varioaciones estratigraficas del marañon

at SciVerse ScienceDirect

Journal of South American Earth Sciences 38 (2012) 147e158

Contents lists available

Journal of South American Earth Sciences

journal homepage: www.elsevier .com/locate/ jsames

Stratigraphic variations across the Marañón Fold-Thrust Belt, Peru: Implicationsfor the basin architecture of the West Peruvian Trough

Arne F. Scherrenberg a,*, Javier Jacay b, Rodney J. Holcombe a, Gideon Rosenbauma

a School of Earth Sciences, The University of Queensland, St. Lucia, QLD 4072, AustraliabUniversidad Nacional Mayor de San Marcos, Av. Venezuela, cda. 34 s/n, Lima, Peru

a r t i c l e i n f o

Article history:Received 25 January 2012Accepted 5 June 2012

Keywords:Basin inversionGrowth strataStratigraphyMarañón Fold-Thrust BeltWest Peruvian TroughBasin architecture

* Corresponding author. Tel.: þ61 7 33469798; fax:E-mail addresses: [email protected] (

unmsm.edu.pe (J. Jacay), [email protected] (R.Juq.edu.au (G. Rosenbaum).

0895-9811/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.jsames.2012.06.006

a b s t r a c t

Basin inversion has played a major role in the structural evolution of the Andean Orogeny. We presentnew observations from the Marañón Fold-Thrust Belt (MFTB) in central Peru that show Cretaceous facieschanges and thickness variations that may have been controlled by a series of faults in the basement ofthe West Peruvian Trough (WPT), separating this basin into smaller sub-basins. We present three newstratigraphic columns and a revised geological map, highlighting stratigraphic relationships within theMFTB. Our results show that a major boundary exists within the MFTB, across which stratigraphic unitsshow distinct facies and thickness changes. This boundary is a long-lived intrabasinal structure (ChontaFault), and its palinspastic reconstruction is of a half-graben geometry, with the graben floors tilted to theeast. Our results indicate that the architecture of the WPT in central Peru during the Late Cretaceous mayhave been made up of three relatively small basins.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Stratigraphic facies changes and thickness variations in fold-thrust belts are commonly associated with the inversion of earlierbasins (Coward, 1994). In this setting, some of the reverse faults arereactivated normal faults, which have also been active during basindevelopment. Identifying and understanding these stratigraphicboundaries, their nature, and their extent is important for inter-preting the basin evolution, and the structural framework of theorogenic belt.

The Marañón Fold-Thrust Belt (MFTB) in central Peru is anexampleof anorogenwhere stratigraphic boundaries haveplayedanimportant role during basin inversion. However, geological infor-mation from this orogenic system is limited, with most work con-ducted prior to the 1980s (e.g. McLaughlin, 1924; Steinmann, 1929;Harrison, 1943; Benavides-Cáceres, 1956; Wilson, 1963; Wilsonet al., 1967; Myers, 1975; Cobbing, 1976, 1978) and relatively fewstudies completed in the last three decades (e.g. Janjou et al., 1981;Mégard, 1984, 1987a, 1987b; Benavides-Cáceres, 1999; Rodríguez,2008; Carlotto et al., 2009). The geology of the area is characterisedby distinct facies changes and thickness variations across the fold-

þ61 7 33651277.A.F. Scherrenberg), jjacayh@. Holcombe), g.rosenbaum@

All rights reserved.

thrust belt (Mégard, 1984), with the most pronounced variations inthe Lower Cretaceous sedimentary cover of the West PeruvianTrough (WPT) (Fig. 1a and b). This has been interpreted to reflectinversion of major basin-bounding faults with basement involve-ment (Mégard, 1987b) (Fig. 1c). A distinct eastward thinning andfacies change has been recorded in Lower Cretaceous sandstone andshale units (Wilson, 1963). These variations may have beencontrolled by a series of faults in the basement, separating the WPTinto two smaller basins (intra-arc and back-arc basins in Fig. 1a).However, the location and architecture of the basin-bounding faultand the level of basement involvement are not fully understood.

This paper presents three new stratigraphic columns anda revised geological map, highlighting stratigraphic relationshipswithin the MFTB between Huánuco and Lima. Our results unravelthe architecture of the Late Cretaceous basin prior to the onset ofCainozoic Andean contractional deformation, and highlight theinfluence of a long-lived basement structure on both sedimentationpatterns and the subsequent basin inversion.

