Palaeoenvironmental implications of ferruginous deposits related to a Middle–Upper Jurassic discontinuity (Prebetic Zone, Betic Cordillera, Southern Spain)

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Sedimentary Geology 2

Palaeoenvironmental implications of ferruginous deposits related to aMiddle–Upper Jurassic discontinuity (Prebetic Zone,

Betic Cordillera, Southern Spain)

Matías Reolid ⁎, Isabel Abad, Juan Manuel Martín-García

Departamento de Geología, Facultad de Ciencias Experimentales, Universidad de Jaén, Campus Las Lagunillas, 23071 Jaén, Spain

Received 4 December 2006; received in revised form 17 October 2007; accepted 23 October 2007

Abstract

The Middle–Upper Jurassic boundary in the westernmost Tethyan basins is marked by a discontinuity. A thin iron crust with ferruginous ooidsand pisoids and an overlying ferruginous oolitic limestone lithofacies occur in a genetic relationship to this discontinuity with a reduced thickness(b50 cm) and very local distribution in the Prebetic Zone (Betic Cordillera).

The ferruginous coated grains are subdivided into two types. Type A ooids are characterised by thin, regular lamination in concentric layersenclosing a nucleus; they are dominant in the top of the iron crust (100% of the ferruginous ooids) and in the ferruginous oolitic limestone (82%).Type B ooids typically have thick, irregular lamination in a few discontinuous concentric layers enclosing a variable nucleus including bioclastsand foraminifera; they are exclusive to the ferruginous oolitic limestone (18% of the ferruginous ooids). The bulk chemical composition variesbetween 80% Fe2O3 by weight in the iron crust and 67% by weight in the coated grains. In the ferruginous ooids, the contents in SiO2 (5.4%),Al2O3 (6.5%), P2O5 (3.6%), and CaO (4.7%) are higher than in the crust. Trace elements (V, Cr, Co, Ni, Zn, Y, Mo, and Pb) in both the crust andooids show enriched values compared with the bulk composition of the upper continental crust. The mineral composition of the iron crust andooids is primarily goethite, with small amounts of Al-hydroxide (bohemite) and apatite, whereas hematite is identified only in the iron crust.

The Type A ooids are interpreted as having an origin related to the iron crust. Since there is no evidence to support a marine genesis for the ironcrust, the possibility of a subaerial origin is presented here. The crust has characteristics (chemical and mineralogical composition) similar to thoseof ferruginous pisolitic plinthite (highly weathered redoximorphic soil), and goethite shows an Al-substitution range (5–10 mol%) that indicatespedogenic conditions. Soil processes under periodic hydrous conditions are suggested; groundwater soils with hydrous conditions are congruentwith the formation of the Type A ferruginous ooids and pisoids. In this situation, a coastal plain with periodically flooded soils would be thelikeliest scenario. Callovian shallow carbonate shelf was possibly emerged and weathered, followed by marine sedimentation during the MiddleOxfordian, associated with major flooding of the Prebetic shelf and the erosion of ferruginous pisolitic plinthite. The first marine deposit wasferruginous oolitic limestones. Fragments of iron crust and Type A ferruginous ooids were reworked and incorporated into the marine sediments.A second phase of ferruginous ooids (Type B) with clear marine features developed, benefiting from iron-rich microenvironments due to theredistribution from iron crust fragments and Type A ferruginous ooids.© 2007 Elsevier B.V. All rights reserved.

Keywords: Ferruginous coated grains; Goethite; Iron crust; Plinthite; Hydromorphic conditions; South-Iberian Palaeomargin

1. Introduction

Discontinuous sedimentation is very characteristic ofepicontinental shelf environments of the South-Iberian Palaeo-margin during the Middle–Upper Jurassic transition. Numerousregional works indicate the presence of a discontinuity related to

⁎ Corresponding author.E-mail address: [email protected] (M. Reolid).

0037-0738/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.sedgeo.2007.10.001

this boundary in the Prebetic Zone of the Betic Cordillera (fromBehmel, 1970 to Reolid, 2005), with an iron crust withferruginous ooids and pisoids recorded in the Sierra de Cazorlaarea. Yet studies focusing on these deposits are scarce. Thisrecord of ferruginous ooids and pisoids related to the iron crustand the overlying limestones may provide significant informa-tion for analysing the discontinuity and interpreting the firstdeposits after sedimentary interruption. Acosta (1989) identi-fied clayey, iron-rich deposits between the hardground and the

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2 M. Reolid et al. / Sedimentary Geology 203 (2008) 1–16

first Oxfordian deposits; and although he interprets the depositsas plinthite (highly weathered redoximorphic soil), he does notprovide data in support of that claim. If these ferruginousdeposits are indeed plinthite, this finding would have importantgenetic implications for the palaeoclimatology and the sedi-mentary dynamics of the basin.

The study of ferruginous coated grains is a very complex andcontroversial research topic. The nomenclature of ooliticironstone facies is based on the definition by Kimberley(1978) for chemical sedimentary rocks that contain over 15%iron. Although Van Houten (1985) and Chan (1992) relate thegenesis of oolitic ironstones to eustatic events and use them asstratigraphic markers, considerable controversy still surroundsthe origin of oolitic and pisolitic ironstones (see Young, 1989for a review). In contrast, the genesis of ferruginous ooids andpisoids has been extensively investigated for many years(Deverin, 1945; Kelly, 1951; Mellon, 1962a,b; Dubois, 1979;Kimberley, 1980; Maynard, 1986; among others). Ooliticironstones, identified throughout much of the stratigraphicrecord ranging from the Precambrian to the present, can form inmore than one way: (a) in shallow to deeper marineenvironments (Kogbe, 1978; Kimberley, 1979, 1994; Chauveland Massa, 1981; Gygi, 1981; Berendsen et al., 1992; Ramajoet al., 2002; Collin et al., 2005), (b) during early subaqueousdiagenesis or subaerial pedogenesis (Dubois and Icole, 1977;James and Van Houten, 1979; Nahon et al., 1980), (c) byerosion of lateritic soils and reworking of particles to form aplacer concentration of laterite ooids (Siehl and Thein, 1978,1989; Aurell et al., 1994), and (d) even in relation to shallow-marine volcanism (Soussi and M'rabet, 1991; Heinkoop et al.,1996; Sturesson et al., 2000; Sturesson, 2003). Moreover, thesemodels may not be mutually exclusive (Young, 1989).

