Diagenesis to metamorphism transition in an episutural ...grupo179/pdf/Abad2010.pdf · Diagenesis...

Diagenesis to metamorphism transition in anepisutural basin: the late Paleozoic St. Mary’sBasin, Nova Scotia, Canada

Isabel Abad, J. Brendan Murphy, Fernando Nieto, and Gabriel Gutierrez-Alonso

Abstract: The Late Devonian – Early Carboniferous St. Mary’s Basin in the Canadian Appalachians consists of HortonGroup fluviatile and lacustrine clastic rocks. The basin occurs along the boundary between the Avalon and Meguma ter-ranes and developed during coeval dextral shear along that boundary. X-ray diffraction reveals that the rocks contain ubiq-uitous quartz, K-white mica, and albite; illite–smectite mixed layers and chlorite are very common and Na–K mica,kaolinite, chlorite–smectite mixed layers, K-feldspar, berthierine, and rutile occur in some samples. Crystal-chemical pa-rameters of white mica indicate the pressure and temperature of mineral growth and discriminate between diagenetic, an-chizone, and low-grade metamorphic processes. Kubler index values measured in the 5 A peak and the presence ofchlorite–mica stacks are indicative of high-anchizone–epizone grades, with a crystallinity (crystal size and number of de-fects) that increases towards the Chedabucto Fault, which defines the northern margin of the basin. Kubler index valuesmeasured in the 10 A peak indicate that a late fluid-rich event could have produced the observed illite–smectite mixedlayers. The overall clay-mineral content and the b-cell dimension of the K-white micas are typical of postdepositional evo-lution in extensional sedimentary basins with high heat flow (>35 8C/km). Taken together, our data record two superposedevents related to deformation along the basin margins and coeval regional fluid flow, in which retrograde reactions at tem-perature T < 200 8C were superimposed on a pre-existing prograde assemblage typical of high-anchizone – lowergreenschist-facies conditions (T > 300 8C). Regional syntheses indicate that this fluid flow may have occurred during epi-sodes of Late Carboniferous dextral shear along the Avalon–Meguma terrane boundary.

Resume : Le bassin St. Mary’s (Devonien tardif – Carbonifere precoce) dans les Appalaches canadiennes est compose deroches clastiques fluviatiles et lacustres du Groupe de Horton. Le bassin se situe le long de la bordure entre les terranesd’Avalon et de Meguma et il s’est developpe le long de cette bordure durant une periode de cisaillement dextre de memeage. Des diffractions aux rayons X revelent que les roches contiennent du quartz, du mica blanc (K) et de l’albite de ma-niere generalisee; des couches d’un melange illite–smectite ainsi que de la chlorite sont tres frequentes et on retrouve,dans quelques echantillons, du mica Na–K, de la kaolinite, des couches d’un melange chlorite–smectite, du feldspath K,de la berthierine et du rutile. Les parametres chimiques de cristaux du mica blanc indiquent la pression et la temperaturede la croissance minerale et distinguent entre les processus diagenetiques, d’anchizone et un metamorphisme de faible in-tensite. Les valeurs de l’indice de Kubler mesurees dans un pic de 5 A et la presence d’eperons chlorite–mica indiquentdes degres metamorphiques eleves, d’anchizone superieur a epizone, avec une cristallinite (taille du cristal et nombre dedefauts) qui augmente en direction de la faille Chedabucto, laquelle forme la limite nord du bassin. Les valeurs de l’indicede Kubler mesurees dans un pic de 10 A indiquent qu’un evenement tardif, riche en fluides, pourrait avoir produit les cou-ches melangees illite–smectite observees. Le contenu global en mineraux d’argile et le parametre b de la maille du micablanc K sont typiques d’une evolution post-depot dans des bassins sedimentaires d’extension a gradient thermique eleve(> 35 8C/km). Prises ensemble, nos donnees indiquent deux evenements superposes, relies a la deformation le long desbordures du bassin, et un ecoulement regional contemporain de fluides, dans lequel des reactions metamorphiques retrogra-des a T < 200 8C ont ete superposees a un assemblage metamorphique progressif existant typique de conditions anchizoneelevee – facies inferieur des schistes verts (T > 300 8C). Des syntheses regionales indiquent que cet ecoulement de fluidess’est possiblement produit durant des episodes de cisaillement dextre, au Carbonifere tardif, le long de la bordure du ter-rane Avalon–Meguma.

[Traduit par la Redaction]

Received 1 July 2009. Accepted 15 December 2009. Published on the NRC Research Press Web site at cjes.nrc.ca on 15 February 2010.

Paper handled by Associate Editor V. Owen.

I. Abad.1 Departamento de Geologıa, Universidad de Jaen, 23009 Jaen, Spain.J.B. Murphy. Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS B2G 2W5, Canada.F. Nieto. Departamento de Mineralogıa y Petrologıa e IACT, Universidad de Granada, CSIC. Av. Fuentenueva, 18002 Granada, Spain.G. Gutierrez-Alonso. Departamento de Geologıa, Facultad de Ciencias, Universidad de Salamanca, 37003 Salamanca, Spain.

1Corresponding author (e-mail: [email protected]).

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Can. J. Earth Sci. 47: 121–135 (2010) doi:10.1139/E09-079 Published by NRC Research Press

IntroductionDistinguishing between diagenesis and low-grade meta-

morphism in basins that formed in active tectonic regimesis crucial to the understanding of basin evolution and its re-source potential (Merriman and Frey 1999; Merriman 2005).Unraveling the thermal history of any sedimentary basinaids in the assessment of its potential for hydrocarbon orore resources and also constrains the orogenic evolution ofmountain belts. Regional syntheses indicate that the LateDevonian – Early Carboniferous St. Mary’s Basin, NovaScotia, Canada, developed during dextral-strike faultingalong the boundary between two major Appalachian terranesand is, therefore, classified as an episutural basin (Murphy2000).

