sem and tem evidence of mixed-layer illite-smectite …grupo179/pdf/do campo 2016.pdfof la poma in...

SEM and TEM evidence of mixed-layerillite-smectite formed by dissolution-crystallization processes in continentalPaleogene sequences in northwestern

Argentina

MARGARITA DO CAMPO1 BLANCA BAULUZ2 FERNANDO NIETO3 CECILIA DEL PAPA4 AND FERNANDO HONGN5

1 INGEIS (CONICET ndash Universidad de Buenos Aires) y FCEyN - UBA Pabelloacuten INGEIS Ciudad Universitaria(1428) Buenos Aires Argentina

2Departamento de Ciencias de la Tierra Universidad de Zaragoza Pedro Cerbuna 12 50009 Zaragoza Spain3Departamento de Mineralogiacutea y Petrologiacutea and IACT Universidad de Granada-CSIC Avda Fuentenueva sn

18002-Granada Spain4 CICTERRA CONICET-Universidad Nacional de Coacuterdoba Coacuterdoba Argentina

5 IBIGEO (CONICET-Universidad Nacional de Salta) Avda Bolivia 5150 4400 Salta Argentina

(Received 3 March 2016 revised 19 July 2016 Associate Editor Juan Cornejo)

ABSTRACT In the northernmost Calchaquiacute Valley (Salta Argentina) the Paleogene continentalsediments show a transition from smectite at the top to R3 I-S (gt90 illite) through R1 I-S (65ndash80illite) in contrast to the remaining sectors containing smectite up to the bottom Samples at the base ofthe succession were characterized by high-quality step-scan X-ray diffraction (XRD) scanning electronmicroscopy (SEM) and analytical high-resolution transmission electron microscopy (HRTEM)Analysis by SEM demonstrated dissolution of primary phases (feldspars micas and quartz) andcrystallization of illite I-S and kaolinite As this alteration is not pervasive an intermediate fluidrockratio could be inferred The lattice-fringe images of the samples from upper parts of the sequence showabundant I1-rich areas whereas in the lower parts of the sequence illite packets and I3 I-S coexist andcompositions evolve towards muscovite (tetrahedral-charge increase principally compensated by Mg-by-Al substitution in octahedral sites and by a slight decrease in Ca in interlayer sites) As burialtemperatures were probably similar in all the samples depth was not responsible for the illite formation atthe bottom The TEM textures suggest that illitization proceeded mainly by dissolution-crystallizationThe active faults close to the northern Calchaquiacute Valley probably promoted the circulation of hot deepfluids favouring illitization

KEYWORDS illite-smectite Paleogene Argentina SEM TEM dissolution-crystallization

The transformation from smectite to illite throughmixed-layer illite-smectite (I-S) is one of the main

mineral reactions during the burial diagenesis ofsiliciclastic sediments Since the pioneering work ofHower et al (1976) the conditions and mechanismsinvolved in the transformation have been the subject ofextensivework in many basins worldwide (egAltaneret al 1984 Ahn amp Peacor 1989 Jiang et al 1990

E-mail Docampoingeisubaarhttpsdoiorg101180claymin2016051503

copy 2016 The Mineralogical Society

Clay Minerals (2016) 51 723ndash740

Veblen et al 1990 Dong amp Peacor 1996 Nieto et al1996 Altaner amp Ylagan 1997 Dong et al 1997Bauluz et al 2000 Abad et al 2004 Cuadros 2006)and has also been reproduced in laboratory experi-ments (Whitney amp Northrop 1988 Huang et al1993) In addition several studies have demonstratedthe importance of fluidrock ratios in this transform-ation (Whitney 1990 Middleton et al 2015)

A recent study on the evolution of PaleogeneAndean foreland basins in NW Argentina revealedenhanced diagenetic evolution in sediments croppingout in the northernmost Calchaquiacute Valley (Saladilloarea Figs 1 2) (Do Campo et al 2014) Thesesediments show a complete evolution from smectite atthe top to R3 I-S mixed layers plus authigenic kaoliniteat the bottom through R1 I-S-type mixed layers inbetween indicating the deep diagenetic transforma-tions (Fig 2b) The study section comprises sedimentsrepresenting the infill of the Los Colorados basin(Quebrada de los Colorados Formation QLC) recentlydefined as part of a broken foreland basin (del Papaet al 2013) as well as the underlying post-riftsequences (Maiacutez Gordo Formation MG Marquillaset al 2005)

Several indicators show that the R3 I-S formation inthese Paleogene continental sediments is exceptionaland not controlled by burial temperature aloneDo Campo et al (2010 2014) reported that equivalentlevels of QLC in southern areas of the Los Coloradosbasin including a control point located as near as15 km contain smectite and no mixed-layer I-S in thebasal fine-grained levels Thus according to the claymineralogy Paleogene sequences were affected byshallow diagenesis only in most parts of the LosColorados basin in agreement with the burial depths ofsim3000 m estimated for the sediment column and thelow geothermal gradient characteristic of forelandbasins (Dorsey et al 1988 Allen amp Allen 2005)Furthermore shallow diagenetic conditions were alsoreported for the underlying sediments of the MaiacutezGordo Formation in several localities (Do Campoet al 2007)

The aim of the present paper was to understand therole of fluids and the transformation mechanismsinvolved in smectite illitization in the Paleogenecontinental sediments cropping out in the northern-most Calchaquiacute Valley employing high-quality step-scan XRD SEM and HRTEM techniques The use ofanalytical and high-resolution techniques combinedwith deconvoluted XRD traces can be useful inunderstanding the complex burialthermal history ofbroken foreland basins in northwestern Argentina

GEOLOG ICAL FRAMEWORK

The Calchaquiacute Valley is located in the EasternCordillera of the southern Central Andes ofArgentina The Eastern Cordillera is a morphotectonicunit characterized by thick- and thin-skinned tectonics

The northern Calchaquiacute Valley (Saladillo area) is anarrow NndashS striking depression bound by upwards-convergent reverse faults (Calchaquiacute and Toro Muerto)(Marrett et al 1994) The stratigraphy of this areacomprises Neoproterozoicndashlower Cambrian very low-grade metamorphic rocks of the PuncoviscanaFormation (Turner 1960) This unit is overlainunconformably by Upper CretaceousndashPaleogene sedi-ments corresponding to the post-rift facies infilling theextensional basin (Salta Group Turner 1959 Salfity ampMarquillas 1994 Marquillas et al 2005) An uncon-formity separates the Salta Group from the overlyingMiddle Eocene Quebrada de Los Colorados (QLC)Formation (Payogastilla Group Diacuteaz ampMalizzia 1983del Papa et al 2004) of the Andean foreland basinStructural and sedimentary facies analyses showedconsiderable evidence at Saladillo of synorogenicdeposition of the Quebrada de los ColoradosFormation (Hongn et al 2007) These authorsconcluded that folding of the Saladillo syncline waspartially coeval with deposition of the QLC FormationDeformation events and therefore fault activity alsotook place in the Neogene and Quaternary as recordedby the intersection of fold axes (see fig 1 in Hongnet al 2007) and by the Quaternary faulting controllingthe eruption of the Los Gemelos and SaladilloQuaternary volcanic centres (Marrett et al 1994Guzmaacuten amp Petrinovic 2008) These structures belongto an active fault system as demonstrated by historicalearthquakes in the area one ofwhich destroyed the townof La Poma in 1930

