Palaeogeography, Palaeoclimatology, Palaeoecology 160 (2000) 105–122www.elsevier.nl/locate/palaeo

Evaporitic constraints on the southward drifting of the westernGondwana margin during Early Cambrian times

J.J. Alvaro a,*, J.M. Rouchy b, T. Bechstadt c, A. Boucot d, F. Boyer e,F. Debrenne f, E. Moreno-Eiris g, A. Perejon g, E. Vennin b

a UPRESA 8014 CNRS, Cite Scientifique SN5, Universite de Lille, I, 59655 Villeneuve d’Ascq, Franceb CNRS-ESA 7073, Lab. Geologie, MNHN, 43, rue Buffon, 75005 Paris, France

c Geolog.-Palaontolog., Institut Universitat Heidelberg, INF 234, 69120 Heidelberg, Germanyd Oregon State University, 3029 Cordley Hall, 97331 Corvallis, OR, USA

e Lab. Geologie Appliquee, Universite Paris VI, 4, place Jussieu, 75005 Paris, Francef UMR 8569 CNRS, Lab. Paleontologie, MNHN, 8, rue Buffon, 75005 Paris, France

g Instituto Geologıa Economica (UCM-CSIC), Universidad Complutense, 28040 Madrid, Spain

Received 22 July 1999; received in revised form 15 November 1999; accepted for publication 13 December 1999

Abstract

Lower Cambrian evaporites and carbonates are reported from nearly all the platforms of the western Gondwanamargin, which comprises the Souss, Ossa–Morena, Cantabro-Iberian, Armorican and Montagne Noire–SardinianBasins. Both lithologies were deposited in climatically restricted belts and their changing palaeogeographic distributions,according to recent biostratigraphic correlations, are used to infer the latitudinal motion of this margin. As a result,the time span involved in the Cordubian–Ovetian–Marianian interval (8–10 m.y.) requires a relative high rate ofdrifting, which supports the high apparent polar wander path rates (defined by palaeomagnetic data) proposed forEarly Palaeozoic times. The varied and abundant relics of primary to early diagenetic evaporites (gypsum, anhydriteand halite) demonstrate that extensive evaporitic conditions were associated with carbonate and mixed platformsystems in an Early Cambrian arid subtropical belt. Evaporites were originally more abundant than suggested by thereported remains because the deposits have undergone a multistep diagenesis that erased most of the formermorphologies. Some petrographic criteria are proposed for recognizing silica pseudomorphs after evaporites, such asthe development of ‘chicken-wire’ and enterolithic structures, and the presence of lenticular to lozenge-shaped crystalsof gypsum and anhydrite relics. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: carbonates; evaporites; Lower Cambrian; palaeogeography; petrography; W Gondwana

1. Introduction polar wander path (APWP) for some Gondwanamargins is very incomplete. This is due to the lackof precise stratigraphic ages, the faunal differencesExamination of Cambrian palaeomagnetic datathat characterize biogeographic provinces (Palmer,sets reveals that the knowledge of the apparent1998), and the scarcity and variable quality ofpalaeomagnetic data sets. However, several* Corresponding author. Tel.: +33-3 2033 6392;attempts have been made to construct thefax: 33-3-2043-6900.

E-mail address: Jose-Javier.Alvaro@univ-lille1.fr (J. Alvaro) Cambrian APWP for Gondwana (Scotese and

0031-0182/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S0031-0182 ( 00 ) 00061-4

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Barret, 1990; Torsvik et al., 1990; McKerrow et al., 1992), in which several distinct platforms havebeen recognized (Fig. 1C).1992; Kirschvink, 1992; Powell et al., 1993; Storey,

1993; Dalziel, 1997; Kirschvink et al., 1997). (1) Morocco. The uppermost Proterozoic (?)–Cambrian succession of the Anti-Atlas and centralEvidence from different geologic and palaeon-

tologic disciplines must be used to improve this High Atlas mountains was deposited in theso-called Souss Basin (Geyer, 1989), whichpalaeomagnetic approach. Our knowledge of

Cambrian palaeogeographic evolution in the west- includes additional outcrops in the Jbilet andRehamma regions, and the coastal Plateau. Theern Gondwana margin (which comprises Morocco

and SW Europe) has progressed considerably in axis of the basin roughly coincides with the moderntrend of the Anti-Atlas (SW–NE). Common west-the last decade, propelled by rigorous systematic

studies of trilobites, acritarchs, archaeocyaths, to-east facies changes throughout the Cambriansuccessions reflect the eastern setting of proximalbrachiopods, echinoderms, small shelly fossils and

trace fossils, which have improved the biostratigra- areas (Geyer et al., 1995; Geyer and Landing,1995).phy of these areas. Recent changes in the accuracy

and precision of event stratigraphy, biostrati- (2) The Iberian Peninsula. The Cambrian tecto-nosedimentary evolution of the Iberian Massif,graphic correlation and chronometric dating have

had an even greater impact on our perception of which comprises the Cantabrian, West Asturian–Leonese, Galician–Castilian, East Lusitanian–the ‘Cambrian explosion’ in this area. Refinement

of bio- and chrono-stratigraphy to the stage and Alcudian and Ossa–Morena Zones (Lotze, 19451961), and its eastern prolongation into the Iberiansubstage level is providing a consistent picture of

the palaeogeographic history of the western Ranges (Demanda and Iberian Chains), has beentraditionally interpreted in terms of two distinctGondwana margin. This supports the idea that

palaeontologic data are of sufficient quality to troughs: the Cantabro-Iberian and the AndalusianBasins, the latter including the Ossa–Morenapropose general statements about how the south-

ward migration of this margin took place during Platform.The geodynamic affinity of Ossa–Morena is oneEarly Cambrian times.