The typical widespread paragenesis of the rocks filling theSt. Mary’s Basin (white mica + chlorite + quartz + feld-spars + carbonates) covers a thermal range from deep dia-genesis to the onset of biotite formation (greenschist facies)and so cannot by itself discriminate between the differentconditions in which the rocks matured and evolved. Becauseof the sensitivity of the crystal-chemical parameters of phyl-losilicates to temperature, pressure, and deformation (finitestrain) changes, illite (or white mica) ‘‘crystallinity’’ analysisis a powerful and widespread tool for the study of the evolu-tion of diagenetic to low-grade metamorphic basins (Frey1987; Merriman and Peacor 1999). The crystal-chemical pa-rameters of white mica have been widely used to deducepressure and temperature conditions of mineral growth andto discriminate between diagenetic, anchizone, and low-grade metamorphic processes (e.g., Abad et al. 2003; Stoneand Merriman 2004) and, therefore, are useful in decipher-ing basin evolution from diagenesis through to regional de-formation (Merriman 2005). In addition, this approach mayassess the potential relationship of the diagenetic–metamor-phic gradient with either the burial depth or the strain in therocks. In some studies, a clear relationship between illite‘‘crystallinity’’ and strain has been determined where suit-able strain markers in the rocks have been examined (e.g.,Frey et al. 1980; Johnson and Oliver 1990; Gutierrez-Alonsoand Nieto 1996; Abad et al. 2003). In other studies, how-ever, this relationship could not be established, and the illite‘‘crystallinity’’ variations are interpreted to be related to bur-ial (Nieto et al. 1996).

This paper provides a complete characterization of thevery low-grade metamorphism in the Upper Devonian –Lower Carboniferous rocks of the St. Mary’s Basin of NovaScotia. We have been able to decipher two superposed proc-esses related to the deformation of the basin. Both processesare most probably fluid-driven, and our data provide con-straints for the hydrocarbon potential in the basin.

Regional settingThe evolution of the Appalachian Orogen in Maritime

Canada is characterized by the early Paleozoic telescopingof the Laurentian margin, the accretion of suspect terranesat various times in the early-to-mid Paleozoic, followed bylate Paleozoic terminal oblique collision between Laurentiaand Gondwana and the final amalgamation of Pangea (e.g.,van Staal et al. 1998, 2009; Murphy et al. 2002, 2006).Mainland Nova Scotia exposes two terranes, the Avalon and

Meguma terranes, which accreted to Laurentia before theEarly Devonian and together define the northern margin ofthe Rheic Ocean (Williams and Hatcher 1983; Keppie 1985;Stockmal et al. 1987; van Staal 1994; Murphy et al. 1995,1996; Keppie et al. 1996; van Staal et al. 1998). The St.Mary’s Basin (Fig. 1) developed along the Minas FaultZone, which defines the boundary between these two ter-ranes, and consists almost entirely of the Upper Devonian –Lower Carboniferous Horton Group continental clastic rocks(Murphy 2000). Horton Group rocks extend over much ofMaritime Canada and overstep the boundary between theAvalon and Meguma terranes. Basin configuration isthought to have been influenced by episodic dextral motionalong the Minas Fault Zone during the latest Devonian andCarboniferous (Keppie 1982; Gibbons et al. 1996; Murphyet al. 1995; Reynolds et al. 2004; Murphy and Collins2008), which has been attributed to the oblique convergencebetween Laurentia and Gondwana during that time (e.g.,Keppie 1982; Keppie et al. 1996; Hatcher 2002).

The basin fill is described in detail in Murphy and Rice(1998) and consists entirely of Upper Devonian – LowerCarboniferous 3000–4000 m continental clastic rocks of theHorton Group that were deposited in fluvial and lacustrineenvironments after the peak of the Acadian orogeny.Although not preserved in the studied basin, these HortonGroup rocks are overlain by the Visean Windsor Group,which consists of a predominantly marine sequence of lime-stone, evaporite, and clastic rocks that are in turn overlainby thick accumulations (ca. 10 km) of Upper Carboniferous –Lower Permian clastic rocks (Durling and Marillier 1993).Where preserved, the contact between the Horton and Wind-sor groups varies in character from a sharp angular uncon-formity in some regions to a concordant contact in others(e.g., Boehner and Giles 1993; Boehner 1994). Fossil evi-dence also indicates the presence of a gap in the depositio-nal record across the contact (e.g., Giles and Boehner 1982).

The St. Mary’s Basin is bounded to the north and southby major east–west-trending faults, which belong to theMinas Fault Zone, the Chedabucto Fault to the north andthe West River St. Mary’s Fault to the south (Fig. 1). Thebasin-fill rocks are divided into six partially laterally equiv-alent formations (Fig. 1) that were deposited in a fluviatileto lacustrine environment (Murphy and Rice 1998). Eachformation contains fossils of Upper Devonian (Famennian)to Tournaisian age (Benson 1967, 1974; G. Dolby, writtencommunication, 1994), which indicate deposition betweenca. 365 and 350 Ma (e.g., Tucker et al. 1998). Contact rela-tionships between these formations are conformable and gra-dational with interlayering of the characteristic lithologies.Clast composition, lithogeochemistry, and detrital zirconpopulations all indicate that the clasts were predominantlyderived from the Meguma terrane (Murphy 2000; Murphyand Hamilton 2000; Jennex et al. 2000).

The Little Stewiacke River Formation comprises thestratigraphically lowest rocks and is predominantly exposedin the central part of the basin in a series of east-northeast –west-southwest en echelon anticlines. The formation pre-dominantly consists of thinly bedded, fine-grained, darkgrey and black clastic rocks that range from mudstone toslate depending on the development of cleavage. The over-lying Barrens Hills Formation is characterized by grey sand-

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Fig. 1. Geological map of central Nova Scotia emphasizing the Minas Fault Zone and the St. Mary’s Basin. The cross sections sampled along St. Mary’s Basin and used in the inter-pretation of the results are depicted as A–A’, B–B’, and C–C’. The sampling locations are represented by stars.