The aim of this paper is to characterize the iron crust andcoated grains associated with the Middle–Upper Jurassicdiscontinuity in the South-Iberian Palaeomargin, in order tointerpret the palaeoenvironmental conditions of their genesis.This will contribute to a better knowledge of the evolution of thewesternmost Tethyan basins and the significance of widelydistributed oolitic ironstones. We report sedimentological,chemical, and mineralogical data regarding these iron deposits.Such characteristics are important because they provide proxydata that enable us to interpret hydrologic and palaeoclimateconditions.

These iron deposits are particularly relevant because of theexceptionally well preserved record of the iron crust bed. Insouthern Europe, most records of ferruginous ooids from theMiddle–Upper Jurassic boundary are related only with calcar-eous deposits: the Iberian Range (Goy et al., 1973; Aurell et al.,1994, 1999; Ramajo and Aurell, 1997; Ramajo et al., 2002;Meléndez et al., 2005), the western Subalpine Basin (Dromart,1989), the Côte D'Or (Courville and Collin, 1997), Schaignay(Scouflaire et al., 1997), the southeastern Paris Basin (Lorinet al., 2004; Collin et al., 2005), the Jura region of Switzerland(Ziegler 1962; Gygi, 1981; Huber et al., 1987) and Swabia(Gygi and Persoz, 1987). Therefore, this research will alsoimprove our knowledge of this interval in epicontinentalEuropean margins.

2. Geological setting

The outcrops studied are located in the Sierra de Cazorlaarea, in the External Prebetic (Fig. 1A and B). The PrebeticZone is the most external northern part of the Betic Cordillera,the westernmost European Alpine chain. The External Zones ofthe Betic Cordillera are subdivided into two well-differentiateddomains – Prebetic and Subbetic – as well as the IntermediateUnits between the two (Fig. 1A). In the Jurassic palaeogeo-graphy of the South-Iberian Palaeomargin, the Prebeticrepresents a comparatively proximal epicontinental shelf,whereas the Subbetic represents distal epioceanic swell andtrough areas. The Prebetic is subdivided, in turn, into Externaland Internal Prebetic (corresponding to the more proximal ordistal areas of the shelf, respectively).

Samples were collected at the contact between the ChorroFormation and the Lorente Formation, in the Chorro sheet(Foucault, 1971). The Chorro Formation consists of nearly400 m of Lower Jurassic dolomitized limestones and 50 m ofMiddle Jurassic oolitic limestones. The Lorente Formationcomprises Upper Jurassic (Middle Oxfordian to LowerKimmeridgian) deposits and constitutes the first interval ofpelagic–hemipelagic sedimentation in the southeastern epicon-tinental system, which took place in Iberia during the Mesozoic.In the most proximal sectors of the shelf that are preserved(External Prebetic), the Lorente Formation is mainly repre-sented by a 70–100 m thick succession of spongiolithiclimestones and marl-limestone rhythmites (Rodríguez-Tovar,1993; Reolid, 2005).

The iron crust in question is related to the discontinuity(Fig. 1C) associated with the boundary between the Lower–Middle Jurassic megasequence and the Oxfordian–MiddleKimmeridgian megasequence, identified in the Prebetic byseveral authors (López-Garrido and García-Hernández, 1988;García-Hernández and López-Garrido, 1988; García-Hernándezet al., 1989). This discontinuity may in fact comprise severalminor discontinuities (forming a “complex unconformity”) atthe base of the OX.I depositional sequence of Marques et al.(1991). The crust appears terrigenous or clayey at the bottom,ranging in colour from tan to brown. The upper part of the ironcrust mostly consists of ferruginous ooids and pisoids (Fig. 2).

In the External Prebetic, the discontinuity is recorded onMiddle Jurassic oolitic limestones as a planar surface withabundant iron oxides and, locally, a slightly irregular erosivesurface. This observation is congruent with the palaeokarst thatLinares-Girela (1976) identified in the Eastern ExternalPrebetic. In the Sierra de Cazorla area (Central ExternalPrebetic), this discontinuity is fossilized by the ferruginousoolitic limestone lithofacies, which constitutes the first UpperJurassic deposits (Olóriz et al., 2004). The surface geometry isgenerally a paraconformity, and sometimes a slightly angularand erosive discordance where the iron crust occurs. Suchfeatures are also described in the Callovian-Oxfordian boundaryand associated deposits from the Iberian Range (Ramajo andAurell, 1997; Aurell et al., 1999; Meléndez et al., 2005). In theInternal Prebetic, this discontinuity is recognized in an irregularsurface that is somewhat obscured by dolomitization.

Fig. 1. (A) Geological map of the Prebetic Zone, the externalmost part of the Betic Cordillera (SE Spain). (B) Detailed geological map of Sierra de Cazorla, indicatinglithologies and the studied outcrops. (C) Stratigraphic columns of the three outcrops (RGCH, Riogazas-Chorro; RGCHSP, Riogazas–Chorro–Esponjas; CHO,El Chorro), showing the position of the Fe-crust and the Middle–Upper Jurassic discontinuity.

3M. Reolid et al. / Sedimentary Geology 203 (2008) 1–16

Fig. 2. (A) View of the upper part of the iron crust in which the ooid content is the highest. (B) Ooids from the iron crust (see description in text). Scale bar: 1 mm.


