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The Geology of Jezero Crater, Observations & Findings
Marz
post Apr 8 2021, 03:47 AM
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QUOTE (Gladstoner @ Apr 7 2021, 03:36 PM) *
Conglomerate bed:

Some of the clasts are rather large.


excellent! That indicates that a main delta channel swept through at high flow and long enough duration to round the stones.
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tdemko
post Apr 8 2021, 05:07 AM
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QUOTE (Marz @ Apr 7 2021, 10:47 PM) *
excellent! That indicates that a main delta channel swept through at high flow and long enough duration to round the stones.


The clinothems seem to be built out of both sandy, cross-bedded material, and poorly sorted structureless conglomeratic material. This may reflect a subaqueous delta front environment that was characterized by lower-concentration sediment gravity flows that deposited the sandy material, and higher-concentration sediment gravity flows (debris flows) that deposited the coarser strata. I've thought we'd see evidence of antidune cross-strata, and it looks like some possible backsets peeking out. Fingers crossed!


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Andreas Plesch
post Apr 8 2021, 06:45 AM
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Wow, a poorly sorted conglomerate with well rounded, seemingly huge clasts, somewhere midsection in the delta sequence. I think sampling the clasts would be very valuable if there is a way to get to a bed like this, since we know the source area, from the crater wall, tens of kilometers to the west. In a way, it is free delivery from a large area without having to travel.
Speculating, the bed could be the result of a catastrophic flooding event upstream (glacial dam breach ???) or a basal conglomerate of a transgression cycle, after a period of non-deposition or perhaps erosion.
Very cool.


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MarkL
post Apr 8 2021, 01:41 PM
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QUOTE (Andreas Plesch @ Apr 8 2021, 07:45 AM) *
Wow, a poorly sorted conglomerate with well rounded, seemingly huge clasts, somewhere midsection in the delta sequence. I think sampling the clasts would be very valuable if there is a way to get to a bed like this, since we know the source area, from the crater wall, tens of kilometers to the west. In a way, it is free delivery from a large area without having to travel.
Speculating, the bed could be the result of a catastrophic flooding event upstream (glacial dam breach ???) or a basal conglomerate of a transgression cycle, after a period of non-deposition or perhaps erosion.
Very cool.


Wondering if we now have any additional insight into the mechanics of the deposition of the delta given this new visual data.

Is there a possibility this delta formed as the result of glaciation - ice rather than water carrying the sediment? Would Mars' lower gravity make it easier for glaciers to flow or to retain the clastic material we see now in the delta?

However warm Mars was in the past, it wasn't that warm for a long long stretch of its history, and so isn't it more likely that any flow would have been in the form of glaciers? The amount of force required to breach the crater rim must have been immense. Is it possible that this force was generated by a massive glacier with somewhere (lower) to go. If it was a glacier, would the water ice have sublimed away during the erosion process, or should we see evidence of vestigial water ice within the delta?

It seems strange to me that these outlier remnants remain at km distance from the eroding delta front. Why did they not erode at the same rate? We also observe a lot of scouring on the top of the delta which seems to suggest to me some mechanical interaction between a glacier and the deposit. Perhaps the outlying remnants are the remnants of eskers rather than part of the original delta?

I know this is speculative, but is it possible?
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tdemko
post Apr 8 2021, 06:50 PM
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QUOTE (tdemko @ Apr 8 2021, 12:07 AM) *
The clinothems seem to be built out of both sandy, cross-bedded material, and poorly sorted structureless conglomeratic material. This may reflect a subaqueous delta front environment that was characterized by lower-concentration sediment gravity flows that deposited the sandy material, and higher-concentration sediment gravity flows (debris flows) that deposited the coarser strata. I've thought we'd see evidence of antidune cross-strata, and it looks like some possible backsets peeking out. Fingers crossed!


Looking a little closer, and squinting mightily, I now think that the conglomeratic unit is crudely cross-stratified, with backsets! I could also believe there is some imbrication and alignment of clasts along the backsets. I would really like a little better idea of scale, but just eyeballing, I would now say that the sandy units may be characterized by cross strata from the migration of antidunes, and the coarser units by cross strata from the migration of cyclic steps. These don’t negate a delta front setting, but these kinds of features are also quite common in glaciolacustrine, and subglacial settings on earth. It does look like it was a very energetic setting, with supercritical flow conditions as the rule. I can post some references to similar cross-stratification formed in marine and glacial deltaic settings here on earth, if anyone is interested. I’ve specialized on identifying and interpreting these types of cross-stratification and the bedforms that create them for the past 8 years, so maybe I see them where they ain’t, too!
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Andreas Plesch
post Apr 8 2021, 08:55 PM
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I definitely agree that there appears to be crossbedding in the coarse conglomerate, and would be interested in terrestrial analogues of a sedimentary environment that can generate a crossbedded, poorly sorted conglomerate with well rounded clasts, in a sandy delta, because this seems a bit out of the ordinary.

I had thought about a glacial lake at some distance upstream, at higher elevation. Such lakes are known to form behind icy dams which get eventually breached and can cause catastrophic flooding events. But it is intriguing to think about glaciation as a mechanism to initially erode the crater wall as well.

