Geomorphology 69 (2005) 242 – 252
www.elsevier.com/locate/geomorph
Clustered streamlined forms in Athabasca Valles, Mars:
Evidence for sediment deposition during floodwater ponding
Devon Burr*
Astrogeology Program, U.S. Geological Survey, Flagstaff AZ 86001, USA
Received 2 September 2004; received in revised form 21 January 2005; accepted 24 January 2005
Available online 10 March 2005
Abstract
A unique clustering of layered streamlined forms in Athabasca Valles is hypothesized to reflect a significant hydraulic event.
The forms, interpreted as sedimentary, are attributed to extensive sediment deposition during ponding and then streamlining of
this sediment behind flow obstacles during ponded water outflow. These streamlined forms are analogous to those found in
depositional basins and other loci of ponding in terrestrial catastrophic flood landscapes. These terrestrial streamlined forms can
provide the best opportunity for reconstructing the history of the terrestrial flooding. Likewise, the streamlined forms in
Athabasca Valles may provide the best opportunity to reconstruct the recent geologic history of this young Martian outflow
channel.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Mars; Floods; Bed forms; Deposits; Sedimentation
1. Introduction
Athabasca Valles is located between Elysium Mons
to the northwest and the Cerberus plains immediately to
the east (Fig. 1). It originates at the Cerberus Fossae,
volcanotectonic fissures that also supplied some of the
very young Cerberus lavas (Plescia, 1990, 2003;
Keszthelyi et al., 2000; Sakimoto et al., 2001). It is
the youngest known outflow channel system on Mars
* Now at The SETI Institute, 515 N. Whisman Road, Mountain
View, CA 94043, USA. Tel. +1 928 556 7143; fax: +1 928 556
7014.
E-mail address: [email protected].
0169-555X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.geomorph.2005.01.009
(Tanaka, 1986). The most recent activity has been dated
by crater counting at less than a few million years ago
(Berman and Hartmann, 2002; Burr et al., 2002b;
Werner et al., 2003). Although these dates may be
questioned on the basis of secondary cratering (McEwen et al., 2005) and possible exhumation (Edgett and
Malin, 2003), Athabasca Valles remains the channel
system with the youngest derived age on Mars.
Athabasca Valles exhibits seemingly pristine floodformed (bdiluvialQ) geomorphology. The clearest mesoscale geomorphic indicator of flooding may be the
streamlined forms (Burr et al., 2002a,b). Streamlined
forms are teardrop-shaped features in planview, recognized in catastrophic flood terrains both on Earth and
D. Burr / Geomorphology 69 (2005) 242–252
243
Fossae. Fig. 2 shows this area, which for simplicity will
be referred to in this paper as the blower proximal
reach.Q These streamlined forms are each up to several
kilometers long (average length=~6.3 km) and up to a
few kilometers wide (average width=~2.7 km), with an
average length:width (L:W) ratio of ~2.5.
These 11 forms are located within a linear distance
along the channel of ~40 km. Ratioed to the length of
channel that they occupy, these clustered forms in the
lower proximal reach have a linear density of 0.28
forms per kilometer of distance along the channel.
These forms in the lower proximal reach account for
about half of all the streamlined forms in the main
channel of Athabasca Valles visible in either Mars
Orbiter Camera (MOC) or Thermal Emission Imaging
Spectrometer (THEMIS) images acquired to date. The
total length of the main channel is ~350 km. The
streamlined forms throughout the entire channel (the
lower proximal reach included) have a linear density
of 0.06 forms/km. Thus, the streamlined forms are
clustered noticeably more densely in the lower
proximal reach than elsewhere. This clustering is
unique in Athabasca: no such concentration of
streamlined forms is visible elsewhere in the channel
(Fig. 1).
Fig. 1. Gridded MOLA topography of the Cerberus region of Mars
(top) with the black box showing the location of the zoomed view of
the upper ~275 km of Athabasca Valles (bottom). Zoomed view of
topography is centered near 98N 1568E, and the contour interval is
50 m. Arrows show paleoflow direction; the wrinkle ridge stretches
along the channel’s southern side. The white box marks the location
of Fig. 2.
on Mars, that result from erosion and/or deposition
during flood flow (summarized by Baker, 1982). Their
planview form derives from the tendency of the
flowing water to minimize resistance to flow (Komar,
1983). This is accomplished through minimizing both
form drag (which tends to produce long, narrow forms
with minimal cross section) and bskinQ drag (which
tends to produce shorter, rounder forms with minimal
surface area). The result of these two countervailing
tendencies is a teardrop-shaped form with an average
length to width ratio of ~3–4, although wider or thinner
forms can be derived depending on flow and initial
conditions (Komar, 1983).
