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 not dis- cussed further here. pography and bedding in left-central part of shallow, subaqueous flows with moderate cur- upper unit, indicating primary origin of stra- rent velocities, fine- to medium-grained sand Trough cross-stratification in the upper unit tal (lamina) geometries. DEM was created will spontaneously form highly sinuous-crested forms a bed set with at least 3–4 cm of ap- by first holding fixed position and angles for image 1M192204573IFF64KWP2956M2F1 ripples with amplitudes of as much as a few parent thickness. Blue arrows in Figure 2A point to three distinct troughs, as indicated by (sol 721) and then adjusting positions and centimeters (Southard, 1973; Southard and angles for all other images (from sols 721 Boguchwal, 1990). The diagnostic attribute of basal truncation and concave-upward geome- and 723) to fit first image. DEMs were cre- such ripples is exposed in cuts transverse to try. Several superimposed sets show basal ated for each stereo pair and merged to- flow, where laminae have a trough-shaped or scouring and backfilling by concave-upward gether. Image processing on merged DEM to occasionally convex-upward cross-laminae removed seams, and DEM was overlain festoon geometry (Rubin, 1987a). Cross- transparently on image mosaic. lamination of this type and scale is not known (red arrows). Truncation surfaces and backfill- to develop in eolian flows and, to our knowl- ing of subjacent sets are well expressed in the edge, has not been observed in deposits re- left-center part of the upper unit, where scal- A mosaic of MI images was constructed to sulting from volcanic- or impact-generated loped cross-lamination (Rubin, 1987b) is de- coincide with the middle and upper units of base surges. The recent suggestion that the veloped; each trough-shaped subset truncates upper Overgaard; this provides additional sup- Meridiani deposits may result from base surge (scallops) the subset to its left and shows a porting detail of the grain-scale variability in processes (McCollom and Hynek, 2005; small angle of climb from left to right. primary lamination (Fig. 2B). Furthermore, Knauth et al., 2005) is inconsistent with both Two processes, fluctuating flow and superim- the MI mosaic was used to construct a digital textural and geochemical data (Squyres et al., posed bedforms, can form scalloped cross- elevation model (DEM) of the outcrop topog- 2006). Very likely, small (centimeter) scale, lamination. Unfortunately, it is not possible to raphy (Fig. 2C); the rock surface is flat at the sinuous-crested ripples are not stable under distinguish between the two in this case due relevant scale across the left-center region the conditions of high particle sedimentation to a lack of bedding-parallel exposures. where the upper unit is exposed, and other-

1086 GEOLOGY, December 2006 Figure 3. Centimeter-scale, trough (or fes- toon) cross-lamination is abundant in Corn- ville. Center part is highlighted and approx- imates area of Figure 4A. Exposed surface is largely transverse to flow, with small com- ponent of climb from right to left. Note abun- dant scour surfaces that truncate underly- Figure 4. Enlargement of center part of Fig- ing concave-up laminae, overlain by ure 3. A: Well-developed trough cross- downlapping cross-laminae of next set. lamination. In left center of image, set has Rock is ~30 cm wide and 50 cm long. Note classic concave-up lower bounding surface moderately steep dips of laminae exposed (up arrow), and upper boundaries are along left side of crack at center part of low- marked by two intersecting concave-up sur- er boundary of rock. These indicate that true faces (down arrows). Internal cross-laminae thickness of this cross-laminated unit must are truncated by upper bounding surface on be less than apparent thickness, which right, and that bounding surface is then equals long dimension of rock (~50 cm); downlapped by cross-laminae of superja- Figure 5. Penecontemporaneous cracks typ- correcting for dip (~30؇), true thickness of cent set. Cross-laminae