The basal levels of the study section correspond tothe Maiacutez Gordo Formation (post-rift stage of the Saltarift) which is overlain in angular unconformity by thethe QLC Formation sediments In the Saladillo areathe QLC Formation consists of a 1400 m-thickupwards-thickening and upwards-coarsening contin-ental succession of claystones siltstones sandstonesand conglomerates representing sedimentation influvial-alluvial plains and alluvial fan settings (delPapa et al 2013) (Fig 2) Based on unconformitiesand abrupt changes in the sedimentary facies patternsthree main depositional sequences have been recog-nized at the Saladillo site Los Colorados I LosColorados II and Los Colorados III (LC I LC II andLC III respectively Fig 2) (Hongn et al 2007 del

724 M Do Campo et al

Papa et al 2013) The authigenic phases identified inthe fine-grained levels of the study section (according toprevious XRD and SEM data) are summarized in Fig 2b(for more details see also Do Campo et al 2014)

SAMPL ING AND ANALYT ICALTECHNIQUES

Five samples were selected from the previous study byDo Campo et al (2014) and are described here fromshallowest to deepest sample 411-4 corresponds to athin pelite level of the LC I sequence located 237 mabove the discordance between the QLCMG forma-tions sample 1611-1 is from a thick pelite bed 150 mabove the discordance between the QLCMG forma-tions samples 5-7-08-4W and 5-7-08-6W correspondto pelite beds 8 and 20 m above the discordancerespectively and sample 5-7-08-1W is a siltstone fromthe MG Formation collected a few metres below thediscordance separating the two units

The five samples were studied in depth with an XRDstep scan using a PANalytical XrsquoPert Pro diffractom-eter (Cu-Kα radiation 45 kV 40 mA) equipped with

an XrsquoCelerator solid-state linear detector using a stepincrement of 0008deg2θ and a count time of 10 sstep(Department of Mineralogy and Petrology Universityof Granada Spain) The XRD traces were deconvo-luted using the MacDiff software (Petschick 2004)

The textures and mineralogy of the deepest samples(5-7-08-1W 5-7-08-4W and 5-7-08-6W) were exam-ined by SEM on polished thin sections using back-scattered electron imaging and X-ray dispersive (EDS)analysis with a ZEISS DSM 950 scanning electronmicroscope (SEM) (Scientific Instrument CentreUniversity of Granada CIC) and Carl ZeissMERLINtrade Field Emission SEM (Servicio de Apoyoa la Investigacioacuten-Universidad de Zaragoza Spain)

Furthermore three samples were selected for the TEMstudy Two claystones (411-4 and 1611-1) belonging tothe QLC LC I sequence correspond to intermediate stepsin smectite illitization and with high I-S contents and asiltstone (5-7-08-1W) from the underlying Maiacutez GordoFormation taken a few metres below the discordanceseparating the two units These samples were treated withLR Whitersquos resin following the procedure of Kim et al(1995) in order to prevent the collapse of smectite-like

FIG 1 (a) Location and geological map of the northern Calchaquiacute Valley (b) Geological map of the Saladillo area(c) Detailed geological map of the Saladillo area and location of the stratigraphic log described in Figure 2

725Fluid-driven mixed-layer illite-smectite in an Eocene succession

FIG 2 (a) Simplified stratigraphic log of the Paleogene Saladillo sequences (b) The main authigenic phases (kaolinitesmectite R0 R1 and R3 type I-S and illite) in each level are indicated along the stratigraphic column Mineralogical

data from the upper part of the stratigraphic column are from the previous study by Do Campo et al (2014)


interlayers in the vacuum of the ion mill and TEM andthereby facilitate the differentiation of illite and smectiteinterlayers in TEM images Direct contact with waterduring sample preparation was avoided in order toprevent smectite swelling which would result in sampledamage Sticky wax-backed thin sections were preparedwith surfaces normal to bedding and first examined byoptical microscopy Typical areas were removed for TEMobservation via attached Cu washers thinned in an ionmill and carbon coated The TEM data were obtainedusing a JEOL-2000 FXII instrument equipped with anOxford instruments dectector (EDS) at the University ofZaragoza (Spain) The TEMwas operated at 200 kVwitha beam current of 20 μA Through-focus image serieswere obtained from 1000 Aring underfocus (approximateScherzer defocus) to 1000 Aring overfocus in part to obtainoptimum contrast in I-S ordering (overfocus) Howeverbecause the initial focus was controlled manually byminimizing contrast small deviations from exact under-focus or overfocus numbers were inevitable

In order to obtain chemical analyses of individualgrains with an EDAX microanalysis system sampleswere studied by TEM using a Philips CM-20 operated at200 kV (University of Granada) on scattered powdersdeposited on gold-coated grids Atomic concentrationratios were converted into formulae according tostoichiometry (oxygens in the theoretical formulae ofminerals) Accordingly the structural formulae of mixed-layer I-S were calculated on the basis of 22 negativecharges The tetrahedral sites were filled with Al to makeSi +Al = 4 and the remaining Al was assigned tooctahedral positions All the Fe was assumed to beferric and all Mg was assigned to octahedral sites

Nomenclature for mixed-layer I-S

Ordering in mixed-layer I-S is usually characterizedby its Reichweite value on the basis of XRD data Suchmeasurements of I-S ordering represent long-rangeordering averaged over all layers that scatter X-raysShort-range ordering of interlayers within I-S packetscan be identified in TEM images however TheReichweite nomenclature is therefore inappropriatefor specific layer sequences as observed by TEM Thenomenclature proposed by Bauluz et al (2000) is usedherein which utilizes the symbol In where n is thenumber of illite-like layers associated with a givensmectite-like layer ie layers of IS are denoted as an I1unit ISI as an I2 unit and so on This nomenclature isanalogous to the Reichweite nomenclature but isapplied only to specific layers

An ideal rectorite-type sample for which R = 1having 50 illite-like layers is identical to a sequenceof I1 units However a sequence with R = 1 and Igt50 would consist mainly of I1 units but would alsohave I2 I3hellip units (n gt 1)

RESULTS

Clay mineralogy ndash XRD results

Sample 411-4 (QLC Formation) is composed ofquartz K-feldspar plagioclase kaolinite mixed-layerI-S illite-mica and scarce chlorite with calcite cementand minor hematite Deconvolution of the XRD tracein the region 16ndash17deg2θ (Fig 3a) revealed two peakswith 531 and 516 Aring indicating the presence of R1 I-Swith illite contents of sim65 and 85 respectivelyaccording to Moore amp Reynolds (1997)Deconvolution of the low-angle region indicates thatR1 is the predominant type of I-S and shows a veryweak peak at 1557 Aring corresponding to minor R0 I-S

Sample 1611-1 (QLC Formation) consists of quartzplagioclase illite-mica kaolinite mixed-layer I-S scarcechlorite and hematite Deconvolution of XRD traces inthe region between 16 and 17deg2θ (Fig 3b) discriminatestwo peaks at 530 and 514 Aring corresponding to R1 I-Swith illite contents of sim70 and 85 respectively(Moore amp Reynolds 1997) In contrast deconvolution inthe region between 8 and 10deg2θ differentiates two peaksmatching those identified between 16 and 17deg2θ but alsoweak peaks at 917 and 1068 Aring the former indicatingsubordinate R0 I-S with 55 illite layers and the lattercorresponding to minor R3-type I-S

