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This article was published in an Elsevier journal. The attached copyis furnished to the author for non-commercial research and
education use, including for instruction at the author’s institution,sharing with colleagues and providing to institution administration.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies areencouraged to visit:
Clay mineral assemblages and analcime formation in a Palaeogenefluvial–lacustrine sequence (Maíz Gordo Formation Palaeogen)
from northwestern Argentina
M. Do Campo a,⁎, C. del Papa b, J. Jiménez-Millán c, F. Nieto d
a INGEIS and Departamento de Geología FCEN (CONICET – UBA), Pab. INGEIS, Ciudad Universitaria (1428) Buenos Aires, Argentinab CONICET-Facultad de Ciencias Naturales, Universidad Nacional de Salta, Buenos Aires 177, (4400) Salta, Argentina
c Departamento de Geología, Universidad de Jaén, 23071 Jaén, Spaind Departamento de Mineralogía y Petrología and I.A.C.T., Universidad de Granada-CSIC, Avda. Fuentenueva s/n, 18002-Granada, Spain
Received 28 August 2006; received in revised form 16 March 2007; accepted 20 April 2007
Abstract
The Palaeogene Maíz Gordo Formation is one of the main lacustrine events recorded in northwestern Argentina. It consists ofsandstone, mudstone, and limestone beds 200 m thick, deposited in a brackish–alkaline lake and braided alluvial systems. The MaízGordo Lake evolved mainly as a closed system, with brief periods as an open one. X-ray diffraction (XRD) and scanning electronmicroscopy (SEM) were used to study samples from seven sites, corresponding respectively to proximal, intermediate, andtransitional positions of the fluvial environment and marginal and inner-lake environment, focusing on the clay mineralogy andanalcime formation. The basinward zonation of diagenetic minerals identified in theMaíz Gordo Lake was: mordenite→ analcime→K-feldspar. Although not a typical zonation of saline–alkaline lakes, it does indicate an increase in salinity and alkalinity towards thecentre.
In proximal fluvial settings, smectite predominates at the base of the sequence, with scarce kaolinite. Towards the top, a strikingincrease in kaolinite content suggests a change from a relatively arid climate with alternating humid and dry seasons, towards awarm and humid climate. Kaolinite content clearly decreases in a basinward direction. Such a variation is attributable to changes inhydro-geochemistry, denoting the progressive influence of the brackish and alkaline lake water on interstitial pores. SEM images ofintermediate fluvial samples reveal authigenesis of illite at the expense of kaolinite booklets.
Considerable attention has been paid in recent years tomineralogical studies of Cenozoic and Recent lacustrinesediments. Particular notice has been given to clayminerals and zeolites, as these minerals representimportant keys to deducing changes in source area,palaeoclimate, and sedimentary environments (see, forexample,Mayayo et al., 1996; Inglès et al., 1998; English,2001; Sáez et al., 2003). While many studies haveexamined the mineralogy of lacustrine sediments, lesscommonly have such studies considered the compositionof both alluvial and lacustrine sediments, and the lateralchanges occurring between the two environments.
The Maíz Gordo Formation constitutes, due to itsthickness, extension, and time span, one of the mainPalaeogene lacustrine events recorded in northwesternArgentina (del Papa, 1999). However, although manystratigraphic and sedimentological studies have fo-cussed on this unit, detailed mineralogical studies arestill lacking. Previous works, centering on the carbonaterocks (Matheos and del Papa, 2003), indicated theoccurrence of analcime, which, in fact, is common inmany Palaeogene lakes of northwestern Argentina (delPapa and Menegatti, 1993; del Papa et al., 2002;Novara, unpublication).
In this paper we analyse the mineralogy of rep-resentative alluvial and lacustrine samples of the MaízGordo Formation to study the relationship betweendepositional environments, climate, and mineral com-position. In addition, we examine changes in the claymineralogy between alluvial and lacustrine sediments,with the aim of establishing the influence of hydro-geochemical changes on these minerals. We also explorein detail the alkaline–brackish lake sediments of theMaíz Gordo Formation to determine possible mechan-isms in analcime authigenesis.
Analcime is a widespread mineral in all lacustrinefacies of the Maíz Gordo Formation. This occurrence hasmineralogical importance as, in general, analcime is con-sidered to form from precursor zeolites derived fromvolcanic glass altered in saline alkaline-lake water (Hay,1966; Sheppard and Gude, 1969). However, in this par-ticular area, volcaniclastic material is scarce and analcimeis almost the sole zeolite identified, making it clear thatanalcime must have a different origin in this case.
Despite analcime being common in lacustrinesediments, literature on the physico-chemical require-ments for its formation from non-volcanic precursors isnonetheless scarce. Recently, English (2001) pointedout the importance of saline groundwater and its timeof residence in sediments for analcime precipitation.
Additionally, he identified a siliceous gel as a precursorof zeolite formation.
The aim of this contribution is to discuss how changesin climate and sedimentary setting control hydro-geochemistry and consequently rule the transformationof clay minerals from alluvial to lake settings. We alsoanalyse the conditions of analcime authigenesis in sedi-ments devoid of volcanoclastic material.
2. Geological setting
The Maíz Gordo Formation is the middle unit of theSanta Bárbara Subgroup of the Salta Group (Turner,1959) (Fig. 1a). The Salta Group was deposited in anintracontinental rift (Galliski and Viramonte, 1988)that evolved from the lower Cretaceous to the middlePalaeogene. The deposits are mainly composed of con-tinental sediments with minor interbedded marine levels(Salfity, 1982). In the synrift stage (lower Cretaceous–Maastrichtian), a thick succession of red conglomeratesand sandy facies accumulated near the faulted blocks.The post-rift stage (Maastrichtian–Eocene) is charac-terised by widespread marine transgression and fluvial–lacustrine environments (Marquillas et al., 2005). Lakescharacterised both the synrift and post-rift stages, but itwas during the late post-rift stage (Palaeocene–Eocene)that thick lacustrine successions were produced.
2.1. Maíz Gordo Formation
In order to consider the sedimentary setting and itscontrol on the origin and transformation of the clayminerals in the Maíz Gordo Formation, we present anoverview of the environment and subenvironmentsrecognized. For a detailed description of lacustrinefacies and their sedimentary interpretation see del Papa(1999).
