sedimentary rocks, JOHN P. GROTZINGER, California Institute of Technology, evaporite minerals, KEN E. HERKENHOFF, accretionary lapilli, diagenetic, sedimentary record, sulfate, Central Michigan University, US Geological Survey Astrogeology Science Center, MARS LAUREN A. EDGAR, Erebus crater, Purdue University, Lakehead University, Duck Bay, USA STEVE W. SQUYRES, US Geological Survey, Victoria, HAYES Division of Geological and Planetary Sciences, JOHN GROTZINGER, Martian Meteorites, Sedimentary Geology of Mars, JOHN W. HOLT, KATHRYN M. STACK, Deposition, Sedimentary Rock Record, Ralph E. Milliken, deposits, Steno, JAMES F. BELL Astronomy Department, Stratigraphy JOHN P. GROTZINGER, Meridiani Planum, Victoria crater, Distributary Network, California Institute of Technology ABSTRACT, environmental history, Massachusetts Institute of Technology, JOHN P. GROTZINGER Department of Geological and Planetary Sciences, Planetary Sciences, meteorites, weathering products, aqueous geochemistry, Laterally Continuous Heterolithic, Laterally Continuous Sulfate, Michigan State University, Complexly Stratified Clay, NICHOLAS J. TOSCA Department of Earth Sciences, Pasadena, California, Mars Exploration Rover Opportunity, flowing water, erosional surface, Cornell University, aqueous fluid, sediment, Cambridge, Massachusetts, MICHAEL P. LAMB, University of Cambridge, JOHN B. SOUTHARD Department, Reynolds numbers, Endurance crater
Sedimentary Geology of Mars John P. Grotzinger and Ralph E. Milliken, Editors CONTENTS Overview The Sedimentary Rock Record of Mars: Distribution, Origins, and Global Stratigraphy JOHN P. GROTZINGER AND RALPH E. MILLIKEN.................................................................................................................. 1 An Atlas of Mars Sedimentary Rocks as seen by HIRISE ROSS A. BEYER, KATHRYN M. STACK, JENNIFER L. GRIFFES, RALPH E. MILLIKEN, KEN E. HERKENHOFF, SHANE BYRNE, JOHN W. HOLT AND JOHN P. GROTZINGER ........................................................................................................... 49 Weathering, Diagenesis, Geochemistry Aqueous Alteration in Martian Meteorites: Comparing Mineral Relations in Igneous-Rock Weathering of Martian Meteorites and in the Sedimentary Cycle of Mars MICHAEL A. VELBEL ............................................................................................................................................................... 97 Geochemistry of Sedimentary Processes on Mars SCOTT M. MCLENNAN........................................................................................................................................................... 119 Sediment Transport and Deposition Were Aqueous Ripples on Mars Formed by Flowing Brines? MICHAEL P. LAMB, JOHN P. GROTZINGER, JOHN B. SOUTHARD AND NICHOLAS J. TOSCA ................................. 139 Source-to-Sink: An Earth/Mars Comparison of boundary conditions
for Eolian Dune Systems GARY KOCUREK AND RYAN C. EWING ............................................................................................................................... 151 Duststones on Mars: Source, Transport, Deposition, and Erosion NATHAN T. BRIDGES AND DANIEL R. MUHS ..................................................................................................................... 169 case studies
Focusing the Search for Biosignatures on Mars: Facies Prediction with an Example from Acidalia Planitia DOROTHY Z. OEHLER AND CARLTON C. ALLEN .............................................................................................................. 183 Stratigraphic Architecture of Bedrock Reference Section, Victoria Crater, Meridiani Planum, Mars LAUREN A. EDGAR, JOHN P. GROTZINGER, ALEX G. HAYES, DAVID M. RUBIN, STEVE W. SQUYRES, JAMES F. BELL AND KEN E. HERKENHOFF .......................................................................................................................................... 195 Terrestrial Analogs Potential Recognition of Accretionary Lapilli in Distal Impact Deposits on Mars: A Facies Analog Provided by the 1.85 Ga Sudbury Impact Deposit PHILIP FRALICK, JOHN GROTZINGER AND LAUREN EDGAR ........................................................................................ 211
Early Diagenesis by Modern Acid Brines in Western Australia
and Implications for the History of Sedimentary Modification on Mars BRENDA B. BOWEN, KATHLEEN C. BENISON AND STACY STORY ............................................................................... 229 Characteristics of Terrestrial Ferric Oxide Concretions and Implications for Mars MARJORIE A. CHAN, SALLY L. POTTER, BRENDA B. BOWEN, W.T. PARRY, LAURA M. BARGE, WINSTON SEILER, ERICH U. PETERSEN AND JOHN R. BOWMAN .................................................................................................................... 253
