geological society of america special papersdiagenesis has occurred in several stages, as evidenced...

Geological Society of America Special Papers

doi: 10.1130/2011.2483(15) 2011;483;231-247Geological Society of America Special Papers

J.D.A. Clarke and M.C. Bourke recognizing Martian springs

Implications for−−Travertine and tufa from Dalhousie Springs (Australia)

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231

The Geological Society of AmericaSpecial Paper 483

2011

Travertine and tufa from Dalhousie Springs (Australia)—Implications for recognizing Martian springs

J.D.A. Clarke1, † and M.C. Bourke2

1Mars Society Australia, Box 327, Clifton Hill, Victoria 3068, Australia, and Australian Centre for Astrobiology, Biological Science Building, University of New South Wales, Kensington, NSW 2052, Australia2Planetary Science Institute, 1700 East Fort Lowell Road, Suite 106, Tucson, Arizona 85719, USA, and School of Geography and the Environment, University of Oxford, South Parks Road, Oxford OX1 3QY, UK

ABSTRACTThermal spring deposits are features of considerable interest to Mars scientists because of their potential as astrobiological oases and as records of the paleohydrology of the planet. Terrestrial counterparts can assist in recognizing such features on Mars and in developing technologies for their study and sampling. In this paper, we describe one such analog, the Dalhousie Springs complex in central Australia. The Dalhousie Springs complex is one of largest groundwater discharge landforms known on Earth. It is a carbonate-limited precipitation system due to the non-supersaturated ascend-ing water. Spring carbonates are deposited as discrete mounds and outfl ow channels resting unconformably on older units. Although subject to postformation geomorphic modifi cation, the spring deposits persist in the landscape and are recognizable long after the parental spring has shut down. We identify 14 specifi c microfacies belong-ing to seven facies, which form three environmental associations related to specifi c depositional environments. Diagenesis has occurred in several stages, as evidenced by distinctive textures on the deposits. Spring deposits on Mars would potentially be recognized by similar textures (although compositions may be quite different) and similar geomorphic relationships. However, in satellite images, spring deposits may be diffi cult to differentiate from deposits resulting from other processes that produce similar geomorphic features, including mud volcanoes, pingos, and rootless cones. Mineralogical data may assist, but ultimately ground truth will be required.

Clarke, J.D.A., and Bourke, M.C., 2011, Travertine and tufa from Dalhousie Springs (Australia)—Implications for recognizing Martian springs, in Garry, W.B., and Bleacher, J.E., eds., Analogs for Planetary Exploration: Geological Society of America Special Paper 483, p. 231–247, doi:10.1130/2011.2483(15). For per-mission to copy, contact [email protected]. © 2011 The Geological Society of America. All rights reserved.

†[email protected].

INTRODUCTION

The evaporation of salts from the groundwater discharge of chemically saturated waters was an important process in Mars’ past. This process may have formed the extensive

chloride salt deposits of the southern highlands (Glotch et al., 2010; Osterloo et al., 2008, 2010). Carbonates can also be chemically precipitated and have been detected in Martian dust and soil, in rock exposures along escarpments, and in craters. The carbonate in soil and dust is thought to be due to



232 Clarke and Bourke

spe 483-01 page 232

as a guide to the recognition of such features on Mars and their differentiation from superfi cially similar surface constructs (such as other cratered cones). We show that despite signifi -cant diagenesis and landform denudation, microscale fabric can be used to identify depositional environments associated with mound springs (e.g., pools and channels). The classifi ca-tion presented here and the microscale features identifi ed can be utilized by future ground-survey missions to Mars such as the Mars Science Lander.

Figure 1. The Great Artesian Basin. (A) Extent of Great Artesian Ba-sin, showing location of the Dalhousie Springs complex and its re-charge areas (modifi ed from Sibenaler et al. [2000] and used by per-mission). (B) Schematic cross section of Great Artesian Basin from NE to SW showing highland recharge and location of mound springs such as the Dalhousie Springs complex along structural controls. NSW—New South Wales; NT—Northern Territory; QLD—Queensland; SA—South Australia.

the interaction of atmospheric carbon dioxide with fi lms of liquid on particle surfaces, or it may be the product of deeply buried carbonate ejecta. Exposures of carbonate strata may have been formed by hydrothermal aqueous alteration asso-ciated with impact or volcanic events. They may also result from weathering of olivine-rich strata or be the deposits of shallow ephemeral lakes (Pollack et al., 1990; Bandfi eld et al., 2003; Boynton et al., 2009; Ehlmann et al., 2008, 2009; Michalski and Niles, 2010; Morris et al., 2010). An additional environment of surface salt precipitation is at hydrothermal springs. Morphologies and deposits proposed to be formed by hydrothermal springs have been identifi ed on Mars (Squyres et al., 2008; Allen and Oehler, 2008; Rossi et al., 2008; Brak-enridge et al., 1985). However, given the antiquity of Mars landforms and the effective denudation rates over large time scales, pristine carbonate landforms such as mound springs are expected to be rare. In addition, a recent study suggests that extensive carbonate deposits may be buried up to 6 km in the Martian crust, exposed only by impact crater formation (Michalski and Niles, 2010). Therefore, ground-based lander or rover microscale image data may be required to confi rm the mode of carbonate emplacement.

Here, we present a detailed petrographic analysis of a spring system located at the Dalhousie Springs complex in central Australia, one of many springs that are associated with discharge from the Great Artesian Basin (Habermehl, 2001). Artesian springs such as those of the Dalhousie Springs com-plex often construct large mounds of precipitated, deposited, and trapped material. The height of the mound is equivalent to the hydraulic head of the artesian basin. These mounds give rise to the common term for such features, mound springs. Similar mounds have been reported from many locations, including Africa (e.g., Roberts and Mitchell, 1987; Renaut et al., 2002), North America (Gardner et al., 1996), and Europe (Linares et al., 2010). Such deposits can provide important insights into the past and present hydrology and climates of artesian basins and preserve records of the environments at the point of dis-charge (e.g., Linares et al., 2010; Renaut et al., 2002). Conse-quently, there is an extensive literature on the petrography of such deposits worldwide (see reviews of Pedley, 1990; Pen-tecost and Viles, 1994; Ford and Pedley, 1996). However, the study of the full range of carbonate spring deposits is incom-plete. Here, we present a detailed analysis and classifi cation of the microscale fabric of deposits from a carbonate-limited spring system located at the Dalhousie Springs complex in central Australia.