The Maíz Gordo Formation consists of 200 m ofsandstone, mudstone, and limestone beds deposited in abraided plain and in a brackish (slightly saline) andalkaline lake. The lake evolved mainly as a closedsystem, but for short periods became open. Its sedi-mentary facies and palaeogeographic reconstructionindicate a lake with a ramp-type margin with alternatingperiods of low and high energy (del Papa, 1999).
Regional studies attest that the Maíz Gordo basindeveloped during a period of tectonic calm character-ized by a low subsidence rate (Gómez Omil et al., 1989;Salfity and Marquillas, 1994).
Coarse-to-fine sandy fluvial systems surrounded thelake, especially to the west and southwest of the basin(Fig. 1b and c), where excellent outcrops allow reliable
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palaeoenvironmental reconstructions. The palaeocurrentpattern indicates the source area was to the south,southwest and west (Fig. 1b); petrographic analysissuggests a provenance from crystalline rocks (both
granitic and metamorphic). Since no change has beendetected in this provenance, we conclude that the sourcearea remained constant, with no significant variations inthe type of material supplied to the basin.
Fig. 1. (a) Stratigraphic chart of Salta Group. (b) Map showing the study areas and their relative positions in accordance with the environments andsub-environments. (c) Schematic block diagram showing the spatial relationship of facies and the depositional model for Maíz Gordo Formation (seeFig. 3 for legend).
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2.2. Fluvial setting
A succession of white very coarse- to fine-grainedsandstones was deposited in the areas surrounding thelake (Figs. 1c and 2). It comprises stacked, fining-upwardsuccessions 2–5 m thick. Each individual facies succes-
sion is made up, from base to top, of coarse sandstoneswith scoured bases containing coarse lag deposits,massive, trough, and planar cross-bedding (Fig. 3a–b),to fine rippled sandstones and/or fine-grained heterolithiclevels. Many beds exhibit mottled oxidation andpedogenic carbonate (rhizoconcretions and concretions).
Fig. 2. Generalized logs of fluvial and lacustrine environments and units of the Maíz Gordo Formation indicating the relative contents of kaolinite,illite, and smectite in the clay mineral association of the different levels (see Fig. 3 for legend).
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In the upper part of the Maíz Gordo Formation, asuccession of superposed ferric palaeosols can be observedin both channel and in floodplain facies, constituting aremarkable guide level for this unit (del Papa, 1999).
The sandstones are arkose and sub-arkose (Fig. 3c)and in a few cases lithic arkose has been observed. Lessfrequently, feldspathic greywackes were noted, associ-ated with floodplains and overbank deposits.
The association of channels and sand bars facies fillingthe scour features and the minor floodplain preservationsuggest periodically active channels that developed in acoarse-to-medium sandy braidplain. In this setting, thepresence of superposed palaeosol levels is remarkable. Nopalaeosol horizons have been identified; instead, thepresence of ferric and carbonate rhizoconcretions andmotley features suggest poorly developed palaeosols.
Fig. 3. Field photographs and photomicrographs showing the main characteristics of fluvial and lacustrine settings. (a) Coarse fluvial conglomeratesandstones proximal to the source area (outcrop is 30 m thick). (b) Channel-bar association of sandy fluvial system (scale bar=2 m)(c) Photomicrograph of well-sorted sub-arkose, note the subrounded grains and sparite as cement, M: carbonate mud intraclast probably fromfloodplain (scale bar=1 mm). (d) Fine-grained sediments dominated the littoral lake setting. (e) Photomicrograph of brecciated horizon showinglinear and circular fractures and secondary precipitation of sparite (scale bar=1.7 mm). (f) Main facies of inner-lake subenvironment. (g) Detailphotomicrograph showing Anl: euhedral to sub-euhedral crystals (in black) of analcime; B: micritic boundstone; Cal: sparry calcite (scalebar=0.5 mm).
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These characteristics are consistent with soils originatinginmainly aggradational fluvial systems (Kraus andBown,1993).
2.3. Lake-littoral setting
The marginal areas mainly comprise red to reddishbrown siltstones, less frequently mudstones, carbonatemudstones, sandstones, and stromatolite levels. Fine-grained sediments are structureless or show mud-cracks,bioturbation, and brecciated horizons (Figs. 1c and 3d).The latter form hard strata due to secondary precipitationin open fractures of spar with drusy mosaic texture,dolomite and, in same cases, gypsum (Fig. 3e). Carbonatemudstones and marls consist of homogeneous micrite,dolomicrospar, or microspar. Sandstones facies are fine-grained and sheet-like, displaying parallel lamination ormassive beds with a silty matrix.
This facies association is interpreted as an extensivemud-flat, pounded by very shallow water with periodsof contraction and desiccation (Fig. 1c). The presence ofbrecciated horizons with drusy-type cement indicatesthe effect of a meteoric phreatic zone.
Laterally, deltaic facies intertongue with mud-flatsiltstones. The facies comprise 5 to 7 m of a coarsening-and thickening-upward succession of fine-to-mediumsandstones with wavy stratification, tabular cross-bedding, and current-rippled lamination.
The presence of Oenotheraceae – Corsinipollenitesmenendezi – and Graminidites sp. support the littoralsubenvironment (Volkheimer et al., 1984). Additionally,the remains of turtles (Pelomeducidae) (Pascual et al.,1981) and Crocodylia Sebecosuchia Bretesuchus bona-partei (Gasparini et al., 1993) recovered in this settingsuggest vegetated and periodically flooded areas(Pascual et al., 1981).
2.4. Inner lake
The inner lake is composed of green laminatedmudstone, massive marls, wackstone, packstone, andoolitic grainstone (Fig. 1c). Features include wavy andlenticular bedding, wave-reworking structures, anddomal stromatolites (Figs 2 and 3f–g). This associationis typical of environments with periods of free watercirculation and good oxygenation.
Moreover, meter-thick successions of well-laminatedmudstones – wackstones with a mean thickness of0.8 mm suggest periods of water stratification.