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stone and conglomerate and interbedded dark grey shale and(or) siltstone. The Lochiel Formation is considered to belargely a lateral facies equivalent of the Barrens Hills For-mation and dominated by *500 m of grey-green feld-spathic, micaceous sandstone, and grey micaceous siltstone.The northwestern portion of the St. Mary’s Basin is domi-nated by the Graham Hill Formation, which consists of redconglomerate, sandstone, and siltstone. Along the southernflank of the basin, the Cross Brook Formation consists ofgrey-green sandstone interstratified with minor siltstone,shale, conglomerate, and limestone. The West River St.Mary’s Formation consists of red and green orthoconglomer-ate and siltstone. Clast lithologies (schist, phyllite, meta-sandstone, and micaceous granite) are typical of rocksexposed in the Meguma terrane (Murphy 2000).

Horton Group rocks probably accumulated in a longitudi-nal drainage system (Murphy and Rice 1998), in which theLittle Stewiacke Formation was deposited in a lacustrine en-vironment overlain by the Barrens Hills and Lochiel forma-tions that were laid down in braided fluvial environments(Fig. 2). To the north, the Graham Hill Formation is thoughtto have been deposited in a highly sinuous fluvial system(Murphy and Rice 1998). Along the southern flank of thebasin, the West River St. Mary’s and Cross Brook litholo-gies are typical of proximal and distal alluvial-fan deposits(Fig. 2).

The origin and evolution of the St. Mary’s Basin is attrib-uted to either discrete or progressive episodes of dextralstrike-slip tectonics along the Minas Fault Zone in the latePaleozoic. Based on structural and sedimentological analy-sis, this margin of the St. Mary’s Basin is interpreted tohave undergone dextral transcurrent motion and uplift be-tween 370 and 365 Ma (Keppie 1993; Murphy 2000). Thisinterpretation is indicated by the presence of 380–370 Madeformed granites along the southern basin margin that dis-play dextral C–S fabrics and the presence of Meguma ter-rane clasts in the unconformably overlying Horton Grouprocks (Murphy 2000). 40Ar/39Ar data from muscovites indextral shear zones near the contact with the St. Mary’s Ba-sin indicate that cooling through *400 to 350 8C continueduntil *345 Ma (Keppie and Dallmeyer 1995; Reynolds etal. 2004). Deformation associated with this event may be re-lated to the unconformable contact between the Horton andWindsor groups. This contact is most pronounced in a nar-row (2 km wide) zone dominated by less-competent, fine-grained clastic rocks of the Little Stewiacke Formation andproduced east-northeast-trending periclinal folds and associ-ated reverse faults. The orientation of these structures rela-tive to the east–west-trending Minas Fault Zone isconsistent with their origin by coeval dextral shear alongthe fault zone. To the north, the rotation of these structuresinto parallelism with the Chedabucto Fault could be attrib-uted to a progressive continuation of the same event orcould reflect a later phase of dextral shear.

To the west of the St. Mary’s Basin, a megabreccia withinthe Minas Fault Zone contains slabs of the Namurian MabouGroup, which implies that a further episode of faulting oc-curred between *330–320 Ma (Gibbons et al. 1996). Inwestern mainland Nova Scotia and southern New Bruns-wick, 320–300 Ma dextral motion along the Minas FaultZone has been documented by 40Ar–39Ar dating of fabrics

(Culshaw and Liesa 1997; Culshaw and Reynolds 1997). Arecent 40Ar–39Ar study of Siluro-Devonian siliciclastic rocksin both the Meguma and Avalon terranes detected the ca.320 Ma growth of fine-grained white mica related to fluidflow during regional dextral shear along the Minas FaultZone (Murphy and Collins 2008).

Samples and methodsTwenty three shale samples, mainly black and grey, be-

longing to the Horton Group were collected from outcropsalong three cross sections (Fig. 1), taking care that theywere free of surface-alteration effects.

These samples were washed and, after coarse crushing,homogeneous rock chips were used for preparation of X-raydiffraction (XRD) samples to determine the overall trends inmineral assemblages and record any variations in metamor-phic grade along the studied cross sections using the Kublerindex (KI; Kubler 1968). Currently, KI is the most commonmethod used for determining the grade and identifying an-chizone conditions between diagenesis and low-grade meta-morphism in metapelitic sequences. This index measures thefull width at half maximum intensity of the first (10 A) X-ray powder-diffraction peak of K-white mica and is ex-pressed as D8q in the Bragg angle.

Whole-rock samples and clay fractions (<2 mm) werestudied using a Philips PW 1710 powder diffractometerwith CuKa radiation, graphite monochromator, and auto-matic divergence slit at the Departamento de Mineralogıa yPetrologıa of the Universidad de Granada, Spain. The <2 mmfractions were separated by repeated extraction of superna-tant liquid subsequent to settling. Oriented aggregates wereprepared by sedimentation on glass slides. Ethylene glycol(EG) and dimethyl sulfoxide (DM), on some samples, treat-ments were carried out to corroborate the identification ofillite–smectite (I/S), chlorite–smectite (C/S), and kaoliniteon the basis of the expandibility of these phases. Preparationof samples and experimental conditions for illite ‘‘crystal-linity’’ (KI) measurements were carried out according toIGCP 294 IC Working Group recommendations (Kisch1991). Our KI measurements (x) were transformed into crys-tallinity index standard (CIS) values (y) according to theequation y = 1.918x – 0.0723 (r = 0.999, where r is the cor-relation coefficient), obtained in the laboratory using the in-ternational standards of Warr and Rice (1994). KI valueswere measured for the <2 mm fractions, <2 mm EG-treatedfractions, and the bulk-rock samples. In addition to the tradi-tional 10 A peak, 5 A reflection was also measured to checkthe effect of other mineral phases on the former measure-ment (Nieto and Sanchez-Navas 1994; Battaglia et al.2004). The b-cell parameters of micas and chlorites wereobtained from the (060) peaks measured on slices of rockcut normal to the sample foliation. For all spacing measure-ments, quartz from the sample itself was used as internalstandard.

