Parasequence development in the Ediacaran ShuramFormation (Nafun Group,Oman): high-resolutionstratigraphic test for primaryorigin of negativecarbon isotopic ratiosErwan Le Guerrouen, Philip A. Allenn,1 and Andrea Cozzin,2

nDepartment of Earth Sciences, ETH-Zentrum, Zˇrich, Switzerland

ABSTRACT

Neoproterozoic carbonates are known to show exceptional variations in their carbon isotopic ratios,and in the absence of biostratigraphy and a ¢rm geochronological framework, these variations areused as a correlation tool. However, it is controversial whether the carbon isotope record reveals aprimary oceanographic signal or secondary e¡ects such as diagenesis.The Shuram Formation of theNafunGroup of Oman allows a stratigraphic test of this problem.TheNafunGroup (HuqfSupergroup, Oman) in theHuqf area of east-central Oman consists of inner carbonate ramp facies ofthe Khufai Formation overlain by marine, storm-generated, red and brown siltstones of the ShuramFormation.Towards its top, the ShuramFormation is composed of distinctive shallowing-upward, 4^17-m-thick parasequences cropping out continuously over 35 km, which show recessive swaley cross-strati¢ed siltstones capped by ledges comprising wave-rippled, intraclast-rich ooidal carbonate.These storm-dominated facies show a regional deepening in palaeobathymetry towards the south.The carbonates of theShuramFormation are marked by an extreme depletion in13C in bulk rock. d13Cvalues quickly reach a nadir of �12% just above theKhufai-Shuram boundary and steadily return topositive values in the overlying mainly dolomitic Buah Formation.The Shuram excursion is thoughtto be ca. 50Myr in duration and extends over 600m of stratigraphy. Carbon isotopic values show asystematic variation in the parasequence stack, with values varying both vertically through thestratigraphy (�2% per 45m) and laterally in the progradation distance (�1% over 35 km).Thissupports a primary, oceanographic origin for these extremely negative carbon isotopic values andindependently argues strongly against diagenetic resetting.

INTRODUCTION

Neoproterozoic sedimentary rocks record periods of ex-treme swings in carbon isotopic ratios.These swings havecommonly been associated with putative snowball Earthevents (Ho¡man et al., 1998; Ho¡man & Schrag, 2002), butlong-lived (tens ofMyr) major negative excursions are alsofoundwhere there is no evidence for widespread glaciation(Saylor et al., 1998; McKirdy et al., 2001; Zhang et al., 2005;Le Guerroue¤ et al., 2006b). Such excursions show extremevariation from 110 to �12% and appear to be up to tensof millions of years in duration (Kaufman et al., 1993; Hal-verson etal., 2005;LeGuerroue¤ etal., 2006b; S.A. Bowring,

J.P. Grotzinger, D. Condon, J. Ramezani & M. Newall, inreview). The fact that a carbon isotope excursion of thismagnitude can be detected in marine sedimentary rocksfrom several continents suggests that it represents a globaloceanographic phenomenon (Saylor et al., 1998; Condon etal., 2005; Halverson et al., 2005; Le Guerroue¤ et al., 2006b),re£ecting the composition of seawater from which carbo-nate minerals were precipitated. However, the presence ofa carbon isotopic excursion of this amplitude and durationis di⁄cult to explain in terms of mass balance. Such nega-tive values (� 12%) are also well below those associatedwith a complete cessation of biological productivity(Broecker & Peng, 1982). Phanerozoic negative anomaliesare usually short-lived and represent excursions of lessthan � 2% (e.g. Hayes et al., 1999; Hesselbo et al., 2000;Galli et al., 2005).There is therefore continuing debate asto the primary origin of these long-lived excursions understeady-state conditions and their legitimate use as a basi-nal and global correlative tool in the Precambrian.

The Shuram Formation (Nafun Group, Huqf Super-group) crops out extensively in theHuqf area of east-central

Correspondence: Erwan Le Guerroue¤ , Department of EarthSciences, Haldenbachstrasse, 44, ETH-Zentrum, CH-8092Zˇrich, Switzerland. E-mail: erwan@erdw.ethz.ch1 Present address: Department of Earth Science and Engineer-ing, Imperial College London, South Kensington Campus,London SW72AZ, England.2 Present address: ENI ^ Exploration & Production Division,SanDonatoMilanese, Italy

BasinResearch (2006) 18, 205–219, doi: 10.1111/j.1365-2117.2006.00292.x

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Oman. It is extremely well exposed for over 35km in aroughly N-S-oriented escarpment in the north of theHuqfarea, where it displays a stack of shallowing-upward cycles orparasequences (sensu Vail, 1987; Mitchum & Van Wagoner,1991). Parasequences are common in both siliciclastic- andcarbonate-dominated Phanerozoic stratigraphy, but se-quence stratigraphic studies dealing with Precambrian ex-amples are scarce (Christie-Blick et al., 1995; Gehling,2000). The Shuram parasequences are of mixed lithology,with deeper-water siliciclastics passing up into shallower-water carbonate deposits. Siliciclastic input and carbonateproduction were most likely controlled by a subtle balanceof environmental factors driven by relative sea-level change(Tucker, 2003; seeModelling section below).

Carbon isotope stratigraphy assists in shedding light oncorrelation problems in Precambrian strata where biostra-tigraphy and radiometric age data are lacking (Knoll et al.,1986;Kaufman etal.,1993; Cozzi etal., 2004a).The particu-lar bene¢t of an investigation of the parasequences of theShuram Formation is that their carbonate componentsyield carbon isotopic values embeddedwithin a long-termsecular trend that is reproduced throughout the outcropareas of Oman and in the subsurface borehole records ofthe Oman salt basins (Burns &Matter, 1993; Amthor et al.,2003; Cozzi & Al-Siyabi, 2004; Allen & Leather, 2006; LeGuerroue¤ et al., 2006b). At this high-resolution scale, bulkrock inorganic carbon isotopic values are shown to re£ectstratigraphic position within the parasequence stack, andeach individual parasequence shows a predictable trendin d13C values in the direction of sediment progradation.This study therefore provides the sedimentological andstratigraphic context that, independent of diagenetic ar-guments, validates the primary origin of the large negativecarbon isotopic excursion in Oman as an oceanographicphenomenon. By doing so, it supports the use of thePrecambrian highly negative d13C values as a valid correla-tion tool, but of course does not imply that allNeoproter-ozoic carbon isotopic excursions can be used in such amanner.