The upper part of the Maíz Gordo Formation iscomposed of green laminated mudstone and lenticular tocontinuous decimetre-thick crystalline carbonate beds
with irregular bases and rippled tops (Fig. 2). Relictooids suggest that the primary texture was an ooliticgrainstone.
Inner-lake facies are rich in palynomorphs, especiallyfine-grained levels such as those in themiddle section (seeFig. 2 — second and third units). The communities arecomposed, among others, of Palmae (Spinizonocolpitessp.), Pandanaceae (Pandaniidites sp.), Classopollis sp.and non-marine dinoflagellate cysts, suggestingbrackish water. The Palmae are adapted to growth insaline soils (Petriella and Archangelsky, 1975). How-ever, the presence of Azolla sp. and Haloragaceae(Myriophyllumpollenites sp.) implies fresh water(Quattrocchio and del Papa, 2000), but they probablyrepresent elements transported from the littoral settingwhere they dwelt under a fluvial freshwater influence(Quattrocchio, pers. comm.).
Requirements for the formation of a brackish tosaline–alkaline lake are a hydrologically closed basinand high evaporation rates associated with a semi-arid toarid climate. Furthermore, it has long been recognizedthat the alteration of volcanoclastic material contributesto the alkalinity of the pore waters through hydrationand solution of the silicic volcanic glass, which is highlyreactive. However, in the Maíz Gordo Basin the sourceof the ions must have been the catchment basin rocks,with a probable minor contribution of the thin ash levelsidentified in the lake settings.
3. Basin evolution
The Maíz Gordo Formation records a number ofenvironmental changes interpreted as climate driven(del Papa, 1999) and documented through modificationsin the sedimentary patterns and facies. In the lakesystem, for instance, fluctuations in the water levelproduced drastic changes in sedimentary facies thatcorrelate quite well with changes in the fluvial patterns.The sedimentological study identified four main evolu-tionary stages (here termed units) in the Maíz GordoBasin (Fig. 2); del Papa, 1994).
The basal unit (first stage) corresponds to a low base-level stage. In the fluvial setting, it constitutes isolatedchannel-fill facies with alluvial plain fines. In the lakesystem this interval is represented by mud-flat facies(mainly dry) with minor pounded facies (Fig. 2).
The second unit is characterized by a rise in the baselevel, which is evident in the lake environment as lakewaters flooded the littoral areas. In the lake, carbonatelaminites precipitated and a carbonate mud-flat devel-oped towards the margin (del Papa, 1999). In the fluvialenvironment, the successions are dominated by the
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superposition of channel fill and bar facies with minoralluvial plain preservation (Fig. 2).
The third unit corresponds to a relative lowering ofthe base level in which the fluvial setting is dominatedby channel fill, sandy bar facies, and a sandy alluvialplain with carbonate palaeosols. In the lake, the faciesare predominantly carbonate with well-developed het-erolithic structures (Fig. 2), representing a carbonatemud-flat. The littoral areas are characterized by sub-aerial exposure, which generated extensive brecciatedhorizons.
The fourth unit and last stage developed towards thetop of the Maíz Gordo Formation, and is recognized by awidespread transgression of the lake that flooded thesurrounding areas, including fluvial settings like theValle Encantado site (Figs. 1b and 2). On higher land, aconspicuous level of ferric palaeosols developed,evoking poorly drained soils (Retallack, 1990).
4. Sampling and analytical techniques
Samples from seven localities were collected for X-ray diffraction (XRD) analysis of bulk samples and claymineral analysis of the b2 μm sub-fraction. In addition,selected samples were studied with scanning electronmicroscopy (SEM). Three localities – Tonco (To), ValleEncantado (Ve), and Ayuzos (Ay) – respectively rep-resent proximal, medial, and distal-transitional positionsof the fluvial environment. Two sites – Chuñapampa(Chu) and Corralito (Co) – are representative of themarginal lake subenvironment and two sites –Garabatal(Ga) and Las Cañas (Lc) – of the inner lake sub-environment (Fig. 1b). The study sites are locatedapproximately in a southwest–northeast section fromthe proximal area to the inner basin and each localitylaterally correlates with the next (Fig. 1c), thus allowingthe detection of mineralogical changes and diminishingthe effect of different source-area provenances.
Sampling covered fine material from the alluvialplains and from lacustrine deposits, as well as tuff andcarbonated levels from the lake. Standard petrographicanalyses were carried out on all samples to determinelithology, general composition, and texture.
The mineralogical composition of 50 powderedsamples was determined by X-ray diffraction (XRD)using a Philips PW1050 diffractometer (INGEIS) withCu Kα radiation operated at 40 mA and 30 kV. Claysub-samples (b2 μm; 31 samples) were prepared inaccordance with the guidelines of Moore and Reynolds(1989). Calcium carbonate was removed from sampleswith Acetic Acid–Sodium Acetate buffer, with pHadjusted to 5, prior to clay separation by centrifugation.
Clay minerals were identified according to the positionof the (00l) series of basal reflections on XRD patternsof air-dried, ethylene-glycolated, and heated specimens(at 500 °C for 4 h).
The reflective factors of Moore and Reynolds (1989)were employed for the semi-quantitative analysis of theclay minerals. Furthermore, the textures and mineralogyof selected mudstones, claystones, and tuff wereexamined by scanning electron microscopy (SEM)employing polished thin sections using back-scatteredelectron imaging and X-ray dispersive (EDS) analysiswith a ZEISS DSM 950 scanning electron microscope(Centro de Instrumentación Científica, University ofGranada, CIC) and a Jeol scanning electron microscope(University of Jaén). Analyses were carried out with aLINK QX2000 microanalyzer attached to a ZEISS DSM950 scanning electron microscope (CIC). The followingcompounds were used as calibration standards: albite(Na), orthoclase (K), periclase (Mg), wollastonite (Si),and synthetic oxides of Al2O3 (Al), Fe2O3 (Fe), andMnTiO3 (Ti and Mn). Atomic concentration ratios wereconverted into formulae according to stoichiometry(number of oxygens in the theoretical formulae ofminerals). For the finest-grained samples, textures werestudied with a Field Emission Scanning ElectronMicroscope (FESEM) (CIC) employing polished thinsections and unprocessed chips of the samples coatedwith 50 Å of carbon.