show small angle of ically are narrow and cut across lamination centimeter-scale cross-lamination is likely climb from right to left. B: Terrestrial trough for more than 5 cm. A: Lower Overgaard. still to be several tens of centimeters, thick- cross-lamination (Cretaceous Mesa Verde Note that truncated laminae adjacent to est observed so far at Meridiani. Super- Group) shows strong geometric similarity cracks (small arrows) are deflected upward resolution image of Cornville acquired sol with that preserved in Cornville (cf. A). along crack margins, and cracks have lat- 705 using Pancam’s (panoramic camera) Note similar (centimeter) scale of cross- eral spacing of several centimeters. This su- 482 nm filter (sequence id p2576). lamination. However, these cross-laminae perresolution image was acquired on sol differ from those in A in that they show 698 using Pancam’s (panoramic camera) small angle of climb from left to right (rather 482 nm filter, sequence id p2572. B: Skull wise bedding shows no systematic relation- than right to left) and cut is more parallel to Valley. Numerous penecontemporaneous ship with respect to the minor local slopes. flow direction. cracks (small arrows) crosscut lamination, some oblique to bedding. Note characteris- This demonstrates that the cross-lamination is tic upward-deflected laminae along crack a primary attribute of the rock rather than an margins; termination of prominent crack in artifact of erosion. SHRINKAGE CRACKS center of rock at discrete bedding plane; Trough cross-lamination is also well dis- Olympia outcrops expose distinct cracks and truncation of upward-deflected laminae played at a rock named Cornville (Fig. 3). that developed during or shortly after deposi- along discrete bedding planes in center and upper parts of rock (large arrows with T). Cornville is ϳ1 m from Overgaard and has a tion of sandy sediment. The cracks are present This superresolution image was acquired on moderate dip toward the upper part of the in several rocks, including lower Overgaard sol 713 at 13:48:35 using Pancam’s 482 nm rock. The entire rock is cross-laminated (Fig. and Skull Valley (Fig. 5). Cracks are oriented filter, sequence id p2589. C: Prism cracks 3), representing a true stratigraphic thickness perpendicular to bedding and typically extend formed in terrestrial eolian interdune de- of 20–30 cm. Centimeter-scale trough cross- to depths of 10 laminae ( 5 cm or more). pression, Jurassic Navajo Sandstone, Tuba ϳ Ͼ ϳ City, Arizona. In cross section, cracks are lamination is well developed near the center Crack width generally does not exceed 1 mm, expressed as narrow lines, with spacing of of the rock. The geometry and scale of these and remains approximately constant for the several centimeters. Note upward deflection cross-laminae sets are nearly indistinguishable length of the crack. Crack spacing typically is of laminae adjacent to cracks (arrows), sim- from terrestrial analogs formed in subaqueous several centimeters. Truncated laminae com- ilar to what is seen in Overgaard and Skull flows (Fig. 4). Cornville trough cross-laminae monly are deformed along crack margins; Valley at Erebus crater. Ruler is subdivided in centimeters. Inset shows polygonal pat- sets are as thick as 1–2 cm and as wide as 2– edges typically are rotated upward, forming tern in plan view; coin is 18 mm in diameter. 3 cm in sections that are most likely transverse curl-up structures (see Allen, 1982, Figs. 13– to flow. The rock surface exposes a cut that is 25). In some cases, cracks splay upward into largely transverse to flow, with a small com- zones of massive or convolute bedding, but dition to related faults. Textures that result ponent of climb from right to left. The scale then are capped abruptly by succeeding un- from the impact process include crush zones, and geometry of this cross-lamination are crit- deformed laminae. small-scale thrust faults, and the authigenic ically different from the structures formed in One possible interpretation of the cracks is breccia described by Shoemaker and Kieffer base surge deposits, as illustrated by Mc- that they are related to the Erebus crater- (1974). However, these impact-related textures Collom and Hynek (2005) and Knauth et al. forming impact, which might be expected to postdate and overprint the features described (2005). include swarms of small-scale fractures, in ad- here, which are instead bounded and truncated