THE SEDIMENTARY ROCK RECORD OF MARS: DISTRIBUTION, ORIGINS, AND GLOBAL STRATIGRAPHY JOHN P. GROTZINGER Division of Geological and Planetary Sciences, California Institute of Technology
, Pasadena, California 91108 USA e-mail: [email protected]
AND RALPH E. MILLIKEN Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, Indiana 46556 USA ABSTRACT: The last decade of Mars exploration produced a series of discoveries that provide compelling evidence for the existence of sedimentary rocks on Mars. Previously, Mars was regarded principally as a volcanic planet, the dominant surface processes of which were eruption of lavas and pyroclastic deposits, although early studies did recognize valley networks, enormous outflow channels, and the required transport of sedimentary materials to the northern plains of Mars. In contrast, our new view of Mars shows a rich history of interactions between water and the surface, with weathering, transport, and deposition of sediments by water as well as eolian processes. Surprisingly thick accumulations of stratified rocks extend back into the Noachian Era--the oldest of which were likely formed over 4 billion years ago, making these rocks much older than any sedimentary rocks preserved on Earth. Some sedimentary rocks were formed and deposited locally, whereas others accumulated as vast sheets that can be correlated for hundreds of kilometers or farther. Local deposits were formed in alluvial fan, deltaic, sublacustrine fan, and lacustrine environments in addition to deposits that fill canyons and valleys possibly carved during catastrophic floods. These former deposits indicate more gradual erosion and sedimentation, perhaps even involving meteoric precipitation, and they provide support for the notion of clement conditions on early Mars. In contrast, rapid erosion and sedimentation may have occurred within large, regional outflow channels thought to have resulted from outbursts of groundwater. Regionally extensive sedimentary deposits have less obvious origins, but the presence of hydrated sulfate minerals indicates that some of these deposits may have formed as lacustrine evaporites, particularly in the Valles Marineris network of open and closed basins. Others may have involved eolian reworking of previously deposited sulfates, or perhaps aqueous (groundwater) alteration of previously deposited basaltic sediments. Another major type of regionally extensive sedimentary deposit occurs as meter-scale stratification with highly rhythmic organization. These deposits occur in several places in the Arabia Terra region of Mars and are also observed at the top of a 5-km-thick stratigraphic section in Gale Crater. The significant lateral continuity of relatively thin beds, their distribution over broadly defined highs as well as lows, and the lack of strong spectral absorption features indicate that these rocks may be duststones, formed by weak lithification of fine particles that settled from the Martian atmosphere. The most ancient sedimentary deposits on Mars may be dominated by stacked, impact-generated debris sheets, similar to those seen on the Moon, and may include impact melts. In the absence of plate tectonics, it appears that the flux of sediment on Mars has declined over time. Early on, primary sediments may have consisted mainly of impact- and volcanic-generated particles that would have been transported by fluvial and eolian processes. Chemical weathering of fragmented bedrock in the presence of circumneutral pH fluids would have generated Clay Minerals
and carbonates, though the latter are surprisingly rare; weathering under more acidic conditions generated dissolved salts that precipitated as sulfates, halides, and oxides. With time, Mars is regarded to have evolved from a rather wet planet, in which chemical weathering by circumneutral pH fluids was common, to a regime in which more acidic chemical weathering took place and, eventually, to a cold, dry environment dominated by physical weathering. As the flux of impactors and volcanism declined, and as the planet's hydrologic cycle decreased in vigor, the formation of sedimentary rocks also declined. Today the Martian highlands appear to be in a net state of erosion, and outcrops of sedimentary rocks are exposed as a result of wind-driven denudation. This erosion is likely balanced by deposition of sediments in the Martian lowlands.