The Dalhousie Springs complex was fi rst identifi ed as a potential Mars analog by Clarke and Stoker (2003), emphasiz-ing its parallel to larger Martian outfl ow features; in particular, the Dalhousie Springs complex lies within a groundwater-eroded depression similar to Martian outfl ow channels, it is a geothermally heated point source, again similar to those postulated for some Martian outfl ow features, and microbial-ites occur at the spring vents, which would on Mars be astro-biological targets. Our object in this paper is to document the petrographic characteristics of the Dalhousie Springs complex



Travertine and tufa from Dalhousie Springs (Australia) 233

spe 483-01 page 233

FIELD SETTING OF THE DALHOUSIE SPRING COMPLEX MARS ANALOG

Location

The Great Artesian Basin underlies 22% of the Australian continent and covers 1.7 million square kilometers (Fig. 1). The Dalhousie Springs complex occurs at the southeastern margins of the Great Artesian Basin. The complex consists of a cluster of more than 60 active springs formed by natural discharge from the Great Artesian Basin (Smith, 1989). A complex mosaic of active and ancient spring deposits and channels is spread over ~1500 km2, and the springs occur in a core zone of ~150 km2 (Fig. 2). Total measured discharge from the Great Artesian Basin is 1.74 GL/d; the unfocused natural leakage through the aquaclude is thought to be approximately equal to this fi gure. Some 54 ML/d are currently discharged by the Dalhousie Springs complex, which equals 3% of the measured total for the Great Artesian Basin. The discharged artesian waters are of low to moderate salinity (700–9400 ppm), near neutral pH (6.8–7.3), and warm (20–46 °C). The elevated temperatures are due to passage of groundwater through deeply buried (up to 3 km) aquifers. The waters also contain high levels of dissolved iron and H2S and <1 ppm dissolved oxygen. The main aquifers of the Great Artesian Basin are the Late Jurassic Alge-buckina Sandstone and earliest Cretaceous Cadna-owie Forma-tion, confi ned by the aquaclude of the Cretaceous Bulldog Shale (Table 1). The aquifers are brought near the surface by the early Neogene Dalhousie anticline, and the groundwater fl ow is focused along a series of faults that breach the anticline’s crest (Krieg, 1985, 1986). Unlike most other springs of the Great Artesian

Figure 2. False-color Landsat 5 image of the Dalhousie Springs complex showing bright blue areas of carbonate and salt precipitation, defi ning the spring core surrounded by maroon-colored silcrete uplands with yellowish-green dunes of the Simpson Desert to the north and east.

TABLE 1. STRATIGRAPHY OF THE DALHOUSIE SPRINGS COMPLEX (SUMMARIZED FROM KRIEG, 1985, 1986)

Outside the Dalhousie Springs complex Inside the Dalhousie Springs complex

Unit Age Lithology Comment Unit Age Lithology Comment ,stlis ,sdnas enilaS enecoloH ahQ

and muds Saline marsh

dna suoeraclaC enecoloH mQ gypsiferous sands,

silts, and muds

Modern spring deposits

Pedirka Formation

Late Cenozoic Silt mantled by gibbers

Eolian Dalhousie Formation

(Qpd)

Late Cenozoic Tufa and travertine Spring limestones

atrenilA Gravel (Qpt)

Late Cenozoic Gypsiferous gravels

Slope deposits, interbedded with Dalhousie

Formation citaremolgnoC ciozoneC

sandstone and hard limestone

Oldest spring-related channel deposits (not

shown on map) and spring limestones (Czm)

Eyre Formation (Tsi)

Eocene Silcreted kaolinitic quartz sandstone

with minor pebbles

Forms upland rim surrounding

the complex

Oodnadatta Formation (Kmo)

Cretaceous (Albian)

Marine gray silty shale, minor

sandstone and limestone beds

dratauqA

Bulldog Shale (Kmb)

Cretaceous (Neocomian-

Aptian)

Marine carbonaceous shale

dratauqA

Cadna-owie Formation (Klc)

Cretaceous (Neocomian-

Aptian)

Marine fine sandstone, fining

upward to siltstone

refiuqA




spe 483-01 page 234

Basin, those of the Dalhousie Springs complex are fed by waters derived from recharge areas in the southern part of the Northern Territory (Habermehl, 2001). The meteoric nature and warm tem-peratures of the Dalhousie Springs complex deposits place them in the superambient meteogene category of Pentecost (2005).

The Dalhousie Springs complex consists of several nested geomorphic units. On the broadest scale, it occurs as a depres-sion in the crest of a breached anticline, the Dalhousie anti-cline of Krieg (1985, 1986). The ovoid depression formed by the breach is ~45 km long and 35 km wide, elongated along a north-south axis. The depression is at least 60 m below a rim composed of low cliffs of silcreted Eyre Formation sandstones. Groundwater discharge, augmented by rainfall, supports fl ow in Spring Creek, a channel ~50 km long and 5 km wide that fl ows eastward from the Dalhousie Springs complex (Figs. 2 and 3). The central part of the spring complex consists of a north-northeast– to south-southwest–oriented ovoid that is 20 km

Figure 3. Ikonos panchromatic image of Dalhousie Springs complex showing location of places and sites mentioned in the text. Mounds are dispersed throughout the depression; active mounds are identifi ed by the fl ush of black-toned vegetation. Saline deposits are light-toned in the image. Spring outfl ow coalesces to the east along Spring Creek. Locations referred to in text are marked by arrows.

long and 9 km wide; it is a palimpsest mosaic of ancient and modern spring depositions and outfl ows expressed as limestone-topped mesas and ridges, low, wide channels incised into exposed Cretaceous sediments, and the pools and vegetated mounds of active springs (Fig. 4).

The spring deposits rest unconformably on older units, the Cretaceous sediments of the Great Artesian Basin. The main aquifer, the Cadna-owie Formation, is exposed in the core of the Dalhousie anticline (Krieg, 1985), and progressively younger aquatard units, the Bulldog Shale and Oodnadatta For-mation, are exposed at progressively greater distances from the anticline core. Extensive bedrock fractures in these units allow artesian waters to reach the surface.

Within the central part of the spring complex, the springs themselves occur as clusters of both of modern and ancient deposits. Here, we distinguish between pools, and active, rel-ict, and residual mounds (see Table 2). “Pools” are depressions at ground level with active outfl ow channels. Pool margins are well vegetated, and the pools are generally several meters deep. “Active mounds” are well vegetated and characterized by pools, both shallow and deep (several meters), and both outfl ow channels and seepage zones. “Relict mounds” lack pools and outfl ows and are often poorly vegetated. Minor seepage may still occur along the tops and fl anks in some cases. “Residual mounds” consist of eroded limestone-capped mesas.

