I have revised the whole paper - this file is a checklist to make sure that I have taken account of
each of Howard’s specific comments.
I think “clastic wedge” refers only to fluvially-transported materials and thus would not include
the bulk of the MFF materials.
Fixed (“sedimentary wedge”).
You may need to carefully define here. There are longer valleys as well as longer outflow
channel deposits.
Fixed (“the longest known chain of Martian channel deposits formed by sustained flows with
distributed tributary inputs”).
Seems out of place – here you are talking about Pre-river deposits.
Shifted to section 2.2. (River-containing deposits).
This is a strange location. They could be layers, or potentially some sort of deformation
structure (slumping, flow). Similar ?structural? patterns occur at ~4.1S 154.8E
Deleted this discussion.
Yes, I interpret Neves as being pre-fluvial. The rounded ridges to the south of this crater, plus a
few highpoints on the rim might be post-impact deposits that are either pre- or post-fluvial, Or the
southern ridges might be scattered ejecta lobe deposits as you suggest.
I think the simplest explanation is that everything here is Neves ejecta (based on terminal
ramparts and radial grooves). I agree that the highest points on the lobes are not (by themselves)
diagnostic of anything.
Also clearly pre-fluvial and partially infilled with material equivalent to your Lower Unit.
Agree.
Clearly pre-dates the fluvial activity and influenced the flow pattern.
Agree.
see my comment about Neves, above. It would have implications for this discussion. These
incised valleys locally transition to inverted channels. This expression of some valleys as incised
is very enigmatic to me. They only occur on materials equivalent to what I interpret to be
equivalent to your Fluvial 2 materials.
Valleys on DiCE postdate the Neves / Obock / Kalba impacts, but may correlate with F1.
Although they are higher-standing than F1, I am uneasy about correlating them to F2 because of
the possibility of differential compaction.
Maybe this is discussed later, but I am not sure what you mean by “knicks”. In fluvial geomorph
“knickpoints” refer to abript increases in downstream river gradient, that generally progress
upstream during subsequent erosion. I wonder if you are referring to the channels and/or
floodplain materials that are now in partially inverted relief. We would typically call these
terraces or straths if caused by subsequent fluvial dissection, or benches for a more processneutral term
Removed the discussion of ‘knicks.’
again my caution here – need to distinguish between this, valleys, and outflow channel deposits.
It is presumably the longest known channel deposit formed by sustained flows with distributed
tributary inputs.
Fixed (“the longest known chain of Martian channel deposits formed by sustained flows with
distributed tributary inputs”).
Comment related to footnote 2: I like this qualification. I think one could break out two
“presentations” of this unit. In the northern part of the mapped unit the topography is flat, and
traversed by numerous narrow, often discontinuous meandering and possibly branching channels
of diverse orientation. This, to me, is consistent with deposition by numerous, ephemeral
distributaries on a broad alluvial plain. In the southern part of the mapped area, particularly
around the margins of the unit, the presentation is as semi-parallel channels and associated
floodplain deposits (including cutoffs) that are weakly dendritic (low junction angles). I suggest
these are individual valley deposits in a dendritic channel system converging generally to the
basin center and northward. DiBiase et al., however interpret (in my view wrongly) some of this
unit as a distributary system. If the convergent valley interpretation is correct, then there must
have been low highlands between the individual valleys in this subunit that were the dominant
source of sediment and constrained the width of the valley bottoms and floodplain deposits.
Brought footnote 2 into the main text and expanded it (2nd paragraph of section 2.2), and renamed
F (Fluvial) units as “R” (River-containing) units.
YES, now headlong into the most enigmatic of the units within the Aeolis area. Are these fluvial
deposits or is this a mantling deposit that embeds earlier fluvial deposits corresponding to your
Fluvial 1 unit? The latter is how I have been interpreting thia unit. The regional presentation of
this unit is as broad domes with a generally smooth surface at km scale, but densely covered by
aligned ridges (dunes) at decameter (HiRISE) scale. Your mapping of this unit corresponds
almost exactly with my mapping of “whatever this is”. But it clearly shows many fluvial
features. See my more extended write-up about this unit.