The conglomerate was deposited when the delta was well established, long after the crater wall was initially compromised. But the deep incision of the crater wall shows that it continued to be eroded representing a significant source of sediment, and therefore potentially of some of the clasts.

I agree that the preservation of the existing piece of the delta needs explanation. It reminds me of much larger scale escarpments of slightly tilted, Mesozoic sedimentary units which form a distinct landscape (Schichtstufen). It may suffice to have a somewhat more erosionally resistant unit at the top to form the step at the current limit of the preserved delta.


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serpens
post Apr 9 2021, 12:18 AM
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Perhaps it is worth stepping back and looking at the features in context (image from Neo56 below). Is it possible that this gravel/large cobble deposit could be scour fill? Certainly the size and rounding of some of the clasts indicates long transportation and deposition in a surge environment but whether this was an unusual event or the norm will need further investigation. The catchment area has been disrupted and eroded but I don't think there is evidence of glacial activity. A surge could be caused by an ice dam, melting of a snow pack by volcanic activity, collapse of an erosion resistant part of the input channel or a sudden increase in precipitation caused by impact into the purported ocean.

There has been significant erosion and step back of the delta front since the Hesperian deposits and the preservation of the remains is potentially due to the transition to a benign environment through loss of atmosphere.


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tdemko
post Apr 9 2021, 04:11 AM
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[attachment=47514:26E7008B...42CD9F59.jpeg]
QUOTE (serpens @ Apr 8 2021, 07:18 PM) *
Perhaps it is worth stepping back and looking at the features in context (image from Neo56 below). Is it possible that this gravel/large cobble deposit could be scour fill? Certainly the size and rounding of some of the clasts indicates long transportation and deposition in a surge environment but whether this was an unusual event or the norm will need further investigation. The catchment area has been disrupted and eroded but I don't think there is evidence of glacial activity. A surge could be caused by an ice dam, melting of a snow pack by volcanic activity, collapse of an erosion resistant part of the input channel or a sudden increase in precipitation caused by impact into the purported ocean.

There has been significant erosion and step back of the delta front since the Hesperian deposits and the preservation of the remains is potentially due to the transition to a benign environment through loss of atmosphere.


Here is one of the terrestrial analogs I was thinking of, the Pleistocene Porta fan in Germany, worked by my friend and colleague Jorg Lang:

Lang and Winesmann, 2013

And here is another example that both Jorg and I, plus George Postma, Dave Hoyal, Juan Fedele, Vitor Abreu, and Keriann Pederson worked on, this one an Eocene fan delta from Spain:

Postma et al., 2020
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serpens
post Apr 12 2021, 01:25 AM
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Tim, harking back to your post link on the Jurassic Tank/XES experiment, the breaching of the crater wall by the outflow channel would have lowered the water level in the crater reasonably quickly which would seemingly have caused increased flow velocity and erosion of the delta at the time. Bit of a stretch I know but could this have resulted in concentration of a layer of gravel/cobbles through top down erosion, negating the need for surge from the inlet channels?
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JRehling
post Apr 12 2021, 04:26 AM
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Per the mention of catastrophic events, we know that Syrtis Major, to the west and upslope, was very likely the source of some volcanic activity which could have contributed catastrophic flooding across the site. However, I'm not sure if that would show up in sediments created while the lake was present, or if that would have begun when the lake was no longer in existence. The minerals found in Meridiani by Opportunity suggest that perhaps aqueous environments that began before planetary volcanic activity increased might have continued in existence into a new epoch. So perhaps units formed in an aqueous environment in Jezero cover a really wide range of eras.
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tdemko
post Apr 13 2021, 07:51 PM
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QUOTE (serpens @ Apr 11 2021, 08:25 PM) *
Tim, harking back to your post link on the Jurassic Tank/XES experiment, the breaching of the crater wall by the outflow channel would have lowered the water level in the crater reasonably quickly which would seemingly have caused increased flow velocity and erosion of the delta at the time. Bit of a stretch I know but could this have resulted in concentration of a layer of gravel/cobbles through top down erosion, negating the need for surge from the inlet channels?


Yes, and entrenchment of the feeder channel system and reworking of previously deposited material. Depending on the speed of the level drop, and the sediment supply coming in, the shoreline/delta system could have 1) followed the water level down and deposited at lower and lower elevations (this is called a forced regression), 2) incised into the delta topsets and fixed the entrance into the lake in an incised valley, or 3) built an alluvial fan/new fan delta on the exposed delta topsets with its apex at the incised crater rim inlet. These are the types of stratal geometries that the XES experiments, and subsequent physical and numerical models, investigated by varying discharge, sediment flux, and base level changes. I did some numerical modeling looking at just these kind of upstream and downstream controls, and on the area in the system affected by both.


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Andreas Plesch
post Apr 16 2021, 09:25 AM
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I was notified of this AAPG event:

https://aapg.zoom.us/meeting/register/tJElc...N5h3355itV2_zGX

on 4/22 noon CT, registration required.

I plan to attend if there is no conflict.



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Toma B
post Yesterday, 05:25 PM
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My first try to stitch Supercam images...

Sol 54 Supercam detail by Tomislav Bandin, on Flickr


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My "Astrophotos" gallery on flickr...
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serpens
post Yesterday, 11:38 PM
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Nice effort Tomislav.
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