Lateral Extent of Carbonate Deposition

Field observations indicate that carbonate deposition at the Dalhousie Springs complex is largely restricted to carbonate muds in the pools, crusts in the capillary fringe, and cements in the subsurface, with lesser, patchy carbonate deposition in the soils of islands in the spring outfl ow channels. Modeling of groundwater fl ow and chemistry (Nelson et al., 2007) indicates that ascending waters are at or near saturation, but they are not supersaturated. This is consistent with the fi eld evidence for relatively limited carbonate precipitation.

Large-Scale Features, Facies, and Depositional Environments

Although mounds in the Dalhousie Springs complex may be up to 6 m high, limestones comprise only a small proportion of this. Most of the spring mound is composed of windblown quartz sand and clay pellets that are trapped by vegetation asso-ciated with the spring. The most likely source for these sands is local, the disaggregation of the bedrock in the central part of the Dalhousie Springs complex by weathering and the defl ation of clay-rich surfaces of dry river beds, respectively. Carbonate deposition occurs primarily at the bottom of pools and chan-nels or in the capillary fringe, and there is little or no carbonate precipitation at the surface. Once spring outfl ow has ceased, erosion exposes the limestone cores as mesa-capped outcrops. Erosion of the resistant caps proceeds slower than the weathered



spe 483-01 page 235

TABLE 2. RANGE OF LIMESTONE OUTCROP STYLES AT THE DALHOUSIE SPRINGS COMPLEX Deposit age Observed

environment Deposit morphology Deposit characteristics Interpreted environments

Circular to ellipsoidal ri enotsemil lartseneF sloop demmWarm pools Circular to ellipsoidal deposits Peloidal fossiliferous lime muds

Active deposits

Thermal pools Circular to ellipsoidal senotsemil citilotamortS sloop demmirFlowing channels: peloidal and fossiliferous

sands Proximal outflows on

sides of mound Linear-ellipsoidal, sloping away from

mounds at 5°

Seepage zones: fenestral limestone Distal outflows

(channels) Highly elongate, sometimes sinuous,

sloping <1° Peloidal and fossiliferous sands, reed molds

enotsemil lartsenef ylbbuR sdnuom woL sdnalsi lennahC Relict

deposits Rounded to flat-topped mounds Rubbly fenestral limestone Filled pools

ladiospille ot ralucric latnoziroH rims Stromatolitic limestones Thermal pools ,sporctuo enotsemil ladiospille-raeniL

sloping away from mounds at 5° Rubbly to massive fenestral or nonfenestral

limestone Proximal outflows on sides

of mound suounis semitemos ,etagnole ylhgiH

limestone outcrops sloping <1° Massive nonfenestral limestone sometimes

with sand grains and reed molds Distal outflows (channels)

sdnalsi lennahC enotsemil lartsenef ylbbuR sdnuom latnoziroh woL Residual

deposits Horizontal circular to ellipsoidal

limestone-capped mesas Massive fenestral to nonfenestral limestone Individual spring

ladiospille ot ralucric latnoziroH limestone-capped mesas

Manganiferous travertine limestone breccia-capped mesas

Thermal spring

deppac-enotsemil ladiospille-raeniL mesas, sloping at 5°

Massive fenestral or nonfenestral limestone Proximal outflows on sides of mound

suounis semitemos ,etagnole ylhgiH limestone-capped mesas sloping <1°

Massive fenestral or nonfenestral limestone Distal outflows (channels)

deppac-enotsemil ralugerri ,egraL mesas

Combinations of all residual lithologies Residual spring-outflow complex

Figure 4. Outcrop-scale features of the Dalhousie Springs complex. (A) Aerial view of spring pools. Note the sinuous, vegetation-rimmed discharge channel. The main pool of Dalhousie Springs (160 m long, 50 m wide) is at top center (see Fig. 3 for location). Pools are identifi ed by arrows, active mounds are indicated by x, and relict mounds are indicated by +. (B) Asymmetric, low mesa (a residual mound spring) formed by erosion of a former pool deposit leaving behind a carbonate cap; top is 30 m wide. (C) Oblique aerial view of bedrock fractures highlighted by vegetation patterns tens of centimeters wide and tens of meters long. (D) Gypsum veins (arrowed) in weathered Cadna-owie Formation beneath eroded core of relict spring deposit; fi eld of view is ~2 m.




spe 483-01 page 236

bedrock of the surrounding areas, resulting in the formation of protected pedestals of weathered Cretaceous bedrock beneath the limestone caps.

Spring limestone deposits in the Dalhousie Springs com-plex show a wide range of morphologies, ranging from elongate to equidimensional in shape (Table 2), and in scale, from meters to kilometers in extent. The smaller deposits represent individual springs or outfl ow deposits, and the larger ones represent amal-gamations of many spring and outfl ow deposits. The tops of the deposits range from fl at to sloping, with angles of ~5°. The lime-stones are typically featureless in outcrop and hand specimen, apart from variations in color (white to dark gray) and varying amounts of cavities and quartz sand. There are exceptions, however. Some limestones contain well-preserved impressions of aquatic plants, and those associated with inferred high-temperature vents show a range of features, including crosscutting veins, crusts, stromato-lites, breccias, and multicolored cavity fi lls (Figs. 4–6).

Regardless of their extent, the spring and outfl ow deposits at the Dalhousie Springs complex are carbonate dominant (except for the lowest, fi rst-deposited units, which can be clay-rich) and thin, typically 1–2 m, with the thickest accumulation measured being 3.5 m. All rest either on weathered bedrock or on a thin sand and gravel lag overlying weathered bedrock. The mesa fl anks are composed of weathered bedrock, often extensively veined by gypsum. The limestone exposed in the mesa caps shows a succession of depositional environments, although these can generally be determined only with petrography.