5. XRD and SEM results
5.1. Fluvial environment, mineralogy and textures
Fluvial samples mainly comprise quartz, plagioclase,and detrital micas. K-feldspar, when present, can be amajor or a minor phase. Calcite in variable amountsappears in most of the fluvial sediments, whereas minormordenite seldom occurs in proximal fluvial settings(M10To, Fig. 4a), and minor analcime was identified indeltaic facies. The clay mineral assemblages identifiedby XRD in fluvial samples comprise illite, kaolinite andsmectite in widely variable percentages (Fig. 2).Representative X-ray patterns of the b2 m sub-samplesare illustrated in Fig. 4.
The texture of sandstone from intermediate fluvialsettings is depicted in the SEM photomicrographs ofFig. 6a and b. The rock is composed of clasts of quartz,K-feldspar, plagioclase (albite and intermediate Na–Ca),biotite, and muscovite, with calcite cement and argilla-ceous matrix between the grains (Fig. 3c). Kaoliniteoccurs in booklets filling the pore spaces (Fig. 6b), atexture typical of authigenic kaolinite; illite fibres
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growing at the expense of kaolinite are also visible in thisview. Smectite is present in these samples too (Fig. 6c);according to EDX spectrums, this mineral has K as themain interlayer cation, some Ca, and minor Na.
In turn, feldspathic greywacke corresponding to deltaicfacies contains abundant calcite cement, irregularlyshaped analcime crystals several microns long, and amatrix comprising dioctahedral smectite and detritalmicas(Fig. 6d). SEM analysis also reveals authigenic smectiticclays growing around a quartz grain. Quantitative EDXanalyses of these smectitic clays (Table 1a: C5, C19, C22,C25) display awide range of Si contents and interlayer siteoccupancy. If we consider the strict definition of smectiteas analyses having Si≥3.5 a.p.f.u. and interlayer charges
from 0.6 to 0.2 (Weaver and Pollard, 1973), then two ofthe analyses do not correspond to smectite, but probably tomixed-layer I/S or to a mixture of detrital muscovite andauthigenic smectite. In these analyses, the main interlayercation isK and less commonlyNa,withminor Ca.Apatite,rutile, iron oxides, ilmenite, and monazite were identifiedas accessory minerals.
The mineralogical study of b2 μm sub-fractionsrevealed marked vertical changes in clay mineralogy insites representative of proximal and medial fluvialsettings (Fig. 2). In the former, the clay fraction isdominated by smectite at the base of the sequence, withsubordinate illite and scarce kaolinite. Towards the topof the sequence, however, is a striking increase in
Fig. 4. XRD patterns of fluvial samples. Orientated mounts of clay fractions (b2 μm): (a) X-ray patterns of M10To, from Tonco site; (b) X-raypatterns of M8055Ve, from Valle Encantado site.
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Table 1Quantitative X-ray dispersive (EDS) analyses of (a) smectitic clays and (b) quantitative EDS and electron microprobe (EMP) analyses of analcime
kaolinite abundance, from 4 to 91%. In contrast, inmedial fluvial positions, kaolinite only prevails in onesample from the middle of the sequence, while in theothers illite/mica predominates with subordinate kao-linite (6–27%) and scarce smectite (Fig. 2).
Also remarkable are the lateral changes in clay min-eralogy from alluvial to lacustrine environments. In thetransitional deltaic facies, the clay fraction is dominatedby muscovite with subordinate smectite; kaolinite isabsent, coinciding with the first occurrence of analcime.
5.2. Lacustrine environment, mineralogy and textures
Most of the samples from these settings wereunsuitable for clay mineral analysis as they mainly
comprise calcite or dolomite, with only scarce fine-grained siliciclastic material. Consequently, clay-miner-al analysis was performed in only one-third of thesamples from marginal-lake (Chuñapampa and Corralitosites) and inner-lake settings (Garabatal and Las Cañassites). In this environment analcime was identified inbulk samples and also in b2 μm sub-samples of allfacies identified (Fig. 7).
In siltstones levels from marginal lake sites, therelative analcime content varies from intermediate toabundant (Figs. 5a and 7), with subordinate quartz,muscovite, and minor hematite, whereas plagioclase andK-feldspar frequently represent minor phases.
Many layers of marls containing variable amounts ofcalcite or dolomite plus quartz, analcime, K-feldspar,
Fig. 5. XRD patterns of lake samples. (a) Bulk sample: X-ray pattern of M11 Chu, depicting all major analcime peaks. (b) X-ray patterns of the clayfraction of the same sample.
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and plagioclase are recognized from marginal- andinner-lake settings. Likewise, the carbonate levels(mudstone, wackstone, grainstone, and stromatolite) ofthese settings are composed of calcite or dolomite, plusscarce to moderately abundant analcime (Figs. 3g and 7)and quartz; less frequently, calcite and dolomite coexist.Several levels of crystalline carbonate interbedded in theupper part of the sequence consist of calcite plus quartzand scarce analcime (Fig. 7), together with a scarce clayfraction of smectite and subordinate illite.
The b2 μm sub-fraction of siltstones, mudstones andmarls from littoral and inner-lake settings consists ofillite/muscovite and subordinate smectite or entirely ofillite/muscovite (Figs. 2 and 5b).
Thin levels (3–7 cm thick) of tuff are intercalated inthe succession. They consist of vitroclastic tuff com-
posed of shards, recognized mostly by clear ghostsoutlining their shapes—quartz, analcime, microcline,micas, and minor calcite, which according to the petro-graphic study fill cavities. A volcaniclastic level fromLas Cañas site is an epiclastic tuff that also containsdolomite, cristobalite, and minor gypsum.
Phyllosilicates identified by SEM in siltstones frommarginal lake sites (M11Co and M3Chu) are detritalmicas and minor smectite not identified by XRD. EDXanalysis reveals this smectite has a similar range ofcomposition as in transitional deltaic facies, with K orNa as the main interlayer cation and subordinate Ca. Inthese siltstones, analcime occurs in pores of the rocks orfills veins, forming subhedral square to hexagonal, oranhedral rounded grains (Fig. 6e and f, Tables 1b: G/512, 13, 14 and 15).
