From what we have seen of the delta front and Kodiak it seems to reflect delta growth into a lake under a stable flow regime with episodes of erosion reflecting dry periods and also a few periods of high intensity flow. My obviously deranged imagination pictures gently flowing river dominated deltas under cloudy skies at the edge of a choppy lake with an ocean visible to the East. In reality it was probably a more violent environment but I like the illusion.

Periodic high intensity flows transporting sizeable detritus both rounded and angular does not necessarily imply a desert flash flood or dry environment. It could reflect a gradual increase in flow to high intensity flood levels due to the progressive melting of abnormally large deposits of snow in the catchment or perhaps the collapse of obstructions at the crater inlet breach or the knickpoint further up the channel. Currently we are in the realm of hypothesis and dare I say imagination and Tim's last post highlights the fact that we have two plausible but opposed explanations for A2 from experts in the field although I don't believe they are mutually exclusive across the delta front.

For imagery of the watersheds try
https://www.arcgis.com/home/webmap/viewer.h...78.2144,20.7779

Perhaps I'm wrong in associating wildly different phenomena with just one facet of similarity, but we've known for a long time that Mars had catastrophic, massive floods in its past, and then we had a long campaign of exploration that led us to discover and characterize these other kinds of water flow, due to climate, more typical of Earth, but should we then think of all of the water flow that took place in Jezero as the latter and not the former?

As Mars had (and has) massive deposits of crustal ice, this allowed volcanic activity to unleash massive floods that are endogenous in origin, not due to climate. Similarly, impact melt could accomplish the same result.

We know that the overall topography of Jezero was reworked by the rise of Syrtis Major to its immediate west. We moreover know from the eroded crater present on the delta that there was still significant impact cratering taking place after the time frame of the delta's formation; it's therefore plausible that as much or even (far?) more impact cratering took place during the time frame of the delta. The signs are abundant on Mars of lobate flows created by the impact melt of subsurface ice. For that matter, the kinetic (rather than thermal) result of an impact hitting a watershed would be yet another potential cause of catastrophic water flow.

It seems to me that it remains to be proven if any flash flooding was due to anything we'd associate with climate versus those two non-climate catastrophic scenarios.

In addition, we know that Mars has likely had cycles of axial tilt leading to climate changes wildly more catastrophic than what Earth has experienced which could have created climate epochs where ice would form locally, then other epochs in which there would be a lot of cumulative melt.

It seems to me that all of these and more are in play as potential explanations.

I completely agree. However, the well-organized delta foreset strata at Kodiak, and the organization of channels and lobes from the orbital imagery suggest that sustained, or at least seasonal, flows probably built a significant volume of the deltaic deposits.

On the other hand, the catastrophic events like you've mentioned can also be important in shaping the geomorphology of lakes, and can produce significant deposits in lake basin fills.

Here is one of my favorite examples, from Lake Tahoe in the USA. A huge landslide deposited giant blocks on the lake floor, but also created tsunami and seiche flows which formed fields and channels of antidune and cyclic steps deposits around the lake margin as they returned back into the lake as supercritical flows. The point of my previous posts was that the authors of the Jezero paper don't seem to be familiar with these types of deposits, or even recognize that supercritical sediment gravity flows would also be expected in the types of short, steep, coarse-grained delta deposits that have been seen in Kodiak.

https://www.researchgate.net/publication/25...lifornia-Nevada

I'd add: debris/mud flows,
and
perhaps river/estuary bores? Jezro may be at the Arabian ocean shoreline, so not tidal bore, but ocean impact bore.

After seeing videos of a the debris font of a Utah flash-flood/debris flow, and then the wave front of a Chinese tidal bore-
I realized where Hayao Miyazaki (anime filmmaker) got his inspiration:

-The motion of debris at the front of a desert flash flood-

https://youtu.be/_yCnQuILmsM?t=236
appears to be the influence for the writhing demon Nago-No-Kami in the film "Princess Mononoke"



-The motion of cresting waves at the reflected front of a tidal bore in Qiantang River

https://youtu.be/k6fr6GUSmAA?t=70
appears to be the influence for the water spirit Ponyo-



Debris flow. = rain in the desert, ice dam breaking, impact into ice-rich terrain.
Tusnami / tidal bore.= crater lake impact, marsquake, or a crater wall landslide.