LIMESTONE CLASSIFICATION

Carbonate samples from the Dalhousie Springs complex were collected from active, residual, and remnant spring depos-its. The 25 samples were classifi ed (Table 3) using a system synthesized from Pedley (1990), Pentecost and Viles (1994),

Figure 5. Representative outcrop textures at the Dalhousie Springs complex: (A) Gully wall exposes a section recording an evolving vent to channel sequence. Note rucksack for scale. (B) Brecciated and cavernous thermal vent limestone with internal cements and sediments. Note scriber (130 cm) for scale. (C) Microbial travertines form small stromatolites on the margin of a former high-temperature pool. Scriber (130 cm) for scale. (D) Molds of aquatic plants in former channel deposit. Scriber (130 cm) for scale.




spe 483-01 page 237

Ford and Pedley (1996), and Pentecost (2005) for spring depos-its and of Wright and Tucker (1991) for the overprinting cal-cretes. Our classifi cation relies on the observed macroscopic and microscopic textures to produce a four-level system. This allows a differentiation of spring deposits according to their depositional environments and diagenetic history. Such long-standing approaches in petrography could be readily applied to samples collected from Martian spring deposits by unmanned spacecraft for return to Earth or by astronauts.

The fi rst level of classifi cation divides the textures into travertine and tufa; contrary to Pentecost (2005), we differentiate between travertine and tufa. Travertines are hard, crystalline pre-cipitates consisting of crystal growths, thin laminae, and shrub-like microbial growths (Ford and Pedley, 1996; Pentecost and Viles, 1994). Tufa consists of fi ne-grained carbonate, often with a spongy fabric due to encrustation of macrophytes (Koban and Schweigert, 1993). Other carbonate grains, such as charophytes, invertebrate skeletons, and peloids, may be common (Pedley,

Figure 6. Representative microscopic textures: (A) branching cavities formed by decay of vegetation in fi ne-grained spring limestone from top of mound; (B) travertine fragment in sandy channel limestone; (C) small gastropod on edge of abandoned channel, Dalhousie rock pile; (D) microbial travertine forming small stromatolites with manganese shrubs; (E) platy calcite overgrown by meniscus deposits of pyrolusite; and (F) bladed calcite lining cavity in peloidal wacke-stone, with micritic internal sediment deposited on lower calcite palisade. All photos have a fi eld of view of 2.5 mm; all except C are plane polarized light; C was photographed under crossed polars.




spe 483-01 page 238

1990; Ford and Pedley, 1996). Travertine and tufa fabrics are not mutually exclusive and can be intimately associated because of changing spring discharges and lateral and vertical changes in environments. Where travertine fabrics occur as cavity or frac-ture fi lls in tufa, they are called travertine cements. Tufa cavity or fracture fi lls in travertine are called are called carbonate mud.

The second-level division of travertines is based on whether they consist of microbial fabrics (crusts, stromatolitic shrubs), cements (crystalline crusts and rounded or botryoidal masses), or rafts (thin plates of carbonate; “ice sheets” in terminology of Veysey et al., 2008; Fouke et al., 2000). The travertines in this study include the “sinters” of Koban and Schweigert (1993), which are dense, crystalline limestone, often with a feath-ery texture. Tufas are divided according to the ratio between carbonate mud and other grains. The tufa may be a carbon-ate mudstone with <10% other grains, matrix supported with >10% other grains (wackestone), grain supported with carbon-ate mud (packstone), or grain supported without carbonate mud (grainstone). Lastly, abundant microbial textures, expressed as fi laments, Renalcis-like growths, and clotted textures, form organically bound fabrics (boundstones). Tufa of all composi-tions commonly contains numerous voids (fenestrae) formed from the decay of plant matter. Highly fenestral sediments form boxworks, which contain more open space than sediment.

The third-level subdivision of travertines consists of microbial shrubby and laminated microbialites and palisade and radiating crusts. At this level, tufas are split into massive and

clotted categories. Wackestones, packstones, and grainstones are divided based on composition of their grains. Examples would include peloidal (small rounded grains of amorphous carbonate), wackestone, intraclasts (angular to rounded larger fragments of spring limestone), packstone, or fossiliferous grainstone. Vary-ing amounts of detrital sand and silt can also be present in all tufa types. Quartz is the most common mineral, but plagioclase, potassium feldspar, and micas are also found.

Fourth-level subdivisions are necessary only for fossil-iferous tufa and are based on the nature of the fossil remains present. Examples include: gastropod grainstone, phytoclast wackestone, and ostracod packstone.

GRAINS AND TEXTURES AT THE DALHOUSIE SPRING COMPLEX

While highly variable on an individual textural scale, all the tufa samples consist of fi ve main components in varying ratios: siliceous detritus, carbonate mud, carbonate grains (pel-oids, lumps, and fossils), coarse calcite cements, and voids.

Silicate Grains

The silicate grains consist of silt- to fi ne-sand–sized mate-rial, mainly of angular to subangular, occasionally euhedral quartz. Minor plagioclase and potassium feldspar (microcline)

TABLE 3. CLASSIFICATION OF LIMESTONES FROM DALHOUSIE SPRINGS level dnoceS level tsriF Third level Fourth level

Shrubs Microbial

Laminated Palisade

Crusts Radiating

TRAVERTINES

setalPMassive

Micritic Clotted

Phytoclast Ostracod Mollusc

Fossiliferous

Microbial Peloidal

Intraclastic

Wackestone

Sandy/silty Phytoclast Ostracod Mollusc

Fossiliferous

Microbial Peloidal

Intraclastic

Packstone

Sandy/silty Phytoclast Ostracod Mollusc

Fossiliferous

Microbial Peloidal

Intraclastic

D E T R I T A L

Grainstone

Sandy/silty Clotted

Filamentous

SPRING LIMESTONE

TUFAS ±fenestrae

Boundstone Renalcid




spe 483-01 page 239

of similar size and morphology to the quartz are also present, along with traces of chlorite, muscovite, biotite, pyroxene, and zircon. The grain size is consistent with a source from weath-ered Cretaceous sediments of the marine volcaniclastic sand-stones and siltstones of the Cadna-owie Formation (Krieg, 1985, 1986), as shown in Table 1.

Carbonate Grains

The majority of the carbonate grains seen in these sam-ples are peloids, followed in abundance by lumps, microbial structures, and mollusc fragments. The origin of the peloids is not known. Typically 100 μm in size, they may be the product of local cementation, or they may be fecal pellets excreted by organisms such as molluscs, worms, or crustaceans. The lumps are interpreted as having been formed by the disruption of cemented sediment.

Microbial structures are of two types, calcifi ed fi laments and botryoidal structures forming small shrubs or bushes closely resembling the fossil organism known as Renalcis. Renalcis has been variably identifi ed as a calcifi ed microorganism or a calci-fi ed biofi lm (Stephens and Sumner, 2002).