Fig. 6. BSE and FESEM images: (a) texture of a fluvial sandstone (M3Ve); (b) radial kaolinite around an albite clast and booklets of kaolinite filling apore in a fluvial sandstone; in addition, illite fibres are growing at the expense of kaolinite (M3Ve); (c) smectitic clays in the matrix of a fluvial sample;(d) analcime crystals in a feldspathic greywacke from Ayuzos (FESEM); (e) and (f) typical textures of marginal lake samples showing severalanalcime crystals.
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Seven samples from inner-lake sites were examinedby SEM: carbonate mudstones (M9Ga and M15Ga),crystalline carbonate (M11Lc), marls (Pal 2Lc and Pal3Lc), silicified tuff (M16Ga), and epiclastic tuff (M2Lc).
Carbonate mudstones have medium to abundantrelative analcime contents, in some cases visible underthe light microscope. At SEM scale, the analcime is seento be euhedral to subhedral hexagonal crystals or anhedralrounded crystals associated with micritic to microspariticcalcite, with microcrystalline silica (Fig. 8c), or withdioctahedral smectitic clays (Fig. 8a). Calcite, minutepatches of barium sulfate, and pyrite altered to iron oxidewere also identified in these rocks. Quantitative EDXanalyses of the smectitic clays do not coincide exactlywith smectite, but probably to mixed-layer I/S or to amixture of detrital muscovite and authigenic smectite(Table 1a: L1/1 L5/1). In addition, albite overgrowthswere observed on detrital plagioclase (Fig. 8b).
In crystalline carbonate facies from the inner lake(M11Lc), relicts of impure (Ca–Mg) carbonate inside
the secondary pure calcium carbonate appears clearly inSEM images (Fig 8d); patches of strontium bariumsulphate are visible in this view.
Marl levels from the inner lake corresponding tothe fourth unit contain calcite and abundant detritalmuscovite; the analcime content is also markedly lowerthan that of marls from the third unit (Fig. 7, Pal3Lc–Pal1 Lc and Pal2 Lc).Moreover, marls from the third unitcontain dolomite instead of calcite (see rhombohedraldolomite crystal in Fig. 8e). In these marls smectitecompositions are similar to those in fluvial samples,although occasionally Na or Ca is more abundant than Kin interlayer sites.
At SEM scale, vitric shards recrystallized to micro-crystalline silica in the core were recognized in thesilicified tuff level. Analcime occurs as globular crystalssome tens of micra in diameter (Fig. 8f), and micro-analyses indicate the association with silica at themicron scale. In other cases, the spherical grains consistof a nucleus of silica surrounded by a covering of
Fig. 7. Relative analcime contents and variations between sedimentary facies and between littoral/inner-lake positions.
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analcime (Fig. 8g). In turn, the epiclastic tuff levelcontains subrounded clasts of quartz, intermediateplagioclase, microcline, and scarce micas (muscoviteand weathered biotite) in a matrix of volcanic glass,calcite, dolomite, and silica. Minor quartz grains withtypical volcanic forms were observed at SEM scale. In
addition, analcime is present in anhedral crystals tens ofmicra in diameter (Fig. 8i). Pyrite altered to iron oxides,rutile, iron oxides, barium sulphate, and zircon areaccessory minerals.
The silicified and epiclastic tuff levels are character-ized by the presence of authigenic K-feldspar showing
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a variety of textural habits, including overgrowths,microfracture filling, and microcrystalline cement. K-feldspar overgrowths around sharp detrital quartz,vitric shards, plagioclase, and K-feldspar grains withsubhedral shapes and embayments. Untwinned over-growths (5–10 μm thick) are optically discontinuous withthe detrital grains and partially to completely cover thesegrains (Fig. 8g and h). Some microfractures in feldspargrains have been filled by authigenic K-feldspar (Fig. 8j).Detrital feldspars usually show planes of weakness alongwhich minerals can displacively precipitate. Last, finelycrystalline (1–10 μm) randomly orientated K-feldsparscan be found as microcrystalline cement filling intergran-ular pores (Fig. 8h). These properties are similar to theauthigenic K-feldspar described by Maraschin et al.(2004) and Warnock and van de Kamp (1999).
5.3. Analcime composition
Quantitative analyses of analcime by EDS and EMPwere performed for six of the analcime-bearing rocks(Table 1b). In many cases, analcime crystals present Na/Al ratios lower than 1, while the Si/Al ratio departsconsiderably from the theoretical value of 2, reachingvalues as high as 3. As most of the analyses show Si/Alratios higher than 2.4, they correspond to the silica-richgroup of Coombs and Whetten (1967). When the Si/Alratio is high, distinguishing between analcime and albite isvery difficult, because most of the analcime crystals aresubhedral to anhedral. These lowNa/Al ratios may be duein part to sodium loss during EDS analyses, vacancies inalkaline sites, or minute silicon inclusions, as no elementsother than Si, Al, and O were detected in these analyses.
The composition of analcime crystallised at Earth-surface conditions commonly reflects the composition ofits sourcematerials (Coombs andWhetten, 1967; Ogihara,1996), and it typically forms at relatively low temperaturesunder quartz-supersaturated conditions. These relation-ships lead to low-temperature analcime typically having arelatively silicic composition because, as temperatureincreases and quartz enters the assemblage, there is littlethermodynamic drive for re-equilibration (Neuhoff et al.,2004).
6. Discussion
In order to discuss the significance of the vertical andlateral changes in clay mineralogy recorded in the fluvialand lacustrine levels of the Maíz Gordo Formation, wemust first briefly consider certain aspects of the geologicalsetting. As mentioned in the geological outline, regionalstudies have proven that theMaíz Gordo Basin developedduring a period of relative tectonic calm with a lowsubsidence rate (Gómez Omil et al., 1989; Salfity andMarquillas, 1994). The source areas of the basin, locatedsouth and southwest, were dominated by crystalline rocksand no changewas detected in this provenance.Moreover,the sandstone petrography indicates that the sedimentarysequence experienced eogenetic diagenesis; thus, the clayminerals preserved should be primary or have undergoneonly slight diagenetic changes.