A cluster of 11 kilometer-scale streamlined forms is
located at the lower end of the reach of the Athabasca
Valles main channel that is proximal to the Cerberus
Fig. 2. THEMIS mosaic of the lower proximal reach. The crater
hypothesized to have caused ponding is labeled. The streamlined
forms marked with X’s are considered to have resulted not from
ponding, but from backflooding in the side channel inlet. The white
arrows show the starting locations of the most upstream distributary
channels, and the black arrows show the diversion channel around
the crater’s north side. The white boxes mark the locations of Figs.
3a,b and 4.
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D. Burr / Geomorphology 69 (2005) 242–252
This unique clustering raises the question of its
origin, and this paper presents an investigation into the
possible causes. Given the forms’ location in a flood
channel in a volcanic province, likely origins for the
forms include deposition of flood sediment, erosion of
pre-existing sediment or of low-viscosity lava, and
deposition of lava in a streamlined form. Through
discussion of these alternatives, this investigation
concludes that a flood sedimentary depositional origin
is the most likely. If built up through flood sediment
deposition over time, these forms may provide a record
of geologic processes in Athabasca Valles.
This paper first describes the forms’ morphology. It
then presents evidence for the streamlined forms’
flood sedimentary composition, suggesting a cause for
that sedimentation and discussing possible terrestrial
analogues. The paper then discusses alternative
hypotheses for the forms’ origin, but none is found
as satisfactory as a flood sedimentary origin. Some
reasons for perceived dissimilarities between the
Athabascan and terrestrial sedimentary streamlined
forms are proposed. Finally, this paper characterizes
the sediments in the streamlined forms, including
estimating their volume and outlining their possible
sources.
2. Morphology
The forms each have at their upslope ends some
feature, either an impact crater or a knob of dark,
massively layered material (Figs. 3a,b). Mars Orbiter
Laser Altimeter (MOLA) data show that, in cases for
which data exists, these upslope features stand tens of
meters higher than the downslope ends of the forms.
The knobs of dark material are largely covered with
Fig. 3. a) Subset of MOC image E05-03124 (res. 4.41 m/pixel) and b) subset of MOC image E12-01946 (res. 4.63 m/pixel). Each figure shows a
streamlined form with dark massive layering at the upslope end (black arrow) and thinner layers at the downslope end. The minimum thickness
of the dark massive layering in (a) is 60 m; that in (b) is 80 m. Below the dark, massive layers is less steeply sloping material (white arrows);
interpreted boulders (within the black ellipses) lie on top of this sloping material. Illumination is from the left.
D. Burr / Geomorphology 69 (2005) 242–252
brighter material, which, in this low thermal inertia
region (Mellon et al., 2000), is likely bright dust. The
dark, massively layered material in the knobs exhibits
in some places a spur-and-gully morphology. This is a
similar morphology to that in the walls of Valles
Marineris, interpreted as lava (McEwen et al., 1999),
and similar to the morphology of the walls of the
volcanotectonic Cerberus Fossae (see, e.g., MOC
images E04-01402, E05-00381, and E05-01933) just
to the northeast (Fig. 1). The dark massive material
also appears to form cliffs, standing at a greater angle
than that of the more gently sloping material located
directly (stratigraphically) below it (Figs. 3a,b). The
angle of repose (or residual friction angle) can thus be
used to approximate a minimum height for the cliffs.
Values for angle of repose for cohesionless, granular
material depend on shape, angularity, grain size and
other material characteristics and so are determined
empirically (Selby, 1993). Because the down slope
(driving) force and the normal (resisting) force are
both proportional to gravity (e.g., Ritter et al., 1995,
pp. 98–100), empirically determined values for the
angle of repose of a given material should be similar
on Earth and on Mars, although differences in
atmospheric conditions may produce slight differences in value. If standing at the static angles of
repose of common to terrestrial materials (~258 for
fine sand up to ~408 for angular pebbles; Selby, 1993,
245
pp. 354–355), the exposed portions of these massive,
dark layers are a minimum of ~35–40 m high.
