2007 campos et roser

Geochemistry of black shales from the Lower CretaceousPaja Formation, Eastern Cordillera, Colombia: Source

weathering, provenance, and tectonic setting

N.O. Campos Alvarez *, B.P. Roser

Department of Geoscience, Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504, Japan

Received 1 December 2004; accepted 1 August 2006

Abstract

The major and trace element characteristics of black shales from the Lower Cretaceous Paja Formation of Colombia are broadly com-parable with those of the average upper continental crust. Among the exceptions are marked enrichments in V, Cr, and Ni. These enrich-ments are associated with high organic carbon contents. CaO andNa2O are strongly depleted, leading to high values for both the ChemicalIndex of Alteration (77–96) and the Plagioclase Index of Alteration (86–99), which indicates derivation from a stable, intensely weatheredfelsic source terrane. The REE abundances and patterns vary considerably but can be divided into three main groups according to theircharacteristics and stratigraphic position. Four samples from the lower part of the Paja Formation (Group 1) are characterized byLREE-enriched chondrite-normalized patterns (average LaN/YbN = 8.41) and significant negative Eu anomalies (average Eu/Eu* = 0.63). A second group of five samples (Group 2), also from the lower part, have relatively flat REE patterns (average LaN/YbN = 1.84) and only slightly smaller Eu anomalies (average Eu/Eu* = 0.69). Six samples from the middle and upper parts (Group 3) havehighly fractionated patterns (average LaN/YbN = 15.35), resembling those of Group 1, and an identical average Eu/Eu* of 0.63. The frac-tionated REEpatterns and significant negative Eu anomalies inGroups 1 and 3 are consistent with derivation from an evolved felsic source.The flatter patterns of Group 2 shale and strongly concave MREE-depleted patterns in two additional shales likely were produced duringdiagenesis, rather than reflecting more mafic detrital inputs. An analysis of a single sandstone suggests diagenetic modification of the REE,because its REE pattern is identical to that of the upper continental crust except for the presence of a significant positive Eu anomaly (Eu/Eu* = 1.15). Felsic provenance for all samples is suggested by the clustering on the Th/Sc–Zr/Sc andGdN/YbN–Eu/Eu* diagrams.Averagesof unmodified Groups 1 and 3 REE patterns compare well with cratonic sediments from the Roraima Formation in the Guyana Shield,suggesting derivation from a continental source of similar composition. In comparison withmodern sediments, the geochemical parameters(K2O/Na2O, LaN/YbN, LaN/SmN, Eu/Eu*, La/Sc, La/Y, Ce/Sc) suggest the Paja Formation was deposited at a passive margin. The Pajashales thus represent highly mature sediments recycled from deeply weathered, older, sedimentary/metasedimentary rocks, possibly in theGuyana Shield, though Na-rich volcanic/granitic rocks may have contributed to some extent.! 2007 Elsevier Ltd. All rights reserved.

Keywords: Black shales; Geochemistry; REE, Provenance; Weathering; Tectonic setting; Paja Formation; Colombia

1. Introduction

The Colombian Eastern Cordillera is a NNE-SSW –trending fold belt composed of Precambrian to Paleozoicbasement covered by a thick sequence of Mesozoic – Ter-tiary sedimentary rocks. It is generally accepted that theformation of the Eastern Cordillera during the Miocene–Pliocene involved inversion of two Late Jurassic–Early

0895-9811/$ - see front matter ! 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.jsames.2007.02.003

* Corresponding author. Present address: Department of Earth andEnvironmental Sciences, University of Windsor, Windsor, Ont., CanadaN9B 3P4. Tel.: +1 519 253 3000x2497; fax: +1 519 973 7081.

E-mail address: [email protected] (N.O. Campos Alvarez).

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Journal of South American Earth Sciences 23 (2007) 271–289

mailto:[email protected]

Cretaceous basins, the Tablazo-Magdalena Basin to thewest and the Cocuy Basin to the east. These paleobasinsare separated by the Santander High, which is cut by faultsrunning parallel to the main trend of the cordillera (Col-letta et al., 1990; Dengo and Covey, 1993; Cooper et al.,1995).

Neocomian black shale units in both flanks of the cen-tral sector of the Eastern Cordillera are of special signifi-cance because they host rich ‘‘Colombian-type’’ emeralddeposits, along with hydrothermal mineralization. Theshales have been the subject of diverse studies focusingmainly on the genesis of emerald mineralization (Ulloaand Rodrıguez, 1976; Ulloa, 1980; Cheilletz et al., 1994,1997; Giuliani et al., 1995, 2000; Branquet et al.,1999a,b). However, little work has addressed geochemicalaspects related to provenance and the tectonic setting ofdeposition of the sedimentary rocks within it.

Recent studies show that the main factors a!ecting com-positions of clastic sedimentary rocks are source rock com-position, source area weathering, sorting, tectonic setting,diagenesis, and recycling (e.g., Bhatia, 1983; Nesbitt andYoung, 1984; Taylor and McLennan, 1985; Roser andKorsch, 1986; McLennan et al., 1993).

This study focuses on surface samples of the Lower Cre-taceous (Hauterivian–Aptian) Paja Formation in the Boli-var district, located in the west flank of the easternColombian Cordillera (Fig. 1a and b). The Paja successionconsists predominantly of organic-rich black shales, minorintercalated fine-grained silty sandstones, calcareous blackshales, massive organic-poor beige sandy mudstones, andoccasional massive organic-poor green mudstones. Previ-ous work in the study area by Ulloa and Rodrıguez(1978) and Fabre and Delaloye (1983) examines the occur-rence of widespread base metal, barite, and fluorite miner-alization, which is possibly coeval with that foundelsewhere in the Eastern Cordillera. Other studies carriedout in Bolivar (Santander) focus on regional-scale geo-chemistry, geological mapping, recognition and prospect-ing of copper and base metal mineralization (Munoz,1961; Pulido, 1976; Tellez, 1980), and regional-scalephoto-interpretations (Gomez, 1977). A recent study byCampos (2003) examines the clay mineralogy, geochemis-try, mineral chemistry, organic geochemistry, and microth-ermometry of the Paja Formation and associatedhydrothermal veins.

The purpose of this article is to examine the whole-rockgeochemistry of the Paja Formation in the Bolivar districtto assess its provenance, source weathering, and tectonicsetting on the basis of its major and trace element compo-sitions, including rare earth elements (REE).

2. Geological setting

The geological evolution of Colombia involves severalstages of accretion, subduction, and collision during theinteraction of the South America, Nazca/Farallon, andCaribbean plates. Roeder and Chamberlain (1995) and

Taboada et al. (2000) discuss various tectonic models andgeodynamic aspects related to the evolution of the EastCordillera and the north Andean region. Maze (1984)describes a series of elongated basins and grabens of similarstratigraphy and tectonic setting along the western marginof South America and Central America (including the lateJurassic–early Cretaceous basins in the eastern Cordillera),where sedimentation was associated with rifting. However,on the basis of study of the La Quinta Formation in north-western Venezuela, Maze (1984) reviews the mechanism forformation of continental rifts in the framework of passivemargins, midcontinental regions, and active margins. Hesuggests that during Jurassic times, the tectonic historywas complex and could have involved sedimentationrelated to both convergent plate margin processes and ten-sion related to the opening of the Caribbean Sea.

Jurassic and Cretaceous sedimentation in the EasternCordillera also likely was associated with Triassic continen-tal rifting linked to the separation of North and SouthAmerica and to backarc extension east of the Central Cor-dillera, which arose from subduction of the Farallon Platebeneath South America (Maze, 1984; Fabre, 1987; Tabo-ada et al., 2000). Fabre (1987) argues that extension andcontinental rifting in the Eastern Cordillera, associatedwith thinning of the crust during the lower Cretaceous,could have been active during Triassic–Jurassic times. Atthat time, sedimentation was predominantly continentaland sporadically marine compared with the dominantlymarine sedimentation prevailing during the Cretaceous.In the Eastern Cordillera, sedimentation during riftingoccurred until the Albian. However, there is no consensusregarding whether rifting occurred at a passive margin orwithin the framework of a subduction zone west of SouthAmerica (Fabre, 1987; Dengo and Covey, 1993; Cooperet al., 1995; Villamil et al., 1999).

Reconstructions by Cooper et al. (1995) show that inLate Jurassic–Early Cretaceous times, the depocenter ofthe present-day Eastern Cordillera comprised a subsidingbackarc environment. The largely submerged Central Cor-dilleran arc and its associated accretionary complex lay farto the west, bordering a marginal sea behind the Baudooceanic island arc (Fig. 1c). Sedimentation in the EasternCordillera backarc depocenter was mostly shallow water.The backarc was divided into two parallel basins, theTablazo-Magdalena and Cocuy (Fig. 1c), separated bythe intrabasinal Santander High (Fig. 1a and c). The Sant-ander High is now represented by the Santander and Flor-esta massifs. This configuration persisted until theMaastrichtian–Paleocene, when the Baudo arc wasaccreted to the Central Cordillera (Cooper et al., 1995).