6.1. Environmental significance of vertical and lateralchanges in clay mineral assemblages in fluvial settings
We will discuss first the vertical changes in claymineralogy recorded in proximal fluvial positionsbecause we assume that it is more representative of theincoming material from weathering profiles, and pre-sumably less influenced by the changes at the lake level.
Proximal fluvial sites record a clear increase inkaolinite content upwards from the base to the top of theunit (Fig. 2). This change is coeval with a sharp decreasein smectite, followed by a drop in illite/mica from themiddle levels towards the top of the sequence.
The presence of abundant smectite is generally linkedto a warm climate with alternating humid and dry seasons,although it can also reflect volcanic activity (Chamley,1989). In contrast, kaolinite generally forms in highlyhydrolytic climates and requires a minimum temperatureof 15 °C (Adatte et al., 2002). Therefore, the increase inkaolinite content towards the top of the sequence implies achange from a relatively arid climate with alternatinghumid and dry seasons towards more constant warm andhumid conditions. This increase in humidity inferred fromthe claymineral assemblages in the fluvial environment isin agreement with the palaeoclimatic evolution discerned
Fig. 8. BSE and FSEM images of inner-lake samples. (a) Analcime crystals near dioctahedral smectitic clays in a carbonate mudstone from inner-lakeenvironment; (b) growth of albite over detrital plagioclase in a carbonate mudstone; (c) euhedral to subhedral analcime crystals associated withmicritic to subsparitic calcite or with microcrystalline silica; (d) relicts of impure (Ca–Mg) carbonate inside the secondary pure calcite in a crystallinecarbonate, patches of barite surrounded by silica also present; (e) rhombohedral dolomite crystal in a marl; texture of the silicified tuff from Garabatal(M16Ga): (f) FESEM image of globular analcime crystals; (g) BSE image of spherical grains with a nucleus of silica surrounded by a covering ofanalcime; also note rims of potassium feldspar around plagioclase and vitric shards; (h) BSE image showing finely crystalline (1–10 μm) randomlyorientated K-feldspars as microcrystalline cement filling intergranular pores; and texture of an epiclastic tuff from Las Cañas site: (i) Overgrowths ofK-feldspar (5–10 μm thick) optically discontinuous with a detrital K-feldspar grain, an anhedral crystal of analcime is also shown; (j) detail of amicrofracture in a detrital feldspar grain filled by authigenic K-feldspar.
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from sedimentological studies of the lake environment(Fig. 2; see also del Papa, 1999).
The palynomorphs in the middle section of inner lakefacies (see Fig. 2 second and third units) point to asubtropical climate in which dry and rainy seasonsprevailed (Pascual et al., 1981; Quattrocchio andVolkheimer, 2000). Therefore, the high kaolinite contentrecorded in the middle section of the proximal fluvialsequence probably indicates slightly more humidconditions in the uplands than in the basin itself. Suchclimatic differentiation between the source area(uplands) and the drainage basin are common in alluvialbasins (see synthesis in Miall, 1996, p. 442).
In medial fluvial settings (Valle Encantado, Fig. 1band c) the dominant clay mineral is illite/mica, whereaskaolinite contents are almost always lower than incorrelative proximal fluvial levels. Furthermore, al-though kaolinite is widespread in the fluvial sedimentsof the Maíz Gordo Formation as a dominant orsubordinate phase, it is absent in transitional deltaicand lacustrine sediments. Moreover, it is worth notingthat the absence of kaolinite coincides with the firstoccurrence of analcime in the deltaic sediments.
In order to analyse the physico-chemical parametersthat could have produced kaolinite breakdown in abasinward direction, we need to consider its stabilityfield in near-surface conditions. Kaolinite is formed inan acidic environment associated with SiO2, and itshould therefore be highly unstable in a basic medium.According to stability diagrams, in solutions saturatedwith respect to amorphous silica and under standardconditions of pressure and temperature, kaolinite isstable in environments in which log [K+]/[H+] and log[Na+]/[H+] are lower than ∼13 (Faure, 1998). More-over, with the increase in Na+ and K+activities inrelation to [H+], kaolinite becomes unstable andmontmorillonite is the stable phyllosilicate. Further-more, experiments carried out by Bauer et al. (1998)have demonstrated that kaolinite dissolution takes placein high molar KOH solutions at 35 °C. In thoseexperiments an initial, dissolution-dominant stage wasfollowed by a precipitation step in which the sequenceof reaction products were illite, KI-zeolite, phillipsite,and finally the stable product K-feldspar. In the runconducted at KOH 0.1 molar, the solid phases detectedafter 150 days were kaolinite plus illite. However, whenthe solution employed was KOH 0.5 molar, illite wasdetected after only 75 days. Accordingly, kaolinitedissolution must be a consequence of the interactionbetween sediments and saline alkaline solutions (Faure,1998). We visualize two complementary contexts inwhich such a reaction could have taken place: (i)
through direct interaction of alluvial sediments with thebrackish–alkaline waters of the lake during highstandlake levels; and (ii) later, as a result of early diageneticreactions mediated by K-rich solutions, likely alkaline–saline pore waters.
Considering that the palynomorphs in the middlelevels of the Maíz Gordo Formation (correlative with thesecond and third units of Fig. 2) indicate brackish lakewater (Quattrocchio and del Papa, 2000), the interactionbetween such water and kaolinite-rich sediments shouldcause kaolinite dissolution. As previously mentioned,during highstand lake levels the lake flooded the littoralareas, including medial fluvial sites like Valle Encan-tado. Therefore, according to the sedimentologicalevidence, the low kaolinite content in levels corre-sponding to the second and fourth units were probably aconsequence of the interaction of alluvial sedimentswith the alkaline waters of the lake during highstandlake levels. Likewise, the clay assemblage, smectite plusillite/mica (lacking in kaolinite), recorded in deltaicsediments probably resulted from the mixing reaction ofthe fresh water from the drainage system and thealkaline and brackish water from the lake.
