Download Week 4 Sedimentary Rocks on Mars: An Orbital Perspective

Week 4 Sedimentary Rocks on Mars: An Orbital Perspective
Discussion Leader: Erica Jawin
Malin, M. C., & Edgett, K. S. (2000). Sedimentary Rocks of Early Mars. Science, 290(5498),
1927–1937.
This work identified several distinct outcrops in the equatorial regions on Mars including
layered, massive, and thin mesa units (Fig. 1). Layered units are light-toned and display thin or
thick beds (Fig. 1A-B) that can be stair-stepped, cliff-bench, or banded in appearance. Massive
units can be light-toned and do not show layering or bedding, and appear above layered units
when seen together (Fig. 1B). Massive units are most common in Valles Marineris troughs. Thin
mesa units are dark-toned and thin, lying unconformably over almost all layered/massive units
(Fig. 1D). All three units display characteristics consistent with indurated fine-grained material
that are horizontally bedded. These outcrops are mostly located in equatorial regions in either
crater interiors, intercrater terrain, chaotic terrain, and chasm interiors.
The interpretation is that these units are sedimentary rocks, and the layered and massive
units are generally Noachian in age; thin mesa units lie unconformably on these units and
therefore it is not possible to determine how much younger this unit is than the layered and
massive units. The process that formed these deposits is unclear, although it needs to be regional
in nature and able to form kms-thick deposits. This process is expected to be either subaerial
and/or subaqueous. Atmospheric transport is a viable subaerial option, while alluvial,
submarine/lacustrine, and deltaic action are viable subaqueous options. The lack of corroborating
evidence of either option complicates the problem, but the ancient age of these features suggests
that evidence could have been removed subsequently. The thin mesa unit was likely emplaced by
eolian processes due to their young age and morphology.
Two scenarios are proposed to explain the formation of these units: first, early Mars
contained much more water on its surface and rock erosion was caused by flowing water, and
sediment was trapped in small, closed depressions or in flatlying areas in shallow seas. Dramatic
variations in surface conditions are suggested from repeated layering and changes in layer
thickness. A second scenario is one where the atmospheric pressure is modulated by atmospheric
processes, such that surface pressure could increase for thousands of years. In these instances,
much more material could be suspended and transported in the atmosphere, sourced from impact
cratering. Variations in
pressure would lead to
variable layer thickness and
layering.
Fig. 1. Sedimentary outcrops on
Mars. (A-B) Layered units. (B,
right) Massive unit. (D) Dark
mesa unit. (A-B) from Fig. 4 in
paper; (B, right) from Fig. 3 in
paper; (D) from Fig. 5 in paper.
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Kocurek, G. and R.C. Ewing (2012). Source-to-sink: An Earth/Mars comparison of
boundary conditions for eolian dune systems, SEPM Special Publication, 102, 151-168.
This work assesses eolian systems as source-to-sink complex systems operating within a
set of boundary conditions. From this framework, the authors assess eolian dune systems on
Earth and Mars. Their overall conclusion is that sediment sourcing and grain transport on both
bodies are significantly different, as are modes of accumulation and preservation, but both
planets contain dunes and dune-field patterns that bear many similarities.
On Earth, dune fields are sourced primarily through fluvial systems. Temporal
relationships between sediment source and dune construction can either be contemporaneous,
lagged, or a mix of contemporaneous and lagged (Fig. 3A-C). Other principle factors include the
transport capacity of the wind, and volume of sediment available to transport. On Mars, sediment
could be sourced from relict Noachian (fluvially-derived) sediments, and later episodic
weathering (e.g. cratering, climate cycles, etc). Dune-fields may have been sourced from this
episodic model, where wind and sediment availability and generation are variable through time
(Fig. 3F). On both Earth and Mars, dune formation and evolution are dependent on eolian
processes and sediment availability, although variations in atmospheric pressure on Mars lead to
variations in dune development and morphology. Dune accumulation is mostly dry-dominated on
both bodies as well, although stabilization and preservation of dunes is quite different on Earth
(stabilization by burial/subsidence, vegetation, and water table variations) than on Mars
(stabilization by freezing, and limited water table variations, and burial beneath lava flows/ice
caps).
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Grotzinger, J.P. and R.E. Milliken (2012). The sedimentary rock record of Mars:
Distribution, origins, and global stratigraphy, SEPM Special Publication, 102, 1-48.
This work examines the record of ancient sedimentary rocks on Mars. Specifically, five
distinct sedimentary terrain types were defined and assessed, including (1) underfilled basins; (2)
overfilled craters; (3) chasm and canyon systems; (4) plains-covering deposits; and (5) very
ancient strata. Underfilled basins contain sedimentary rocks or sediments but do not appear to
have been filled close to or beyond their rims. Fluvial systems were an important transport agent
in certain locations, but these basins lack a unique depositional mode. Eberswalde and Jezero
crater are both examples. Overfilled craters contain sequences of rocks, often preserved as
mounds that can extend up to, and above, the rims of the craters in which they are preserved.