A simplified stratigraphy of the Upper Jurassic–Creta-ceous sedimentary rocks in the Middle Magdalena Basinand western flank of the Eastern Cordillera appears inFig. 1b. Continental redbeds of the upper Jurassic GironFormation and lower Cretaceous Tambor Formation con-sist of alternating sandstones, mudstones, and conglomer-atic horizons deposited in a continental rift basin

272 N.O. Campos Alvarez, B.P. Roser / Journal of South American Earth Sciences 23 (2007) 271–289

associated with the separation of North and South Amer-ica (Cooper et al., 1995; Villamil et al., 1999). Clastic sed-iments and restricted fine-grained marginal facies overliethese redbeds, including sandstones and mudstones of theCumbre Formation, limestones of the Rosablanca Forma-tion, and black shales of the Paja Formation. During thisinterval, the Santander High was the main control on faciesdistributions across the Eastern Cordillera depocenter(Cooper et al., 1995).

The study area is located within the Tablazo-MagdalenaBasin, south of the Santander Department, between theBucaramanga and La Salina fault systems in the westernflank of the Eastern Cordillera, and forms part of theVelez-Arcabuco anticlinorium (Fig. 1a). This region is

characterized by widespread synclinal and anticlinal struc-tures and traversed by generally NE-SW-trending, east-ward-dipping, reverse and thrust faults (Royero andClavijo, 2000). Structurally, the study area around Bolivarcity lies within the western flank of the Jesus Maria Syn-cline (Fig. 2). This structure extends approximately 50 kmwith a N20E trend and is complemented by the Jesus MariaAnticline to the east. The district around Bolivar is domi-nated by rocks of the Rosablanca and Paja formations,with a small area of Cumbre Formation cropping out inthe NW corner (Fig. 2). The Cumbre Formation consistsof fine-grained grey sandstone with siliceous cement, greyto white mudstone and siltstone, and red limestone. It isinterpreted to have been deposited in a marginal marine

Fig. 1. (a) Map of the main tectonic elements and stratigraphic units of the northern segment of the Eastern Cordillera, modified from Cooper et al.(1995). Inset: Present-day plate configuration. (b) Simplified stratigraphy of the Middle Magdalena Basin and western flank of the East Cordillera in theSantander Department (Cooper et al., 1995; Royero and Clavijo, 2000). sst, sandstone; mst, mudstone; cgt, conglomerate; cgsst, conglomerate-sandstone;lmst, limestone; cal, calcareous. (c) Schematic tectonic and depositional model for early Cretaceous, showing position of the Eastern Cordillera depocenter(modified after Cooper et al., 1995).

N.O. Campos Alvarez, B.P. Roser / Journal of South American Earth Sciences 23 (2007) 271–289 273

setting bordering a fluvial coastal plain. The Cumbre For-mation is 20–50 m thick in the Santander Department andin transitional contact with the overlying Rosablanca For-mation (Mendoza, 1985). The Valanginian–lower Hauteri-vian Rosablanca Formation (Etayo, 1968; Etayo andRodriguez, 1985) is widely distributed in the SantanderDepartment and consists of limestone with intercalatedevaporitic layers at its base, as well as limestone, sandstone,and calcareous mudstones to its top. It was deposited in ashallow marine environment and varies in thickness from150 to 450 m (Clavijo et al., 1993).

In the study area, the Rosablanca Formation is con-formably overlain by siliciclastic rocks of the Paja Forma-tion (Royero and Clavijo, 2000). The Paja Formation isdominated by carbonaceous black shales, with intercalatedgrey to black sandy mudstone and local pyritic layers. Int-erbedded fine-grained, muddy, quartzofeldspathic sand-stones also occur. Calcareous black shales are commontoward the base and in the middle part. Thickness rangesfrom 125 to 650 m (Royero and Clavijo, 2000). The Pajablack shales are generally massive but occasionally show

very thin lamination parallel to the overall orientation ofthe strata. These laminations usually consist of fine-grainedquartz or other terrigenous detritus. Detailed X-ray di!rac-tion study indicates that the mineral assemblage of the PajaFormation shales consists of quartz, illite, chlorite, mixedlayer illite–smectite ± rectorite ± pyrophyllite ± a brush-ite-like mineral (Campos, 2003). In the Bolivar area, thePaja Formation is often cut by hydrothermal veins (e.g.,calcite, barite, fluorite, quartz). According to Mantilla-Fig-ueroa et al. (2003), these veins are oriented parallel to thestratification and may represent pathways for fluids associ-ated with the development of localized albitization and sul-phide mineralization in the district.

3. Analytical methods

Twenty-four samples were analyzed from the Paja For-mation in theBolivar area (Fig. 2). Sampleswere crushed fol-lowing the procedures described in Roser et al. (1998), andloss on ignition (LOI) determinations were made using themethod outlined in Campos et al. (2002). Total carbon andorganic carbon (Corg) were determined using a FissonEA1108 CHNS elemental analyzer, and inorganic carbon(Cinorg) was calculated by the di!erence, as outlined byCam-pos et al. (2002). Major elements and 14 trace elements weredetermined on the ignited samples from the LOI determina-tions, using a Rigaku RIX 2000 X-ray fluorescence spec-trometer (XRF) equipped with an Rh-anode X-ray tube.The analyses followed the method of Kimura and Yamada(1996), using glass beads prepared with alkali flux compris-ing 80% lithium tetraborate (Li2B4O7) and 20% lithiummetaborate (LiBO2), and a flux to sample ratio of 2:1.

Rare earth elements and additional ultra-trace elementswere determined on 18 selected samples, using a VG Ele-mental PQ3 inductively-coupled plasma mass spectrometer(ICP-MS). The analyses were made on the ignited powdersused for XRF analysis. Solutions were prepared using acombination of alkali fusion and acid digestion and ana-lyzed following the methods and analytical conditions out-lined in Kimura et al. (1995) and Roser et al. (2000).

4. Whole-rock geochemistry

4.1. Major elements

Major element abundances in the Paja Formation mud-rocks (Table 1) show considerable variation. SiO2 contentsrange from 11.7 wt% in sample PES5 (calcareous blackshale) to 73.1 wt% (PES21), though most samples lie inthe narrower range of 55–65 wt%. The Al2O3 contents varyinversely (4.8–27.7 wt%), with most samples containing 20–25 wt%. A Harker variation diagram with samples splitinto three groups on the basis of their Corg content showsa weak negative correlation between SiO2 and Al2O3

(Fig. 3a). The group characterized by low Corg contents(<1 wt%) and the single sandstone analyzed form a lineararray, whereas the Corg-rich group is displaced toward

Fig. 2. Geological map of the study area showing location of the analyzedsamples.


Table 1Whole-rock XRF analyses (hydrous basis) of samples from the Paja Formation, Bolivar (Santander), Colombia

Sa# PES1 PES2 PES3 PES4 PES5 PES8 PES9 PES10 PES11 PES12 PES14 PES15Pstn MU MU MU L MU MU L MU L MU L MULith BS BS BS BS CBS BS BS BS BS BS BS BS

SiO2 65.56 62.61 50.91 53.06 11.68 60.83 64.06 64.29 65.74 59.48 55.83 63.21TiO2 0.21 1.00 0.62 0.94 0.17 0.55 0.66 0.93 0.64 0.80 0.84 0.76Al2O3 5.70 20.28 18.03 22.63 4.81 15.07 20.52 23.99 19.99 21.55 21.18 18.71Fe2O3 1.02 0.51 1.62 5.54 1.51 3.80 3.60 0.51 3.13 5.69 6.39 5.72MnO 0.01 0.00 0.00 0.01 0.02 0.01 0.01 0.00 0.01 0.02 0.01 0.01MgO 0.38 0.33 1.27 0.98 1.21 0.99 1.35 0.27 1.21 2.54 1.29 3.42CaO 5.89 0.03 0.06 0.99 35.83 1.49 0.18 0.04 0.10 0.09 0.69 0.04Na2O 0.02 1.04 0.13 0.91 0.03 0.38 0.39 1.99 0.36 0.22 0.59 0.26K2O 0.73 3.27 2.49 4.73 0.25 1.99 4.35 1.73 4.27 4.37 4.31 3.34P2O5 0.06 0.03 0.64 0.72 0.26 0.61 0.20 0.04 0.10 0.16 0.40 0.09LOI 19.06 10.67 22.95 9.77 41.39 13.99 5.05 6.51 4.86 5.43 8.83 5.09

Total 98.65 99.78 98.74 100.28 97.17 99.69 100.34 100.30 100.39 100.35 100.35 100.66

