Messinian halite and residual facies in the Crotone

basin (Calabria, Italy)

S. LUGLI1, R. DOMINICI2, M. BARONE2, E. COSTA3 & C. CAVOZZI3

1Dipartimento di Scienze della Terra, Universita degli Studi di Modena e Reggio Emilia,

Largo S. Eufemia 19, 41100 Modena, Italy (e-mail: lugli.stefano@unimore.it)2Universita degli Studi della Calabria, Dipartimento di Scienze della Terra, 87036

Arcavacata di Rende (Cosenza), Italy3Dipartimento di Scienze Geologiche, Universita di Parma, Parco Area delle Scienze

157/A, 43100, Parma, Italy

Abstract: The Neogene Crotone basin in eastern Calabria contains extensive Messinian evaporitedeposits, including thick gypsarenite and halite. The halite deposit reaches a maximum thicknessof c. 300 m and in some areas forms relatively small diapirs piercing late Messinian and Pliocenesediments. Halite is strongly modified by folding and recrystallization, but a few primary featuresare preserved. Four primary halite facies have been recognized: (a) banded halite consisting offolded white and dark bands deposited in a salt pan and/or saline mudflat; (b) white facies,massive halite containing anhydrite nodules, probably formed in a variably desiccating salinelake; (c) clear facies made up of a mosaic of large blocky halite crystals separated by mud, possiblythe product of displacive halite growth in a saline mudflat; and (d) breccia facies, a product of dis-solution of halite/mudstone/siltstone layers. Residual facies formed from halite dissolution arepresent as both weld and cap rocks. Weld rocks are thick, undeformed, and composed only of inso-luble phases originally included in the salt, whereas cap rocks are thin, strongly sheared andinclude clasts from the cover rocks.

As one of the less investigated depositsformed during the Messinian Salinity Crisis in theMediterranean, halite was deposited in theCrotone basin, Calabria (south Italy). The extentof the halite deposit is well known, based on theearly works of the 1960s that were producedmostly for mineral extraction purposes (Roda1964), but no detailed description of the halitefacies has ever been produced. Since then, ourunderstanding of the Messinian Salinity Crisis haslargely improved and facies analysis of theevaporites provided us with new information ontheir environmental evolution. Although the signifi-cance of the Mediterranean halite deposits in theSalinity Crisis framework is not yet well con-strained (Manzi et al. 2006), the study of halitefacies in Sicily revealed desiccation surfaces(Lugli et al. 1999, 2006) and significant constraintson their depositional environments (Lugli &Lowenstein 1997).

This paper documents the halite facies and itsdissolution products in the Crotone basin. Thescope of this study is to provide new insights onthe complexity of the Messinian Salinity Crisisand to provide criteria for the recognition of halitedissolution products that may be useful in tracingelusive weld surfaces.

Geological setting and stratigraphy

The Crotone basin is located on the northeasternmargin of the Calabrian arc and represents thefilling of the wedge-top basin of the southernItaly foreland basin system (DeCelles & Giles1996; Critelli 1999; Fig. 1). Sedimentation inthis basin began in the Serravallian?–Tortonianwith deposition of a shallow-marine arenaceous–conglomerate succession (San Nicola Fm) passingupward to marine argillaceous deposits (PondaFm, Tortonian) and organic-rich laminites (TripoliFm, Tortonian–Messinian).

During the Messinian the sedimentation in thearea was dominated by evaporitic conditions produ-cing sulphate deposits and halite. In the Crotonebasin such conditions are recorded mostly byhalite (maximum thickness 300 m) and by extensiveclastic evaporite deposits, such as a thick gypsare-nite body (up to 120 m thick), with minor gypsru-dite and limestone breccias.

These evaporite deposits are associated with asedimentary chaotic complex (SCC), consisting ofmetric blocks of limestone and alabastrinegypsum included in an argillaceous matrix. A lateMessinian succession consisting of gypsarenites,sandstones and pelites with a Lago Mare fauna at

From: SCHREIBER, B. C., LUGLI, S. & BABEL, M. (eds) Evaporites Through Space and Time.Geological Society, London, Special Publications, 285, 169–178.DOI: 10.1144/SP285.10 0305-8719/07/$15.00 # The Geological Society of London 2007.

the top (Gennari & Iaccarino 2004) onlaps boththe chaotic complex and the gypsarenites. An ero-sional surface separates these latest Messiniandeposits from the overlaying fluvial conglomerates(Carvane conglomerates), which are in turnabruptly covered by the off-shore Cavalieri marls(Early Pliocene). The marls grade upward into theshallow-marine Zinga Molasse deposits (Pliocene;Roda 1964; Zecchin et al. 2004).