These mounds may be erosional remnants of formerly more laterally continuous units and may
exhibit distinctive cyclicity in bedding thicknesses. Gale and Becquerel craters are examples.
Chasm and canyon systems are large tectonic systems that contain thick, light-toned interior
layered deposits (ILD) in the walls of some chasms. These deposits may pre-date canyon
formation, and therefore may be very ancient. Candor and Juventae Chasma are examples. Plains
covering deposits are rather unique as they do not appear to have accumulated in topographic
depressions, and may reach substantial thicknesses. Sinus Meridiani and the Medusa Fossae
Formation are examples. Very ancient strata are samples of Noachain crust that are exposed and
appear stratified, often containing strong spectral signatures of clay minerals. Mawrth Vallis and
Nilli Fossae are examples.
In general, sedimentary deposits appear to have been derived through several means,
including local formation and deposition in topographic depressions, as well as accumulation
over widespread, flat-lying areas that are regionally extensive with no clear sediment source.
Both fluvial processes have been a major factor, with evidence of both rapid erosion and
sedimentation (within outflow channels) as well as gradual fluvial processes, evidenced by the
presence of alluvial fans, deltas, and lacustrine environments. In addition, eolian processes have
been credited with forming very regionally extensive deposits with thin bends, potentially
formed of weakly lithified, fine-grained duststones.
In an attempt to aid in the creation of a relative chronology of martian processes and the
formation of various stratified deposits, the authors integrated morphologic and mineralogic
characteristics of martian strata to define several “orbital facies”, including massive breccia (MBR), complexly stratified clay (CSC), laterally continuous sulfate (LCS), laterally continuous
heterolithic (LCH), distributary network (DNW), and rhythmite (RHY). Key reference sections
are described, and the orbital facies in each section are identified, in an attempt to create a more
refined understanding of martian geologic history. The most ancient terrains on Mars (Mawrth
Vallis, Nili Fossae) contain MBR and CSC facies, while the younger regions are dominated by
LCS and RHY facies (Fig. 23). This transition is consistent with the paradigm that early Mars
exhibited clay-forming conditions, which transitioned to sulfate/Fe-oxide-forming conditions.
However, post-Noachian clay-bearing facies (LCH) have been identified and linked with
distributary networks. This suggests that the environmental shift in Mars’ history is likely more complicated than the simple phyllosian-theiikian-siderikian model predicts.
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1. Kocurek and Ewing discuss the dune development on Earth and Mars. Earth shows a much
greater variety of dunes and a commonality of linear dunes, which require an obtuse bidirectional
wind. In contrast, most dunes on Mars are crescentic, and thus form in a more unidirectional
wind regime. What factors on Earth produce a more complex constructive wind regime than
what is required on Mars?
2. Kocurek discusses the utility of calculating the angle of bedform climb on Earth to better
understand the nature of accumulation surfaces over time, but this tool is not discussed for Mars.
Is it easy to calculate or model the angle of bedform climb on Mars to better understand how
accumulation might have changed over time or vary today based on latitude?
3. Thinking in terms of martian sedimentary unit (and dune) production as a function of time,
how did the emplacement of the Hesperian-ridged plains (which cover ~30% of the surface of
Mars) effect sedimentary processes on Mars? Could the transition from a megaregolith surface
(rich in fines that are easily transported by wind) to a bedrock surface at the Late NoachianHesperian boundary have had a dramatic effect on sediment transport and construction of
sedimentary features for the last ~3.6 Gyrs?
4. How would the degree of cementation through time and cementation from groundwater or
intergranular freezing/permafrost effect the morphology of eolian abraded sediments in craters?
5. Are katabatic winds restricted to the highlands and polar caps? What about Arabia Terra?
6. Kocurek and Ewing suggest that localized winds may be more significant than global wind
patterns in sand transport and forming surface features. Is it possible to determine the relative
frequency of global vs. localized winds (e.g., nighttime katabatic winds)? Is the scale of
distribution the only clue in determining if geomorphic surface features stem from global or
localized winds?
7. Do we expect that stratified layers and sedimentary rocks to exist at higher latitudes, mantled
by the LDM and other ice-related features? Or are these features truly equatorial, and other
factors prevented them from forming at higher latitudes?
8. How cautious should we be, when morphologically studying layered structures on Mars from
orbit, of the "hard-won experience of comparative planetology"? On Mars we see geological
features with no terrestrial analog (e.g., km-thick sulfate deposits). We see rhythmic layering
from volcanic processes (e.g., in Oudemans crater) that forms km-thick stacks of layers of equant
thickness. We should see global exchange of geological materials in the early Noachian from
impact gardening. How many non-terrestrial processes forming layer structures may be operative
at Mars? How do we account for these, when many of the martian layered structures can be
accounted for by terrestrial analogs?