Ba 186 464 535 575 132 408 518 376 505 623 502 432Ce 10 108 120 20 14 65 34 112 16 163 59 37Cr 80 135 129 132 74 149 63 140 62 59 96 64Ga 8 32 24 32 5 19 26 34 25 25 30 24Nb 7 24 18 24 5 14 27 28 26 22 23 21Ni 150 34 241 136 217 355 14 8 50 39 32 27Pb 11 30 38 33 1 39 25 49 19 298 42 22Rb 63 184 156 246 17 137 224 108 221 215 225 176Sc 7.6 12.5 12.7 26.1 0.6 14.5 12.5 12.7 13.7 14.2 17.3 16.3Sr 112 91 69 162 464 109 262 263 252 116 327 73Th 3.2 11.0 12.6 16.5 4.6 10.0 14.9 21.7 14.6 13.9 14.4 11.9V 1566 1456 3192 433 1284 2244 99 704 93 102 244 106Y 12 12 96 32 30 27 93 25 86 45 18 23Zr 43 197 108 163 25 106 164 130 168 265 142 183

Corg 11.40 5.95 16.50 4.51 13.69 7.36 1.13 1.96 0.89 0.36 4.27 0.74Cinorg 0.35 – – – 8.19 0.03 0.06 0.08 0.14 0.13 0.29 0.00

CV 1.45 0.31 0.34 0.62 8.12 0.61 0.51 0.23 0.49 0.64 0.67 0.73CIA – 79 86 77 – 79 79 82 79 81 78 82PIA – 91 99 92 – 87 96 87 96 98 93 97

Sa# PES16 PES18 PES20 PES21 PES23 PES24 PES25 PES27 PES28 PES29 PES30 PES31Pstn L L L L MU MU MU L L L L MULith FGSS BS MSM BS BS BS BS BS MGM BS LGS BS

SiO2 67.00 62.04 69.33 73.08 54.92 69.70 57.50 62.40 61.52 57.07 57.95 46.40TiO2 0.56 1.07 0.58 0.70 0.88 0.66 0.62 1.18 1.02 1.10 0.62 0.65Al2O3 12.58 22.97 12.76 14.69 27.27 21.65 18.06 24.55 20.50 25.06 24.92 19.90Fe2O3 8.23 2.85 6.07 2.66 3.78 0.49 0.74 1.24 5.63 0.87 2.14 4.01MnO 0.08 0.01 0.02 0.01 0.00 0.00 0.00 0.00 0.03 0.00 0.01 0.00MgO 6.65 0.94 6.66 1.78 1.40 0.29 0.90 0.48 2.13 0.49 3.81 0.97CaO 0.07 0.04 0.04 0.64 0.13 0.06 0.10 0.15 0.24 0.38 0.05 1.61Na2O 0.05 0.58 0.05 0.24 0.57 0.35 0.40 0.76 0.34 0.78 0.35 0.75K2O 0.40 4.88 0.49 2.24 2.04 1.23 1.96 4.32 4.74 3.76 3.61 3.21P2O5 0.10 0.10 0.02 0.34 0.91 0.11 0.05 0.11 0.28 0.04 0.02 0.60LOI 5.09 4.79 4.76 4.13 8.05 6.45 19.21 4.90 3.99 10.24 6.53 20.73

Total 100.81 100.27 100.78 100.51 99.96 101.00 99.56 100.10 100.44 99.81 100.01 98.84

Ba 96 581 100 263 751 402 59 474 768 490 365 473Ce 72 45 22 28 111 103 31 117 87 81 12 73Cr 40 101 40 56 275 88 178 140 81 151 50 156Ga 16 33 16 18 7 27 25 35 28 37 27 25Nb 16 30 17 19 29 22 17 29 26 27 30 16Ni 19 11 15 22 37 5 183 8 36 10 18 255Pb 557 66 34 15 38 34 24 40 2 39 20 27Rb 25 249 28 127 117 80 120 240 201 214 173 200Sc 9.0 22.7 9.4 9.2 27.7 15.1 11.5 27.3 20.2 20.5 14.8 18.4Sr 22 334 21 185 480 273 278 452 96 616 116 212

(continued on next page)


the origin. One carbon-rich, Al2O3-poor sample (PES1)plots well o! either trend. The SiO2/Al2O3 ratio of thissample is 11.49, well above the range of 2.01–4.04 typicalof the majority of the shales. The other Al-poor sample(PES5) is highly calcareous.

Among the other major elements, only TiO2 shows anyclear correlation with Al2O3 (Fig. 3b). Most shales withlow or moderate Corg and LOI form a group at 0.5–1.2 wt% TiO2 and 20–27 wt% Al2O3 (circled), whereasCorg-rich samples are displaced toward the origin. The sand-stone (PES16) and twoCorg-poormudrocks (PES20, 21) alsolie on this trend. All three samples have SiO2/Al2O3 ratios(5.0–5.4) greater than those of the rest of theCorg-poor group(2.0–3.4).

Abundances of total iron (Fe2O3T) have a wide range(0.51–8.23 wt%) and show no correlation with Al2O3 orCorg content (Table 1). Abundances of MgO (<4%), CaO(! 2%), and Na2O (<2%), are generally low and show nocorrelation with Al2O3 or Corg (Fig. 3c–e), whereas K2Ocontents are moderate (<4.9%) and weakly correlated withAl2O3 (Fig. 3f). P2O5 contents are generally low (<0.2 wt%)in samples with low or moderate Corg, but some higher val-ues ("0.6 wt%) occur in Corg-rich samples.

4.2. Trace elements

The trace elements analyzed can be divided into fourgroups according to their correlation with Al2O3. Mostincompatible elements, including Nb (Fig. 4a), Sc, Th,Ga, and Ta, are strongly correlated with Al2O3 abun-dances, whereas a few (Zr [Fig. 4b], Hf, Ce, and Y) showlittle or no correlation with Al2O3.

A group of more mobile elements including Rb (Fig. 4c),Ba, Sr, Pb, U, and Be show considerable scatter and weakpositive correlation with Al2O3. Weak to moderate positivecorrelations exist between anhydrous Al2O3 and Rb(r = 0.66), Sr (r = 0.65), and Ba (r = 0.43).

The final group of trace elements (Cr, Ni, and V) gener-ally have low abundances (<100, <40, and <200 ppm,

respectively) in samples with <1% Corg and show moderatecorrelation with Al2O3 (Fig. 4d–f). Conversely, sampleswith high Corg contents (Table 1) are clearly enriched inCr (74–178 ppm) and Ni (150–354 ppm) and extremelyenriched in V (1284–3192 ppm).

4.3. Rare earth elements (REE)

Even excluding two shales with anomalous patterns(PES25 and PES30), the Paja Formation REE data showconsiderable variability (Table 2). Total REE abundances(P

REE) range from 43 to 435 ppm, averaging 180 ppm,slightly less than that of average post-Archean Australianshale (PAAS) and greater than average upper continentalcrust (UCC) (183 and 146 ppm, respectively; Taylor andMcLennan, 1985). Chondrite-normalized REE parametersshow similar variability, with LaN/YbN ratios of 1.0–24.9(average 9.52); LaN/SmN 1.23–10. (average 3.69), andGdN/YbN 0.73–2.89 (average 1.47). The averages of theseratios are comparable with those in both PAAS(LaN/YbN 9.17; LaN/SmN 4.27; GdN/YbN 1.36) andUCC (9.21, 4.20, 1.40, respectively) (Taylor andMcLennan, 1985). All shales have distinct negative euro-pium anomalies (Eu/Eu* 0.47–0.77), which average 0.65,identical to PAAS and UCC.

Chondrite-normalized plots show that the variability inthe REE patterns is partially stratigraphically controlledand can be divided into three main groups (Groups 1–3).Samples from the lower part of the Paja succession (within500–700m of the contact with the underlying Rosa BlancaFormation) show two distinct groupings (Fig. 5a, Table 2).Four samples (Group 1) have relatively fractionated pat-terns similar to UCC, with LaN/YbN ratios of up to 11.7and LaN/SmN of 3.1–4.8. Europium anomalies are charac-teristically negative (0.56–0.69) and average 0.63. Fiveother samples (Group 2) are depleted in the light rare earthelements (LREE), with LaN/SmN ratios of 1.23–2.00 andrelatively flat patterns (LaN/YbN 0.97–2.65). They also dis-play small negative Ce anomalies and slight enrichment in

Table 1 (continued )

Sa# PES16 PES18 PES20 PES21 PES23 PES24 PES25 PES27 PES28 PES29 PES30 PES31Pstn L L L L MU MU MU L L L L MULith FGSS BS MSM BS BS BS BS BS MGM BS LGS BS

Th 7.3 17.3 9.7 11.6 30.8 23.1 5.1 18.6 16.0 19.4 20.1 12.5V 65 186 62 78 880 312 2274 626 148 1033 81 2894Y 1 73 11 16 39 10 9 34 42 34 10 28Zr 292 182 240 426 126 94 126 228 227 181 227 112

Corg 0.25 0.48 0.13 0.79 0.66 1.93 13.11 0.25 0.03 3.96 0.09 14.94Cinorg 0.13 0.17 0.11 – 0.18 0.08 0.37 0.13 0.14 0.67 0.11 –

ICV 1.28 0.45 1.09 0.56 0.32 0.14 0.26 0.33 0.69 0.29 0.43 0.56CIA 96 79 95 82 90 92 86 81 78 81 85 76PIA 99 95 99 94 96 97 96 94 96 92 97 86

Major elements wt%, trace elements ppm. Pstn, stratigraphic position; L, lower; MU, middle to upper; Lith, lithology; BS, black shale; CBS, calcareousblack shale; FGSS, fine-grained silty sandstone; MSM, massive sandy mudstone; MGM, massive green mudstone; LGS, light grey shale; LOI, loss onignition. Corg and Cinorg contents (wt%) from Campos et al. (2002). Total iron as Fe2O3. ICV, Index of Compositional Variation (Cox et al., 1995); CIA,Chemical Index of Alteration (Nesbitt and Young, 1982); PIA, Plagioclase Index of Alteration (Fedo et al., 1995).