GEOLOGICAL SETTING

TheNeoproterozoic Huqf Supergroup of Oman crops outmainly in northern (JabalAkhdar) and central (Huqf) areasofOman, the latter representing a palaeohigh duringNeo-proterozoic time, cored by granodioritic crystalline base-ment dated at 8201Ma (Allen & Leather, 2006). TheHuqf Supergroup is subdivided into three groups: theAbu Mahara, Nafun and Ara groups (Glennie et al., 1974;Gorin et al., 1982; Wright et al., 1990; Le Guerroue¤ et al.,2005). The Nafun Group in the Huqf area is consistentlyrepresented by shallower-water facies relative to those inthe Jabal Akhdar (Cozzi & Al-Siyabi, 2004; Le Guerroue¤et al., 2006a, b). The Nafun Group overlies the presumedMarinoan (ending at ca. 635Ma; Allen et al., 2004; LeGuerroue¤ et al., 2006b; S.A. Bowring, J.P. Grotzinger,D. Condon, J. Ramezani &M.Newall, in review) glacigenic

Fiq Member of the Ghadir Manqil Formation (AbuMahara Group), and is bracketed upwards by the AraGroup dated in its middle at 542.0 � 0.3Ma (Amthoretal., 2003).TheNafunGroup,which is thought to have beendeposited during a period of thermal relaxation after therifting phase of the Ghadir Manqil Formation, comprisesat its base the strongly transgressive cap carbonate of theHadash Formation. Above this, it consists of two major si-liciclastic to carbonate cycles (Fig. 1; Allen & Leather,2006; Le Guerroue¤ et al., 2006a).The ¢rst cycle consists ofmarine shales, siltstones and sandstones of the MasirahBay Formation (150^300m thick; Allen & Leather, 2006),which pass gradationally upwards into the carbonate rampof the Khufai Formation (50^300m thick; Gorin et al.,1982;Wright et al., 1990; McCarron, 2000).The second cy-cle comprises thick marine siltstones, shales and subordi-nate carbonates of the Shuram Formation (250^700mthick; LeGuerroue¤ etal., 2006a), which pass up gradation-ally into the carbonate ramp of the Buah Formation (250^350m thick; Gorin et al., 1982; Wright et al., 1990; Cozzi &Al-Siyabi, 2004).This large-scale couplet (Shuram-Buahformations) is associated with a major d13C perturbationin its carbonate record (Fig.1; Burns &Matter, 1993; Cozzi& Al-Siyabi, 2004; Le Guerroue¤ et al., 2006a, b). Thepattern of variation in d13C from the Khufai to the BuahFormations shows a major positive^negative^positive pat-tern.Values as positive as 15% characterize the top of theKhufai limestones, followed by a precipitous fall in the ba-sal Shuram Formation to values as negative as �12%, afew tens of metres above the formation boundary. A long,slow, sub-linear trend to less negative d13C values occursthrough the overlying Shuram and Buah Formations,eventually climbing back to positive values (12%) at thetop of the Buah Formation (Fig. 1; Burns & Matter, 1993;Cozzi et al., 2004a,b; Le Guerroue¤ et al., 2006b). Based onsubsidence modelling and the age distribution of detritalzircons, this peculiar perturbation of the carbon cycle isthought to have persisted for approximately 50 millionyears, representing most of the Ediacaran period (seedetails in Le Guerroue¤ et al., 2006b).

PARASEQUENCES OF THE SHURAMFORMATION

The Shuram Formation, comprising the lower part of thesecond of the Nafun ‘grand cycles’, lies on top of the shal-lowing-upward carbonate ramp of the Khufai Formation(Fig. 1; McCarron, 2000; Le Guerroue¤ et al., 2006a). TheShuram Formation, which reaches 250m in thickness inthe Huqf area, marks an abrupt deepening of the shelf(Fig. 1; Le Guerroue¤ et al., 2006a). It is divided into threemembers. The Lower Member comprises siltstones andsubordinate siliciclastic sandstones in the Jabal Akhdar,but transgressive carbonates in theHuqf area.TheMiddleMember in the Huqf area is characterized by hummockycross-strati¢ed and wave-rippled red siltstones with nu-merous storm-reworked ¢ne-grained limestone beds.

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The Upper Member, particularly well developed in thenorthern extremity of the Huqf area (Wadi Aswad), showsshallowing-upward storm-dominated parasequences(from 4 to 17m thick; Gorin et al., 1982;Wright et al., 1990;McCarron, 2000; Cozzi & Al-Siyabi, 2004; Fig. 1). Faciesare arranged into gradually shoaling packages (Figs 1 and2) that crop out continuously along the 35 km extent ofWadi Aswad. More sections are available farther south ofWadi Aswad (Nafun area of Mukhaibah Dome) but theUpperMember occurs here in slightly deeper water facies(Figs 1 and 2) without well-developed parasequences. Fivedistinctive parasequences have been identi¢ed in the ¢eldinWadi Aswad, overlain by a package of condensed, amal-gamated, m-scale cycles, which in turn pass up into 10mof shales marking a maximum £ooding zone (Cozzi & Al-Siyabi, 2004; Le Guerroue¤ et al., 2006a; Figs 1 and 2).The

UpperMember of the ShuramFormation then passes intodolomites with abundant edge-wise conglomerates of thebasal Buah Formation (Cozzi & Al-Siyabi, 2004; Cozzietal., 2004b), the boundary being de¢ned by the disappear-ance of siltstone intercalations (Gorin et al., 1982).