This paper presents the changing geographic of the most controversial matters within theIberian Massif (Quesada, 1991). One hypothesisdistribution of lithological indicators of climate

(such as evaporites and carbonates) to estimate supposes its accretion to the Iberian Autochthon(which would includes the rest of the northernthe drifting of western Gondwana during Early

Cambrian times, which is determined for correlat- Zones) during the Cadomian orogeny ( lateRiphean–earliest Cambrian; Quesada, 1991;able stratigraphic units based on archaeocyathan

and trilobite zonations. The method serves as a Abalos, 1992), whereas another hypothesis con-siders the Iberian Massif as a complex tectonictest case to constrain the APWP defined using

palaeomagnetic methods. mosaic constructed after two distinct plates col-lided during the Acadian orogeny (early Devonian;Martınez-Garcıa and Rolet, 1991). Because of thisdiscussion, and the possibility of a displacement2. A general outline of the western Mediterranean

area of Ossa–Morena from NW to SE along theBadajoz–Cordoba shear zone during theHercynian orogeny, we will illustrate Ossa–Cambrian successions in the western

Mediterranean area (High and Anti Atlas, Iberian Morena as a neighbouring basin of Gondwana, inwhich the main rifting process took place through-Peninsula, Armorican Massif, Montagne Noire

and Sardinia; Fig. 1A and B) are presently wedged out the Early Cambrian ( Vegas, 1978; Mata andMunha, 1990).between Mesozoic and Cenozoic orogenic belts.

The mosaic of outcrops represents small cratonic On the other hand, the Cantabro-Iberian Basincomprises the Cantabrian, West Asturian–Leonesedomains without well-defined boundaries con-

nected with Gondwana (Courjault-Rade et al., and eastern Galician–Castilian Zones, and their

107J.J. Alvaro et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 160 (2000) 105–122

Fig. 1. Pre-Hercynian outcrops of the western Mediterranean area and tectonosedimentary units (see text for references).

prolongation into the Iberian Ranges (Alvaro (3) The Armorican Massif. The latePrecambrian–Cambrian interval is characterizedet al., 1993b). It was limited to the NE by the

Cantabro-Ebroian Land area, which constituted by an APWP for the Armorican Massif that canbe superimposed on the Gondwana path. Somethe main source of sediments for both the

Cantabro-Iberian and the Pyrenean Basins (Carls, palaeomagnetic data from Armorica, as well asthe existence of a mid-European ocean during1983), and in the SW by some uplifted areas (or

median highs; Lotze, 1961), which episodically Ordovician times, still remain uncertain. However,a counter-clockwise rotation of the massif hassupplied sediments (Vegas, 1978; Aramburu et al.,

1992; Alvaro et al., 1993b). been proposed for Cambrian–Ordovician times

108 J.J. Alvaro et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 160 (2000) 105–122

(Hammann, 1992; Pillola et al., 1994; Vennin et al., 3. Stratigraphy and evaporitic events1998) solving this problem.

(4) The Montagne Noire and SW Sardinia. To In order to avoid nomenclatural confusion onbiostratigraphic correlations, we will use the Lowerthe north of the Cantabro-Ebroian Land area, a

mosaic of platforms are represented in the Cambrian Iberian chart (Linan et al., 1993); seeFigs. 2 and 3 for correlations.Cambrian outcrops of the Pyrenees, Montagne

Noire and SW Sardinia. A S–N-trending sectionof this area comprises the Pyrenean and the 3.1. The Souss BasinMontagne Noire Platforms, the latter one includ-ing the Catalan, Occitan and Albigeois domains Carbonate deposition took place episodically in

the Souss Basin from latest Proterozoic (?) to(Demange, 1994). The main palaeontologic andbiostratigraphic data provided by this paper come Marianian times. This episode has been divided,

from a lithostratigraphic point of view, into thefrom the Minervois and Pardailhan nappes(Occitan domain) due to the scarcity of accurate Adoudou, Lie-de-vin, Igoudine, Amouslek,

Lemdad, Tislit and Issafen Formations (Geyer,correlations for the other outcrops. The Cambrianstratigraphic and biogeographic patterns of SW 1989). Afterwards, carbonate substrates disappeared

in the basin excepting the ‘Breche a Micmacca’Sardinia reveal a close similarity to the MontagneNoire (Geze, 1952), so that this isolated platform (lower Jbel Wawrmast Formation) that contains

bioclastic-rich, bedded and nodular limestones.can be considered neighbouring the MontagneNoire Platform. Platform evaporites are recognized in the

Fig. 2. Lower Cambrian biostratigraphic correlation throughout the western Mediterranean area [after Debrenne (1964), Perejon(1986), Spizharski et al. (1986), Geyer (1990), Linan et al. (1993), Debrenne and Debrenne (1995), Geyer and Landing (1995),Moreno-Eiris et al. (1995), Pillola et al. (1995) and Zhuravlev (1995)].

