Clay mineral assemblages in the Cretaceous Justina bentonite deposit (Neuquén Basin, Argentina): Sedimentological and genetec considerations

PETTINARI, GISELA; MUSSO, TELMA BELÉN; POZO, MANUEL; MARTINEZ, GABRIEL; GONZÁLEZ, RAFAEL

Conferencia; XVII INTERNATIONAL CLAY CONFERENCE; 2022

Anewtypeof sodiumbentonitedepositof Cretaceousagehasbeenrecognizedintercalatedwithdetritalcontinentalfacies in the Neuquén basin (Argentina).The bentoniteoccurs within the AnacletoFormation,the uppermostlithostratigraphicunitof theNeuquénGroup(Cenomanian-Campanian).TheNeuquénGroupcomprisesa sequenceof continentalsedimentsdepositedduringthe initialforelandstageof theNeuquénBasin,conformingthreefnning-upwardfuvialcyclesvaryingin thicknessfrom 500 to 1300 m.Within the study area, the AnacletoFormationcontainsapproximately70 metersof fne-graineddeposits,composedmainlyof peliticandveryfnepsamiticlevels.Peliticlevels are constituteby intercalatedmassivereddishbrown mudstonesand siltstones,while yellowishtograyishpinksandstonesofmediumto fnegrainedsizeconformthe psamiticfaciesof thesequence.Thirty-twosampleswerecollectedforthemineralogicalstudy(XRD,SEMandopticalmicroscopy)of thebentoniteand associatedlithofacies.Bentonitelevelsare intercalatedin siliciclasticfaciesand are easyto identifydue to theirgreyto pinkcolors,sometimeswitha redlamination,or evenred.Thebedsarehorizontalto subhorizontal,0,21-0,8m thick(0.60m averagevalue).Accordingto the bulk and clay mineralogy, three mineralogicalassemblageswere established:Assemblage1.Sodiumdioctahedralsmectite(montmorillonite)with very low proportionof quartz,feldsparsand gypsum.Thesmectiteis relativelywellordered(lowerFWHM,larger crystallitesize).Assemblage2. Sodium,calciumor Ca-Nadioctahedralsmectite(montmorillonite)withvariableproportionof quartz,feldspars,gypsum,micaand analcime.The smectiteis poorerorderedthan that of assemblage1 (higherFWHM,smallercrystallitesize).Assemblage3.Mixedlayersillite-smectite(50-70%Ca-smectite)(R0),illiteandsubordinate7Å phases(chlorite+ kaolinite).Inthebentonites,phyllosilicatesaredominant(74-98%)withameanvalueof92%.Quartz(0.5-11%)andfeldspar(0-19%)occursubordinatewithtracesofgypsumandanalcime(< 2%). In theassociatedfacieslocatedbelowandupperthebentonitelevel,phyllosilicatescontentisvariable(43%-81%)anddependsonthedetritalmineralsproportionsuchasfeldspars(6-28%)andquartz(5-23%).In theselithologiesis noticeablythepresenceof analcimereachingup to7%.Petrographicexamination(thin section)of a representativebentonitesampleshows a laminationconstitutedbylaminarmica grains(phlogopite-biotite)and by relictlaminae,sometimesbroken,with a dirtyappearance.Somequartzgrainsand possibleglassfragmentsare detected.The presenceof euhedralopaquecrystalsis recognizedinsmallaggregates.Secondaryfbrousgypsumhas been also identifedas surfacecoatings.SEM analysisshowedamatrixtype microfabric,showingthe presenceof two groupsof smectiteswith differentparticlesize.The largersmectiteshaveglobularmorphology, reacha sizeof20µm andseemsto originatefromthesmallerones.In samplesfromAssemblage2 canbeidentifedthepresenceofsmallgrain-sizezeolites(analcime)(