Preface: let me suggest that I'm not going on a Catastrophism kick with this. It's that I've been accustomed to Mars' typically subtle and gentle erosional processes that I'm shifting my mindset for more pronounced processes.

I'm sure that for the most part the delta formation has been slow and steady with moderate streamflows and sedimentation rates. But this has been punctuated and accentuated with violent episodic events over thousands or even millions of years. And Mars has has profound climate changes over those years, ranging from moist and temperate to dry and cold.

Engaging discussions here.

--Bill

After a long look at A1,2,3 over a glass of very nice wine I am now moving firmly into Tim's corner. A lot of delta front A is covered by scree but if the dipping beds to the right are indeed foresets then the direction if flow would be primarily right to left and the line of partially exposed horizontal rock leading from its upper edge to the top of the a2 fill could possibly be remnant topsets. Hypothetical of course but that fits Tim's diagnosis.

Well, it helps to remember that Earth's 23° tilt varies over time.
https://climate.nasa.gov/news/2948/milankov...earths-climate/
https://www.smithsonianmag.com/science-natu...rery-180971615/




And Mar's tilt is expected to vary even more often.


First rule of short-term geology - "glaciers were created to annoy geologists by shuffling rocks"

https://www.nasa.gov/mission_pages/msl/mult...a/pia15095.html

And Earth has stabilizing factors such as a large Moon and temperature-regulating oceans that Mars does not have.
https://agupubs.onlinelibrary.wiley.com/doi...29/2008GL034954

But given the increasing evidence, may have had when Jezero was a lake.

Perseverance Rover Mission Update - Katie Stack Morgan - 2021 Mars Society

#AGU21 Press conference: Ten months of Perseverance: Jezero science - 15 Dec 2021
Presentations and questions answered by:

Ken Farley, California Institute of Technology
Sanjeev Gupta, Imperial College London
Briony Horgan, Purdue University
Eva Scheller, California Institute of Technology
Kelsey Moore, California Institute of Technology

Lots of discussion of these results in the "Finishing work in and around Seitah" topic, starting here, which would actually be more appropriate if moved to this topic.

After listening to the AGU press briefing, it seems the team themselves don't have a good explanation for the tilted layering seen in Seitah, and how to reconcile that with the apparently igneous textures that they see in the microscopic images, though they mention how igneous cumulate textures can mimic sedimentary textures. Knowing how the layering and dips were distributed across Seitah would probably be a valuable constraint- perhaps Ingenuity can help out by mapping more of Seitah?

John

Actually, John, the Arm-Waving and discussion of the incoming data from the Rover would be more apropos in the "Finishing Up in and Around" thread. We are observing bits and pieces of incoming information that could be interpreted as many things. Save this Geology topic for conclusive data and discussion of the results from research papers down the road.
Thus far the discussions of observed deltaic facies and features falls into a reasonable realm of reality
My two cents

--Bill

There are many lunar craters with floors filled by what seems to be the same lava flows seen in adjacent maria without the crater rim being visibly breached. It would seem possible (necessary?) that the layers in Jezero would have seeped in, like those lunar craters, laterally through the crater wall, then been tilted either by subsequent lava emplacement below that or the general tilting of the entire area as Syrtis Major formed?

The floor of Gusev Crater that Spirit traversed was "surprisingly" unaltered volcanic or volcaniclastic rocks... probably from intrusive fissures after the wallop of crater formation, and then since buried. The 'water' channels into Gusev brought hopes to find an abundance of clays and cobbles and other such lake and riverine materials. Exposed volcanic crater basements may be a feature and not a bug for non-tectonic aeolian Mars?