Following the XRD and optical study, three samples ofthe C–C’ section (SM2, SM7, and SM12) and one of the B–B’ section (SM20) (Fig. 1) were selected for electron micro-scopy study on the basis of the crystal-chemical parametersand the mineral assemblages. Carbon-coated slices were ex-amined by scanning electron microscopy (SEM), using



Fig. 2. Block diagram showing a paleogeographic reconstruction of the St. Mary’s Basin infill. The formations in the Horton Group represent the lateral sedimentary facies variationswithin the same continental depositional system. Approximate sample locations, plotted downplunge into a representative cross section, with their respective Kubler index (KI) valuescorresponding to the 10.and 5 A peaks are depicted as stars.

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backscattered electron (BSE) imaging and energy-dispersiveX-ray (EDX) analysis to obtain textural and chemical data.These observations were carried out using a Zeiss DSM 950SEM, equipped with an X-ray Link Analytical QX-20 energy-dispersive X-ray system (EDX) at the Centro de Instrumenta-tion Cientıfica (CIC) of the Universidad de Granada. An accel-erating voltage of 20 kV with a beam current of 1–2 nA andcounting time of 50 s were used to analyze the silicates bySEM, using both natural and synthetic standards: albite (Na),periclase (Mg), wollastonite (Si and Ca), orthoclase (K), andsynthetic Al2O3 (Al), Fe2O3 (Fe), and MnTiO3 (Ti and Mn).

The chemical composition of micas are calculated on thebasis of 22 negative charges O10(OH)2. According to Gui-dotti et al. (1994), it is assumed that 75% of the Fe in themicas is Fe3+. The chlorite compositions are calculated onthe basis of 28 negative charges.

Results

Due to the rapid lateral facies documented within theHorton Group (Murphy and Rice 1998; Murphy 2003), allthe studied samples cannot be presented in a single strati-graphic column. Therefore, estimates of the burial depth ofthe samples are derived from their positions in the cross sec-tions, rather than by their stratigraphic positions. For thesereasons, we consider all our results as a single dataset, andthe relative position of each sample is obtained by a down-plunge projection onto cross sections and restoration into asimplified paleogeographic reconstruction (Fig. 2).

Mineralogy and crystal-chemical parametersThe XRD results are given in Table 1. The XRD patterns

of bulk samples show that quartz, K-white mica, and albiteare the main phases in all samples; chlorite and I/S mixedlayers are very common and Na–K mica, kaolinite, C/Smixed layers, K-feldspar, berthierine, and rutile are presentin some samples. The basal spacing of micas (d001) variesin a close range (9.965–9.994 A). An overall average valueof 8.992 A with a standard deviation of 0.005 A was ob-tained for the b-cell parameter XRD data. This low value,together with the d001, suggests a composition close to mus-covite for the micas, with a very low phengitic component.

Figures 2 and 3 show the KI values measured in the 10and 5 A XRD peaks of the <2 mm fraction corresponding toeach sample. Values of 10 A KI indicate deep diagenesis-anchizone conditions (Merriman and Peacor 1999). Figure 4shows the 10 A peaks of three samples, progressively nearerto the Chedabucto Fault. These data show that samples lo-cated closer to the fault have narrower peaks and lower KI.

Nevertheless, the KI values measured in the 5 A peak arerelatively homogeneous and correspond to high-anchizone–epizone grades (£0.31 D82q). In samples for which thewidths of the 10 and 5 A peaks are very similar, K-whitemica is the only 10 A mineral phase present. In sampleswhere the 10 A peaks are wider than the 5 A peaks, othermineral phases are expected to interfere with the 10 A peak(Nieto and Sanchez-Navas 1994; Battaglia et al. 2004). TheKI values measured on <2 mm EG-treated fractions areclearly lower than those of the <2 mm air-dried fractions

Table 1. Crystal-chemical parameters and bulk mineralogy of very low grade metamorphic rocks of St. Mary’s Basin.

Crystallinity index standard (Kublerindex) Mineral composition

10 A 5 A 10 A d001 (A) bulk

<2 mm<2 mmEg <2 mm Bulk

b Ms(A) Ms Chl

b Chl(A)

FeChl

Qtz, K-rich mica, feldspars(all the samples)

SM-1 0.46 0.48 0.27 0.22 8.999 9.974 14.120 Chl, I/SSM-2 0.58 0.35 0.30 0.47 8.993 9.980 Kln, I/S, RtSM-3 0.54 0.46 0.30 0.34 8.994 9.976 14.109 Na–K mica, I/S, Kln, ChlSM-4 0.36 0.31 0.29 8.991 9.984 14.158 ChlSM-5 0.50 0.44 0.31 0.28 8.994 9.980 14.164 Na–K mica, Chl, I/S, C/SSM-6 0.37 0.31 0.40 8.995 9.984 14.136 9.302 2.7 ChlSM-7 0.42 0.36 0.21 0.21 9.984 14.142 Na–K mica, Chl, I/S, C/S, RtSM-8 0.39 0.32 0.29 0.33 8.987 14.131 9.302 2.7 Na–K mica, Chl, I/S, C/SSM-9 0.33 0.34 0.21 0.37 8.993 9.994 14.114 9.298 2.6 Chl, I/S, RtSM-10 0.25 0.20 0.24 8.995 9.984 14.136 Chl, RtSM-11 0.30 0.31 0.19 9.986 Chl, Kln, BrtSM-12 0.31 0.41 0.25 0.27 8.994 9.976 14.120 9.308 2.9 Na–K mica, Brt, Chl, I/S, C/S,

RtSM-13 0.29 0.30 0.23 0.26 9.976 14.120 Chl, I/S, C/S, RtSM-14 0.46 0.35 0.26 0.36 8.991 9.982 14.136 Na–K mica, Chl, I/SSM-15 0.24 0.26 0.18 0.15 9.976 14.136 Chl, KlnSM-16 0.27 0.18 0.28 9.965 14.315 Chl, I/S, C/S, KlnSM-17 0.61 0.39 0.30 0.38 8.991 9.986 14.170 Chl, I/S, KlnSM-18 0.54 0.51 0.21 0.34 8.993 9.980 14.158 Chl, I/S, KlnSM-19 0.45 0.47 0.20 0.35 14.131 Chl, Kln, Brt, I/SSM-20 0.35 0.29 0.25 0.22 8.987 9.988 14.060 9.328 3.6 Chl, I/S, Kln, RtSM-21A 0.28 0.24 0.13 0.20 8.980 9.982 14.109 9.329 3.7 Chl, I/S, Kln, C/S, RtSM-21B 0.26 0.22 0.18 8.984 9.982 14.109 Chl, RtSM-22 0.21 0.18 0.21 8.995 9.992 14.136 9.308 2.9 Chl

Note: Mineral abbreviations according to Kretz (1983): Brt, berthierine; Chl, chlorite; Eg, ethylene glycol; Kln, kaolinite; Ms, muscovite; Qtz,quartz; Rt, rutile; C/S: chlorite–smectite mixed layers; I/S, illite–smectite mixed layers.