2.1. Underlying deposits

Middle Jurassic oolitic limestones with megaripples andoncolitic–bioclastic facies occur directly beneath the disconti-nuity surface (upper part of the Chorro Formation). The textureof this limestone varies from packstone to grainstone, withabundant radial and concentric ooids, intraclasts, oncoids, andbioclasts. Three types of microfacies dominate: intraclasticgrainstone, oolitic grainstone/packstone, and oolitic andoncolitic grainstone. These deposits formed in a shallowinner-shelf environment with carbonate sedimentation domi-nated by oolitic bars. Due to the scarcity of biostratigraphicallysignificant fossil remains and the dolomitization that affects thisformation, no detailed biostratigraphy was carried out. How-ever, the records of Protopeneroplis striata (Jerez-Mir, 1973;Rodríguez-Estrella, 1978), Mesoendothyra croatica, and Tro-cholina palestiniensis (López-Garrido and García-Hernández,1988) confirm the Middle Jurassic age of these materials.

2.2. Overlying deposits

Ferruginous oolitic limestones lie directly over the disconti-nuity surface (Reolid, 2005), but pinch out laterally. Thicknessvaries from 10 to 40 cm, depending on the number of preservedbeds (generally 1 or 2). The lower boundary of these bedsis irregular and related to the iron crust. This iron crust is thin(2–10 cm) and quite localized. The ferruginous ooliticlimestone has a grain-supported bioclastic fabric (packstone)with abundant peloids (30%), ferruginous ooids and pisoids(26%), and bioclasts (26%) (Olóriz et al., 2004, 2005; Reolid,2005). Mineral grains such as quartz are less common. Recently,the Plicatilis Zone (Middle Oxfordian) was recognized in thislimestone (Olóriz et al., 2005).

3. Methods

The samples, obtained from well below the outcrop surfaceand far from joints, were washed and coarse-crushed. Theironstone ooids and pisoids were hand-picked from mechani-

cally disaggregated samples. Ooids and pisoids were brokenat random in an agate mortar by gentle, short blows with apestle.

The mineral composition of both the crust and the fer-ruginous coated grains (ooids and pisoids) was determined byX-ray diffractometry (XRD) using Co-Kα radiation at 40 kVand 30 mA, with a scan speed of 0.06 °2θ min−1, in a SiemensD-5000 diffractometer (University of Jaén). Unoriented pow-ders were prepared using a holder filled from the side with haliteas internal standard. The Al3+ content in the goethite structurewas calculated from these data according to the methodproposed by Schulze (1984) as an indicator of the origin ofgoethite (Fitzpatrick and Schwertmann, 1982). In addition,oriented aggregates were prepared by sedimentation and dryingon glass slides to enhance the tendency of phyllosilicates todisplay preferred orientation of 00l reflections.

Petrographic microscopy was used to determine the micro-facies (textural classification of the rock and semi-quantitativeanalysis of grain proportions) and the morphology of coatedgrains.

Carbon-coated polished thin-sections were examined byscanning electron microscopy (SEM) using back-scatteredelectron (BSE) imaging and energy-dispersive X-ray (EDX)analysis in order to obtain textural and chemical data. Theseobservations were carried out using a Zeiss DSM 950 SEMequipped with an X-Ray Link Analytical QX-20 energy-dispersive X-ray system (EDX), at the University of Granada'sScientific Instruments Centre (CIC).

In addition, Fe-coated grains were split by a gentle blow witha sharp steel blade. The broken pieces, approximately equatorialhalves, were mounted, coated with C, and examined directlyunder the SEM using secondary electrons to study their internalultrastructure and crystal morphology.

Whole-rock analyses of the major elements of the crust andthe ooids were carried out using X-ray fluorescence (XRF) in aPhilips PW 1040/10 spectrometer. Trace elements wereanalysed using an inductively coupled plasma-mass spectro-meter (ICP–MS) Perkin Elmer Sciex-Elan 5000 at the CIC(University of Granada).


4. Structure and morphology of the ferruginous coatedgrains

The ferruginous ooids (ironstone ooids as per Bhattacharyyaand Kakimoto, 1982) are grains in which the nucleus isenveloped by an iron oxide coating. When these coated grains

Fig. 3. Coated grain images from the ferruginous oolitic limestone taken by petrograooid; (D–E) fragmented ooids and pisoids. Type B ooids: (F), (G) and (H). Scale b

are over 2 mm in diameter, they are termed pisoids, in ac-cordance with the usual classifications established for carbonatecoated grains (Leighton and Pendexter, 1962; Choquette, 1978;Donahue, 1978). Regardless of the term used, the morphologyand textural features of the grains studied here (over 600 ooidsand pisoids) are size-independent. In general, the coated grains

phic microscope. Type A ooids: (A) simple pisoid; (B) spastolith; (C) complexar: 1 mm.


from the iron crust and the ferruginous oolitic limestone arebrown or black, with a greyish to brownish-yellow streak and ametallic luster (Fig. 2). They range from 0.3 to 3.5 mm indiameter and are subspherical to elliptical in shape, the largergrains being more elliptical. The grain surface is smooth buttends to be more irregular in the smallest ooids.

Analysis of the ferruginous ooids and pisoids in thin sectionsshows a structure in concentric layers (Fig. 3). According toGuerrak's classification (1987), four types of ferruginous ooidsand pisoids are present: a) simple, b) spastolith (s. Adams et al.,1984; Guerrak, 1987; Cotter, 1992), c) complex or multiple, andd) fragmented. The most common type (N70%) is that of simpleferruginous coated grains with a mean size of 900 μm (Fig. 3A).Spastoliths (coated, distorted and stretched grains that aredeformed by squashing) are the next most frequent type (15%)(Fig. 3B). Complex ferruginous ooids and pisoids are rarer

Fig. 4. BSE images: (A–B) Details of the concentric and regular layers of the Typeooids. White arrows indicate the position of the top of the iron crust.

(b10%), with a mean size of 880 μm (Fig. 3C). The fragmentedooids and pisoids (≈1 mm mean size, 5%) were fragmentedonce or more than once during formation. Later, new layersmight have developed in discordance with older ones (Fig. 3Dand E). These spastoliths and fragmented ferruginous ooids andpisoids are very similar to those described by Kimberley (1983).