Sample 5-7-08-6W (QLC Formation) consists ofquartz plagioclase K-feldspar illite-mica kaolinitemixed-layer I-S and hematite Deconvolution in theregion between 8 and 10deg2θ differentiated between I-SR3 (gt90 illite layers) and R1 I-S However in theregion of 16ndash17deg2θ deconvolution was impossibleindicating that (002) illite and (002003) R3 I-Sreflections almost coincide

Sample 5-7-08-4W (QLC Formation) consists ofquartz plagioclase illite-mica kaolinite minormixed-layer I-S and hematite Deconvolution of thepeaks corresponding to mixed-layer I-S was notpossible due to its low abundance

Sample 5-7-08-1W (MG Formation) consists ofquartz plagioclase illite-mica kaolinite mixed-layerI-S and hematite Deconvolution of the low-angleregion of XRD traces discriminated between I-S R3(gt90 illite layers) and subordinate R1 I-S (Fig 3c)As for sample 5-7-08-6W deconvolution in the region


of 16ndash17deg2θ was not possible indicating that (002)illite and (002003) R3 I-S reflections almost coincide

SEM results

Backscattered electron images (BSE) of sample 5-7-08-6W (QLC Formation) display abundant detritalclasts of albite Na-Ca plagioclase K-feldspar andquartz partially dissolved and replaced by booklets ofkaolinite or mixed-layer I-S (Fig 4andashd) Texturalrelationships between kaolinite and mixed-layer I-S donot allow us to infer a paragenetic sequence

Illitic phases which according to XRD correspondto mixed-layer I-S are the main clay minerals in thematrix According to the microanalyses their composi-tions range from smectite-rich I-S to illite-rich I-S oreven to illite with interlayer charges ranging from 029to 073 epfu (Table 1) Mixed-layer I-S also formsmicrometre-scale intergrowths with kaolinite (Fig 4e)Besides both phases occur closely associated withcalcite (mostly Mg andor Fe-Mg bearing) or dolomitecements in the matrix Kaolinite also occurs fillingpores of the rock

On the other hand in the pelite 5-7-08-4W (QLCFormation) albite Na-rich plagioclase and quartz aremainly replaced by kaolinite (Fig 4f ) in agreementwith XRD data indicating the low abundance of mixed-layer I-S in this sample In this pelite kaolinite partlyreplaces detrital mica laths (Fig 5a) frequentlyshowing displacive precipitation along the cleavageplanes (De Ros 1998 Arostegui et al 2001)

Sample 5-7-08-1W (MG Formation)) comprisesabundant detrital clasts of quartz and mica whereasfeldspars are scarce According to BSE and EDS theauthigenic phyllosilicates identified in the matrix wereillite illite-rich I-S and kaolinite in close association(Fig 5bc Table 1) Mixed layer I-S also occursreplacing irregular quartz grains (Fig 5b) In thissample kaolinite partly replaces detrital mica laths alsoshowing displacive precipitation along the cleavageplanes similar to the previous sample (Fig 5c)Kaolinite also replaces plagioclase clasts (Fig 5dwhite arrow) and occurs in pores in the rock

Transmission electron microscopy (TEM)

Sample 411-4 (QLC Formation) The phyllosilicatesidentified in this sample by TEM were I1 mixed-layerI-S and illite Low-magnification TEM images showthin packets of mixed-layer I-S with curved andanastomosing morphologies depicting high-angleterminations with similar packets of contrasting

orientation (Fig 6a) The SAED patterns (Fig 6ainset) of I-S display 00l reflections corresponding to10 Aring periodicity in the 00l row and ill-defined andnon-periodic reflections or even a line of streaking(diffuseness parallel to c) in rows with kne 0indicating 1Md polytypism Diffuseness normal andparallel to c is ubiquitous in the 00l row the formerbeing a consequence of the changes in orientation ofadjacent packets whereas the latter originates frominterlayering Figure 6bc shows two lattice-fringeimages representative of mixed-layer I-S with packetsonly a few layers thick (sim50ndash80 Aring) depicting thealternating dark and light contrast typical of mixed-layer I-S Adjacent packets frequently have a slightlydifferent orientation with curved and discontinuous(001) fringes with spacings corresponding to the sumof illite- and smectite-like layer spacings (21ndash22 Aringperiodicity) Although deconvolution of the XRD tracediscriminates peaks corresponding to R1 and minor R0(Fig 3a) only the dominant I1 layer sequences wereidentified in I-S mixed-layer packets in TEM imagesAbundant lens-shaped voids are consistent with partialcollapse caused by the TEM vacuum despite sampletreatment with LR White resin Mixed layers areoblique to subparallel to straight or slightly curved withrespect to illite packets in general the two phases areseparated by ellipsoidal or triangular regions lackinglattice fringes (Fig 6c) Illite packets (60ndash120 Aring thick)consist of straight lattice fringes with constant 10 Aringspacings displaying uniform contrast consistent withrelatively constant orientation although they depictabundant layer terminations (Fig 6bc)

Sample 1611-1 (QLC Formation) Phyllosilicatesidentified in this sample by TEM were I1 I-S illite andoccasionally I2 andor I3 I-S sequences kaolinite andchlorite

At low magnification packets exhibit a curved andanastomosing morphology without preferential orien-tation and high-angle layer terminations (Fig 7a)Collapsed layers are abundant Figure 7a depictsdetrital micas gt200 Aring thick next to curved andanastomosing packets of mixed-layer I-S orientedoblique or even perpendicular to the former replace-ment of detrital micas by authigenic I-S are alsoobserved (Fig 7a white arrows) Iron oxides fre-quently occur in close association with authigenic I-S(Fig 7a) The SAED patterns with very diffuse 10ndash104 Aring (001) reflections elongated normal to c as aresult of the curvature of I-S layers were obtainedindicating that I-S collapsed under the vacuum of theion mill and TEM (Fig 7a inset) Less frequentlySAED patterns displaying 00l reflections


FIG 3 The left-hand column shows air-dried and ethylene glycol-solvated XRD patterns of lt2 m sub-samples forthe three samples studied by TEM (a) QLC middle LCI sequence sample 411-4 (b) QLC middle LCI sample1611-1 (c) Maiacutez Gordo Formation sample 5-7-08-1W The right-hand columns show decomposition of the low-angle region of the XRD patterns (obtained using MacDiff ) The percentages of illite layers in R0 and R1 I-S areindicated as subscripted numbers in parts a and b according to Moore and Reynolds (1997) See the text for peak

positions


corresponding to 10 Aring periodicity in the 00l row andill-defined and non-periodic reflections and diffuse-ness parallel to c in rows with kne 3n (indicating 1Md

polytypism) were obtained for mixed-layer I-S Latticefringes have a wavy appearance (Fig 7b) and the

alternating dark and light contrast typical of mixed-layer I-S An along-layer transition from 152 to 102 Aringwas observed (Fig 7c black arrow) Layer sequencescorresponding to I1 and occasionally to I2 andor I3 wereidentified in mixed-layer I-S although I1 is the most

