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Lunar and Planetary Science XLVIII (2017)
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FACIES ANALYSIS AND BASIN ARCHITECTURE OF THE UPPER PART OF THE MURRAY
FORMATION, GALE CRATER, MARS. C. Fedo1 (Earth & Planetary Sciences, Univ Tennessee, Knoxville, TN
37996, [email protected]), J. Grotzinger2, S. Gupta3, N.T. Stein2, J. Watkins2, S. Banham3, K.S. Edgett4, M. Minitti5, J.
Schieber6, K. Siebach7, K. Stack-Morgan8, H. Newsom9, K.W. Lewis10, C. House11, A.R. Vasavada8
1
University of Tennessee, Knoxville, TN, 2California Institute of Technology, Pasadena, CA, 3Imperial College,
London, 4Malin Space Science Systems, San Diego, CA, 5Planetary Sciences Institute, Tucson, AZ, 6University of
Indiana, Bloomington, IN, 7StonyBrook University, Stony Brook, NY, 8JPL, Pasadena, CA, 9University of New
Mexico, Albuquerque, NM, 10Johns Hopkins University, Baltimore, MD, 11PSU, State College, PA
Introduction: The Mars Science Laboratory mission to Gale crater, using the Curiosity rover, provides
the opportunity to directly test hypotheses regarding
past climate through understanding the past environments recorded by sedimentary facies. Gale crater is
located on fluvially dissected, cratered, highlands
crust, along the Martian topographic dichotomy
boundary. The crater formed at ~3.8-3.6 Ga [1]. The
interior plains of northern Gale crater (Aeolis Palus)
contain remnants of eroded alluvial fan deposits, and
sedimentary deposits that form a large mound (Aeolis
Mons, informally known as Mt. Sharp) cored by a central peak; the sediments were deposited, buried, lithified, and exhumed and eroded before 3.3-3.1 Ga [1].
Having discovered evidence for ancient stream and
thin (m-thick) lake deposits within its landing region
[1], Curiosity recently explored the basal strata of Gale
crater’s central mountain, Aeolis Mons (informally
known as Mt. Sharp). Rocks exposed on the lower
slopes of Mt. Sharp include hundreds of meters of strata containing evidence of hydrated minerals. Previous
work has demonstrated a succession of sedimentary
rock types deposited dominantly in river-delta settings
(Bradbury group), and interfingering/overlying contemporaneous/younger lake settings (Murray formation, Mt. Sharp group) [1]. The Murray formation
is on the order of 200 meters thick, composed dominantly of mudstones, and it mostly records lacustrine
deposition in which hematite-phyllosilicate and magnetite-silica mineralogic facies record primary chemical sedimentation [2] and diagenetic overprints [3].
The Stimson formation unconformably overlies the
Murray formation [4].
Here we present recent observtions from an ~105
meter thick interval of the Murray formation, studied
between sols 1350-1565, discuss preliminary interpretations of the facies and stratigraphy, and place this in
the broader context of the Mt. Sharp stratigraphic succession. This pairs with chemostratigraphic work on
the same interval [5].
Facies: A summary stratigraphic column shows all
previous rocks studied, in addition to the interval reported here, between the Naukluft Plateau and the
“Precipice” location (Fig. 1). The Murray formation is
subdivided into four recognizeable facies.
Figure 1. Stratigraphy to-date compiled for rocks encountered
during ascent fo Mt. Sharp. New strata discussed here span from
-4438 to -4330 in elevation. Solid dots = drill holes.
Finely laminated gray-colored rocks interpreted to
be of lacustrine origin that typify the basal Murray
Lunar and Planetary Science XLVIII (2017)
formation are overlain by a thick (~25 m) interval of
cross-bedded siltstone to very fine-grained sandstone.
The cross-bedding occurs as meter scale troughs, with
angle of repose foresets. Sediment grain size is below
the resolution (~64 µm/pixel) of the routine MAHLI
imges. Decimeter-scale cross stratification also is associated with the larger-scale concave-up bodies that
extend for >10 m across the outcrop, such as at
“Baynes Mountain”. The Oudam drill hole indicates
the presence of the most hematite measured so far during the mission, associated with Ca-sulfate [6, 7].
Cross stratification at this scale and grain size is consistent with bedload sediment transport in an aeolian
setting, particularly curved-crested dunes [8], although
other environments may produce deposits of this type.