Rewrote section 3.2.
Your mapping of F3 stops well short of the map boundary on the SW side, although the channel
systems extend into what you indicate as undifferentiated. I consider the F3 materials to be a
dendritic drainage system (not a fan/delta with tectonically reversed gradient as Lefort et al
interpret). It drains to the SE rather than the bulk of the fluvial features that drain northward. I
think there was a NW-SE drainage divide (now inverted) between the F1 and F3 fluvial systems.
Whether they are asynchronous or synchronous is hard to determine. Clearly the NE flowing
drainage on the NE side of the divide is lower, and one would most readily interpret older, but a
possible scenario is a migrating divide,with the higher drainage network flowing SW gradually
being cannibalized by the NE flowing drainage at a lower elevation.
New Figure 19 and accompanying discussion discusses paleodrainage direction options.
Would prefer to defer a definitive statement on paleodrainage direction until more work done (in
support of ‘paleohydrology’ paper.)
ambiguous. Does this mean that only in Aeolis Dorsa are such superpositions found, or that all of
the fan deposits superimpose large-river deposits. I’m not sure where an instance of such
superposition occurs. Maybe shown later in paper.
Fixed. (“In Aeolis Dorsa, alluvial-fan deposits are only found overlying large-river deposits, and
are always directed away from highs in the modern topography (Aeolis Planum Rise, Zephyria
rise; Fig. 2d).”)
Here I agree that the fans post-date some earlier fluvial activity. The question is whether this is
just a climatic pulse of high sediment yield causing fan buildup or whether it is temporally
separated by a significant time period, possibly including deformation, erosion, or MFF
deposition.
“Does this change represent merely a climatic pulse of high sediment yield causing fan buildup
(without a long spell of dry conditions), or does it represent a significant time gap in the record of
surface runoff? Two lines of evidence favor a long time gap on the sub-alluvial-fans
unconformity. The first is based on the low likelihood of the change in river deposit style
occurring at the same time as thrusting, and the second is based on large-scale topography. […]”
I tend to think more that they are loess-like. Easy to dig with a shovel, enough grain-to-grain
contacts per unit volume and weak sedimentation to form nearly vertical bluffs, as in the Midwest
US and China.
Agree, removed references to ‘rock-like strength’.
Your interpretation is clearly the simplest, although not necessarily the only possibility.
A reviewer may want us to soften the language, but I am not imaginative enough to come up with
an alternative explanation.
There is actually a channel breaching the northern crater rim.
Agree, but didn’t change the figure as I’m not 100% sure that this is a crater (although the
material on the E side is old, the material on the west side could just possibly be younger than
Aeolis Serpens).
OK, I buy this story.
Agree!
The right side relationship seems OK, the left side needs to be considered further.
Section 3.2 rewritten. Corresponding figure redrawn.
Yes, I agree with all of this, save the interpretation of F2 as a FLUVIAL mantling unit.
Renamed F (Fluvial) units as “R” (River-containing) units and brought footnote 2 into the main
discussion.
See my discussion of this in the word document and images.
Refers to F3/F1 correlation (new figure 19, new discussion).
Could the resistant unit be an indurated fluvial blanket? It dips in the same direction as the
channels and with about the same gradient.
I’m not sure about this because Antoine Lucas’s HiRISE DTM of F3 shows that the ridges are
10s of m above the locally surrounding plateau.
Again, I don’t necessarily agree.
Refers to F3/F1 correlation (new figure 19, new discussion).
I agree with the fan superposition on older fluvial deposits, but the degree of fan depositional
control by the wrinkle ridges is less clear.
Talked to Howard about this at U. Virginia. Have tightened up the figure. Hopefully it is more
convincing now.