FIG 4 (a) BSE images (andashe) sample 5-7-08-6W (a) Clasts of quartz and Na-rich plagioclase partially dissolved andreplaced by kaolinite and mixed-layer I-S (b) Enlargement of part a kaolinite and mixed-layer I-S replacing a clast ofplagioclase (c) Enlargement of boxed area in part a clast of quartz partially dissolved and replaced by kaolinite andmixed-layer I-S (d) Remnant of a K-feldspar clast surrounded by booklets of kaolinite (e) Enlargement of part bshowing intergrowths of kaolinite and mixed-layer I-S at micrometre scale (f ) Sample 5-7-08-4W Clasts of albiteheavily dissolved and replaced by kaolinite grain of dolomite replaced by kaolinite and laths of detrital mica showing

replacement by kaolinite


common ordering type (Fig 7c) in agreement with XRDdata (Fig 3b) Illite packets (30ndash200 Aring thick) are straightto slightly curved depicting homogeneous contrast along

and across layers (Fig 7c) The white arrow indicates alow-angle boundary between illite and mixed-layer I-Spackets with abundant layer terminations

TABLE 1 Representative structural formulae for I-S according to AEM data

411-4

11-1 11-2 11-6 11-7 11-8 11-9 11-13Si 344 340 350 356 362 340 343IVAl 056 060 050 044 038 060 057VIAl 152 153 163 144 162 163 166Fe 025 025 023 033 021 021 021Mg 034 033 024 028 028 023 026Mn 000 000 000 000 000 000 002Ti 000 000 002 000 000 002 000sumoct 210 211 212 205 211 208 215K 044 047 023 047 023 046 026Na 000 000 000 000 000 000 000Ca 007 007 007 005 005 005 007Intcharge 058 061 037 058 033 056 040

1611-1

11-1 11-2 11-3 11-4 11-5 11-7 11-8 11-9Si 346 343 329 340 347 359 332 357IVAl 054 057 071 060 053 041 068 043VIAl 142 142 156 142 147 159 157 144Fe 028 027 025 025 028 017 023 023Mg 039 035 032 043 030 024 023 037Mn 000 000 000 000 000 000 000 000Ti 000 002 002 002 000 002 002 000sumoct 210 206 214 212 205 202 205 203K 042 051 044 048 049 037 060 010Na 000 000 000 000 000 000 000 000Ca 011 011 007 009 009 010 007 030Intcharge 064 072 058 066 067 058 074 070

1611-1 5-7-08-1W


Analyses were normalized for 10 O and 2 (OH)


Sample 5-7-08-1W (MG Formation) In this sampleillite I1 and I3 mixed-layer I-S occur in adjacentpackets with low-angle boundaries between them inillite-rich areas (Fig 8ab) in agreement with the XRDdata that indicate R1 and R3 (Fig 3c) Illite forms sub-parallel packets (60ndash240 Aring thick) with straight andrelatively defect-free lattice (001) fringes with low-angle boundaries (Fig 8b white arrow) The SAEDpatterns of illite crystals indicate 1Md polytypism(Fig 8c)

In contrast in mixed-layer I-S-rich areas packets ofI-S (60ndash200 Aring thick) and illite (no preferred orienta-tion) with high-angle boundaries between thempredominate (Fig 9) In such areas layer sequencescorresponding to I1 I3 and I gt 3 I-S were singled outfurthermore along-layer transitions between I-S andillite were identified (Fig 9)

Analytical electron microscopy (AEM)

Structural formulae calculated from new data formixed-layer I-S are shown in Table 1 Figure 10 alsoincludes data from the previous study by Do Campoet al (2014) The usual SiAl ratio change with depth(Fig 10a) in tetrahedral layers is clearly present in thesamples examined here

The chemical compositions of mixed-layer I-S ofsamples 411-4 and 1611-1 (intermediate steps in smectiteillitization shallower samples) are similar they depictlarge compositional variations and overlap each other inthe compositional diagrams (Fig 10) This is consistentwith the XRD and TEM data because they are formed byat least two types of dioctahedral clays In contrastmixed-layer I-S of sample 5-7-8-1W (evolved step insmectite illitization deepest sample) show more

FIG 5 (a) Enlargement of Fig 4f laths of detrital mica showing pseudomorphic replacement of the lamellae andadditional displacive kaolinite precipitation between the cleavage planes (sample 5-7-08-4W) (bndashd) Sample 5-7-08-1WMaiacutez Gordo Formation (b) Booklets of kaolinite and bunches of mixed-layer I-S in the matrix and laths of detrital micashowing pseudomorphic replacement of the lamellae (white arrow) (c) Bunches of mixed-layer I-S in the matrix andkaolinite replacing detrital mica grains (d) Clast of quartz (black arrow) and albite partially replaced by kaolinite

(white arrow)


homogeneous compositions (Fig 10) The most signifi-cant differences among dioctahedral clays in thesesamples are the lower Ca and Mg contents in I-S in thedeepest sample (Fig 10bc) and the increase in the K andAl contents (Fig 10ad) from shallower to deepestsamples reflecting an increase in the illitization processAccordingly Si-K Si-Mg and Si-Al compositionaldiagrams (Fig 10acd) reveal three groups of analysisgroup 1 probably corresponds to smectite or R0 I-Scompositions group 2 corresponds to R1 I-S composi-tions and group 3 to illite (or R3 I-S) analysis

D I SCUSS ION

XRD vs TEM results

The XRD data show that the samples analysedconsist of complex mixtures of mixed-layer I-S with

different Reichweite orders In summary samples 411-4 and 1611-1 are mainly composed of R1 mixed-layerI-S with 65ndash80 illite layers R1 coexists with veryscarce R0 in both samples whereas minor R3 I-Smight be present in sample 1611-1 In contrast sample5-7-08-1W consists mainly of R3 I-S (gt90 illitelayers) and minor R1 I-S Thus XRD shows asignificant change from the two shallowest to thedeepest samples (5-7-08-1W) in which there is anapparent increase in diagenetic grade

The TEM images confirm that I1 is the mostabundant dioctahedral clay in samples 411-4 and 1611-1coexisting with discrete illite Furthermore in sample1611-1 this I-S occasionally coexistswith I-S having I gt 1In contrast based on the TEM study sample 5-7-08-1Wconsists of illite and I1 I-S with minor I gt 1 I-S

Both techniques show good agreement in theidentification of R1 I-S and the R3 I-S with a large

FIG 6 TEM images of sample 411-4 (a) Low-magnification TEM image and SAED pattern showing thin packets ofmixed-layer I-S with curved and anastomosing morphologies oriented at high angles to one another (bc) Lattice-fringeimages showing thin packets of mixed-layer I-S (sim50ndash80 Aring) with curved and discontinuous (001) fringes with spacingscorresponding to the sum of illite- and smectite-like layer spacings (21ndash22 Aring periodicity) indicating I1 ordering typeAdjacent packets frequently show a slightly different orientation (b centre) Lens-shaped voids consistent with partial

collapse caused by the TEM vacuum despite sample treatment with LR White resin can be seen (c)


percentage of illite layers (determined by XRD) probablycorresponding to the illite packets imaged by TEM Onthe other hand R0 I-S was not identified in the TEMimages of the two shallowest samples probably becauseit is very scarce but also because more defective phasesdeteriorate easily in the vacuum of the ion mill and TEMAccordingly the type of mixed-layer I-S identified in

TEM images for each sample matches quite well with theresults obtained by peak deconvolution using MacDiffindicating that carefully interpreted good-quality XRDdatamay provide very useful information even in samplescontaining very complex phase assemblages

Dong et al (1997) demonstrated that the trendsobserved in XRD patterns are due to changes in the