On the other hand, the decrease in kaolinite percen-tages between proximal and medial fluvial settingsrecorded in levels correlated with lowstand lake stages(first and third units; Fig. 2) could be a consequence ofearly diagenetic reaction mediated by K-rich solutions,likely alkaline saline pore waters. This conclusion issupported by the textural evidence of kaolinite illitizationfound at SEM scale (Fig. 6b). Furthermore, well-developed floodplain facies mainly composed of redsilty beds at the base of the unit suggest that oxidizingconditions prevailed at that time, which could havefacilitated kaolinite breakdown (Milne and Earley, 1958).
6.2. Environmental significance of vertical and lateralchanges in clay mineral assemblages in lake settings
In siltstones and marls corresponding to lowstand lakelevels (Fig. 2, first and third units), the b2μmsub-fractionis made up entirely of illite/muscovite in lake littoralsettings and of∼90%muscovite— 10%Sm in inner-lakesettings. In turn, the samples corresponding to the fourthunit, characterized by the highest water level stage,contains ∼90% muscovite – 10% Sm in lake littoralsettings and 70% muscovite – 30 % Sm in inner-lakelevels (Fig. 2). These variations in relative abundancesuggest that smectite authigenesis probably occurred ininner lake environments during relative highstand lakelevels. This hypothesis is supported by images ofauthigenic smectitic clays (mixed-layer I/S or a mixture
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of detrital muscovite and authigenic smectite) revealedunder SEM in carbonate mudstone from inner-lake sites(Fig. 8a and b).
Factors controlling smectite stability at near-surfaceconditions have received considerably less attentionthan studies at high temperature. However, recentlyDrief et al. (2002) carried out experiments with K-enriched seawater, demonstrating the effect of Kconcentration and pH on the smectite to illite transfor-mation. These authors obtained illite from smectite after30 days of reaction at 50 °C in a run carried out insolution with [K]: 1 M and pH: 13.40. In contrast,smectite was stable in the runs performed with seawateror 0.1 M [K] solutions. However, in the experimentscarried out with solutions of intermediate pH and [K]contents, a broadening of the smectite peak was evidentin the XRD patterns of the residue. Therefore, thepersistence of smectite in inner-lake settings duringcycles corresponding to highstand as well as lowstandlake levels suggests that the waters were never eitherhypersaline nor highly alkaline.
Further evidence of the change in hydro-geochem-istry prompted by the shift from low- to highstand lakelevels is the markedly higher analcime content in marlsfrom the inner lake of the third unit compared to marlsfrom the fourth unit (Fig. 7).
6.3. Analcime authigenesis
During early studies of saline–alkaline lakes (Hay,1966; Sheppard and Gude, 1969), a common pattern ofmineralogical zoning was observed to be a consequenceof laterally variable water chemistry. This zoning consists
of alkali silicic zeolites such as clinoptilolite, erionite, andphillipsite precipitating in the margins, grading intoanalcime followed by potassium feldspar in the centresof the lakes, creating a “bull’s eye” depositional model.However, in the Maíz Gordo Formation this pattern isonly partial, as analcime was almost the sole zeoliterecognized. In fact, no evidence of K–Ca zeolites,considered the common precursors of analcime (Chiperaand Apps, 2001), was found even at SEM scale. The onlyzeolite other than analcime identified was mordenite,which seldom occurs in proximal fluvial settings from thetop of the sequence. In turn, rims of potassium feldspararound detrital feldspar and plagioclase were identified intuff and carbonate mudstone from the inner lake.Therefore, the incomplete basinward zonation of diage-netic minerals identified in Maíz Gordo Lake was:mordenite→ analcime→ K-feldspar. This mineralogicalzonation indicates an increase in salinity and alkalinitytowards the centre, although in fact it is not typical ofsaline–alkaline lakes (Langella et al., 2001). Theanalcime+sparitic calcite assemblage was detected inthe deltaic sandstones levels and in inner-lake wack-estone/mudstones and grainstone facies (see Fig. 3g).Analcime associated with sparitic calcite would likelyhave formed under rather stable conditions with a highdegree of water saturation, which is generally required forthe development of sparitic carbonate cement.
In sediments from the Maíz Gordo Formation, theoccurrence of analcime is not related to the presence ofvolcaniclastic material, except in the thin levels of tuff.Moreover, no textural evidence of replacement of anyprecursor zeolite was observed at SEM scale. Furthermore,analcime crystals have a very different morphology when
Fig. 9. Activity diagrams for the Ca–Na–K–Al–Si–H2O system balanced with respect to (a) Al for silica activity equal to pyrophyllite+kaolinite at25 °C, (b) Si for Al activity equal to pyrophyllite+kaolinite at 25 °C. In both cases K-feldspar, hematite, and Ca-saponite are also saturation phases.Modified from Birsoy (2002).
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they occur associated with volcaniclastic material, asdocumented by the globular grains imaged by FESEM inthe silicified tuff from the inner lake (Fig. 8 e and f). Thesespherical forms are indicative of homogeneous nucleationat higher degrees of supersaturation (Wilkin and Barnes,2000), which is compatible with crystallization from highlyreactive volcanic glass in contact with brackish alkalinelake waters. While volcanic glass is the most frequentprecursor of zeolites in saline–alkaline lakes (Surdam andSheppard, 1978; Boles and Surdam, 1979), these mineralscan also form at the expense of other materials, such as clayminerals, feldspars, and feldspathoids (Hay and Sheppard,2001). Analcime in particular has been recorded in severalcases in lacustrine sediments free of associated volcani-clastic material (Hay, 1966; Gall and Hyde, 1989; Remyand Ferrell, 1989; Renaut, 1993; English, 2001).
In contrast, in all the levels devoid of volcanicmaterial, analcime crystals are subhedral and lessfrequently euhedral, probably reflecting different chem-ical conditions for crystal growth than in the volcani-clastic rocks. In their experiments on analcime synthesisfrom Na-clinoptilolite and Na-mordenite, Wilkin andBarnes (2000) found that euhedral analcime crystalsdevelop as the particles coarsen at lower degrees ofsupersaturation; while the primary nucleated crystals,roughly spherical in shape, formed at the highestdegrees of supersaturation. Therefore, we consider thatthe textural evidence favours an origin from a materialless reactive than volcanic glass for the analcime presentin siltstones and carbonate rocks.
