BasinResearch Trench-forearcinteractionsreflectedinthe

Trench-forearc interactions reflected in thesedimentary fill of Talara basin, northwest PeruAndrea Fildani,1 Angela M. Hessler and Stephan A. Graham

Department of Geological and Environmental Sciences, Stanford University, Stanford, CA, USA

ABSTRACT

Exceptional exposure of the forearc region of NW Peru o¡ers insight into evolving convergentmargins.The sedimentary ¢ll of theTalara basin spans the Cretaceous to the Eocene for an overallthickness of 9000m and records within its stratigraphy the complicated history of plate interactions,subduction tectonics, terrane accretion, and Andean orogeny. By the earlyTertiary, extensionaltectonismwas forming a complex horst and graben system that partitioned the basin into a series oflocalized depocentres. Eocene strata record temporal transitions from deltaic and £uvial to deep-water depositional environments as a response to abrupt, tectonically controlled relative sea-levelchanges across those depocentres. Stratigraphic and provenance data suggest a direct relationshipbetween sedimentary packaging and regional tectonics, marked by changes in source terranes atmajor unconformities. A sharp shift is recognized at the onset of deepwater (bathyal) sedimentationof the Talara Formation, whose sediments re£ect an increased in£ux of ma¢c material to the basin,likely related to the arc region. Although the modern topography of the AmotapeMountains partiallyisolates theTalara basin from the Lancones basin and the Andean Cordillera to the east, provenancedata suggest that the AmotapeMountains were not always an obstacle for Cordilleran sedimentdispersal.The mountain belt intermittently isolated theTalara basin fromAndean-related sedimentthroughout the early Tertiary, allowing arc-related sediment to reach the basin only during periods ofsubsidence in the forearc region, probably related to plate rearrangement and/or seamounts collidingwith the trench. Intraplate coupling and/or partial locking of subduction plates could be among themajor causes behind shifts from contraction to extension (and enhanced subduction erosion) in theforearc region. Eventually, collisional tectonic and terrane accretion along the Ecuadorian marginforced a major late-Eocene change in sediment dispersal.

INTRODUCTION

Forearc basins record compelling evidence about the age,nature and intensity of subduction-related tectonicevents (Dickinson, 1995). Forearc basin sedimentary ¢llshave low preservation potential because many active mar-gins involve accretionary phases with major shorteningand/or destructive phases (i.e. subduction erosion). TheMesozoic forearc is missing in most of southern Peru,and as much as 148 km in width of the South Americanmargin has been lost since the Eocene at the Lima basinlatitude because of subduction erosion (Clift et al., 2003).However, in NW Peru, approximately 9000m of near-continuous forearc sedimentation preserved in theTalara basin (Figs 1 and 2) records events occurring ata dynamic Andean juncture: between (1) active ridgesubduction (Witt et al., 2006) and terrane accretion

(Spikings et al., 2005) to the north and (2) an area of activesubduction erosion to the south (Lima basin: Clift et al.,2003).

The Peru-Chile arc-trench system extends for morethan 5000 km along the western margin of SouthAmerica,where subduction of the oceanic Nazca plate at a rate ofabout 6^7 cmyear�1 has formed a series of forearc basins(Clift etal., 2003;Witt etal., 2006;Figs1and 2).Despite con-vergence across the interface between the South Americaand Nazca plates, extensional events are common alongthe Peruvian and Ecuadorian margins (Clift et al., 2003;Sage et al., 2006; Witt et al., 2006). In the region of Talara,a series of extensional events accommodated nearly con-tinuous sedimentary in- ¢lling from the Late Cretaceousto the Neogene (Jaillard et al., 2000) (Fig. 3), with subsi-dence controlled by normal faulting and large-scale de-tachment faults. Horst and graben structures fosteredthick sediment accumulation and abrupt lateral facies var-iations, with loci of deep-water sedimentation developingin the middle and late Eocene (Fildani, 2004; Duerichen,2005). Basin inversion began in the late Oligocene (Ser-anne, 1987) and continued through the Neogene (Nobletet al., 1996). Ridge collision and partial subduction (well

Correspondence: Andrea Fildani,Department ofGeological andEnvironmental Sciences, Stanford University, Stanford, CA94305, USA. E-mail: andrea¢[email protected] Present address: Chevron Energy and Technology Company,6001 Bollinger Canyon Rd., Rm. D1192, San Ramon, CA 94583,USA.

BasinResearch (2008) 20, 305–331, doi: 10.1111/j.1365-2117.2007.00346.x

r 2008 The Authors. Journal compilation r 2008 Blackwell Publishing Ltd 305

documented to the north of the study area, i.e. Carnegieridge; Fig.1) is presumed responsible for the uplift and ex-humation of most of theEcuador and northernPeru coast-line (Gutscher et al., 1999a; Pedoja et al., 2006; Witt et al.,2006).

This paper focuses on the sedimentary record in theonshore part of the basin, where the sedimentary ¢ll con-sists of 60^70% shale, with the balance being sandstoneand conglomerate.The purpose of this study is three-fold:(1) to re-evaluate the stratigraphic record of theTalara ba-sin based on evolving depositional environments, (2) todetermine the provenance of distinctive sedimentologicalcycles of the basin and (3) to evaluate large-scale tectonicimplications for the northern Central Andes.We use stra-tigraphic stacking patterns, sediment dispersal patterns,sandstone petrography, mudstone geochemistry, and con-glomerate clast data to relate local forearc development tolarge-scale subduction processes, such as seamount sub-duction (Gutscher etal.,1999a, b;Witt etal., 2006), subduc-tion erosion (Clift et al., 2003; Laursen & Normark, 2003),accretionary tectonics (Spikings et al., 2005), and changesin plate convergence rate (Pardo-Casas & Molnar, 1987;Silver et al., 1998).

TECTONIC HISTORYOF THEHUANCABAMBA ANDES

The sector of the Andes inland of theTalara area, known asthe Huancabamba Andes or the Cordillera Real, stretchesbetween 31S and 81S in southern Ecuador and northernPeru, and represents the connection between the North-ern and the Central Andes (Figs 1 and 2). A distinguishingfeature here is the directional variation in the Andeantrend known as the Huancabamba de£ection (Mourieret al., 1988), from N 201W in the northern part of theCentral Andes to N 201E in the Northern Andes (Fig. 1).The Huancabamba de£ection is a major tectonic linea-ment that separates two intrinsically di¡erent sectors ofthe main Andean Cordillera: the Palaeozoic metamorphicbelts of the Cordillera Real intruded by Mesozoic(?) plu-tons and the PeruvianWesternCordillera (CordilleraOcci-dental, Fig. 1) composed of Mesozoic sediments intrudedby a Mesozoic arc (the Coastal batholiths) with youngerTertiary volcanics (Chew et al., 2007).

Early Senonian onset of contractional deformation andcalc-alkaline arc activity in the Huancabamban Andesis interpreted as the initiation of Farallon-Nazca plate

Tumbes-Guayana Megashear

Huancabamba DeflectionAmazonas Megashear

Axis Peru - Chile Trench

Dolores-Guayaquil Megashear

Cord. O

ccidental

Cord. C

entralC

ord. Oriental

Cord.

Orie

ntalC

ord.

Cen

tral

Cor

d. O

ccid

enta

l

C. R

eal

LIMA

QUITO

BOGOTA

C. C

oast

al

0 500

km

N

PanamaBasin

SouthAmerica

Pacific O

cean Atla

ntic

Oce

an

Carnegie

Ridge

Ocala R.

Sarmiento R.

75°80°

10° S

5° S

5° N

80° 75° 70°

0°

5° S

5° N

0°

TG.F.Z.

6-7 cm/yr

Mendana F.F.

Gulf of Guayaquil

Fig. 2

Fig.1. Distribution of major tectoniclineaments of the northern Andes, andof the Peru^ChileTrenchwithintersecting seamount chains. LocationofTalara basin is in grey andmarkedwithaT, G.F.Z., Grijalba Fracture Zone.C. Real, Cordillera Real.The NazcaRidge location is reported in the insetmap (map modi¢ed from Shepherd &Moberly, 1981).

r 2008 The Authors. Journal compilation r 2008 Blackwell Publishing Ltd,Basin Research, 20, 305^331306

A. Fildaniet al.

subduction (Benavides-Caceres, 1999) following westwarddrift of the South American plate as a consequence of theSouthAtlanticOcean opening since theAlbian (Nurnberg&Muller,1991).Changing tectonic styles along theAndeanmargin since the Senonian are de¢ned by major tectonicphases with not well-de¢ned timing (Fig. 4). In the Peru-vian Andes these tectonic phases have been linked to:(1) changes in convergence rates between the overridingSouth American plate and the Farallon-Nazca plate(Jaillard et al., 1995), (2) successive clockwise rotations inthe direction of convergence (Pardo-Casas & Molnar,1987; Mayes et al., 1990) and/or (3) absolute plate motion(Silver et al., 1998). It is arguable that tectonic phasesmay span only a few million years per event, and perhapsthese phases are better described by a continuum of

convergence interrupted by periods of acceleration(Noblet et al., 1996).

Presently, £at slab subduction and a volcanic gap are re-ported in the Andean portion inboard of theTalara basin(Bernal et al., 2002; Lamb&Davis, 2003).

TECTONIC SETTING AND GEOLOGYOFTALARA BASIN

TheTalara basin sits immediately eastward of the Nazca-South American plate boundary (Fig. 2).To the north, the

Tertiary Arc

TamarindoHigh

Silla dePaita

Cretaceous arc

Jurassic arc

Progreso basin ?

?

Talarabasin

Lanc

ones

basin

Cerro deIllesca

100 km

Trench

DGFS

Cordillera uplift

Basement highs(i.e. Amotape Mts.)

?

N

South America

Pacific O

cean Atlant

ic O

cean

6° S

4° S

82° 80°

Sechura basin

Cor

dille

ra

Amotape-TahuinBlock

Amot

ape

Mts.

Tumbes B

.

Puna IslandJambelibasin

PacificOcean

?

?

G.F.Z.G.G.

R.G.

Fig. 2. Sketch map of the NW Peru Cordillera Real and locationof major basins and tectonic lineaments.Talara and Tumbesbasins extent highlightedwith dashed lines. N.F., Naranjo Fault;R.G., Raspas Group; G.G., Gulf of Guayaquil; P.deZ., Pillars deZorritos; H.F., Huayapira Fault; DGFS, Dolores^GuayaquilFault System (map modi¢ed fromMourier et al., 1988; Jaillardet al., 1999; Fildani et al., 2005; Spikings et al., 2005;Witt et al.,2006; Bourgois et al., 2007).