FIG 7 TEM images of sample 1611-1 (a) Low-magnification lattice-fringe image detritalmicas curved and anastomosingpackets of mixed-layer I-S oriented at high angles to the former replacement of detrital micas by authigenic I-S can be seen(white arrows) Lens-shaped voids consistent with partial collapse caused by the TEM vacuum despite sample treatmentwith LR White resin and iron oxides in close association with authigenic I-S can be observed inset SAED patterndepicting very diffuse 10ndash104 Aring (001) reflections elongated normal to c as a result of curvature of I-S (b) Lattice-fringeimage of mixed-layer I-S packets (001) fringes have a wavy appearance and spacings corresponding to the sum of illite-and smectite-like layer (217ndash23 Aring periodicity) low-angle boundaries between adjacent I-S mixed-layer packets can beseen (c) Illite and mixed-layer I-S depicting low-angle boundaries between them (white arrow) Layer sequencescorresponding to I1 ordering type (225ndash23 Aring periodicity) including five consecutive I-S layers can be observed (lower

left) An along-layer transition from 152 to 102 Aring can be seen (black arrow)


proportion of discrete phases and not in the proportionwithin sequences of layers of individual packets In thepresent study the discrete phases are I1 I-S illite andprobably smectite R0 I-S was not identified inagreement with all the previous images published inthe diagenesis literature and the minor presence ofRge 3 could represent residual layers of smectite notcompletely transformed during illitization

Chemical evolution of I-S with depth

The chemical changes related to the diageneticevolution of mixed-layer I-S have not yet been wellconstrained due to the intrinsic difficulty of analysinglow-crystallinity phases which at the same time havevery small grain sizes and are difficult to separate fromeach other and from other minerals present in the

sample In addition the coexistence in the same sampleof various types of mixed-layer I-S (Arostegui et al2006 SantrsquoAnna et al 2006 Ferrage et al 2011) isanother source of complication To date therefore therespective fields of compositions of the various typesof mixed-layer I-S have not been defined properly anda definitive criterion to classify smectite illite andmixed-layer I-S phases based on chemical compos-ition only does not exist Various authors haveproposed that I-S chemistry does not need to reproduceexactly intermediate compositions between smectiteand illite (Ahn amp Peacor 1986 Jiang et al 1990) in asimilar way as was demonstrated for the equivalentsmectite-chlorite series (Shau et al 1990)

The structural formulae of Saladillo I-S (Table 1Fig 10) show the expected and previously described(Hower et al 1976 Ahn amp Peacor 1986) Si-for-Al

FIG 8 Lattice-fringe images from the illite-rich areas of sample 5-7-08-1W (Maiacutez Gordo Formation) (a) Adjacentpackets of illite and mixed-layer I-S corresponding to I gt 3 and I1 ordering type with low-angle boundaries betweenthem (b) Illite and mixed-layer I-S with I gt 3 ordering type low-angle boundaries between adjacent packets can be seen

(white arow) (c) SAED pattern of the illite crystals indicating 1Md polytypism


substitution with depth In terms of charge balance thischemical change needs to be compensated by otherchanges producing a positive charge increase Thischange is basically given by the Mg to Al substitutionin the octahedral sheet (Fig 10c) Therefore the mostsignificant change overall is the Tschermak substitu-tion [(Si Al-1)

IV (Al Mg-1)VI] which is well known in

other silicates including metamorphic micas TheTschermak component decreases to the bottom of thesequence giving way to more muscovitic compositions

The large compositional variation of mixed-layer I-Sin the shallower samples is characteristic of metastablephases forming under low-temperature conditions Incontrast the more homogenous composition of I-S insample 5-7-8-1W (Fig 10c) probably indicates highertemperatures andor a higher fluidrock ratio

The change in Ca contents expected for a smectite-to-illite evolution has been employed successfully todifferentiate the two end-member minerals and to obtainpartial information about their mixed-layer phases

(Fig 10b) Previous studies (eg Ahn amp Peacor 1986Drief amp Nieto 2000) showed that the Ca contents insmectite or R0 I-S are not as high as assumed before theuse of AEM Therefore even if the overall reduction ofCa is not substantial the differences between shallowerand the deepest more evolved sample (Fig 10b) aresignificant and confirm that the use of Ca contents as adifferential criterion between smectite and illite inprevious studies was justified

To conclude the traditionally accepted increase intetrahedral charge due to the substitution of Si by Al isconfirmed in the Saladillo samples but the compos-itional vector able to explain such a change is not assimple as traditionally assumed The overall chemicaldifferences between smectite the parent material andthe resultant illite are more complex than only the SiAlratio and K content because Mg and Fe also need to beless abundant as they are abundant only in mediumndashhigh-pressure micas Ca is a very minor component inmicas and its substitution by a larger amount of K ions

FIG 9 Lattice-fringe image from a mixed-layer I-S-rich area of sample 5-7-08-1W (Maiacutez Gordo Formation) Packets ofI-S depicting layer sequences corresponding to I1 and I3 ordering type and illite with no preferred orientation and high-angle boundaries between them can be seen An along-layer transition between I-S and illite is shown (white arrow)


in the interlayer (Fig 10bd) is less than necessary tocompensate the significant increase in the tetrahedralcharge Therefore together with the traditionallyaccepted reduction in the pyrophyllitic componentobservable in the Saladillo samples (Fig 10d) adecrease in Tschermak component (Fig 10c) is alsopresent in the illitization of smectite

Evolution and genesis of clays

The evolution with depth in the dioctahedral claysinferred from XRD indicates a change in the diageneticgrade between samples 411-4 and 1611-1 on one handand samples 5-7-08-6W and 5-7-08-1W on the otherThe TEM images also support this trend showing anincrease in the illite vs I1 I-S contents and in thethickness of authigenic illite crystals and a decrease inthe defects of illite crystals In vast areas of the LosColorados basin (including a control point 15 km southof Saladillo) these sequences were only affected byshallow diagenesis (Do Campo et al 2007 2010

2014) Therefore the diagenetic grade attained by thepost-rift and the overlying foreland sediments in thenorthernmost Calchaquiacute Valley must be influenced bylocal factors BSE images provide decisive evidence ofdissolution of primary phases (K-feldspar albite whitemicas and quartz) and crystallization of secondaryphyllosilicates namely illite I-S and kaoliniteTextures evidencing dissolution-crystallization pro-cesses could be recognized in MG Formation and upto the middle levels of LCI sequence in QLC formation(Do Campo et al 2014) As alteration of primaryphases is not pervasive an intermediate fluidrock ratiomight be inferred

According to Do Campo et al (2014) smectitewould have been the common precursor of I-S mixed-layers in these sediments In fact smectite or R0 I-S arethe dominant smectitic phases at the top of thesequence (Do Campo et al 2014)

Remarkably in the Saladillo section beds contain-ing dominant I1 mixed-layer I-S and those composedof illite or I3 mixed-layer I-S plus authigenic kaolinite

FIG 10 Compositional plots for mixed-layer I-S phases (a) Si-total Al graph (b) Si-Ca graph (c) Si-Mg graph (d) Si-Kgraph Rhombs sample 411-4 squares sample 1611-1 and triangles sample 5-7-08-1W Filled symbols correspond to

data from Table 1 and open symbols to data from Do Campo et al (2014)


are separated by lt250 m (Fig 2b) Given thetemperatures estimated for the transformation from R0to R1 I-S in siliciclastic sediments of different basins(75ndash120degC) (Hoffman amp Hower 1979 Pollastro 1993Schegg amp Leu 1996) and the transition temperaturefrom R1 I-S to ordered R3 I-S in sediments older than5 Ma (sim175degC) (Pollastro 1993) a geothermal gradientof gt220degCkm would result which is unrealistic forburial diagenesis As mentioned above considerableevidence indicates that the formation of illite I3 mixed-layer I-S plus kaolinite in the Paleogene continentalsediments at the Saladillo site is exceptional and was notcontrolled by burial temperature alone

