46th Lunar and Planetary Science Conference (2015)
G. Di Achille1 and F. Salese2, 1INAF-Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Teramo, Italy,
[email protected], 2International Research School of Planetary Sciences-Universita’ di Chieti, Italy.
Background and scientific rationale: More than
50 possible ancient deltaic deposits have been reported
on Mars [e.g. 1-6]. Among the latter deltas only a few
(Jezero [1], Eberswalde [2], Sabrina and Hypanis [3],
Idaeus Fossae [4]) show well-developed stratigraphy
with evidence of avulsing channels and multilobate
depositional patterns; they are the largest deposits (in
terms of surface area and volume) and are found exclusively at the mouth of relatively large valleys with tributaries and alluvial plains that were presumably formed
by sustained and persistent surface runoff discharges.
The latter deposits might be strictly interpreted as river-deltas. Whereas, the majority of Martian deltaic
deposits are small (a few km wide and long) singlelobe with deep (and often steep) delta front resembling
terrestrial Gilbert-type fan-deltas. These deposits are
tipically found at the mouth of short (often theatherheaded) valleys lacking tributaries and alluvial plains.
Several questions about the formation and evolution of martian deltas remain to be answered. First of
all, while there is a general consensus on assuming that
the few martian river-deltas formed in > 102 yrs timescales, the time required to form fan-deltas, and thus
their paleoclimatic and paleoenvironmental interpretations, are controversial. Particularly, [7] suggested that
fan-deltas could have also been formed in relatively
short (< 102 yrs) and single depositional events. If this
would be the case martian fan-deltas could not be
uniquely used to assess whether they formed during
extended epochs of clement climatic conditions (and
thus if they are unequivocal indicators of favorable and
durable conditions for life) or during limited (in space
and time) events produced by regional factors, like for
example impact craters [8], volcanism [9], or tectonics
and resultant hydrothermal activities (and thus if they
could have been formed also under climatic conditions
not necessarily different from those of present Mars).
LGC/BA as terrestrial analog: For the abovementioned reasons and since fan-deltas are the majority of
Martian fluvio-lacustrine deposits and often also proposed as landing sites for future missions, we are
studying the fan-deltas at Lake General Carrera/Buenos
Aires (LGC/BA, Chile/Argentina, Fig. 1) to gain a better quantitative understanding about the formation and
evolution of ancient martian fan-deltas.
Due to climate change and the last major deglaciation
of the northern Patagonian ice sheet, LGC/BA experienced significant hydrological modifications which
resulted in a more than 400 m vertical retreat of the
lake during the last 20-15k years. The latter overall
retreat was punctuated by at least 6 highstand periods
which are all testified by a series of six stacked fandeltas in the southwestern part of the lake (Fig. 1-2).
We are studying the latter region in detail through
modeling, fieldwork campaigns and remote sensing
using ASTRIUM Pleiades satellite stereoimages from
which we derived a high-resolution (1 m/pixel) digital
elevation model (DEM, see Fig. 2a) comparable to
HiRISE DEM for Martian deposits.
The site provides access to a concentrate of present and
past coastal lacustrine morphology, erosional and depositional features such as beach ridges, shorelines, and
terraces which can be directly compared with Martian
putative deposits and associated features. Particularly,
the deposits from level 2 and 3 (dated approximately to
13k and 8-6k yrs ago, respectively, Fig. 3) show wellpreserved beach ridges and embayments (Fig. 4a),
shorelines, and terraces on the delta fronts (Fig. 4b).
Therefore, the site offers the opportunity to investigate
the morphometry of coastal/deltaic features to be compared to martian putative analogs and to test the scenarios put forth for explaining their formation. Moreover, the six raised deltas are entirely entrenched thus
enabling potential sampling over the entire thickness of
the sedimentary sequences to reconstruct paleodischarges, based on grain sizes analysis. In summary, the
quali-quantative study of fan-deltas at LGC/BA has
strong implications for several open issues related to
martian fan-deltas and their paleoclimatic and paleohydrologic significance. Particularly, the study of the
deposits at LGC/BA is important for the following
puroposes: i) to understand formation timescales of
fan-deltas from the context geology/sedimentology of
the lacustrine deposits thus placing constraints for the
martian analogs; ii) to assess the observability of martian putative features in remote sensing datasets comparing the evidence of LGC/BA in similar terrestrial
satellite datasets; iii) to investigate the boundary conditions for fan-delta entrenchment from a quantitative
point of view thus for the understanding of the erosion
of the martian deposits and to infer paleoclimatic and
paleohydrologic conditions during their terminal activity; finally, all the above aspects are fundamental for a
iv) better implementation of martian fan-deltas modeling by providing Earth-validated observational and
numerical constraints for the modeled processes.
46th Lunar and Planetary Science Conference (2015)
Figure 1 - Overview of the Lake General Carrera/Buenos Aires (Chile/Argentina) from LANDSAT
satellite images; the red box indicates the area that we
are studying in detail through field work campaigns
and where we targeted and acquired new ASTRIUM
Pleiades satellite images (see Fig. 2)
Figure 3 – Topographic profile (from the 1 m/pixel
stereo derived DEM) across the series of stacked fandeltas; see Fig. 2 for location
Figure 2 – a) 1 m/pixel DEM derived from ASTRIUM
Pleiades satellite images; b) ortorectified stereoimage
draped on derived DEM of the southwestern fan-deltas
assemblage (red line - AB topographic profile of the
sequence, see Fig. 3)
Figure 4 – (a) Terraces on the delta front #2 marking
the lake retreat from about 8000-6000 yrs ago to the
present time; total delta front thickness is about 120 m
and terraces are a few meters high; (b) Exposed gravel
beach beds along road cut
References: [1] Fassett C. I. and Head J. W. (2005),
GRL, 32, L14201. [2] Malin, M. C., and Edgett, K. S.
(2003), Science, 302, 1931-1934. [3] Hauber, E. et al.
(2009), Planet. Space Sc., 57(8), 944-957. [4] Salese et
al., (2014), AGU Fall Meeting, Abstract #1402. [5] Di
Achille G. and Hynek B.M. (2010), Nature Geo., 3,
459-463. [6] Irwin, R. P. et al., (2005). JGR (1991–
2012), 110(E12). [7] Kleinhans M. G. (2005), JGR,
110, 12003. [8] Brackenridge G. R. et al. (1985), Geology, 13, 859-862. [9] Halevy I., and Head, J. W.
(2014), Nature Geo., 7, 865-868.