MesaPA

LE

OG

EN

E

EO

CE

NE

PALE

OC.

Lowe

rBasal Salina Fm. (Ss.)

Balcones Fm. (Sh.)

Mesa Fm. (Ss.)

Mogollon Fm.

Ostrea Fm.

Talara Group

Echinocyamus(Echino) Fm.

Pozo Sh.Verdun Fm. (Ss.)

Chira Fm.

?

Mid

dle

Formations

Mirador Fm.

Carpitas Fm. (Sh.)Mancora Fm.

Uppe

r

OLIG

.

Lowe

r

Petacas Fm. (Sh.)Ancha Fm. (Cong.)

Copa Sombrero Fm. (Sh.)

Amotape Fm. (Met.)

CR

ET

A.

Low.

Uppe

r

Monte Grande Fm. (Sh.)

Pananga Fm. (Carb.)Muerto Fm. (Ml.)

Redondo Fm. (Sh.)

Heath Fm.

PaleGreda Fm.

Salina Group

PaleozoicD

eta

ils in

Fig

. 6S

echu

ra b

. Tum

bes

b.

Fig. 3. Simpli¢ed stratigraphic column of Talara basin.Thisstudy focused on the Eocene section (modi¢ed fromMarsaglia &Carozzi, 1991). Formations present in Sechura and/orTumbesbasins are noted to the right. Sh., Shale; SS., Sandstone; Cong.,Conglomerate;Ml.,Marls; Met.,Metamorphic.

r 2008 The Authors. Journal compilation r 2008 Blackwell Publishing Ltd,Basin Research, 20, 305^331 307

TheTalara Forearc basin

Peru-Ecuador trench is dissected by a major tectoniclineament trending SW^NE that continues inland as theDolores^Guayaquil Fault System (DGFS in Fig. 2). TheDGFS represents a fundamental break in crustal struc-ture along the South American margin (Shepherd & Mo-berly, 1981).The basement of western Ecuador, west of thefault system, may be an accreted fragment of an overthick-ened and buoyant oceanic plateau of Upper Cretaceous toLower Palaeocene age (Reynaud et al., 1999), whereas thebasement south and east of the fault system has continen-tal a⁄nities (Shepherd & Moberly, 1981; Spikings et al.,2005;Witt etal., 2006).TheTalara basin mostly overlies this‘continental-type’ crust based on gravimetric studies(Lonsdale, 1978).

The DGFS delineates the southern end of the NorthAndean Block whose northward movement and subse-quent extensional strain is the likely cause of detachmentfaulting and subsidence in the Gulf of Guayaquil (Fig. 2;Witt etal., 2006). Recent (o12Myr) collision of the Carne-gie Ridge with the trench caused interplate coupling,northward movement of the North Andean Block, and es-cape tectonics in the Gulf of Guayaquil in the last 2Myr(Witt et al., 2006).This has given rise to a tectonic mosaic

of at least four di¡erent detachment-bound basins in theGulf of Guayaquil: Progreso, Jambeli, Esperanza andTumbes. (Witt et al., 2006; Fig. 2).

Nowadays, the Lancones basin is separated from theTalara basin to the east by the Amotape Mountains, atopographic expression of the much larger metamorphicAmotape Tahuin block (Fig. 2; Mourier et al., 1988; Spik-ings et al., 2005).TheTamarindo High is presently coveredby sediments and it is the southern continuation of theAmotape Tahuin block and part of a larger block togetherwith the Silla de Paita and the Cerro de Illescas (Fig. 2),which currently isolates theTalara basin from the Sechurabasin to the south.To the north, theTalara basin appears tobe separated from theTumbes basin by the ‘Pillars deZorri-tos’, a series of enigmatic granitic intrusions encountered indrilling (Bush etal.,1994).The present-day o¡shorewesternand southwestern margins remain conjectural. Active de-tachment faults and active subduction erosion for this mar-gin suggest that theTalara basin could have been partiallydestroyed to the west (Bourgois et al., 2007).

However, theTalara basin was not always isolated fromthese nearbybasins. Intermittent activity along theHuaypiraand other normal faults in the southern Amotape Tahuin

MogollonInferior

MogollonMedio

MogollonSuperior

Ostrea E

Ostrea D

Ostrea C

Pena Negra

TerebratulaLobitos

Talara SS

Cabo Blanco

Helico

Verdun

Marginal Marine(nearshore to offshore)

Outer Shelf toSlope

Deltaic

Fluvial

Marginal Marine

Marginal Marine

Bathyal (turbidites)

Bathyal

Marginal Marine(foreshore to shelf)

Bathyal to Middle Bathyal

Shallow Marine

Marginal Marine ?

Bathyal (slope to basin facies)

Ypresian

Lutetian

Late

Early

Bartonian

FormationAGE

Low

erM

iddl

ePriabonian

TalaraShales

ChiraU

pper

Echino-cyamus

Pozo

Mogollon

Ostrea/Pale Greda

Shallow Marine

Marginal Marine toBrackish??

Talara Br.

?

EO

CE

NE

Change in Convergence Direction

High Subsidence Rate

Partial AmotapeUplift ?

Amotape Uplift(N Talara basin

uplift)

Tectonic Events

Deep Shallow

VerdeBallenaSomatito

Clavel

Major subsidencephase NW Peru

(Talara and Sechura)

Talara BasinPaleobathymetry

1000 m 200 m 0 m

Member Sedimentary Env.

PB

Mac

uchi

Isla

nd A

rcC

ollis

ion

~43

Ma

?H

igh

Pla

te C

onve

rgen

ce R

ate

High Accommodation?

Change inConvergence Direction

End of Inca I Phase(Jaillard et al., 2000)

Fig.4. Detailed stratigraphic section for Eocene strata of Talara basinwith sedimentary environment interpretation for members andformations (this study) and their relation with major tectonic events (from this study, Benavides-Caceres, 1999; Jaillard et al., 2000;Spikings et al., 2005). Chronostratigraphy is based on data fromTable1 (based on Berggren et al., 1995).The reconstructed palaeobathy-metric curve (PB on Fig. 4, data reported onTable1) is superimposed over the global eustatic sea-level curve (in grey) for comparison(sea-level curve fromHardenbol et al., 1998).


A. Fildaniet al.

block (Fig. 2) may have controlled interactions betweentheTalara, Lancones, and Sechura basins (Fildani, 2004).The Lancones basin is ¢lledwith Cretaceous sedimentaryrocks unconformably overlain by upper Eocene strata de-scribed as coeval with theVerdun andChira Formations ofthe Talara region (Valencia & Uyen, 2002), con¢rming aconnection between the two basins during the late Eocene.The Verdun and Chira formations of theTalara basin ex-tend into the Sechura basin (Fig. 3), but the late Oligoceneto Miocene sequences of the Sechura basin do not extendto theTalara basin (Caldas etal., 1980), suggesting completeseparation of these two basins by the lateOligocene.Northof theTalara basin, theTumbes basin contains a thick accu-mulation of Oligocene and Miocene strata exposed alongthe southern edge of the Gulf of Guayaquil (Fig. 2). TheOligocene Mancora Formation ¢lled a depocentre thatstraddled theTumbes and Talara basins.The Progreso Ba-sin, an extensional or pull-apart basin related to activityalong the DGFS (Fig. 2), was contiguous to the Tumbesbasin in the Miocene, but did not likely interact with theTalara basin at any point in its history.

Upper Oligocene andMiocene rocks are missing acrossmost of the central and southernTalara basin, where Plio-cene strata rest unconformably on Eocene strata.

METHODS

The present investigation uses data derived from outcrop-based sedimentary facies and biofacies analyses to build apalaeobathymetric history of the basin; sandstone composi-tion data from modal petrologic analyses; conglomeratecomposition data from clast counts and inorganic geochem-ical analyses of shale and single clasts from conglomerate.

The stratigraphic framework presented here is one re-¢ned from Fildani (2004), Fildani et al. (2005), and Dueri-chen (2005). Foraminifera, nannoplankton, palynomorphsand macrofossils were used to constrain timing, palaeo-bathymetry, and palaeoenvironment of key stratigraphiclevels in the basin (Fildani, 2004). Because of a lack of pub-lished palaeobathymetric zonations for the PalaeogenePeruvian sections, we used the California palaeobathy-metric model of Ingle (1980) with modi¢cations by Ingleet al. (1980).The data, summarized inTable 1, are includedin appendix in Fildani (2004).

Modal composition for each of the 38 medium-grainedsandstone samples was determined using the Gazzi-Dick-inson point-counting method (Dickinson, 1970, 1985; In-gersoll et al., 1984) (Table 2). More than 500-point countswere performed on each thin section by one operator (A.Fildani). Potassium feldspars were stained to facilitatetheir identi¢cation.

Forty shale samples from representative formationsthroughout the Eocene^Oligocene section, were analysedfor major-, trace- and rare-earth elements (REE) with X-ray £uorescence spectrography (XRF) and inductivelycoupled plasma mass spectrometry (ICP-MS) atWashing-ton State University GeoAnalytical Laboratories (Supple-

mentary materials Tables S1 and S2). Conglomerate clastcounts were performed on a regular outcrop 10� 10 cmgrid at eight locations (100 clasts per locations). Clast-count categories included: plutonic clasts (mainly grani-toids), volcanic, white quartzite, black quartzite, sedimen-tary clasts (siltstone, shale and sandstone).

STRATIGRAPHYOF THE TALARA BASIN

The general stratigraphy of theTalara basin to date has de-veloped out of both academic (Bosworth, 1922; Iddings &Olsson, 1928; Travis, 1953; Bellido, 1969; Gonzales, 1976;Marsaglia & Carozzi, 1991; Carozzi & Palomino, 1993; Fil-dani, 2004) and proprietary petroleum-related study.Theoldest outcrops of theTertiary deposits, the Lower EoceneMogollon Formation, sit unconformably on pre-Cenozoicmetasedimentary and granitoid rocks of the Amotapeblock. Upsection, there is fairly continuous marginal mar-ine deposition of the Ostrea/Pale Greda, Echinocyamus,andTerebratula Formations, punctuated by minor uncon-formities and £uctuating (but shallow) water depths (Fig.4). By the middle Eocene, a signi¢cant unconformity andmarine £ooding event separates theTerebratula from theTalara Formation, and the basin remains a deepwater de-pocentre through the middle Eocene Verdun Formation.These formations and unconformities are detailed in thefollowing section.