From the upslope impact craters or knobs, the forms
taper to bpointQ downslope (Fig. 2). In contrast to the
impact craters or knobs at the upslope ends, these
downslope portions of the forms are tens of meters
lower in elevation. This is indicated directly by MOLA
data (where such data is available) or indirectly by
visual comparison between the upper surface of the
down slope portions of the form and the lower level of
the tens-of-meters-thick dark, massive layers, which
are sometimes located directly above them (see, e.g.,
the southwest corner of the knob in Fig. 3a). The
downslope portions have an albedo intermediate
between that of the dark, massively layered material
and the bright dust that covers it. These downslope
portions have flattish tops (hence, bstreamlined mesas;Q
Burr et al., 2002a), which slope gently downward to the
rear via a series of thin layers (Figs. 3a,b and 4). These
finer layers comprising the downslope portions of the
forms have been cited as being a maximum of ~10 m
thick (Burr et al., 2002a), but MOC image resolution
(generally 3–6 m/pixel) limits the accuracy of that
shadow measurement. Measurements from the highest
resolution image available to date (R0100745, res. 1.54
m/pixel, incidence angle 388) show layers down to the
limit of resolution (Fig. 4).
3. Flood sedimentary hypothesis
3.1. Morphological contrast
Fig. 4. Subset of MOC image R01-00745 (res. 1.54 m/pixel). This
image shows fine layering on the southern, downslope side of a
streamlined form down to the limit of resolution. Illumination is
from the left.
Based on their dark albedo and spur-and-gully
morphology, the tall, massive, cliff-forming layers at
the forms’ upslope ends have been interpreted as lava
bedrock (Burr et al., 2002b) although dark, cohesive
sediments could also account for the observations. By
contrast, the layers of the forms’ downslope portions
are an order of magnitude thinner, have a more
moderate albedo, and are lower standing. Because of
this contrast with the dark, massive, upslope material,
the downslope layers of the streamlined forms are
hypothesized to have a different composition, namely,
more finely layered flood sediment. This hypothesis is
supported by (i) a reasonable cause for the flood
sedimentation, and (ii) the forms’ similarities to
terrestrial flood sedimentary streamlined forms.
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D. Burr / Geomorphology 69 (2005) 242–252
3.2. Cause of sedimentation: in-channel crater
A large crater in the channel immediately down
slope of the cluster of streamlined forms (Fig. 2)
provides a reasonable cause for the hypothesized
sedimentation: hydraulic damming. In its present
state, this 6-km-diameter crater and its remaining
ejecta occupy approximately one-half the channel.
This significant reduction in flow cross-sectional area
at the crater would have reduced flow velocity
upstream of the crater. Martian floods likely carried
significant amounts of sediment (Komar, 1980).
When flow velocity at the constriction was reduced
below the sediments’ settling velocity, a depositional
flow regime upstream of the constriction would have
resulted. In its original state, the crater ejecta would
have added to this hydraulic damming. The ejecta
height alone would likely not have been a significant
impediment (crater ejecta height rapidly decreases
with distance from the crater; Melosh, 1989), but the
irregular ejecta surface would have increased the
frictional resistance for several kilometers along the
channel. However, the majority of the hydraulic
damming would have been caused by the constriction
of the crater itself. The erosion and diversion channel
around its northern side shows that the crater both
preceded the floodwaters and significantly altered
their flow (Figs. 1 and 2). (For simplicity, this crater
will be referred to in the text as the bdam craterQ).
The channel’s present topography supports this
interpretation. The dam crater sits on the northern side
of the channel. Contours derived from gridded MOLA
data show the floor on the northern side of the channel
to be shallower than on the southern side (Fig. 1). The
northern portion of the channel also contains the
majority of the streamlined forms (Fig. 2). These
effects would be a reasonable result of the greater
damming effect of the crater, resulting in more
sedimentation on the northern side and less energetic
outflow. In contrast, the terrain in the southern part of
the channel would have experienced less hydraulic
damming and provided the passage for the outflowing
water. This would have resulted in lesser sedimentation, more energetic outflow, possibly more scouring
or entrainment of sediment, and consequently the
observed greater depth of the southern channel floor.