The textures observed in TEM images (Fig 7a)strongly suggest that illitization mainly proceeded bydissolution-crystallization (1) packets exhibit a curvedand anastomosing morphology without preferentialorientation and (2) packets with frayed borders displayhigh-angle layer terminations with adjacent packets(Fig 7a) However few examples were found of layer-by-layer replacement among the tens of imagesobtained (Fig 6c) According to Altaner amp Ylagan(1997) the fluidrock ratio and permeability influence thereaction mechanisms of smectite illitization with highfluidrock ratios favouring dissolution-crystallizationprocesses In the same way the fan-like texture depictedby authigenic kaolinite is indicative of precipitation froma fluid phase It is in agreement with the evidence ofdissolution and crystallization of kaolinite mixed-layerI-S and illite observed at SEM scale

The wide range of compositional variation depictedby the mixed-layer I-S studied is characteristic ofmetastable phases formed under low-temperatureconditions which follow the lsquoOstwald Step RulersquoThe composition of each individual grain is aconsequence not only of temperature and pressureconditions but also of the parent material and chemicalenvironment at the micrometre scale Another indicatorof thermodynamic disequilibrium is the coexistence ofI-S with different Reicheweite values and illite in asingle sample at a distance of around 50 nm (Fig 9)The faults that bound the northern Calchaquiacute Valley(Toro Muerto and Calchaquiacute reverse faults) are only4 km apart in the Saladillo area whereas the samplingpoints are lt1 km from the fault planesSyndepositional deformation features preserved inthe Paleogene sediments of the study area theNeogene refolding of the Eocene folds the link withQuaternary volcanic centres and historical earth-quakes (Marrett et al 1994 Hongn et al 2007)corroborate the fact that these faults were active atseveral stages during the Cenozoic In fact the faults

may have facilitated the circulation of hot deep fluidsthat increased the fluidrock ratios and enhanced heattransport thus favouring the illitization of smectite bydissolution-crystallization processes Furthermorepermeable sandstone and conglomerate sandstonelayers interbedded with claystone and siltstone levelsmay have facilitated fluid flow near the faults Stressmight also have contributed to the attainment of deepdiagenesis at the illite (or I3 mixed-layer I-S) stage inthese young continental sediments

The connection between areas showing advancedreaction progress in I-S evolution and the activity offluids in faulted zones has been mentioned previouslyfor different basins (Uysal et al 2000 Abid amp Hesse2007 Abad et al 2010 2012 Middleton et al 2015)In several cases the authors could not conclude withcertainty whether fluids were involved directly insmectite illitization or if they only acted as heat carriers(Uysal et al 2000) On the other hand Abid amp Hesse(2007) concluded that fluids channeled by faults wereinvolved directly in diagenetic reactions yieldinganomalous areas of enhanced illitization in the offshoreJeanne drsquoArc Basin of Canada Likewise Middletonet al (2015) described several generations of illite(Rge 3) precipitation from MerrimeliandashInnaminckaRidge (WarburtonndashCooper basins central Australia)sediments associated with the reactivation of fracturenetworks that would have acted as highly permeableconduits for hydrothermal fluid flow during theCarboniferous and Late Triassic tectonothermalevents Middleton et al (2015) suggested that theprecipitation of authigenic clays in these sedimentsproceed from alteration of detrital feldspars and micasduring the repeated periods of fault activity and fluidflow which brought high waterrock ratios and a led toa high palaeogeothermal gradient (sim100degC kmminus1)

ACKNOWLEDGEMENTS

The help of MM Abad-Ortega (Centro de InstrumentacioacutenCientiacutefica University of Granada) with the TEM and AEMMaria Angeles Laguna with TEM and Cristina Gallego withFESEM (University of Zaragoza) was essential for thepresent work The authors wish to acknowledge the use ofthe Servicio General de Apoyo a la Investigacioacuten-SAIUniversity of Zaragoza Spain The authors are grateful toC Brime and an anonymous reviewer for their valuable andconstructive comments This work was financed in part bythe grants CONICETndashPIP 489 and ANCyTndashPICT 0407Research Projects CGL2011-30153-C02-01 and CGL2013-46169-C2-1-P (Spanish Ministry of Science) and theGobierno de Aragoacuten and the European Social Fund


(Grupos Consolidados) The stay of M Do Campo at theUniversity of Granada and the field work of Fernando Nietoin Argentina were supported by AECI project A771207

REFERENCES

Abad I Murphy BJ Nieto F amp Gutieacuterrez-Alonso G(2010) Diagenesis to metamorphism transition in anepisutural basin the late Paleozoic St Maryrsquos BasinNova Scotia Canada Canadian Journal of EarthSciences 47 121ndash135

Abad I Nieto F Gutieacuterrez-Alonso G Murphy BJBraid JA amp Rodriacuteguez-Navarro AB (2012) Fluid-driven low-grade metamorphism in polydeformed rocksof Avalonia (Arisaig Group Nova Scotia Canada)Swiss Journal of Geosciences 105 283ndash297

Abid IA amp Hesse R (2007) Illitizing fluids as precursorsof hydrocarbon migration along transfer and boundaryfaults of the Jeanne drsquoArc Basin offshoreNewfoundlandCanada Marine and Petroleum Geology 24 237ndash245

Abid IA Hesse R amp Harper JD (2004) Variations inmixed-layer illitesmectite diagenesis in the rift andpost-rift sediments of the Jeanne drsquoArc Basin GrandBanks offshore Newfoundland Canada CanadianJournal of Earth Sciences 41 401ndash429

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-llitetransition Clays and Clay Minerals 34 165ndash179

Ahn JH amp Peacor DR (1989) Illitesmectite from GulfCoast A reappraisal of the transmission electronmicroscopy images Clays and Clay Minerals 37542ndash546

Allen PA amp Allen JR (2005) Basin Analysis Principlesand Applications Second edition BlackwellPublishing Oxford UK 549 pp

Altaner SP amp Ylagan RF (1997) Comparison ofstructural models of mixed-layer illitesmectite andreaction mechanisms of smectite illitizationClays andClay Minerals 45 517ndash533

Altaner SP Whitney G Aronson JL amp Hower J (1984)A model for K-bentonite formation evidence fromzoned K-bentonites in the disturbed bed of MontanaGeology 12 412ndash415

Arostegui J Irabien MJ amp Nieto F (2001) Microtexturesand the origin of muscovite-kaolinite intergrowths insandstones of the Utrillas Formation BasqueCantabrian Basin Spain Clays and Clay Minerals49 529ndash539

Arostegui J Sanguumlesa FJ Nieto F amp Uriarte JA (2006)Thermal models and clay diagenesis in the Tertiary-Cretaceous sediments of the Alava block (Basque-Cantabrian basin Spain)ClayMinerals 41 791ndash809

Bauluz B Peacor DR amp Gonzaacutelez Loacutepez JM (2000)Transmission electron microscopy study of illitizationin pelites from the Iberian Range Spain layer-by-layerreplacement Clays and Clay Minerals 48 374ndash384

Cuadros J (2006) Modeling of smectite illitization inburial diagenesis environments Geochimica etCosmochimica Acta 70 4181ndash4195