Storm-generated siltstone facies

The siltstone facies is volumetrically dominant within theShuram parasequences, occurring in the lower part ofeach cycle (Figs 1, 2 and 3).This facies comprises pinkishto yellowish siltstones, grading up to subordinate very¢ne to ¢ne sandstones (Fig. 4). In terms of primary sedi-mentary structures, the siltstone facies is dominatedby swaley cross-strati¢cation (SCS; Fig. 4b) (Leckie &Walker, 1982). Long wavelength swales, up to 5m, are

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Fig.1. General geological setting of theNafunGroup of theHuqf area of east-centralOman. (a)Map ofOman showingNeoproterozoicoutcrops. (b) Summary stratigraphic column of theNafunGroup in theHuqf areawith associated d13C record. (c)Geological map of theHuqf area showing the localities studied. (d) Composite sedimentary log of the Shuram Formation and detail of the parasequences intheWadi Aswad sections.

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symmetrically to slightly asymmetrically ¢lled above asharp, erosional and locally highly irregular, lower-bound-ing surface. Locally, undulating erosional surfaces, cutinto dark brown argillaceous siltstones, have overhangingsteps; overlying swaley cross-strata downlap onto this un-dulating surface and ¢ll the positive step. Flat and verylow-angle laminae commonly occur below or pass gradu-ally upwards into SCS packages, in laterally continuousplanar-laminated, locally gently undulating units (Fig. 4i).Small wave ripples locally ornament amalgamation sur-faces between swaley cross-sets. Parting and primary cur-rent lineation (PCL) are abundant on the surfaces ofplanar laminae as well as on very low-angle swaley cross-strata (Figs 3 and 4a).Wave ripples are 5^10 cm in spacing,and are commonly trochoidal in pro¢le and linear in crest-line (Figs 4c, d and j; Allen,1997), locally reworking tops ofbeds. Some examples are ladder-back, with primary andsecondary crestline patterns preserved. Aggradationalclimbing ripples are found in cosets with grain sizeand wavelength diminishing upward, locally draped byshales (Fig. 4j).

Ubiquitous soft- sediment deformation occurs in theform of complexly folded laminae, load balls and £uidiza-tion pipes (Fig. 4h, i and j), locally homogenized with ves-tiges of deformed laminae. Asymmetrically deformed(sheared), slumped and convoluted layers are commonlyerosionally truncated above. In some places, arrays of smallmicro-normal faults cut the laminae. Flat to low-angle de-tachment surfaces are also present, showing limited exten-sional adjustment (slip of 4 cm) of the sediment pile,together with thin rafts of red siltstone deformed intolarge irregular folds (30^70 cm).

The abundance of low-amplitude, large wavelengthswales and the rare preservation of hummocky antiformssuggest proximal storm-in£uenced water depths as shal-low as several metres (Leckie & Walker, 1982; Datta et al.,1999). Their association with £at laminae and PCL sup-ports the activity of high-energy currents, typical of theshoreface during storms (Dott & Bourgeois, 1982). Tro-choidal wave ripples are also diagnostic of wave action inshallow water depths (Allen, 1997).Wave ripple crestlinesare approximately perpendicular to the direction of PCL

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Fig. 2. Parasequence correlation panel. (a) Southern sections (MD3 andNa1) showing d13C record. (b) Correlated sections ofWadiAswadwith primary current lineation (PCL) palaeocurrent indicators compiled per parasequence (rose diagrams). Distances betweenlocations are given in kilometres.The main cross-strati¢cation palaeocurrent mode is to the SW (see text). See Fig.1for location of logs.(c) Field panorama around localityWA1showing the parasequences ofWadi Aswad. Carbonate facies form prominent ledges in thelandscape. Parasequence 4 and 5 are not exposed all the way alongWadi Aswad.The top parasequence is about10m thick.

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(SW-NE), showing that high-energy currents acted in thesame direction as wave propagation, indicating that waterdepths were shallow enough for friction to dominate overgeostrophic turning (Allen, 1997). Sea-bed currents wereclearly powerful and erosive, cutting steep notches in themore cohesive, argillaceous sea-bed sediments and broadundulating scours in the less cohesive coarse silts to very¢ne sands. Such currents involved a net mass transport tothe SW, as indicated by the downlap of swaley cross-strataover lower-bounding surfaces. Currents reduced in inten-sity to allow wave rippling of the sediment surface beforeburial under new episodes of bedform migration. Cur-rents also reduced in intensity for longer time periods, al-lowing the accretion of climbing wave-ripple cosets, whenthe oscillatory component of the £ow ¢eld dominated overthe steady component (Scott, 1992; Cheel & Leckie, 1993).

The overall picture is therefore of an alternation of rela-tively quiet water deposition of ¢nely laminated argillac-eous siltstones, with strong, initially erosive, pulsating

storm currents. Storm currents, which caused a netsouth-westward sediment transport, reduced in intensityto allow residual wave action to ripple the rapidly aggrad-ing sea-bed. Post- stormwater depths were very shallow, asindicated by the ladder-back ripples, which were formedbywave propagation under local winds rather than the ap-proach of open marine swells. The resulting m-thickstacked units point to the high amount of deposition asso-ciatedwith major storms taking place inwater depths typi-cal of the upper shoreface.

The range of soft- sediment deformation features indi-cate water escape and gravitational collapse, typical of ra-pidly deposited sediments such as storm deposits (Mills,1983). Shortly after deposition, disturbance and dewater-ing of the bed mayhave been caused by cyclic wave loadingunder storm conditions (Staroszczyk, 1996), or by seismicshaking (Allen, 1986). It is certain that the original sedi-ment was a well-laminated siltstone, as the lamination isintricately folded and locally destroyed. In places, £uid es-cape velocities were high enough to £uidize the bed, lead-ing to homogenization. In general, however, £uid escapevelocities were lower, allowing folding by liquefaction. Ar-rays of small brittle fractures (slip ofo1cm) and low-angledetachments demonstrate local extension. However, thepervasive and ubiquitous folding shows that most of thedeformation involved gravitational collapse and ductileshortening.

