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Short Communication

Sulphate clastic injectites at Sinus Meridiani on Mars

Forese Carlo Wezel a,n, Davide Baioni a,b

a Planetary Geology Research Group, University of Urbino, Campus Scientifico Sogesta, 61029 Urbino, Italyb Dipartimento di Fisica e Scienze della Terra, Università di Parma, Parco area delle Scienze 157a, 43100 Parma (PR), Italy

a r t i c l e i n f o

Article history:Received 20 June 2013Received in revised form1 October 2013Accepted 6 November 2013Available online 13 November 2013

Keywords:MarsSinus MeridianiDykesInjectitesEvaporite remobilizationDiapirism cyclicityUplift and exhumation

a b s t r a c t

Based on stratigraphic sections, the 1500 m-thick sulphate-based “Sinus Meridiani complex” appears tobe composed of a superposition of three lithologically similar stratigraphic units, each ending with aresidual mesa caprock associated with planation surfaces. The three exposed plains display polygonalfault networks, nodular anydrite/gypsum dykes located along subvertical polygonal fractures, andpresumed pipe-like fluid flow structures. Such vertically intrusive fluidized bodies are interpreted asindicative of subsurface evaporite remobilization and injection, which probably occurred near activelyrising evaporite diapirs originating from an autochthonous “mother” layer situated underneath theMeridiani region. Renewal of diapirism is inferred to have caused repeated post-depositional cycles ofuplift and erosional exhumation during relatively recent times.

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1. Introduction

As documented from orbit, the units exposed in MeridianiPlanum represent the uppermost portion of a thick sequence oflayered succession (Hynek et al., 2002; Arvidson et al., 2003;Edgett, 2005). Such beds have been further investigated in detailby the Mars exploration rover (MER) Opportunity, revealing thatthe bedrock- units exposed on the Meridiani Planum surface arecomposed of layered sulphate deposits, interpreted as being depos-ited subaqueously in a playa-like or interdune lake setting (Squyreset al., 2004; Grotzinger et al., 2005; Jolliff et al., 2006). The sulphatedeposits were subsequently modified by rising groundwater toproduce secondary diagenetic features such as hematite-rich con-cretions (Squyres et al., 2006; Arvidson et al., 2006).

Alternative explanations involving non-aqueous and non-evaporative formation and alteration mechanisms for the Meri-diani sulphates include the “volcanic hypothesis” (Hynek et al.,2002; McCollom and Hynek, 2005), the “surge deposits fromimpact origin” theory (Knauth et al., 2005), and the “ice-weath-ering model” (Niles and Michalski, 2009; Hynek, 2009). Suchalternative non-marine hypotheses appear to have been proposedin an attempt to avoid addressing the enormous amount of evapo-rite precipitation which would have necessarily occurred in a hugebody of briny water to produce the observed features.

In our opinion these interesting formation hypotheses sug-gested for the deposition and alteration of the Meridiani sulphate

beds contain serious problems related to their inconsistency withgeological and mineralogical field characteristics.

The article of Edgett and Parker (1997) is one paper that doesattribute the sedimentary deposits of Meridiani Planum to marinesedimentation occurring sometime in early Martian history. Thehypothesis presented is generally rejected by the Mars sciencecommunity, in part because of arguments that an ocean coveringMeridiani Planumwould have much more water in it thanwould bereasonable for Mars. However, more recent work by Perron et al.(2007) has revealed that the assumptions were likely not correct,reaffirming the plausibility of an ancient northern ocean on Mars.

In this paper we report the presence in the sulphate outcrops ofpost-depositional sedimentary features indicating the occurrence ofsignificant clastic remobilization processes in the Sinus Meridianiregion. In order to obtain a firm footing in geologically observed facts,we first review the stratigraphic description of the sulphate succes-sion that has been studied in an intra-crater section of an unnamedcrater, called crater “A”, located near 2113′02″N, 2130′06″W (Fig. 1).The stratigraphic context provides reliable data for interpretation ofthe observed clastic structures, which are described in the secondportion of this study. Our stratigraphic observations documentimportant phenomena such as the cyclic nature of the sulphatesuccession, planation surface development, and regional uplift.