GEOLOGY, December 2006 1087 by younger laminae, and are associated with would then extend upward through the sedi- mineralogy of outcrops at Meridiani Planum: Earth and Planetary Science Letters, v. 240, zones of soft-sediment deformation, including ment veneer. In this manner the cracks are p. 73–94. the curl-up structures that show clear evidence able to maintain their narrow width, yet cut Demicco, R.V., and Hardie, L.A., 1994, Sedimen- for plastic deformation. across bedding for many laminae. In terrestrial tary structures and early diagenetic features of The cracks described here are interpreted as analogs, precipitation of salts is a key part of shallow marine carbonate deposits: Tulsa, Oklahoma, SEPM (Society for Sedimentary shrinkage cracks formed during desiccation of the process, and results in the addition of fresh Geology), 265 p. damp sediments. They have direct analogs in material to the crack void space (Kocurek and Fischer, A.G., 1964, The Lofer cyclothems of the terrestrial environments where intermittent Hunter, 1986). Once cracked, and filled with Alpine Triassic, in Merriam, D.F., ed., Sym- wetting and drying of laminated sediments re- sediments and/or precipitated salts, thermal posium on cyclic sedimentation: Kansas Geo- logical Survey Bulletin 169, p. 107–149. sult in development of narrow but deep cracks stresses may cause the crack to close. How- Grotzinger, J.P., Arvidson, R.E., Bell, J.F., III, Cal- (Chavdarian and Sumner, 2006; Demicco and ever, because the addition of fresh material re- vin, W., Clark, B.C., Fike, D.A., Golombek, Hardie, 1994; Kocurek and Hunter, 1986). In duces the crack volume, the margins may be M., Greeley, R., Haldemann, A., Herkenhoff, K.E., Jolliff, B.L., Knoll, A.H., Malin, M., plan view these cracks form well-developed subjected to compressive forces that can lead McLennan, S.M., Parker, T., Soderblom, L., polygons, with widths of several centimeters to buckling of adjacent laminae. Sohl-Dickstein, J., Squyres, S.W., Tosca, N.J., to several decimeters. Because the cracks ex- and Watters, W.A., 2005, Stratigraphy and tend to significant depths, the crack-bounded CONCLUSIONS sedimentology of a dry to wet eolian deposi- Rocks exposed at the Olympia outcrop, Er- tional system, Burns Formation, Meridiani sediment volumes have shapes similar to Planum, Mars: Earth and Planetary Science prisms, and thus these cracks are commonly ebus crater, show a distinct set of primary Letters, v. 240, p. 11–72. referred to as prism cracks (Fischer, 1964). In stratification geometries and shrinkage cracks Knauth, L.P., Burt, D.M., and Wohletz, K.H., 2005, terms of process, prism cracks differ from or- that are consistent with deposition and sub- Impact origin of sediments at the Opportunity landing site on Mars: Nature, v. 438, dinary desiccation cracks in that they involve sequent modification under aqueous condi- p. 1123–1128. multiple desiccation events, as frequently as tions. Trough cross-lamination is distributed Kocurek, G., and Hunter, R.E., 1986, Origin of po- after deposition of each lamina or after de- on a regional basis, at least across the ϳ4 km lygonal fractures in sand, uppermost Navajo position of a few laminae (Chavdarian and traverse examined by Opportunity, and this and Page Sandstones, Page, Arizona: Journal of Sedimentary Petrology, v. 56, p. 895–904. Sumner, 2006; Demicco and Hardie, 1994; suggests that overland aqueous flows were McCollom, T.M., and Hynek, B.M., 2005, A vol- Kocurek and Hunter, 1986). Prism cracks are widespread. Prism cracks provide strong evi- canic environment for bedrock diagenesis at developed in a broad range of environments, dence for iterative shrinkage and expansion of Meridiani Planum on Mars: Nature, v. 