Aqueous Deposits Related to Formation of Hale Crater in Southern

46th Lunar and Planetary Science Conference (2015)
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AQUEOUS DEPOSITS RELATED TO FORMATION OF HALE CRATER IN SOUTHERN
MARGARITIFER TERRA, MARS. J. A. Grant1, and S. A. Wilson1, Smithsonian Institution, NASM CEPS, 6th
at Independence SW, Washington, DC, 20560 ([email protected]).
Introduction: Hale crater (35.70S, 323.60E) is a
137 km-diameter impact structure that formed near the
Amazonian-Hesperian boundary [1] or in the early-tomiddle Amazonian [2]. The formation of Hale destroyed the probable head of the Uzboi-Ladon-Morava
meso-scale outflow channel and blocked the southern
end of the Uzboi Vallis segment of the system. During
and after the impact event that formed Hale, fluidized
debris flows and channels sourced near the crater rim
incised or modified the ejecta deposit [2]. Mapping
reveals these water-driven flows [2] extend considerably further north than previously mapped, reaching the
floor of Uzboi Vallis just south of Nirgal Vallis (Fig.
1) [3-5].
Fig. 1. Southern Margaritifer Terra north of Hale crater
(centered on 30.50S, 323.50E) shows the extent of Hale
flow deposits (light teal) beyond the Hale ejecta (yellow along bottom right side) [6]. Cratered uplands and
plains are brown, valleys are purple, and lacustrine and
ponded water deposits are green. North towards top.
Expression of Late Flow Deposits: The deposits
associated with the Hale impact vary in expression
from relatively thin veneers along newly incised segments and within pre-existing valleys and topographic
lows (Fig. 2) to thicker lobes where materials accumulated behind and below topographic constrictions (Fig.
3). Incised reaches and streamlined forms are sometimes present downstream of constrictions that appear
to have slowed and partially blocked the flows [4, 5].
In other locations, close to two crater diameters north
of Hale, the material ponds and shallowly embays local
relief in a large crater. The material deposited by the
flows is darker-toned than bounding surfaces and is
mostly smooth and featureless at scales of 10’s to 100’s
of meters. The deposits embay Hale secondary craters
and appear to generally thin northward (distally).
Fig. 2. Hale deposits (Ahs) within pre-existing valleys
and topographic lows in the Noachian Highlands and
Terra units (Nh and NHt). North towards top.
Fig. 3. Hale deposits (Ahs) accumulating in and below
a topographic constriction in Nh. North towards top.
Thicker flow lobes have distinct margins and capping ridges are typically oriented perpendicular to the
flow direction [5]. Where exposed at flow fronts, crosssections show no obvious layering and deposits are
often crossed by fractures (Fig. 4). Despite an overall
morphology that sometimes appears broadly similar to
volcanic flows, the local erosion of the digitate, distal
46th Lunar and Planetary Science Conference (2015)
margins produces isolated remnants by what appears to
be deflation due to the absence of bounding erosional
deposits (Fig. 4). This implies a significant finegrained component: there are few boulders or other
coarse fragments observed with the exception of the
tops of some thicker flow lobes.
Fig. 4. Digitate ends of Hale deposit in Fig. 3. Erosion
and absence of associated deposits implies deflation of
constituent fines. North towards top.
Timing Relative to Other Regional Events: One
of the last, regional events in southern Margaritifer
Terra prior to the formation of Hale crater was the occurrence of a 4000 km2 lake within Uzboi Vallis [7].
Hale flow deposits superpose the Uzboi lake deposits
and in the southern reaches of Uzboi extend across a
degraded crater where a series of meter-scale bedforms
occur [7]. These may represent primary depositional
structures or later eolian reworking of the Hale deposits
[7]. If the former, the bedforms imply water depths
greater than the scale of the structures was present locally over a crater diameter from Hale. The Hale deposits were the last water-driven activity in the region.
Origin of the Deposits: Because most deposits occur within topographic lows and some can be traced
back to the channels emerging from the Hale ejecta,
their origin appears related. There are more isolated
occurrences where the connection to Hale is not obvious (Fig. 1). The apparent absence of layering in flow
fronts points to their emplacement during a single event
Nonetheless, pulses of flow cannot be ruled out. Emplacement likely occurred shortly after the Hale impact
based on the embaying relation between the deposits
and Hale secondary craters.
The water-driven flows north of Hale occurred near
the Amazonian–Hesperian boundary or even later when
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climate was likely cold and characterized by mostly
below freezing temperatures [8], thereby implying rapid freezing would be expected. Almost half the distance
traversed by the flows, however, was across warm Hale
ejecta that would have slowed or even precluded freezing [9]. Additional water sourced from the Hale ejecta
may have contributed to discharge. Once beyond the
ejecta, freezing at the surface of the flows likely created an insulating layer of ice that enabled the underlying
water-sediment mix to continue downslope [10]. Freezing may also have occurred at the bottom of the flows,
but constrictions and steeper grades likely created
higher velocities and the local incision observed. Slower moving, locally thicker accumulations (Fig. 3) could
have developed a thicker, more rigid ice crust that fractured to create observed capping boulders. Continued
cooling and further contraction caused additional fracturing. Once halted, the water in the flows could sublime away, leaving behind the thin deposits observed in
many locales.
Gradients along the depressions traversed by the
flows are often relatively low and some reaches include
local relief the flows would have had to overcome.
Two relatively long reaches trending SSW-NNE just
west of Bond crater (Fig. 1) have average gradients of
slightly less than 10 m/km. Coupled with the long
runout, this suggests flow viscosities and velocities
were relatively low, characterized by transport rates
that may not have exceeded several meters/second. If
correct, these velocities are broadly consistent with the
inference that the flows carried mostly fine-materials
[11, 12], perhaps aided by freezing at the bed that limited entrainment of large fragments from all but the
locally steepest reaches. Nevertheless, such velocities
would enable the flows to reach two crater diameters
from Hale in a day or two, thereby implying they could
relate to very short-lived conditions and events.
References: [1] Cabrol et al., 2001, Icarus, 154,
98–112, doi:10. 1006/icar.2001.6661. [2] Jones et al.
(2011), Icarus, doi:10.1016/j.icarus.2010.10.014. [3]
Wilson et al., (2013), Abs. Ann. Mtg. Planet. Geol.
Mappers, Washington, DC. [4] Wilson et al. (2013),
LPSC Abst. 2710. [5] Wilson et al., (2013), Abs. Ann.
Mtg. Planet. Geol. Mappers, Flagstaff, AZ. [6] Wilson
et al. (2015) LPSC Abst. 2492. [7] Grant et al. (2011),
Icarus, doi:10.1016/ j.icarus.2010.11.024. [8] Carr
(2006), The Surface of Mars, 307 pp., Cambridge
Univ. Press, Cambridge, UK. [9] Mangold (2012),
PSS, doi:10.1016/j.pss.2011.12.009. [10] Gregg and
Greeley (1993), JGR, 98, 10,873-10,882. [11] Burr et
al. (2006), Icarus, doi: 10.1016/j.icarus.2005.11.012.
[12] Baker et al. (1988), Flood Geomorphology, 503
pp., Wiley Press, NY.