9. During discussion on the role of surface water and fluvial networks' relationship to martian
strata, Grotzinger and Milliken (2012) suggest that "sediments passed through the alluvial fans
could then be deposited as shoreline deposits if bodies of standing water were present". Inferring
from figure three and the apparent large amount of sedimentary rocks near the potential
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shoreline, does this hypothesis show support of a Late Noachian northern ocean (i.e. can the dots
related to fluvial systems be used to construct a shoreline)? Similarly, have these features been
mapped to correspond with rims of potential Noachian open-basin lakes?
10. It is noted in Grotzinger and Milliken (2012) that the ice-rich LDM could be responsible for
forming stratified rocks at high-latitudes. Could this process also be responsible for forming
stratified rocks at lower latitudes under conditions of high obliquity when ice deposits are
mobilized towards the equators (implying that some stratified rocks seen across Mars could be
lag deposits from ancient ice deposits)?
11. Malin and Edgett (2000) describe a series of sedimentary units found on Mars as "layered,"
"massive," and "thin mesa" units. They conclude that two general scenarios might be responsible
for the formation of these units, those being (1) "a Mars environment capable of sustaining liquid
water on its surface, and the movement by this water, of substantial amounts of eroded rock
material." (2) An early Mars with a much thicker atmosphere which underwent large periodic
variations in atmospheric pressure which may have allowed the atmosphere to suspend and
transport substantially more material at some times an not others (and more than the present).
Given that Mount Sharp in Gale crater can be described as one of these sedimentary deposits,
what evidence has been observed by the Curiosity rover that might be consistent with either of
these hypotheses (e.g. any evidence of chemical weathering which would argue against the subaerial model)?
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Sedimentary Rock Cycle of Earth and Mars Week 4 Discussion Summary
There were three papers assigned this week regarding global-scale sedimentary rocks on
Mars. Discussion surrounding the Malin and Edgett (2000) paper primarily regarded the
discussion of their three distinct units, “layered”, “massive”, and “thin mesa” units, and the
nomenclature regarding the distinction between these units. Specifically, “beds” refer to much
smaller-scale features than those identified in the work, which were up to tens of meters thick. In
reality, stratification could exist on much finer scales in these sedimentary deposits than were
identified from orbital data, and the existence of finer-scale laminations would have implications
for the formation of these distinct layers. Moreover, the distinction that each of these thick
meter-scale units is a distinct bed implicates a single depositional period or environment for each
bed, with no erosional surface or other discontinuity between beds. Relatively few geological
processes are able to create a single, coherent meter-scale bed in this manner (avalanches/debris
flows and impact breccia were some examples given). As a final point, it was noted that
taxonomy should be separate from interpretation when naming geologic features, and distinct
units should have distinct morphologies. As all layers described in this work are somewhat
stratified, there can be confusion in distinguishing between units. How does the scale of the
layering relate to the feature description (i.e. are massive units actually massive, or could they be
finely bedded but not resolvable at orbital scales or could they contain amalgamated beds)?
The discussion of the Grotzinger and Milliken (2012) paper covered many topics relating
to sediment trapping and deposition. Sediment will primarily become trapped in topographic
lows, which on Mars are predominantly craters and basins. The largest sink is the northern
lowlands. Overfilled craters, one of the specific terrain types, were discussed at length with the
example of Gale crater. The nature of the mound was discussed, specifically relating to whether
or not the mound initially filled the entire crater. A specific aspect relating to this is the nature of
drilled mudstones at the base of Mt. Sharp, and competing hypotheses about their formation. If
these mudstones are an erosional window, then the mound could have been much larger in the
past. If, however, this is a younger unit onlapping the crater wall, the mound may not have been
significantly larger in the past, and would not have completely filled Gale crater. Fractures in the
mudstone may require burial by up to at least a kilometer in order to form. This could suggest
that the erosional window model is correct.
Eolian processes were discussed in the context of dune formation, as discussed in the
Kocurek and Ewing (2012) paper. The source of basaltic dunes is an interesting question –
basaltic sands were not necessarily derived directly from bedrock. These sediments could have
been derived from previous generations of dunes that were cemented or otherwise stabilized for a
period of time, then later liberated and mobilized. In addition, primary sediments (mechanically
eroded but not chemically altered) are more likely to be second generation sediments derived
from dunes rather than derived directly from bedrock. This is because sand-sized sediments
derived from a cohesive unit (i.e. bedrock) are more likely to undergo a degree of chemical
alteration in addition to physical, as it is relatively more difficult to erode a rock into sand-sized
particles as opposed to a dune composed of mechanically eroded particles.