Detailed analysis of thin sections allowed us to distinguishtwo types of coated grains based on differences in morpholo-gical features and lamination:

Type A: Ooids with very thin, regular lamination in con-centric layers enclose a nucleus, which is typically a fragment ofan older ferruginous ooid or an indeterminate ferruginous lump(Figs. 3A–E, 4A and B). They include all four of Guerrak'sooid types (1987). The nucleus-to-coating ratio is low inthese grains, with a mean size close to 1 mm. Secondary elec-tron images reveal these coated grains to have a homogeneous

A ooids; Iron crust: (C–D) Textural aspect of the matrix; (E–F) Type A simple


appearance, in some cases with filamentous structures inside theporous nuclei or between the stacked laminae of the coating(Fig. 5). Type A ooids constitute 100% of ferruginous grains inthe iron crust and 82% in the ferruginous oolitic limestone.Fragments of this type of ooids and of the iron crust are alsopresent in the ferruginous oolitic limestone. Most of the ooids inthe iron crust are located in the upper millimeters, where thematrix is characterised by disrupted layers (Fig. 4C–F).

Type B: Ooidswith irregular lamination and few layers (5–12).These laminae are thicker than in the Type A ooids and pisoids,and thickness is variable (15 to 50 μm) as the laminae pinch out.Carbonate laminae occur between ferruginous layers. The laminaedo not form continuous, concentric layers (Fig. 3F–H), as smallgrains are sometimes agglutinated among the laminae (Fig. 6). In

Fig. 5. Secondary electron images: (A) Simple pisoid with a homogeneous coat aroFilament structures can be observed in images C and E.

some cases, the externalmost layers of the ferruginous ooidscontain encrusting foraminifera (nubeculariids), whose chambersare occupied by oxides (Fig. 3F). Ooids may also comprise onlyone or two layers, where the total diameter mainly corresponds tonucleus thickness. The nucleus may be a lump, a bioclast or – lessoften – ferruginized foraminifera (Figs. 3H and 6). In addition, inabout 6%of the studied ooids, the nucleus is formed by a fragmentof iron crust or a ferruginous Type A ooid. In these grains, the ironis irregularly distributed in the layers and voids among thelaminae, which are sometimes replaced by carbonate. Due to thescarcity of layers, the TypeB ooids are smaller (600μmmean size)than the Type A ooids. The Type B ferruginous coated grains areabsent in the iron crust and constitute 18% of ferruginous ooidsand pisoids in the ferruginous oolitic limestone. According to

und a porous nucleus; (B–C) Details of the nucleus; (D–E) Details of the coat.

Fig. 6. BSE images: Type B simple ooids.

Table 2Trace element contents (ppm)

Crust Ooids

Rb 5.15 10.36Cs 0.41 0.81Sr 44.53 65.86Ba 27.29 51.69Sc 14.23 15.10V 1414.42 1908.24Cr 186.09 1058.94Co 143.36 121.09Ni 384.68 369.06Cu 54.94 35.25Zn 289.60 311.37Ga 7.44 13.44Y 66.28 107.45Nb 9.57 40.42Ta 0.31 1.02Zr 62.52 147.71Hf 1.39 4.31Mo 56.07 10.18Sn 0.64 3.01Tl 0.13 0.56Pb 150.74 120.89


Guerrak (1987), these coated grains mainly correspond to simple(≈95%) and, to a lesser extent, to complex ferruginous ooids(5%).

In the ferruginous oolitic limestone, some Type A ooidsshow a layer with features close to Type B ooids. Moreover,although recrystallization features and loss of iron are morecommon in Type B ooids, they are also present in Type A ooids.Where there has been iron loss, the iron is found impregnatingsurrounding grains. Locally, ferruginous ooids and pisoids aredolomitized, partially or totally losing lamination as a con-sequence of the dolomite crystal growth among the layers.

Table 1Major element contents (weight%)

Crust Ooids

SiO2 3.84 5.39Al2O3 2.52 6.47Fe2O3 80.48 67.28MnO 0.03 0.05MgO 0.33 0.52CaO 2.31 4.69K2O 0.10 0.24TiO2 0.14 0.53P2O5 1.75 3.58LOI 10.88 12.90

5. Chemical and mineralogical composition

Tables 1 and 2 show the bulk chemical composition of thecrust and the ferruginous coated grains within it. The mostabundant component is Fe2O3, which reaches its highest valuesin the lowest part of the iron crust (80.5% by weight). In theooids, the Fe2O3 content is lower (67.3%), whereas the contentsin SiO2 (5.4%), Al2O3 (6.5%), P2O5 (3.6%), and CaO (4.7%)are higher than in the crust. The mobile elements K andMg showvery low concentrations (b0.5%) and Na is below detection. Theooids contain higher concentrations of trace elements than thecrust (Table 2). In addition, both the crust and the Fe-ooids presenthigher values of V, Cr, Co, Ni, Zn, Y, Mo, and Pb as compared tothe bulk composition of the upper continental crust (Post-ArcheanAustralian Shales, PAAS, published by Taylor and McLennan,1985). Rare earth elements present higher concentrations in theooids than in the crust (Fig. 7) and in both cases light rare earthelements (LREE) are more abundant than heavy rare earthelements (HREE). The chondrite-normalized patterns accordingto the CI carbonaceous chondrite (McDonough and Sun, 1995) ofboth crust and ferruginous ooids are very similar (Fig. 7) and

U 13.17 8.02Th 6.97 27.87La 29.58 55.09Ce 13.79 31.30Pr 4.42 10.38Nd 19.03 43.83Sm 4.09 10.35Eu 0.95 2.30Gd 5.13 11.09Tb 0.87 1.81Dy 5.76 12.07Ho 1.41 2.67Er 4.01 7.14Tm 0.60 1.06Yb 3.47 6.02Lu 0.55 0.83

Fig. 7. Chondrite-normalized REE patterns for the studied materials.


show two negative anomalies: in Eu (Eu/Eu⁎crust=0.63; Eu/Eu⁎ooids=0.65) and Ce (Ce/Ce⁎crust=0.29; Ce/Ce⁎ooids=0.32);this last anomaly is not present in PAAS. A comparison of the ironcrust and PAAS chondrite-normalized patterns shows a relativeimpoverishment in LREE (from La to Eu) but a slight enrichmentin HREE (from Gd to Lu) in the crust. Nevertheless, except forEu, ooids are enriched in REE with respect to the average uppercontinental crust composition.