Orbiter observations of depositional framework, bed-scale textural/morphologic attributes, and mineralogy provide the basis for an "orbital facies" Classification scheme
. Orbital facies include Massive Breccia (MBR); Complexly Stratified Clay (CSC); Laterally Continuous Sulfate (LCS); Laterally Continuous Heterolithic (LCH); Distributary Network (DNW); and Rhythmite (RHY). These orbital facies are observed in several key reference sections, and their succession allows for correlation between widely separated regions of Mars, leading to a more refined understanding of environmental history. The oldest terrains on Mars are dominated by MBR and CSC facies, whereas younger terrains are characterized by LCS, DNW, and RHY facies. However, some occurrences of claybearing DNWand LCH facies may be contemporaneous with large sulfate deposits of the LCS facies, which are typically regarded as Hesperian in age. This indicates that the climatic evolution of Mars may be more complex than a simple global alkalineacidic transition and that important regional variations in aqueous geochemistry and the relative roles of surface waters and groundwaters may be preserved in the Martian sedimentary record.
AN ATLAS OF MARS SEDIMENTARY ROCKS AS SEEN BY HIRISE ROSS A. BEYER Sagan Center at the SETI Institute and NASA Ames Research Center
e-mail: [email protected]
KATHRYN M. STACK AND JENNIFER L. GRIFFES Department of Geologic and Planetary Sciences, California Institute of Technology RALPH E. MILLIKEN University of Notre Dame KEN E. HERKENHOFF United States Geological Survey SHANE BYRNE Lunar and Planetary Laboratory, The University of Arizona JOHN W. HOLT University of Texas at Austin
AND JOHN P. GROTZINGER Department of Geologic and Planetary Sciences, California Institute of Technology ABSTRACT: (a collection of photographs)
AQUEOUS ALTERATION IN MARTIAN METEORITES: COMPARING MINERAL RELATIONS IN IGNEOUS-ROCK WEATHERING OF MARTIAN METEORITES AND IN THE SEDIMENTARY CYCLE OF MARS MICHAEL A. VELBEL Department of Geological Sciences, 206 Natural Science Building, Michigan State University
, East Lansing, Michigan 48824-1115 USA e-mail: [email protected]
ABSTRACT: Many of the minerals observed or inferred to occur in the sediments and sedimentary rocks of Mars, from a variety of Mars-mission spacecraft data, also occur in Martian meteorites. Even Martian meteorites recovered after some exposure to terrestrial weathering can preserve preterrestrial evaporite minerals and useful information about aqueous alteration on Mars, but the textures and textural contexts of such minerals must be examined carefully to distinguish preterrestrial evaporite minerals from occurrences of similar minerals redistributed or formed by terrestrial processes. Textural analysis using terrestrial microscopy provides strong and compelling evidence for preterrestrial aqueous alteration products in a number of Martian meteorites. Occurrences of corroded primary rockforming minerals and alteration products in meteorites from Mars cover a range of ages of mineralwater interaction, from ca. 3.9 Ga (approximately mid-Noachian), through one or more episodes after ca. 1.3 Ga (approximately midlate Amazonian), through the last half billion years (late Amazonian alteration in young shergottites), to quite recent. These occurrences record broadly similar aqueous corrosion processes and formation of soluble weathering products over a broad range of times in the paleoenvironmental history of the surface of Mars. Many of the same minerals (smectite-group clay minerals, Ca-sulfates, Mg-sulfates, and the K-Fesulfate jarosite) have been identified both in the Martian meteorites and from remote sensing of the Martian surface. This suggests that both kinds of samples--Martian meteorites and Mars' surface rocks, regolith, and soils--were altered under broadly similar conditions. Temporarily and locally occurring but likely stagnant aqueous solutions reacted quickly with basaltic/mafic/ultramafic minerals at low waterrock ratios. Solutes released by primary mineral weathering precipitated locally on Mars as cation-rich clays and evaporite minerals, rather than being leached away, as on Earth. The main secondary host minerals for Fe differ between Martian meteorites and Mars' surface materials. In Martian meteorites, sideriticankeritic carbonate is the predominant secondary host mineral for Fe, whereas in Mars' surface materials, ferric oxides and ferric sulfates are the predominant secondary host minerals for Fe. Differences in the initial compositions of the altering solutions are implied, with carbonate/bicarbonate dominating in the solutions that altered Martian meteorites and sulfate dominating the solutions that altered most Mars' surface materials. During impact on and ejection from Mars, Martian meteorites may have been exhumed from depths sufficient to have isolated them from large quantities of Mars' surface solutions. Preejection weathering of the basaltic rocks occurred in grain-boundary fracture microenvironments at high pH values in aqueous solutions buffered by reactions with basalt minerals.
