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Geomorphology of Gale Crater, Rock on!
serpens
post Jun 7 2017, 08:40 AM
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Julius, I think that Curiosity will need to get up close and personal with the clay bearing trough before they can assess how and when it was formed. In their brilliant Geological mapping and characterisation of Gale as a potential landing site, Anderson and Bell depicted the clay as a thin bedding plane with a segment exposed by the trough and that characterisation has carried forward. As I understand it the clay signature seems to indicate smectites. The overlying hydrated sulphates would likely have formed in an acidic environment and smectites are pretty good at consuming acidity with the end product being amorphous silica. So if the smectites had been exposed to the acidic waters during sulphate deposition, wouldn't hydrated silica and kaolinite have been detected, unless the clay had been covered by an impervious layer? An alternative is that the clay was formed following deposition of the sulphates as a function of erosion of the sulphate and formation of the fan. Could this clay be a localised deposit formed from pooled water that had leached Mg from the higher sulphate deposits? Both the hematite ridge and the clay trough are going to tell interesting stories.
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serpens
post Nov 9 2018, 12:14 AM
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QUOTE (HSchirmer @ Nov 8 2018, 05:06 PM
...Given the elevation of Gale crater, and recent northern-ocean papers, is there any way to differentiate whether Gale was lacrustine or perhaps an interior estuary?

All evidence to date implies lacustrine. Dichotomy elevation differences aside, the northern crater rim is heavily degraded compared to the northern rim and the area directly north of Gale is somewhat atypical compared to adjoining topography. it is possible that it was overtopped by lake water or by catastrophic events such as impact driven tsunami although there is no evidence of such. The rim is above the elevation assessed for proposed shorelines for the northern ocean(s).

Hmm, reminds me of the Newark Basin paradox- a 2-mile deep deposit of shallow water mudstone interbedded with fanglomerates and sandstones. (It is a paradox because with a 2-mile deep basin [~about the average depth of the Atlantic or Pacific] you'd expect to start with deep water sediments which get shallower as you fill in the basin over time. Instead, the Newark basin was shallow during the entire time that 2 miles of sediment were deposited.)

I thought the Newark basin was the result of a slow graben process? As the land sank the rate was matched by shallow lake deposition.
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HSchirmer
post Nov 9 2018, 12:57 AM
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QUOTE (serpens @ Nov 9 2018, 01:14 AM) *
The rim is above the elevation assessed for proposed shorelines for the northern ocean(s).


Curious about that, a recent paper suggested that early shorelines make sense, IF you remove the Tharsis bulge, and the corresponding antipodal bulge near Gale.



That puts the Arabia shorline (purple line) right around the elevation of Gale


QUOTE (serpens @ Nov 9 2018, 01:14 AM) *
I thought the Newark basin was the result of a slow graben process? As the land sank the rate was matched by shallow lake deposition.


Yep. Newark is a hanging wall/graben.
Which is interesting with the possible faulting/slumping of Gale's north crater wall.
But the twist for Newark - clays interbedded with fanglomerates and sand, might illuminate what happened at Gale.
If you have sedument from 2 sources, young rivers cutting into the hanging wall side, providing fanglomerates; while flat rivers from the seaboard side providing clay sediments.

Earlier papers discuss the hypothesis of an Elysium lake overtopping the north rim,
https://vdocuments.mx/hydrogeologic-evoluti...biological.html
but either way, the notch in the north crater rim would let north draining rivers flow into Elysium lake/Arabia sea during wet periods,
but would/should also result in shallow areas to the north draining fine clays back INTO Gale-

I'm thinking of the paradox that ~250 million years ago, US east coast rivers- Potomac, Susquehenna, Schuylkill and Delaware, initially drained NORTHWEST.
When the Atlantic ocean opened up, the rivers reversed course and began cutting back from the new ocean, causing the same valleys to drain SOUTHEAST.

I wonder if something similar applies to Peace Vallis, where the notch in the northern rim allowed drainage from the southern highlands into Elysium lake or Arabia sea,
but when those streams dried up, the notch allowed the lake and sea to drain back into Gale.

http://spaceref.com/mars/curiousmars-box-s...stinations.html
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serpens
post Nov 11 2018, 05:33 AM
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I suspect that this would represent the maximum fill with your suggested high ocean level, with the northern rim less degraded and above water level. Occasional tsunami drain back cutting the few possible channel like features but who knows. Certainly the mount would not exist at this time and the central uplift would be the sole feature within the lake. There is some evidence from tsunami features that the proposed first ocean was not iced over although the second, shallower ocean was.
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HSchirmer
post Nov 11 2018, 04:12 PM
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Theory - Gale Crater was a bay when the Arabia Ocean filled the northern lowlands of Mars.


QUOTE
Certainly the mount would not exist at this time and the central uplift would be the sole feature within the lake.


Perhaps but if Gale was a beach environment, why wouldn\'t there be sand, and perhaps dunes?
If there was an ocean, then there had to be an atmosphere, and had to be wind, and had to be waves,
and had to be sand, and sand plus wind means dunes.

If there was a northerly wind, why wouldn\'t the central peak of Gale trap a static barchan style dune?
Is there a name for the barchan-dune looking half-crater north of the Gale notch?


QUOTE
I suspect that this would represent the maximum fill with your suggested high ocean level, with the northern rim less degraded and above water level.


A bit of digging finds that early plaotting of the "Arabia Sea" aka "Contact 1" goes right through Gale crater.

QUOTE (PALEOSHORELINES AND THE EVOLUTION OF THE LITHOSPHERE OF MARS)
https://eprints.ucm.es/33193/1/3-Marte%20SL.pdf]
Parker et al. (1989, 1993) also proposed an older, higher-standing Contact 1, later on
renamed Arabia shoreline (Clifford and Parker, 2001). This shoreline, which would be of
Noachian age (see Clifford and Parker, 2001), is roughly coincident with the Martian
dichotomy separating the lowlands from the highlands,


figure #2 from the paper is enlarged as the first image below

At the Arabia shoreline, Gale crater would have been a bay.
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HSchirmer
post Nov 11 2018, 05:27 PM
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QUOTE
I suspect that this would represent the maximum fill with your suggested high ocean level, with the northern rim less degraded and above water level.


Actually, "just overtopping" is the Arabia Ocean level, Meridiani would be higher, making Gale the entrance to an estuary.

QUOTE (Topographic evidence for lakes in Gale Crater)
https://www.lpi.usra.edu/meetings/lpsc2013/pdf/1844.pdf
Figure 1: Three lake levels at Gale:at - 2277 m spilling over the northern rim


The shorelines suggest 3 oceans, Deuteronilus, Arabia, and Meridiani.

A zoom in from the Paeloshorelines paper shows the various shorelines in black in the images attached below-

QUOTE
Figure 6. The Deuteronilus (yellow), Arabia (green) ... shorelines after Clifford and Parker (2001), represented on the Martian topography (scale in km). Also represented (black) are the contour of the a) -1.5 km, cool.gif -2.09 km, and c) -3.792 km elevation levels.


Three images below (which of course are out of order...) To judge the ocean height, look at Elysium.
Working left to right- Gale crater is due north of the left most yellow dot at the bottom center of the images.

At the Arabia shorline, (left image) Elysium is an island with wide flanks, and Gale is a circular bay.

At the lowest shoreline Deuteronilus, (middle image) Elisum is still attached to the continent, and Gale is a notch at the fall-line.

At hightest shoreline, Meridiani, (right image) Elysium is a small island, and Gale is the entrace to a large estuary including the lowlands to the east.

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