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Sedimentary Textures Formed by Aqueous Processes, Erebus Crater, Meridiani Planum, Mars

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Sedimentary Textures Formed by Aqueous Processes, Erebus Crater, Meridiani Planum, Mars Sedimentary textures formed by aqueous processes, Erebus crater, Meridiani Planum, Mars J. Grotzinger Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA J. Bell III Department of Astronomy, Space Sciences Building, Cornell University, Ithaca, New York 14853, USA K. Herkenhoff United States Geological Survey, Flagstaff, Arizona 86001, USA J. Johnson A. Knoll Botanical Museum, Harvard University, Cambridge, Massachusetts 02138, USA E. McCartney Department of Astronomy, Space Sciences Building, Cornell University, Ithaca, New York 14853, USA S. McLennan Department of Geosciences, State University of New York, Stony Brook, New York 11794-2100, USA J. Metz Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA J. Moore National Aeronautics and Space Administration Ames, Space Science Division, Moffett Field, California 94035, USA S. Squyres Department of Astronomy, Space Sciences Building, Cornell University, Ithaca, New York 14853, USA R. Sullivan O. Ahronson Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA R. Arvidson Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130, USA B. Joliff M. Golombek Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA K. Lewis Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA T. Parker Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA J. Soderblom Department of Astronomy, Space Sciences Building, Cornell University, Ithaca, New York 14853, USA ABSTRACT vide such a test; the data were collected at a New observations at Erebus crater (Olympia outcrop) by the Mars Exploration Rover significant distance from Endurance crater Opportunity between sols 671 and 735 (a sol is a martian day) indicate that a diverse suite (ϳ4 km) and probably at a different strati- of primary and penecontemporaneous sedimentary structures is preserved in sulfate-rich graphic level. These new observations indi- bedrock. Centimeter-scale trough (festoon) cross-lamination is abundant, and is better cate an abundance of centimeter-scale trough expressed and thicker than previously described examples. Postdepositional shrinkage cross-lamination, intimately associated with cracks in the same outcrop are interpreted to have formed in response to desiccation. other facies characterized by probable desic- Considered collectively, this suite of sedimentary structures provides strong support for cation cracking and soft-sediment deforma- the involvement of liquid water during accumulation of sedimentary rocks at Meridiani tion. The cross-lamination is better expressed Planum. here than at Eagle crater, where the original observations of trough cross-lamination were Keywords: Mars, water, cross-lamination, shrinkage cracks, sedimentary structures. made, and probable sediment desiccation cracks are observed for the first time. INTRODUCTION Each successive crater encountered by Op- Because of its relevance to habitability, the portunity as it crossed the Meridiani plain pro- SETTING AND METHODS search for sedimentary rocks formed in aque- vided new bedrock exposures that enable us Erebus crater is located ϳ4 km south of En- ous depositional and diagenetic settings is a to test and refine this developing model. Op- durance crater (Fig. 1). Both Mars Orbiter La- prime objective of the Mars Exploration Ro- portunity’s investigations at Erebus crater pro- ser Altimeter and stereo Mars Orbiter Camera ver Mission. Outcrops at Eagle and Endurance craters investigated by the rover Opportunity have provided substantial evidence for in- Figure 1. Location of volvement of liquid water in the formation Olympia outcrop. A: Mars and subsequent diagenetic alteration of sedi- Orbiter Camera (MOC) mentary rocks formed in eolian dune and in- image of Erebus crater. Image is subframe of terdune depositional environments (Squyres et MOC image S05–00863 al., 2004; Clark et al., 2005; Grotzinger et al., and its location is near -McLennan et al., 2005). The data have 2.0؇S, 5.8؇W. Sunlight il ;2005 been interpreted to record the formation of luminates scene from left, scale bar is 100 m, sulfate-rich materials during aqueous, acid- and north is toward top. sulfate weathering of precursor basalts supple- B: Complete rover tra- mented by accumulation of salts within pla- verse path as of sol 727, yas. Reworking and transport of these near end of Olympia cam- materials as sand grains by eolian and fluvial paign. Image credit: NASA/JPL/MSSS/Ohio processes to their current site were followed State University. by later alteration via groundwater diagenesis. ᭧ 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; December 2006; v. 34; no. 12; p. 1085–1088; doi: 10.1130/G22985A.1; 5 figures. 