GEOCHEMISTRY OF SEDIMENTARY PROCESSES ON MARS SCOTT M. MCLENNAN Department of Geosciences, State University of New York
at Stony Brook, Stony Brook, New York 11794-2100 USA e-mail: [email protected]
ABSTRACT: Mars has an extensive, long-lived sedimentary record that is complimentary to the terrestrial record, bearing both first-order similarities and first-order differences. The igneous record is composed of basaltic rocks, in fundamental contrast to the granodioritic upper continental crust of the Earth, which in turn dominates the provenance of clastic and chemical sedimentary rocks. The crust and sedimentary mass of Mars on average are older than the terrestrial records, and Mars provides exceptional potential for understanding processes that were active during the earliest history (>3.5 Gyr) of the solar system. Numerous sedimentary minerals have been identified both from orbit and by rovers/landers and include a variety of clays, sulfates, amorphous silica, minor carbonates, and possibly chlorides. The Martian sedimentary mineralogical record is Fe- and Mg-enriched and Na- and K-depleted compared to the terrestrial record, reflecting differing crustal compositions and differing aqueous surficial environments. There is evidence for three distinct sedimentary mineralogical epochs: an early clay-rich era, intermediate sulfate-rich era, and a younger era dominated by secondary iron oxides. This mineralogical evolution likely records desiccation, acidification, and oxidation of the surface over geological time. There is also evidence that surficial processes were controlled by a sulfur cycle, rather than the Carbon Cycle
, over much of Martian geological time, leading to low-pH aqueous conditions. The nature of this S cycle changed over time as volcanic sulfur sources and amounts of near-surface water diminished. There is a linkage between the S cycle and iron/oxygen cycles related to diagenetic oxidation of iron sulfates to form iron oxides. Where studied in detail, weathering is dominated by low pH, with mobility of ferric iron being common. Lack of evidence for expected aluminum mobility indicates that low waterrock ratio conditions prevailed. In Noachian terrains, where clay minerals are common, it is more likely that aqueous conditions were closer to circum-neutral, but detailed study awaits future landed missions. Numerous depositional environments are recognized, including fluvial, deltaic, lacustrine, eolian, and glacial settings. Evaporitic rocks appear common and are characterized by distinctive suites of Mg-, Ca-, and Fe-sulfates and possibly chlorides. A system of chemical divides can be constructed and indicates that the range of observed evaporite minerals can be explained by typical water compositions derived from acidic weathering of Martian crust, and with variable initial pH controlled by HCO3-/SO42- ratios. Several diagenetic processes have also been identified, including complex groundwater diagenetic histories. One process, consistent with experimental studies, that explains the correlation between sulfate and iron oxide minerals seen from orbit, as well as formation of hematitic concretions in the Burns Formation on Meridiani Planum, is oxidation of iron sulfates to form iron oxides. In general, the diagenetic record that has been identified, including incomplete iron sulfate oxidation, limited clay mineral transformations, and absence of amorphous silica recrystallization, indicates highly water-limited postdepositional conditions. Among the most important outstanding questions for sedimentary geochemistry are those related to the quantification of the size and lithological distribution of the sedimentary record, the detailed history of near-surface water, and the origin and history of acidity in the aqueous environment.
WERE AQUEOUS RIPPLES ON MARS FORMED BY FLOWING BRINES? MICHAEL P. LAMB, JOHN P. GROTZINGER Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125 USA e-mail: [email protected]
JOHN B. SOUTHARD Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology
, Cambridge, Massachusetts
02139 USA AND NICHOLAS J. TOSCA Department of Earth science
s, University of Cambridge
, Cambridge CB2 3EQ UK ABSTRACT: The discovery in 2004 by Mars exploration rover Opportunity of sedimentary rocks with centimeterscale trough cross-bedding is one of the compelling lines of evidence for flowing water on the Martian surface. The rocks contain a significant evaporite component mixed with weathered mafic silicates, suggesting that the aqueous fluid in contact with the sediments must have been of very high ionic strength because dissolution features are not observed. Recent thermodynamic modeling indicates that these brines could have had higher densities (by up to a factor of 1.3) and significantly higher viscosities (by up to a factor of 40) than pure water. Because fluid density and viscosity can significantly affect sediment transport mechanics, herein we analyze whether ripples could have been stable bed forms under flowing Martian brines. To this end, we compiled bed form stability diagrams with an emphasis on those studies that have considered high-viscosity fluids. For the case of viscous Martian brines, we find that ripples are stable under modest Shields numbers and low particle Reynolds numbers
. These conditions translate into sediment sizes ranging from sand to gravel, and they are substantially coarser than sediment sizes for equivalent ripple-forming flows in freshwater. It is likely that ripples might also form in silt sizes under viscous brines, but these conditions (i.e., particle Reynolds numbers < 0.1) have not yet been explored in flume experiments, motivating future work. Using flow-resistance equations and assuming steady uniform flow, we calculate that Marian brines must have had flow depths ranging from 0.01 to 1 m and flow velocities of 0.01 to 1 m/s, and been driven by gravity on slopes of 10-4 to 10-2 in order to generate the bed stresses necessary to produce ripples. These conditions seem reasonable given the interdune environment that has been proposed for the Burns formation. In addition to the potential for ripples in much coarser sediments, ripples formed by viscous brines also might be larger in height and wavelength than their freshwater counterparts by as much as a factor of 12. Thus, large (>10 cm heights) and finegrained (<1 mm particle diameter) cross strata would be compelling physical evidence for flowing brines in the Martian past, provided that independent evidence could be provided for a subaqueous (i.e., not eolian) origin of the cross-stratification. Smaller centimeter-scale ripples can also be formed by brines due to flow-depth limitations or lower-viscosity fluids, and therefore the physical sedimentological evidence in support of brines versus freshwater flows may be ambiguous in these cases.