De Ros LF (1998) Heterogeneous generation andevolution of diagenetic quartzarenites in the Silurian-Devonian Fumas Formation of the Paran Basinsouthern Brazil Sedimentary Geology 116 99ndash128

del Papa C Hongn F Petrinovic I amp Domiacutenguez R(2004) Evidencias de deformacioacuten pre-miocena mediaasociada al antepaiacutes andino en la Cordillera Oriental(24deg35primeSndash66deg12primeO) Asociacioacuten Geoloacutegica ArgentinaRevista 59 506ndash509

del Papa C Hongn F Powell J Payrola P Do CampoM Strecker MP Petrinovic I Schmitt A amp PereyraR (2013) Middle Eocene-Oligocene broken forelandevolution in the Andean Calchaqui Valley NWArgentina insights from stratigraphic structural andprovenance studies Basin Research 25 574ndash593

Diacuteaz JI amp Malizzia DC (1983) Estudio geoloacutegico ysedimentoloacutegico del Terciario superior del ValleCalchaquiacute (Departamento de San Carlos Prov DeSalta) Boletiacuten Sedimentoloacutegico 2 8ndash28

Do Campo M del Papa C Jimeacutenez-Millaacuten J amp Nieto F(2007) Clay mineral assemblages and analcimeformation in a Paleogene fluvial-lacustrine sequence(Maiz Gordo Formation) from NorthwesternArgentina Sedimentary Geology 201 56ndash74

Do Campo M del Papa C Nieto F Hongn F ampPetrinovic I (2010) Integrated analysis for constrain-ing palaeoclimatic and volcanic influences on clayndashmineral assemblages in orogenic basins (PaleogeneAndean foreland Northwestern Argentina)Sedimentary Geology 228 98ndash112

Do Campo M Nieto F del Papa C amp Hongn F (2014)Syn- and post-sedimentary controls on clay mineralsassemblages in a tectonically active basin AndeanArgentinean Foreland Journal of South AmericanEarth Sciences 52 43ndash56

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite I-S and illite ofshales evidence for McEwan relations in crystallitesand dominance of 2M1 polytypism Clays and ClayMinerals 44 257ndash275

Dong H Peacor DR amp Freed RL (1997) Phaserelations among smectite R1 illite-smectite andillite American Mineralogist 82 379ndash391

Dorsey RJ Buchovencky EJ amp Lundberg N (1988)Clay mineralogy of Pliocene-Pleistocene mudstoneseastern Taiwan Combined effects of burial diagenesisand provenance unroofing Geology 16 944ndash947

Drief A amp Nieto F (2000) Chemical composition ofsmectites formed in clastic sediments Implications forthe smectite-illite transformation Clay Minerals 35665ndash678

Ferrage E Vidal O Mosser-Ruck R Cathelineau M ampCuadros J (2011) A reinvestigation of smectiteillitization in experimental hydrothermal conditions


Results fromX-ray diffraction and transmission electronmicroscopy American Mineralogist 96 207ndash223

Guzmaacuten S amp Petrinovic I (2008) Pucarilla-Cerro TipillasVolcanic Complex the oldest recognized caldera in thesoutheastern portion of Central Volcanic Zone ofCentral Andes Collapse Calderas Workshop IOPConference Series Earth and Environmental Science3 012003 IOP Publishing doi1010881755-130731012003

Hoffman J amp Hower J (1979) Clay mineral assemblagesas low grade metamorphic geothermometersApplication to the thrust faulted Disturbed Belt ofMontana U S A Pp 55ndash79 in Aspect of Diagenesis(PA Scholle amp PS Schluger editors) SpecialPublication 26 Society of Economic Paleontologistsand Mineralogists Tulsa Oklahoma USA

Hongn F del Papa CE Powell J Petrinovic IA MonR amp Deraco V (2007) Middle Eocene deformationand sedimentation in the Puna-Eastern Cordilleratransition (23degndash26degS) Control by preexisting hetero-geneities on the pattern of initial Andean shorteningGeology 35 271ndash274

Hower J Eslinger EV Hower ME amp Perry EA (1976)Mechanism of burial metamorphism of argillaceoussediments Mineralogical and chemical evidenceGeological Society of America Bulletin 87 725ndash737

Huang W-L Longo JM amp Pevear DR (1993) Anexperimentally derived kinetic model for smectite-to-illite conversion and its use as a geothermometerClays and Clay Minerals 41 162ndash177

Jiang W-T Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed-layer illite-smectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449ndash468

Kim JW Peacor DR Tessier D amp Elsass F (1995) Atechnique for maintaining texture and permanentexpansion of smectite interlayers for TEM observa-tions Clays and Clay Minerals 43 51ndash57

Marquillas R del Papa C amp Sabino I (2005)Sedimentary aspects and paleo-environmental evolu-tion of a rift basin Salta Group (Cretaceous-Paleogene) northwestern Argentina InternationalJournal of Earth Sciences 94 94ndash113

Marrett RA Allmendiger RW Alonso RN amp Drake RE (1994) Late Cenozoic tectonic evolution of thePuna plateau and adjacent foreland northwesternArgentine Andes Journal of South American EarthScience 7 179ndash208

Middleton AW Tonguc I Uysal S amp Golding D (2015)Chemical and mineralogical characterisation of illitendashsmectite Implications for episodic tectonism andassociated fluid flow central Australia Geochimicaet Cosmochimica Acta 148 284ndash303

Moore DM amp Reynolds RC (1997) X-ray Diffractionand the Identification and Analysis of Clay MineralsOxford University Press Oxford UK

Nieto F Ortega-Huertas M Peacor DR amp Arostegui J(1996) Evolution of illitesmectite from shallowdiagenesis through incipient metamorphism in sedi-ments of the Basque-Cantabrian Basin Clays andClay Minerals 44 304ndash323

Petschick R (2004) httpwwwgeol-paluni-frankfurtdeStaffHomepagesPetschickclassicsoftwarehtmlMacDiff

Pollastro RM (1993) Considerations and applications ofthe illitesmectite geothermometer in hydrocarbon-bearing rocks of Miocene to Mississippian age Claysand Clay Minerals 41 119ndash133

Salfity JA amp Marquillas RA (1994) Tectonic andsedimentary evolution of the Cretaceous-Eocene SaltaGroup Basin Argentina Pp 266ndash315 in CretaceousTectonics of the Andes (JA Salfity editor)Earth Evolution Sciences Monograph SeriesFriedrich Vieweg amp Sohn BraunschweigWiesbaden Germany

SantrsquoAnna LG Clauer N Cordani UG Riccomini CVelaacutezquez VF amp Liewig N (2006) Origin andmigration timing of hydrothermal fluids in sediment-ary rocks of the Paranaacute Basin South AmericaChemical Geology 230 1ndash21

Schegg R amp Leu W (1996) Clay mineral diagenesis andthermal history of the Thonex Well Western SwissMolasse BasinClays and Clay Minerals 44 693ndash705

Shau Y-H Peacor DR amp Essene EJ (1990) Corrensiteand mixed-layer chloritecorrensite in metabasalt fromnorthern Taiwan TEM AEM EMPA XRD andoptical studies Contributions to Mineralogy andPetrology 105 123ndash142

Turner JC (1959) Estratigrafiacutea del cordoacuten de Escaya yde la sierra de Rinconada (Jujuy) Revista de laAsociacioacuten Geoloacutegica Argentina 13 15ndash39

Turner JCM (1960) Estratigrafiacutea de la Sierra de SantaVictoria y adyacencias Boletiacuten Academia Nacional deCiencias de Cordoba 41 163ndash196

