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Hirise Imaging of Impact Megabreccia and Sub-Meter Aqueous Strata in Holden Crater, Mars John A

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Hirise Imaging of Impact Megabreccia and Sub-Meter Aqueous Strata in Holden Crater, Mars John A Downloaded from geology.gsapubs.org on January 18, 2016 HiRISE imaging of impact megabreccia and sub-meter aqueous strata in Holden Crater, Mars John A. Grant Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Rossman P. Irwin III Washington, D.C. 20560, USA John P. Grotzinger Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, Ralph E. Milliken California 91125, USA Livio L. Tornabene Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA Alfred S. McEwen Catherine M. Weitz Planetary Science Institute, Tucson, Arizona 85719, USA Steven W. Squyres Department of Astronomy, Space Sciences Building, Cornell University, Ithaca, New York 14853, USA Timothy D. Glotch Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA Brad J. Thomson ABSTRACT part of the crater (Smith et al., 1999; Pondrelli High Resolution Imaging Science Experiment (HiRISE) images of Holden crater, Mars, et al., 2005) and data from the MRO Compact resolve impact megabreccia unconformably overlain by sediments deposited during two Reconnaissance Imaging Spectrometer for Mars Noachian-age phases of aqueous activity. A lighter-toned lower unit exhibiting phyllosilicates (CRISM) indicate that phyllosilicates likely was deposited in a long-lived, quiescent distal alluvial or lacustrine setting. An overlying compose at least ~5% by weight of all three darker-toned and often blocky upper unit drapes the sequence and was emplaced during later members. However, the broadly similar expres- high-magnitude fl ooding as an impounded Uzboi Vallis lake overtopped the crater rim. The sion of outcrops coupled with observed changes stratigraphy provides the fi rst geologic context for phyllosilicate deposition during persistent in spectral absorptions from member to member wet and perhaps habitable conditions on early Mars. suggest that phyllosilicate abundances are vari- able (Milliken et al., 2007). Keywords: Mars, stratigraphy, aqueous, megabreccia. The lowest member is darker toned, lacks com- plex bedding geometries such as cross-bedding, INTRODUCTION west, damming its lower reach. Water impounded and displays uniform, meter-scale, fl at-lying beds Images from the High Resolution Imaging Sci- in Uzboi Vallis eventually overtopped the crater devoid of large clasts that would be observable at ence Experiment (HiRISE) on the Mars Recon- rim, incising a full entrance breach. The −2300 m >26 cm/pixel scale. Though beds are traceable naissance Orbiter (MRO) (McEwen et al., 2007) elevation of the crater fl oor is the lowest exposed for hundreds of meters, the maximum thickness at ~26−52 cm/pixel scales reveal a sequence of surface of its size within an ~700 km radius, of the lower member is poorly constrained, as exposed impact megabreccia and sedimentary favoring ponding of any emergent groundwater. outcrops do not expose the underlying crater units in Holden crater in Margaritifer Terra, fl oor. Middle member beds are similar to those Mars (26°S, 326°E, 154 km diameter; Fig. 1). IMPACT MEGABRECCIA in the lower member, but lighter toned (Fig. 3), A lower light-toned sedimentary unit display- Multiple HiRISE images reveal outcrops of and larger exposures allow individual beds to be ing meter- to sub-meter-scale bedding is asso- impact materials in the crater walls: variably traced for kilometers. The contact between the ciated with alluvial fans (Moore and Howard, rounded, poorly sorted, chaotically arranged lower and middle members appears gradational 2005) and is capped by an upper dark-toned unit blocks up to 50 m across within a fi ner matrix and conformable, and both lower and middle exhibiting distinct alluvial morphology. These (Fig. 2) often characterized by clastic dikes. The member beds are often expressed as slopes, sug- sedimentary facies are at least 150 m thick and albedo of the blocks is often intermediate between gesting low resistance to erosion. were emplaced during two wet phases: early pro- the brighter matrix and darker eolian drift, and Upper member beds contain lensoidal accumu- longed erosion of crater walls and basin deposi- they commonly stand in negative relief relative lations of meter-scale, darker-toned blocks that tion in a distal alluvial or lacustrine setting was to the more resistant matrix. These characteristics separate some beds, distinguishing them from followed by high-magnitude fl ooding, the timing suggest a possible origin for many blocks as sedi- beds in the lower members. Upper member beds of which can be constrained to the late Noachian mentary materials excavated from the pre-impact are also fl at lying, thinner, and lighter toned than epoch (e.g., Reiss et al., 2004). Using >1 m/pixel Holden basin. Blocks of comparable size occur in lower member beds and can be traced for kilome- images, previous workers hypothesized that the walls of Popigai Crater, Russia (Vishnevsky