A range of small mollusc species has been identifi ed in the pools and outfl ow channels of the Dalhousie Springs complex (Ponder, 1989). Small amounts of nonidentifi able fragments of mollusc shells are present in a number of samples. The small number of remains is surprising, given how common these organisms are in the modern pools and channels. This may present a preservational bias; mollusc shells are typically ara-gonitic, and this may be metastable in the spring waters. Cer-tainly, the shell fragments present have undergone leaching and replacement by calcite, supporting this possibility.

Carbonate Mud

Carbonate mud forms very early by evaporation in the cap-illary fringe of the springs, forming what has been described as “spring chalk” (Pedley, 1990). Such carbonate mud is com-monly indurated from the start. Carbonate mud can also form by direct precipitation in the water column of the pools, as shown by the calcareous silt of the pool bottoms. Pool carbon-ate mud is initially unconsolidated, and it can be reworked into peloids and lumps by early cementation and ingestion by detritovores such as molluscs, worms, and crustaceans living in the pools and mound soils. Care must be taken to distinguish between true granular peloids and peloid-like textures formed by the development of clotted fabrics through compaction and aggregation.

Cavities

Most cavities are elongate, rounded, and branching. This strongly suggests that they were formed as a result of sediment deposition around plant material. After lithifi cation, the plant

matter decayed, leaving voids. This is consistent with the obser-vations in the splash zone and capillary fringes of the main Dalhousie Springs pool and at Witjerrie spring mound at Dal-housie, where carbonate precipitates encrust roots and stems close to the waterline. The proportion of cavities of this sort may be taken as a proxy for the amount of vegetation originally present during deposition of the sediment. Thus, tufas that form in vegetated spring-edge environments are interpreted as being highly fenestral. Fenestrae are likely to be absent from the pool sediments because of the absence of pool-bottom vegetation. Other cavities are simply fractures that have undergone varying degrees of recementation.

Cements

Cements are predominantly of calcite. The diverse textures include: blocky to bladed isopachous calcite overgrowths, often contemporaneous with internal sediments, meniscus and pen-dant cavity-lining cements of both micritic and bladed texture of varying thickness, again sometimes associated with internal sediments, and micritic cements.

Noncalcite cements are of clay, goethite, manganese, and gypsum. Clay, manganese, and goethite both form cavity lin-ings and overgrowths and typically show meniscus fabrics. Gypsum occurs as blocky to euhedral lining and (in some cases) fi ll in remnant cavities.

DEPOSITIONAL ENVIRONMENTS

The results of the petrography study are summarized in Table 4. Microfacies, defi ned by the textures in Table 3 and the grains and crystalline components in the previous section, are the result of specifi c microenvironments. Fourteen micro-facies were recognized, belonging to six facies deposited in three major environment types: tufa springs and their outfl ows (ambient), mixed tufa-travertine springs (hot 40–80 °C), and travertine springs (very hot >80 °C). Only tufa spring environ-ments are presently active at Dalhousie, with the exception of the Dalhousie pool, where spring water temperatures can exceed 40 °C. Limestones from the bottom of the pool con-tained rare fragments of travertine and are stained by manga-nese oxides and hydroxides and encrusted by small manga-nese-rich microstromatolites. The two major types of spring limestone observed at Dalhousie Springs complex, crystalline travertines and fi ne-grained tufas, are here interpreted to result from different environmental temperatures (Ford and Pedley, 1996; Pentecost and Viles, 1994). Travertines are interpreted as being associated with higher temperatures (thermogene), and tufas are interpreted as being associated with ambient or near-ambient temperatures (meteogene). Therefore, we identify three main associations—tufa spring deposits, tufa-travertine spring deposits, and travertine spring deposits. Representative examples of microfacies seen in thin sections of limestones from the Dalhousie Springs complex are shown in Figure 6.




spe 483-01 page 240

TABLE 4. MICROFACIES, FACIES, AND ENVIRONMENTAL ASSOCIATIONS OF SAMPLES FROM DALHOUSIE SPRINGS aFnoitaterpretnIseicaforciM cies Depositional environment

Highly fenestral mudstone and wackestone

Wet soil associated with springs, seeps, and stagnant channels

Highly fenestral mudstone-wackestone tufa Marsh

Sparsely fenestral mudstone and wackestone

Fenestral boundstone Vegetated pool

looppeeDenotsekcawdnaenotsduM

looPafutlartsenefnonotylesrapS

Fenestral grainstone and packstone Vegetated channel lennahcnepOenotskcapdnaenotsniarG

lennahccitiropavEkrowxobenotsniarGGrainstone-packstone tufa

Tufa spring

association

Channel

Mudstone-grainstone + travertine Mudstone-grainstone mixed

travertine-tufa Margin

Boundstone + travertine Resurgent hot spring system

looptoHafutcitilotamortshcir-esenagnaMlooPafut-enitrevartenotsdnuoB

Meniscus wackestone-grainstone + travertine Margin of outflow channel

Wackestone-grainstone mixed travertine-tufa

Tufa-travertine spring

association Channel

looPenitrevartlaiborciMtneVenitrevartytalP

“Felted travertine” Organic-rich vent )?(nigramgnirpSenitrevartdeniarg-eniF

Travertine Travertine spring association

Tufa Spring Deposits

Tufa spring microfacies are dominated by fenestral mud-stones and wackestones, with less common boundstones. The abundance of branching vugs was used to differentiate between highly vegetated (shallow-water and terrestrial environments) (Fig. 6A) and less vegetated environments (deep-water, fl owing-channel environments). The presence of boundstone fabrics and relatively abundant charophyte and invertebrate fossils indicate subaqueous environments. Grainstones and packstones imply con-centration of coarse grains and winnowing of mud, and they were used to infer fl owing water associated with outfl ow channels. Tufa pool and outfl ow deposits make up most of the samples, and there is little to distinguish between those formed by present or recent spring activity and those from old spring deposits of mesas.

Tufa-Travertine Spring Deposits

Tufa- travertine spring deposits contain a mixture of tufa and travertine. The framework is constructed of tufa, and the travertine occurs as overgrowths and cavity or fracture fi lls. Two classes are observed, those containing in situ travertine, and those containing transported travertine clasts. In both types, the tufa forms the framework of the limestones.

In samples with in situ travertine, the travertine occurs as cavity and fracture fi lls or as surface coatings. The traver-tine represents an overprint, often localized, by ascending or out fl owing hot waters. Tufa-travertine deposits with traver-tine clasts are interpreted as outfl ow deposits from hot springs (Fig. 6B), or deep pools where travertine forming on the sides has been dissolved and incorporated into detrital sediments.