The channel system’s distributary network also
supports this interpretation of ponding. The main
channel is largely confined along its southern side by
a linear wrinkle ridge (Fig. 1), although some smaller
distributary channels breach that ridge (Fig. 2) (Burr
et al., 2002a). Ponded water rises higher than flowing
water, so that the location of the hypothesized
ponding should provide the most likely place for the
water to have overtopped the ridge. Mapping (Keszthelyi et al., 2004) shows the location slightly
upstream from the dam crater is indeed where the
most upstream distributary channels are found
through the wrinkle ridge (Fig. 2).
Two or three streamlined forms are located slightly
downslope along the channel from the dam crater (Fig.
2). These forms sit at the head of the largest distributary
channel (Keszthelyi et al., 2004). As discussed below,
sedimentary streamlined forms are commonly noted in
the mouths of side channels in terrestrial catastrophic
flood terrain, where flow competence decreases in
association with flow stagnation during backflooding
(Baker, 1978; Carling et al., 2002). This interpretation
agrees with the Athabascan configuration, and these
three forms are attributed to sedimentation not during
ponding but during backflooding in the side channel
mouth. The linear frequencies given in Section 1
exclude these forms.
3.3. Similarities between Athabascan streamlined
forms and terrestrial diluvial sedimentary streamlined
forms
The processes hypothesized for Athabasca Valles –
namely, that constriction of flood flow produced
backwater ponding, sediment deposition, and the
creation of sedimentary forms – have been documented to have occurred in terrestrial catastrophic
flood terrains. The Athabascan streamlined forms
exhibit both situational and stratigraphic similarities
to those terrestrial diluvial sedimentary forms.
3.3.1. Situational similarity: locus upstream of a flow
constriction
Sedimentary deposits in catastrophic flood landscapes on Earth are found where the water was
flowing more slowly than the predominant main
channel flow. Deposition occurs because the sediment
transportable by the faster main channel flow is too
heavy for transport by more slowly flowing water.
Slower flow leads to deposition in three common
D. Burr / Geomorphology 69 (2005) 242–252
locations. Deposits downstream of bedrock projections are referred to as pendant bars; deposits in the
mouths of side channels or main channel alcoves are
referred to as eddy bars; and deposits from decelerating flow in an expanding reach are referred to as
expansion bars (Baker, 1973, pp. 35–42).
Such flood deposits are well-documented in
catastrophic flood tracts in the Altai Mountains of
Siberia (Baker et al., 1993; Rudoy and Baker, 1993;
Carling et al., 2002) and in the Channeled Scabland,
Washington state, USA (e.g. Baker, 1978; Baker et
al., 1991; Bjornstad et al., 2001). The Channeled
Scabland was carved by Pleistocene flooding from
glacial Lake Missoula (summarized in Baker, 1982)
and possibly other sources (e.g. Shaw et al., 1999;
Komatsu et al., 2000). The deposition of sediment
upstream of a flow constriction has been perhaps
best documented for the Pasco Basin, a major locus
of flow convergence for the region’s numerous
anastomosing channels situated just upstream of the
Wallula Gap (Fig. 5) (O’Connor and Baker, 1992;
Bjornstad et al., 2001). During the flooding, water
ponded in this basin behind the constriction of
Wallula Gap, creating the ephemeral Lake Lewis
(Allison, 1933). This slower flow during ponding
resulted in thick and extensive accumulations of
flood deposits (O’Connor and Baker, 1992; Bjorn-
Fig. 5. Digital elevation model of the Pasco Basin. The flood water
entered the basin through multiple channels from the northwest and
east and exited through the Wallula Gap off the image to the
southeast. PR denotes the Priest Rapids Bar, GM denotes the Gable
Mountain Bar, and CC denotes the Cold Creek Bars. X’s denote
locations of some other catastrophic flood deposits (after Bjornstad
et al., 2001).