The consistently strained load balls erosionally trun-cated by swaley cross-strata indicate that soft- sedimentdeformation took place in near-surface sediment, and thatthe upper portion of the sea-bed underwent deformationunder the powerful shear of overlying currents. An originof the deformation by £uid^sediment interaction duringpowerful storms therefore appears more likely than an ori-gin by seismic shaking.

Intraclast-rich carbonate facies

The vertical transition between the siltstone and carbo-nate facies is commonly gradual over a few dm.The thick-ness of the carbonate facies varies from a few centimetresin the older parasequences to up to 7m in the youngest(Figs 1^3). The facies is made up mainly of recrystallizedand partially dolomitized ooidal-pisoidal grainstones con-taining numerous cm- to dm-size rounded to angular in-traclasts arranged along the main £ow direction (locallyimbricated) or more commonly organized into edgewisenests (Fig. 5).

Ooidal grainstones also occur as planar, tabular andcross-strati¢ed o10-cm-thick beds. Beds show evidenceof in situ brecciation, with a transition from allochthonousclast-rich units passing laterally into undisturbed cross-strati¢ed beds.The broken limestone clasts show a progres-sion from angular pieces close to the intact parent bed, toroundedwhere they are far separated (Fig. 5).This range ofstates of break-up into intraformational breccias is espe-cially noticeable in the ¢rst carbonate beds marking the 1^2m thick transition between the two facies. Ooids vary con-

Siltstones CarbonatesFacies:

Swaley cross-stratification

Swaley cross-stratification

Linear crested climbing wave-ripple and trochoidal wave-ripple

Flat laminae withparting and primarycurrent lineations

Undulating laminae

Soft-sediment deformation folded laminae, load ballsfluidization pipes

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Imbricated intraclasts

Edgewise nests

Ooids

Swaley cross-stratification

Fluidisation

Asymmetrically strained soft-sediment deformation

Trochoidal wave-ripples

Rippled top surface

Planar, tabular bed

5 m

Fig. 3. Idealized sedimentological section of parasequence1showing all the characteristic structures within one mixed-lithology cycle. See discussion and interpretation in the text.

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siderably in size from bed to bed, ranging from 0.1 to 5mmin diameter, and are generallywell sorted, the younger para-sequences containing the larger examples (Fig. 5c). Radialfabrics of ooids are preserved as very thin micritic lamellae,but originally were made of radiating CaCO3 minerals(Medwede¡&Wilkinson,1983).Orange to brown siliciclas-tic silt mixedwith the ooids is commonly preferentially pre-

served underneatho5 cm-sized limestone intraclasts andin interstices between intraclasts.

The facies shows abundant small- scale planar(o10 cm) cross-sets with angular or tangential cross-stra-ta, as well as undulatory styles of cross-strati¢cation (SCS)and trains of trochoidal wave ripples (Fig. 5a and d). Someripple cross-sets evolve from undulatory lamination to a

Fig.4. Siltstone facies. (a) Primary current lineation. Note (arrows) two di¡erent orientations on di¡erent laminae. (b) Swaley cross-strati¢cation.The book is about15 cm long. (c) Bedding plane coveredwith trochoidal, irregular wave ripples. (d) Low-anglestrati¢cation grading to swaley cross- strati¢cation overlain by climbing symmetrical wave ripples.The pencil is 15 cm. (e)Water-escapepipe within lique¢ed laminated siltstones. (f) Foundered balls and pseudonodules of laminated siltstone within £uidized(homogenized) bed truncated by swaley cross-strati¢ed siltstones. (g) Swaley cross- strati¢ed coset showing one set with pervasive softsediment deformation. Coin for scale. (h)Wavy laminated and draped surface eroding into planar laminae. On all pictures, the coin isabout 2.5 cm in diameter.

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Fig. 5. Carbonate facies. (a) Typical parasequence carbonate facies with a cross-strati¢ed siltstone to very ¢ne sandstone interbed.Note the low-angle, planar and swaley cross- strati¢cation in the light grey intraclast-rich carbonate bed. (b) Bidirectional cross-strati¢cation.The main direction is to the SW. (c) Ooidal grainstones. Note the presence of ooids in both matrix and carbonateintraclasts. (d) Large-scale swaley cross-strati¢cation. Swiss army knife for scale. (e) Surface view of edge-wise conglomerate. (f) Cross-strati¢ed carbonate bedwith a trochoidal wave rippled top. Low-angle large-scale cross-strati¢ed siltstones occur below and above,where they drape the underlying ripple topography. (g) Edge-wise conglomerates in a nest in cross-section. (h) Carbonate with orangesilt ¢lling pockets and crevices between clasts. Lens cap is 4 cm. (i) Swaley cross-strati¢ed siltstones truncated by carbonate bed (belowcompass) passing from intact bed to breccia of intraclasts towards the left.The compass is of 8 cmwidth. On all pictures, the coin isabout 2.5 cm.

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bidirectional build-up with a symmetrical upper surface.In general, the tops of beds are either rippled, or erosiveand irregularly covered with limestone clasts derived lo-cally from the underlying bed. Palaeocurrents from thethin limestones intercalated with laminated siltstones arebidirectional, with a dominant mode to the SW.Planar setslocally contain rippled cross-strata lined with a ¢ne sedi-ment drape, indicating a pause in bedform migration.