As noted previously, these deposits are not found at present in Dalhousie, except possibly the hottest springs near the Dalhou-sie pool (Smith, 1989). Tufa-travertine deposits at Dalhousie are therefore interpreted as representing deposition from waters of at least 40 °C (Folk, 1994). Black carbonates formed by segregations

or impregnations of manganese phases similar to those reported from other thermal spring systems (Gruszczynski et al., 2004) occur as muddy infi ll of internal cavities. These occur in the center of fossil spring deposits and are surrounded by colorless travertine. The black carbonates are interpreted as the core parts of spring sys-tems where upfl ow was strongest (Gruszczynski et al., 2004). The closest contemporary example observed was stromatolitic black carbonate from the main pool at Dalhousie.

Travertine Spring Deposits

In travertine spring deposits, the travertine constitutes the framework of the sediment. This contrasts with the tufa- travertine spring deposits, where the tufa constitutes the frame and the travertine overgrows and infi lls the framework.

Only one example of travertine was observed in the area, located in a small terrace that may have originally been a rimmed pool (Fig. 5C). Textures are diverse, with stromato-litic and platy calcite fabrics similar to those commonly formed by very hot to boiling springs (Renaut et al., 2002; Jones and Renaut, 1998; Folk, 1994; Pentecost, 1990). The stromatolitic limestone is stained black in hand specimen. In thin section, the black stain is composed of laminae of vertically oriented shrubs of manganese phases (Fig. 6D). These have also been reported from other hot spring stromatolitic limestones (Chafetz et al., 1998). A caveat exists: In some carbonate-rich springs, such textures are found at lower (but still >45 °C) temperatures (Veysey et al., 2008; Fouke et al., 2000). The low carbonate content of Dalhousie Springs complex waters suggests that higher temperatures are a more reasonable explanation than high carbonate levels for these particular samples.

DIAGENETIC PROCESSES AND SEQUENCE

Diagenesis of the young, surfi cial carbonates is subtle. The principal fabrics are those that construct the framework.




spe 483-01 page 241

The youngest samples, from active mounds and pools, are composed largely or completely of framework-constructing tufa. These are overprinted by later cavity- and fracture-fi lling cements of various types. The cements are composed of four types of material.

1. Blocky to bladed calcite overgrowths of isopachous cements (equal thickness), typical of the phreatic zone (Flügel, 2004), are interpreted as forming either

a. in the saturated zone beneath in pools or channels, or b. in the phreatic zone surrounding springs when the buildup

of mounds resulted in former capillary-zone tufas becom-ing submerged beneath the water table.

Contemporaneous with some of these cements, there are also internal sediments in cavities composed of carbonate mud.

2. Cavity-lining meniscus and pendant cements are typical of vadose zone environments (Flügel, 2004). They include both micritic and bladed cements of varying thickness. These are interpreted as either

a. occurring in cavities that were never submerged beneath the water table, or

b. overgrowing the phreatic cements described point 1, indi-cating a lowering of the water table, probably associated with the reduction or cessation of spring discharge.

Contemporaneous internal sediments of carbonate mud may also be associated with these cements.

3. Clay- and goethitic-rich cavity linings typically show menis-cus fabrics. These are interpreted as forming during the most recent phase of diagenesis when the limestone was subject to pedogenic processes only.

4. Precipitation of gypsum in remnant cavities is associated with the evaporation of ephemeral water.

In addition to precipitation, many samples show indica-tion of minor dissolution, typically through the enlargement of existing vugs and fenestrae. No clear timing relationship between dissolution and cementation is evident in the samples, since the dissolution events are seen to both pre- or postdate cementation.

The formation of travertine overgrowths and cavity or frac-ture fi lls of the tufa-travertine association can be seen as a special case of point 1a. Where tufa is infi ltrated by high-temperature (>45 °C) waters, travertine may overprint the earlier tufa. This can occur either through an increase in spring discharge over time, or a shifting in the position of the main discharge location. Coincident with the deposition of travertine, there may be depo-sition of manganese-rich muds as internal sediments or growth of manganese-rich stromatolitic tufa crusts.

No clear signs of neomorphism or recrystallization that can be attributed to the diagenesis of original aragonite were observed in thin section. Aragonitic fossils are progressively recrystallized, however, so such neomorphism would be expected if aragonite were present. The one possible exception would be the “fi ne-grained travertine” from the oldest spring deposits of the Dalhousie rock pile. These possibly could be

formed through neomorphism of aragonite to calcite; however, this interpretation has not been tested against further evidence.

Illustrative textural evolution of Dalhousie Springs com-plex limestones is shown in Figure 7. Two alternate diagenetic pathways are shown, the fi rst (1) showing evolution through distal diagenetic facies, the other (2) showing evolution proxi-mal to upwelling of thermal waters.

DISCUSSION OF DALHOUSIE SPRING DEPOSITS

Limestone-Depositing Environments

We have documented the facies of a poorly described mem-ber of the carbonate spring system at the Dalhousie Springs complex, that of carbonate-limited precipitation. The following are unique aspects of this type of deposit:

1. There is a low abundance of travertine, except in localized areas of high-temperature discharge;

2. there is an absence of cascades and stone waterfalls;3. the majority of preserved deposits are tufa-spring and out-

fl ow deposits; and4. deposition is typically very proximal to outfl ow.

Our evidence suggests that the depositional environments at Dalhousie today are similar to the earliest deposits we docu-mented. This would suggest that the facies are independent of climate change and that any indication of past climatic condi-tions in these deposits is to be found in their trace-element and isotopic chemistry, rather than in their morphology, facies, or microfacies. With respect to the textures, we note that:

1. Microfacies correlate closely with depositional environ-ments and can usefully test hypotheses about the origin of particular deposits formulated from outcrop data.