2. Stratigraphic setting

2.1. Outcrop description of section inside crater “A”

No visible plain is exposed at the base. The section referred toas the “Lower Sulphate Group” is approximately 800 m thick andconsists of three formations, which from bottom to top are:

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Unit 1—Stratified sulphate bedsThis formation is composed of undulose parallel beds display-ing low-angle—cross-bedding. When unaltered, the beds dis-play white-cream-coloured nodular outcrops under theyellowish–brown patina and exhibit distinct scalloped edges.Unit 2—Massive nodular intervalThis formation is characterized by a massive nodular intervalapproximately 150 m thick, covered by blue aeolian sand.Unit 3—Bedded mesaThe capped resistant mesa displays large-scale polygonal crackpatterns bounded by clastic dykes. The mesa forms a protectivecapping over the faster-weathering lower beds, particularlyinterval 2. A dissected white-cream-coloured “residual crust”carapace lies directly over the flat plain, originating in theupper bedding surface.

The “Middle Sulphate Group” section overlies a plain surfaceaffected by polygonal structures and consists of a bedded sedi-mentary sequence approximately 750 m thick and containing thefollowing distinctive litho-stratigraphic units:

Basal breccia bedThis unit consists of disorganized white angular/subangular

blocks approximately 3–4 m in size.

Unit 1—Stratified sulphate bedsIt is formed from packets of 4–5 undulating beds exhibitinglow-angle trough cross-stratification and possibly hummockycross-stratification, which is considered to be associated withstorm-dominated shelf settings.

Unit 2—Massive nodular intervalThis interval is approximately 250 m thick and is composed ofwhite nodular sediments covered by dark sand and/or likelyaltered material.Unit 3—Bedded mesaThe sequence is capped by a resistant bedded mesa originatingas a plain surface. The mesa is overlaid by an erosional relict ofwhite “residual crust” carapace.

2.2. Extra-crater outcrops

The outcrops outside crater A represent the “Upper Sulphate Group”of the complete evaporite succession. Because of the lack of naturalrock cuts (sections), it is impossible to estimate the thickness of units1 and 2. The vertical exposure of the strata involves unit 3 as follows:

Unit 1—Stratified sulphate bedsOnly the upper bedding surface of this unit is visible. The erosionalplain appears to be smooth and composed of white-ivory nodularsulphates (likely gypsum outcrops). The plain surface is polygonallyfractured, with the fractures filled with vertical clastic dykes.Unit 2—Massive nodular intervalThis unit is exposed in plain view and is composed of greynodular sulphates which exhibit several circular pipe-likestructures and deformed nodular dykes.Unit 3—Bedded mesaThe resistant white cap overlies a thin abraded grey “residual crust”(yellowish brown in the colour images) characterizedby the presence of striation and large cavities resulting fromdissolution. The upper mesa surface is affected by small polygonalnetworks.

The dominantly evaporitic sulphate succession cropping out in theSinus Meridiani area (referred to here as the “Sinus MeridianiComplex” or SMC) is formed from the superposition of three litholo-gically similar groups of stratigraphic units, with each individual groupending in a mesa cap (Figs. 2,3 and 5). A repetitive arrangement suchas this emphasizes the cyclic nature of the sulphate units.

The predominant lithotype of the complex is a diageneticallygenerated nodular anhydrite, formed as the modified dehydrationproduct of precipitated primary gypsum, possibly lately re-gypsified by water (Wezel and Baioni, 2010). The hypothesizedgypsification is supported by spectral signatures characteristic ofhydrated sulphates that were detected in exposed deposits innorthern Sinus Meridiani (Wiseman et al., 2010).

Another significant point is the great total thickness of the entiresulphate succession, which exceeds 1500 m. If that estimate is correct,wemust consider that the Sinus Meridiani succession of sulphate bedsis probably indicative of submarine precipitation in a huge marinearea, comparable to the ancient “saline giants” of Earth (see Warren,2010). Considering that the sulphate deposits exhibit a spatial exten-sion of more than 3�105 km2 (McCollom and Hynek, 2005) and areat least 700 m thick with a maximum thickness of 1500 m, we arriveat an enormous estimate for the volume of the evaporites present inthe Meridiani region, ranging from 2.1 to 4.5�106 km3. Even largeroriginal volumes may be hypothesized if we consider post-depositional erosion of the anhydrite succession.

Finally, the excellent correlation of the upper mesa surfaces atdistances of hundreds of km demonstrates the remarkable lateralpersistence of the sulphate units.