Consensus on stratigraphic nomenclature and correla-tion in theTalara basin has been hindered by (1) structuralpartitioning due to horst and graben subsidence through-out the earlyTertiary, (2) abrupt lateral facies changes dueto the nature of marginal marine depositional systems and(3) the �20 000 km2 blanket of Pliocene to Quaternarymarine terraces or ‘tablazos’ (DeVries, 1988; Bourgoiset al., 2007) that obscure the Eocene section, except whereincised by £ash- £ood quebradas (Fig. 5). Based on allavailable information, however, the Talara basin can besub-divided into southern and northern palaeogeo-graphic domains. Age control between the two domains ispoor, but Seranne (1987) proposed a correlation betweenthe northern and southern basin using a proprietary data-base, which has been modi¢ed using newdata fromFilda-ni (2004) (Fig. 6).

Palaeobathymetry for theTalara Basin was constructedfrom facies analysis combined with palaeontological(benthic foraminifera) and palaeoecological (ichnofossilsand/or fossils) observations (Table 1; Fig. 4; Fildani, 2004).This palaeobathymetric curve strictly relates to the north-ern part of the basinwheremajor deep-water sedimentationoccurred and the majority of the data was collected, but itcan tentatively be extended to the southern part of the basin.

DEPOSITIONAL ENVIRONMENTS ANDPALAEO-DISPERSAL PATTERNS

The sedimentary ¢ll of Talara basin consists of threemajor intervals, from bottom to top: (1) 4000^6000m of



Tab

le1.

Micropaleon

tologicaldataforselectedform

ations

ofTalarabasin(m

icrofaun

adescriptionrepo

rted

inFildani(2004))

Form

ations

Benthon

icforam

Calcareou

snann

oplankton

Palyno

logy

Environ

ment

Age

Age

Zon

es(O

kada

&Buk

ry,1980)

ChiraFm.(outcrop)

Probablebathyal

Indeterm

inate

Barren

Verdun

(11/8-2)

Middle/lowbathyal

Indeterm

inate

Barren

Verdun

(11/22-6)

Middle/lowbathyal

Indeterm

inate

Barren

Pozo

(11/27-11)

Bathyal

Indeterm

inate

Indeterm

inate

PZ1-11/6/01(PozoSh

.)Bathyal(/)

Indeterm

inate

Barren

Pozo

(Well3265)

Probablebathyal

MiddleEocene

Barren

Talarass.(11/16-3)

Marine

Indeterm

inate

Barren

B.Talara(11/13-3)

Middle/lowbathyal

B.Talara(11/13-2)

ProbableBathyal

Indeterm

inate

TalaraSh

.(Mon

te-11/27-2)

Middle/lowbathyal

Indeterm

inate

EarlymiddleEocene

CP10-13B

TalaraSh

.(11/20-6)

Bathyal

HelicoMb.(TOO-H

2)Middle/lowbathyal

MiddleEocene

MiddleEocene

ProbableC14A

HelicoMb.(TOO-H

1)Bathyal

MiddleEocene

Indeterm

inate

HelicoM.(TO1-H7wellA

A80)

MiddleBathyal(?)

MiddleEocene

Indeterm

inate

HelicoMb.@

P.LaCruz

MiddleBathyal(?)

MiddleEocene

Indeterm

inate

Lob

itos

(11/23-4)

Middle/lowbathyal

Indeterm

inate

EarlymiddleEocene

CP10-13B

Lob

itos

(11/17-5)

Middle/lowbathyal

Indeterm

inate

Lob

itos

(11/17-3)

Middle/lowbathyal

Indeterm

inate

Lob

itos

(Well1768)

Bathyal

MiddleEocene

MiddleEocene

CP12B-C

P13C

BasalTalara(11/27-1)

Bathyal?

Indeterm

inate

Non

-Marine(Pinacee)

TopEchino@

Ter.(11/22-2)

Indeterm

inate

Indeterm

inate

Indeterm

inate

Echino(11/27-6)

OuterNeritic

Indeterm

inate

Clavel(11/29-1)

Marine(?)

Indeterm

inate

Indeterm

inate

Clavel(11/27-4)

Marginalm

arine

Indeterm

inate

Clavel(WellE

A9296)

Indeterm

inate

Indeterm

inate

Marginalm

arine(Eocene)

Clavel(outcrop)

Indeterm

inate

Indeterm

inate

Indeterm

inate

ProbableEocene-Olig

ocene

PaleGreda

?(11/15-1)

ProbableMiddleBathyal

Lateearly

Eocene

CP10-CP12A

OstreaII(W

ellA

X11)

Marginalm

arine

Indeterm

inate

Barren

Ostrea(W

ellA

A80)

Marignalm

arine

Indeterm

inate

Barren

MogollonMedio(W

ell1944)

Indeterm

inate

Indeterm

inate

Barren

Marginalm

arine(Eocene)


A. Fildaniet al.

Palaeocene to middle Eocene shallow-marine mudstonepunctuated by £uvio-deltaic to shallow-marine coarsesiliciclastic sediments, (2) 1500^3000m of middle to lateEocene deep-water turbiditic sandstone and conglomer-ate units within ¢ne-grained sandstone and mudstone

background and (3) 1500m of latest Eocene to Oligoceneshallow-marine siliciclastic sediments (Marsaglia &Carozzi, 1991; Carozzi & Palomino, 1993; Fildani, 2004;Duerichen, 2005). This study focuses on outcrop datafrom the earliest two intervals.

Table2. Raw point-count data for sandstone from theTalara basin, Peru

No. Sample Location Qm Qp Cht. Kspar Plags Lv Lm Lsheav/opaq

Detritalmicas Interstitial

TotalGr.

Mogollon Fm.1 11/25-2 Well EA1764 144 42 3 19 101 52 71 10 3 6 49 5002 11/25-4 Well EA1764 128 36 2 16 113 57 47 15 3 16 66 5003 11/25-5 Well EA1764 177 56 5 16 69 60 42 17 8 5 45 5004 3/16-1 S 4 31038.700W814055.700 160 23 0 3 105 45 38 3 6 33 84 5005 3/16-2 S 4 31038.700W814055.700 121 33 5 7 97 89 42 5 4 8 89 5006 TM2-00 S 4 320600W814041.200 139 41 1 18 114 51 44 4 5 22 61 5007 TM4-00 S 4 320600W814041.200 188 41 2 15 82 72 34 8 2 1 55 5008 TM10-00 S 4 28024.900W80 59053.200 174 47 0 7 92 62 25 8 10 11 64 5009 TM12-00 S 4 32028.600W81 04046.800 128 19 1 13 101 61 32 3 11 32 90 500

HelicoM.1 T02HR1 Punta Restin 70 16 3 19 77 70 44 19 7 4 153118 5002 T01-PLC2 Punta Restin 102 17 9 34 83 77 25 9 5 10 12217 5003 T01-N2 S 4 2804700W811804000 98 20 10 28 73 124 30 14 5 8 8515 5004 T02PLON9B Punta Lobitos North 99 29 4 23 73 113 23 16 3 3 11014 5005 T02Hrest1 Punta Restin 65 21 6 71 31 100 24 28 1 1 140112 5006 T0192-4217 Punta Restin 95 23 9 29 69 150 53 13 1 4 51 5007 T0192-4123 Punta Restin 88 23 6 23 98 124 31 11 4 4 8414 5008 T01-PLC3 Punta La Cruz 75 29 5 22 71 93 32 10 5 9 133116 500

Verdun Fm.1 11/22-7 S04 32055.000W8111028.20 227 19 5 13 91 13 24 7 4 8 89 5002 11/24-1 S04 08003.100W80 58005.60 214 26 6 10 59 37 33 7 2 14 92 5003 11/24-5 S04 10030.200W80 54 57.100 226 19 2 32 89 30 31 11 2 3 55 5004 11/29-5 Well EA 8003 230 35 5 26 31 43 19 14 6 8 83 5005 11/29-7 Well EA 8004 182 32 6 19 62 50 38 35 2 5 69 5006 11/30-6 S04 10056.600W81 080 00.400 210 32 7 24 46 25 36 12 0 11 97 5007 3/16-3 Quebrada Parinas 215 9 2 16 53 31 34 23 22 50 61 5008 3/20-3 Taime Field 225 40 5 8 64 18 19 12 16 101 500

Echino Fm.1 11/17-1 S04 25022.000W8115053.100 137 27 0 29 72 35 42 7 10 27 114 500

Terebratula Fm.1 11/18-1 S 04 25036.100W8115052.300 152 3 2 37 128 37 29 1 3 4 104 5002 11/18-3a S 04 25036.100W8115052.300 124 20 1 24 70 67 59 3 1 16 115 5003 11/18-3b S 04 25036.100W8115052.300 111 15 1 29 85 67 39 4 1 16 132 5004 11/22-3 S 04 25037.000W8115051.400 138 8 7 28 86 40 36 2 0 10 145 500

Ostrea Fm.1 4/11-01 S 4 1602500W811509.600 168 28 3 18 79 78 32 2 6 9 77 5002 T01-00 S 4 1602500W811509.600 182 25 5 20 58 57 45 14 4 12 78 5003 3/20-1 S 4 1602500W811509.600 162 31 2 17 68 55 36 10 1 7 111 500

Pale Greda Fm.1 11/11-2 S 04 31024.800W81 05032.500 183 21 2 6 83 27 39 1 2 31 105 5002 11/10-3 S 04 31034.500W81 04058.800 212 27 2 5 97 25 20 1 1 5 104 500

Cabo BlancoM.1 TCB1-00 S 04 13040.6W 8112013.100 310 24 5 18 20 13 50 0 0 1 59 5002 TCB3-00 S 04 13039.2W 8112007.200 313 36 3 21 7 16 45 5 0 6 48 5003 11/21-3 S 04 13039.2W 8112007.200 374 29 2 3 12 13 36 1 0 2 28 500

Counted parameters.Qm,Quartz monocrystalline; Qp, Quartz polycrystalline; Cht, Chert; Kspar, Potassium feldspar; Plags, Plagioclase feldspar; Lv,Volcanic lithic; Lm,Metamorphic lithic; Ls, Sedimentary lithic; Heav, Heavy minerals; cmt, cement; mat, matrix.