HREE in the segment Tm–Lu (Fig. 5a). Despite these con-trasting patterns, all Group 2 samples have significant neg-ative Eu anomalies (Eu/Eu* 0.63–0.77, average 0.69).

Overall, samples from the middle and upper parts of thePaja Formation (Group 3) have REE patterns similar toUCC, with LaN concentrations 120–220 times those ofchondrite, with the exception of PES15 (Fig. 5b). TotalREE abundances (

PREE = 117–435 ppm) are similar to

or greater than that of UCC and considerably greater thanthose in Group 2. Eu anomalies are significant (Eu/Eu*

0.47–0.72), and ranges in LaN/YbN (5.51–24.88) andLaN/SmN (2.56–10.67) are wider than those in Group 1.

The GdN/YbN ratios are also generally low (1.11–2.89),and most samples in this group show heavy rare earth ele-ment (HREE) enrichment in the segment Tm–Lu.

The single sandstone analyzed (PES16) has REE param-eters that closely match UCC (Fig. 5c) in terms of

PREE

(138 ppm), LaN/YbN (9.52), LaN/SmN (4.43), andGdN/YbN (1.47). However, the positive Eu anomalyobserved (Eu/Eu* = 1.15) contrasts conspicuously withthe black shales in which it occurs and with UCC.

Two other samples not included in the above groups(black shale PES25, middle Paja; light grey shale PES30,lower Paja) have anomalous concave REE patterns, with

10

30

50

70

90

0 10 20 30

Low CLow C sstModerate CHigh C

SiO2

DT

PES1

PES5

Dilution

0

4

8

0 10 20 300

4

8

0 10 20 30

CaO

0

2

4

6

0 10 20 30

Na 2O

Al2O3 wt%

0.0

0.5

1.0

1.5

0 10 20 30

TiO 2

Dilution

PES21PES16

oxid

e w

t%

0

4

8

0 10 20 30

K 2O 0.9 0.3Range of K2O/Al2O3 ratios

in K-feldspars

Range in clays

MgO

Al2O3 wt%

Fig. 3. Selected major element-Al2O3 variation diagrams for the Paja Formation suite (hydrous basis), di!erentiated by Corg content (low C <1%;moderate 1–5%; high >5%). Solid line in (a) illustrates probable detrital trend (DT), drawn by eye; dashed arrows in (a) and (b) illustrate dilution trends.Ranges of K2O/Al2O3 ratios in K-feldspars and clays in (f) from Cox et al. (1995).


marked depletion in the MREE to approximately 3 timeschondrite, well below the UCC levels (Fig. 5d). TheLaN/YbN ratio of PES30 (1.62) is similar to that of theGroup 2 shales, whereas that of PES25 (12.7) is compara-ble with Group 3 samples.

5. Discussion

Major and trace element abundances in the Paja Forma-tion samples reflect the combined e!ects of sorting of theirclastic component, varyingCorg and LOI contents, and silicaor carbonate dilution. Linear relationships for SiO2 andTiO2 with Al2O3 in carbon-poor samples suggest detrital

sorting trends (Fig. 3a and b). Displacements toward the ori-gin of the Al2O3–SiO2 plot for samples rich in carbon reflectdilution by that component and their high LOI. One Al2O3-poor sample (PES5) has been diluted by a carbonate compo-nent. Their high SiO2/Al2O3 ratios and displacement of theanalyzed sandstone (PES16) and two Corg-poor mudrocks(PES16, 20) toward the origin of the Al2O3–TiO2 plot(Fig. 3b) suggest they have been a!ected by silica dilution,probably by a greater siliciclastic component.

The general lack of correlation between the group ofmore mobile oxides (Fe2O3T, MgO, CaO, Na2O, andK2O) and Al2O3 likely reflects combined di!erences in theirclay mineralogy and proportions; varying siliciclastic

ppm

Al2O3 wt% Al2O3 wt%

0

1000

2000

3000

0 10 20 30

V

0

100

200

300

400

0 10 20 30

Ni

DT

0

100

200

300

400

0 10 20 30

RbDT?

0

100

200

0 10 20 30

Cr

DT?

0

20

40

0 10 20 30

Low CLow C sstModerate CHigh C

Nb

DT

0

200

400

0 10 20 30

Zr

DT

Fig. 4. Selected trace element-Al2O3 variation diagrams for the Paja Formation (hydrous basis), di!erentiated by Corg content, as in Fig. 3. Solid linesillustrate probable detrital trends (DT), drawn by eye.


Table 2ICP-MS REE and ultratrace element analyses (ppm, hydrous basis) of selected Paja Formation samples

Sa# Lith Lower Paja Group 1 Lower Paja Group 2 Middle and upper Paja Group 3 Sandstone and others

PES14 PES20 PES28 PES29 Mean PES4 PES9 PES11 PES18 PES21 Mean PES2 PES3 PES8 PES10 PES15 PES24 Mean PES16 PES25 PES30BS MSM BS BS BS BS BS BS BS BS BS BS BS BS BS FGSS BS LGS

La 40.68 8.08 51.19 59.23 39.79 15.25 18.99 11.41 22.10 6.30 14.81 71.03 81.52 43.70 66.39 23.95 62.74 58.22 31.14 36.02 5.72Ce 75.89 17.46 96.99 83.22 68.39 26.84 30.93 17.11 37.87 11.86 24.92 89.26 150.83 81.14 93.41 45.10 90.55 91.71 57.24 37.03 8.51Pr 9.08 2.25 11.83 13.05 9.05 3.50 5.47 3.28 6.28 1.69 4.04 13.07 21.75 9.68 13.46 5.60 12.75 12.72 6.50 4.72 1.02Nd 33.95 9.26 47.54 55.38 36.53 16.18 27.11 18.10 31.08 8.03 20.10 48.02 100.17 36.41 50.49 21.69 49.24 51.00 23.41 14.62 3.46Sm 5.88 1.57 10.45 7.80 6.42 6.03 8.26 5.84 6.96 2.73 5.96 4.19 20.08 6.00 6.87 4.06 8.70 8.32 4.43 1.20 0.61Eu 1.14 0.30 2.40 1.36 1.30 1.66 1.94 1.53 1.63 0.63 1.48 0.51 4.68 1.27 1.15 0.87 1.58 1.68 1.58 0.28 0.20Gd 5.08 1.47 10.80 6.99 6.09 7.28 10.57 8.07 8.00 2.88 7.36 2.66 19.81 5.57 4.60 4.01 6.17 7.14 4.01 1.13 0.84Tb 0.71 0.25 1.53 0.81 0.82 1.24 1.86 1.51 1.26 0.47 1.27 0.25 2.63 0.75 0.45 0.65 0.65 0.90 0.60 0.14 0.15Dy 3.98 1.83 8.22 4.91 4.73 7.29 12.22 10.30 8.00 2.86 8.13 1.89 14.78 4.44 2.92 4.12 3.18 5.22 3.56 1.13 1.38Ho 0.76 0.44 1.48 1.10 0.95 1.41 2.66 2.30 1.80 0.63 1.76 0.41 2.66 0.87 0.64 0.83 0.48 0.98 0.70 0.29 0.39Er 2.22 1.40 3.84 3.49 2.74 3.73 7.85 6.67 5.30 1.86 5.08 1.43 6.96 2.43 2.09 2.48 1.38 2.80 1.99 1.12 1.52Tm 0.35 0.23 0.57 0.60 0.44 0.57 1.30 1.11 0.86 0.32 0.83 0.26 1.01 0.39 0.34 0.41 0.23 0.44 0.32 0.24 0.31Yb 2.36 1.63 3.76 4.24 3.00 3.89 9.38 7.99 6.18 2.34 5.96 1.93 6.73 2.63 2.51 2.94 1.73 3.08 2.21 1.91 2.38Lu 0.36 0.24 0.55 0.72 0.47 0.58 1.35 1.18 0.94 0.39 0.89 0.30 1.01 0.41 0.43 0.45 0.26 0.48 0.35 0.33 0.38