The original stratigraphic relationships of halitewith other evaporite sediments (the main gypsare-nite body) in the Crotone basin is unclear atpresent due to diapiric condition of the salt.Seismic and well data from salt mining areas(Belvedere di Spinello; Roda 1964) show thathalite is interposed between organic-rich laminitessimilar to the Tripoli Fm at the base and latestMessinian gypsarenites, arenites and pelites onthe top. A few boreholes show that in someareas halite probably lay on top of the ChaoticComplex. Unfortunately these cores are no longeravailable for scientific examination. Furtherinvestigations and planned new cores will help toreconstruct the stratigraphic position of salt.

The investigated diapirs are located along N 408-and N 808-oriented morphostructural highs andpierce the late Messinian and early Pliocenedeposits (Fig. 1).

Materials and methods

To describe the halite facies we directly sampleddiapiric and wall structures in the Zinga villagearea (Fig. 1). In addition, we also examined a fewremaining samples from cores drilled in the pastin the salt mining areas (Belvedere di Spinello,Crotone). Detailed provenance and stratigraphicposition of these samples are not known and weconsidered them just for the purpose of faciesdescription completion.

For thin section preparation, halite samples werecut into 4 � 5 � 3 cm slabs using a diamond sawwith a small amount of water to prevent formationof microcracks and significant dissolution artefacts(Schleder & Urai 2005). The slabs were polisheddry on grinding paper, mounted on glass platesusing epoxy resin and then cut with a wet

Fig. 1. Schematic geological map of the Zinga-Belvedere di Spinello area of the Crotone basin, eastern Calabria.

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diamond saw to a thickness of about 5 mm. The sec-tions were then ground down with dry grindingpaper to a thickness ranging from less than 1 toabout 2 mm, depending on the specific sample.Residual samples were impregnated in epoxyresin (araldite) and then prepared using the standardtechniques for thin section production.

Halite facies: description

In diapiric structures halite clearly shows the actionof moderate to high plastic deformation and flow:sheared marl clasts, crude imbrication of marlclasts, isoclinal folds, shear folds (Fig. 2) andalignment of halite crystals (Fig. 3). Four distinctoriginal (primary) halite facies were recognized.

Banded facies

This is a folded alternance of centimetre- todecimetre-thick dark and white bands (Fig. 2); thewhite bands consist of up to 5 mm elongate clearcrystals, whereas the dark bands contain variableamounts of non-halite clasts; the clasts are up to10 cm in size and consist of mudstones, contortedmudstone/anhydrite laminites and gradedsiliciclastic-carbonate siltstones. The halite crystalsare free of primary fluid inclusions (Fig. 3).

White facies

This is a massive halite rock formed by large, equant,blocky crystals (up to 5 cm in size) containing sul-phate nodules up to 1 cm in size (Fig. 4); the halitecrystals are clear but most of them contain awhitish core rich in fluid inclusions. Some of thesecores show halite with chevron-like shapes (markedby fluid inclusions, Fig. 4). Anhydrite nodules andthin films of mud and microcrystalline anhydriteare located at the grain boundaries and within theclear overgrowth zone of the large halite crystalsbut are not present in the fluid inclusion-rich core.These characteristics suggest that the fluid inclusion-rich milky core represent remnants of primary halitecrystals, which underwent rim recrystallization toform a clear overgrowth by grain boundary migration(Schleder & Urai 2005).

Clear facies

This rock is composed of a mosaic of large blockyirregular and elongate crystals (up to 5 cm) separ-ated by thin films of mud (Fig. 5). This facies hasbeen documented only in cores. The zones withthicker mud partings (a few millimetres across)contain small displacive halite cubes and hexagonalshapes filled by microcrystalline anhydrite; some ofthe large halite crystals have fluid inclusion-richcores and thick, clear, overgrowths. The clear

Fig. 2. Salt diapir cropping out along the Vitravo valley (* 1 in Fig. 1). The diapir pierced the marls of the PlioceneCavalieri Fm and consists of the banded halite facies. White bands are pure halite, dark bands contain variableproportions of mudstone clasts. Notice the relatively thin cap rock cut by satin spar veins on top of the salt.