(Fig. 5a), which demonstrates the presence of R3 I/S mixedlayers in some samples (Nieto and Sanchez-Navas 1994).The presence of other 10 A phases as the I/S mixed layersproduces a tail in the 10 A peak towards low angles, causingasymmetrical broadening that disappears after EG treatment.The convolution of the mica and I/S produces a 10 A peakwider than that corresponding exclusively to mica and is inpart responsible for the difference in width between thispeak and the 5 A one, which mainly corresponds to mica(Fig. 5b).

A comparison of the KI values of the <2 mm fraction withthose corresponding to the bulk-rock samples shows, in gen-eral, no systematic difference according to grain size, sug-gesting an insignificant contribution of detrital mica in theuntreated samples.

In some samples, the peak at 14 A corresponding tochlorite is wide, small, and ill-defined and even disappearsafter EG and DM treatments, reflecting low quantities andpoor ‘‘crystallinity.’’ Nevertheless, in other samples, thispeak is intense and asymmetric and its change after EGtreatment confirms the presence of some smectite layers in-terstratified within chlorite. The b-cell parameter of chloritesuggests high-Fe contents (2.6–3.7 atoms per formula unit(apfu), see Nieto 1997), which is also confirmed by high-intensity ratios between even and odd basal reflections(Shata and Hesse 1998).

SEM observations

Textural aspectsBackscattered electron (BSE) images of selected samples

show that these are quartz-rich rocks of fine-grained texturewith microdomains of phyllosilicates (predominantly whitemica and Fe-rich chlorite) that may form stacks with nomore than 20 mm in thickness and show curved shapes.Some open spaces are filled by kaolinite (Fig. 6a). Iron andtitanium oxides are also common accessory minerals. TheSEM characterization confirms the general mineralogicalcomposition determined by XRD.

Most of the studied samples display an incipient slatycleavage, defined by subparallel, preferentially orientedpackets of phyllosilicates and elongated quartz grains(Figs. 6b, 6c). These features are typical texture of tectonicand (or) metamorphic origin. Other samples, however, pre-serve no evidence of fabrics that could be attributable to atectonic and (or) metamorphic origin (Fig. 6d).

Chemical composition of silicatesThe chemical compositions of dioctahedral micas are pre-

sented in Table 2. These are quite variable within each sam-ple. The sum of interlayer cations (Sinter.) varies over therange of 0.8–1 apfu and some of the white micas analyzedwere found to have significant Na content (up to 0.40 apfu).

Fig. 3. Simplified geological map of St. Mary’s Basin showing the position of the main faults and the Kubler index (KI) values of thestudied samples.

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In addition to the presence of Na–K intermediate micas, Nais generally present in K-rich dioctahedral micas, whereasCa is invariably £0.03 apfu. Although Ti is not usual inwhite micas, it has been measured in almost all the analyses

(£0.07 apfu); its presence is probably linked to minor rutile in-tergrowths within the white micas. The phengitic content isvariable and presents an average value of Si equal to3.14 apfu. In the intermediate Na–K micas, the phengitic com-

Fig. 4. The 10 A X-ray diffraction peaks of three samples, progressively nearer to the Chedabucto Fault. The closer to the fault are thesamples, the narrower are the peak and the smaller are the Kubler index (KI).

Fig. 5. Scatter plots showing the relation between (a) the Kubler index values obtained in air-dried versus ethylene–glycol- (EG) treatedsamples, respectively, and (b) the values of the 10 versus 5 A peaks.



ponent is lower than in the K-rich dioctahedral micas. No correla-tion between the interlayer population and Si exists (r = –0.13),which indicates an almost complete absence of illitic substitution.

Trioctahedral chlorites have a high Fe/(Fe + Mg) ratio(0.51–0.82) corresponding to a chamosite variety (Table 3),which is coherent with the Fe content obtained from the X-ray data. Most of the analyses show a large range of excessAlVI (up to 0.52 apfu) over AlIV, which is characteristic oflow-grade metamorphic chlorite in clastic sedimentary rocks(Li et al. 1994). Mn was detected in all the chlorite analyses,and Ti only in some of them. The sum of octahedral cations(Soct.) is almost always <6 apfu and a slight contamination

by interlayer cations (K, Ca, and Na) has been detected inalmost all the chlorite analyses. These data are, in part, theresult of the unavoidable overlapping of the beam on mate-rial other than the mineral under investigation due to thevery fine grained nature of the samples and, in part, the re-sult of interstratification of smectite layers within the chlor-ite, previously described in the XRD section.

Kaolinite shows the typical Al-rich composition, withvery small quantities of Fe, Mg, and alkaline elements prob-ably due to the contamination with other phases such aswhite mica. Analyzed feldspars display a composition nearto the end-member albite.

Fig. 6. Backscattered electron images showing the fine-grained texture of quartz-rich rocks with microdomains of phyllosilicates (whitemica and Fe-rich chlorite). Ab, Albite; Chl, chlorite; Kln, kaolinite; Ms, muscovite; Qtz, quartz; Ti-ox, titanium oxide.

Abad et al. 129


Discussion

Horton Group rocks (Fig. 1) were deposited in a longitu-dinal drainage system during either discrete or progressive

episodes of dextral strike-slip tectonics along the MinasFault Zone in the Late Devonian – Early Carboniferous(Murphy 2003). Our study of the mineral assemblages, crys-tal-chemical parameters, and chemical composition of phyl-

Table 2. Structural formulae for K-rich dioctahedral micas normalized to O10(OH)2 on the basis of scanning electron microscopy(SEM) data.