ters. These layers form cliffs (Fig. 3) and hence these deposits were lacustrine, air fall, or even and Montanari, 1999), and are impact-fragmented are likely stronger than lower and middle mem- glacial in origin (e.g., Parker, 1985; Malin and megabreccia (Grieve et al., 1977) buried beneath bers. The boundary with the middle member is Edgett, 2000; Grant and Parker, 2002; Pondrelli younger crater-fi lling deposits (Melosh, 1989). typically abrupt but conformable, although there et al., 2005; Irwin et al., 2005). Hence, the Holden outcrops are likely the fi rst is one possible geometric discontinuity where Holden crater formed in the Noachian period recognized impact megabreccia on Mars. underlying beds may be truncated. The upper (Scott and Tanaka, 1986; Grant and Parker, member is capped by a thin, dark-toned layer 2002; Moore et al., 2003), interrupted the previ- LOWER UNIT STRATIGRAPHY commonly exhibiting 4–5-m-diameter polygonal ously through-fl owing Uzboi-Ladon-Margaritifer The lower sedimentary unit overlying the fractures (Fig. 3), some of which extend meters (ULM) outfl ow channel system (Grant and megabreccia (Fig. 3) includes three members into the subsurface. A HiRISE image of the east- Parker, 2002), and excavated sediments deposited distinguished by varying albedo, bed thick- ern crater fl oor reveals exposures of the upper by ULM within the preexisting (Early Noachian) ness, and apparent lateral extent. Prior map- member, suggesting that the lower unit is widely Holden impact basin (Schultz et al., 1982) ping shows outcrops and landforms associated distributed in the crater. (Fig. 1). The newly formed crater rim was ~900 m with the lower unit reach a common elevation The thin bedding, lateral continuity, and topo- higher than the fl oor of Uzboi Vallis to the south- of –1960 m (within ~10 m) in the southwestern graphic restriction of the lower unit suggest a © 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, March March 2008; 2008 v. 36; no. 3; p. 195–198; doi: 10.1130/G24340A.1; 5 fi gures. 195 Downloaded from geology.gsapubs.org on January 18, 2016 Figure 3. A: Upper unit and upper and middle members of the lower unit. Thin layer cap- ping the upper unit commonly exhibits 4–5-m-diameter polygonal fractures in map view (inset), including these from ~4 km to the south. B: Middle and lower members (above and below dashed line, respectively) of lower unit. C: Idealized stratigraphic sec- tion for Holden crater. Sedimentary section in C is ~150 m thick, though the unknown thick- ness of lower unit, lower member, makes this a minimum estimate. Stratigraphic positions of units in A and B are indicated in C by con- necting lines. North is up in A and down in B. High Resolution Imaging Science Experiment (HiRISE) image PSP_001468_1535_RED was used for A and B and image PSP_002154_1468 was used for inset in A (image scale of both is 26 cm/pixel). members. Occasional pulses of channelized alluvium account for lenses of blocky material in the upper member (Howard et al., 2007), and the poly gonally fractured material capping the lower unit may represent a terminal playa phase. The lower unit predates the Uzboi rim breach Figure 1. Elevation data (inset) and mosaic of Mars Global Surveyor Mars Orbiter Camera images over THEMIS (thermal emission imaging system) data of southwest Holden crater, (drainage divide) and introduction of alloch- layered sedimentary deposits, and rim breach created by Uzboi Vallis. Inset covers 19°S–32°S, thonous sediments into the crater, requiring 27°W–40°W and elevations from +2500 m (red) to –2500 m (purple). Contours indicating that observed phyllosilicate spectral signatures –1960 m (white) and –2060 m (black) are indicated. High Resolution Imaging Science Experi- (Milliken et al., 2007) were derived locally. If ment (HiRISE) images within fi gure boundaries are labeled in yellow. White numbers indicate the phyllosilicates result from in situ weather- locations of Figures 2–5. North is up. ing, then their varying abundance upsection likely records changing environmental condi- tions. Alternatively, the phyllosilicate-bearing contention (Glotch, 2006). The generally block- sediments may predate the crater and were poor nature, parallel bounding surfaces, and ele- eroded from crater walls, and the varying abun- vation distribution of the deposits argue against dance refl ects changing diffusional degradation their emplacement as impact ejecta. of the walls, runoff, and/or erosional exposure Distinguishing a distal alluvial versus lacus- of more resistant materials. trine depositional environment is a challenge for the Holden lower unit deposits and some ter- UPPER UNIT STRATIGRAPHY restrial strata (Winston, 1978; McCormick and A second, shorter aqueous phase occurred dur- Grotzinger, 1993). The lower unit is exposed in ing breaching of Holden’s rim by Uzboi Vallis. eroding fan fronts, and the upper member incor- Associated discharge into the crater locally eroded porates some large rocks, suggesting an alluvial lower unit deposits and emplaced an upper unit origin.
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