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Fig. 3. Stratigraphic chart of the Lower Cambrian platforms recognized in the western Mediterranean area [after Schmidt-Thome(1973), Bechstadt et al. (1988), Bechstadt and Boni (1989), Geyer (1989), Alvaro et al. (1993a, 1995, 1998a,b), Linan et al. (1993),Geyer and Landing (1995), Pillola et al. (1995) and Alvaro and Vennin (1998)].

Adoudou, Lie-de-vin and Tislit Formations the Tislit Formation (up to 300 m thick), definedfor proximal areas of the Souss Basin, contains(Destombes, 1952; Lotze, 1957). The carbonates

of the Adoudou Formation (up to 150 m thick) thick intervals of largely dolomitic carbonates, inwhich pseudomorphs after halite crystals arereflect deposition under shallow subtidal to inter-

tidal marine conditions, which include stromato- known from the El Graara massif (Destombes,1952; Geyer and Landing, 1995) overlying shallow-litic dolostones with scattered anhydrite moulds at

the western El Graara massif (Buggisch and ing-upward cycles and associated with desiccationcracks (Siegert, 1986).Flugel, 1986). The carbonate intercalations of the

Lie-de-vin Formation (300–950 m thick) weredeposited under persistent low energy, shallow, 3.2. The Cantabro-Iberian Basin (Iberian and

Cantabrian Platforms)relatively hypersaline conditions (Monninger,1979), which led to rhythmic deposition of tidalflat dolostones with microbial laminites and halite Two phases of episodic carbonate deposition

are identified in the Iberian Platform (Iberianand anhydrite/gypsum (Schmitt, 1979). Finally,

110 J.J. Alvaro et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 160 (2000) 105–122

Chains, NE Spain): the late Ovetian–Marianian the existence of a mosaic of platforms or troughs(Delgado-Quesada et al., 1977; Linan andand late Bilbilian–early Middle Cambrian

intervals, which are bounded by deposition of Quesada, 1990). Both in the Alconera (Linan andPerejon, 1981; Moreno-Eiris, 1987) and thesiliciclastic sediments associated with the Daroca

regression (Alvaro and Vennin, 1998). The second Cordoba (Linan, 1978) troughs, mixed sediments(50–1000 m thick) rich in trilobites, archaeocya-episode lacks evaporitic remains and will not be

described here. In contrast, the first episode is than and microbial build-ups were deposited fromOvetian to Marianian times. However, in spite ofrepresented by mixed (carbonate–siliciclastic)

deposits, comprising the Jalon, Ribota and the common record of intertidal to shallow subti-dal deposits, no relics of evaporites have beenHuermeda Formations. The Jalon Formation

(250–325 m thick) is composed of stromatolitic found.dolostones, sandstones and variegated shalesdeposited under shallow marine environments 3.4. The Armorican Basinranging from shallow subtidal into intertidal andsupratidal (Schmidt-Thome, 1973; Alvaro et al., The Lower Cambrian sedimentary successions

of the North Cotentin Peninsula are the most1993b), which yield common halite pseudomorphsin muddy shales. This formation is overlain by representative of the entire Armorican Massif. In

this SW–NE-trending trough a thick Lower30–120 m of dolostones and marls (RibotaFormation), which represent the establishment of Cambrian succession has been divided into four

formations (Dore, 1994; Fig. 3). The Saint-Jean-shoals and subsequent development of back-shoal,peritidal deposits with scattered halite moulds, de-la-Riviere Formation is the only unit exhibiting

carbonate lithologies. It consists of alternatingfrequently submitted to subaerial conditions; incontrast, the distal shoal dolostones have yielded dark limestones and siltstones, the latter containing

pseudomorphs after halite. Limestone beds wereanhydrite/gypsum pseudomorphs (Alvaro et al.,1995). deposited in subtidal to tidal environments, and

record common archaeocyathan and stromatoliticThe Cantabrian Platform (NW Spain) hasrecorded a unique carbonate episode from late build-ups of early Ovetian age.Ovetian to early Middle Cambrian: the Lancaraand Vegadeo Formations (50–300 m thick). Their 3.5. Sardinia and the Montagne Noirelower members contain distinct facies rich in bird-seyes, microbial laminites and oolitic grainstones On the northern side of the Cantabro-Ebroian