SOURCE-TO-SINK: AN EARTH/MARS COMPARISON OF BOUNDARY CONDITIONS FOR EOLIAN DUNE SYSTEMS GARY KOCUREK Department of Geological Sciences, University of Texas, Austin, Texas 78712 USA e-mail: [email protected]
AND RYAN C. EWING Division of Geological & Planetary Sciences, California Institute of Technology, MC 17025, Pasadena, California 91125 USA *Present address: University of Alabama, Department of Geological Sciences, Tuscaloosa, Alabama 35487 USA ABSTRACT: Eolian dune fields on Earth and Mars evolve as complex systems
within a set of boundary conditions. A source-to-sink comparison indicates that although differences exist in sediment production and transport, the systems largely converge at the dune-flow and pattern-development levels, but again differ in modes of accumulation and preservation. On Earth, where winds frequently exceed threshold speeds, dune fields are sourced primarily through deflation of subaqueous deposits as these sediments become available for transport. Limited weathering, widespread permafrost, and the low-density atmosphere on Mars imply that sediment production, sediment availability, and sand-transporting winds are all episodic. Possible sediment sources include relict sediments from the wetter Noachian; slow physical weathering in a cold, water-limited environment; and episodic sediment production associated with climatic cycles, outflow events, and impacts. Similarities in dune morphology, secondary airflow patterns over the dunes, and pattern evolution through dune interactions imply that dune stratification and bounding surfaces on Mars are comparable to those on Earth, an observation supported by outcrops of the Burns formation. The accumulation of eolian deposits occurs on Earth through the dynamics of dry, wet, and stabilizing eolian systems. Dry-system accumulation by flow deceleration into topographic basins has occurred throughout Martian history, whereas wet-system accumulation with a rising capillary fringe is restricted to Noachian times. The greatest difference in accumulation occurs with stabilizing systems, as manifested by the north polar Planum Boreum cavi unit, where accumulation has occurred through stabilization by permafrost development. Preservation of eolian accumulations on Earth typically occurs by sediment burial within subsiding basins or a relative rise of the water table or sea level. Preservation on Mars, measured as the generation of a stratigraphic record and not time, has an Earth analog with infill of impact-created and other basins, but differs with the cavi unit, where preservation is by burial beneath layered ice with a climatic driver.
DUSTSTONES ON MARS: SOURCE, TRANSPORT, DEPOSITION, AND EROSION NATHAN T. BRIDGES Applied Physics Laboratory, MP3-E171, 11101 Johns Hopkins
Road, Laurel, Maryland 20723 USA e-mail: [email protected]
AND DANIEL R. MUHS US Geological Survey
, MS 980, Box 25046, Federal Center, Denver, Colorado 80225 USA ABSTRACT: Dust is an abundant material on Mars, and there is strong evidence that it is a contributor to the rock record as "duststone," analogous in many ways to loess on Earth. Although a common suite of dust formation mechanisms has operated on the two planets, fundamental differences in environments and geologic histories have resulted in vastly different weighting functions, causing distinct depositional styles and erosional mechanisms. On Earth, dust is derived predominantly from glacial grinding and, in nonglacial environments, by other processes, such as volcanism, eolian abrasion, and fluvial comminution. Hydrological and biological processes convert dust accumulations to loess deposits. Active hydrology also acts to clean dust from the atmosphere and convert loess into soil or erode it entirely. On Mars, glacial production of dust has been minor, with most fine particles probably produced from ancient volcanic, impact, and fluvial processes. Dust is deposited under arid conditions in which aggregate growth and cementation are the stabilizing agents. Thick accumulations result in duststone.