Mogollon and Pale Greda formations

The lower Eocene Mogollon Formation is the oldestformation to crop out continuously in the eastern portion

of theTalara basin with a maximum thickness of �700m(Fig. 5). It is interpreted as a braided £uvial depositbased on thick cross-bedded sandstone, conglomerateand the occurrence of large £ow-parallel tree trunks

Scale1:100 000

0 5 10 15 kilometers

Legend

Tertiary

Ce

no

zoic

Paleozoic

Era Period Epoch

Basement (Undiff.)

Basement (Undiff.)

Deep-waterDeposits

Cretaceous

TumbesBasin

Amot

ape

Und

iffer

entia

ted

N81° 00'

4° 30'

4° 00'

81° 30' 81° 00'

Mancora

Los Organos

Cabo Blanco

Punta Restin

Punta La Cruz

Punta Lobitos

Talara

Negritos

Pac

i f ic

Oce

an

Punta Nautilus

Considered Tumbes Basin

El Nuro

Border Talara-Tumbes

?

1

2

3

4

5

Fig. 5. Geologic map of the Peru coastal area (scale1 : 100 000 ^ modi¢ed and simpli¢ed from Bellido, 1969 and INGEMMET,1999).Only the Eocene interval is discussed in details in this paper. (1) Location of measured section on Fig. 7a; (2) Location for sketchedmeasured section fromQuebrada Angostura on Fig. 7b; (3) Reported Pale Greda section in Fig. 7c; (4) Cabo BlancoMember outcropscharacterized in Fig.9a; (5) location of measured section of Terebratula Fm. in Fig.9b.


A. Fildaniet al.

(Figs 7a, b and 8) (Fildani, 2004). Palaeocurrent datafrom clast imbrication and cross-sets clearly indicate apredominant west and northwestward palaeo- £owdirection (Figs 7a, b and 8a^d). From unpublished sub-surface data (courtesy of Perez Companc) the MogollonFormation is correlatable to shallow marine facies tothe west and south. Together, these relations de¢ne anNE^SW transition from non-marine to marine facies(Fig. 6).

The uppermost Mogollon Formation, composed ofmedium- to ¢ne-grained sandstone with faint planarlaminations, may represent the gradual transition from£uvial to marginal marine deposition of the overlyingPaleGreda Formation.The contact between theMogollonand Pale Greda Formations is marked by the ¢rst appear-ance of unequivocal marine deposition: a thick shellfragment-rich bed (mostly gastropods and oysters)interpreted as coquina bed (Figs 7c and 8e).ThePaleGredaFormation is composed of marine shale and siltstonewith intercalated sandstone and common coquina beds,which contain progressively fewer gastropods andmore oyster shells upsection. Because of its sedimentarypackaging and biofacies associations (Weiss, 1955;Fildani, 2004), this formation has been interpreted as ashallow-marine deposit that deepens upsection (Fig. 7c)(Fildani, 2004). The equivalent of the Pale Greda

Formation to the north is the Ostrea Formation (Fig. 6),which crops out very sparsely but has been studied in coreand logs in the subsurface (Perez Companc, nowPetrobrasS.A., proprietary data). Both the Ostrea and Pale GredaFormations are characterized by shallow-marine shelf ele-ments; however, the more sand-rich Ostrea Formationwas likely deposited closer to a source of coarse detritalmaterial (Fildani, 2004). The Ostrea Formation reaches amaximum thickness of �1100m (Picarelli et al., 2003).Transition to the unconformably overlying EchinocyamusFormation is only observed in the subsurface (Fig. 11)(Fildani, 2004).

The Echinocyamus formation

TheEchinocyamusFormation (hereafter shortened to Echi-no) is composed of ¢ve members: Clavel, Cabo Blanco,Verde, Ballena and Somatito for an overall thickness of�400m (Daudt et al., 2004).The Clavel and Cabo Blancomembers are the only Echino Formation members to cropout, other than a few metres of the Somatito Member ac-cessible in a limited outcrop along the coast. The ClavelMember is a green^grey shale barren of microfossils (Fil-dani, 2004; seeTable 1). The top few metres of the ClavelMember are pervasively bioturbated, overlain by shell lagsand metre-scale cross-bedded, quartz-rich sandstone of

S N

OstreaPale Greda

Salinas-Mogollon

Parinas

Chacra

Lobitos

HelicoTalara Shale

Pozo Shale

Chira

Verdun

San Cristobal

Tablazo

Mogollon

1

2

3

4

5

Sed

imen

tary

Cyc

les

Talara Sandstone

Basal Salina

Eo

ce

ne

?

?

Lo

we

rM

idd

leU

p.

Manta (?)

Terebratula

Echino

Clavel

Sandstone

Sandstone (with conglomerate)

Conglomerate

Shale

?

?

Fig. 6. Correlation betweennorthern and southerndepositional areas and mainsedimentary cycles of Talarabasin (Modi¢ed afterSeranne, 1987; unpublisheddata PetroPeru).



6070 50 40 30 20 10 090 80

ms

fm

c

ss

Mog

ollo

n F

orm

atio

n(M

ogol

lon

Sup

erio

r M

embe

r)Lo

c. Q

uebr

ada

Sal

ado

S 0

4° 3

2' 6

"W

81°

4' 4

1.2"

40 30 20 10 0m

sf

mc

ss

Mog

ollo

n F

orm

atio

n(M

ogol

lon

Sup

erio

r M

embe

r)Lo

c. Q

uebr

ada

Ang

ostu

raS

04°

28'

32.

9"W

80°

59'

54.

9"S

impl

ified

rep

rese

ntat

ive

sect

ion

(tot

al e

stim

ate

~30

0 m

eter

s)

Pal

eocu

rren

ts fr

om im

bric

atio

n: fl

ow to

war

d w

est

Tran

sitio

n to

Pal

e G

reda

Fm

.

Lege

nd

Cro

ss-b

edde

d sa

ndst

one

Con

glom

erat

e (S

andy

mat

rix)

Mas

sive

san

dsto

ne

Fos

silif

erou

s in

terv

als

(Coq

uina

bed

s)

Mud

ston

e

(c)

(b)

(a)

Pla

ne la

min

atio

ns

ssss

Con

glom

erat

e (C

last

sup

port

ed)

68 4 02101214

18 1624 20222628

ms

fm

cpe

bm

sf

mc

peb

Low

er P

ale

Gre

da F

orm

atio

n(O

stre

a E

Mem

ber)

Loc.

Que

brad

a S

alad

o

Oys

ter

inliv

ing

posi

tion

N=

8im

br.

N=

11x-

beds

N=

6x-

beds

N=

7x-

beds

N=

7x-

beds

N=

7x-

beds

N=

28im

br.

N

N

Fig

. 8E

Fig

. 8A

Fig

. 8B

Fig

. 8D

Fig.7.

(a)S

impli¢ed

measuredsectionoftheUpp

erMogollonForm

ationin

theQuebradaSalado

withtransition

toPaleGreda

Form

ationand(b)sim

pli¢ed

measuredsectionfrom

Quebrada

Ang

ostura.(c)MeasuredsectionofthelowerPaleGreda

Fm.intheQuebradaSalado

(Locations

onFig.5).R

osediagramsindicatepersistent

NW

directed

palaeo£ows.Thicknessin

metres.


A. Fildaniet al.

(a) (b)

(c) (d)

(e)

Fig. 8. Mogollon Formation typical outcrops. (a) Faint cross-beds in pebble conglomerate (arrows indicating beds, 1.5m Jacob’s sta¡for scale). (b) Pebble-conglomerate in sandy matrix. (c) Large (40^50 cm) rip-up mud clasts in cobble-conglomerate with sandy matrix.(d)Large cavity left by a tree-trunk in cross-bedded sandstone. (e)Coquina beds of the PaleGredaFormation overlain by cross-strati¢edsandstone.



05101520253035404550

50556065

ms

fsm

scs

peb

ms

fsm

scs

peb

6070 50 40 30 20 10 0

130

140

120

110

100

90 80

170

180

160

150

Coq

uina

bed

(in s

cour

with

peb

bles

)

Coq

uina

bed

(Ter

ebra

tulid

i)

Silt

y sh

ale

Cro

ss-b

edde

d(w

ith s

hells

)

Tree

logs

(w

ith T

ered

olite

s)

Nau

tilus

100

Tree

logs

(with

Ter

edol

ites)

Larg

e cr

oss-

sets

with

peb

bles

Coq

uina

bed

(Ter

ebra

tulid

i)

Lobi

tos

Sha

le M

b.(T

alar

a F

m.)

190

ms

fsm

scs

peb

ms

fsm

scs

peb

Tere

brat

ula

For

mat

ion

(Loc

. Pun

ta N

autil

us)

S 0

4° 2

5' 3

6.1"

W 0

81°

15' 3

1.9"

Bio

turb

atio

n

Cla

vel M

b.

She

ll La

gs

(a)

(b)

Ech

ino

Cab

o B

lanc

o M

embe

r(E

chin

o F

orm

atio

n)S

4°

13' 4

0.6"

W 8

1° 1

2' 1

3.1"

Bio

turb

atio

n

She

lls+

qtzi

te p

ebs

N

N

N

Tere

brat

ula

N=

11C

last

s im

br.

N=

12C

last

s im

br.

N=

13X

-bed

s

N=

8X

-bed

s

N=

3Lo

gs o

rien.

N=

12X

-bed

s

N=

9P

ebbl

e im

br.

N=

7Lo

g-or

ien.

Pal

eocu

rren

ts p

ublis

hed

by P

alom

ino

and

Car

ozzi

, 197

9

N=

98

Pal

eocu

rren

ts m

easu

red

for

this

stu

dyS

4°

15' 5

.1"

W 8

1° 1

4' 8

.8"

N=

10x-

beds

Fig.9.

(a)S

impli¢ed

measuredsectionoftheCaboBlancoMem

bersandstone

incoastaloutcrop

sofElN� uro.R

osediagramsindicatewestd

irectedpalaeo£ows.(b)D

etailedmeasuredsectionofthe

TerebratulaForm

ationwithtransition

sfrom

theEchinoFm.(bottom

)and

Lobitos

Shale(to

p)(L

ocations

onFig.5).T

hicknessin

metres.


