace erosional surface/horizon continues beneath the surface (and under where the rover now sits). Occasionally, as at Endurance and Victoria, the solid bedrock is uplifted to the surface by large meteor impacts. Otherwise, most of the 'bedrock' observed is just more of the jumbled ejecta.

The smaller impact craters are mostly or entirely within the ejecta deposit.

Where Opportunity now sits, the bedrock is relatively thin. Further out (eg. at Victoria Crater), the bedrock is much thicker (not shown in the drawings).

The ancient rim of Endeavor, on the other hand, is made up of more resistent stuff, as is readily apparent. Here, it acts as a 'canvass' for the Meridiani 'paint'.

[Juramike]

I'm not sure, but I think the uplifted rims of Endeavour were subjected to erosion and probably made a local mantling deposit at the base of the rim crests. Then additional materials were laid on top of that too. Then it was exhumed as you've described in your drawing.

(So take the drawing you did on June 18, then add a layer on top that then gets exhumed.)

[Gladstoner]

I'm not sure, but I think the uplifted rims of Endeavour were subjected to erosion and probably made a local mantling deposit at the base of the rim crests. Then additional materials were laid on top of that too. Then it was exhumed as you've described in your drawing.
(So take the drawing you did on June 18, then add a layer on top that then gets exhumed.)

Do you mean like this?:







The wonderful thing is that we will soon find out....

[centsworth_II]

....Over time, these sedimentary rocks are eroded and covered with ejecta (made up of the same rock)...

I'm not too sure about the ejecta layer you talk about. Over Opportunity's route, every time rock is exposed it appears to be a pavement of the originally deposited layers, not a bed of ejecta. The only significant ejecta seen is around some of the fresher craters and that is sparsely spread around the crater, not forming a layer. I don't think the sulphate rocks are durable enough to build up an ejecta layer. The only layer build up on the surface of Meridiani seems to be dust, sand and berries.

[CosmicRocker]

centsworth_II: There was an ejecta layer at Victoria. There's not a continuous deposit across Meridiani as the drawing suggests, but at Victoria we drove over the planed surface of the ejecta blanket and also saw it in cross section on the capes. It should be drawn as a discontinuous layer. I agree that on many of the ancient craters the ejecta has been removed (Erebus, for example), but I think it is ok to show it schematically.

[centsworth_II]

...There was an ejecta layer at Victoria.

The Victoria ejecta was highly localized, as is any of the ejecta on Meridiani as I see it. I could have it wrong, but here's the way I see it:

Ninety percent of the Meridiani that Opportunity has shown us is characterized by a fractured sulphate pavement covered by a thin ripple layer of sand. I think this sulphate pavement is unmoved since it was formed from wind deposited sediment layers, subsequently acted on by water. I don't think it is a bed of ejecta.


Just adjacent to Cape York, this sulphate pavement seems to be a little jumbled. But I still think it is essentially the original wind placed Meridiani layers, not redistributed ejecta. Of course, I could be wrong.

[Juramike]

So you are thinking that the Endeavour ejecta is buried deep below the "Meridiani wind deposited then aqueous-altered" layers and probably not well exposed?

Below the yellow and brown layers in Gladstoner's graphic (which I really like, BTW).

(...Makes sense to me.)

[centsworth_II]

So you are thinking that the Endeavour ejecta is buried deep below...

To explain my understanding, I've matched a geologic map of Merridiani (from here) with the corresponding area from Google Mars. Endeavour crater (red arrow) is buried in Meridiani deposits and subdued compared to the craters (yellow arrows) outside of the area of the layered deposits. The craters near the edge of the deposits (blue arrows) are special cases. I don't know if they were never fully covered, have been almost fully uncovered, or punched through the deposits. The smaller (blue arrow) crater near Endeavour is Lazu.

From these images -- and many other papers on Meridiani -- it is clear that Endeavor and its ejecta is buried in up to hundreds of meters of Meridiani deposits. The only visible part of the original crater are the rim peaks (including Cape York!).

Some quotes from the paper:
"We have completed a regional analysis of the hematite deposit in Terra Meridiani and
conclude that the unit is in the midst of a 600-m-thick stack of friable layered materials
superposed on Middle and Late Noachian cratered terrain."

"The hematite (P2) is in the midst of a complex stratigraphic sequence of layered materials. These younger
layered deposits (P1, P2, P3, E, I) are superposed on ancient cratered terrain..."

"The layered deposits clearly bury the underlying cratered terrain..."

[Julius]

Cant recall where but i had read that there was a possibility that the water activity in meridiani could be related to lava coming into contact with ice underground when the tharsis bulge was forming... meridiani lies on a slope from the tharsis bulge and mariner valley is close by!

[Gladstoner]

I suppose I should have limited the 'ejecta' to the jumble surrounding Cape York. My main point was that the bench edge could be an erosional surface. The deposit could be ejecta (as it appeared to me), or it could be fractured bedrock that had jostled as it crept downslope.

In any case, I appear to have jump-started an interesting discussion.

[john_s]

I don't see any reason to invoke a significant ejecta layer on top of the Meridiani sulfates near Cape York. The only places we've seen significant ejecta have been very close to the individual craters that created the ejecta, and there are no large craters near Cape York that post-date the Meridiani sediments. The surface bedrock here does look kinda broken up, but that could be due to the ubiquitous shrinkage fracturing that we've always seen, coupled with downslope movement and settling stresses associated with being draped over the Endeavour rim.

John

[Gladstoner]

There is a mostly buried, larger-than-Victoria-sized crater just to the north of Cape York, but then it is not certain if any of its ejecta would have covered Oppy's location.

I assumed that there are many of these craters with little surface expression lurking beneath much of Meridiani.



(Sorry about the small image size. I was in a hurry....)