Hf 3.93 6.52 5.46 4.69 5.15 4.00 4.79 4.35 3.95 11.20 5.66 3.95 2.51 2.70 2.74 4.80 1.98 3.11 7.11 2.65 6.51Ta 2.12 1.58 2.19 2.62 2.13 2.23 2.32 2.22 2.52 1.91 2.24 1.91 1.30 1.16 2.30 1.79 1.95 1.73 1.44 1.29 2.67Th 13.84 9.31 14.46 21.65 14.81 12.49 13.32 12.15 15.95 12.59 13.30 11.34 13.85 9.53 21.62 10.48 24.29 15.18 9.80 5.15 20.37U 5.13 3.01 3.45 10.40 5.50 11.16 2.66 2.36 3.11 5.34 4.93 6.89 19.04 21.87 4.78 2.85 4.13 9.93 3.09 5.07 6.27Be 3.16 0.54 2.48 3.54 2.43 3.55 3.77 3.52 3.20 1.97 3.20 2.07 1.70 2.06 2.47 2.70 1.71 2.12 0.80 2.13 2.17P

REE 182.4 46.4 251.2 242.9 180.7 95.5 139.9 96.4 138.3 43.0 102.6 235.2 434.6 195.7 245.8 117.2 239.6 244.7 138.0 100.2 26.9LaN/YbN 11.65 3.36 9.20 9.43 8.41 2.65 1.37 0.97 2.42 1.82 1.84 24.88 8.18 11.22 17.85 5.51 24.47 15.35 9.52 12.71 1.62LaN/SmN 4.35 3.25 3.08 4.78 3.87 1.59 1.45 1.23 2.00 1.45 1.54 10.67 2.56 4.58 6.09 3.72 4.54 5.36 4.43 18.87 5.87GdN/YbN 1.74 0.73 2.33 1.34 1.53 1.52 0.91 0.82 1.05 1.00 1.06 1.12 2.38 1.71 1.48 1.11 2.89 1.78 1.47 0.48 0.29Eu/Eu* 0.64 0.61 0.69 0.56 0.63 0.77 0.63 0.68 0.67 0.68 0.69 0.47 0.72 0.67 0.63 0.66 0.66 0.63 1.15 0.73 0.83

Chondrite normalizing factors from Taylor and McLennan (1985). Eu anomaly calculated following Taylor and McLennan (1985): Eu/Eu* = EuN/(SmN · GdN)1/2.

N.O

.Cam

posAlvarez,

B.P.Roser

/Journal

ofSouth

American

Earth

Sciences

23(2007)

271–289279

components; and the dilution e!ects of Corg, LOI, carbon-ate, and silica. The weak correlation between Al2O3 andK2O and depletion in CaO and Na2O suggests that Kwas preferentially incorporated into aluminous clay miner-als (Fedo et al., 1995, 1997), whereas Ca and Na were lea-ched during source weathering and diagenesis (Nesbittet al., 1980). Preponderance of clay minerals is indicatedby low K2O/Al2O3 ratios (<0.3), comparable with theranges of clays (Fig. 3f) but distinct from the higher ratios(0.3–0.9) typical of feldspars (Cox et al., 1995).

Among the trace elements, the strong correlation of theincompatible elements Nb (Fig. 4a), Sc, Th, Ga, and Tareflects their immobile nature, association with the clayfraction, and the e!ects of dilution. The four incompatibleelements that show poor correlation with Al2O3 (Zr[Fig. 4b], Hf, Ce, and Y) are typically enriched in resistantheavy minerals, such as zircon, monazite, and apatite(McLennan, 1989), so their abundances may be controlledby variable distributions of these phases in the fine fraction.Zircon control of Zr and Hf abundances is supported bythe strong correlation between these elements (r = 0.98);the lowest concentrations of both are in samples with mod-erate or high Corg contents. However, Ce and Y do not cor-

relate well with P2O5, suggesting that their abundances arenot controlled solely by monazite or apatite.

Weak positive correlations between more mobile traceelements (Rb [Fig. 4c], Ba, Sr, Pb, U, and Be) and Al2O3

suggest these elements mainly reside in the clay fraction,but abundances also were influenced by dilution e!ects(Corg and silica) and by redistribution during weathering.Nesbitt et al. (1980) demonstrate that Ba and Cs behavesimilarly to Rb in weathering profiles, and K may be fixedin clays. Camire et al. (1993) suggest that K and Rb arepreferentially incorporated into clays during chemicalweathering, in contrast with Ca, Sr, and Na, which are lea-ched. Positive correlations between K and Rb (r = 0.98)and K and Ba (r = 0.75) in the Paja shales suggests thatRb preferentially occurs in K-bearing clays, as does Ba tosome extent. Low contents of Cr, Ni, and V and weak posi-tive correlations with Al2O3 in carbon-poor samples(Fig. 4d–f) indicate a detrital trend. However, the scatterto very high values in carbon-rich samples also suggeststhat these three ferromagnesian elements have been signif-icantly redistributed.

The results overall show that the major element compo-sitions of the analyzed samples are typical of shales and

100

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

PES25PES30UCC

Others100


PES16UCC

Paja sandstone

1010

10 10

sam

ple/

chon

drite

100


PES4PES9PES11PES14PES18

PES20PES21PES28PES29UCC

Lower Paja

100


PES2PES3PES8

PES10PES15PES24UCC

Middle & Upper

Fig. 5. Chondrite-normalized REE plots (hydrous basis) compared to average UCC (Taylor and McLennan, 1985). (a) Lower Paja: Group 1, opensymbols; Group 2, solid symbols. (b) Middle and upper Paja, Group 3. (c) Sandstone PES16. (d) Anomalous patterns: PES25 (upper Paja) and PES30(lower Paja). Chondrite values from Taylor and McLennan (1985).


mainly controlled by the abundance of clay minerals rela-tive to non-clay phases. This trend is illustrated by the val-ues for the Index of Compositional Variation (ICV) of Coxet al. (1995), in which

ICV # $Fe2O3 %K2O%Na2O% CaO%MgO%MnO

% TiO2&=Al2O3:

Excluding three calcareous or sandy samples, the ICV val-ues in all but one of the Paja shales are low (0.14–0.69;Table 1). Such values are typical of minerals such as kaol-inite, illite, and muscovite and lower than the higher values(>1) expected in rock-forming minerals such as plagioclase,K-feldspar, amphiboles, and pyroxenes (Cox et al., 1995).Together with the low K2O/Al2O3 ratios observed, thesevalues emphasize the importance of the clay fraction indetermining the bulk compositions of the Paja shales.Nevertheless, some stratigraphic geochemical variation isevident within the sample suite.

A comparison of the average compositions by strati-graphic position with UCC on multi-element plots showsthat many elements are present at or near average crustallevels (Fig. 6a), especially Nb, U, Zr, Th, and the Ce–Tisegment. In contrast, the more mobile elements Ca andNa are strongly depleted (0.1 times) relative to UCC(Fig. 6a), which indicates intense source area weathering.Nickel, Cr, and V are strongly enriched in some samples,most commonly in samples in the middle and upper partof the sequence, where average V enrichment is almost 27times that of the UCC, compared to 8 and 3 times in theGroups 1 and 2 lower Paja samples (Fig. 6a). Normaliza-tion of samples grouped by carbon content shows thathigher concentrations occur in Corg-rich shales (Fig. 6b),which are more common in the middle and upper partsof the succession. Feng and Kerrich (1990) note that Cr,V, and Ni may be fractionated during weathering. Assum-ing that these elements were remobilized from the sourceduring weathering and preferentially incorporated into clay

0.01

0.1

1

10

Nb Pb U K Ca Zr Th Na Sr Rb Mg La Ce Si Al Ba Sc Y Fe Ti Ni Cr V

Group 2 (lower)

Group 1 (lower)

Group 3 (middle-upper)

0.01

0.1

1

10

Nb Pb U K Ca Zr Th Na Sr Rb Mg La Ce Si Al Ba Sc Y Fe Ti Ni Cr V

High-C content average(excluding PES1, 5)

Moderate-C average

Low-C average

aver

age

/ UC

C

Fig. 6. UCC-normalized multi-element plots for Paja Formation averages, grouped by (a) REE patterns, as in Fig. 5a and b, and (b) Corg content, as inFigs. 3 and 4. Elements arranged from left to right in order of increasing normalized abundance in average Mesozoic–Cenozoic greywacke (Condie, 1993)relative to UCC (Taylor and McLennan, 1985) following Dinelli et al.’s (1999) methodology. Major elements are normalized as oxides.


minerals may account for the higher Cr, Ni, and, especially,V contents. However, Vine and Tourtelot (1970) observethat in black shales, Cr and V may be associated with theorganic fraction, as is also the case in the Paja Formation,because the higher V, Cr, and Ni contents are clearly linkedto samples with higher Corg content.