The association of undulatory (swaley) cross-strati¢ca-tion, wave ripple cross-lamination and planar, tabularlimestone beds are indicative of storm and fair-weatherprocesses in shoreface water depths. Intense current orwave activity is shown by the high-energy structures, bi-directional palaeocurrents, local break-up of the partiallylithi¢ed sea-bed and formation of edgewise conglomerates(Mount & Kidder, 1993). The interbedding of carbonateand siliciclastic beds in the transition zone below the car-bonate ledge, and the mixing of ooids with siliciclastic siltand very ¢ne sand, demonstrate that the carbonate factoryoperated at the same time as siliciclastic deposition, in lat-erally time-equivalent locations. The preservation of or-ange-brown quartzose silt in pockets and crevicesbetween limestone clasts suggests late in¢ltration into aclast-supported framework. This might have taken placeon the sea-bed during quiet periods, or by aeolian trans-port onto an emergent, rubbly, sediment surface. Thesesurfaces may be diagnostic of sediment bypass, but noclear evidence for surface exposure such as karsti¢cationfeatures has been recognized.

The lack of evidence for soft- sediment deformation inthe carbonate facies indicates a lithological control onhigh-pore £uid pressure in the siltstone facies.

Although depositional water depths were shallow, faciesremain uniform throughout the parasequence stack atWadi Aswad.No mudstone, stromatolite or peritidal facieshave been recognized, and there is little evidence for ero-sional surfaces, soil formation or karsti¢cation in theupper parts of shallowing-up parasequences.

PALAEOCURRENTANDWATER DEPTHINDICATORS

The orientation of PCL indicates currents oriented NE-SW. This orientation is regionally consistent among allparasequences (Fig. 2). The main cross-strati¢cation di-rection is to the SW, with a subordinate mode to the NE(Fig. 5b and f). The wave propagation direction is alsoSW-NE based on crestlines of vortex ripples on the topsof beds.The ¢lling direction of troughs by siltstones andvery ¢ne sandstones above the wave-rippled top surfaceof carbonate beds is also to the SW.These di¡erent palaeo-current indicators are mutually consistent: assuming thatthe net sediment transport was o¡shore-directed, the di-rection of storm attack was from the SWonto a NW-SE-oriented coastline.

The preservation of wave ripples with steep trochoidalpro¢les allows the estimation of the wave period of forma-

tive waves and a range of depositional water depths (seediscussion of technique in Allen, 1981and Allen, 1984, anda recent application in Allen & Ho¡man, 2005a).Taking asteep, trochoidal wave ripple with spacing 250mm andheight 45mm (vertical form index of 5.5) preserved abovea limestone bed made of medium-coarse ooids, a maxi-mum wave period of 5.5 s has been calculated, assumingthe wave ripple was formed at the threshold condition.Formation at the threshold is supported by the preserva-tion of the trochoidal top pro¢le before cessation of sedi-ment movement.However, if the dimensionlesswave shearstress was in excess of the threshold value (Jerolmack &Mohrig, 2005, and reply by Allen & Ho¡man, 2005b), theformative wave period reduces to ca. 4 s. Using the 5.5 s es-timate of thewave period, formativewaves must have actedin water depths of less than 20m, and most likely of lessthan 10m.This supports the view that the parasequenceswere formed on a storm-dominated shoreface. By extend-ing this argument, the swaley cross-strati¢ed siltstoneswere also formed in similar or slightly greater waterdepths, and the carbonate components of parasequenceswere deposited in water depths shallower than10m. How-ever, the period of thewaves that generated the primary se-dimentary structures in the siltstones and theirsubsequent soft sediment deformation, or that assistedthe in situ break-up of the limestone beds on the sea £oor,is not known. As the trochoidal wave ripples signify theend of deposition following bedform migration andbreak-up of the sea-bed, it is highly likely that storm con-ditions were more intense in the preceding phase.

THE SPACE-TIME EVOLUTION OF THEPARASEQUENCES

The strike of the outcrop belt is oriented parallel to the pa-laeocurrent directions (Figs 1 and 2) and extensive expo-sures provide a rare opportunity to trace parasequencearchitecture in the down/up-dip direction. Based on the¢ve most complete parasequences of Wadi Aswad, thewater depth trend suggested by lateral thickness and faciesvariations shows a primary deepening towards the south,which is consistent with the palaeocurrent direction (Fig.2).This is supported by sections in the Mukhaibah DomeandNafunvillage areas (MD3 andNa1; Fig.1), which showdeeper water facies (Fig. 2). Local variation in parase-quence thickness is present alongWadiAswad and is inter-preted as being due to basinal relief. In addition, section10located north of Wadi Aswad records the northward dee-pening of depositional environments at the northern limitof theHuqf palaeohigh described in other regional studies(Le Guerroue¤ et al., 2006a). The increasing parasequencethickness upwards in the stack, combined with the in-creased carbonate preservation in the same direction(Fig. 2 andTable1), suggest a trend in time of increased ac-commodation, whichwas easily ¢lled by a plentiful supplyof both siliciclastic and carbonate sediment.This accom-modationwasmost likelygenerated byeustasy superimposed

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on background basin subsidence (Vail, 1987; Adams &Grotzinger, 1996; see alsoModelling section below).

PROGRADATION AND THE CARBONISOTOPE RECORD

Methodology

Bulk-rock inorganic carbon and oxygen isotope measure-ments have been carried out on each of the carbonateledges within the Wadi Aswad parasequences (measure-ments are shown inTable1) aswell as on the carbonate bedsof the southern sections at Mukhaibah Dome and Nafun(MD3andNa1;Figs 2 and 3).Hand specimenswere drilledwith 1^5mm dental drill bits from freshly cut rock slabsavoiding sparry cement and vein material. Ooid-rich sam-ples were preferentially analysed, andwhere possible ooidshave been directly micro-drilled for isotopic measure-ments.The C and O isotope composition of powder fromthe carbonate samples was measured with a GasBench IIconnected to a Finnigan MAT DeltaPlus XL mass spec-trometer, using a He-carrier gas system (methods adapted

after Spoetl & Vennemann, 2003). Samples were normal-ized using an in-house standard calibrated against d13Cand d18O values of NBS-19 (11.95 and � 2.20%, relativeto PDB). External reproducibility for the analyses esti-mated from replicate analyses of the in-house standard(n5 6) is � 0.07% for d13C and � 0.08% for d18O.