Uysal IT Glikson M Golding SD amp Audsley F (2000)The thermal history of the Bowen Basin QueenslandAustralia Vitrinite reflectance and clay mineralogy ofLate Permian coal measures Tectonophysics 323105ndash129

Veblen DR Guthrie Jr GD Livi KJT amp ReynoldsRC Jr (1990) High-resolution transmission electronmicroscopy and electron diffraction of mixed-layerillitesmectite Experimental results Clays and ClaysMinerals 38 1ndash13

Whitney G (1990) Role of water in the smectite-to-illitereaction Clays and Clay Minerals 38 343ndash350

Whitney G amp Northrop HR (1988) Experimentalinvestigation of the smectite to illite reaction Dualreaction mechanisms and oxygen-isotope systematicsAmerican Mineralogist 73 77ndash90

Whitney DL amp Evans BW (2010) Abbreviationsfor names of rock-forming minerals AmericanMineralogist 95 185ndash187


Veblen et al 1990 Dong amp Peacor 1996 Nieto et al1996 Altaner amp Ylagan 1997 Dong et al 1997Bauluz et al 2000 Abad et al 2004 Cuadros 2006)and has also been reproduced in laboratory experi-ments (Whitney amp Northrop 1988 Huang et al1993) In addition several studies have demonstratedthe importance of fluidrock ratios in this transform-ation (Whitney 1990 Middleton et al 2015)

A recent study on the evolution of PaleogeneAndean foreland basins in NW Argentina revealedenhanced diagenetic evolution in sediments croppingout in the northernmost Calchaquiacute Valley (Saladilloarea Figs 1 2) (Do Campo et al 2014) Thesesediments show a complete evolution from smectite atthe top to R3 I-S mixed layers plus authigenic kaoliniteat the bottom through R1 I-S-type mixed layers inbetween indicating the deep diagenetic transforma-tions (Fig 2b) The study section comprises sedimentsrepresenting the infill of the Los Colorados basin(Quebrada de los Colorados Formation QLC) recentlydefined as part of a broken foreland basin (del Papaet al 2013) as well as the underlying post-riftsequences (Maiacutez Gordo Formation MG Marquillaset al 2005)

Several indicators show that the R3 I-S formation inthese Paleogene continental sediments is exceptionaland not controlled by burial temperature aloneDo Campo et al (2010 2014) reported that equivalentlevels of QLC in southern areas of the Los Coloradosbasin including a control point located as near as15 km contain smectite and no mixed-layer I-S in thebasal fine-grained levels Thus according to the claymineralogy Paleogene sequences were affected byshallow diagenesis only in most parts of the LosColorados basin in agreement with the burial depths ofsim3000 m estimated for the sediment column and thelow geothermal gradient characteristic of forelandbasins (Dorsey et al 1988 Allen amp Allen 2005)Furthermore shallow diagenetic conditions were alsoreported for the underlying sediments of the MaiacutezGordo Formation in several localities (Do Campoet al 2007)

The aim of the present paper was to understand therole of fluids and the transformation mechanismsinvolved in smectite illitization in the Paleogenecontinental sediments cropping out in the northern-most Calchaquiacute Valley employing high-quality step-scan XRD SEM and HRTEM techniques The use ofanalytical and high-resolution techniques combinedwith deconvoluted XRD traces can be useful inunderstanding the complex burialthermal history ofbroken foreland basins in northwestern Argentina

GEOLOG ICAL FRAMEWORK

The Calchaquiacute Valley is located in the EasternCordillera of the southern Central Andes ofArgentina The Eastern Cordillera is a morphotectonicunit characterized by thick- and thin-skinned tectonics

The northern Calchaquiacute Valley (Saladillo area) is anarrow NndashS striking depression bound by upwards-convergent reverse faults (Calchaquiacute and Toro Muerto)(Marrett et al 1994) The stratigraphy of this areacomprises Neoproterozoicndashlower Cambrian very low-grade metamorphic rocks of the PuncoviscanaFormation (Turner 1960) This unit is overlainunconformably by Upper CretaceousndashPaleogene sedi-ments corresponding to the post-rift facies infilling theextensional basin (Salta Group Turner 1959 Salfity ampMarquillas 1994 Marquillas et al 2005) An uncon-formity separates the Salta Group from the overlyingMiddle Eocene Quebrada de Los Colorados (QLC)Formation (Payogastilla Group Diacuteaz ampMalizzia 1983del Papa et al 2004) of the Andean foreland basinStructural and sedimentary facies analyses showedconsiderable evidence at Saladillo of synorogenicdeposition of the Quebrada de los ColoradosFormation (Hongn et al 2007) These authorsconcluded that folding of the Saladillo syncline waspartially coeval with deposition of the QLC FormationDeformation events and therefore fault activity alsotook place in the Neogene and Quaternary as recordedby the intersection of fold axes (see fig 1 in Hongnet al 2007) and by the Quaternary faulting controllingthe eruption of the Los Gemelos and SaladilloQuaternary volcanic centres (Marrett et al 1994Guzmaacuten amp Petrinovic 2008) These structures belongto an active fault system as demonstrated by historicalearthquakes in the area one ofwhich destroyed the townof La Poma in 1930

The basal levels of the study section correspond tothe Maiacutez Gordo Formation (post-rift stage of the Saltarift) which is overlain in angular unconformity by thethe QLC Formation sediments In the Saladillo areathe QLC Formation consists of a 1400 m-thickupwards-thickening and upwards-coarsening contin-ental succession of claystones siltstones sandstonesand conglomerates representing sedimentation influvial-alluvial plains and alluvial fan settings (delPapa et al 2013) (Fig 2) Based on unconformitiesand abrupt changes in the sedimentary facies patternsthree main depositional sequences have been recog-nized at the Saladillo site Los Colorados I LosColorados II and Los Colorados III (LC I LC II andLC III respectively Fig 2) (Hongn et al 2007 del



















RESULTS









SEM results












positions











411-4


1611-1


1611-1 5-7-08-1W










(white arrow)



D I SCUSS ION

XRD vs TEM results













































ACKNOWLEDGEMENTS




REFERENCES







































































RESULTS









SEM results












positions











411-4


1611-1


1611-1 5-7-08-1W










(white arrow)



D I SCUSS ION

XRD vs TEM results













































ACKNOWLEDGEMENTS




REFERENCES























































SEM results












positions











411-4


1611-1


1611-1 5-7-08-1W










(white arrow)



D I SCUSS ION

XRD vs TEM results













































ACKNOWLEDGEMENTS




REFERENCES























































positions











411-4


1611-1


1611-1 5-7-08-1W










(white arrow)



D I SCUSS ION

XRD vs TEM results













































ACKNOWLEDGEMENTS




REFERENCES































































411-4


1611-1


1611-1 5-7-08-1W










(white arrow)



D I SCUSS ION

XRD vs TEM results













































ACKNOWLEDGEMENTS




REFERENCES




























































(white arrow)



D I SCUSS ION

XRD vs TEM results













































ACKNOWLEDGEMENTS




REFERENCES























































D I SCUSS ION

XRD vs TEM results













































ACKNOWLEDGEMENTS




REFERENCES



























































































ACKNOWLEDGEMENTS




REFERENCES























































REFERENCES






















































sem and tem evidence of mixed-layer illite-smectite …grupo179/pdf/do campo 2016.pdfof la poma in...

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