Other than this association of V, Cr, Ni, and Corg, how-ever, there is little di!erence between the UCC-normalizedpatterns from the lower and middle to upper parts. Thispattern is rather surprising considering the contrastingREE patterns in the lower part, which suggest contrastsin provenance within it, as well as with the middle andupper parts of the sequence. The possible causes of the geo-chemical anomalies in the lower part of the Paja Formationare discussed further next.

5.1. Weathering and metasomatism

The Chemical Index of Alteration (CIA) is well estab-lished as amethod of quantifying the degree of sourceweath-ering (Nesbitt andYoung, 1982, 1984).With the exception oftwo calcareous samples (PES1, 5), for which realistic valuescannot be calculated, the Paja shales have high CIA values(77–96; Table 1), significantly greater than PAAS (70). Onan A–CN–K ternary plot, the data cluster near the A–Kedge, near illite composition, and spread toward the A apex(Fig. 7a).Although scattering of the data prevent a definitionof an accurate ideal weathering trend (IWT), it probably liesbetween UCC and more felsic protoliths (shaded zone),trending toward illite before proceeding along theA–Kedge.Some samples are displaced toward theKapex andbelow theIWT, suggesting the addition of K during diagenesis (Fedoet al., 1995) or, alternatively, during circulation of basemetalmineralizing brines that a!ected the lower Cretaceous strataof the Eastern Cordillera (Fabre, 1983a,b).

The role of source weathering and elemental redistribu-tion during diagenesis also can be assessed using the relatedPlagioclase Index of Alteration (PIA), which quantifies thedegree of destruction of plagioclase and can identify albiti-zation (Fedo et al., 1995, 1997). The Paja shales have veryhigh PIA values (86–99; Table 1), indicating that plagio-clase was almost totally destroyed during source areaweathering and transport. Most samples plot toward theA–K apex, along the albite field in the (A–K)–C–N space(Fig. 7b). Assuming a felsic UCC provenance, as suggestedby the multi-element plots and REE patterns (Figs. 5, 6),this trend indicates that most plagioclase in the sourcewas converted to albite by Na metasomatism (Fedoet al., 1997). Further removal of Na by weathering pro-duced more aluminous clays.

Th/U ratios also give some indication of source weath-ering, as surface weathering produces elevated ratios dueto oxidation of U4+ to soluble U6+ (McLennan and Taylor,1991; McLennan et al., 1993). Nevertheless, U can bereconcentrated during sedimentation in reducing condi-tions, which lowers Th/U ratios. Th/U ratios greater than3 usually reflect weathering e!ects, whereas Th/U ratios

less than 3 may be interpreted in terms of provenance(McLennan and Taylor, 1991). Recent muds derived fromactive margins have Th/U ratios ranging from 1 to 6, com-bined with low Th contents (McLennan et al., 1993). Th/Uratios below 3.5–4.0 (typical of upper crust igneous rocks)are common in post-Archean turbidites but absent in theirArchean counterparts (McLennan and Taylor, 1991).Ratios in PAAS are typically high (4–7; Fig. 8), reflectingthe loss of U during source weathering.

The Paja black shales have Th/U ratios ranging from0.44 to 5.88, high Th contents similar to UCC, and over-lap the PAAS (Fig. 8) and modern passive margin mud

An AbBy La Ad Og

A-K

UCC

PAAS

C N

b

A

KCN

100

80

60

40

CIA

BA

FGPl Ksp

Illite

Muscovite

Ka, Gb, Chl

Smectite

Middle & Upper

LowerPAAS

UCC

UCC

PAAS

IWT

a

100

80

60

40

PIA

Fig. 7. Ternary weathering diagrams. (a) A–CN–K ternary plot.A = Al2O3; CN = CaO* + Na2O; K = K2O (molar proportions). CIA,Chemical Index of Alteration. Mineral compositions: Pl, plagioclase; Ksp,K-feldspar; Ka, kaolinite; Gb, gibbsite; Chl, chlorite, after Nesbitt andYoung (1984). Linked stars: B, basalt; A, andesite; F, felsic igneous rock;G, granite, represent typical primary source trend (Condie, 1993). UCC,upper continental crust; PAAS, post-Archean Australian shale (Taylorand McLennan, 1985). The ideal weathering trend (dashed line; IWT)implies a felsic source close to UCC, but the shales could have beenderived from even more evolved sources (IWT within the grey zone).Samples displaced toward the K apex (arrow) may have been influencedby K-metasomatism. (b) AK–C–N ternary plot (after Fedo et al., 1997).A–K = Al2O3–K2O; C = CaO*; N = Na2O; (molar proportions). PIA,Plagioclase Index of Alteration; An, anorthite; By, bytownite; La,labradorite; Ad, andesine; Og, oligoclase; Ab, albite. Dashed line indicatespossible path from UCC due to diagenetic transformation of plagioclase(albitization) and weathering.


compositions (McLennan et al., 1990). The combination oflow Th/U and low Th in muds from depleted mantlesources in active arc-margin settings is not observed, sup-porting derivation from a weathered, felsic, post-Archeansource. The scatter of values below that of UCC probablyreflects U enrichment during deposition under reducingconditions, because samples with the highest carbon con-tents have the lowest Th/U ratios (Fig. 8).

The high CIA and PIA values observed in the Pajashales suggest derivation from an intensely weathered felsicsource similar in composition to present-day UCC. More-over, clustering of the data on the A–CN–K plot (Fig. 7a)suggests a tectonically stable source region, with detritussupplied only from the upper zones of weathering profiles(Nesbitt et al., 1997).

5.2. Provenance

The multi-element plots, weathering indices, and mostREE patterns are consistent with derivation from a felsicsource with a composition close to UCC. It has been sug-gested that the main source of sediments during the lowerCretaceous was easterly, located toward the Guyana shield(Cooper et al., 1995). Julivert (1968) suggests that the Pro-terozoic Roraima Formation was the likely source.

General reviews of the lithostratigraphy of the GuyanaShield appear in Toussaint (1993) and Voicu et al. (2001),among others. The oldest units are represented by gneissesand migmatites of the Archean Imataca Complex (in Ven-ezuela) and Paleoproterozoic granitoid-greenstone belts.The Trans-Amazonian Orogeny was characterized by amain phase of syntectonic sodic intrusions, followed bylate, post-tectonic potassic intrusions. The K-feldspar richSanta Rosalia, Surucucu, and Parguaza batholiths were

emplaced at the end of the Trans-Amazonian event. TheRoraima Formation, found in the west and southeast ofthe Guyana Shield, was deposited at the end of this orog-eny. This unit consists of conglomerates, sandstones, anddark shales deposited in coastal and fluvial-deltaic environ-ments. All have been a!ected by low-grade metamorphism.Diabasic dikes, sills, and sheets intrude the Trans-Amazo-nian terranes and Roraima Formation.

Precambrian rocks of the Guyana Shield in Colombiainclude migmatites and granitoids of the Mitu Complex,which formed during the Trans-Amazonian Orogeny(Toussaint, 1993). Granites and monzogranites of the Par-guaza Batholith also intrude the Mitu Complex, which isunconformably overlain by fine- to coarse-grained sand-stones, mudstones, metasandstones, metaconglomerates,and phyllites of the Pedrera and Piraparano formations.These units have been regionally correlated with the Rora-ima Formation (Toussaint, 1993).

It is generally thought that REE and other immobilehigh field strength elements are transferred quantitativelyfrom source rocks to the sedimentary record, so abun-dances in sediments provide evidence of their bulk sourcecomposition (McLennan, 1989). However, reliable prove-nance signatures are di"cult to evaluate because local geo-chemical data are limited. Available analyses ofProterozoic sediments from the Guyana Shield (Gibbset al., 1986) include one sample from the base of theintracratonic basin (G949 Marrawahni Formation) andtwo shale samples from the Roraima Formation itself(SPR-59, ECR-10). Sample G949 is characterized by asmall negative Eu anomaly and relatively flat REE pattern,and Gibbs et al. (1986) indicate that the underlying green-stone belts were significant sources for these sediments. Incontrast, the two Roraima shales display larger negative Euanomalies (0.71, 0.53) and significant chondrite-normalized

0

2

4

6

8

0.1 1 10 100

High CLow and moderate C

Th (ppm)

Upper Crust

Depleted Mantle sources(arcs)

Weathering trend

PAAS

AS

U gain

Th/U

Fig. 8. Th–Th/U plot (McLennan et al., 1993) for Paja samples (ICP-MSdata from Table 2). Dashed lines: Th/U ratio and Th content of UCC;star: PAAS (Taylor and McLennan, 1985); fields for depleted mantlesources and Australian shales (AS) from McLennan et al. (1993). Arrowsindicate direction of trends for weathering (U loss) and enrichment (Ugain).