3. Mesa caprock and gypcrete carapace

Each individual sulphate group exhibits at the top a flat-topped,steep-sided, and duricrust-capped summit plateau or mesa composed

Fig. 1. (A) Northern Sinus Meridiani (NSM) area with location of impact craternamed crater “A” containing the stratigraphic section described in this paper (whitebox) at 2113′02″N latitude and 2130′06″W longitude. Image NASA/USGS/ESA/DLR/FUBerlin (G. Neukum) taken from Google Mars (www.google.com/mars). (B) Impactcrater called crater “A” in NSM described in this study. Numbers highlight thelocation of the observed units: (1) lower sulphate group; (2) middle sulphate group;(3) upper sulphate group. Image CTX P15_006926_1819_XI_01N002W (north direc-tion is toward top of image).

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of an altered anhydrite cap. The caprock probably consists of insolublecomponents of the rock sulphate sequence.

Representing a type of duricrust (cemented weathering crust),the variable thickness brownish gypcrete carapace covers the

surface of the plateau (Figs. 3 and 5). The gypcrete (also calledgypcrust) results from the alteration of the caprock. It may beassumed that the gypcrete carapace consists of gypsum-cementeddissolution residues such as iron-rich ferruginous material atopevaporite deposits from the mesa. The weathering of the evaporitematerial consisted of rehydration of anhydrite that had beengypsified by meteoric waters.

Our interpretation of the mesas as caprock plateaus implies theoccurrence of previous evaporite diapirs in the Sinus Meridianiregion. Residual caprock constitutes the uppermost part of mostevaporite diapirs on Earth (see Jenyon, 1986), and caprock forma-tions signify the arrival of a diapir from depth to the dissolutionlevel (Weinberger et al., 2006).

Furthermore, many salt domes such as the Iranian diapirs arecovered by a thick brownish gypcrete carapace (Bosak et al., 1998).The fractured strata in the crestal zone of the dome were saturatedwith flowing water. These meteoric waters continually dissolvedthe top of the rising evaporite diapir, leaving insoluble impuritiesand less-soluble components such as anhydrite and gypsum,

Fig. 2. Schematic diagram of “sulphate group”, indicating the stratigraphic sub-division into formations, as well as recognition of associated structures exposedmainly on the plains. The white lower plain consists of hydrated sulphate layers(spectral data after Wray et al., 2009), which we interpret as nodular anhydriterocks rehydrated into gypsum (secondary gypsum) at the surface by meteoricwater or below the surface by groundwater during exhumation and exposure. PF,polygonal fractures; CD, nodular clastic dykes; PC, pipe-like conduits; KF, karsticforms. Note that the sulphate evaporite succession consists of a vertical stacking ofthree of these sulphate groups (the lower two of which are located within thecrater under investigation), revealing the megacyclical arrangement of the eva-poritic sequence of Sinus Meridiani. In the upper part the cycles display. significantsub-aerial planation surfaces, interpreted as exhumed plains. They correspond tosedimentary cycles (T–R cycles) composed by marine sedimentation followed byregressive deposition and sub-aerial emergence. Note the possible occurrence ofhummocky-type cross-stratified beds in the Unit 1. See text for details anddiscussion.

Fig. 3. Perspective outcrop view of the “middle sulphate group”, composed of athree-unit stratigraphy represented, from bottom to top, by stratified sulphate beds(unit 1), a massive nodular interval (unit 2), and a bedded mesa (unit 3), which iscovered by a residual gypcrete crust. The succession rests directly upon the flat-topped mesa capping the “lower sulphate group” illustrated in Fig. 5. Image MROHiRISE ESP_011277_1725 (north toward top).

Fig. 5. Irregular patterns of polygonal fractures also affect the flat surfaces atop themesas, for example at the summit of the mid-mesa unit. Note that the verticalpolygonal fractures appear to have been intruded by tabular dykes composed oflight-coloured nodular sulphate sandy material. The occurrence of a dyke approxi-mately 1400 m long is noteworthy. The resistant mesa caprock is almost entirelywrapped in a residual crust carapace referred to as gypcrete or gypcrust, whichcovers many salt domes on Earth such as those in the Iranian salt diapirs (plugs).Image MRO HiRISE ESP_011277_1725 (north toward up).