Land area, the palaeogeographic evolution is bestcontaining dolomite rhombs, pseudomorphs aftercalcium sulphate minerals (Russo and Bechstadt, known in SW Sardinia. Carbonate sediments were

deposited from middle Ovetian to early Middle1994), and scattered idiomorphic quartz interpre-ted as being characteristic of hypersaline condi- Cambrian times, and are included in the Matoppa,

Punta Manna, Santa Barbara, San Giovanni,tions (Zamarreno, 1972, 1975). These facies weredeposited under peritidal conditions on homoclinal Planu Sartu and Campo Pisano Formations

(Pillola et al., 1995; Perejon et al., 2000). Theramps (Aramburu et al., 1992; Russo andBechstadt, 1994), and were locally topped by an geodynamic and palaeogeographic framework of

the area can be summarized as the successiveunconformity (Mohr, 1969; Gietelink, 1973) asso-ciated with the Lower–Middle Cambrian boundary establishment of a homoclinal ramp, a rimmed

shelf and an isolated platform (Gandin, 1987;(Alvaro et al., 1993a).Bechstadt et al., 1985, 1988; Bechstadt and Boni,1989, 1994), with evaporitic precipitation in the3.3. The Ossa–Morena Basinlast phase. The Santa Barbara Formation containsevaporites found in two kinds of environmentalThe Lower Cambrian sedimentary successions

of the rifting Ossa–Morena Basin are differentiated setting: (i) arid, restricted tidal environments on aplatform well-documented by the presence ofaccording to various structural units, which show

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moulds after anhydrite or gypsum, rosettes of Some of the most representative relics of evapo-rites are found in the Iberian Chains and theformer gypsum, and local idiomorphic gypsum/

anhydrite crystals, probably of diagenetic origin Cantabrian Mountains (N Spain), and in theMontagne Noire (southern France), and will be(Schledding, 1985; Bechstadt and Boni, 1989).

These alternate with stromatolitic and tidal depos- illustrated below. They appear as various diage-netic chert facies (scattered nodules, aggregates ofits displaying common subaerial exposures

(Carannante et al., 1984; Bechstadt et al., 1985; coalesced nodules and discontinuous layeredmasses), which are disseminated within well-Bechstadt and Boni, 1989; Cocozza and Gandin,

1990); (ii) tidal environments characterized by bedded to finely laminated dolostones. Althoughless abundant, carbonate pseudomorphs after len-subaerial tufas containing microbial carbonates,

laminated fenestrae and evaporitic moulds on plat- ticular gypsum, moulds of halite cubes filled bymudstone and carbonate pseudomorphs afterform margins. Local cyclic sediments have been

deposited during repeated, small-scale tilting that cubic minerals (probably halite) are locally present.formed tectonically enhanced grabens with smallponds infilled with partly slumped material and 4.1. Mud-filled or carbonate pseudomorphs after

halitefinally covered by laminated tidal to supratidalcarbonates (Bechstadt and Boni, 1989).

In the southern Montagne Noire, episodic car- Two major types of pseudomorph after halitecrystals have been identified: one in the Jalonbonate deposition expanded from latest Ovetian

to early Middle Cambrian times, which forms the Formation (Iberian Chains) and the other in theLancara Formations (Cantabrian Mountains). ThePardailhan, Lastours, Pont de Poussarou and

La Tanque Formations (Alvaro et al., 1998a). first type consists of mud-filled, cube-shapedmoulds, about 0.5 mm in average size, which areEvaporitic cauliflower-like nodules of anhydrite/

gypsum occur in the upper member of the Lastours disseminated on the surface of mudstone layers(Fig. 4A). The second type is petrographicallyFormation (‘serie schistodolomitique’ et ‘calcaires

en plaquettes’; Boyer, 1962), preserved within silica ambiguous: it is represented by different cubicsections of small size, up to 100 mm acrossnodules. The upper member was deposited in

peritidal environments submitted to episodic sub- (Fig. 4B), now preserved as dolomite, which couldrepresent remains of either halite or late diageneticaerial exposures marked by erosive surfaces. Silica

nodules occur on the top of shallowing-upward anhydrite crystals, both of them displaying similarfeatures (Rouchy et al., 1984). The almost exclu-cycles (Alvaro et al., 1998c). Finally, an upper

Ovetian microbial–archaeocyathan build-up com- sively square shapes of the sections and the hopper-like depressed faces of some of them indicate thatplex has been recognized and biostratigraphically

dated in an isolated nappe of the eastern Pyrenees they represent sections of halite crystals, whichwere filled (after dissolution) by carbonates.(Terrades area; Perejon et al., 1994; Abad et al.,

1996). Despite the differences in crystal size, and natureof the host-sediment and mould filling, both typesseem to correspond to displacively formed halitecrystals, similar to those described by Gornitz and4. Petrographic features of evaporitesSchreiber (1981) from the Holocene sediments ofthe Dead Sea. They were formed by intrasedimen-The Lower Cambrian deposits described above

contain a variety of evaporitic remains that have tary crystallization in clays or carbonate host-sediment as the consequence of the capillary con-been silicified, replaced by carbonates or filled by

silty material. Observations of samples and thin centration of pore fluids. Their replacement implieslarge fluctuations in salinity, resulting in alternat-sections reveal the presence of precursor primary

gypsum, anhydrite and halite, whose original tex- ing precipitation and dissolution. In the first type,halite crystals grew through fine-grained terrige-tures allow reconstruction of their mode of forma-

tion and environments of deposition. nous deposits where the moulds of the crystals,