438, sediments during desiccation. This may have p. 1129–1131. ranging from tidal flats to interdune McLennan, S.M., and 30 others, 2005, Provenance depressions. occurred whenever the capillary fringe of the and diagenesis of the Burns formation, Meri- The terrestrial shrinkage cracks shown in groundwater table rose to intersect the surface. diani Planum, Mars: Earth and Planetary Sci- Figure 5C are developed along an interdune Subsequent drying would have induced crack- ence Letters, v. 240, p. 95–121. Rubin, D.M., 1987a, Cross-bedding, bedforms, and bounding surface in the Jurassic Navajo Sand- ing, as commonly happens in playa lake or paleocurrents: Tulsa, Oklahoma, Society of stone and form discrete prisms 10–15 cm sabkha-type environments. Intermittent wet- Economic Paleontologists and Mineralogists, wide extending to depths as great as 11 cm. ting and transient flooding of interdune de- 187 p. pressions during groundwater migration pro- Rubin, D.M., 1987b, Formation of scalloped cross- These prism cracks formed by wetting and bedding without unsteady flows: Journal of drying of a truncation surface that separates vide a simple model that is consistent with Sedimentary Petrology, v. 57, p. 39–45. sets of eolian strata. Such surfaces are regard- observations from all three craters (and sev- Shoemaker, E.M., and Kieffer, S.W., 1974, Guide- ed to have formed during hiatuses in sedi- eral intervening outcrops) examined to date. book to the geology of Meteor Crater, Arizo- na: Tempe, Arizona State University, Center mentation, followed by deflation to the water for Meteorite Studies, 66 p. table (Kocurek and Hunter, 1986). By analo- ACKNOWLEDGMENTS Southard, J.B., 1973, Representation of bed config- gy, Martian strata in lower Overgaard and We gratefully acknowledge the entire Mars Ex- uration in depth-velocity-size diagrams: Jour- ploration Rover (MER) science and engineering nal of Sedimentary Petrology, v. 41, Skull Valley are interpreted to have become teams for making this investigation possible. We p. 903–915. intermittently wet so that prism cracks could thank Mark Rosiek, Donna Galuszka, and Bonnie Southard, J.B., and Boguchwal, L.A., 1990, Bed form along discrete surfaces. Minor oscilla- Redding for constructing the microscope imager configurations in steady unidirectional flows. tions of the water table would have been suf- digital elevation model of upper Overgaard. Dawn Part 2. Synthesis of flume data: Journal of Sumner provided valuable discussion, and Jim Sedimentary Petrology, v. 60, p. 658–679. ficient to accomplish this. The upward deflec- Cowher helped with field work at terrestrial analog Squyres, S.W., Grotzinger, J.P., Arvidson, R.E., tion of laminae to form curl-up structures is sites. We also thank Philip Allen, Mike Leeder, and Bell, J.F., III, Calvin, W., Christensen, P.R., developed because a desiccating layer under- an anonymous referee for helpful reviews. The Na- Clark, B.C., Crisp, J.A., Farrand, W.H., Her- kenhoff, K.E., Johnson, J.R., Klingelhofer, G., goes more evaporation than the underside of tional Aeronautics and Space Administration funded the overall MER project. Knoll, A.H., McLennan, S.M., McSween, the same layer, leading to differentially greater H.Y., Jr., Morris, R.V., Rice, J.W., Jr., Rieder, R., and Soderblom, L.A., 2004, In situ evi- shortening, and thus curling, of the upper REFERENCES CITED surface. dence for an ancient aqueous environment at Allen, J.R.L., 1982, Sedimentary structures, their Meridiani Planum, Mars: Science, v. 306, Curl-up structures are a well-known, almost character and physical basis, Volume 2: Am- p. 1709–1714. diagnostic, feature of desiccated terrestrial sterdam, Elsevier, 663 p. Squyres, S.W., Knoll, A.H., Arvidson, R.E., Clark, Bell, J.F., III, Joseph, J.R., Sohl-Dickstein, J., Ar- sediments (see Demicco and Hardie, 1994, B.C., Grotzinger, J.P., Jolliff, B.L., McLennan, neson, H., Johnson, M., Lemmon, M., and Sa- S.M., Tosca, N., Bell, J.F., III, Calvin, W., Far- their Fig. 46). 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1088 GEOLOGY, December 2006