Fig. 4. Image illustrating the ivory etched plain located at the top of Unit 1, whichexhibits pervasive polygonal fracture networks that are intruded into polygonalfracture plans by subvertical tabular dykes. Although most of the dykes are dust-covered by mantles of aeolian sand, they are thought to be clastic anhydrite/gypsum dykes based on evidence from outcrops with better exposure. Many dykepolygons are quadrangles or triangles, while others display a radial fracture pattern.The average fracture spacing is approximately 400 m. This interpretation requiresthat the clastic intrusions, composed of a sulphate sand/fluid mixture, wereremobilized and injected upwards from depth, exploiting the polygonal tensionalfractures. The occurrence of sulphate intrusive features leads us to infer thepossibility of a mobile evaporite “mother” layer at depth. Image MRO HiRISEESP_011277_1725 (north toward top).

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which accumulated as an evaporite caprock. It is possible that therehydration of diagenetically generated nodular anydrite could becaused by liquid water flow during ice melting.

The implication of our diapiric interpretation is that the SinusMeridiani region appears to have been disrupted and broken intohundreds of small mesas. The bedded mesa units representdeposition that occurred much later (in post Noachian-earlyHesperian times) than the deposition of the stratified sulphatesand massive nodular units (see Malin and Edgett, 2000).

4. Polygonal fracturing

The surface of the Sinus Meridiani plain displays irregularpolygonal fault networks, with a typical spacing of 100–500 mbetween fractures (Fig. 4). On Earth, three main types of verticalfeatures are recognized (Collinson and Thompson, 1982): desicca-tion cracks, sub-aqueous shrinkage cracks, and ice-wedge poly-gons which are interpreted as post-depositional soft sedimentdeformation structures. Hypotheses for their origin include dryingof exposed sediment to produce desiccation shrinkage cracks,shrinkage of mud during syneresis resulting in sub-aqueousshrinkage cracks, and thermal contraction of the host sedimentin extremely cold conditions producing ice wedges. However, noneof these non-tectonic processes appear to be a satisfactoryexplanation for the Martian polygonal patterns (Pechmann, 1980).

The genesis of terrestrial polygonal faults associated withsediment volumetric contraction and water expulsion (de-water-ing of evaporite deposits) in the absence of external tectonic stressis still a topic of debate (Cartwright et al., 2003; Van Rensbergenet al., 2003). We believe that the influence of local or regionaltectonic stress (such as that exerted by the rise of an evaporitediapir) may be necessary for the production of deep fractures andthe propagation of intrusive sand bodies (see Head, 2007).

Diapiric movements may generate a state of stress which canbe reconstructed (Cartwright, 2009). The intrusion of evaporitediapirs is associated with structural changes in the sedimentarycover consisting of fracture along basic tectonic lines, followed byopening of structures (Bosak et al., 1998). The normal faultingaffecting the Meridiani plains indicates a pervasive horizontaltensile stress field, similar to experiments demonstrating exten-sion of the rocks overlying rising diapirs (Davison et al., 1993).

5. The diapiric suite of Sinus Meridiani

Bedding-discordant clastic intrusions may form by remobiliza-tion and subsequent injection of fluidized sandy material intofractures within sufficiently compacted host sediments. Verticalintrusions occurring along polygonal fracture systems cross-cutting the host sulphate succession are manifested as evaporiteclastic dykes and possibly fluid pipes. Both of these intrusivebodies are due to the post-deposition escape of fluid from thesediment. The vertical structures are conduits of sulphate evapor-ite remobilization and injection. The surface and near-surfacemanifestations appearing at Sinus Meridiani and belonging tothe “diapiric suite” are described below.

5.1. Nodular sulphate dykes

Sulphate dykes, probably gypsum dykes, occur on horizontalplanation surfaces as positive, sub-vertical tabular intrusive bodies,whereas pipe-like vertical conduits are visible in profile in verticalsections. Such features are thought to have formed on Earththrough post-depositional remobilisation of nodular sulphate sandfrom deeper levels by fluidisation, upward injection, and

propagation of fluidized nodular sand mixtures (clastic injectites,see Hurst and Cartwright, 2007). We suppose that the dykes andpipes emanated from a deeper diapiric mass composed of a mobileevaporite substratum (Morley, 2003a) underlying the entire Mer-idiani region (autochthonous evaporite layer).