A. Fildaniet al.

the lower Cabo Blanco member (Fig. 9a). Upsection theCabo Blanco member contains lenticular sandstone beds,¢ne-sandstone beds with oyster bioherms,Turritellae shelllags and bioturbated siltstone are present; inferred sedi-ment dispersal is westward (Fig. 9a). The Cabo BlancoMember has been interpreted as a mixture of £uvial anddeltaic conglomeratic sandstone deposited from east towest (Palomino and Carozzi, 1979; Daudt et al., 2004;

Fildani, 2004). Combined outcrop and proprietarysubsurface data for the Echino Formation support theinterpretation of the ¢ve members as successive £uvio-deltaic to deltaic and/or shallow marine deposits stackedin a prograding fashion (Fig. 10) (cf. Van Wagoner et al.,1990).This interpretation is in agreement with the Daudtetal. (2004) interpretation of the Echino being a ‘regressivemegacycle’.

CLAVEL

CABO BLANCO

VERDE

SOMATITOINFERIOR

SOMATITOSUPERIOR

LOBITOS

LAGOON OSTREA

ShaleTALARA

Type Well from ZAPOTAL-CENTRAL field Well EA 9526

Member Formation

EC

HIN

OC

YAM

US

Env.

Flo

od p

lain

Flu

vio-

delta

icD

elta

ic (

Sha

llow

-mar

ine)

Dee

pw

ater

(777

m)

(747

m)

(716

m)

(686

m)

(655

m)

(625

m)

(594

m)

(564

m)

(533

m)

Lago

on?

Fig.10. Resistivity and g-ray well log record from thenorthern portion of thebasin (BlockX), showing thetypical stacking pattern ofthe Echino Formation(courtesy of Perez Compancdel Peru). Env., Interpreteddepositional environment.



TheTalara Group:Terebratula and TalaraFormations

A major basin-wide depositional transition occurs be-tween the Echino, Terebratula, and Talara Formations.The Terebratula Formation (�200m) overlies the cross-bedded sandstone of the Somatito Member (Echino For-mation) in apparent angular discordance (Fildani, 2004).Gutierrez & Arriola (2002) indicated a �2m.y. hiatusbetween the Echino Formation and TerebratulaFormation using well data. The Terebratula Formationis characterized by 2- to 3-m-thick cross-beddedsandstone beds with pebble- and shell- ¢lled scouredbases. Conglomerate beds are also present. Coquina bedscontain common brachiopods (terebratulidi) and Turritellae.Upsection, pervasively bioturbated shaly siltstonebecomes predominant with interbedded cross-beddedthin layers of pebbly sandstone and shells (Fig. 9b). Thisformation is interpreted to re£ect progressive deepeningfrom foreshore to outer shelf depositional environments(Fildani, 2004).

The overlying Talara Formation represents the ¢rstdeep-marine deposits in the Talara basin (Fig. 4) with amaximum thickness of�700m (Picarelli et al., 2003).Theshaly LobitosMember (�80m), deposited in a deep-waterenvironment on the palaeoslope of the basin (see palaeo-bathymetric reconstruction fromTable 1), unconformablyoverlies the shallow marineTerebratula Formation. Basedon benthic foraminifera (Table 1; details reported in Ap-pendix for Fildani, 2004), deepening of the basin appearsto have been rapid, with water depth changing fromo200m during deposition of theTerebratula Formationto upper bathyal depths of the Lobitos Member and mid-dle bathyal of the overlying HelicoMember.

Interbedded shale and pebbly sandstone deposits of theHelico Member (�300m) are re-sedimented gravity £ows(sensu Bouma, 1962; Lowe, 1982) and show palaeocurrentpatterns indicating a northwestward trend very similar tothat of the Mogollon Formation (Fig. 11b). However, theoverlying deepwater turbidite of the Talara SandstoneFormation, which outcrops as discontinuous sandstonebeds within the background shale of theTalara Formation(Fildani, 2004), indicate a WSW palaeo£ow direction(Fig.11c; Fildani, 2004; Duerichen, 2005).

Eventually, the Pozo shale blanketed the entire basinto end the ¢rst stage of basin-wide deep-water deposi-tion. Biofacies assemblages con¢rm a widespread deep-water environment across Talara during Pozo time(Gutierrez & Arriola, 2002) and are supported by thisstudy (Table 1).

Verdun and Chira formations

The second and ¢nal deep-water episode in theTalara ba-sin deposited the Verdun-Chira formations immediatelyfollowing the Talara Group. Gutierrez & Arriola (2002)proposed a hiatus of �1m.y. between the Talara Groupand the Verdun Formation. The Verdun Formation varies

in thickness, from about 400 to 620m, and was depositedbasin-wide (Duerichen, 2005). It is composed of thickconglomerate and sandstone intervals interbedded withthin-bedded sandstone and mudstone and it is inter-preted as a turbidite system, where palaeocurrent dataconsistently indicate a southwestward palaeo£ow (Fig.11d). This palaeodispersal pattern di¡ers signi¢cantlyfrom that observed in underlying formations (Fig. 11).Using nannoplankton data, Duerichen (2005) attributedthe Verdun Formation to the late Eocene. TheVerdun Formation is overlain by the Chira Formation, abasin-wide marine mudstone (Carozzi & Palomino, 1993).Bio-facies associations indicate that uppermostsediments of this formation were deposited in a shallowmarine environment (Fildani, 2004; seeTable 1).

PROVENANCE ANALYSIS

Conglomerate clast composition

Throughout theTalara basin, conglomerate units consistof mature, well-rounded pebbles of highly resistant rocktypes like quartzite, chert, and low-grade metamorphicrock. However, variations between the major sedimentaryunits are observed in detailed clast counts (Fig.12).

Mogollon Superior conglomerate units contain abun-dant quartzite, in addition to lesser felsic volcanic andgranitoid clasts. Intermediate volcanic clasts of greenishcolour (described by the local geologists as ‘andesita’) arepresent in both localities but were most easily identi¢ablein the Quebrada Angostura outcrop (Fig.12).

HelicoMember conglomerate units are rich in quartziteand contain volcanic clasts (sub-rounded to rounded in-termediate volcanic clasts showing microlitic, porphyritic,lathwork, and vitric textures) and very few plutonic and re-cycled sandstone pebbles. Overall, the Helico Memberconglomeratic units show a �20% increase in volcanicclasts with a marked increase in intermediate varieties(Fig.12).

Verdun Formation conglomeratic units contain easilyrecognizable sandstone intraclasts (¢ne-grained sand-stone and shale matrix supported), microcrystalline chert,and granitic cobbles.Very few intermediate volcanic clastswere observed in outcrops but never occurred withinanalysis grids (Fig.12).

Sandstone petrography

Previous petrographic studies in theTalara basin (Marsa-glia & Carozzi, 1991; Carozzi & Palomino, 1993) havereached comparable conclusions: derivation from a fairlyuniform mineralogical province producing quartz-richsand with only minor volcanic input. However, only theBasal Salina Formation, a pre-Mogollon (Palaeocene)fan-delta deposit restricted to the subsurface, has beenstudied in detail (Marsaglia & Carozzi, 1991; Fig. 13d). Ba-sal Salina sandstone plots within the recycled orogen, andthe presence of plutonic and high-grade metamorphic


A. Fildaniet al.

Gross paleo-currents dispersalpatterns

Cabo Blanco

Talara

Pacific Ocean

Talara Sandstone

N

Talara

Pacific Ocean

Verdun Formation

Amotape undifferentiated

Cenozoicsediments

(c) (d)

N

Outcrop and Core Locations

Pacific Ocean

Mogollon Formation

Outcrop

N

Cabo Blanco

Talara

Pacific Ocean

Helico Member

N

(a) (b)

Fig.11. Palaeocurrent distribution patterns for theMogollon Formation (a), the HelicoMember of theTalara Formation (b),TalaraFormation (c), and Verdun Formation (d). Arrows indicate gross palaeo-dispersal trends. Figure modi¢ed fromDuerichen (2005).Data forTalara Formation, HelicoMember and Verdun Formation are fromDuerichen (2005).



fragments is interpreted to re£ect derivation from anancient Amotape highland and its cover sequences (Mar-saglia & Carozzi, 1991) (Fig.13d).

Petrographic data are presented in Table 2, and sand-stone detrital modes are summarized on ternary diagramsin Fig.13.

Mogollon Formation arenites are weakly cemented bycalcite and clay minerals. Quartz (angular to subangular)and feldspar are the dominant framework grains, with pla-gioclase more common than potassium feldspar (Fig.13c).Quartz grains exhibit varying degrees of undulose extinc-tion, and polycrystalline quartz grains are common. Mo-

VERDUN

HELICO

(Talara Group)

MOGOLLON

0

10

20

30

40

50

60

70

Mogollon 1

WHT QZT

0

20

40

60

80

100

120

Mogollon 2

0

20

40

60

80

100

120 Mogollon 3

0

10

20

30

40

50

60

0102030405060708090

100

UP

SE

CT

ION

BLK QZT

VOLC PLUT SS SHALE

WHT QZT

BLK QZT

VOLC PLUT SS SHALE WHT QZT

BLK QZT

VOLC PLUT SS SHALE

WHT QZT

BLK QZT

VOLC PLUT SS SHALE

WHT QZT

BLK QZT

VOLC PLUT SS SHALE

Fig.12. Conglomerate clast counts results of major conglomeratic units of Talara basin.WHTQZT, white quartzite; BLKQTZ, blackquartzite;VolC, volcanic rocks; PLUT, plutonic rocks; SS, sandstone; SHALE, shale. Data from Fildani (2004).


A. Fildaniet al.

gollon Formation sandstones contain a diverse popu-lation of lithic volcanic and metamorphic grains.Volcanicgrains range from angular to sub-rounded and are ¢ne-grained, microlitic, and intermediate to basaltic incomposition. Fine-grained quartz-rich volcanic lithicfragments and some tu¡aceous grains are present. Meta-morphic fragments include metaquartzite and schist(Table 2).

The Cabo Blanco Member sandstone is composed al-most entirely of monocrystalline unstrained quartz. A fewfeldspars (mainly potassium-feldspar) and lithic meta-morphic grains are present. Lithic volcanic grains are rare.

The Helico Member sandstone contains abundantlithic volcanic (vitric volcanic and microlitic intermediatewith some lathwork grains) and only minor plutonic frag-ments and sedimentary lithics (siltstone and ¢ne-grainedsandstone). Metamorphic fragments are schistose (Table2). Many of the volcanic grains in the Helico Member aresigni¢cantly altered but still recognizable.