Diagenesis

In thin section, carbonates show partial dolomitization ina texture that is mainly recrystallized, all samples showingsimilar microfacies. Some carbonates of the ShuramFormation were screened under a cathodoluminescencemicroscope. Good preservation of low-Mg calcitic carbo-nates in the Shuram Formation is indicated by the radialfabrics of the ooids (McCarron, 2000).The lack ofCLzon-ing in the cements suggests that they did not re-equili-brate under anoxic conditions in the sulphate-reducingzone (McCarron, 2000). Some bulk carbonate sampleswere geochemically screened (Burns&Matter,1993;Burnset al., 1994; McCarron, 2000; Leather, 2001; see data in LeGuerroue¤ et al., 2006a). Mn/Sr ratios in screened sampleswereo10, with many sampleso3, which has been taken

Table1. Carbon and oxygen isotope data (from locality1to 9) for each parasequence, and average parasequence thickness and carbonate(Carb.) to siltstone (Silt.) facies ratio.

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to imply that negligible diagenetic alteration has occurred(Kaufman&Knoll,1995, but seeMcKirdy etal., 2001). Ad-ditional isotopic data, which are consistentwith the valuesin the samples screened for diagenetic alteration, weretherefore usedwithout further screening.

On the basis of geochemical screening, therefore, car-bon isotopic values maybe interpreted as primary,whereasoxygen values show values randomly ranging from � 6 to� 9% (Table1), indicating probable mixing with meteoric£uid (Fairchild et al., 1990). The rarely preserved positived18O values occur in a di¡erent part of theShuramd13C ex-cursion (at the base of the excursion; see data in Le Guer-roue¤ etal., 2006a).During early diagenesis, alteration of theoxygen isotopes is common, leaving the carbon isotopicvalues intact (Kaufman &Knoll, 1995).

The strongest indication that the d13C values are pri-mary is the regional reproducibility of the signal, with lit-tle variation regardless of facies changes between outcropsections and well data (Burns & Matter, 1993; McCarron,2000; Cozzi & Al-Siyabi, 2004; Cozzi et al., 2004a; LeGuerroue¤ et al., 2006a, b). The analysis of carbon isotopictrends within the parasequence stack of the Upper Mem-ber of the Shuram Formation strongly supports this view.

Results

Systematic measurements of d13C along individual parase-quences inWadiAswad showa persistent trend of less nega-tive valueswhenmoving laterally to the south (Fig.6). Someimprecision occurs because of analytical errors (the instru-mental precision is � 0.07% for d13C measurement), pos-sible local minor alteration of the primary d13C signal andsedimentary reworking of older material along the parase-quence. Given the southward progradation direction, timelines cut across facies packages, with lateral facies changesbetween co-existing shallow-water carbonates and rela-tively deeper-water siliciclastics (Fig. 7).The carbon isoto-pic ratio is isochronous along individual time lines,makingits variations predictable as a result of the development ofthe prograding parasequence stack (Fig. 7).

If we assume a 50Myr duration of the principal NafunGroup d13C excursion (LeGuerroue¤ etal., 2006a), the linearrate of carbon isotope recovery over the Shuram to Buahstratigraphic interval is about 0.3%/Myr. Assuming a d13Cvariation from �10% in the oldest Shuram parasequenceto � 7.5% in the youngest (Fig. 6), this implies a total dura-tion of about 8Myr for the ¢ve parasequences and an aver-age of ca.1.6Myr per parasequence.This average duration isdi⁄cult to attribute to orbital forcing, but cycles in theupper Ara Group (Amthor et al., 2003; Amthor et al., 2005)are radiometrically constrained to last 1.3Myr per cycle (3.5cycles present) (S.A. Bowring, J.P.Grotzinger,D. Condon, J.Ramezani &M.Newall, in review).The greater thickness oftheAra cycles (50^200m)would require considerablyhighertectonic subsidence and sediment accumulation rates dur-ing Ara times comparedwith the Shuram. Lateral variationin d13C is of the order of o1% over a 25 km N-S distancealong each parasequence.This gives an average variation of

0.04%per km and a progradation rate of �7.5 km/Myr.Thetime-averaged vertical sediment accumulation rate, cor-rected for compaction, was of the order of �12m/Myr,which is typical of the late stages of failed rifts or passive-margins (Allen &Allen, 2005).

MODELLING

Methodology

The stack of parasequences in the Shuram Formation atWadi Aswad has been investigated using a one-dimen-sional (1-D)model based on simple algorithms for accom-modation and sediment supply (see Allen & Allen, 2005,p. 269^275, and code in supported online material). Themodel allows prediction of the thickness and water depthvariation through a stack of parasequences based on inputof the eustatic variation, maximum sediment supply rateand tectonic subsidence rate (Fig. 8a). No distinction be-tween siliciclastic and carbonate sediment is made, andno sediment dynamics are implied. Furthermore, cyclicityis assumed to be driven by a eustatic control rather than byunforced internal dynamics (cf. Burgess et al., 2001).Whenwater depth is very low, and accommodation tends to 0,further autogenic sediment production or allogenic in£uxis assumed to be transported into deeper water and notpreserved in the1-D pro¢le.

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Fig. 6. Composite isotope plot for the carbonate components of¢ve parasequences atWadi Aswad.The colour key shows thelocation of a section in a north to south pro¢le. For eachparasequence, isotopic ratios become less negative towards thesouth, which is the progradation direction.

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Long-term tectonic subsidence, balanced roughly bysediment supply expressed as a vertical velocity, is as-sumed to be linear (11m/Myr) over the time span of the de-position of the parasequence stack. To replicate thecyclicity of the parasequence stack, two eustatic sinusoidswere combined: one with an amplitude and a duration of30m and 1.6Myr, and the other with 10m and 16Myr(Fig. 8a). Sediment supply is coupled to the eustatic varia-tion, with a maximum (14m/Myr; i.e. 6 and 8m per cycle)at times of relative sea-level fall and aminimum at times ofrelative sea-level rise (Fig. 8a). The water depth variationthrough time plotted against the potential sediment accu-mulation gives an illustration of cycle developmentthrough the stack (Fig. 8b). Facies distributions are in£u-enced by the choice of bypass depth (x) and the palaeowaterdepth of the carbonate-siliciclastic transition (Fig. 8c).