5.2. Polygonal dyke network

In the study area a number of prominent straight dykes,generally in the NE-SW direction, are traceable over distances ofmore than 1 km. Another group of NW-SE dykes is less apparent,scattered, and slightly curved. Typical surface dyke spacing isapproximately 400 m (Fig. 4). Small 3rd order polygons (up to100 m wide) fit into larger 2nd order polygons (200–500 m wide).A 1st order polygon (1–2 km wide) displays a quadrangular shapewhich is bounded by a good example of an exposed clastic nodulardyke (Fig. 5). Several dykes in the upper plain surface alongpolygonal fractures are mantled by aeolian sand (Fig. 4). In goodexposures the unmantled dykes exhibit positive vertical tabularfield characteristics and smooth and sharp sidewall rocks. Thecommon presence of white nodular sands in exposed flat walls isindicative of fracture-filling sulphate nodular material. Thus, it isbelieved that dykes constitute large networks of clastic injectionsof nodular material encased within low permeability fine-grainedevaporites. The clastic sulphate dykes appear to have exploited thepolygonal fractures traversing the three Sinus Meridiani plainsurfaces. The polygonal arrangement of the injectites suggeststhat their geometry and distribution are structurally controlled.Dykes and host sediments are lithologically similar, both beingcomposed of evaporitic sulphates. In our interpretation, the hostsulphates were characterized by a greater compactness at the timeof intrusion.

5.3. Fluid pipe-like conduits

Hollow semi-cylindrical (eroded) columns are visible withinthe stratified sulphate succession. These columns are circular toelliptical in vertical section and range from a few hundred metresto approximately 350 m in internal diameter (Fig. 6B). We believethat the columns could represent escape pipes for fluidizedevaporite vertical intrusions in the host rocks, which are similarto the structures described in terrestrial outcrops by Morley(2003b). The pipe-like structures terminate upwards in small,low-relief mounds with narrower circular to ovoid depressions(approximately 50 to 200 m in internal diameter), which areapparently collapse structures exposed on the upper plain surface.In the partially eroded terminal depressions, salt-glacier-likeextrusions of material are discernible (Fig. 6A), recording the flowof fluidized nodular evaporite through the pipe-like vents.

Piping, focused fluid injection, and sediment venting are there-fore inferred to have occurred in the Martian Sinus Meridiani onoutcrop-based visible structures. Structural control of the fluidflow pathways seems likely, for example through evaporite diapirsand faults, similar to features documented on Earth (e.g., Gay et al.,2007).

5.4. Small diapiric mound

Inside the polygonal fault networks are cone-shaped structureswith a central depression, surrounded by several smaller satellitedepressions. The central elliptical depression is approximately100–200 m in diameter, while the satellite depressions areapproximately 10–20 m across. We tentatively interpret these“pockmark”-like structures as possibly formed by the expulsionof subsurface fluids and/or gas seepage, for instance methane.

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5.5. Structural discontinuities

The presence of discontinuities in the sedimentary section isnecessary to permit the upward migration of fluid sediments. Themajor flow pathways are always associated with tectonic struc-tures, such as fault-fracture zones, evaporite diapirs, and polygonalfaults (Gay et al., 2007 and references therein) which exertstructural control over fluid flow focusing.

6. Clastic remobilization/injection above and near risingevaporite diapirs

The rise of evaporite diapirs is associated with structuralchanges in the sedimentary cover (the overburden). Localizeddeformation within the overburden can generate weakness zones(Jackson and Vendeville, 1994) and abundant faults (Vendevilleand Jackson, 1992). The host sediments are fractured along basictectonic lines, opening fault structures which are exploited formigration of viscous flows of evaporite. Not only is significantfracturing associated with rising evaporite diapirs but also massiveremobilization and injection, as determined from seismic sectionsand cores obtained near the Nini salt diapir in the Danish Northsea (Svendsen et al., 2010 and references therein). The previousauthors describe the injection of mobilized sand upward intoextensional fault planes generated by the tectonic activity of thedome. The fault-guided injected sands, protruding 20–40 m intothe overburden, are considered one of the three different remo-bilization features recognized by Svendsen et al. (2010). Fieldinvestigations west of the Mount Sedom diapir led Marco et al.(2002) to hypothesize that the radial and tangential vertical

fracture system was formed during the rising of a hidden saltdiapir. Clastic dykes in the form of intruding vertical fractureswere associated with the rising of an underlying evaporite dome.

Despite the absence of direct evidence for such intrusive diapirsdue to erosion of the pertinent dome structures, their existence issupported by the previously-described indicators (i.e., subverticalextensional faults, clastic dykes, pipes, caprocks, brownish gyp-crete carapaces, karstic landforms, and megacyclicity).