MESSINIAN HALITE AND RESIDUAL FACIES IN CALABRIA 171

rims commonly include growth bands marked bythin mud sheets, suggesting a displacive overgrowthin the mud after initial mud-free growth (Fig. 6).

Breccia facies

This rock is composed of clear irregular halitecrystals (a few centimetres across) cementing

decimetric clasts of graded laminites of siliciclas-tic–carbonate siltstone/mudstone/anhydrite; inthe larger clasts the mudstone contains smalldisplacive halite cubes and nodular anhydrite(Fig. 7); this rock has been observed only in coresand probably represents the result of earlydissolution collapse of the interbedded halite/mudstone/siltstone layers.

Fig. 3. Photomicrograph showing the white part of the banded halite facies. Crystals are elongated, fluid inclusion-freeand meet at triple junctions with angles approaching 1208, suggesting complete recrystallization. Plane polarizedlight. Salt diapir, * 1 of Figure 1 (see also Fig. 2).

Fig. 4. Outcrop view of the white halite facies (diapir * 2 in Fig. 1); large equant blocky crystals contain awhitish core marked by fluid inclusions. Some of these cores evidence chevron-like shapes. Several anhydritenodules are present (arrows). Thin white lines crossing the rock are ephemeral salt efflorescence crusts grown alongfractures. Finger for scale.

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Halite facies: interpretation

Although most salt is deformed by flow and recrys-tallization, a few primary features that may help toreconstruct the original sedimentary setting are stillpreserved in some of these halite rocks (Shearman1970; Hardie & Lowenstein 1985; Handford1991). The original facies were massive halite andhalite/mud/anhydrite sequences.

The massive facies (white facies) may representformer primary halite crusts cut by dissolution pitsthat were then filled by precipitation of clearhalite cement. This facies possibly represents depo-sition in a saline lake which underwent stages ofdesiccation to form a dry salt pan (Lowenstein &Hardie 1985).

The banded facies can be interpreted as theresult of flow, disruption and complete

recrystallization of former alternance of purehalite (white bands) and sequences of halite/mud-stone/siltstone laminites (dark bands). Deformationand recrystallization completely obliterated theprimary features of the halite. The presence ofmud and anhydrite laminite intercalations,however, suggests repeated flooding and dilutionof the salt basin by undersaturated, muddy,waters. Although no specific sedimentary featuresare preserved in this type of halite, with the excep-tion of dissolution breccias (breccia facies), thetypical halite/mud alternation strongly suggestsdeposition in a salt pan and a saline mudflat (Low-enstein & Hardie 1985). The presence of these thinbeds of graded siliciclastic siltstone may be theproduct of sheet floods on both/either a salt flat ora mudflat.

Fig. 5. Clear halite facies. Core slab consisting of a mosaic of large irregular and elongated crystals, separated bymud. Some crystals show a milky core marked by fluid inclusions (white arrow) and a clear overgrowth (gray arrow)(see also Fig. 4). Belvedere di Spinello hydrodissolution plant, stratigraphic position of sample unknown.

MESSINIAN HALITE AND RESIDUAL FACIES IN CALABRIA 173

The clear halite facies probably formed fromsubaqueous primary crystals (either cumulate orbottom nucleated) that underwent further displaciveovergrowth within the mud, together with minoramounts of gypsum, in a saline mudflat.

Residual facies: description

Both weld rocks (Jackson & Cramez 1989) and caprocks crop out in the studied area allowing for thefirst time, a direct comparison of their macroscopicand microscopic features. Weld rocks mark removalhorizons from which salt flowed, whereas cap rocksrepresent horizons of salt dissolution at the diapircrest. For this reason, the two horizons differ in

position within the sedimentary sequence and inlateral continuity, as well as in their composition.