2. Fourteen specifi c microfacies have been identifi ed, belong-ing to seven facies:

a. highly fenestral mudstone-wackestone tufa facies (highly fenestral mudstone and wackestone microfacies);

b. sparsely to nonfenestral tufa facies (sparsely fenestral mudstone and wackestone, fenestral boundstone, mud-stone, and wackestone microfacies);

c. grainstone-packstone tufa facies (fenestral grainstone and packstone, grainstone and packstone, grainstone boxwork microfacies);

d. mudstone-grainstone mixed travertine-tufa facies (mud-stone-grainstone + travertine microfacies);

e. boundstone-grainstone mixed travertine-tufa facies (bound-stone + travertine, manganese-rich stromatolitic microfacies);

f. wackestone-grainstone mixed travertine-tufa facies (wackestone-grainstone + travertine); and

g. travertine facies (microbial travertine, platy travertine, also “felted” [feathery-dendritic] travertine and fi ne-grained [microsparry] travertine microfacies correspond-ing to the sinter of Koban and Schweigert [1993]).




spe 483-01 page 242

Figure 7. Conceptual diagram showing alternative textural evolutionary pathways of a representative spring limestone during diagenesis. Each diagram is ~5 mm across: (A) Depositional fabric— peloidal silty wackestone formed by capillary-zone precipitation surrounding plant material. (B) Phreatic diagenesis—(1) Rise in groundwater results in decay of plant material, leaving fenestrae, cavities lined by phreatic sediment, localized infi lls by internal sediment, and micritization (fi ne-grained crys-tallization) of aragonite in gastropod shell. Alternatively, (2) rise in high-temperature spring water results in fracturing of sediment and coprecipitation of zoned travertine and black, manganese-rich internal sediment, and micritization of aragonite in gastropod shell. (C) Vadose diagenesis—(1 and 2) Cessation of spring activity and fall in water table lead to precipitation of meniscus and pendant cements of carbonate, clay, and iron oxides, and cavity fi lling by gypsum.




spe 483-01 page 243

3. These were deposited in three environmental associations: a. tufa spring association (highly fenestral mudstone-

wackestone, sparsely to nonfenestral, grainstone- packstone tufa facies) representing deposition at <40 °C;

b. tufa and travertine association (mudstone-grainstone, boundstone-grainstone travertine-tufa facies) represent-ing deposition at 40–80 °C; and

c. travertine association (travertine facies) representing deposition at >80 °C.

4. Diagenesis has occurred in several stages. a. The fi rst stage was phreatic diagenesis, consisting of: (i) blocky to bladed isopachous calcite overgrowths and

associated internal sediments formed either in pools or channels, or in the phreatic zone when buildup of spring mounds resulted in former capillary-zone tufas becoming submerged beneath the water table. Alter-natively, and in some cases, this is overprinted by

(ii) neomorphism of aragonite to fi ne-grained calcite. (iii) In some cases, these textures are overprinted by

deposition of travertine as overgrowths and cavity or fracture fi lls, manganese-rich muds as internal sedi-ments, or growth of manganese-rich stromatolitic tufa crusts (Koban and Schweigert, 1993) and shrubs. This is interpreted as occurring when tufa was infi l-trated by high-temperature (>45 °C) waters, and these processes may overprint the earlier tufa through an increase in spring discharge over time, or a shifting in the position of the main discharge location.

b. The second stage was formation of meniscus and pendant vadose cements and associated internal sediments typical of vadose zone environments, including both micritic and bladed cements of varying thickness. These overgrow the phreatic cements (if present) and indicate a decrease in the water table associated with the cessation of spring discharge.

c. The third stage is evidenced by clay and goethitic-rich cavity linings, typically showing meniscus fabrics and gypsum in remnant cavities. These are interpreted as hav-ing been formed by pedogenic processes.

Figure 8. Vertical environmental succession in selected representative limestone outcrops with fi eld descriptions. Evolving channel, infi lling pool, and thermal spring are from different sections through Dalhousie Rock Pile mesa, and evolving pool is from small mesa near main pool of Dalhousie Springs.

Evolution of Limestone Depositional Environments

In both outcrop and thin section, the limestone deposits show poorly developed vertical environmental successions (Fig. 8). In sections with mixed tufa-travertine lithologies, man-ganese-rich lithologies occur in the upper part of the section only, indicating more reducing conditions deeper in the suc-cession and less reducing conditions higher up. There is also a tendency for the lowermost parts of the section to be more clay rich, indicating less strong carbonate precipitation, and for the uppermost sediments to be highly fenestral, indicating infi ll of pools and channels and proliferation of aquatic vegetation. In the main, however, there are few changes between top and bottom of limestone deposits. We interpret this as indicating a rapid onset and cessation of limestone deposition, linked to rapid pool formation and rapid termination of groundwater sup-ply, possibly occurring over a few years to decades.

Evolution of the Dalhousie Spring Complex Sediments

The age of the oldest deposits in the Dalhousie Springs complex is unknown, but ages are taken to be Miocene and younger (Krieg, 1985, 1986) based on geological and landscape relationships. Spring deposition at various locations within the Dalhousie Springs complex is likely to have been continuous through to the present. We observed the following relationships between age and preserved textures.

Samples from youngest spring deposits are dominated by microfacies deposited in marshy environments. We regard this to be a sampling bias. Microfacies formed in subaqueous envi-ronments are, for the most part, likely to have been too inacces-sible or too poorly indurated for sampling.

The older deposits, from both relict and residual mounds, record almost the full range of microfacies types. Some deposits that are almost absent, however, are the microfacies associated with marshy environments. This too is interpreted as a preserva-tional bias. Marshy environment microfacies will be distributed on the top and margins of spring deposits, where they will be




spe 483-01 page 244

most vulnerable to erosion. The nodular nature of these deposits increases their vulnerability to erosion, whereas the other tufa and travertine types have more massive expressions and therefore tend to be more resistant to erosion.

APPLICATION AS A MARS ANALOG

Importance of Recognizing Possible Martian Springs

Spring deposits are high-value targets in the surface explo-ration of Mars because of their astrobiological signifi cance and potential to elucidate details of groundwater processes and history (e.g., Walter and Des Marais, 1993; Pirajno and van Kranendonk, 2005; Bishop et al., 2004). While a number of pos-sible spring features have been identifi ed from orbit (e.g., Allen and Oehler, 2008; Rossi et al., 2008; Crumpler, 2003), follow-up of remotely sensed features with ground truth from landers and rovers requires a hierarchy of recognition features, from the resolution of satellite images to the microscopic, from kilometers to nanometers. At the smallest scale, Allen et al. (2000) further identifi ed the way in which a range of microfabrics in spring sediments can be used as biomarkers, including submicron min-eralized spheres, mineralized biofi lms, and distinctive surface textures. However, morphological evidence on its own can be a problematic guide (Garcia-Ruiz et al., 2003). Before such micro- and nanometer-scale investigations can even be carried out, pos-sible spring sediments have to be identifi ed in satellite imagery and then followed up by ground-level investigations. A range of possible spring features in orbital imagery is shown in Figure 9, with comparisons from the Dalhousie Springs complex.