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after dissolution, became filled downward by the cally stacked and display enterolithic structures(Fig. 4E and F). The larger nodules are commonlyoverlying material (Plaziat and Desprairies, 1969;

Haude, 1970). In the second type, which grew in formed by aggregation of small individual nodules,commonly less than 1 cm in size, separated fromhomogeneous fine-grained carbonate sediments,

the halite moulds are filled by secondary, precipi- each other by dolomitic laminae deformed aroundthe nodules or squeezed between them to formtated carbonates.irregular carbonate patches or partings. At themicroscopic scale, the silica nodules display4.2. Dolomite pseudomorphs after gypsum crystalsdifferent varieties of quartz, such as dense aggre-gates of small interlocked, anhedral grains, mosaicsPseudomorphs after gypsum crystals appear in

the Ribota Formation (Iberian Chains) as ghosts, of equant anhedral to subhedral megaquartz,spherulites of megaquartz with radial extinctiondiscoidal to lozenge-shaped, 1 to 5 mm in length,

commonly replaced by a mosaic of sparry dolomite and, in some cases, aggregates of chalcedony.(grains up to 500 mm in size) within a fine-grainedto micritic dolomite matrix (Fig. 4C and D).

Such a crystal habit is very common in evapo- 5. Interpretation of evaporitic pseudomorphsritic deposits, and is usually interpreted as a resultof early diagenetic interstitial precipitation from The features that indicate former evaporites

replaced by silica are as follows.hypersaline pore-fluids. However, in the depositsstudied, the aggregates of randomly interlocked (1) The morphologies of the cherts show that

they were initially composed of individual to tightlycrystals clearly indicate they formed by intrasedi-mentary growth. The formation of the gypsum packed nodules separated by carbonate laminae or

partings, which were deformed by the growth ofcrystals can be interpreted as a result of crystalliza-tion in supratidal deposits, either in the phreatic nodules suggesting that the nodules initially grew

within a soft carbonate matrix. These features areor capillary zones.very reminiscent of the ‘chicken-wire’ structure ofanhydrite (Fig. 4E) and even, in some cases, of4.3. Silica replacement of nodular calcium-

sulphates enterolithic structures (Fig. 4F). The presence insome of the nodules of anhydrite relics (Fig. 5E)allows us to interpret them as resulting from theThe cherts of the upper member of the Lastours

Formation (Montagne Noire) display a great vari- replacement of nodular anhydrite, similar to thatformed in supratidal deposits by interstitial growthety of morphologies, such as millimetre- to several

centimetre-sized scattered nodules, irregular aggre- related to evaporation in the capillary to vadosezones (Shearman, 1963, 1966; Butler et al., 1982).gates of coalesced nodules (decimetric in size), and

discontinuous layered masses some centimetres to Similar features have been reported in sedimentarysuccessions of different ages (Folk and Pittman,several decimetres in length. The chert nodules

may be spherical, ovoid or flattened, can be verti- 1971, Siedlecka, 1976; Schreiber, 1977; Ulmer-

Fig. 4. Relics of evaporites. (A) Mud-filled moulds of halite (arrows) embedded in purple shales, Jalon Fm. (At 20/15), IberianChains. Sample surface; scale bar: 1 cm. (B) Dolomite pseudomorphs after halite in limestones, lower Lancara Fm. (BL1/26),Cantabrian Mountains. Thin-section photomicrograph, plain light; scale bar: 100 mm. (C) Carbonate pseudomorphs after lenticulargypsum, Ribota Fm. (91/Bo/A), Iberian Chains. Thin-section photomicrograph, plain light; scale bar: 1 mm. (D) Carbonate pseudo-morphs of lenticular ( l ) to prismatic (p) gypsum crystals (arrows), upper Lastours Fm. (GR 9.1), Montagne Noire. Thin-sectionphotomicrograph, plain light; scale bar: 500 mm. (E) Silicified coalesced nodules displaying a chicken-wire-like aspect, upper LastoursFm. (GR 14), Montagne Noire. Sample surface; scale bar: 2 cm. (F) Enterolithic-like features (arrows) in a silicified layer, upperLatours Fm. (GR 13), Montagne Noire. Sample surface; scale bar: 5 cm. (G) Vertically arranged silicified nodules, with elongatedangular shapes, indicating replacement of former selenitic gypsum crusts, upper Lastours Formation (GR 10), Montagne Noire.Thin-section photomicrograph, crossed nicols; scale bar: 1 mm.