Si AlIV AlVI Fe Mg Mn Ti Soct. K Ca Na Sinter. Al total Fe+MgSM2/1_1 3.03 0.97 1.95 0.03 0.04 0.00 0.01 2.03 0.79 0.01 0.13 0.92 2.92 0.06SM2/1_3 3.12 0.88 1.70 0.16 0.11 0.00 0.03 2.01 0.94 0.01 0.04 0.99 2.59 0.27SM2/1_6 3.12 0.88 1.57 0.26 0.15 0.00 0.03 2.02 0.96 0.00 0.05 1.01 2.45 0.42SM2/1_13 3.31 0.69 1.78 0.10 0.07 0.01 0.02 1.98 0.73 0.00 0.10 0.84 2.47 0.17SM2/2_9 3.31 0.69 1.49 0.26 0.23 0.01 0.01 2.00 0.97 0.02 0.00 0.99 2.18 0.49SM2/2_10 3.36 0.64 1.25 0.45 0.28 0.00 0.01 2.00 1.02 0.00 0.01 1.03 1.90 0.73SM2_4_3 3.15 0.85 1.75 0.11 0.12 0.01 0.01 2.00 0.95 0.01 0.01 0.98 2.60 0.23SM2_4_5 3.22 0.78 1.67 0.17 0.14 0.00 0.04 2.01 0.85 0.00 0.04 0.89 2.45 0.31SM2_4_7 3.13 0.87 1.63 0.25 0.09 0.00 0.03 2.01 0.92 0.02 0.04 0.97 2.51 0.35SM7_3_6 3.21 0.79 1.85 0.06 0.07 0.00 0.02 2.01 0.59 0.02 0.23 0.84 2.65 0.13SM7_4_1 3.16 0.84 1.68 0.20 0.12 0.01 0.03 2.04 0.82 0.01 0.03 0.86 2.52 0.32SM7_4_3 3.24 0.76 1.87 0.07 0.06 0.00 0.01 2.01 0.57 0.01 0.23 0.81 2.63 0.12SM7_4_5 3.40 0.60 1.65 0.18 0.14 0.01 0.01 1.99 0.73 0.00 0.08 0.82 2.24 0.32SM7_6_3 3.08 0.92 1.75 0.15 0.09 0.01 0.03 2.03 0.87 0.01 0.06 0.94 2.67 0.25SM7_6_4 3.07 0.93 1.80 0.12 0.06 0.01 0.06 2.05 0.68 0.00 0.14 0.82 2.73 0.19SM7_8_2 3.04 0.96 1.91 0.09 0.03 0.00 0.01 2.05 0.68 0.01 0.18 0.87 2.87 0.13SM7_9_4 3.01 0.99 1.89 0.12 0.04 0.00 0.02 2.06 0.44 0.01 0.40 0.85 2.88 0.16SM12_2_1 3.08 0.92 1.92 0.03 0.04 0.00 0.01 2.01 0.88 0.00 0.04 0.93 2.84 0.07SM12_2_3 3.13 0.87 1.80 0.14 0.09 0.01 0.01 2.05 0.78 0.00 0.07 0.86 2.67 0.23SM12_2_10 3.12 0.88 1.87 0.08 0.05 0.00 0.02 2.02 0.73 0.00 0.14 0.86 2.75 0.13SM12_3_5 3.09 0.91 1.79 0.08 0.10 0.00 0.03 2.00 0.83 0.02 0.15 1.00 2.70 0.18SM12_5_8 3.07 0.93 1.96 0.04 0.02 0.00 0.01 2.03 0.64 0.00 0.22 0.87 2.90 0.05SM12_7_3 3.09 0.91 1.61 0.23 0.13 0.01 0.04 2.02 0.95 0.01 0.04 1.00 2.52 0.35SM12_7_6 3.18 0.82 1.83 0.07 0.04 0.00 0.06 2.00 0.76 0.01 0.06 0.83 2.66 0.10SM12_8_4 3.15 0.85 1.86 0.09 0.08 0.00 0.01 2.04 0.44 0.03 0.36 0.82 2.71 0.17SM12_10_1 3.14 0.86 1.66 0.16 0.16 0.00 0.03 2.02 0.88 0.01 0.07 0.96 2.52 0.33SM20_2_2 3.01 0.99 1.87 0.07 0.06 0.00 0.02 2.02 0.83 0.00 0.15 0.98 2.86 0.13SM20_2_4 3.17 0.83 1.80 0.11 0.03 0.00 0.07 2.01 0.59 0.01 0.20 0.81 2.63 0.14SM20_5_1 3.02 0.98 1.85 0.09 0.06 0.00 0.02 2.02 0.88 0.00 0.11 0.99 2.83 0.15SM20_5_6 3.18 0.82 1.79 0.15 0.10 0.00 0.02 2.05 0.72 0.01 0.05 0.78 2.61 0.25SM20_5_7 3.13 0.87 1.80 0.11 0.10 0.00 0.03 2.03 0.81 0.00 0.06 0.87 2.68 0.20SM20_5_8 3.10 0.90 1.75 0.13 0.09 0.00 0.05 2.02 0.82 0.01 0.09 0.92 2.65 0.22SM20_6_1 3.04 0.96 1.99 0.02 0.01 0.00 0.01 2.03 0.51 0.01 0.35 0.87 2.95 0.04SM20_6_2 3.25 0.75 1.61 0.25 0.12 0.01 0.03 2.01 0.85 0.00 0.03 0.88 2.37 0.36

Note: Soct., sum of octahedral cations; Sinter., sum of interlayer cations.

Table 3. Scanning electron microscopy (SEM) data for chlorites normalized to O10(OH)8.

