Jezero Delta Campaign, Sols 414-1000, 21 Apr 2022- 23 Dec 2023 Options

[Julius]

Think we can kick off the new phase of exploration at Jezero.

[tau]

Sol 522 SuperCam Remote Micro-Imager mosaic.
The outcrops remind me of glacial till with erratic blocks.

[Bill Harris]

Sol 522 SuperCam Remote Micro-Imager mosaic.
The outcrops remind me of glacial till with erratic blocks.

Yes, or talus with float. I'm looking upslope and wondering how resistant that white fine-grained unit is.

--Bill

[tdemko]

Yes, or talus with float. I'm looking upslope and wondering how resistant that white fine-grained unit is.

--Bill

In my previous reply, I purposely used the term "dilute" sediment gravity flow. The proportion of sediment to water in a subaqueous sediment gravity flow determines the rheology and fluid mechanics of the flow. In a dilute flow (in terrestrial settings that is ≤ around 15% sediment), the flow is turbulent, and the sediment concentration is greater at the base of the flow (dense basal layer), and decreases upward, with a mixing zone at the top where ambient water gets mixed into the flow. The flow largely behaves as a Newtonian fluid. Mixing of ambient water and deposition of the suspended sediment decreases the concentration of the flow, which can eventually remove the potential energy of the density difference, and the flow stops. The flow may also spread out once it exits a channel, and the slope angle usually also decreases downflow, also contributing to lower velocity, turbulence, and therefore sediment carrying capacity. This is the typical evolution of turbidity currents in submarine and sublacustrine fans. The deposits of lower concentration sediment gravity flows reflect these conditions, and often produce a characteristic fining-upward texture (graded), and a fabric of predictable sedimentary structures (including the famous Bouma sequence), called turbidites.

The rheology and fluid mechanics of higher concentration flows (> 15-20% sediment) gets interesting. They may act as non-Newtonian fluids, plastics, or even plug-like flows with increasing concentration. In general, in higher-concentration subaqueous sediment gravity flows, turbulence is suppressed, and they can act as laminar flows (like a fluid mud), or even slide and/or hydroplane on a thin later of ambient water trapped beneath the flow as it moves downslope. The deposits of higher-concentration subaqueous sediment gravity flows (sometimes called debrites) are often characterized by very poor sorting (little or no grading), floating outsized clasts, rafted blocks of disparate materials, deformed internal fabrics, or can be structureless and massive.

I think these outcrops reflect an initial episode of high concentration sediment gravity flow deposition (the structureless, poorly sorted units in the foreground) followed by lower concentration sediment gravity flow deposition (the bedded units in the background). Both could have been in the lake, or the initial deposits could have been part of a subaerial debris flow during a lake-level lowstand, followed by a lake level rise and progradation of the delta. It is interesting that the poorly-sorted unit contains a lot of rounded clasts...it apparently was sourced by fluvial deposits somewhere in the drainage basin, maybe from a catastrophic flash flood.

[serpens]

..... It is interesting that the poorly-sorted unit contains a lot of rounded clasts...it apparently was sourced by fluvial deposits somewhere in the drainage basin, maybe from a catastrophic flash flood.

From the context image the flow here was from left to right. The rounded clast rich unit does not seem to extend far laterally and sits below the topsets. My initial thought was that this was channel fill.

[tdemko]

From the context image the flow here was from left to right. The rounded clast rich unit does not seem to extend far laterally and sits below the topsets. My initial thought was that this was channel fill.

The story gets even more interesting...the latest image show that the inclined units below the topset truncation surface are much more poorly sorted than the overlying topsets. If the two sets of units are related ala Walther's Law, this is counterintuitive: deltaic and other sediment gravity flow deposits are relentlessly better sorted as one goes downflow. Flow stripping of the finest-grained material in from that in suspension, and deposition of the coarsest material from bedload, drives the sorting of the deposits to some better-sorted mean (or stated differently, losing the coarse and fine tails of the grain size distribution). So, one would expect that the topsets, deposited in a more up flow proximal position, would be more poorly sorted than the more distal foresets, again, if they are related in a Waltherian sense. I would say, in this case, they are not. The foresets were deposited by progradation of poorly sorted, debris-rich flows, while the apparent topsets were deposited by better sorted sandy flows. We may be looking at the vertical juxtaposition of parts of two different delta lobes with very different sediment sources, from a grain size and sorting sense.

The other view of the cobble/debris rich unit looked massive and structureless, the debris-rich inclined foresets here look bedded. Some interesting stratal architecture and geometries, as we process stratigraphers would say, are not yet well known...