The XRD patterns of unoriented powders indicate that thesematerials basically comprise ferric oxyhydroxides: goethite andhematite in the iron crust, and almost entirely goethite in thecoated grains, as well as some Al-hydroxide (bohemite) and

Fig. 8. XRD patterns of the iron crust and coated grains. Mineral abbreviations accordiand Hl = halite (internal standard).

apatite (Fig. 8). XRD patterns of oriented aggregates clearlyshow the absence of clay minerals. Following the methodproposed by Schulze (1984), the Al3+ content in the goethitestructure was calculated based on the position of the peakscorresponding to the d110 and d111 of goethite, which weredetermined from the diffractograms. The results indicate anAl-substitution in goethites of 5 mol% in ooids and 10 mol%in the iron crust.

Although EDX resolution is insufficient for analysingindividual layers or minerals in ooids, the data obtained fromthe ooids and matrix analyses (Table 3) reveal a majoritypresence of ferric oxyhydroxides, with Fe2O3 contents ranging

ng to Kretz (1983): Ap = apatite, Bhm = bohemite, Gt = goethite, Hm = hematite,

Table 3EDX analyses of iron oxides (wt.%)

Fe2O3 MgO Al2O3 SiO2 P2O5 Na2O K2O CaO TiO2 VO2 CrO2

Pli-3/oo 1/1 78.98 0.60 4.81 2.53 0.85 0.00 0.10 0.46 0.56 1.21 0.21Pli-3/oo 1/2 75.67 0.77 5.28 1.93 2.46 0.00 0.04 3.22 0.29 1.17 0.18Pli-3/oo 1/3 78.14 0.68 5.41 2.66 1.11 0.00 0.10 0.57 0.45 0.85 0.35Pli-3/oo 1/4 78.82 0.58 5.04 2.70 0.92 0.22 0.01 0.67 0.27 0.97 0.14Pli-3/oo 2/1 80.54 0.70 5.00 2.36 0.62 0.26 0.11 0.27 0.42 1.32 0.16Pli-3/oo 2/2 78.08 1.18 6.10 2.23 0.85 0.28 0.06 0.21 0.33 0.99 0.30Pli-3/oo 2/4 74.71 0.70 6.24 3.92 0.78 0.00 0.11 0.50 0.13 0.58 0.00Pli-3/oo 3/1 74.20 0.62 6.51 3.99 0.71 0.15 0.07 0.59 0.16 0.60 0.05Pli-3/oo 3/2 76.60 0.85 5.94 3.13 0.80 0.11 0.06 0.43 0.22 0.70 0.16Pli-3/oo 3/3 76.58 0.85 5.63 3.04 0.78 0.12 0.02 0.41 0.22 0.92 0.07Pli-3/oo 3/4 73.94 0.62 6.69 4.69 0.71 0.04 0.17 0.42 0.15 0.67 0.07Pli-3/oo 3/5 73.01 0.65 5.83 2.79 3.22 0.00 0.06 3.71 0.11 0.85 0.14Pli-3/oo 3/6 74.94 0.82 6.20 3.17 2.41 0.22 0.08 2.69 0.09 0.65 0.18Pli-3/oo 3/7 74.90 0.67 6.38 4.41 0.76 0.00 0.08 0.48 0.00 0.76 0.09Pli-3/oo 3/8 77.69 0.73 6.45 3.00 1.02 0.18 0.07 0.52 0.20 0.70 0.18Pli-3/oo 4/1 80.26 0.87 3.16 2.34 0.80 0.18 0.07 0.38 0.15 0.56 0.14Pli-3/oo 4/2 81.23 0.97 3.57 2.21 0.88 0.09 0.00 0.43 0.15 0.58 0.09Pli-3/oo 4/3 81.34 0.63 3.24 2.06 0.78 0.00 0.00 0.66 0.20 0.54 0.14Pli-3/m/ 1 76.07 0.63 6.06 1.69 1.37 0.00 0.04 1.48 1.38 0.79 0.74Pli-3/m 81.02 0.42 2.16 3.54 1.02 0.05 0.00 0.55 0.00 0.25 0.00Pli-3/m 3 68.02 0.45 2.47 4.14 0.83 0.00 0.06 0.71 0.00 0.09 0.12Pli-3/m 4 86.33 0.25 2.08 4.69 0.52 0.15 0.12 0.63 0.00 0.36 0.11LIM 1/oo1/1 72.44 1.13 3.43 3.51 2.41 0.35 0.10 2.60 0.29 0.27 0.14LIM 1/oo1/3 66.03 1.48 4.34 3.36 1.44 0.40 0.13 0.98 0.35 0.47 0.18LIM 1/oo1/4 62.30 1.00 3.71 3.73 1.04 0.00 0.20 0.67 0.27 0.34 0.25LIM 1/oo1/5 65.89 0.97 4.51 4.31 1.16 0.19 0.30 0.67 0.38 0.32 0.19LIM1/oo1/6 67.67 1.20 4.00 3.17 1.99 0.22 0.06 2.00 0.47 0.38 0.37LIM1/oo2/4 75.11 1.05 2.92 3.43 1.63 0.36 0.07 1.26 0.04 0.18 0.05LIM1/oo2/5 71.93 0.87 2.55 3.32 4.14 0.12 0.02 4.24 0.13 0.42 0.07LIM1/oo3/3 67.74 0.90 2.82 3.45 3.93 0.18 0.23 4.20 0.05 0.20 0.04LIM1/oo3/5 69.30 0.90 3.83 3.02 1.85 0.20 0.07 1.86 0.25 0.27 0.19


from 62% to 86% by weight. The Al values (b10 mol%) fromthese analyses are coherent with the values obtained byXRD. Thepresence of P and Ca corroborates the notion that phosphates canbe easily adsorbed on the surface of iron oxyhydroxides (Gehring,1985). The alkaline elements K and Na, when present, appear invery small quantities, whereas Mg, V, Ti, and Cr are common,although also in small amounts.