Quartz is the dominant constituent of the Verdun For-mation sandstone (Fig. 13), mainly represented by un-strained angular quartz with minor undulose strainedgrains. Grains are sub-angular to sub-rounded with

10^15% open pore space area between grains and occa-sional traces of mud matrix. Some quartz grains arerounded to sub-rounded. Plutonic grains contain perthi-tic, microperthitic and myrmekitic intergrowths.Volcanicgrains are present but not common. Metamorphic frag-ments include metasandstone (and siltstone) and schist.Rare fragments of siltstone and argillite are also present(Table 2).

The composition of theTalara Sandstone is similar tothe sandstone of the Verdun Formation but slightly richerin volcanic lithic grains (Duerichen, 2005).The values re-ported by Duerichen (2005) are not included in statisticalanalyses here owing to di¡erences in petrographicmethods.

Shale geochemistry

REE geochemistry of shale was used to track componentsof the source area that might not be represented in thesand fraction. Of the sedimentary size fractions, shale isthought to give the best estimate of average provenance(Taylor & McLennan, 1985). Immobile elements (e.g.REE) are only slightly a¡ected by geochemical andmineralogical changes during source-area weathering

Qp

Lv Lsm

Arc orogensources

Collision suture &fold-thrust belt

sources

Helico

Mogollon

Verdun

Q

F L

Cabo Blanco Mb.Verdun Fm.

Mogollon Fm.

Helico Mb. (Talara Fm.)

P K

Qm

Helico Mb.

Verdun Fm.

Circum-Pacific volcanoplutonicsuites

Limit o

f detr

ial m

odes

Mogollon Fm.

Recycledorogen

Dissected arc

Transitionalarc Undissected

arc

Basementuplift

Transitionalcontinental

Cratoninterior

(a)

(c) (d)

(b)

Ostrea Fm.

Echino Fm.

Pale Greda Fm.

Terebratula Fm.Q

F L

Basal Salina Fm.(Marsaglia & Carozzi, 1991)

C. B.

C. B.

Fig.13. Sandstone detrital mode normalized into grain categories for comparative petrography and displayed on ternary diagramwiththeir mean values and their standard deviation; (a) quartz, feldspar and lithic components (QFL), (b) quartz policrystalline, lithicvolcanic and lithic sedimentary1metamorphic (QpLvLsm), and (c) quartz monocrystalline, plagioclase and k-feldspar (QmPK) as theapexes. (d) QFLdiagram displaying samples (not enough for statistical mean and standard deviation) fromTerebratula, Echino, Ostrea,and Pale Greda fms compared to mean value for Basal Salina Fm. Compositional ¢elds are fromDickinson (1985) (Table 2). Arrowsindicate sequential stacking of formations from bottom (Mogollon Fm.) to top (Verdun Fm.). C.B., Cabo BlancoMember.



(McLennan, 1989; Cullers et al., 1993; Hessler & Lowe,2006). REE in shale are considered particularly reliableprovenance tracers because of their relative insolubilityduring weathering and sedimentary transport (McLen-nan,1989).We investigated REE abundance and fractiona-tion assuming that a high mass-related REE fractionationcorresponds to silt and clay weathered from felsic sourcerocks, whereas low mass-related REE fractionation im-plies a ma¢c source for silt and clay (Nesbitt, 1979).

On a plot of average, chondrite-normalized REEvalues(Fig.14), the oldest shale (MogollonFm.) shows the highestdegree of mass-fractionation, and therefore the most felsiccomposition of the basin shales. Upsection, the Echinoshale has a slightly smaller mass-fractionation than theMogollon shale, showing increased relative input of morema¢c-derived clays. REE show intermediate mass fractio-nation for the overlyingTalara group,making this shale themost ma¢c in the basin.The overlyingVerdun shale showsa signi¢cant increase in felsic material but still does not at-tain the high light REE values of the older Mogollon andEchino groups.

We assume that variations in shale geochemistry are re-lated to changing composition of source terrane rather thanto changing weathering conditions. Al2O3, CaO1N2O,K2O (A^CN^K) relationships in shale provide an estimateof the degree of weathering of the source terrane before ¢ -nal deposition and burial (Fig. 15; Nesbitt & Young, 1989).With progressive weathering, major-element geochemis-try of plutonic source rock shifts away from the CN apexand towards theA apex, as a function of feldspar weatheringand mobile cation (Ca, Na) depletion (Nesbitt & Young,1989). During early phases ofweathering no relative loss ofK2O occurs. Throughout the Talara basin stratigraphy,shales exhibit a consistent degree of weathering (Fig. 15),enabling geochemical di¡erences to be attributed to prove-nance rather than to climate and/or weathering changes.

Signi¢cant variations in REE fractionation throughoutthe Eocene stratigraphy is illustrated inFig.16. Shale unitsshow signi¢cant shifts inREEmass fractionation (expressedby the ratioLaN/LuN;Rollinson,1993) at major unconfor-mities. The most notable shift from high (felsic) to low(ma¢c) fractionation occurs at the Echino Talara contact.

DISCUSSION

Palaeogeographic evolution

The Lower Eocene section is characterized by a series ofdepositional systems ranging from terrestrial to shallowmarine: Mogollon, Pale Greda/Ostrea, and Echino fms.(Fig. 3).TheMogollon Formation (Figs 3 and 4) is an allu-vial fan to £uvial system transgressed by the overlyingshallow marine Pale Greda Formation (Fildani, 2004).TheMogollon depositional system consists of a £uvial sys-tem in proximal areas (eastern portion of the basin),whereas equivalent shallow-marine facies can be foundin the southern and western parts of the basin (Salinas-Mogollon Fm.; Fig. 6). A similar relationship is truefor the Echino deltaic system and its shaly, deeper-watercounterparts to the south (Fig. 6).With the exception of agradual deepening of theTalara area to outer shelf or slopedepths duringOstrea/PaleGreda deposition, water depthsare generally shallow through the higher-order deltaicprogradational cycles of the Echino Formation (Fig. 4).Fluvial and shallow marine sediment transport directionis consistently to theWNW, in accordance with the distri-bution of deeper water environments to the west (Fig.17a).

A major palaeogeographic disruption is marked by theangular unconformity between the Echino and overlyingTerebratula Formations, following uplift, tilting, and ero-sion of parts of the underlying sedimentary succession be-

5

25

45

65

85

105

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

Felsic

Mafic

Mogollon

Echino

Talara

Verdun

(ppm

/ppm

cho

ndrit

e)

Mogollon

Echino

Verdun

Talara

REE (Rare Earth Elements)

Fig.14. Fractionationdiagram for REE comparingmajor formations of Talarabasin. Patterns are shown asaverage (dark-line), with onestandard deviation (shadedcolours) (SupplementaryTables S1and S2). REE,rare-earth elements.


A. Fildaniet al.

fore the major mid-Eocene deepening of theTalara basin.Subsurface data are in accordance; in at least one PerezCompanc oil ¢elds (Carrizo ¢eld in Block X), the EchinoFormation is absent and theTalara Group is reported tounconformably overlie the Ostrea Formation (Gutierrez& Arriola, 2002).

Deepening of the middle Eocene shelf (Terebratula For-mation) and deposition of the overlying deep-water (bath-yal) shale (Lobitos Member, Talara Formation) is the ¢rstmajor phase of deep-water deposition in theTalara basin(Fig. 9b). The Talara Sandstone, which is comprised ofhighly discontinuous, lenticular sand bodies encapsulatedin shales, is interpreted as the ¢ll of channels that carveda palaeo-slope environment where the background de-position is represented byTalara Formation shale (Fildani,2004) (Fig.17).This palaeo-environment setting is similarto the modern slope described by Laursen & Normark(2003) and Spinelli & Field (2003).The channels ¢lled byTalara Sandstone may have occurred between sub-basinswhere intervening ridges were covered by mud and fed bylocal sources from the shelf, either recycling shelf sand orbypassing the shelf and tapping into more distant sources(Fildani, 2004).

Basin con¢guration during this deepening event is un-certain due to likely lateral heterogeneities along the slope;however, palaeocurrent data suggest a westward sloping

gradient (Figs 11 and 17). Abrupt changes in lateral faciesassociations encountered in the ¢eld suggest segmenta-tion of the larger basin into sub-basins perhaps along ma-jor border faults.

The deep-water (bathyal) upper middle Eocene PozoFormation shales cover the entire basin. This may be re-lated to the shutting o¡ of a distal coarse sediment supplyduring widespread deepening of the basin and/or to up-dip sequestration of the coarser material.

The �1m.y. hiatus proposed by Gutierrez & Arriola(2002) between the Pozo and Verdun Formations corre-sponds with a major palaeogeographic change at theboundary. The overlying Verdun Formation representscontinued deposition into a deep-water setting but with asigni¢cant shift in sediment dispersal to the south (Figs11and 17). Sediment transport along the axis of a north^south elongated basin is in accordance with observationsof the Verdun Formation extending to the southernLancones basin (Fig. 2; Valencia & Uyen, 2002) and theSechura basin (Dunbar et al., 1990) as well as withsubsurface indications of a north^south con¢ningfault system (Gutierrez & Arriola, 2002; Picarelli et al.,2003; Duerichen, 2005).

Deepwater sedimentation ended in theTalara basin bythe upper Eocene, with deposition of the Chira Forma-tion. Biofacies associations in the overlying Chira

Verdun Group

Al

wea

ther

ing

trend

Talara Groupwea

ther

ing

trend

Ostrea/Echino Groupwea

ther

ing

trend

Mogollon Group

Up-

sect

ion

CaNa K

wea

ther

ing

trend

(Al 50%)

Fig.15. Ca1Na, Al, K ternary diagrams fordi¡erent shale groups to show thatweathering trend did not a¡ect Talara shalefractionation (Supplementary Table S1).



Formation indicate a shoaling upward history (Fildani,2004). By the time of deposition of theOligoceneMancoraFormation (Fig. 3), the basin was again characterized byshallow-marine water depths, with theMancora sequenceinterpreted as a deltaic system (Kraemer et al., 1999). Atthat time, activity along theDGFS already controlled sub-sidence of the Tumbes basin as the locus of depositionshifted to the north (Fig. 2).The southern end of theTalarabasin was uplifted in the late Oligocene as depocentresshifted northward, with upper Oligocene strata preservedonly in the northern part of the basin (Fildani, 2004).