247
stad et al., 2001). These sediments were subsequently streamlined into dbarsT during the outflow of
ponded water as the water incised downward through
the sediments (Baker et al., 1991). Although the
bedrock or sedimentary composition of all the
streamlined forms in the Channeled Scabland has
not been exhaustively mapped, the sedimentary fill
of the Pasco Basin is well-documented and contrasts
with the evidence for erosive flow elsewhere in the
Channeled Scabland (Baker, 1978; Patton and Baker,
1978; Bjornstad et al., 2001).
Sediment deposition during ponding upstream of a
flow constriction is also evidenced in southern
Siberia. Pleistocene outburst floods of approximately
equal or larger discharge than the Channeled Scabland
flooding emanated from glacial Lake Kuray–Chuja in
the Altai Mountains (Baker et al., 1993). These floods
deposited extensive tracts of large gravel dunes and
bars downstream from Lake Kuray–Chuja in the
Chuja and Kuray River gorge (Baker et al., 1993;
Rudoy and Baker, 1993; Carling et al., 2002). After
the Chuja gorge intersects the Katun River, the river
turns abruptly to the east and the gorge width is
halved. This significant constriction would have
impeded any floodwater and caused ponding for a
significant distance upstream (Carling et al., 2002).
Evidence of this ponding is seen in the extensive
sediment deposits. Sediment deposits are located in
main channel alcoves or on the inside of valley bends,
in positions similar to that of riverine point bars
(Carling et al., 2002). In addition, each side channel
mouth along the 70-km stretch of the gorge is blocked
by a giant gravel bar (Carling et al., 2002).
3.3.2. Stratigraphic similarity: layering
Like the Athabascan streamlined forms, terrestrial
diluvial sedimentary streamlined forms are composed
of layers. Recent work on the streamlined forms in the
Pasco Basin has documented multiple sedimentary
layers as distinguished by radiometric age dating,
reversals in magnetic polarity, and/or degree of soil
development (Bjornstad et al., 2001). Although the
thickness of the individual layers in the streamlined
forms is not stated, other sedimentary slack water
deposits around the edge of the basin have an average
thickness of ~1 m (e.g. Waitt, 1980). This is similar to
the layer thickness discernable in the highest resolution
image of the Athabascan forms.
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D. Burr / Geomorphology 69 (2005) 242–252
In the Kuray–Chuja flood tract, sedimentary
deposits are also comprised of layers. Layering in
the bars consists of multiple, subparallel, planar sheets
of gravels that are between a few decimeters and 2 m
thick each (Carling et al., 2002). This is again similar
to what is discernable in Athabasca Valles. Some of
the Siberian bars show distinct benches (see Carling et
al., 2002, Fig. 7), as seen on the Athabascan streamlined forms (Figs. 3a,b).
The formation of the layers in these diluvial
deposits, whether from a few or many floods, is
actively debated (e.g., Waitt, 1980; Shaw et al., 1999;
Komatsu et al., 2000). The work in the Pasco basin
indicates that the creation of the layered streamlined
bars occurred in multiple discrete episodes over the last
2.5 Ma (Bjornstad et al., 2001). If this formation
mechanism applies to the layered streamlined forms in
Athabasca’s lower proximal reach, they too may have
formed in multiple flooding episodes. This would
substantiate earlier speculation based on other geomorphic evidence (Burr et al., 2002b) and modeling
(Manga, 2004) that Athabasca Valles were carved by
multiple floods.
4. Alternative hypotheses
4.1. Erosional streamlined forms
Although some terrestrial streamlined forms show
benches (see Carling et al., 2002, Fig. 7), others are
rounded and do not have individual flood layering
exposed at the surface. The jagged, stair-stepped
benches of the Athabascan forms are dissimilar to
the rounded streamlined bars in the Channeled Scabland, and they appear very similar to the edges of
flood channels there scoured into basalt bedrock (B.
Bjornstad, Pacific Northwest National Laboratory,
personal communication, 2004). Thus, an alternative
hypothesis for these streamlined forms is that they are
a result of flood erosion of pre-existing lava flows.
