79th Annual Meeting of the Meteoritical Society (2016) 6474.pdf

GEOLOGIC MAPPING OF BAKHUYSEN CRATER, MARS: ANALOGIES TO THE RIES IMPACT EJECTA WITH INSIGHTS INTO MARTIAN IMPACT MELT. C. M. Caudill1, G. R. Osinski1,2, L. L. Tornabene1, 1Centre for Planetary Science and Exploration / Dept. Earth Sci- ences, University of Western Ontario, London, ON, Canada, 2Dept. Physics and Astronomy, University of Western Ontario, London, ON, Canada. Introduction: Impact ejecta deposits allow an understanding of subsurface lithologies, volatile content, and other compositional and physical properties of a planetary crust [1], yet development and emplacement of these deposits on terrestrial bodies throughout the solar system is still widely debated. The Ries impact structure in Germany is one of very few on Earth with preserved and well-studied impact melt-bearing ejecta deposits [2]. Mars possesses hundreds of well-preserved impact craters and ejecta [3], and with similarities to terrestrial atmospheric conditions and crustal volatiles [4] presents an opportunity to: 1) understand these structures at the meter to sub-meter scale using current remote sensing datasets; and 2) draw analogies from the Ries impact structure. Current models of ejecta and impact melt emplacement [e.g., 1, 5] are suggested as a framework for understanding ejecta formation on all terrestrial planets [1]. However, relating Martian ejecta morphology to those preserved at the Ries impact structure is an area of active research [e.g., 4, 6, 7]. In this study, we report the mapping and geologic interpretation of 150-km diameter Bakhuysen Crater, Mars, and discuss the implications for multi-unit ejecta emplacement for Martian impact structures. Textural, compositional, and thermophysical mapping point to two distinct ejecta units, where an overlying pitted layer shows ubiquitous pond and flow features. Mapping and Observations: Bakhuysen crater and ejecta map was constructed covering 90,000 km2 at 1:125,000-scale or better utilizing high-resolution visible imagery [8, 9], thermophysical data [10, 11], and HiRISE blue-green (BG) composite and infrared band-ratio (IRB) images as a first-order indication of mineralogy [12]. Two main units (e1 and e2) comprise the continuous ejecta. The overlying unit (e1) is always observed with a pitted texture and is exposed as patchy, isolated outcrops. The pitted texture of e1 is consistent with degraded Martian crater-related pitted material interpreted as volatile-rich impact melt, widely observed and described by Tornabene et al. [6, 13]. Visible imagery coupled with thermophysical and elevation data indicate e1 are ponded deposits with flow morphol- ogies and where best preserved are similar to lunar impact melt flows [e.g., 14]. The e1 unit occupies topographic lows within the ejecta (<5° slopes) and thins out at its margins, gradually exposing the basal ejecta unit. Contacts between the units are strongly delineated by thermophysical properties; thermal inertia values of the basal ejecta unit (e2) in comparison to e1 indicates that its composition is finer grained, more homogenous, and/or having general looser packing configuration of grains. The majority of the ejecta is mapped as the basal ejecta unit, which is comprised largely of a hummocky facies. Discussion: Mapping of the ejecta deposits of Bakhuysen Crater reveals the presence of patchy volatile-rich im- pact melt deposits (pitted material) overlying a continuous hummocky ejecta facies. This is analogous to the Ries impact ejecta. Sharp contacts between Ries ejecta deposits indicate some temporal separation between emplacement of the two units, where volatile-rich impact melt-bearing breccia (suevite) overlies the Bunte Breccia conformably and without evidence of turbulence [15]. This is in direct comparison to Bakhuysen, where ejecta contacts are sharp and flow and lobe features are observed in e1 with no evidence of mixing with e2. Although observations of Martian multi-unit ejecta, where units are temporally-separated with an overlying melt- bearing layer are rare, we suggest that these features are observed in Bakhuysen Crater due to distinct geological conditions. First, the late Noachian-aged impact (~3.7 Ga) [6] was formed at the end of the Late Heavy Bombardment, just after a period which formed large-basin impacts and had high erosional rates with fluvial activity. Bakhuysen was a large enough impact crater to have produced a significant volume of melt [16], yet uniquely preserved. This supports previous work [17] which suggests that given similarities in volatile content and subsurface stratigraphy, similar mech- anisms of multi-unit ejecta emplacement may extend to impact cratering processes on other comparable rocky bodies. References: [1] Osinski, G.R. et al. 2011. Earth and Planetary Science Letters 301: 167-181. [2] Osinski, G.R., Grieve, R.A.F., and Spray, J.G. 2004. Meteoritics & Planetary Science 39: 1655–1683. [3] Tornabene L. L. et al. 2012. Abstract #7069. 3rd International conference on Early Mars. [4] Kenkmann, T. and Schonian, F. 2006. Meteoritics & Planetary Science 41: 1587–1603. [5] Stoffler, D., et al. 2013. Meteoritics and Planetary Science 48: 515-589. [6] Tornabene, L.L. et al. 2012. Icarus 220: 348–368. [7] Sturm, S. et al. 2013. Geology 41: 531-534. [8] Malin, M.C. et al. 2007. Journal of Geophysical Research 112: E05S04. [9] McEwen, A.S. et al. 2007. Journal of Geophysical Research 112: E5. [10] Chris- tensen, P.R. et al. 2004. Space Science Reviews 110: 85-130. [11] Christensen, P. R. et al. 2013. Abstract# 2822. 44th Lunar and Planetary Science Conference. [12] Delamere, W. A. et al. 2010 Icarus 205: 38–52. [13] Tornabene, L.L. et al. 2007. Abstract #3288. 7th Mars Conference. [14] Bray, V.J. et al. 2010 Geophysical Research Letters 37: L21202. [15] Newsome et al. (1986) Journal of Geophysical Research 91: E239-E251. [16] O’Keefe, J.D. and Ahrens, T.J. 1977. Proceedings of the Lunar Science Conference. 8: 3357-3374. [17] Osinski, G.R. 2005. Meteoritics & Planetary Science 41: 1571-1586.