Si AlIV AlVI Fe Mg Mn Ti Soct. Fe/Fe+Mg Na K CaSM7_5_1 2.76 1.24 1.27 3.06 1.22 0.01 0.21 5.76 0.72 0.00 0.01 0.01SM7_6_1 2.72 1.28 1.48 2.70 1.60 0.04 0.02 5.83 0.63 0.05 0.06 0.00SM7_8_1 2.65 1.35 1.49 3.09 1.31 0.02 0.00 5.92 0.70 0.00 0.02 0.00SM12_3_1 2.78 1.22 1.25 3.35 1.31 0.04 0.00 5.95 0.72 0.05 0.00 0.01SM12_3_2 2.86 1.14 1.52 3.12 1.08 0.03 0.01 5.77 0.74 0.01 0.03 0.01SM12_7_1 2.76 1.24 1.40 3.35 1.10 0.03 0.00 5.88 0.75 0.05 0.01 0.01SM12_7_2 2.66 1.34 1.41 2.29 2.19 0.03 0.00 5.93 0.51 0.02 0.01 0.01SM12_7_4 2.82 1.18 1.37 3.28 1.15 0.03 0.02 5.85 0.74 0.05 0.02 0.01SM12_8_1 3.09 0.91 1.42 3.21 1.04 0.05 0.01 5.72 0.76 0.01 0.01 0.00SM12_10_5 2.59 1.41 1.46 3.25 1.21 0.04 0.00 5.97 0.73 0.00 0.00 0.00SM20_3_3 2.72 1.28 1.25 3.64 1.06 0.04 0.01 5.99 0.77 0.02 0.01 0.00SM20_3_6 2.66 1.34 1.58 3.46 0.78 0.05 0.00 5.86 0.82 0.02 0.02 0.00SM20_4_1 2.60 1.40 1.32 3.35 1.30 0.03 0.00 6.00 0.72 0.02 0.02 0.01

Note: Soct., sum of octahedral cations.



losilicates of the fine-grained rocks is of relevance to theunderstanding of the diagenetic–metamorphic evolution ofthese rocks after their deposition.

In general, the mineral assemblage is typical of very lowgrade metamorphism, but also some typical sedimentaryphases are present in minor quantities. The clay-mineralcharacteristics of these rocks (K- and Na–K micas, chlorite,and chlorite–mica stacks) together with the b-cell dimensionof the K-white micas (<9.01 A) are typical of postdeposi-tional evolution within extensional sedimentary basins (Mer-riman 2005) that have a high heat flow (>35 8C/km). Thiskind of evolution was called diastathermal metamorphismby Robinson and Bevins (1989) and extensional metamor-phism by Merriman and Frey (1999) to describe the devel-opment of diagenetic to epizonal rocks caused by earlyheating of the basin fill in an extensional setting. The b pa-rameter of K-white micas is homogeneous and consistentwith a low-pressure regional metamorphism (Guidotti andSassi 1986).

The widths of the two first XRD peaks of mica compo-nents in samples representative of the geological environ-ments in which KI is widely used (diagenesis and incipientmetamorphism) are very similar when I/S mixed layers areabsent because they are a consequence of the same physicalcauses: crystal size and the number of defects (Nieto andSanchez-Navas 1994). Nevertheless, the presence of mixedlayers adds a variable degree of broadening to the 10 A dif-fraction peak producing an asymmetrical and wider peak inthe air-dried sample than in the EG-treated one. In such acase, 10 A peak is significantly wider than the 5 A peak.This broadening is useful for discriminating the degree ofdiagenesis because the quantity of mixed layers and their il-lite/smectite ratio are related to diagenetic maturity, makingthe KI very sensitive to small differences in grade. Hence,both measurements complement each other as the peaks pro-vide different information: the 10 A peak is more sensitiveto diagenetic changes and the 5 A peak is more representa-tive of the crystallite size and number of defects of the micadomains.

In the St. Mary’s Basin, the KI measured on the 10 and5 A peaks differ significantly; the former is indicative ofdeep diagenetic – low anchizone conditions, and the latterof high-anchizone–epizone (Fig. 5).

The chemical composition of dioctahedral micas, althoughquite variable within each sample, shows an almost com-plete absence of an illitic component (Table 2), which ismore in accordance with the metamorphic grade indicatedby the 5 A peak. In addition, the observed texture of thesamples and the presence of chlorite–mica stacks are alsotypical of anchizone or low-epizone conditions. In conclu-sion, the micas of the St. Mary’s Basin show crystallinity(crystal size and number of defects), textural characteristics,and chemical composition typical of high-anchizone–epi-zone grade.

However, the presence, in the same samples, of C/S and I/S mixed layers, berthierine and kaolinite are inconsistentwith those features typical of higher grade assemblagesformed in typical prograde reactions during a regional meta-morphism process. Different authors have interpreted thepresence of kaolinite, berthierine, and mixed layers in clasticrocks, affected by metamorphism, as the result of ‘‘retro-

grade diagenesis’’ (Nieto et al. 2005 and references therein),that is, a fluid-mediated process occurring within the rangeof temperature normally attributed to diagenesis but subse-quent to regional metamorphism. In the St. Mary’s Basin,in addition to the coexistence of mineral phases correspond-ing to different pressure–temperature (P–T) conditions, the10 A KI values suggest that a late fluid-driven event couldhave produced I/S mixed layers.

The reaction of fluids with the mineral phases that previ-ously grew during the regional metamorphism (progradeevent) is interpreted to have been favoured by the fluid ac-tivity associated with a major fault zone, which is a com-mon source of these reactions. Jiang et al. (1990) describedin North Wales a process in which illite reacted locally withfault-related fluids to form interstratified illite–smectite; thiswas, in fact, one of the cases in which the definition of ret-rograde diagenesis by Nieto et al. (2005) was based. The St.Mary’s Basin is a narrow basin bounded to the north andsouth by major east–west-trending faults, which belong tothe Minas Fault Zone (Fig. 1). This overall scenario is con-sistent with the regional syntheses that provide abundantevidence for episodic dextral motion and related fluid flowalong the Minas Fault Zone during the Carboniferous.