6. Discussion

6.1. Depositional environment of the ferruginous deposits

Combining mineralogical, geochemical, and sedimentologi-cal data for the iron deposits related to the Middle–UpperJurassic discontinuity in the Prebetic Zone gives a comprehen-sive view of the environment they originated in.

The mineralogical data presented in this work agrees withdata described in materials with similar characteristics (May-nard, 1986; Guerrak, 1987; Cotter, 1992) except for theabsence, in our case, of phyllosilicates such as chamosite and/or kaolinite. This absence shows as a lack of phyllosilicatediffraction peaks in the X-ray diffractograms of the orientedaggregates. Coated grains from the crust and the overlyinglimestone basically comprise goethite, but hematite is alsopresent in the crust (Fig. 8). Only the presence of spastoliths inthe crust suggests the possibility of an early presence of clay

minerals. According to many authors, only clay-mineral ooidswould be sufficiently plastic to undergo compactional squash-ing to spastoliths (Guerrak, 1987; Cotter, 1992; among others).Chan (1992) suggests the colloidal nature of the clay minerals tobe partly responsible for the deformation features of the ironcoats. In accordance with the classification by Guerrak (1987),the Fe2O3 contents (67.3–80.5%) in the ferruginous materialscorrespond to the “Fe-rich ironstones” type. P2O5 and CaOcontents are related to apatite occurrence (detected by XRD andSEM), indicative of a biogenic origin. Apatite is very commonin Phanerozoic ironstones (Gehring, 1985; Chan, 1992; Ramajoet al., 2002). Furthermore, the high contents in trace elementssuch as Th and Sr in ooids of the iron crust can be explained bythe presence of apatite (Taylor and McLennan, 1985). Alkalineelements (K and Mg) are present in very low concentrations asin all types of ironstones. The highly mobile elements Co, Ni,and Cr appear at higher concentrations than average for theupper continental crust, as do Zn and Pb. The presence of Zn isaccounted for by the fact that it is easily absorbed by Fe-oxidesand hydroxides. The origin of Pb is more difficult to explain,although its source might be found in meteoric waters. REEpatterns are similar in both the crust and the ooids (Fig. 7),although the absolute values are higher in the coated grains.

As described in Section 4, the ferruginous coated grains(ooids and pisoids) have been classified into two types based onmorphology and internal microstructure (Fig. 9): A and B. Both

Fig. 9. Descriptive characteristics and depositional environments of the ferruginous coated grains studied.


types have an iron-oxide coat composed of laminae reflectingsuccessive growth phases with an interruption between theformation of adjacent layers. The chemical and mineralogicalcomposition of Type A ooids is very similar to that of thesurrounding matrix (see above). Therefore, the data suggest thatthe genesis of these ooids and the crust are related. Theexistence of Al-substitution in goethites from the crust and inthe Type A ooids, and the proportion of this substitution, can beused as a parameter to determine the origin of the goethite(Fitzpatrick and Schwertmann, 1982). In our own samples, withvalues of Al-substitution of 5 mol% in ooids and 10 mol% in thecrust, this parameter is an indicator of pedogenic conditions.Nevertheless, filament structures of possible fungal or bacterialorigin observed inside the porous nuclei or between layers ofthese ooids (Fig. 5) are not exclusively related to subaerialenvironments, since the influence of microbial activity in the

precipitation of ferric oxyhydroxides has been described in bothsubaerial and marine environments (e.g. Dahanayake andKrumbein, 1986; Folk and Milliken, 2000; Préat et al., 2000).

The first carbonate deposits of the ferruginous ooliticlimestone contain fragments of the crust (sometimes includingcoated grains) and many Type A ferruginous ooids, which thendecrease upwards in both abundance and size. These ooids werereworked during the erosion of the iron crust, and fragments arecommon. The morphology of the spastoliths is indicative of adeformation process favoured by a slight consolidation of thereworked ooids, probably still in a colloidal and plastic stage.This deformation was apparently not diagenetic (by load ortectonic), as it only affects some Type A ferruginous ooids butnot the surrounding carbonate grains.

The Type B ooids, less abundant, are only present in thelimestones. The existence of encrusting foraminifera in the


laminae and the nature of the nuclei undoubtedly indicate amarine origin for these ooids. The scarcity of laminae and theirirregularity suggest an environment with episodic energeticperiods alternating with calm intervals, which favoured theencrusting of foraminifera. In some samples, two oolitic growthphases can be observed, the initial phase being Type A and thelast, irregular layers corresponding to Type B. Moreover, theabsence of Type B ferruginous ooids in the iron crust confirmsthat their origin was subsequent to that of the Type A ooids. Partof the iron belonging to ferruginous grains (Type A ferruginousooids included) was removed and incorporated into thesurrounding carbonate grains (inside foraminiferal chambersand microborings) and Type B ferruginous ooids. The in-corporation of iron to Type B ooids could be related to simpleimpregnation (as occurs in some bioclasts, e.g., echinodermsand foraminifera), or it could be favoured by the presence ofbenthic microbial communities. The mechanical pattern of theType B ooids resembles that of carbonate oncoids or Bahamianooids (Peryt, 1983) insofar as the absence of Type B spastolithsand the presence of components (microbial benthic commu-nities and nubeculariids), which are similar to the carbonate-coated grains described by Reolid et al. (2005) in the Oxfordianlithofacies of the Prebetic. The deformation of Bahamian ooidsis unlike that of spastoliths, the former being indurated as theyadd new calcitic laminae.

Therefore, whereas there is clear evidence for the marineorigin of the Type B ooids, the iron crust and Type A ooids pointto a subaerial origin. The chemical composition (Table 1,Fe2O3=80% by weight) and the mineralogy of this iron crust(mainly hematite and goethite) agree with the ferruginouspisolitic plinthite described by Mohr et al. (1972). Allen (1977)states that the pisolitic plinthite has high contents in iron oxidesand hydroxides, predominantly hematite and goethite, and asignificant percentage of aluminium hydroxides, gibbsite, andbohemite.

