DETECTING ASTROBIOLOGICALLY SIGNIFICANT OCEAN FLOOR SEDIMENTS IN THE TSUNAMI-BATTERED COASTS OF EARLY MARS. J. A.P. Rodriguez1, M. Zarroca2, R. Linares2, G. Ko-matsu3, D. Oehler1, A. Davila4, V. Baker5, D. Bernman1, Hideaki Miyamoto6, 1Planetary Science Institute, Tucson, AZ; alexis@psi.edu; 2Universitat Autònoma de Barcelona, Barcelona, Spain; 3Università d’Annunzio, Pescara, Ita-ly; 4NASA Ames Research Center, Moffett Field, CA; 5University of Arizona, Tucson, AZ; 6University of Tokyo, Tokyo, Japan.

Introduction: Numerous investigations indicate

that, approximately 3.4 Ga, an enormous ocean likely covered most of the Martian northern plains [1-2]. The geological record of sedimentary deposits laid down within this early Mars ocean could help decipher its characteristic types of submarine environments, there-by shedding light on its potential habitability. A major obstacle towards the remote-sensing detection of these purported ocean floor materials, however, is that de-tailed geologic maps position them within the northern plains buried stratigraphy, most likely beneath the Ear-ly Amazonian Vastitas Borealis Formation (VBF, Fig. 1 [3, 4]).

Fig. 1 Schematic reconstruction of the northern plains (1) and adjoin boundary cratered regions in northwest-ern Arabia Terra (2). The VBF is depicted as overlying Late Hesperian marine sediments, which in turn lie on the cratered Noachian basement. The adjoinging tsu-nami debris fields are interpreted to be composed of displaced ocean floor strata intermixed with debris captured during overland flow. The lower boundary of the tsunami lobes represents the paleoshoreline loca-tion.

The VBF is thought to consist of an ice-rich geo-logical unit [5, 6], which has been proposed to have initially formed during the ocean’s freezing [7]. The unit could still retain some residual marine ice; howev-er, its upper strata have been subject to intense resur-facing driven by numerous episodes of periglacial and aeolian processes [3,7], and thus the precise prove-nance of its surface lithological character and volatile content remain difficult to determine.

We propose that the recently discovered debris fields, interpreted to have been emplaced by tsunami waves that propagated enormous inland distances [8, 9] might allow access to shored-up ocean floor sedi-ments (Fig. 1). On Earth long run-up tsunami deposits typically include significant lithological overprints generated by debris ingested during the overland prop-agation [10]. Tsunami deposits consisting exclusively of displaced submarine strata and biomass have been identified draping sizable ridges exposed above sea level [11]. Inspired by this analogy, we considered the Mars ocean paleo-geographic reconstructions devel-oped by Rodriguez et al. [8], in which the paleo-shoreline level is set at -3800 m. This analysis high-lighted an area of potential tsunami sedimentation as particularly distinctive within the entire surveyed area: an extensive deposit draping over an ocean-facing cape on the outer margin of an enormous bay located in northwestern Arabia Terra (Fig. 2).

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Fig. 2 (A) Distribution of lithic tsunami deposits ex-tending from a paleo-shoreline high stand at -3800 m (light blue area) (modified from Rodriguez et al. [8]). (B) Paleo-shoreline reconstruction of an area in north-western Arabia Terra, where we find that a bay would have secluded shallow coastal plains. The paleo-ocean is shown in light blue. (C) Perspective view of the bay facing south away from the paleo-ocean and towards the highlands. The red line outlines the extent of the proposed tsunami deposit. The white arrows identify two locations where the deposit overflowed devides and intruded into the bay’s interior.

The cape’s north-facing side comprises a broad ramp, which is overlain by extensive superposed sedi-mentary lobes with their frontal margins oriented in the upslope direction (Fig. 3A). The lithic composition of the lobes is indicated by a lack of evidence for duc-tile deformation, which for a glacial origin would re-sult in flow elongation and topographic relaxation of superposed impact craters. Here, we propose that these sedimentary lobes consist of tsunami materials that were displaced from the ocean floor and deposited as pulses of density flows. The bay’s interior floor is flanked by widespread benches that are cut into the interior crater walls as well as the frontal margins of the tsunami lobes (Fig. 3B), indicating that their for-mation must have post-dated the tsunami event.

Fig. 3 (A) Highland-facing lobes, which locally over-flow the crater rim (yellow) and into the bay’s interior (red). The dashed red line represents an erosional con-tact along the deposit’s ocean-facing margin, which we

propose would have developed due to intense marine erosion following the deposit’s emplacement. (B) View of an interior area of the bay, where an intruded tsunami flow and adjoining crater walls appear to be cut by terraces (blue arrows). Relative locations shown in Fig. 2C.

Characterizing the potential habitability of early Martian paleo-environments comprises a fundamental goal in ongoing and future space exploration. Here, we propose that interior plains of a bay in northwestern Arabia Terra constitute a landing site candidate of prime astrobiological significance. These plains offer in-situ accessibility to a potentially diverse tsunami-derived and non-tsunami-derived marine geologic rec-ord (Fig. 4), which, based on Earth analogues, might contain important, organic biosignatures [12].

Fig. 4 Sketches showing paleo-geographic reconstruc-tions leading to the emplacement of the proposed tsu-nami deposit.

References: [1] Parker et al. (1989) Icarus, 82, 111-145. [2] Parker et al. (1993) JGR, 98, 11061-11078. [3] Tanaka et al. (2005) USGS SIM-2888. [4] Tanaka et al. (2014) USGS SIM-3292. [5] Tanaka (1997) JGR, 102, 4131-4149. [6] Mouginot et al. (2012) GRL, 39. [7] Kreslavsky and Head (2002) J. Geophys. Res. 107, 5121. [8] Rodriguez, J. A. P., et al. (2016), Nat. Sci. Rep., 6, 25106, doi:10.1038/srep25106. [9] Costard et al. (2017), J. Geophys. Res. Planets, 122, 633–649.[10] Goto et al. (2012) Marine Geology, 40, 887-890. [11] Ilayaraja and Krishnamurthy (2010). Jour. Coastal Conserv. v. 14 (3), 215-230. [12] Oehler and Allen (2012) SEPM (Society for Sedimentary Geology) Special Publication No. 102 (Sedimentary Geology of Mars), 183-194.

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