100


Lower Paja Grp 2

Lower Paja Grp 1

Middle & Upper Paja Grp 3

Marrawahni (G949)

Roraima average (SPR-59; EC-1066)

10sam

ple/

chon

drite

Fig. 9. Chondrite-normalized plot of average REE abundances in blackshales from the Paja Formation compared with intracratonic sedimentary/metasedimentary rocks from Guyana Shield (Roraima and Marrawahniformations). Chondrite values from Taylor and McLennan (1985), datafor the intracratonic sediments from Gibbs et al. (1986). Gadolinium wasnot reported by Gibbs et al. and therefore was calculated using theformula GdN # $SmN ' Tb2

N&1=3 (Condie, 1993).


LREE fractionation, with LaN/SmN ratios of 3.6 and 3.4(Gibbs et al., 1986).

Fig. 9 compares chondrite-normalized plots of the aver-age REE abundances in each subgroup of the Paja Forma-tion with the patterns from the Roraima and Marrawahniformations. Average REE patterns of the more fraction-ated samples from Paja Groups 1 and 3 are similar to theRoraima Formation average (Fig. 9), particularly in LREEand Eu/Eu*, suggesting the latter could be a suitablesource. These two Paja groups have the same averageEu/Eu* (0.63), almost identical to the Roraima average(0.62), but somewhat greater

PREE contents (180.73,

244.7, respectively) than the Roraima average (160 ppm)and slightly more fractionated LREE (LaN/SmN 3.87,5.36). HREE fractionation is more marked in the Pajashales (GdN/YbN 1.53 and 1.78, respectively) relative tothe Roraima Formation (GdN/YbN 1.16). LaN/YbN ratiosare also greater (average 8.41 and 15.35) than the Roraimaaverage (LaN/YbN 6.15). These features suggest thatthough the Roraima Formation is a possible source forthe Paja shales, an additional and more fractionated com-ponent must also have been present.

In contrast, the average REE pattern of the lower Pajasamples with flat REE distributions (Group 2) is similarto Marrawahni sample G949 (Fig. 9), with comparableP

REE content (103 and 89 ppm), LaN/SmN (1.54 and1.80), and GdN/YbN (1.06 and 1.27). However, these sam-ples are distinguished by more significant negative Euanomalies (average Eu/Eu* = 0.69) than that in G949(0.84). Although the Group 2 shales could have beenderived from a relatively mafic source similar in composi-tion to the Marrawahni Formation, their larger Eu anom-alies are problematic. Moreover, the Group 2 shales havemulti-element patterns similar to the UCC-like Group 1shales with which they are interbedded (Fig. 6), implyingthat their source composition did not di!er greatly.

Diagenetic redistribution may be responsible for thecontrasting REE patterns of the Group 2 shales comparedwith Group 1. Although REE generally are believed to betransferred quantitatively from source to sediment(McLennan, 1989), recent studies show that in sedimentaryrocks, significant diagenetic redistribution can occur subse-quently (Milodowski and Zalasiewicz, 1991; Cullers et al.,1997; Hannigan and Basu, 1998; Lev et al., 1999; Kidderet al., 2003; Bock et al., 2004). Dissolution and reprecipita-tion of phosphate minerals, including apatite and monazite,are factors, and both LREE and MREE may be lost at ahand-specimen scale. Although there is no clear relationshipof mineralogy, bulk chemistry, and the occurrence of theGroup 2 samples, the wide variation of P2O5 observed inthe Paja suite (0.02–0.91 wt%) suggests this element has beenmobile. The range in Th/U observed in the shales; markedCr,V, andNi enrichment; andpositiveEu anomaly observedin the analyzed sandstone also suggest that other elementswere mobile. It is thus quite likely that the flat Group 2 pat-terns were produced by preferential loss of LREE ratherthan derivation from a more mafic source.

Diagenetic redistribution may also account for theanomalous concave MREE-depleted patterns of samplesPES25 and PES30 (Fig. 5d). These are strikingly similarto those reported by Cullers et al. (1997) from the low-REE Zones 2 and 3 of the Perry Mountain Formation ofwestern Maine, which were interpreted to reflect significantLREE and HREE loss during diagenesis. Group 2 patternsresemble the less depleted patterns of Perry Mountain low-REE Zone 1, whereas the UCC-like Groups 1 and 3 pat-terns compare with those from high-REE Perry Mountainsamples, inferred to have retained their original granitoidsource signatures. Cullers et al. (1997) conclude that sam-ples collected at wide spacings (0.1 km+) could produce avariety of REE patterns, ranging from highly modified tooriginal, as observed here. It is thus likely that the twoanomalous REE patterns and those of Group 2 are diage-netic in origin and do not fully reflect their provenance.This explanation also is probably the cause of the anoma-lous positive Eu anomaly in the single sandstone analyzed,the REE pattern of which otherwise closely matches that ofUCC (Fig. 5c). Eu enrichment is normally interpreted asthe result of plagioclase enrichment during sedimentarysorting (McLennan, 1989; McLennan et al., 1993), thepresence of mafic detritus, or a concentration of Eu-richphases such as epidote. However, the overall geochemicalcomposition of this sample, XRD analysis (Campos,2003), and petrography do not support the presence ofany of these components. Diagenetic mobilization andreconcentration of Eu under anoxic conditions (MacRaeet al., 1992) may be the cause of the positive Eu anomaly.Given the presence of variable REE patterns, the Paja For-mation would be a good target for more specific studies ofREE redistribution.

Th/Sc–Zr/Sc relations (Fig. 10) also suggest a relativelyuniform source within the Paja suite. It is generallyaccepted that Sc and Th are transferred quantitatively fromsource to sediment, and consequently, Th/Sc ratios reflectbulk source compositions (Taylor and McLennan, 1985;McLennan and Taylor, 1991). Scandium is concentratedin mafic components, and variations in the Th/Sc ratiosreflect magmatic di!erentiation. Zirconium is mostly con-centrated in zircons, which accumulate during sedimenta-tion while less resistant phases are preferentiallydestroyed. The Zr/Sc ratio therefore can be used as a tracerfor zircon or heavy mineral concentration (Taylor andMcLennan, 1985; McLennan, 1989). In first-cycle sedi-ments, Th/Sc ratios show an overall positive correlationwith Zr/Sc, depending on the nature of the source rock,whereas Zr/Sc ratios in mature or recycled sediments dis-play considerable variation with little accompanyingchange in Th/Sc (McLennan et al., 1993).

Th/Sc ratios of the Paja Formation indicate a highlyfractionated and uniform source with composition closeto UCC and PAAS, irrespective of stratigraphic positionand REE pattern grouping (Fig. 10a). Zr/Sc ratios showsignificant variation, but the general Th/Sc–Zr/Sc trend isconsistent with zircon concentration during recycling of


older sedimentary components (McLennan et al., 1993).Th/Sc ratios in the Paja samples are also comparable withthe values in the two Roraima samples (0.70, 0.89; Gibbset al., 1986) and di!er from the lower ratio (0.22) of themore mafic G949 from the Marrawahni Formation. Thesefeatures further suggest that the flat REE patterns ofGroup 2 Paja shales are not due to the presence of a maficcomponent.

All Paja samples but sandstone PES16 (Eu/Eu* = 1.15)have Eu/Eu* ratios less than 0.85, a value characteristicof sediments recycled from old upper continental crust(McLennan and Taylor, 1991), and most are less than 0.7(Fig. 11a). Most Paja samples have GdN/YbN ratios of lessthan 1.0–2.0, indicating relatively flat HREE patterns. TheGdN/YbN of samples PES3, 24, and 28 are greater than 2.High ratios are characteristic of rocks with steep REE pat-

terns, as commonly found in Archean Na-rich volcanic/granitic rocks (McLennan and Taylor, 1991). Therefore,rocks of this type may represent another likely source,directly exposed to erosion or through sedimentary recy-cling, which consequently produced the steeper REE pat-terns observed in some samples. Despite the variabilityobserved within the Paja Formation, the overall REE dis-tributions are typical of sediments of cratonic derivation(Taylor and McLennan, 1985; McLennan and Taylor,1991).