The existence of Al-substitution in the analysed ferric oxidesis an important finding that corroborates its edaphic origin.Nahon et al. (1980) concluded that a lack of aluminium ingoethites and hematites, both in microprobe and X-ray dif-fraction analyses, indicates a soil origin to be unlikely. Never-theless, when there is Al-substitution in goethite, the degree ofsubstitution will vary according to the soil environment(Fitzpatrick and Schwertmann, 1982). According to theseauthors, an Al-substitution in goethites of 5–10 mol% is char-acteristic of redoximorphic environments in which Fe accumu-lation predominates. Mohr et al. (1972) and Retallack (2001)interpret hydrous conditions during the formation of ferruginouspisolites and matrix. In hydromorphic soils (groundwater soilsas per Mohr et al., 1972), the oversaturated conditions generallyrestrict leaching due to insufficient drainage. The alternation ofvery dry periods with floods lasting long enough to favour soildevelopment is congruent with the generation of ferruginouscrusts with a nodular or pisolitic structure (Type A ooids andpisoids), such as those described herein. On the other hand,hydromorphic conditions, which favour the precipitation of ironoxyhydroxides (mainly goethite) alternating with dry periods,which favour interruptions in growth, would explain the genesis

of the layered texture characteristic of the Type A ooids.Reducing fluids that take up the iron eventually mix withoxidizing groundwater. Under freatic conditions where all poresare saturated with the reducing fluid, diffusion at the interfacebetween the oxidizing and reduced solution causes oxidation ofiron and precipitation of goethite (Chan et al., 2006). Thisphenomenon may be mediated by microbes (Fortin and Langley,2005).

Fitzpatrick and Schwertmann (1982) describe plinthite as asoil with fossilized hydromorphic features in which – accordingto Van Breemen (1988) – gley development spans decades(forming mottled horizons due to periodic water saturation) oreven thousands of years or more (to form plinthite or ground-water soil with an absolute accumulations of iron due to supplyof dissolved Fe2+ from elsewhere). This explanation is con-gruent with the hiatus affecting the Lower Oxfordian and pos-sibly theUpper Callovian (López-Garrido andGarcía-Hernández,1988; Olóriz et al., 2005).

Modern examples of plinthite are reported from coastal plainenvironments by Wood and Perkins (1976) and Aide et al.(2004). The palaeokarsts identified in the Eastern ExternalPrebetic (Linares-Girela, 1976) and the slight palaeoreliefs(Reolid, 2005) are congruent with subaerial origin in a coastalplain having periodically flooded soils.

The interpretation of this iron crust as plinthite may also haveimportant palaeoclimatic implications, as modern plinthiteoccurs in tropical or subtropical regions (Mohr et al., 1972;Allen, 1977; dos Anjos et al., 1995; Sokolov, 1997). Moreover,on the basis of global and regional palaeoclimatic indications,numerous authors have proposed warm and dry conditionscorresponding to the intertropical zone for the South-IberianPalaeomargin during the Late Jurassic (Hallam, 1984, 1985;Ramalho, 1988; Moore et al., 1992). More recently, Olóriz(2000) has also postulated the existence of a warm and humidsubtropical climate for this time interval. The Jurassic climatehas been described as a typical greenhouse climate with mini-mal equator-to-pole gradients (Hallam, 1993). Notably, noconsensus exists about the climate of the Middle–Late Jurassictransition. The Callovian–Oxfordian transition is marked by asignificant temperature increase which is well documented athigh latitudes (Rais et al., 2007). Evidences for considerablewarming during the Oxfordian are the oxygen isotopecomposition of coleoids from the Russian platform (Riboulleauet al., 1998), and sporomorph data from North Sea (Abbinket al., 2001) among others. Studies on palynology, claymineralogy and distribution of evaporites and coal (Rioultet al., 1991; Parrish, 1993, Abbink et al., 2001; Hautevelle,2005) indicate a warming accompanied by an expansion of thearid climate belt in the northern hemisphere. In contrast,Dromart et al. (2003) and Lécuyer et al. (2003) found evidenceof a severe, brief and global cooling that affected sea surfacetemperatures during the Middle–Late Jurassic transition.

Finally, the Ce negative anomaly in the crust and Type Aooids indicates that Ce-depleted REEs moved into the soilsolution (probably in a reducing environment with cerium asCe3+), as described by Braun et al. (1990, 1998) for top soiland ferruginous nodular horizons from the lateritic soil of

Fig. 10. These diagram blocks show an idealized evolution of the coastal plainenvironment with periodic flooding that favoured the development of theplinthite during the Upper Callovian–Lower Oxfordian (A) to the oceanizationrecorded in the Tethyan Domain during the Middle–Late Oxfordian (B). Erosionpredominated between the two phases.


Cameroon. The relative impoverishment of LREE and enrich-ment of HREE with respect to the upper continental crust(Taylor and McLennan, 1985) could be due to the chemicalfeatures of the ferruginous crust parent rock, since this ferru-ginous material shows an opposite tendency to that of charac-teristic REE fractionation during weathering processes (Nesbitt,1979).

6.2. Shelf evolution around the Middle–Upper Jurassicboundary

The controls on this major discontinuity and other possibleassociated minor discontinuities were both eustatic and tectonic(García-Hernández et al., 1989; Marques et al., 1991). Thediscontinuity is related to distensive tectonic activity associatedwith an advanced rifting stage, followed by an oceanization andexpansion phase recorded in the Tethyan Domain in theMiddle–Late Oxfordian (García-Hernández et al., 1989). Thesea-level fall at the Middle–Late Jurassic transition can beviewed as globally well documented, of eustatic origin, with amaximum in the Late Callovian (Aurell et al., 2003; Dromartet al., 2003). The Middle Jurassic shallow carbonate shelf mayhave then emerged and weathered. Sharp erosive surfaces formedatop the Upper Callovian oolitic limestones, and ferruginouspisolitic plinthite developed in a coastal plain environment withperiodic minor flooding (Fig. 10A). The discontinuity (DIII afterMarques et al., 1991) marked the end of the Callovian–Oxfordiancrisis, in which emersion, erosion, or non-deposition have beendescribed in very different sectors (see Marques et al., 1991;Norris and Hallam, 1995; Aurell et al., 2003).