FOCUSING THE SEARCH FOR BIOSIGNATURES ON MARS: FACIES PREDICTION WITH AN EXAMPLE FROM ACIDALIA PLANITIA DOROTHY Z. OEHLER AND CARLTON C. ALLEN Astromaterials Research and Exploration Science Directorate, National Aeronautics and Space Administration, Johnson Space Center, 2101 NASA Parkway, Houston, Texas 77058 USA e-mail: [email protected]
ABSTRACT: The search for martian biosignatures can be enhanced by focusing exploration on locations most likely to contain organic-rich shales. Such shales both concentrate and preserve organic matter and are major repositories of organic geochemical biomarkers in sediments of all ages on Earth. Moreover, it has been suggested that for Mars, accumulations of organic matter may be the most easily detected and least ambiguous of possible biosignatures (Summons et al. 2010). Since current surface conditions on Mars are unfavorable for preservation of organic matter, focusing exploration on locations predicted to contain ancient organic-rich shales would offer one of the best chances of detecting evidence of life--if it ever evolved on the planet. Orbital data can be used to evaluate regional sediment sources and sinks on Mars, and, based on that, facies can be predicted and locations identified that are most likely to contain organic-rich sediments. An example is presented from Acidalia Planitia, in the martian lowlands, where this approach led to the conclusion that facies in southern Acidalia were likely to be dominated by fine-grained, muddy sediments. That conclusion added weight to the hypothesis that mounds in Acidalia are martian versions of mud volcanoes as well as the suggestion that organic materials, if present, would have been deposited in the same area as the mounds. This allowed speculation that potential mud volcano clasts in Acidalia could include preserved, organic biosignatures and, thus, that the mounds in Acidalia constitute an untested class of exploration target for Mars. Facies prediction using orbital data is particularly applicable to planetary exploration where ground truth is most often lacking but orbital data sets are increasingly available. This approach is well suited to the search for potential geochemical biomarkers in organic-rich shales. The approach additionally could be applied to exploration for other categories of biosignatures (such as stromatolites or morphologically preserved microfossils) and to more general planetary objectives, such as the search for hydrothermal sediments, carbonates, or any particular type of geologic deposit.
STRATIGRAPHIC ARCHITECTURE OF BEDROCK REFERENCE SECTION, VICTORIA CRATER, MERIDIANI PLANUM, MARS LAUREN A. EDGAR, JOHN P. GROTZINGER, AND ALEX G. HAYES Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125 USA e-mail: [email protected]
DAVID M. RUBIN US Geological Survey Pacific Science Center, Santa Cruz, California 95060 USA STEVE W. SQUYRES AND JAMES F. BELL Astronomy Department, Cornell University, Ithaca, New York 14853 USA AND KEN E. HERKENHOFF US Geological Survey Astrogeology Science Center, Flagstaff, Arizona 86001 USA ABSTRACT: The Mars Exploration Rover Opportunity has investigated bedrock outcrops exposed in several craters at Meridiani Planum, Mars, in an effort to better understand the role of surface processes in its geologic history. Opportunity has recently completed its observations of Victoria crater, which is 750 m in diameter and exposes cliffs up to ~15 m high. The plains surrounding Victoria crater are ~10 m higher in elevation than those surrounding the previously explored Endurance crater, indicating that the Victoria crater exposes a stratigraphically higher section than does the Endurance crater; however, Victoria strata overlap in elevation with the rocks exposed at the Erebus crater. Victoria crater has a well-developed geomorphic pattern of promontories and embayments that define the crater wall and that reveal thick bedsets (37m) of large-scale cross-bedding, interpreted as fossil eolian dunes. Opportunity was able to drive into the crater at Duck Bay, located on the western margin of Victoria crater. Data from the Microscopic Imager and Panoramic Camera reveal details about the structures, textures, and depositional and diagenetic events that influenced the Victoria bedrock. A lithostratigraphic subdivision of bedrock units was enabled by the presence of a light-toned band that lines much of the upper rim of the crater. In ascending order, three stratigraphic units are named Lyell, Smith, and Steno; Smith is the light-toned band. In the Reference Section exposed along the ingress path at Duck Bay, Smith is interpreted to represent a zone of diagenetic recrystallization; however, its upper contact also coincides with a primary erosional surface. Elsewhere in the crater the diagenetic band crosscuts the physical stratigraphy. Correlation with strata present at nearby promontory Cape Verde indicates that there is an erosional surface at the base of the cliff face that corresponds to the erosional contact below Steno. The erosional contact at the base of Cape Verde lies at a lower elevation, but within the same plane as the contact below Steno, which indicates that the material above the erosional contact was built on significant depositional paleotopography. The eolian dune forms exposed in Duck Bay and Cape Verde, combined with the geometry of the erosional surface, indicate that these outcrops may be part of a larger-scale draa architecture. This insight is possible only as a result of the larger-scale exposures at Victoria crater, which significantly exceed the more limited exposures at the Erebus, Endurance, and Eagle craters.