This could be consistent with the location of the
channel. Between the Elysium Mons shield volcano to
the northwest and the Cerberus volcanic province
directly to the east, Athabasca Valles appears eroded
into Late Amazonian Cerberus lavas (Plescia, 1990;
Keszthelyi et al., 2000; Sakimoto et al., 2001; Berman
and Hartmann, 2002). With rootless cones at the
channel’s distal region (Lanagan et al., 2001) and lava
flows near their origin at the Cerberus Fossae (Burr et
al., 2002a, Head et al., 2003), Athabasca Valles’
geology from source to sink is closely associated with
lava flows.
However, the distinct difference between the
upslope and downslope ends of the streamlined forms
does not support the alternative hypothesis that the
streamlined forms are eroded lava flows. If the
massive, dark, spur-and-gully-forming layers in the
upslope ends are eroded lava flows, then the thinner,
lighter-colored layers in the down slope ends must
either be a different material (e.g., sediment), or a
different type of lava flow. If they signify a different
type of lava flow, this would require an associated
change in the local style of volcanism. This might be
possible, but no corroborating evidence from research
on the Cerberus plains lavas can as yet be adduced to
support it.
As a second erosional alternative hypothesis, the
streamlined forms could be a result of erosion of
cohesive sediment. The best candidate for cohesive
sediment is the Medusae Fossae formation (MFF),
located a few hundred kilometers to the south of
Athabasca Valles (Fig. 1). Although the origin of the
MFF remains enigmatic, a strong hypothesis is
volcanic airfall (see Bradley et al., 2002 and
references therein). The layering in the MFF is
variable (Bradley et al., 2002), so that layer thickness
could locally match that of the streamlined forms,
according with this second alternative hypothesis.
However, any erosional origin – whether of lava or
of sediments – is inconsistent with the location of the
forms. Their unique clustering, if a result of erosion,
would require a locally restricted deposit of erodible
material and/or locally erosive flow. However, neither
of those two conditions is likely. First, no obvious
reason exists to expect a locally restricted deposit of
erodible material. The MFF is a very extensive
formation covering N1106 km2 (Bradley et al.,
2002), and lava flows are apparent from the source
to sink of the channel. Second, the visible geomorphology is not consistent with locally erosive
flow. As shown above, the constriction created by the
large in-channel crater downstream would have
produced ponding, which would have led upstream
to depositional, not erosive, flow. Corroborating
evidence exists for depositional flow: three fields of
D. Burr / Geomorphology 69 (2005) 242–252
flood-formed sedimentary dunes, located within a few
kilometers of one of the lower proximal channel
streamlined forms (Burr et al., 2004).
In comparison to these issues weighing against an
erosive origin, the objections outlined above regarding
a sedimentary origin of the forms may be reasonably
explained as resulting from the differences between the
Earth and Mars. The sedimentary bars in the Channeled
Scabland, although composed of layers (Bjornstad et
al., 2001), are smoothly rounded, without stair-stepped
benches (B. Bjornstad, Pacific Northwest National
Laboratory, personal communication, 2004). However,
benches are apparent on at least a few of the Siberian
streamlined forms (Carling et al., 2002). The rounded
morphology is therefore not an inherent characteristic
of diluvial sedimentary bars, and may be at least in part
due to local climate or other nonflood factors. The
manifestation of the layering as benches, if smoothed
out in some terrestrial cases by rainfall, would
reasonably be more prominent in a young channel on
Mars. The stair-stepped appearance of the benches, if
they are composed of sediment, would seem to require
some cohesion of grains. This cohesion could be an
inherent characteristic of the sediment if it were very
fined grained (e.g., Selby, 1993). However, at modeled
flow velocities of a few meters per second (Burr, 2003;
Burr et al., 2004), such fine-grained sediment would be
transported as washload in autosuspension (Komar,
1980) and therefore very unlikely to deposit. Another
possibility is chemical cementation. Cementation of the
layers through alteration/weathering would be unlikely
on Earth in only several million years (the time since
the Athabasca channel formation), but Martian soil is
enriched in sulfur and other cementing material
compounds (Landis et al., 2004). Cementation was
hypothesized to have occurred in the Cerberus plains
region based on analogy with terrestrial volcanic gases
(Plescia, 1993) and was observed by the Mars
Exploration Rovers at Gusev crater, ~1800 km southwest of Athabasca Valles (Arvidson et al., 2004). The
steep layer scarps on the Martian streamlined forms
could thus be cemented even during the short time since
formation from enrichment in such compounds.