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Scholle et al., 1993). Although this resemblance spiky texture due to the projection into the dolo-mite matrix of lath-shaped blades reaching 500 mmcannot be a determining criterion for the replace-

ment of previous evaporitic nodules, the association in length (Fig. 5C). The outward development ofthese laths can erase the morphology of the parentin several samples with relics of anhydrite (see

below) indicates a previous evaporitic origin. gypsum crystal, resulting in a micronodular aspect.This process may lead to the transformation of(2) Some quartz aggregates exhibit a vertically

standing pattern of elongated angular-shaped nod- aggregates of gypsum crystals into irregular nodu-lar masses. Many individual laths are also dissem-ules, up to 1 cm high, which clearly mimic the

structure of palissadic aggregates of gypsum crys- inated within the dolomitic matrix and some ofthem clearly display the typical rectangular cross-tals (Fig. 4G). The presence of anhydrite relics in

some megaquartz crystals enclosed in the nodules section of anhydrite (Fig. 5D). The dominant ori-entation of the laths parallel to the bedding indi-also indicates pseudomorphs after gypsum crystals,

which have been converted to anhydrite before cates that they were developed before thecompaction of the sediment.their replacement by silica. This type of gypsum is

known to be formed by subaqueous growth of This texture typically results from the expansivegrowth of anhydrite replacing former gypsum crys-gypsum crystals nucleated above the sediment–

water interface (Schreiber,1977; Shearman and tals where the anhydrite produced is much moreimportant than could have been provided by theOrti Cabo, 1978).

(3) Pseudomorphs after lenticular to lozenge- original gypsum (Shearman, 1983). This type ofreplacement has been reported in both the modernshaped crystals, several millimetres to 1 cm in

length, commonly observed as scattered grains and ancient sedimentary records, and is interpretedas being formed by syndepositional transformationwithin the dolomite matrix, very likely correspond

to former gypsum crystals (Fig. 5A). Although of gypsum into anhydrite crystals, which initiallygrew interstitially in a pre-existing sediment, as inmore difficult, similar crystal shapes can be recog-

nized within the massive quartz nodules where the Persian Gulf sabkhas (Holliday, 1968;Shearman, 1983). The excess of anhydrite impliesthey are sometimes surrounded by dolomite

matrix, indicating that these nodules were initially a continuous supply of calcium and sulphate ionsfrom the trapped brines (Shearman, 1983). Themade up of aggregated gypsum crystals (Fig. 5B).

Some nodules show angular outlines that seem to dominant parallel orientation of the anhydritelaths could have been produced by early processesbe related to the apex of former crystals pointing

outward into the carbonate host-sediment. These of mechanical compaction of this very soft materialcomposed of porous aggregates of crystals retain-crystals are similar to the above-mentioned lenticu-

lar gypsum replaced by dolomite, and they formed ing large quantities of pore water (Shearman andFuller, 1969)mostly by intrasedimentary growth from hypersa-

line pore-fluids in the phreatic to capillary zones. (5) Some quartz crystals exhibit a near-cubictermination, which has been commonly associated(4) Some lenticular to lozenge-shaped ghosts of

gypsum crystals, up to 2 mm in length, display a with evaporites (Milliken, 1979; Arbey, 1980).

Fig. 5. Relics of evaporites. (A) Pseudomorphs after lenticular to lozenge-shaped crystals of gypsum (arrows) within a silicifiednodule, upper Lastours Fm. (GR 1), Montagne Noire. Thin-section photomicrograph, plain light; scale bar: 500 mm. (B) Silicifiedrelics of gypsum crystals (stars), whose boundaries are underlain by dark carbonate, upper Lastours Fm. (GR 13), Montagne Noire.Thin-section photomicrograph, plain light; scale bar: 1 mm. (C) Relics of lenticular to lozenge-shaped gypsum crystals displayingspiky outlines. The rectangular shape of the spikes is typical of anhydrite and indicates that the gypsum crystals have been convertedinto anhydrite before their silica replacement, upper Lastours Fm. (GR 11.1), Montagne Noire. Thin-section photomicrograph, plainlight; scale bar: 1 mm. (D) Detail of (C) showing the rectangular shape of the spikes (a). The sample is rich in pyrite, upper LastoursFm. (GR 11.1). Thin-section photomicrograph, plain light; scale bar: 200 mm. (E) Anhydrite inclusions (arrowed) in a megaquartzcrystal, upper Lastours Fm. (GR 7). Thin-section photomicrograph, crossed nicols, scale bar: 100 mm. (F) Cube-shaped quartzcrystals (arrows), Ribota Fm. (91/Bo/93), Iberian Chains, Thin-section photomicrograph, crossed nicols; scale bar: 100 mm.