Therefore, the 5 A peak KI values represent the progradeevolution, a burial metamorphic process with local influenceof the deformation associated to the faults, characterized byhigh-anchizone–epizone grades (£0.31 D82q), which is incoherence with the incipient cleavage of the slates; whereasthe 10 A KI values define a retrograde event under lowertemperature conditions. Kaolinite crystallization is, nor-mally, favoured by the circulation of low-temperature waterswithin the rocks and its presence is considered an additionalevidence for low-temperature alteration subsequent to pro-grade evolution. Berthierine has been interpreted in other se-quences as a replacement product of chlorite underretrograde metamorphic conditions (Merriman and Peacor1999; Mata et al. 2001).

The importance of fluid-driven processes in the evolutionof the micas in the St. Mary’s Basin is supported by 40Ar–39Ar (white mica, infrared laser (IR) single grain, total fu-sion) data from Upper Ordovician – Lower Devonian sedi-mentary strata in both the Avalon and Meguma terranes,which yield widespread ca. 322 Ma ages and are interpretedto reflect distributed fluid flow coeval with dextral shearalong the Avalon–Meguma terrane boundary (Murphy andCollins 2008).

The important and complex role of basin-bounding faultsin influencing the paragenesis of the St. Mary’s Basin rocksis indicated by KI values projected onto a cross section ofthe basin (Fig. 7). The lowest values of the KI are in sam-ples of the Little Stewiacke Formation, which is located inthe central part of the St. Mary’s Basin and represents thestratigraphically lowest rocks of the Horton Group. Thecross section clearly shows that KI values generally decreasetowards the Chedabucto Fault (Fig. 7), implying an increasein the diagenetic–metamorphic grade towards the fault.However, an exception to this overall trend occurs in thetwo samples closest to the St. Mary’s Fault (SM15 andSM16). These samples are intercalated with permeable con-glomerate and sandstone layers, that is, they are in a high-permeability area, in which the fluids related with the faults

Abad et al. 131


would have arrived to the samples with high facility. In con-clusion, the pattern of evolution of the 5 A KI values is co-herent with a very fast burial metamorphism in atranscurrent context with rapid subsidence. The geothermalgradient, anomalously elevated, was mediated by the gener-alized presence of fluids in relation to the synsedimentary

faults and locally favoured by higher permeability litholo-gies than shales.

Lastly, we interpret that the effect of the sedimentaryloading of the studied rocks by >10 000 m of Lower Carbon-iferous (Visean) marine sediments from the overlying Wind-sor Group, assuming that a similar thickness on Windsor

Fig. 7. Kubler index (KI) index values projected onto three cross sections of the basin. Note the clear increase of grade towards the Cheda-bucto Fault.



Group strata overlay the St. Mary’s Basin (Boehner andGiles 1993), is probably represented by the prograde meta-morphic process recorded in the 5 A diffraction peaks. Sub-sequent, strike-slip movement along the Minas Fault Zone,especially along the Chedabucto Fault, caused the exhuma-tion of the formerly buried rocks of the Windsor Group andthe fluid-driven retrogression recorded in the 10 A whitemica diffraction peaks.

Hydrocarbon potential in the St. Mary’s BasinAccording to Merriman and Peacor (1999), three types of

data are useful to deduce the sedimentary-basin maturity: theclay-mineral assemblage, quantification of mixed-layer min-erals, and the clay-mineral ‘‘crystallinity.’’

In this study, the potential for hydrocarbon exploration inthe basin depends directly on its prograde evolution.Although the data indicate the existence of retrograde reac-tions at T < 200 8C, the existence of a previous metamor-phic peak corresponding to high-anchizone–epizoneconditions and the onset of greenschist facies (T > 300 8C)indicates that the St. Mary’s is a supermature basin thatlacks significant hydrocarbon potential.

As a cautionary tale, we note that a lack of comparisonbetween 10 and 5 A KI values would have led to the incor-rect characterization of the low-grade metamorphic condi-tions of the basin-fill rocks. Although the mineralassemblage and the crystallinity of mica are indicative of atleast anchizonal conditions, the origin and timing of mixedlayers, responsible for the 10 A KI, and other minerals askaolinite and berthierine might have been incorrectly inter-preted within a prograde context. Therefore, it is essentialto compare the 10 A KI values with the <2 mm 5 A andthe <2 mm EG-treated fractions KI data to confirm the I/Sidentification when the samples contain coexisting matureillite–muscovite and chlorite with clays such as kaoliniteand mixed layers.

Conclusions

Two successive processes having affected the St. Mary’sBasin rocks have been identified based on the correlation ofgeological characteristics with mineral paragenesis andcrystal-chemical parameters of micas.

The first one is an extensional metamorphism, accordingto the Merriman and Frey (1999) designation, completed byMerriman (2005). Our data from the Horton Group rocks areconsistent with rapid burial metamorphism in a transcurrentsetting accompanied by rapid subsidence and an anoma-lously high geothermal gradient (>35 8C/km), mediated bythe generalized presence of fluids that migrated along synse-dimentary faults.

The second one is a retrograde diagenesis, according tothe Nieto et al. (2005) definition, which affected the HortonGroup rocks after the exhumation of the formerly buriedrocks of the Windsor Group; strike-slip movement along theMinas Fault Zone, especially along the Chedabucto Fault,caused the fluid-driven retrogression. To our knowledge,this is the first example of retrograde diagenesis for whichthe geologic timing and origin of the fluids has been accu-rately identified.

AcknowledgmentsFinancial support has been supplied by the Spanish Minis-

try of Science and Technology (Research Project GL2007-66744), Research Groups RNM-179 and RNM-325 of theJunta de Andalucıa and Natural Sciences, and EngineeringResearch Council Discovery and Research Capacity grantsA0623 to JBM. G.G.-A. funding comes from Spanish Edu-cation and Science Ministry Project Grant CGL2006-00902.G.G.-A. also acknowledges funding from the Mobility Pro-gram Grant PR2007-0475 and hospitality at St. Francis Xav-ier University, Antigonish, Nova Scotia. The authors arealso grateful to the referees D. Keighley and R. Hisccot andto the Associate Editor V. Owen for their helpful comments.

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