Cap rocks

In the studied area the halite diapirs pierce the latestMessinian and the Pliocene sediments and arecharacterized by thin cap rocks (maximum thicknessis about 2 m) composed of a silty–marly matrixcemented by prismatic gypsum crystals (Fig. 8) con-taining anhydrite and gypsum nodules and rosettesand including blocks of sandstone, marls and gypsar-enites (Fig. 2). Also, in some places pebbles andcobbles from the Carvane Conglomerate Fm areincluded within the cap rock matrix. The cap rocksare deformed, sheared and cut by satin-spar veins.

Fig. 6. Clear halite facies of Figure 5 under the optical microscope. The core of the halite crystal contains primaryfluid inclusion marking growth bands (black arrows), but the clear further overgrowth is inclusion-free andcontain growth bands marked by thin sheets of mud (white arrows). These characteristics suggest a displacive growthinto the mud after an initial subaqueous growth. Plane polarized light. Belvedere di Spinello hydrodissolutionplant, stratigraphic position of sample unknown.

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Weld rocks

A weld horizon is located in between theSE-verging anticline and the northernmost diapirs,at Cozzo Uliveto (Fig. 1). It is represented by achaotic complex with a maximum visible thicknessof about 10 m, lying just below the latest MessinianLago Mare sediments (Gennari & Iaccarino 2004).

This deposit is composed of prismatic gypsum crys-tals, anhydrite and gypsum nodules, rosettesand gypsarenite clasts floating in a marl matrix(Fig. 9). The gypsum nodules formed by hydrationof anhydrite nodules and are similar to thoseincluded in halite (Figs 9 & 4). In contrast togypsum nodules, which include anhydrite relics,however, prismatic gypsum is relic-free, suggesting

Fig. 7. Photomicrograph of the breccia halite facies showing hollow halite crystals grown displacively into themud together with gypsum now replaced by anhydrite (hexagonal shape at left). Plane polarized light. Belvedere diSpinello hydrodissolution plant, stratigraphic position of sample unknown.

Fig. 8. Photomicrograph of cap-rock residual facies of the diapir of Figure 2. Idiomorphic gypsum crystals intoa fine-grained matrix. The crystals are devoid of anhydrite relicts suggesting direct displacive growth into thematrix. This facies also contains Pliocene foraminifera. Plane polarized light.

MESSINIAN HALITE AND RESIDUAL FACIES IN CALABRIA 175

a pristine displacive growth (de novo) into thematrix (Fig. 9).

Residual facies interpretation: salt weld

vs cap rock

Welding surfaces may be caused by passive saggingof the salt sedimentary cover, by normal faultingand/or associated rollover synclines. They mark ahiatus lasting the duration of salt deposition. Theweld faces are located between the basement andcover rocks (the sub-salt and supra-salt rocks,respectively).

The composition of weld-rocks is dominated bythe insoluble phases originally inter-layered in thesalt and left behind when the salt flows away.Weld rocks may also contain the lowermost partsof the cover collapsed as result of salt withdrawal.By contrast, cap rocks may contain portions of thecover rocks intruded by diapirs at various strati-graphic levels, which form the insoluble phasestogether with the insoluble residue originally strati-fied within the vanished salt.

Weld horizons play an important role in pet-roleum geology, as they allow hydrocarbonmigration from the sub-salt to the cover rocks.They are often easily recognized in calibratedseismic sections, even when they separate geometri-cally conformable sequences. It is more difficult torecognize weld horizons in the field, where few

markers remain after the salt flowed out fromunder its overlying load.

The main features distinguishing weld from caprocks in the Crotone basin are their thickness, com-position and deformation pattern. Weld rocksare much thicker (possibly more than 10 m in thick-ness), contain only insoluble phases that wereincluded in the salt, and appear relatively unde-formed. In contrast, cap rocks are thin (up to2 m-thick), include clasts from the latest Messinianand Pliocene cover rocks and are deeply deformedby shearing between the rising diapir and thepierced cover (Fig. 2).