116 J.J. Alvaro et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 160 (2000) 105–122

These features are observed at the periphery of tionally associated with carbonate platform sys-tems in the arid subtropical belt. The modernsome nodules from the silicified deposits of thesupratidal flats of the Persian Gulf provide aMontagne Noire, but also as single cube-shapedvaluable analogous for the interpretation ofquartz crystals in the Lancara Formation fromancient environments (Shearman, 1963; Kinsman,NW Spain (Fig. 5F). In the latter example, there1969; Butler et al., 1982; Warren, 1989). Thus, theare also a few idiomorphic quartz crystals display-different modes of formation and related deposi-ing a double termination. Both the cubic andtional settings of the original evaporitic featuresdoubly terminated crystals contain inclusions ofcan be summarized as follows.dolomite, but no anhydrite relics are found, which1. The layers of vertically standing gypsum crys-could prove their initial relation to evaporitic

tals characterize subaqueous crystallization onconditions; however, these morphologies are com-hypersaline systems, which may have beenmonly observed associated with evaporiteseither ephemeral hypersaline ponds (in the(Milliken, 1979; Arbey, 1980; Ulmer-Scholle et al.,supratidal zone), or more perennial systems as1993; Chafetz and Zhang, 1998).intertidal lagoons.The anhydrite inclusions in the megaquartz do

2. The growth of the lenticular to lozenge-shapednot seem to display any oriented fabric whichgypsum crystals usually occurs in supratidalcould suggest that the quartz replacement occurredsediments close to the phreatic table, either inafter a phase of compaction. In contrast, theirthe phreatic or in the lower part of the capillarydistribution suggests a felted fabric typical of thezones. Gypsum crystals grow displacively withinformer crystal growth arrangement that wouldthe carbonate matrix, as scattered crystals orindicate the megaquartz formed before the anhy-aggregates composed of either randomly inter-drite could be compacted. The synsedimentary orlocked crystals or dense crystalline masses withearly diagenetic precipitation of megaquartznodular outlines. The gypsum is commonlyrelated to evaporitic conditions, mostly in the formaltered to anhydrite during hot and dry periods,of euhedral doubly terminated habit, has beenand the growth of new anhydrite crystals indescribed from the sediments of the modern hyper-and around the former crystals, due to a con-saline lagoon of Fernan Vaz, Gabon (Giresse,tinuous supply of calcium and sulphate ions1968), and from the evaporite-bearing Pleistocenefrom the concentrated pore fluids, commonlysabkha deposits of the Persian Gulf (Chafetz andleads to nodular morphologies.Zhang, 1998).

3. The nodular to chicken-wire facies are formed(6) Tiny relics preserved as anhydrite, appearingin supratidal deposits above the phreatic level,as rectangular cross-sections, 20 to 50 mm in size,in the capillary to vadose zones.are common, but they are restricted to megaquartz

4. The scattered halite crystals grew displacivelycrystals, spherulitic aggregates or mosaics of anhe-within carbonate or terrigenous deposits neardral megaquartz (Fig. 5E). The presence of typicalthe surface of the sediments.doubly terminated quartz crystals has not beenExcept for subaqueous crystallization ofobserved in the silicified nodules of the Montagne

gypsum crusts composed of vertically standingNoire. The inclusions of anhydrite are easily distin-crystals of gypsum, all the evaporitic remainsguished from the dolomite by their typical rectan-reported here, from the Montagne Noire (France)gular outline or fibrous features, and sometimesand the Iberian Chains and the Cantabrianby a low birefringence due to a partial rehydrationMountains (Spain), characterize processes of inter-to gypsum. The association of various types ofstitial growth from hypersaline pore fluids concen-

megaquartz with anhydrite has been commonly trated by mechanisms of capillary evaporation.observed in many ancient and modern silicifieddeposits (Siedlecka, 1972; Chowns and Elkins,1974; Schreiber, 1977; Milliken, 1979; Arbey, 1980; 6. ConclusionsChafetz and Zhang, 1998).

The main evaporitic facies described here clearly The varied and abundant relics of primary toearly diagenetic evaporites (gypsum, anhydrite andtypify different types of platform setting, conven-

117J.J. Alvaro et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 160 (2000) 105–122

halite) demonstrate that extensive evaporitic condi- The lack of extensive carbonate depositiontions were locally associated with the evolution of across the Proterozoic–Cambrian transition inthe Early Cambrian carbonate-dominant and south-western Europe can be explained by appeal-mixed platforms of the western Gondwana margin. ing to a combination of active orogenic phasesThese relics appear as ghosts of cubic halite crys- [represented by the angular unconformitiestals, lenticular/lozenge-shaped gypsum crystals, described by Martınez Catalan (1985), San Josecrusts of vertically standing gypsum crystals, and et al. (1990) and Truyols et al. (1990)] and activenodular to mosaic masses of anhydrite. The source regions supplying conglomerates andcalcium sulphate deposits have been extensively coarse-grained sandstones, which inhibited car-replaced by either dolomite or various types of bonate production in these areas.quartz, whereas the halite appears as moulds of Another diachronous occurrence of climaticallycubic crystals, dissolved and filled by silty material sensitive facies is present in the westernor replaced by carbonates. Mediterranean area, where pseudomorphs of evap-