Nevertheless, for several authors the Middle-Late Jurassictransition is a period of sea-level rise on a global scale (Legarreta,1991; Norris and Hallam 1995; Hallam, 2001). Jacquin et al.(1998) consider a second-order peak transgression close to theCallovian–Oxfordian boundary as one of the most correlatabledrowning events of the European craton. This interpretation is inapparent contradiction with the purported sea-level fall at theMiddle–Late Jurassic transition (Aurell et al., 2003; Dromartet al., 2003).

In the area studied, sedimentation began during the MiddleOxfordian, (Fig. 10B). According to García-Hernández et al.(1989), the ferruginous oolitic limestone represents the floodingdeposits of the Prebetic shelf during the Late Jurassic. Olórizet al. (2004, 2005) and Reolid (2005) interpreted, based on datafrom taphonomy and palaeoecology, a fast and episodic depositof the ferruginous oolitic limestone in high-energy marineconditions related to fast flooding of the platform. This litho-facies developed in a context of sedimentation dominated bystorms (Olóriz et al., 2005; Reolid, 2005), which favoured thereworking and incorporation of the Type A ferruginous ooidsand pisoids from the source area to a marine environment. Theflooding reached the emerged areas and favoured the erosion ofthe iron crust, corresponding to ferruginous pisolitic plinthitethat had developed in marshy zones of the coastal plain. Ferru-ginous ooids and pisoids (Type A) were transported, distributedand finally reworked into the overlying limestones. The pisoliticplinthite is preserved only locally in Sierra de Cazorla as a thin

iron crust lying above the discontinuity surface. A second phaseof ferruginous ooids (Type B) with clear marine featuresdeveloped, taking advantage of the iron-rich microenvironmentdue to the redistribution of iron crust fragments and Type Aferruginous ooids and pisoids.

In our opinion, the relative sea-level fall produced the emer-sion and weathering of the Middle Jurassic shelf deposits in theSouth-Iberian Palaeomargin; later, sedimentation began inrelation to the flooding of the shelf in the Middle Oxfordian(Plicatilis Zone). We should underline that no consensus existsas to eustatic history at the Middle-Late Jurassic transition ata worldwide scale. All these discrepancies may show thatblock tectonics also played an important role in separate western


European basins during this interval, thereby interfering witheustatic control.

7. Conclusions

Iron crust and ferruginous coated grains related to theMiddle–Upper Jurassic transition in the Prebetic are distinctive and re-cognizable deposits, marking the discontinuity between theshallow facies deposited during Middle Jurassic and thehemipelagic facies deposited during Late Jurassic. The study ofsedimentologic, mineralogic and geochemical features of the ironcrust and coated grains, as well as the overlying ferruginousoolitic limestone, has led us to the following conclusions:

1. The record of the iron crust is exceptional compared withother iron deposits related to the Middle–Upper Jurassictransition from the western Tethyan basins, in which onlyallochthonous ooids are recorded and an iron crust is absent.These iron deposits present a reduced thickness (b50 cm)and very local distribution.

2. The ferruginous coated grains (ooids and pisoids) can besubdivided into two types: Type A ooids, with a thin, regularlamination in concentric layers enclosing a nucleus, domi-nant in the top of the iron crust (100%) and in the ferruginousoolitic limestone (82% of the ferruginous ooids); and Type Booids, with a thick, irregular lamination in a few discontin-uous concentric layers enclosing a variable nucleus thatincludes bioclasts and foraminifera, exclusively found in theferruginous oolitic limestone (18% of the ferruginous ooids).

3. The chemical and mineralogical composition of the iron crustshares characteristics with the ferruginous pisolitic plinthite(hydromorphic soil). The Al-substitution range of goethite(5-10 mol%) indicates pedogenic conditions. As a result,pedogenesis under periodic hydrous conditions is suggested.The Type A ooids and pisoids are interpreted as having anorigin related to the iron crust, and a context of groundwatersoils with hydrous conditions is consistent with their genesis.

4. The Middle Jurassic shallow carbonate shelf was emergentand subject to weathering. A coastal plain with periodicallyflooded soils would be the likeliest scenario. The alternationof very dry periods with floods lasting long enough to favoursoil development would be compatible with the redox-imorphic conditions that generated the iron crusts withpisolitic structure (Type A ooids and pisoids).

5. The erosion of ferruginous pisolitic plinthite occurred througha major flooding of the Prebetic shelf during the MiddleOxfordian, and Type A coated grains were winnowed. Thefirst marine deposit was ferruginous oolitic limestones withreworked fragments of iron crust and Type A ferruginousooids.

6. A second phase of ferruginous ooids (Type B) with clearmarine features developed in the first stages of the transgres-sion, benefiting from iron-rich microenvironments due to theredistribution of iron crust fragments and Type A ferruginousooids.

7. The interpretation of this iron crust as a plinthite has im-portant palaeoclimatic implications, as modern plinthite

occurs in tropical or subtropical regions. Such paleoclimaticconsiderations may extend along the western Tethyan basins.Eustatic and tectonic control played a major role in deter-mining the sedimentological features and stratigraphic posi-tion of these ferruginous deposits.

Acknowledgements

This research was carried out with the financial support ofResearch Projects CGL2005-01316/BTE, CGL2005-06636-C0201/BTE and CGL2007-66744-C02-02/BTE (Spanish Min-istry of Science and Technology) and Research Groups RNM-178 and RNM-325 of the Junta de Andalucía. The authors areindebted to M. Aurell (Universidad de Zaragoza) and M.A.Chan (University of Utah) for their valuable comments andsuggestions regarding an early version of this paper. We are alsograteful to Christine Laurin and Jean Louise Sanders for theirassistance in reviewing the English.

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Palaeoenvironmental implications of ferruginous deposits related to a Middle–Upper Jurassic discontinuity (Prebetic Zone, Betic Cordillera, Southern Spain)