Late gypsum cement

Associated with evaporites in the Crotone Basin,but not related to halite dissolution, are fine-graineddeposits that appear similar in general aspect andcomposition to the residual sediments. These depos-its consist of an argillaceous matrix including pris-matic and lenticular gypsum crystals. No anhydriterelics are present and no other insoluble residueafter halite dissolution could be recognized.Therefore, caution must be used in interpreting fine-grained sediments including sulphate crystals asproducts of halite dissolution. This is because latedisplacive prismatic gypsum is relatively commonin argillaceous formations surrounding the manyItalian gypsum deposits of both Triassic (Lugli

Fig. 9. Photomicrograph of the weld-rock at Cozzo Uliveto. Gypsum nodules (left and right) and euhedra (centre)included into a fine-grained matrix. The nodules appear light grey-coloured due to the presence of numerous anhydriterelicts, indicating that they formed by hydration of anhydrite nodules originally included into the vanished halite. Theidiomorphic gypsum crystals are devoid of anhydrite relicts and grew displacively into the matrix. Planepolarized light.

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2001) and Messinian age (Testa & Lugli 2000).These fine-grained sediments, cemented bygypsum, are always located in the vicinity ofexisting sulphate deposits and do not containresidual remnants of halite dissolution such as mud-stones and carbonate clasts or anhydrite nodules(and their hydration products). These gypsum crys-tals are not related to halite dissolution, but are lateprecipitates from gypsum-saturated groundwaterseeping through fine-grained sediments afterflowing into sulphate outcrops.

General discussion

The array of salt-related facies that have beenrecognized in the Crotone basin is the productof depositional features that have been modifiedat various level of complexity by shearing, flow,recrystallization and, finally, dissolution. Themain depositional facies that have been observedpoint to a saline basin that evolved into a saltpanand a saline mudflat through repeated episodes offlooding and desiccation.

After burial, it seems that the salt began movingvery early as a consequence of an early Plioceneextension phase. Salt flowed away from the north-ernmost area (downbuilding, sensu Barton 1933;Jackson et al. 1988; Jackson 1997) producingweld rocks and forming diapirs in the adjacentsector further south. In the diapirs the salt flowscause varying degrees of destruction of the primaryfeatures of the salt facies so that they becamestrongly modified with complete recrystallizationand loss of primary relics (banded facies). Theonly possibility to read through and try to recon-struct the primary depositional environment in thiscase is studying the associated sediments that areintercalated in the salt.

Dissolution in the sub-surface and at surfaceexposure finally produced the residual cap rockfacies that commonly appear very similar to weldrocks. This is because the main component ofboth weld and cap rocks are the non-salt com-ponents originally intercalated in the salt, such asanhydrite nodules and anhydrite/mudstone layers.In the case of weld rocks, the non-salt elementsare left behind by the migrating salt, whereas inthe case of cap rocks they are accumulated as theinsoluble residue of salt dissolution. However, dis-tinguishing the two type of rocks after halite hasvanished is possible by considering the differentmechanism of formation and the relative strati-graphic position. As a rule, cap rocks also containclasts deriving from the sedimentary cover thatwere sheared and pierced by the diapirs, whereasweld rocks are devoid of rock fragments youngerthan the salt itself.

Conclusions

In this paper we describe and consider the halitefacies of the Crotone Basin and their associatedresidual products. The halite is strongly modifiedby folding and recrystallization, but some preser-ved features reveal clues on its origin. Four quasi-primary halite facies were recognized:

(1) banded facies consisting of pure halite andhalite including mudstone and anhydriteclasts; this facies was deposited in a salt panand a saline mudflat;

(2) white facies of massive halite containinganhydrite nodules, probably deposited in asaline lake which underwent phases ofdesiccation;

(3) clear facies composed of coarse blocky haliteseparated by thin mud stringers, possibly theproduct of displacive halite overgrowth in asaline mudflat after initial subaqueousnucleation; and

(4) breccia facies, a dissolution collapse productof halite/mudstone/siltstone layers.

Halite flow and dissolution also produced tworesidual facies: weld and cap rocks. Criteria forthe recognition of weld rocks and cap rocks in theCrotone Basin are thickness, composition anddeformation pattern. Weld rocks are thick (possiblymore than 10 m), contain only the insoluble phasesincluded in the salt and appear relatively unde-formed. In contrast, cap rocks are thin (up to2 m-thick), include clasts from the cover rocksand are strongly sheared.

This study benefited from the kind cooperation offered bythe staff of the Syndial hydrodissolution plant (Belvederedi Spinello): L. Musso, C. Fortugno U. Rizzo. Thisresearch was founded by a Cofin grant (E. Costa).

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