2. Geological setting

The geological history of the MFTB in central Peru involveda pre-orogenic period of extension and crustal thinning, followedby episodic contraction during Andean orogenesis (Mégard, 1984,1987b). The early phase of rifting in the WPT started in the LateTriassic but was superimposed on an earlier (Palaeozoic) rift

Delta:1_given name

Delta:1_surname

Delta:1_given name

Delta:1_surname

mailto:[email protected]






www.sciencedirect.com/science/journal/08959811

http://www.elsevier.com/locate/jsames

http://dx.doi.org/10.1016/j.jsames.2012.06.006



A.F. Scherrenberg et al. / Journal of South American Earth Sciences 38 (2012) 147e158148

system. The major phase of deposition started in the Late Jurassicwith subsidence and deposition continuing until the Late Creta-ceous (Mégard, 1984; Benavides-Cáceres, 1999). The WPT is thewesternmost of two depositional basins; it was separated fromthe East Peruvian Trough by the Marañón High and was flanked tothe west by the Paracas High (Mégard, 1984; Benavides-Cáceres,1999) (Fig. 1a). An obscure boundary is generally considered todivide the WPT into a western intra-arc trough, dominated byigneous rocks of the coastal batholith, and an eastern back-arcbasin, where continental clastic rocks and marine carbonates arediscordantly overlain by volcanic rocks (Wilson, 1963; Wilson et al.,1967; Mégard, 1987a). The latter was subsequently deformed intothe present MFTB.

Fig. 1. Map of central Peru and cross sections through the Cretaceous sedimentary cover (Benavides-Cáceres, 1999). (a) Map illustrating the main Late Cretaceous tectonic units. (b)across the WPT during the Cretaceous. (c) Schematic cross-section (BeB0) of the current arstratigraphic variations across a thrust in the MFTB: the sandstone-dominated unit thickensthe Upper Cretaceous marl-limestone within the WPT and red bed sequence across the MFTBthe reader is referred to the web version of this article.)

The eastward tapering wedge of sedimentary rocks in theback-arc basin represents Upper Jurassic to recent rocks, whichare separated from a Palaeozoic basement by a major unconfor-mity (Mégard, 1984; Benavides-Cáceres, 1999). Within the MFTB,Lower Cretaceous facies and thickness changes are thought tooccur across a reverse fault (reactivated normal fault) that sepa-rates the sedimentary wedge sequence into eastern and westernfacies (Mégard, 1987b) (Fig. 1c). Mégard (1987b) suggested thatthis reverse fault initiated as a listric growth fault that divided theback-arc basin and its strata into a thick sequence and a thin shelfsequence, and may have been basement-involved. The position ofthis reverse boundary does not overlap with the westernboundary of the MFTB (Fig. 1a) (Wilson, 1963; Wilson et al., 1967)

modified with elements from Wilson, 1963; Wilson et al., 1967; Mégard, 1984, 1987b;Schematic cross-section (AeA0) showing the prominent facies and thickness changeschitecture of the inverted back-arc basin illustrating the simplified (Lower) Cretaceousto the west, whereas the shale unit disappears toward the east. Note that the extent ofis poorly constrained. (For interpretation of the references to color in this figure legend,

Fig. 1. (continued).

A.F. Scherrenberg et al. / Journal of South American Earth Sciences 38 (2012) 147e158 149

nor does it overlap with its eastern boundary (Romero, 2008;Carlotto et al., 2009).

The lower part of the thick sequence is generally comprised ofUpper Jurassic to Lower Cretaceous shale and sandstone interca-lated occasionally by a limestone unit. The Upper Cretaceoussequence is dominated by limestone (Mégard, 1987b) (Fig. 1b).Contrastingly, the shelf sequence is characterised by a muchthinner succession of Cretaceous sandstone and limestone hori-zons. The beginning of Andean orogenesis is marked strati-graphically by the cessation of marine input and the appearance offoreland red beds. Episodic orogenic pulses, crustal thickening anduplift were responsible for bringing the former back-arc basin to itspresent-day altitude (Mégard, 1984, 1987b; Benavides-Cáceres,1999) (Fig. 1c). Foreland red beds and arc-related volcanic rocksdominate the Cainozoic stratigraphy and blanket the deformedbasin.

The distribution of pre-Cainozoic stratigraphic units variesconsiderably across the MFTB (Fig. 2). Palaeozoic and early Meso-zoic units are exposed on the belt’s eastern margin and are rarelyexposed within the belt so the nature of basement rocks underlyingthe Cretaceous units is not entirely understood. The Cretaceoussequences show variations in several lower and upper units in an‘east-west’ direction, both in northern and central Peru (e.g.Wilson, 1963; Janjou et al., 1981; Benavides-Cáceres, 1956) (Fig. 2).This has led to suggestions that the MFTB forms the boundary zoneto the Western and Eastern cordilleras (Janjou et al., 1981) and that

the inverted West Peruvian Trough was comprised of numeroussmaller basins (e.g. Mégard, 1987b; Carlotto et al., 2009).