What is the age of the larage crater relative to F1 deposition. Some inverted floodplains flow
from just below the crater rim on the W and SW side of the crater. They appear to end abruptly
just below the crater rim and have a trend commensurate with the F1 fluvial deposits in the center
of your illustration. I wonder if the crater ejecta on the W side has been eroded away.
Relationships are somewhat complicated by the smooth material at the top center of your image,
which may be F2 material. If the crater post-dates earlier fluvial activity it pre-dates fan activity,
which may have been directed by the impact ejecta in addition to the thrust faults. It is interesting
that the F1 fluvail deposits have been eroded to form inverted ridges, whwereas the fan deposits
show only modest erosion. Is this due to relative resistance of the fan and inter-F1 channel
deposits or did the inversion of the F1 channels happen in the interval between the F1 activity and
the fan deposition?
I think the 9km-diameter crater has an elliptical rim because of deformation by the thrust. I have
cropped the associated figure so that crater is no longer visible. Agree that total postdepositional
erosion of (R1 + R2) >> Total postdepositional erosion of alluvial fans, but the inverted channels
on the alluvial fans are locally 20-30m above adjacent hollows on the same fans which is the
same order of magnitude as the elevation of the R1 meander belts above inter-meander-belt
terrain.
I do not follow this. On one hand, you talk about a change occurring during one Mars year, then
you emerge from the discussion with s 40 ma time gap.
Refers to alluvial fans time gap. 40 Ma is the bottom line, one Mars year was a strawman (poorly
labeled as such). Rewrote and shortened discussion.
We should look at this more carefully. I have not done so at this time but will look at regional
relationships on the East side of the Aeolis fluvial features soon.
sounds reasonable, but the apparent age of the Peace Vallis Fan seems to be
Fragmentary comment(?) – see later comment on alluvial fan ages.
Edwin, I am going to skip this section. Firstly because I want to get this back to you, and
secondly because I can’t follow it on initial reading. We should discuss later.
Rewrote and compressed this section. I have asked Kevin Lewis (who works down the hall) to
give me a second opinion.
we can revisit this in discussion.
Refers to summary stratigraphy figure (revised to match other changes; F3 omitted)
need reference for this.
References added.
The fans in Gale seem to have a range of ages, with the Peace Valles fan perhaps being the
youngest possibly extending into the Amazonian. There are a range of possible lake levels in
Gale suggesting a complicated hydrologic history from the Early Hesperian crater age to the
Amazonian. I think that most of the larger alluvial fans in the equatorial region were more or less
synchronous, but there may have been some tailing-stage activity. We can discuss.
I have started work on a stochastic Mars climate evolution model combining GCMs, orbital
models, increasing solar luminosity, and simple weathering and water-availability models. The
working title is “Late bursts of habitability on Mars-like planets.” Based on an unpublished zerodimensional toy model of carbonate formation, I expect that the output will feature either one or
several “late bursts” of wet conditions, depending on the supply of cations for carbonate
formation. But each “burst” should be regionally/globally synchronous. However publishable
results are at least six months away (likely more).
Lets discuss a variety of scenarios. Is there stereo coverage here?
Moved to “Supplementary Discussion” (maybe omit completely).
Sediment supply may also be a factor. Channels in the area shown in Fig 5C were aggrading
during meandering, and there are at least two levels of channels. Also the channel deposit in Fig
5A is above the level of the surrounding floodplain. Aggrading fluvial systems generally occur
either at the downstream end of a fluvial system or they imply variations in the relative
sediment/water supply (e.g. terrace sequences). Variation in runoff can account for alternation of
erosion and deposition, but the net aggradation suggests to me that airfall deposition events may
have episodically provided fresh, easily-eroded sediment to the system.
Agree, and folded this possibility in to the discussion. I left the orbital forcing hypothesis in the
abstract though (without pointing out that sediment supply is an alternative), because sediment
supply could itself be modulated by orbital forcing even if water availability is not limiting for
some reason.