49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1328.pdf

Crater-fill on Mars: Sediment Traps and Long Term Stratigraphic Record: R. A. De Hon, Department of Geography, Texas State University, San Marcos, TX ([email protected])

Introduction: Martian craters are mixed with material of other origins are to be natural closed basins that exhibit a variety of expected. Aeolian deposits are the latest secondary floor materials which manifest deposits following earlier periods of maritime, themselves as smooth, shallow floors; humid climate. Dune fields of remobilized concentric rings; fans and deltas; and central clastics are common in many mid-latitude mounds. The origin of these crater-filling craters. Aeolian deposits are superposed on materials may be ascribed to various processes all other materials. operating on Mars in the past including Fluvial and fluviolacustrine infill is volcanic, aeolian, fluvial, alluvial, lacustrine, incurred by water flowing into the crater from groundwater, and glacial processes. an outside source. A onetime influx during a Identification of some of these materials in catastrophic flood may flood the crater surficial exposures by morphologic details is without breaching the rim and leave a standing possible, but ambiguity exists for others. In body of water to seep out by infiltration the early days of Mars studies, much of the through the bottom of the crater or evaporate smooth crater fill was attributed to volcanic over time. A graded deposit of rapidly settled action, but as understanding of the importance debris may be expected as a blanket across the of water and wind action in Mars history has crater floor with a top layer of evaporitic grown, these interpretations require re- materials. evaluation. The planet-wide dust cover makes On the other hand, if a sustained flow identification of surface units by chemical spills into the crater for a length of time, a signature difficult. Mars rovers are providing breach is eroded in the crater wall and a delta in-the-field observations of the textural and or alluvial fan will form [2]. Depending on mineralogical detail to make refined the duration and discharge, a standing lake conclusions. may be established and a delta will encroach Floor Materials: Various types of across the crater floor [3, 4]. If the lake fills crater infill are examined in the following the crater, spillover of the downslope crater paragraphs. rim erodes a breach to allow the flow to Volcanic materials may be in the form continue downslope. Once breached, of fluid lava flows or pyroclastic materials continued flow cuts a channel across the floor. either of which may produce a blanketing Pluvial lakes formed by accumulation layer or may mound over vents on the floor. of precipitation during a wetter period on Volcanic ash layers are important as wide- Mars lead to standing-crater-lakes with no spread, intra-crater, marker beds within rather fluvial inlet or outlet. Evidence from Gale undistinctive, wide-spread loess deposits. crater suggests a complex and long lacustrine Aeolian infill produces fine-grained history of hundreds to thousands of years silt or clay-sized materials in flat-lying beds. duration. Sediment source is localized to Abundant dust storms [1] on Mars produce erosion of the crater walls and air fall deposits blanketing layers of aeolian materials in all of dust storms or volcanic ash. Interior fill is craters. Thick loess-like (duststone) deposits at the expense of the crater rim and walls as 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1328.pdf

wall material is shed onto the floor of the craters are more likely to interact with the crater by mass wasting, rain fall runoff, and water table. Crater size controls catchment groundwater seepage. Sediment is coarsest on area. Strata in small craters will show more the edges of the floor and graded to finer lateral continuity than in large craters. material in the interior. Crater floor deposits record episodic Groundwater lakes are expected in influx of water—either fluvial, precipitation, craters that excavate below the water table or or fluctuation in water table height. Deposits in existing craters by a rising water table representing more widespread factors during wetter intervals. Little sediment other including planet-wide dust storms and than evaporates will be deposited from changing climatic conditions are preserved, groundwater sources, but the combination of including the declining water availability and groundwater and surface runoff from the the desertification of the planet. Evaporitic interior rim may produce mixed phyllosilicate deposits are expected as climate becomes and clastic materials. McLaughlin Crater more arid and crater lakes shrink. Correlation contains evidence for Mg–Fe-bearing clays by planet-wide markers will be important in and carbonates that probably formed in an establishing a planet-wide geologic column. alkaline, groundwater-fed lacustrine setting In the absence of deep drilling capabilities, [6]. Long-lived lakes require continuing current and near future assessment of crater surface input or groundwater effluent to floor stratigraphy must rely on exposures by maintain water levels for thousands or cross-crater channels; intercrater faults; or millions of years. Large craters are more erosional remnants (such as in Gale Crater). likely to interact with ground water; therefore, References: [1] Gotzinger, J. P. and lakes in large craters are able to maintain R. E. Milliken, in Sedimentary Geology of water levels for extended periods. Mars, SEPM spec. Publ. 102, 1-48. [2] Groundwater saturation of sediments is Cabrol N. A. and E. A.Grin (2010) in Lakes important to cementation of sedimentary on Mars (Cabrol and Grin) Elsevier, 1-30. materials. [3] Malin M.C. and K. S. Edgett (2003) Glacial and fluvioglacial deposits are Science,, 302 (5652): 1931–1934. [4] Carr M. formed by melting of standing ice within the H. and J.W. Head (2010) in Lakes on Mars crater or by glacial streams breaching the (Cabrol and Grin) Elsevier , 31-68 . [5] crater wall [8]. Crater-bound ice will Grotzinger J.P. et al. (2014) Science , 343 eventually sublimate or form melt-water (6169), 1242777. [6] Michalski J. R., et al. pools. Concentric crater fill is interpreted to (2013) Nature Geoscience 6, 133–138. [7] have formed by glacial ice flow and debris Pondrelli M. et al. (2015) Geol. Soc. Am. Bul., covering [9]. Sediments are unsorted, debris B31225.1. [8] Garvin J. et al. 2002. Lunar carried in the ice which maybe overlain by Planet. Sci: 33. Abstract # 1255. [9] Fastook glacial flour of finely-ground rocks suspended J. L. and J. W. Head (2014) Planetary and in melt water pools. Space Sciences, 91, 60-76. Discussion: Crater size controls important factors in lake deposits. Larger