7. Cycles of uplift and exhumation

It seems probable that the erosion and uplift of the region arerelated to cycles alternating between breaks (temporary cessationof rise) and renewal of evaporite diapirism. Regional uplift ofbroad arches is believed to have caused the regional exhumationthat affected the Meridiani area. We interpret the great degree ofexhumation as due to broad uplift, likely involving multipletopographic domes, of which the flat crestal portion is indicatedby the dissected remnants of the resistant capping mesa (orcaprock plateau) unit which non-conformably overlies the morequickly weathered lower sulphate series.

Based on remnants of three regional planation geomorphicsurfaces denoting temporary interruption of diapiric rise activity,three main uplift phases are hypothesized to have given rise to therespective exhumation episodes. A probable fourth plain surface,which is covered by aeolian basaltic sand, is located at the base ofthe sulphate succession in the crater A. The erosional truncationwas likely sufficiently intense to remove thick sections of exposedsedimentary cover rocks, with formation of planation surfaces(etchplains). The great volume of detritus from the eroded sectionswas deposited elsewhere in a still unknown depressed region(Edgett, 2005). It is possible that the polygonal tensional fracturespresent on the three visible plains surfaces are the result of theseuplift phases triggering disruptive tectonism and lineament reac-tivation in the domed areas. Each of these cycles is regionallyregressive or shallowing upwards, ending at the top in subaerialemergence revealed by karstic forms (Baioni and Sgavetti, 2013).During periods of planation surfaces development the evaporitedeposits at the surface of the diapirs were altered to brownishgypcrete caprock (also called gypcrust). This was also the time inwhich karstification of evaporites occurred (Baioni et al., 2012a).

The karst landforms observed in northern Sinus Meridiani arewell preserved and apparently lack deep reworking or modifica-tion by aeolian erosion. The freshness of the karst landforms andthe absence of features with evidence of aeolian erosion, such asdegraded depression margins or ventifacts, suggest a youngerosional age. Moreover, their observation suggests that solutionalprocesses and water penetration into bedrock persisted over time,and that karstification processes were not as short lived assupposed for karst landforms in other regions on Mars (Baioniand Sgavetti, 2013). Hence, the formation of the brownish sandygypcrust is interpreted as related to the stabilization of the diapiricuplift with consequent chemical weathering of diapir material.The long stable phase of low relief was followed by a shorter andrelatively unstable phase of renewed evaporite diapirism charac-terized by the emergence, formation of high topographic relief,and erosional stripping of regoliths. Moreover, the topographicelevation alone affected climate at a regional scale and might havetriggered a regional climate change causing the onset of a periodof cooling and glaciation (see on Earth Ruddiman, 1997; Ollier andPain, 2000). Ice accumulation near the present-day Martianequator during periods of high obliquity has been speculated byNiles and Michalski (2009) and Hynek (2009). Morphologieswhich seem possibly indicative of mountain glaciers have beenidentified next Sinus Meridiani (Baioni et al., 2012b).

Fig. 6. Features interpreted to be fluid pipe-like structures, observed in plan (A) andsection views (B), suggesting subsurface fluid circulation in this region. (A) Circular toelliptical mounded features outcropping on grey upper plain, located at the top of Unit2, which are interpreted as surface expressions of pipe-like vents (upper left). Theinferred discharge appears to indicate the fluid nature of the extruded material. Thepossible presence of degassing structures cannot be excluded. Image MRO HiRISEESP_011277_1725 (north toward up). (B) Subvertical, roughly semi-cylindrical, hollowconduits, ranging from a few metres to 200–350 m or more in internal diameter (leftside). We suggest a connection of these chimneys with the mounded tops, in spite ofthe narrowing of the last specimens. We interpret the hollow semi-columns present inunit 1 as possible vertical flow pathways for fluidized sulphate evaporites whichflowed into Unit 2 over the grey upper plain surface (see (A)). Image MRO HiRISEESP_011277_1725 (north toward top).

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We suppose that at least the younger exhumation episodeoccurred during Amazonian times, as suggested by the recenterosional age of the karst landforms on the last planation surface.So the cyclic pulses of diapiric uplift, followed by erosional exhuma-tion events exposing the older (late Noachian-early Hesperian)evaporitic sulphate sections, ended in the Amazonian period.