POTENTIAL RECOGNITION OFACCRETIONARY LAPILLI IN DISTAL IMPACT DEPOSITS ON MARS: A FACIES ANALOG PROVIDED BY THE 1.85 GA SUDBURY IMPACT DEPOSIT PHILIP FRALICK Department of Geology, Lakehead University, Thunder Bay
, Ontario, P7B 5E1 Canada e-mail: [email protected]
AND JOHN GROTZINGER AND LAUREN EDGAR Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125 USA ABSTRACT: Our understanding of the significance and abundance of sedimentary strata on Mars has increased considerably during the last decade. The highly cratered surface of Mars leads to the prediction that impact ejecta deposits, possibly containing accretionary lapilli, should be part of the sediment record. While no impact-induced base surge deposits have been confirmed on Mars, it is likely that they will one day be discovered, and it is important to establish criteria for their recognition in the rock record. The recognition of ejecta deposits containing accretionary lapilli on Mars requires reliable facies models developed from known impact-generated strata on Earth. Sections through ejecta layers formed by the 1850 Ma Sudbury impact event provide data to begin development of such models. These deposits are laterally variable but generally show a vertical decrease in lithic clast size and, where present, an upward fining in accretionary lapilli. In thicker deposits, the accretionary lapillibearing portion of sections generally progresses upward from decimeter-scale beds of clast-supported lapilli interlayered with centimeter-scale sandstone beds, to parallel and undulatory laminated lapilli, and sandstones. These are overlain by lapilli stringers and isolated lapilli in parallel-laminated to cross-stratified sandstone. Both grain size and sedimentary structures indicate a succession deposited by an impact-generated base surge during decelerating flow. Thinner deposits of ejecta, possibly laid down on topographic highs, are commonly massive with reverse and normal grading. We compare the accretionary lapillibearing strata in the Sudbury ejecta deposits to proposed impactgenerated base surge deposits in the Burns formation at Meridian Planum, Mars. Units comprising the Burns formation do not have the internal organization of spherule-bearing layers exhibited by the Sudbury ejecta deposits. Comparison with Sudbury ejecta layers and theoretical considerations indicate that the spherules developed in the Burns formation do not represent grains deposited by a base surge and are most likely diagenetic in origin. However, impact ejecta layers should be present in the sedimentary successions on Mars, and comparison with similar strata on Earth may lead to their eventual identification.