Provenance

TheMogollon Formation unconformably overlies the de-formedCretaceous and older rocks of the central and low-ermost Amotape Mountains. This relationship suggeststhat the Amotape Mountains probably did not partitionthe Lancones and Talara basins during the lower EoceneMogollon deposition, and that sediment from the Lan-cones basin did reach theTalara basin. The proximity ofthe Lancones basin to the east, and the SE^NW-directedpalaeocurrents of theMogollonFormation (Figs11and17),suggest it may have been a signi¢cant source for theTalarabasin. The petrography of the Mogollon sandstone, theclast composition of its conglomerate, and the overall geo-chemical signature of the shale and pebbles re£ect a mixedsource terrane (Figs 12, 13 and 14). Highly fractionatedREE values of the shale indicate a strong felsic igneous

component (Fig. 14). Sandstone petrography indicates anintermediate volcanic source area, and sandstone modesplot between the arc and recycled orogenic ¢elds (Fig.13a).The occurrence of a signi¢cant amount of intermedi-ate volcanic detritus in the sandstone (410%) suggeststhe presence of a volcanic terrane among other sources.The Lancones basin does include a thick cover of Cretac-eous (?) intermediate and basaltic volcanic £ows (Valencia& Uyen, 2002). Recycled arc-derived materials from theLancones basin may provide the partial arc and felsic sig-natures ofMogollon Formation sandstones and shales, re-spectively.

Above the Mogollon Formation, sandstone detritalmodes of the Ostrea/Pale Greda formations continue toplot in the ‘recycled orogenic’ ¢eld with little in£uencefrom an Amotape-type source and only partial recyclingof Cretaceous and lowerTertiary sandstone (Fig.13d).TheLancones basin may have still been linked with theTalarabasin through Pale Greda/Ostrea deposition. Detritalmodes of the Cabo Blanco member (lowermost memberof theEchino group), however, indicate amature sandstonemainly composed ofmedium- towell-roundedmonocrys-talline quartz, which also plots very closely with thePalaeocene Amotape-derived Basal Salina sandstone(Marsaglia & Carozzi, 1991). We infer that the AmotapeMountains were a topographic high at this time, isolatingtheTalara basin from the Lancones basin and serving as aprimary sediment source for the Echino Formation.Thispotential Amotape uplift corresponds with renewed

La/Lu

Chira

Verdun

Talara ss

Helico

Monte

Lobitos

Terebratula

Echino/Chacra

Chacra

Verde

Clavel

Pale Greda

Mogollon

San Cristobal

Verdun/ChiraGroup

Talara Group

Echino Group

Mogollon/OstreaGroup

10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00

Fig.16. La/Lu fractionation plot indicatingvariation of fractionation of rare-earthelements (REE) at major unconformitiesthroughout the Eocene stratigraphy (datafrom supplementary Table S2).


A. Fildaniet al.

Talara Basin

Lancones Basin

Andean CordilleraAmotapeMountains

Verdun Fm.(late-middle Eocene)

NTr.?

Active Subductionchannel

Talara Basin

Lancones Basin

Andean CordilleraAmotapeMountains(lower relief)

NTr.?

Mogollon Fm.(early Eocene)

Talara Basin

Lancones Basin

Andean CordilleraAmotapeMountains(lower relief)

NTr.?

Talara Fm.(middle Eocene)

A

B

C

Underplating

Forearc subsidence

Fig.17. Block diagram summarizing the di¡erent stages of Talara basin evolution during the Eocene. Arrows indicate gross palaeo-dispersal patterns (see details Fig.11). (A) In the early Eocene (Mogollon time) sediments from the Lancones basin were able to bypassthe AmotapeMountains.Mogollon Fm. palaeo-dispersal patterns indicate northwestward directions with sediments wedging outeastward. (B) By the middle Eocene (Talara time), the basin is highly partitionedwith areas of deep-water sedimentation. Palaeo-dispersal patterns are more mature with diverse sources (mixed Amotape sources and Lancones-related detritus able to bypass theAmotapeMountains to the south - see Fig.11 for details). (C) By the late middle Eocene (Verdun time) the dominant palaeo-dispersalpattern is towards the southwith the AmotapeMountains separating the fully deep-water Talara basin from the Lancones basin to theeast. Diagram inspired by Shepherd &Moberly, 1981.



east^west deltaic progradation of the Echino Formationonto the outer shelf environment of the Pale Greda/OstreaFormations.

The late-Middle Eocene Terebratula Formation restswith angular unconformity on the Echino Formation andshows a marked shift towards more lithic-rich sandstone(Fig. 13b) and a more ma¢c source terrane (Fig. 16). Thepresumed uplift and erosion associated with the angulardiscordance may have provided the increased lithiccomponent inTerebratula sandstone. As well, the onset ofTalara Formation deep-water sedimentation above theTerebratula Formation correspondswith a signi¢cant shiftin detrital composition. Sandstone, conglomerate, andshale of theTalaraGroup indicate amore intermediate vol-canic (or partly ma¢c) source (Figs12^14).Talara shaleREEre£ects a partly ma¢c source terrane (Fig. 14); sandstonedetrital modes of the Helico sandstone also indicate asource enrichedwith intermediate to ma¢c volcanic mate-rial (Fig. 13).The palaeogeographic model proposed herefor the Talara Formation includes a westward-dippingincised slope system, possibly partitioned into fault-controlled sub-basins. Possible intermediate- to ma¢c-volcanic source areas could be either the arc to the east,the Raspas Group to the north (Fig. 2; Arculus et al., 1999;Spikings et al., 2005), or the Lancones basin to the east, ifsediment was again able to bypass the Amotape barrier. Ifthe Talara deepening event was regional, and subsidenceinvolved the entire forearc region, perhaps the AmotapeMountains were a lower relief feature and sediment fromLancones basinwas able to reach the outboard basin ofTa-lara (Fig. 17).Work from Duerichen (2005) indicates thatpalaeocurrents of the Helico Member have dominantlynorthwestward directions indicating the Lancones basinas a very promising source area (Fig. 11).We cannot dis-count that the largely unmapped Cordillera to the eastmay have contributed arc-related detritus, bypassing theLancones basin. Preliminary detrital zircon data indicatepresence of an active arc throughout the Eocene (Hessler& Fildani, 2005).

Another shift in sediment composition and in palaeo-current dispersal pattern is evident in the late middle Eo-cene Verdun Formation; a highly fractionated (felsic) REEsignature (Figs14 and16), the sandstone composition (Fig.13), the presence of meta-granodioritic cobbles (Fig. 12),and north^south palaeocurrents (Fig. 11) set this forma-tion apart from the underlyingTalara Formation.The pre-sence of sandstone cobbles in the Verdun Formation,which contain the sameVerdun index microfossils (Lepido-cyclina peruviana), suggest partial recycling of the VerdunFormation itself. The Verdun Formation is recognizedacross the entire basin, but with the more distal faciesin the southern basin (Fildani, 2004; Duerichen, 2005).This palaeogeography, together with the occurrence ofintraformational clasts, indicate that inversion occurredin the north, where Verdun beds were uplifted, eroded,and recycled into the southern part of the basin. The�1m.y. hiatus between theTalara Group and the VerdunFormation indicated by Gutierrez & Arriola (2002) may

have speci¢cally involved partial uplift and erosion ofthe Eocene strata of the northern end of the basin in thelate Eocene.

During the early Oligocene, the locus of depositionshifted to the north with deposition of the thick £uvio-deltaic sequence of the Mancora Formation spilling intotheTumbes basin (Kreamer et al., 1999; Fildani, 2004; Fil-dani et al., 2005).The Mancora Formation sediments sug-gest a shift to more ‘ma¢c’ source material with thepresence of andesitic cobbles in the conglomerate (Filda-ni, 2004). Shale geochemistry con¢rms a shift to ma¢csources that could be the geographically nearby RaspasComplex (Table S1; Fig. 2).

Basin tectonics

Evolution of theTalara basin is directly tied to regional in-teractions between theSouthAmerican andFarallon-Naz-ca plates. However, the peculiar forearc position, with amodern quiescent arc (Andean Cordillera), escape tec-tonics features to the north (DGFS and Gulf ofGuayaquil), and the present day £at-slab subduction(Bernal et al., 2002)make for a complicated tectonic recon-struction of the basin’s Eocene history.

Eocene deep-water deposition in theTalara basin is un-ique relative to other coastal basins of northern Peru andsouthern Equador, which are characterized by shallow-marine deposition or non-deposition during the late Eo-cene (Jaillard etal.,1995; Jaillard&Soler,1996).This abruptdeepening ( �1000m of bathymetric change) is almostone order ofmagnitude greater than global eustatic sea-le-vel £uctuations (Hardenbol etal.,1998) that occurredwith-in the same time frame, emphasizing its tectonic driver(Fig. 4).Moreover, eustatic £uctuations are dominantly re-gressive during this time interval, requiring signi¢cantlyenhanced, tectonically driven subsidence (Fig. 4).The me-chanisms behind these deep-water events are not clear,but they are probably related to a combination of wrenchtectonism and subduction erosion.Eustacy appears to haveplayed a minor role in sedimentation inTalara basin.

Wrench tectonics may be manifest in the origin and be-haviour of the Amotape Tahuin block. The tectonics aremore complicated than the simplistic scenario that pro-poses the Amotape Tahuin block as accreted to SouthAmerica (Feininger, 1987; Mourier et al., 1988).The Amo-tape TahuinBlock north of theNaranjoFault zone is com-posed of a regional-scale me¤ lange which encloses tectonicblocks of high-pressure, low-temperature rocks inter-preted as a partially subducted and now exhumed oceanicassemblage of an accretionary prism (Fig. 2; Arculus et al.,1999; Spikings et al., 2005, among others). The Amotape^Tahuin block south of the Naranjo fault is a continentalblock that rotated clockwise as consequence of multipleterrane collisions along the Ecuadorian margin (Spikingset al., 2005). Cooling and rotation of the block at 43^39Macould be directly related to the collision of theMacuchi Is-land Arc with the Ecuadorian margin (Spikings et al.,2005). Recently published ages from granitoids in the


A. Fildaniet al.

AmotapeMountains indicate aTriassic crystallization agerelated to the break-up of western Gondwana (Jaillardet al., 1990; Noble et al., 1997) and continental a⁄nities forthe S-type, granitoid intrusions of theTres Lagunas blockin Ecuador (Sanchez et al., 2006) suggesting its para-auto-chthonous origins as a translated sliver of the CordilleraReal (Aspden & Litherland, 1992; Aspden et al., 1992; Spik-ings et al., 2005).