8. Summary of the results

The “Sinus Meridiani Complex” (SMC) appears to be consti-tuted by the superposition of three shallowing-upward sulphatemegacycles, separated by planation surfaces expressing unconfor-mities a caused by subaerial erosion and karstification. Theplanation surfaces were subsequently buried during the followingcycle of sedimentation, which is characterized as the others by atransgressive–regressive nature (that is T–R. cycles). The cycles areattributed to major pulses of diapiric rise and exhumation epi-sodes (Fig. 7). The younger upper cycle with shallow-marinesediments in the lower part, is considered to be Amazonian in age.

We have not identified evidences concerning the age of the twopossibly pre-Amazonian cycles.

The cyclic, dominantly anhydrite SMC succession, possibly partlyweathered to gypsum, reaches a thickness of at least 1.5 km incrater “A”.

A variety of evidence such as sulphate megacycles, mesacaprocks with gypcrete carapaces, and surface and near-surfacemanifestations resulting from intrusion of fluidized clastic mate-rial into fractures (i.e., nodular sulphate dykes and pipe-like fluidconduits), are inferred to document subsurface remobilization/injection processes which likely occurred above uprising evaporitediapirs. We suggest also that the polygonal fracturing affecting thebrittle sedimentary cover is associated with the upward motion ofunderlying intrusive evaporite domes (Marco et al., 2002). Wehypothesize that significant evaporite diapirism, accompanied byinjected sulphate nodular sands and upward fluid migration,affected large parts of the Sinus Meridiani region. This activitymay have formed an evaporite-dome province that was subse-quently removed through deep erosion.

Finally, a similarity is expected between the interior evaporitedomes within the Valles Marineris troughs (see Wezel and Baioni,2010) and the gypsified caprock mesas located inside and outsidethe Sinus Meridiani impact craters. We believe that in both regionsancient buried evaporites recently emerged at the surface fromdeeper levels in the form of mobile allochthonous diapirs, perhapstaking advantage of cratered or troughed interior depressions.Furthermore we assume that the ancient evaporites were depos-ited in similar submarine environments.

The observed outcrop features appear to be problematic forpreviously proposed hypotheses such as volcanic activity(McCollom and Hynek, 2005), impact surge (Knauth et al., 2005),

or ice-weathering (Niles and Michalski, 2009; Hynek, 2009) forthe deposition and alteration of the Meridiani sulphate succession.

Concerning the playa-depression origin attributed by previousauthors (e.g., Squyres et al., 2006; Andrews-Hanna et al., 2007) tothe primary mega-sulphate deposits in the Sinus Meridiani region, wenote that the playa setting does not produces thick-layered evaporitesequences (Sonnenfeld, 1984). Besides, groundwater precipitationleads to the development of crusts in soils, but does not generatebedded evaporite deposits (Sonnenfeld, 1984). Moreover, the diage-netic replacement of precursor gypsum, originating with the coales-cence of nodules which compose the anydrite beds, must occur in asubmarine setting following lengthy contact with brines (Sonnenfeld,1984). Here, we propose a shallow marine platform origin for theformation of the very thick, extensive, and sulphate-dominant eva-poritic “Sinus Meridiani Complex”. This proposal is also based on thepossible evidence in Unit 1 of sedimentary structure, similar tohummocky cross-stratification (HCS), which is thought by Collinsonand Thompson (1982) to be most common in a storm-wave domi-nated shelf setting.

Finally, we believe that the alternative hypothesis presented heremay provide an opportunity for testing again the stratigraphic anddepositional interpretation of the 7 m-thick stratigraphic section ofsulphate strata, explored by the rover Opportunity at MeridianiPlanum. We stress that the middle unit (aeolian sand sheet) andupper unit (damp/wet interdune) observed at Endurance-Eaglecraters, jointly with the younger transition from interdune to aeoliandune exposed at Erebus–Victoria craters (Grotzinger et al., 2005),compare in scaled-down version with the transgressive–regressivephases of our sedimentary cycles. It is to remark that they areexplained on the contrary as cycles of water table rise and fall, inspite of lacking terrestrial analogs of current-ripple cross-laminationexposed in aeolian deposits (Grotzinger et al., 2005). The accumu-lated evidence suggests that the transgressive sulphate sequence atSinus Meridiani, inclusive of the upper part at Meridiani Planum,consists of water-lain sediments deposited in shallow (lower shore-face to inner shelf) seas in different times afterward exhumationepisodes.

Acknowledgement

Vic Baker critically reviewed the manuscript and providedvaluable insights.

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