Model results

The model has been run as a simple simulation exercise inorder to identify the most likely range of values of the for-cing mechanisms. These parameter ranges can then be

directly comparedwith the estimates derived from analysisof sedimentary facies.Two superimposed eustatic signalsare required to produce a thickening-upward parase-quence stack assuming a constant subsidence rate. Thelonger scale eustatic variation allows progressive minordeepening, less bypass and thicker carbonate depositionthrough time (Fig. 8).The shorter scale eustatic variationdrives the primary stratigraphic cyclicity.

Simulations have been run with di¡erent values of thebypass threshold depth (probably between 1 and 2mwaterdepth) and palaeowater depth of the facies transition fromcarbonate to siliciclastics. Best results are obtained whencarbonate deposition is constrained in a zone 3m belowthe bypass threshold depth (x� 3; Fig. 8b). Siltstone de-position is constrained within a zone between 3 and�20m below the bypass threshold depth. The bypassthreshold could be as shallow as 0mwater depth.These re-sults are in agreement with the palaeowater depths inter-preted from facies andwave-ripple analysis.

The m-scale package of cycles overlying the parase-quence stack (Figs 2 and 8) represents a sharp change indepositional setting. Field observations show that the

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Fig.7. Conceptual model of the Shuram parasequence stackwith isotope variation following a linear secular trend de¢ned in Figs1and6. Four parasequences are drawn, delimited by parasequence boundaries (PB) and labelled by circled number1^4. (a)Lithostratigraphicdepositional model representing dip section architecture, showing less negative carbon isotope ratios in the direction of sedimentaryprogradation andvertically through the stack of parasequences. (b) Samemodel represented as aWheeler diagram showing hiatuses dueto non-deposition or sediment bypass, as predicted by numerical modelling (Fig.8).d13Cvalues are identical along individual time lines(numbered 0^10) and follow a secular trend.Moving along one carbonate facies crosses di¡erent time lines, resulting in an expectedvariation of the d13C record through time.

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m-scale package of cycles slowly deepens upwards, even-tually reaching10m of shales marking amaximum£oodingzone.To simulate these relatively deeper water cycles, twoscenarios were tested: (a) reduced sediment supply causesa decrease in cycle thickness and carbonate deposition tobe replaced entirely by siltstone deposition, whereas an in-crease in sediment supply causes more bypass and preser-vation of shallowwater carbonates at the expense of deeperwater siltstones; and (b) reduction of the amplitude andperiod of the shorter eustatic component allows the pre-servation of thin cycles comprising both carbonate andsiltstone (two eustatic sinusoids are combinedwith ampli-tude, duration and coupled sediment supply of 10m,0.32Myr and 5m/Myr, and 30m, 16Myr and 5m/Myr;Fig. 8).The gradual upward transition into the maximum£ooding zone may be due to a longer-term rise in absolutesea level. The change from a thickening-upward parase-quence stack into condensed cycles followed by a maxi-mum £ooding zone therefore suggests a fundamentalchange in the eustatic forcing of cycle development.

DISCUSSION

The simulation results described above are clearly non-unique. Nevertheless, the presence of a clearly decipher-able eustatic signal and the observation that mixed carbo-nate-siliciclastic sequences are more typical of icehouse

rather than greenhouse periods (Tucker, 2003) raises thepossibility that the UpperMember of the Shuram Forma-tion was deposited during a period of glaciation. Glacia-tion is known during Ediacaran time in the form of the‘Gaskiers’ event. The Gaskiers Formation is dated at ca.580Ma (Krogh et al., 1988; Bowring et al., 2003), but theages of possible correlatives are loosely constrained (seeHalverson et al., 2005; Le Guerroue¤ et al., 2006a for a re-view). Subsidence analysis of the Nafun Group suggestsdeposition of the top Shuram parasequences at around580Ma, making them possibly synchronous with the‘Gaskiers’ glacial period (see Le Guerroue¤ et al., 2006b).AsLeGuerroue¤ etal. (2006b) have shown, theGaskiers gla-cial event is unnoticed in the Huqf Supergroup of Omanin terms of direct sedimentological evidence, such as a sig-ni¢cant sea-level lowering, or in terms of its impact on thecarbon cycle, but it might be recognized in terms of an en-hanced eustatic signal in the stratigraphic architecture ofthe Upper Member of the Shuram Formation. Specula-tively, the end of glacioeustasy may coincidewith the sharptransition between the stack of thick parasequences andthe package of m-scale cycles that leads eventually to themaximum £ooding zone marking the Shuram-Buahboundary (Fig. 8). If so, theKhufai-Shuram-Buah succes-sion of the Nafun Group may represent an alternation ofgreenhouse/arid and icehouse/pluvial periods. However,as a caveat, it must be emphasized that there are other me-chanisms possible for the relative sea-level changes seen in

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Fig. 8. Numerical modelling parameters and results. (a) Various parameters used in the models plotted against time. PSA, potentialsediment accumulation. (b) PSA plotted against water depth. Bypass threshold depth and facies transition limit (x� 3m) are chosen inorder to obtain the best ¢t with ¢eld observations. Note that two models are runwith di¡erent parameter values. (c) Best ¢t obtainedwith parameters de¢ned in a, comparedwith ¢eld observations (see average thickness inTable1).More detailed explanations in text. Seecomputer code in online supported material.

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E. Le Guerroue¤ et al.

the upper Shuram stratigraphy, especially bearing in mindthe long duration of the Shuram cycles (�1.6Myr).