EARLY DIAGENESIS BY MODERN ACID BRINES IN WESTERN AUSTRALIA AND IMPLICATIONS FOR THE HISTORY OF SEDIMENTARY MODIFICATION ON MARS BRENDA B. BOWEN Department of Earth and Atmospheric Sciences, Purdue University
, West Lafayette, Indiana 47907 USA e-mail: [email protected]
KATHLEEN C. BENISON Department of Earth and Atmospheric Sciences, Central Michigan University, Mt. Pleasant, Michigan 48859 USA AND STACY STORY Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907 USA ABSTRACT: Mineralogical and geochemical data collected from multiple sites on Mars suggest that acid saline surface waters and groundwater existed there in the past. The geologic context and sedimentology suggest that these acid saline waters were associated with groundwater-fed ephemeral lakes. Ephemeral acid saline lakes in southern Western Australia (WA) are some of the few known Natural Systems
that have the same combination of extreme acid brine chemistry and lacustrine depositional setting as is observed on Mars. Thus, the WA acid saline environments provide a modern analog for understanding past depositional and diagenetic processes that may have occurred on Mars. Here, we examine surface sediments and sedimentary rocks that have been in contact with acid (pH down to ~1.5) and saline brines (total dissolved solids up to ~32%) in southern Western Australia. Through sedimentological, mineralogical, geochemical, and petrographic analyses, we identify the impacts of early diagenesis in and adjacent to eight acid saline lakes and evaluate the processes that have been important in creating these deposits. The combination of extreme chemistry, spatial variability, arid climate, and reworking by winds and floods contributes to make spatially complex depositional products that are a combination of siliciclastics and chemical sediments. Important syndepositional and very early diagenetic processes in these settings include the chemical precipitation of minerals from shallow groundwaters to form displacive crystals and cements, dissolution/partial dissolution of chemical sediments, replacement/partial replacement of some minerals, cracking due to repeated wetting and drying, and the formation of iron-oxide concretions. Minerals observed in these sediments include a variety of chlorides, sulfates, iron oxides, and phyllosilicates, many of which have textures and mineral associations that suggest authigenic formation. These observations are supported by the chemistry of the modern acid brines, which appear to be supersaturated with respect to these minerals. The range of early diagenetic products, compositions, and textures that are apparent in the WA acid saline lake sediments may provide insights into the processes that influenced the sediments on Mars and the timing of sedimentary formation processes on Mars.
CHARACTERISTICS OF TERRESTRIAL FERRIC OXIDE CONCRETIONS AND IMPLICATIONS FOR MARS MARJORIE A. CHAN AND SALLY L. POTTER University of Utah, Department of Geology and Geophysics, 115 South 1460 E. Rm. 383 FASB, Salt Lake City, Utah 84112-0102 USA e-mail: [email protected]
BRENDA B. BOWEN Department of Earth and Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, Indiana 47907 USA W.T. PARRY University of Utah, Department of Geology and Geophysics, 115 South 1460 E. Rm. 383 FASB, Salt Lake City, Utah 84112-0102 USA LAURA M. BARGE University of Southern California
, Department of Earth Sciences, 3651 Trousdale Parkway, Los Angeles
, California 90089 USA *Present address: Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109 USA WINSTON SEILER University of Utah, Department of Geology and Geophysics, 115 South 1460 E. Rm. 383 FASB, Salt Lake City, Utah 84112-0102 USA *Present address: Chevron North America
Exploration and Production, 9525 Camino Media, Bakersfield, California 93301 USA ERICH U. PETERSEN AND JOHN R. BOWMAN University of Utah, Department of Geology and Geophysics, 115 South 1460 E. Rm. 383 FASB, Salt Lake City, Utah 84112-0102 USA ABSTRACT: Concretions are diagenetic products of cementation that establish significant records of groundwater flow
through porous sedimentary deposits. Common spheroidal ferric oxide concretions form by diffusive coupled with advective mass transfer and share similar physical characteristics
with hematite spherules from Meridiani Planum (Mars "blueberries"), investigated by the Mars Exploration Rover Opportunity. Terrestrial concretions from the Jurassic Navajo Sandstone are not perfect analogs to Mars, particularly in terms of their geochemistry. However, the Navajo Sandstone contains exceptional examples that represent typical concretion characteristics from the geologic record. Both ancient and modern analogs provide information about concretion forming processes and their relationship to porosity and permeability, fluid flow events, subsequent weathering, and surficial reworking. Concretions on Earth possess variable mineralogies and form in a variety of lithologies in formations of nearly all geologic ages. Despite the prevalence of concretions, many unknowns exist, including their absolute ages and their precise nucleation and growth mechanisms. Some opportunities for future concretion research lie in three approaches: (1) New analytical techniques may show geochemical gradients and important textures reflecting biotic (role of bacteria) or abiotic origins. (2) Concretion modeling can determine important formation mechanisms. Sensitivity tests and simulations for different parameters can help show the magnitude of influence for different input factors. (3) New age-dating methods that remove preservational bias and expand the supply of datable material may yield quantitative limits to the timing of diagenetic events beyond what relative cross-cutting relationships can show. The discovery of hematite spherules on Mars has driven efforts to better understand both terrestrial examples of ferric oxide concretions and the competing mechanisms that produce spheroidal geometries. The integration of geologic and planetary sciences continues to encourage new findings in the quest to understand the role of water on Mars as well as the tantalizing possibility that extraterrestrial life is associated with mineral records of watery environments.
JP Grotzinger, RE Milliken