Proposed models for the evolution of the Eastern Cor-dillera during the Cretaceous generally agree that theTablazo-Magdalena and Cocuy basins were separated bythe Santander paleohigh until the Aptian (Fig. 1c; Collettaet al., 1990; Cooper et al., 1995; Villamil, 1999). Hence, theSantander High could represent another potential sourceof sediments, especially considering clasts from the Jurassicsedimentary rocks have been found within the TamborFormation (Ramon et al., 2001). The Tambor Formationis the equivalent of the Los Santos Formation, whichunderlies the Cumbre Formation in the western flank ofthe Eastern Cordillera (Fig. 1b). It is also time-equivalentto the Cumbre Formation when the stratigraphy of themiddle Magdalena Valley is compared with that of the wes-tern flank of the Eastern Cordillera at a regional scale

0.01

0.1

1

10

1 10 100

Lower (PES16 sst)Upper (PES25)

Middle-Upper (Grp 3)Lower (Grp 2)Lower (Grp 1)

Lower (PES30)

UCC

PAAS

PES25

PES30

Roraima

G

0.01

0.1

1

10

1 10 100

TE sandTE mud

CA sandCA mud

FA sandFA mud

Zr/Sc

F

B

UCC

PAAS

G

Th/

Sc

Th/

Sc

Fig. 10. Zr/Sc–Th/Sc plots (McLennan et al., 1993) for (a) PajaFormation; (b) modern muds from trailing edge (TE, passive margin),continental arc (CA), and forearc (FA) settings (data from McLennanet al., 1990). Solid line connecting stars B (basalt) and F (felsic volcanicrock), illustrates the trend expected in first-cycle sediments due tomagmatic evolution from mafic to felsic end members; star G isaveragegranite (Condie, 1993). UCC and PAAS from Taylor and McLennan(1985). Arrow illustrates the trend produced by zircon concentrationduring sedimentary sorting and recycling. Shaded zone in (a) is the rangein ratios in two Roraima Formation cratonic shales (Gibbs et al., 1986).

0.4

0.6

0.8

1.0

1.2

1.4

0 1 2 3 4

TE sandTE mudCA sandCA mud

FA sandFA mud

Eu/Eu*=0.85

Modern turbidites

0.4

0.6

0.8

1.0

1.2

1.4

0 1 2 3 4

Mid-Upper (Grp 3)Lower (Grp2)Lower (Grp1)

Lower sst PES16Upper PES25Lower PES30Cratonic shales

Eu/Eu*=0.85

PES16

Eu/

Eu*

Eu/

Eu*

Gd /YbN N

Fig. 11. GdN/YbN – Eu/Eu* plots for (a) Paja Formation, (b) modernmuds from trailing edge (TE, passive margin), continental arc (CA), andforearc (FA) settings (data from McLennan et al., 1990).


(Cooper et al., 1995). However, considering the very highCIA and PIA ratios, highly fractionated REE patterns inGroups 1 and 3, high Zr/Sc ratios, and GdN/YbN–Eu/Eu* relations, the results overall suggest that the GuyanaShield was the main source of the Paja sediments.

5.3. Tectonic setting

Several geochemical discriminants and diagrams con-structed to identify the tectonic setting of deposition of sed-iments (especially sandstones) have been applied in theliterature (e.g., Bhatia, 1983; Bhatia and Crook, 1986;Roser and Korsch, 1986). They divide sediments into threeor four categories (e.g., oceanic island arc, continentalisland arc, active continental margin, passive margin; Bha-tia and Crook, 1986) on the basis of bulk geochemistrycontrasts. Passive margin sediments are those derived fromstable continental areas and deposited in plate interiors,Atlantic-type continental margins, failed rifts or grabens,or general depocenters along continental edges (Bhatiaand Crook, 1986; Roser and Korsch, 1986). Active marginsediments are deposited at subduction arc margins, strike-slip margins and in proximal positions in backarc basins.

The application of discriminant schemes based on majorelements is limited in the Paja shales because of dilutione!ects, the possibility of diagenetic elemental redistribu-tion, and fine grain sizes, because chemical contrastsbetween mudrocks of di!erent settings are smaller thanthose between sandstones. However, SiO2–K2O/Na2O rela-tions in the Paja shales at least suggest their tectonic settingof deposition. K2O/Na2O ratios are typically high (>3),and almost all samples plot in the passive margin field(Fig. 12) of Roser and Korsch (1986). Although K2O/Na2O ratios in some samples have almost certainly beenincreased by post-depositional K-metasomatism (Fig. 7),

high K2O/Na2O ratios (generally >1) are typical of passivemargin muds (Roser and Korsch, 1986) and are character-istic of highly mature recycled sediments.

An evaluation of immobile trace element ratios alsofavors a passive margin setting for the Paja shales.Th/Sc–Zr/Sc relations in sands and muds from modernturbidites of known tectonic setting (McLennan et al.,1990) show some contrasts between active arc settings(forearc, continental arc) and passive margins (Fig. 10b).Arc sands and muds lie along the source evolution linedefined by igneous rocks, whereas passive margin equiva-lents cut across this trend at high Th/Sc and variable Zr/Sc, as do the Paja shales (Fig. 10a). A similar distinctioncan be made between arc and passive margin sedimentson the GdN/YbN–Eu/Eu* plot (Fig. 11b). Although the dis-tributions overlap in the GdN/YbN (1–1.4) and Eu/Eu*

(0.65–0.75) regions, arc sediments have much more vari-able and smaller Eu anomalies and lower GdN/YbN ratios.Distribution of the Paja samples in Fig. 11a is comparableto that of passive margin sediments.

The fractionated LREE-enriched patterns of PajaGroups 1 and 3 shales, inferred as una!ected by diagenesis,are typical of passive margin muds. Although the data formodern muds of known setting (McLennan et al., 1990) islimited, average LaN/YbN and LaN/SmN ratios and Euanomalies increase from forearc through continental arcto passive margin settings (Table 3). The Paja Groups 1and 3 averages for all three parameters compare withpassive margin muds (Table 3). Similar trends occur inthe La/Sc and La/Y ratios, which increase from forearcto passive margin in modern muds, and the Paja averagesare again compatible with passive margin deposition(Table 3), as is also true for the Ce/Sc ratios.

Collectively, these results indicate that the Paja shaleswere deposited in a passive margin setting, in either a riftor backarc environment, but the Paja depocenter receiveddetritus primarily from continental sources and thus islikely to have been near the cratonic margin (Fig. 13).No arc geochemical signatures, such as low or intermediateTh/Sc or Zr/Sc ratios, are evident. Within this frameworkand considering the complex tectonic evolution duringthe lower Cretaceous in Colombia, the results overall sug-gest Paja Formation shales are highly mature sediments,largely recycled from deeply weathered older sedimentary

0.1

1

10

100

50 60 70 80 90 100

SiO2 wt%

ARC ACM

PM

KO

/Na

O2

2

Fig. 12. SiO2–K2O/Na2O tectonic setting discriminant plot for the PajaFormation suite. Field boundaries after Roser and Korsch (1986): PM,passive margin; ACM, active continental margin; ARC, oceanic islandarc. Symbols as in Fig. 11a.

Table 3Comparison of average ratios in Groups 1 and 3 Paja Formation shaleswith averages in modern trailing edge (TE, passive margin), continentalarc (CA), and forearc (FA) muds (data from McLennan et al., 1990)

Ratio Group 1 Group 3 TE mud CA mud FA mud

LaN/YbN 8.41 15.35 10.02 6.26 3.19LaN/SmN 3.87 5.36 3.61 2.75 1.96Eu/Eu* 0.63 0.63 0.68 0.74 0.88

La/Sc 2.2 4.3 3.6 1.7 0.6La/Y 1.5 3.1 1.3 1.0 0.5Ce/Sc 3.8 6.8 7.3 3.2 1.1


or metasedimentary rocks (e.g., the Guyana Shield), asproposed in previous work (e.g., Julivert, 1968; Cooperet al., 1995) and depicted in Fig. 13.

6. Conclusions

Black shales from the Paja Formation of Colombia havegeochemical compositions broadly similar to upper conti-nental crust but are strongly enriched in V, Cr, and Niand markedly depleted in Ca and Na. Enrichment in Cr,V, and Ni is associated with high organic carbon contents,whereas low Na and Ca contents reflect intense sourceweathering, as shown by CIA and PIA values greater thanthose of PAAS. The LREE-enriched REE patterns withsignificant negative Eu anomalies (Groups 1 and 3) suggestderivation from a felsic upper continental crust source.Anomalous flat REE patterns in some samples from thelower part of the succession (Group 2) and strongly con-cave MREE-depleted patterns in two other samples arelikely of diagenetic origin. A uniform felsic source through-out is supported by consistent immobile element ratios inall samples. The REE patterns and ratios of Groups 1and 3 shales compare well with intracratonic sedimentsfrom the Roraima Formation of the Guyana Shield, sug-gesting derivation from a similar source. Discriminantratios and comparison with modern muds of known tec-tonic setting indicate the Paja shales were deposited at apassive margin rather than adjacent to an arc. They thusrepresent highly mature sediments recycled from tectoni-cally stable, deeply weathered, older sedimentary ormetasedimentary rocks, possibly the Guyana Shield.

Acknowledgements

This work represents part of an M.Sc. study byN.O.C.A. at Shimane University, sponsored by a scholar-ship from the Inter-American Development Bank. All anal-yses were undertaken in the Department of Geoscience,Shimane University, supported by grants to B.P. Roser.The authors thank Jun-Ichi Kimura for ICP-MS determi-nations and Yoshihiro Sawada for access to the XRF.Samples were collected during a field excursion organizedby Dr. L.C. Mantilla of the School of Geology, Universi-dad Industrial de Santander (UIS), Colombia, to whomwe are indebted.

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