There is general agreement that analcime forms byreplacement of earlier zeolites, including clinoptilolite.Wilkin and Barnes (1998), for instance, arrived at thisconclusion based on the spatial distribution of clinopti-lolite and analcime in sedimentary and volcanic-hosteddeposits. In turn, research on the kinetics andmechanismsof analcime formation from Na-clinoptilolite and Na-mordenite indicates that the latter mineral is similar to Na-clinoptilolite in both composition and solubility (WilkinandBarnes, 2000). Furthermore, solubility experiments inquartz-saturated solutions (and even aqueous silicaactivities between quartz and amorphous silica satura-tions) show that at 25 °C Na-clinoptilolite is unstablerelative to stoichiometric analcime (Wilkin and Barnes,1998). Therefore, the mordenite identified in proximalfluvial sediments (Fig. 4a) would have been the precursorof analcime in the Maíz Gordo Basin. However,mordenite was identified only in one level and, what ismore, no textural evidence of analcime formation throughreplacement of mordenite or other precursor zeolite wasfound. In consequence, without excluding this mecha-nism, we consider it more probable that analcime formed
by direct authigenic precipitation or through the reactionbetween interstitial brines and clay minerals or plagio-clase. This mechanism has been interpreted as beingresponsible for analcime precipitation in marginal faciesof Bogoria Lake, Kenya (Renaut, 1993).
According to the stability diagrams calculated byBirsoy (2002), mordenite, kaolinite, K-feldspar, andsmectite could coexist at a range of silica activitieslower than that for amorphous silica saturation and greaterthan that for cristobalite saturation (Fig. 9a). The influenceofAl activities on the Ca–Na–K–Al–Si–H2O systemwasalso considered by Birsoy, who presented activitydiagrams balanced with respect to Si for four differentAl activities at 25 °C.As in the case of the Turkish depositsstudied byBirsoy (2002), themost suitableAl saturation isamorphous silica+pyrophyllite (compositionally similarto smectite), as it accounts for the coexistence of smectiteswith zeolites in the Maíz Gordo Formation (Fig. 9b). Thecoexistence of smectite and analcime has been documen-ted, for example, in the Rocky Brook Formation ofWestern Newfoundland, Canada (Gall and Hyde, 1989).
Lake Natron, Tanzania, probably represents a closemodern analogue of the Maíz Gordo Formation, as thisalkaline lake (pHN9.5) is devoid of potential precursorvolcanogenicmaterial and no zeolites other than analcimewere identified (Hay, 1966). This author suggested thatanalcime formed in Recent and Pleistocene sediments ofthe lake by direct precipitation and that it is presentlyforming as a primary precipitate at the sediment-waterinterface in the bottom mud of the lake (Hay, 1966).Another example of direct analcime formation withoutassociation with volcaniclastic material is Lake Lewis, amodern saline lake in Australia in which analcimeauthigenesis occurs in lacustrine sediments below thewater table in the presence of Na-rich brines (English,2001). Taking into account the cases of Lake Natron andLake Lewis, we interpret that analcime would haveformed in the siltstones and carbonated rocks of the MaízGordo Formation by direct authigenic precipitation orthrough the reaction between interstitial fluids and clayminerals or plagioclase soon after deposition of thesediments, probably at the bottom of the lake. In somelevels, however, analcime could have formed throughreplacement of mordenite. On the other hand, texturalevidence indicates that in volcaniclastic levels, analcimeformed through the dissolution of highly reactive volcanicglass under high degrees of supersaturation.
7. Conclusions
The clay mineralogy of the proximal fluvial settingindicates a progressive increase in humidity for the
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basin, in agreement with the palaeoclimatic evolutionindicated by previous sedimentological studies.
Comparing the clay mineral assemblages present inproximal fluvial positions with those in correlativefluvial levels in a basinward direction (Valle Encantado,Ayuzos) reveals a clear decrease in kaolinite contents.Such variations are not attributable to climate imprint,but instead to changes in hydro-geochemistry, denotingthe progressive influence of the brackish and alkalinelake waters on interstitial pore waters.
Kaolinite, widespread in the fluvial sediments of MaízGordo Formation as a dominant or subordinate phase, iscompletely absent in lacustrine sediments. Its disappear-ance in the latter coincides with the first occurrence ofanalcime in deltaic sediments. We deduce that kaolinitedissolution must be a consequence of the interactionbetween sediments and brackish alkaline solutions. Weconsider that such a reaction could have taken place undertwo complementary contexts: (i) through direct interac-tion of alluvial sediments with brackish alkaline waters ofthe lake during highstand lake levels; or (ii) later, as aresult of early diagenetic reactions mediated by K-richsolutions, likely alkaline saline pore waters.
The basinward zonation of diagenetic minerals iden-tified inMaízGordo Lake is mordenite→ analcime→K-feldspar. Although not typical of saline–alkaline lakes(Langella et al., 2001), this mineralogical zonation doesindicate an increase in salinity and alkalinity towards thecentre of the lake.
The textural evidence favours an origin for the analcimepresent in siltstones and carbonate rocks by directauthigenic precipitation or through the reaction betweeninterstitial brines and clay minerals or plagioclase.
On the other hand, in the volcanoclastic levelsFESEM reveals that analcime is spherical, indicative ofhomogeneous nucleation at high degrees of supersatu-ration (Wilkin and Barnes, 2000), which is compatiblewith crystallization from highly reactive volcanic glassin contact with brackish alkaline lake waters.
Acknowledgements
The authors are grateful to Sheilagh Douma (JournalManager) and two anonymous reviewers for their valuableand constructive comments. We thank Alicia GómezSegura (Centro de Instrumentación Científica, Universityof Granada) for her help with the SEM. The stay ofM. DoCampo at the University of Granada was supported by anAECI fellowship awarded in 2003 (MAE-AECIprogramme). This work was partially financed byCONICET-PEI 6091 and ANCyT – PICT 12419 grantsand Research Project BTE2003-07867 (Spanish Ministry
of Science and Technology). This paper is a contributionto INCE and IBIGEO (FCN—Universidad de Salta). Ourthanks to Christine Laurin for revising the English.
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