Depletedvalues ofNeoproterozoic carbonates have beententatively explained by a biological pumping in a strati¢edocean (Grotzinger & Knoll, 1995; Kaufman et al., 1997).Turnover of the deep-water, 13C-depleted reservoir duringdeglaciation would cause the precipitation of carbonateswith light d13C values in shallow waters of the continentalshelf. Interestingly, Shen etal. (2005) reported a carbon iso-topic variation of �3% along a palaeoenvironmentaltransect from shelf to deep basinal sedimentary facies intheMarinoanNantuo cap carbonate at a given stratigraphictime. However, with respect to the Shuram excursion, nolateral gradient is decipherable between the d13C data setsfrom shallow and deep water facies (separated by 500 km;Le Guerroue¤ et al., 2006a). At the parasequence scale, thelimited depth range and lateral extent of facies preservedin the stack inWadi Aswad do not allow the existence of alateral gradient in carbon isotopic values to be tested.

The entire excursion occupying the stratigraphic inter-val from top Khufai Formation to Buah Formation is es-sentially in phase with longer term relative sea-levelchange (LeGuerroue¤ etal., 2006a), with a nadir in d13C oc-curring at the level of the maximum £ooding zone of thelower Shuram, and the cross-over to positive values occur-ring within the Buah highstand.This feature is also foundin other post-Marinoan and post-Sturtian transgressive‘cap sequences’ (see compilation in Halverson et al., 2005).At the parasequence scale, however, the carbon isotopicratios are una¡ected by the ca. 1.6 Myr frequency cycles ofrelative sea-level change, whichwere most likely driven byeustasy. Any explanation of the Shuram shift must involvean exceptionally long residence time or lag comparedwithPhanerozoic examples of perturbations of the carbon cycleand should involve a su⁄ciently large reservoir of 13C-de-pleted material (e.g. organic carbon). Although such an ex-planation is hard to demonstrate (but see the non-steady-state mathematical model from Rothman et al., 2003), anylinkage between Ediacaran-aged (Gaskiers) glaciation andthe Shuram carbon isotopic excursion can be ruled out, asthey have di¡erent ages and durations.

The fact that the carbon isotopic record varies system-atically and predictably through the stack of progradingparasequences is a powerful justi¢cation for regarding thecarbon isotopic ratios as re£ecting a primary variation inthe chemistry of ocean water (Fig. 7). It is inconceivablethat post-depositional processes could mimic this, as thecarbonate facies comprising the upper parts of parase-quences are lithologically identical (ooidal grainstones).

CONCLUSIONS

Neoproterozoic extremely negative d13C values have beeninvestigated at the scale of a stack of thickening-upwardparasequences in the Upper Member of the Shuram For-mation of the Nafun Group of Oman. Although a numberof hypotheses exist for the origin of extremely negative

carbon isotopic values in marine carbonate, there is cur-rently no satisfactory explanation for such long-lived andstrongly negative d13C values available. Consequently,there is considerable debate as to whether such values areprimary (and so re£ect the carbon isotopic compositionof the ocean water from which carbonates were precipi-tated) or the result of diagenetic alteration.

The parasequence stack consists of shallow-water,storm-dominated, mixed-lithology sedimentary cyclesbounded by £ooding surfaces. Siltstones typi¢ed bylarge-scale swaley cross-strati¢cation and ubiquitoussoft- sediment deformation pass up intowave-rippled, cal-careous ooidal grainstones with abundant intraclasts, re-£ecting various stages of break-up of carbonate beds anddispersal by vigorous currents. Five such parasequencesshow a systematic trend of progradation away from a pa-laeohigh situated in the northern Huqf region towardsdeeper palaeowater depths in the south.

The occurrence of extremely well-developed parase-quences at this level of the stratigraphy suggests the super-imposition of eustatic change on a long-term regionalbasin subsidence, although unforced internal cyclicitycannot be ruled out. The approximate age of the UpperMember of the Shuram Formation is essentially uncon-strained by radiometric dates from within the NafunGroup, but a subsidence analysis bracketed by dates ofthe underlyingMarinoan-aged FiqMember and overlyingAra Group suggests that it is in the region of 580Ma.Thisdate is similar, perhaps coincidentally, to the reported ageof the Gaskiers glaciation recorded in Laurentia. Themixed carbonate-siliciclastic parasequences may thereforere£ect increased icehouse conditions at this time in theEdiacaran period.

The carbonate-dominated tops of the parasequencesshow a systematic trend in carbon isotopic values, becom-ing less negative from the oldest parasequence to theyoungest, and d13C values also become less negative in thedirection of sedimentary progradation. These combinedstratigraphic-carbon isotopic observations validate the ge-netic stratigraphicmodel, but in addition corroborate a pri-mary, oceanographic origin for the carbon isotopic ratios.

The progradational architecture of the parasequencestack results in a 0.04% per km lateral variation along anindividual parasequence. This lateral variation is simplycaused by facies boundaries crossing inclined time lineswithin the parasequence stack.This coeval isotope varia-tion and stratigraphic evolution demonstrates a primaryorigin of the extremely negative carbon isotopic values inthe Shuram Formation. Parasequence-scale variations inaccommodation generation, freshwater in£ux, bioproduc-tivity and upwelling had negligible e¡ects on the carbonisotopic composition of seawater.

ACKNOWLEDGEMENTS

Wewish to thank PetroleumDevelopmentOman for long-term logistical assistance during the duration of this work.

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Erwan Le Guerroue¤ was funded by ETH-Zˇrich(TH-Projekt Nr. TH-1/02-3). We thank Matthias Pappfor help in the ¢eld. Carbon isotope measurements weremade in theTorsten Vennemann laboratory in Lausanne.Guy Simpson is thanked for his help with the numericalcode. Martin Kennedy, James Etienne, John Grotzinger,Adam Maloof, David Fike and Galen Halverson arethanked for their useful input. The manuscript bene¢tedfrom reviews byMauriceTucker, PaulWright andGrahamShields.

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Manuscript received 23 December 2005; Manuscript accepted7May 2006

Supplementary material

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Appendix S1

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Shuramparasequences