The next higher stratigraphic interval, which is ~30
m thick, returns to dominantly finely laminated, purple-colored mudstone through very fine-grained sandstone. These rocks resemble the lowermost Murray
formation, except in color and presence of distinct diagenetic features, including patches of cm-scale concretions. Additionally, there are irregular cracks with tapered terminae that are restricted to the interiors of
fracture-defined polygons, which are filled with lighttoned Ca-sulfate based on ChemCam data. Compositionally, these rocks differ significantly from the crossstratified facies in that they contain decreased amounts
of hematite and have significant amounts (20-25%) of
phyllosilicate based on the Marimba and Quela drill
holes [9]. The similarity with the lowermost Murray,
and particularly, the presence of finely laminated mudstone points to suspension fallout of detrital material in
a lake environment.
The uppermost unit thus far includes ~50 meters at
the top of the composite section, designated as “heterolithic mudstone-sandstone.” The rocks are exposed in a
region of subdued topography, where outcrops are
small and mostly exposed as slabs and plates of broken
bedrock, with some blocks and slabs having been rotated from their original orientation. This makes facies
assessment difficult, and particularly the recognition of
important stratigraphic bounding surfaces, so the rocks
are grouped according to what attributes are observed
at smaller scale, including: dark red, finely laminated,
fine-grained mudstone, cm-scale ripple crosslaminated siltstone or very fine sandstone, dm-scale
cross-stratified siltstone and very fine grained sandstone; massive intervals marked by extensive development of concretions that disrupt faint relicts of primary lamination. In one case, a MAHLI image showed
clear evidence for a sandstone with fine-to-medium
grain size, but all the rest were very fine grained. In
particular, cm-scale ripple cross-lamination is abundant, whereas it was observed at one locality in the
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lowermost Murray formation [1]. This stratigraphic
interval also shows distinctive, small-scale polygonal
fractures, that resemble desiccation cracks [10]. Compositionally, one drill sample of the facies (Sebina)
provides evidence for very similar composition to
Quela and Marimba; again, phyllosilicate dominated,
with subordinate hematite and Ca-sulfate. Although
this facies is difficult to assess, the potential presence
of mud cracks associated with contemporaneous gypsum precipitation raises the possibility of a lacustrine
environment dominated by suspension fallout with less
common traction deposition, perhaps along a desiccating lake.
Basin Architecture: The broad facies arrangement
of the sedimentary fill of Gale crater beginning at the
landing site is consistent with progradation of terrestrial deposits from the crater margin to a lake that occupied crater [1]. Overall, the facies types and architecture are consistent with an overfilled lake basin [11].
In such basins, precipitation and sediment yield exceeds the rate of evaporation leading to perennial
standing water fed by active fluvial systems and relatively poorly defined parasequences.
Above the Naukluft Plateau (Fig. 1), the upper
Murray formation facies could represent a change in
the expected environments relative to that of an open
hydrologic system. The presence of nearly 30 m of
large-scale trough cross stratified sandstone of possible
aeolian orgin suggests climate aridification. In additional support of subaerial exposure, is the potential
presence of mud cracks, which form from the desiccation of fine-grained sediments, and coeval gypsum
infilling. The association of these facies is more similar
to underfilled lake basins, where evaporation exceeds
precipitation [11]. A possible record of climate change
is not particularly surprising given the stratigraphic
thickness (and presumed duration of sedimentation) of
the sedimentary succession.
References: [1] Grotzinger, J.P., et al. (2015) Science, 350, aac7575. [2] Hurowitz, J.A., et al., (2016)
47th LPSC Abstract #1751. [3] Rampe, E.B., et al.
(2106) 47th LPSC Abstract #2543. [4] Banham, S. et
al., (2017) 48th LPSC. [5] Milliken, R.E., et al. (2017)
48th LPSC. [6] Rampe, E.B., et al. (2017) 48th LPSC.
[7] Vaniman, D.T., et al. (2017) 48th LPSC. [8]
Brookfield, M.E., Silvestro, S. (2010) Facies Models
4, GAC GEOtext6, 139-166. [9] Bristow, T.F., et al.
(2017) 48th LPSC. [10] Stein, N.T. et al. (2017) 48th
LPSC. [11] Bohacs, K.M., et al (2000) AAPG Studies
in Geology 46, 3-34.