Rotation of the Amotape Tahuin block during the LateCretaceous^earlyTertiary (Mourier et al., 1988; Jaillard etal.,1999) andmore speci¢cally in the lateEocene (Spikingset al., 2005) indicates that coeval wrench tectonismwas ac-tive in the Talara forearc region as terrane collisions af-fected the plate boundary.The change in palaeo-dispersalpatterns and possible uplift in the northern source areasduring Verdun time deposition could be related to terranecollision along the Ecuadorian margin (Spikings et al.,2005). Evidence for right-lateral strike^slip movement isreported for theDolores^Guayaquil system from theMio-cene to recent (Kreamer etal.,1999) but mayhave occurredearlier based on the presumed mid-Eocene uplift of theAmotape Tahuin block that served to isolate the Talaraand Lancones basins (Fig. 17). Large-scale strike^slipfaulting can result in rapid vertical tectonism and lateralshifting along the boundary of large and small blocks overdistances of hundreds of kilometres (Ryan & Coleman,1992); resultant deep, partitioned basins create high ac-commodation space for sudden deep-water depositionand rapid lateral facies changes, as is seen in the mid-Eo-cene Talara section (Figs 4 and 17). During this time theAmotape blockwas segmented and partially submerged.

As a modern analogue, right-lateral motion is behindthe formation of the northern Progreso-Tumbes basins.Witt et al. (2006) reported evidence of large-scale exten-sional faults accommodating the thick sedimentary ¢ll inthe Gulf of Guayaquil (Witt et al., 2006).The recent rapidopening of the Gulf is explained by escape tectonism andnorthward movement of the North Andean Block (Wittet al., 2006), but the area was already a locus of sedimenta-tion during theOligocene (Mancora Formation ofTumbesbasin; Fildani, 2004) and the Miocene (Progreso basin;Kreamer et al., 1999).

Another mechanism for Eocene subsidence could beextension related to subduction erosion (sensuVon Heune& Scholl, 1991).This mechanism may have caused the re-gional forearc deepening during Verdun deposition (Fig.17). Sediment accretion, sediment subduction, and sub-duction erosion seem to take place at the same time alongthe western margin of South America (Sage et al., 2006).Basal subduction erosion of the forearc crustal basementinboard of an accretionary prism is quite common (see re-view in Clift & Vannucchi, 2004) and deep-water forearcbasins may form as a result of enhanced thinning of theupper plate through basal subduction erosion along theplate interface, as explained by Laursen &Normark (2003).For example, subsidence of the Lima basin is controlled bysubduction erosion related to the subducting Nazca plate

underneath South America causing hundreds of metres ofvertical excursion in the forearc position (Clift etal., 2003).

Sediment in the trench seems to play a major role in ‘lu-brication’ of the subduction channels controlling intra-plate coupling and de-coupling. Forearc basins can formdue to subduction erosion even in over¢lled trenches(Wells et al., 2003). It is worthwhile to explore the possibi-lity that enhanced trench- ¢lling caused bywrench tecton-ism during ‘Talara Formation time’ may have locked theintraplate zone, thereby forcing subduction erosion andsubsequent regional deepening during ‘VerdunFormationtime’ (Fig. 17). In any case, we speculate that locking andunlocking related to variable coupling in the intraplate po-sition causes extensile or contractile phases (as reported inBourgois etal., 2007), resulting in dramatic vertical forearcmovements such as that observed in the EoceneTalara ba-sin (this study) or in the Lima Basin (Clift et al., 2003).

Modern subduction erosion along the Peruvian trenchis well documented (Clift et al., 2003) even at Talara basin’slatitude (Bourgois et al., 2007). Bourgois et al. (2007) docu-mented major detachment faults o¡shore of Talara andspeculated on their direct connection with the subductionchannel. Local events in the trench, such as subduction ofseamounts, could cause major erosion underneath theoverriding plate and enhance the extensional fabric in theforearc region.TheTalara basin may have evolved in aVal-paraiso basin-type setting (sensu Laursen & Normark,2003), very similar to that associated with Carnegie ridgesubduction occurring o¡ the modern coast of Ecuador(Fig. 2).

Subduction erosion events in the Talara basin corre-spond with and may have been enhanced by the late Mid-dle Eocene contractional Inca II tectonic phase (Jaillardet al., 1995; Benavides-Caceres, 1999). Plate rearrangementand changes in convergence rate would encourage subduc-tion erosion with subsidence in the upper plate and majorsediment accumulation in the forearc position (Fig. 4)(Fildani, 2004).

We speculate that the modern Talara basin is what re-mains of a once much larger forearc basin. Bourgois et al.(2007) estimated that subduction erosion removed �25to100 km3Myr km�1 for the past �5.2Myr.The presenceof Precambrian and Palaeozoic basement in the Ciclayocanyon on the Peruvian slope just south of the study area(Sosson etal.,1994) con¢rms subduction erosion along thismargin possibly for even longer periods.

Today, £at- slab subduction, collision and partial sub-duction of seamounts into the Peruvian trench (Fig. 1), aswell as intensi¢ed coupling along subduction channelscaused by intraplate locking (Bourgois et al., 2007), allcombine to cause dramatic vertical excursions theTalararegion. Emergent Pleistocene terraces show up to�400m of uplift along the Talara coast (Pedoja et al.,2006; Bourgois et al., 2007), and this uplift has been linkedto the Carnegie ridge subduction (Pedoja et al., 2006) andto poor lubrication and locking along the subductionchannel in the intraplate position (Bourgois et al., 2007).



CONCLUSIONS

The Eocene section of Talara basin records rapid shifts ofdepositional, stratigraphic and provenance patterns. De-position in terrestrial to shallow-marine environmentspersisted until the abrupt deepening of the basin startingin theMiddle Eocene.This deepening event loosely corre-spondswith a reported change in the direction of plate con-vergence (Pardo-Casas &Molnar, 1987; Somoza, 1998). Re-organization of plate motion vectors and new convergencedirection may have enhanced subduction erosion of theoverriding plate (sensuClift etal., 2003) or wrench tectonismand reactivated strike^slip faults thatwere active during theLate Cretaceous, causing the major extension in the LateMiddle Eocene that correspond to the deposition of thedeep-water sequences of the Talara Formation. Strati-graphic and provenance data suggest a direct relationshipbetween sedimentary packaging and regional tectonicswith changes in source terranes and sediment dispersalpatterns at major unconformities. A sharp shift is recog-nized at the onset of deep-water sedimentation within theTalara Formation (Lobitos shale Member) by an increasedin£ux of ma¢c material to the basin related to the segmen-tation and partial subsidence of theAmotapeBlock.A laterdeepening phase is re£ected by the deposition of the deep-water Verdun Formation that shows a di¡erent dispersalpattern than theTalara Formation and was deposited aftera hiatus of �1Ma (Gutierrez & Arriola, 2002).The activewrench tectonism of the ‘Talara Formation time’ may havefavored the sediment delivery in the trench, initially lock-ing and eventually lubricating and unlocking the intraplatezone, thereby forcing subduction erosion and subsequentregional deepening during ‘Verdun Formation time’. Lock-ing and unlocking related to variable coupling in the intra-plate position causes extensile or contractile phases (asreported in Bourgois et al., 2007), possibly resulting in ver-tical forearc movements such as the ones observed in theEocene of Talara basin. Variation of intraplate couplingand partial locking of subduction plates have been identi-¢ed as some of the major causes behind switching tectonicsetting from contraction to extension (andvice versa) in theforearc region. Collisions of terranes along the Ecuadorianmargins (Spikings et al., 2005) forced inversion of the basinstarting in the late Eocene (Verdun time). Eventually activedeposition shifted to the north in the modern Gulf ofGuayaquil area. Talara basin sedimentary ¢ll is thus theresult of a complicated forearc history: itwas never comple-tely isolated from surrounding basins and its con¢gurationand sediment source areas changed through time. TheAmotape Tahuin block was not a persistent barrier be-tween theTalara basin and the arc area, and so arc-deriveddetritus was able to reach the basin episodically.

ACKNOWLEDGEMENTS

We thank Perez Companc del Peru (now Petrobras delPeru) for ¢nancial support and help in the ¢eld. In particu-

lar, A. F. would like to thank the geologists of Perez Com-panc del Peru (Pedro Raul Arriola, Gerardo Pozo, Peter‘Pedro’ McGregor, the late Juan Leyva, Fabian Gutierrez,Ricardo Savini, Jose Daudt and many others) that helpedto improve our understanding of the basin through dis-cussions about the geology of NW Peru.This manuscriptis solidly built on the results of two PhDdissertations sup-ported by Perez Companc and based on extensive ¢eldmapping and facies analysis, integrated with proprietaryunpublished subsurface data (Fildani, 2004; Duerichen,2005). ErinDuerichen is thanked for sharing ideas. Specialthanks go to SPODDS (Stanford Project On Deep-waterDepositional Systems), a consortium of 15 oil-companies.Bill Normark, Don Lowe and Jim Ingle are thanked for re-viewing early versions of this manuscript. Constructivecomments from Reviewers Kathie Marsaglia, Je¡ Trop,Dave Scholl and Editor Peter DeCelles largely improvedthis manuscript. Agustin Cardona and AnthonyTankardare thanked for insightful conversations about the generaltectonic setting of South America. Brad Ritts (Chevron) isthanked for reading the latest version of this paper.

A. F. andA.M.Hwould like to dedicate this manuscriptto their son Cesare, whose arrival joyfully delayed thispublication.

Supplementary Material

The following material is available for this article online:Table S1.Major and trace element data for shales in the

Talara Basin. XRF analyses performed by WashingtonState GeoAnalytical Laboratories.

Table S2. Unnormalized rare-earth element data forshales in theTalara Basin. ICP-MS analyses performed byWashington State GeoAnalytical Laboratories.

This material is available as part of the online articlefrom: http://www.blackwell- synergy.com/doi/abs/10.1111/j.1365-2117.2007.00346.x (This linkwill take you to the arti-cle abstract).

Please note: Blackwell Publishing are not responsiblefor the content or functionality of any supplementary ma-terials supplied by the authors. Any queries (other thanmissing material) should be directed to the correspondingauthor for the article.

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Manuscript received 9 April 2007; Manuscript accepted19 October 2007.



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