Potential Recognition Criteria

The textures described here from the Dalhousie Springs com-plex are common to many different spring deposits in highly vary-ing settings, deposits of very different mineralogy, formed by fl uids of varying temperature and chemistry. They appear independent to be of composition to a large degree. While the deposits of the Dalhousie Springs complex are predominantly calcite, with minor gypsum and pyrolusite, similar textures can be found in most min-eral precipitates associated with cold springs and hydrothermal vents, for example, spring deposits composed of sulfates (Bonny and Jones, 2008), fi ne-grained silica and sulfi des (Rohrlach et al., 1998), opal (Jones and Renaut, 2003), and iron oxides (Fernández-Remolar, 2005). This suggests that, despite the unknown composi-tion of the Martian springs, the textures and relationships described by us and by others (e.g., Allen et al., 2000; Bishop et al., 2004) may hold valid for the recognition of Martian spring deposits.

We suggest fi ve criteria, from the largest to the smallest features:

Kilometers to Decameters (Medium-Resolution Satellite Images)

Spring deposits form disconformable to unconformable surface caps that may be topographic highs. This may be due

to their formation as deposited mounds of precipitated ground-water salts or because the well-cemented deposits have become relatively elevated through lowering of the surrounding land-scape. Apical pits are sometimes present, and their margins can be precipitous.

At the same scale (104–102 m), compositional contrasts between the surrounding rocks and the spring deposits are likely (for example, carbonate over silicate) and should be vis-ible in medium- to high-resolution hyperspectral data, unless obscured by surface dust.

Decimeter to Meter Scale (High-Resolution Satellite and Ground-Level Images)

Spring deposits will be generally conformable to the sur-face but locally crosscutting. Convex-upward layering may be evident. Onlap point source deposits may be present on the original mound fl ank.

Meter to Millimeter Scale (Close-Range, Ground-Level Images)

Mineral assemblages are apparent and include features such as: vein stock works, cavities, internal sediments, lami-nated, botryoidal, and mammaliated crusts, and cavity fi lls; vugs are also common.

Millimeter to Micrometer scale (Close-Up Images, Low-Power Microscopy)

Spring deposits show highly complex internal textures with crosscutting stratigraphic relationships. Possible micro-bial textures may be present, as may biomorphic but otherwise inorganic mineral precipitates.

Micrometer to Nanometer Scale (SEM [Scanning Electron Microscope], AFM [Atomic Force Microscope])

Possible preservation of microbial textures may be present, along with biomorphic inorganic precipitates.

Caveats Regarding Biological Features

The Dalhousie Springs complex is an oasis in the desert, and the sediments contain abundant evidence for complex life forms including molluscs, ostracods, stromatolites, Renaclis, and carbonate-encrusted plant remains. The obvious question therefore is: Do these features reduce the signifi cance of the Dalhousie Springs complex sediments as a Mars analog? We think not. First, the main precipitation mechanisms for deposi-tion of spring sediments are cooling (Bonny and Jones, 2008) and degassing (Jones and Renaut, 1998; Linares et al., 2010). Consequently, the bulk of the textures shown in this study are associated with rapid, inorganic precipitation of carbonate. Although the role of microbes in precipitation remains prob-lematic (see Folk, 1994), in the Dalhousie Springs complex, biogenic components are, for the most part, incidental compo-nents, best typifi ed by the plant remains, where the plants have only provided a substrate for precipitation. However, if these



spe 483-01 page 245

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spe 483-01 page 246

are subtracted, the remaining textures—the carbonate mud, cavity fi lls, crusts, and cements—would, we suggest, appear much the same in abiotic systems as they do here.

Implication for Mars Exploration

Orbital images, especially high-resolution images such as from High Resolution Imaging Science Experiment (HiRISE), are essential to providing the regional context of Mars surface features. Much can be achieved on the surface of Mars to test hypotheses generated from orbital data using rovers, as the out-standing successes of the Spirit and Opportunity missions have shown. However, such remotely operated observations have their limitations. In studying a Dalhousie-like deposit on Mars, present and future rovers are likely to be hampered by the steep (in some cases precipitous) nature of the outcrop, and the dif-fi cult slopes, which can be both very rocky and very soft at the same time. Robot rovers may be restricted to examining out of context allochthonous blocks on the slopes surrounding ancient spring deposits, rather than accessing in situ material.

Furthermore, much of detailed fabric of spring limestones can only be identifi ed on a scale where the fi eld of view is a few millimeters or less and the resolution is a few microns. This is an order of magnitude higher than can be obtained using instruments onboard current rovers. Higher-resolution images are expected from instruments on future spacecraft, such as the Mars Hand Lense Imager (MAHLI), with a pixel resolution of 13.9 μm, which would be suffi cient to image the structures in Figure 6. The chemistry and mineralogy (CheMin) spectrom-eter has X-ray diffraction (XRD) and X-ray fl uorescence (XRF) capabilities and thereby would yield useful data on composi-tions. However, even if future imagers were linked with a tool such as the RAT (Rock Abrasion Tool) carried by the MER (Mars Exploration Rover) rovers, the microscopes will only be able to examine rough-cut surfaces. By contrast, petrographic thin sections offer micron-scale textural and compositional data, but their manufacture is beyond the capabilities of even the most advanced robotic rovers yet conceived.

Once spring deposits have been identifi ed on Mars through a combination of remote sensing and robotic rover exploration, their further study will require the capabilities of sample return missions and, potentially, human visits to adequately document and interpret these complex features.

ACKNOWLEDGMENTS

This work was funded by National Aeronautics and Space Administration (NASA) grant NNG05GL37G. We would like to thank the Irrwanyere Aboriginal Corporation, the Eringa Native Title claimants, the Wankanguru Native Title claimants, the Department of Aboriginal Affairs and Reconciliation, and the Department for Environment and Heritage of South Austra-lia for allowing us to work in the area. We also personally thank Dean Ah Chee, ranger at Dalhousie Springs, for his personal

assistance and advice during fi eld work. Mars Society Australia provided support during the initial visit to the area. Jim Skinner and an anonymous reviewer provided many helpful corrections.

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Allen, C.C., Albert, F.G., Chafetz, H.S., Combie, J., Graham, C.R., Kieft, T.L., Kivett, S.J., McKay, D.S., Steele, A., Taunton, A.E., Taylor, M.R., Kathie, L., Thomas-Keprta, K.L., and Westall, F., 2000, Microscopic physical bio-markers in carbonate hot springs: Implications in the search for life on Mars: Icarus, v. 147, no. 1, p. 49–67, doi:10.1006/icar.2000.6435.

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