Streamlined forms are relatively lighter in night IR
images (Burr and Keszthelyi, 2004), possibly indicating cementation. However, the higher thermal inertia
signature could also indicate larger sized sediment
grains.
249
4.2. Lava depositional streamlined forms
Whereas Martian outflow channels have been
suggested to have formed through erosion by low
viscosity lava (e.g., Carr, 1974; Leverington, 2004,
and references therein), streamlined forms are also
suggested to have been created by lava deposition,
i.e., bduring the same event in which surrounding
basalt units were emplaced. . . in the complete
absence of waterQ (Leverington, 2004, p. E10011).
This suggests the idea of streamlined form creation
by purely lava deposition. No specific mechanism
was detailed for this process, so it is difficult to test
this suggestion, but creation purely by lava deposition (i.e., without subsequent erosion) does not
appear the most reasonable explanation for the
Athabascan forms. Since the Athabascan forms have
dark, massive material at their upslope ends, the
deposition of thinly layered lava at their downlope
ends would require (i) deposition of the dark,
massive material, (ii) erosion of most of that material
to leave knobs, and (iii) deposition of low viscosity
lava behind those knobs. If the dark, massive
material is lava, this would require a distinct change
in the style of volcanism in the area. As noted above,
this is possible, but no analysis has yet been
proposed to support it. Also, the streamlined form
layers appear stair-step in profile and jagged in plan
view (e.g., Fig. 4) with pointed downslope tails. This
is unlike the rounded, lobate morphology produced
at the front or sides of a flowing fluid with a finite
viscosity like lava, and ascribed to lava flows on
Earth and Mars (e.g., Wadge and Lopes, 1991).
For these reasons, neither a lava or a sedimentary
(MFF) erosional origin nor a lava depositional origin
appears consistent with the morphology and location
of the streamlined forms. In contrast, flood sediment
deposition is a very reasonable and even expected
phenomenon. Flood-deposited sediments are held to
be the most like composition of the forms.
5. Characterization of the sedimentation
5.1. Amount of sedimentation
The entire area in the lower proximal reach could
have been the site of sediment deposition, with the
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D. Burr / Geomorphology 69 (2005) 242–252
sediment being preferentially retained and streamlined
behind flow obstacles during the ponded water outflow. This area of possible sediment deposition as
outlined laterally by the edge of the channel and
longitudinally by the extent of the streamlined forms
equals ~800 km2. The height of the sediment can be
assumed equal to the height of the highest downslope
portions of the streamlined forms, which stand at least
60 m above the present channel floor. The total
volume of sedimentation is thus roughly estimated to
have equaled ~48 km3. This number could be an
underestimation if sedimentary layers underlie the
streamlined forms.
The sediment could also have been deposited only
or primarily behind the flow obstacles as the floodwater was filling the basin. This would give a
minimum estimate for the amount of sediment
deposited, which is about 10% that of the first
scenario. However, the first scenario, involving sediment deposition during ponding and then sediment
erosion and streamlining during efflux, has a direct
analog in the processes of the Pasco Basin (Baker et
al., 1991) and so is considered a more plausible
description for actual events.
These estimates assume no erosion of sediment
during or between depositional episodes. The evidence for the large erosion and transport capacity of
the Athabascan flood(s), such as the erosion of the
crater ejecta (Fig. 1), shows this assumption to be
false. Thus, these volumetric sedimentation values are
likely minima and are intended as order of magnitude
estimates only.
lower slopes or at the bases of the knobs (Figs. 3a,b).
Thus, these lava boulders may comprise a significant
component of the sedimentary portion of the form.
An exclusively volcanic origin for the streamlined
forms has been discounted in favor or a predominately
flood sedimentary origin as discussed in Section 3.
However, the forms’ layer thickness in a few locations
is equal to that of the thinnest individual lava flows in
this region of the Cerberus plains detectable with
MOLA data (on the order of 10 m; Lanagan and
McEwen, in revision). Volcanic and aqueous activity
were apparently closely interleaved in time here
(Lanagan et al., 2001; Berman and Hartmann, 2002;
Burr et al., 2002a,b; Head et al., 2003). Thus, a
volcanic (either thin lava flow or volcaniclastic) origin
for some of the layers is viable. If the layers were
deposited over time in multiple floods, thin lava flows
or volcanic airfall deposits likely interbed with the
diluvially deposited sediment.