Originally, evaporites were seemingly more orites were precipitated during the late Vendianabundant than suggested by the remains that have (?)–Ovetian interval in the Souss Basin, the latebeen recognized, because these deposits have Ovetian–early Marianian in the Iberian Platform,undergone multistep diagenesis which erased most and the latest Marianian–early Bilbilian (?) in theof the former morphologies. Silicified facies, sim- Cantabrian, Armorican and Montagne Noire–ilar to those described here, are conventionally Sardinian Platforms. Although each Cambriancriteria for identification of vanished evaporite platform in the western Gondwana margin hasdeposits (Schreiber, 1977), and have even been recorded different relative sea level fluctuations,qualified as a ‘silicified evaporite syndrome’ due to differences in geodynamic settings and(Milliken, 1979). This phenomenon is well sedimentation rates, the occurrence of evaporitesillustrated in some Carboniferous deposits of the seems to be directly related to climatic conditionsFrench Ardennes (Rouchy et al., 1984, 1986),

associated with the drifting of this margin fromwhere calcium-sulphate deposits, up to several

inter-tropical towards higher temperate latitudes.hundreds of metres thick, were discovered in sub-According to Scotese and Barret (1990), thesurface while their coeval outcropping deposits

latitudinal abundance of climatically sensitivecontained only scattered carbonate pseudomorphsfacies can be estimated by statistical techniques:and silicified nodules, associated with collapsecarbonates have a maximum likelihood of occur-breccias.ring between 10 and 30° latitude, and evaporitesCarbonates were deposited in the Souss Basinat 25–35° latitude. The time span involved in theduring the latest Proterozoic (?)–Marianian,Cordubian–Ovetian–Marianian interval [8–whereas in the Ossa–Morena Basin they are of late10 m.y.; adapted from Landing et al. (1998)]Ovetian–Marianian age, and in the Cantabro-requires a relative high rate of drifting, whichIberian, Armorican and Montagne Noire–would support the high APWP rates proposed forSardinian Basins of late Ovetian to early MiddleEarly Palaeozoic times.Cambrian age (Fig. 6). The latter episode was

Therefore, the existence of an Early Cambrianlocally punctuated by the Daroca regressionsouthern Hemisphere arid belt is envisaged in(Alvaro and Vennin, 1998) and deposition ofterms of widespread evaporites in the westerncoarse siliciclastic sediments in the IberianGondwana margin. The presence of other LowerPlatform. The episodic carbonate deposition of theCambrian evaporites in North America, thenorthern platforms was replaced by terrigenousArgentine Precordillera, Bolivia, Iran and parts ofsedimentation at the beginning of the Middlethe Middle East, south-western Asia, China andCambrian and reappeared only in the Ashgill (LateSiberia [summarized in Scotese and Barret (1990)]Ordovician), except in the Montagne Noire, whereand the absence of glacial deposits indicate thatcarbonate deposition occurred across the

Cambrian–Ordovician transition. the global Lower Cambrian climatic gradient was

118 J.J. Alvaro et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 160 (2000) 105–122

Fig. 6. Sketch map of the Souss, Ossa–Morena, Cantabro-Iberian, Armorican and Montagne Noire–Sardinian Basins, illustratingthe palaeogeographic evolution throughout the Early Cambrian and occurrence of evaporitic sediments (summarized from referencesreported in the text); the AG line indicates the tectonic boundary between the Anti-Atlas domain and the Hercynian fold belt; theBC line represents the Badajoz–Cordoba shear zone.

119J.J. Alvaro et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 160 (2000) 105–122

Alvaro, J.J., Linan, E., Vizcaıno, D., 1998b. Biostratigraphicalvery low and the overall climate warmer thansignificance of the genus Ferralsia (Lower Cambrian, Trilob-today.ita). Geobios 31, 409–504.

Alvaro, J.J., Vennin, E., Vizcaıno, D., 1998c. Depositional con-trols on Early Cambrian microbial carbonates from theMontagne Noire, southern France. Trans. R. Soc. Edin-Acknowledgementsburgh: Earth Sci. 89, 135–143.

Aramburu, C., Truyols, J., Arbizu, M., Mendez-Bedia, I.,The authors are very grateful to F. Ortı and Zamarreno, I., Garcıa-Ramos, J.C., Suarez de Centi, C.,

J.J. Veevers for helpful advice and comments, to Valenzuela, M., 1992. El Paleozoico Inferior de la ZonaCantabrica. In: Gutierrez-Marco, J.C., Saavedra, J.,Mr. Destarac for the photography, and to W.Rabano, I. (Eds.), Paleozoico Inferior de Ibero-America.Hammann and E. Villas for fertile discussions inUNEX Press, Merida, pp. 397–422.the field which stimulated further debates about

Arbey, F., 1980. Les formes de la silice et l’identification desEarly Palaeozoic palaeogeography. This paper is evaporites dans les formations silicifiees. Bull. Cent. Rech.a contribution to the Spanish Projects PB 96-0842 Explor.-Prod. Elf Aquitaine 4, 309–365.and PB 98-1625, and to IGCP Project ‘Ecological Bechstadt, T., Boni, M., 1989. Tectonic control on the forma-

tion of a carbonate platform: the Cambrian of southwesternaspects of the Cambrian radiation’.Sardinia. In: SEPM Spec. Publ. 44, 107–122.

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