1085 (MOC) data suggest that the plains surround- rates and rapidly decelerating current veloci- ing Erebus are ϳ10–15 m higher in elevation ties that are characteristic of base surge flows. than those surrounding Endurance. The cur- For these reasons, the presence of trough rent investigation at Erebus centered on the cross-lamination in the Burns formation at Ea- northwestern margin of the crater, where an gle and Endurance craters is regarded as com- outcrop named Olympia (Fig. 1) was studied pelling evidence for overland water flow comprehensively by Opportunity during sols across the ancient surface of Mars (Squyres et 671–735 (a sol is a martian day). al., 2004; Grotzinger et al., 2005; Squyres et Noting the elevation gain between Endur- al., 2006). This conclusion is independent of ance and Erebus, and with the assumption that the difference in gravity between Mars and bedding is approximately horizontal, Erebus Earth, which affects only the transitions be- should expose a stratigraphic level that is tween bedforms and not their existence; for somewhat higher than the uppermost level ob- constant grain size and flow depth, a given served at Endurance. However, imaging of the transition will occur at a lower flow velocity western rim reveals that the bedrock there has on Mars as compared to Earth, by a factor of been deformed, suggesting folding and/or 0.74 (Grotzinger et al., 2005). faulting, which frustrates detailed correlation Recent observations at Olympia confirm with rocks to the north. these earlier interpretations and suggest that Outcrops at Olympia were imaged under water flow was more extensive across the an- conditions of late afternoon lighting to en- cient Meridiani surface than is evident from hance surface relief, using Pancam’s (pano- Eagle and Endurance craters alone. The Over- ramic camera) L7 and R1 (blue) filters, and gaard locality consists of two rocks studied employing lossless compression for maximum during sols 708–723: upper Overgaard and resolution. Several targets were also imaged lower Overgaard. The bedding sequence in using superresolution sequences as described upper Overgaard contains three units: a lower in Bell et al. (2006). In addition, one rock cross-stratified unit, a middle planar-laminated Figure 2. Trough (or festoon) cross- (Overgaard) was selected for study using the unit, and an upper unit with centimeter-scale lamination in upper Overgaard. A: Note microscopic imager (MI). trough cross-lamination (Fig. 2A). Laminae cross-stratified lower unit, overlain by The exposed surfaces of all rocks analyzed that define the middle planar-stratified unit on- planar-laminated middle unit (indicated by yellow bar), and trough cross-laminated up- here are flat, due to relatively recent eolian lap a scour surface from right to left. Note that abrasion. In contrast, the structural dips of per unit. See text for discussion. Image of five laminae define the middle unit on the Overgaard is one of several taken at differ- these rocks are moderately steep, ϳ15Њ–30Њ. right part of the rock (yellow bar); however, ent lighting geometries using Pancam’s These dips were likely acquired during due to onlap, these decrease in number to one (panoramic camera) 432 and 436 nm filters. impact-generated deformation of the crater or two laminae at the left part of the rock. Low-angle light image was acquired on sol 716, sequence id p2593, 432 nm filter. B: Mi- rim and differential rotation of breccia blocks. Trough cross-laminae of the upper unit scour Nevertheless, key structures and laminae can croscope imager (MI) mosaic of middle and down into the middle unit, from left to right. upper units. Note generally granular texture be traced from rock to rock in many cases, Note additional, stratigraphically higher, pla- and excellent sorting, right-to-left pinch out and stratigraphic facing directions are provid- nar laminae in the middle unit at the far left of middle unit, and stratal truncation and ed by crosscutting relations indicated by stra- downlap of cross-laminae in upper unit (cf. (green arrows), where the scour surface rises tal truncations and onlap. A, red arrows). Mosaic was constructed up through the section. All units are affected from images obtained on sols 721 and 723. in a small way by what is interpreted as soft- C: Digital elevation model (DEM) of MI mo- TROUGH CROSS-LAMINATION saic. Note lack of correlation between to- When subjected to shear stresses created by sediment deformation, but this is
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