Finally, the sediment may have originated with the
floodwater from the Cerberus Fossae. Any such
sediment, coming from within fissures that were
recently both volcanically (Keszthelyi et al., 2000;
Sakimoto et al., 2001; Plescia, 2003; Head et al.,
2003) and aqueously (Burr et al., 2002a,b; Head et al.,
2003) active, may have been hydrothermally altered
and may bear record of recent subsurface hydrothermal alteration. The sediment may also carry
evidence for a putative subsurface biosphere (Fisk
and Giovannoni, 1999).
5.2. Provenance of the sediment
6. Summary and implications for future surface
exploration of Mars
The sediment in the streamlined forms may have
had a variety of sources. Some of the sediment likely
came from the flow obstacles themselves or from the
surrounding terrain, as suggested by experiments
(Komar, 1983) and the author’s observations of
paleoflood deposits in Iceland. Thus, some of the
sediment in the streamlined forms may be crater ejecta,
either from the large in-channel crater that caused the
ponding, or from smaller craters in the streamlined
forms. If extant within the forms, this ejecta is likely
buried beneath subsequent flood deposits. The knobs
of lava bedrock at many of the forms’ upslope ends
appear to be shedding boulders, which are seen on the
Streamlined forms clustered in the lower proximal
Athabasca Valles are interpreted as sedimentary. Their
clustering may be most reasonably explained by
hydraulic damming of the floodwaters behind a large
in-channel crater and resultant sedimentation during
ponding. Sedimentation was likely deposited throughout the area of ponding, with subsequent streamlining
of the sediment behind flow obstacles occurring
during the outflow of the water.
Martian outflow channel morphology is thought to
be dominated by erosive landforms. Investigations
based on Viking results (e.g., Baker, 1979) have
emphasized erosive processes, denoting sedimentary
D. Burr / Geomorphology 69 (2005) 242–252
processes only as probable (Baker, 2002). Other
interpretations based on MOC and THEMIS data
concluded that there are no depositional bed forms,
specifically denoting the streamlined forms in the
circum-Chryse outflow channels as erosive (Rice et
al., 2003). However, calculations show that the
Martian floods should have been capable of moving
large amounts of sediment (Komar, 1980). These
depositional streamlined bed forms in Athabasca
Valles, along with sedimentary diluvial dunes (Burr
et al., 2004), support these earlier calculations.
In the Channeled Scabland, thick deposits of flood
sediment in the Pasco and other basins (Allison, 1933;
O’Connor and Baker, 1992; Bjornstad et al., 2001)
stand in contrast to other stretches (e.g., the CheneyPalouse tract) where erosive regimes prevailed (Patton
and Baker, 1978). Because the Pasco Basin was a
single locus of flow convergence for all flood channels
in the Channeled Scabland, its sedimentary deposits
provide the best preserved long-term history of that
region. Likewise, the streamlined forms in lower
proximal Athabasca Valles sit below the confluence
of two tributary channels and immediately at or above
the first distributary channels (Fig. 1), so that all the
Athabascan floods must have flowed past or over these
forms. They may thus offer the best opportunity to
untangle that channel system’s geologic history. If the
streamlined forms were created during multiple floods,
weathering horizons between the layers would constrain their relative timing. The size and orientation of
the sediment in the layers would provide data on the
hydraulics of the flood(s) that emplaced it. Interbedded
lava or volcanic airfall layers would indicate the
amount and character of recent volcanic activity in
the region. The sediment in the streamlined forms, if it
came from within the Cerberus Fossae, may offer the
best opportunity to access hydrothermally altered
sediments containing evidence of a possible subsurface biosphere (Fisk and Giovannoni, 1999).
Acknowledgements
I thank Bruce Bjornstad, Jim Head, and Jim
Zimbelman for constructive critiques of an earlier
version of the manuscript, and Goro Komatsu and Vic
Baker for very helpful reviews. This work was
supported by the NASA Planetary Geology and
251
Geophysics Program through the E.M. Shoemaker
Fellowship at the U.S. Geological Survey.
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