3. Stratigraphy and revised geological maps

We conducted extensive fieldwork and produced a revisedgeological map of a w130 km long segment of the MFTB fromapproximately La Unión to Churín, and three stratigraphic columnsfrom three different localities (Fig. 3). The choice of the three typelocalities for stratigraphic analysis was governed by access,complete stratigraphy and coverage of the key tectonic units in thestudy area. The localities are: (1) near Margos, where outcropsseem to include a complete sequence from the Palaeozoic base-ment to the Cainozoic red beds; (2) near Cauri, where the sequenceseems similar to that near Margos, except for the missing Carbon-iferous succession and Upper Cretaceous marl and red beds, and (3)near the Iscaycruz mine, where the sequence appears representa-tive for the Cretaceous in the western part of the study area; rocksolder than the Cretaceous, as well as PalaeogeneeNeogene volcanicrocks, are not exposed in this sequence.

Strata thicknesses of individual units in the Palaeozoic to Cai-nozoic section are predominantly field estimates, complementedby information from the literature and map pattern. We think thatthe stratigraphic columns represent the overall stratigraphy aver-aged over large areas within the study area. However, we note thatunit thicknesses are highly variable across the region.

Fig. 2. A comparison of the Cretaceous stratigraphy across the MFTB in central and northern Peru showing the rock units that outcrop in the study area (modified with sections fromBenavides-Cáceres, 1956; Wilson, 1963; Janjou et al., 1981). The last column illustrates the basic stratigraphic order of units described in this study, divided into five segments thatrefer to sections 3.1e3.5.


3.1. Metamorphic units

The lowermost unit of the study area is extensively folded, andconsists of phyllite, sericitic schistwith dark to light grey phlogopite,andmetavolcanic intercalations. These rocks occupy approximately40% of the surface geology of the study area and are exposed alongthe eastern margin of the FTB. The rocks are foliated, stronglysilicified, and contain abundant quartz veins parallel and across thefoliation. This unit is part of theNeoproterozoic-PalaeozoicMarañónComplex (Wilson and Reyes, 1964; Dalmayrac et al., 1980; Chewet al., 2007; Cardona et al., 2009; Carlotto et al., 2009).

A thick sequence (w160m)of basaltic volcanic rocks discordantlyoverlies the Marañón Complex. Thin layers of shale and cross-bedded sandstone intercalate 0e10 m thick basic lava flows. Thisunit outcrops locally at the base of the canyons on the eastern side ofthe studied area, e.g. along the Lauricocha River between Cauri andJesús. We think that this volcanic sequence is a local flow, called theCauri volcanics (Scherrenberg and Jacay, 2006). It may correlateregionally with the Vijus Formation in the Pataz area, and probablywith the volcanic members of the Ollantaytambo Formation in theCuzco region, giving it a likely LowereMiddle Ordovician age.

Discordantly overlying these volcanic rocks is a 150e200 mthick schist sequence of metamudstone intercalated by fine- tocoarse-grained, cross-bedded metasandstone. It is only exposedlocally above the Cauri volcanics. Regionally, we think that theserocks pinch out westward, and correlatewith the graptolite bearingContaya Formation (Newell and Tafur, 1943), suggesting that thisunit reflects a MiddleeUpper Ordovician marine transgression.

3.2. PalaeozoiceMesozoic sequences

The base of the PalaeozoiceMesozoic sequence lies discordantlyon the Marañón Complex near Margos, but is missing from the

sections near Cauri and probably farther west. The Lower Palae-ozoic to Lower Cretaceous section is well exposed in the canyonnear Cauri, but lacking the unit that makes up the base of thePalaeozoiceMesozoic sequences (Fig. 4). Therefore, we suggest thatthis basal unit pinches out to the west. The rocks of this unit gradefrom muddy argillaceous sandstone and shale up to sandstone andconglomerate with thin lenses of coal (containing plant remains).An upper sequence of reworked tuffs completes the 200e300 mthickness of this group. The entire succession is exposed east ofthe uplifted block exposing theMarañón Complex in the east of thisstudy area. We interpret this lowermost sedimentary group topinch out to the west, and to correlate stratigraphically with theCarboniferous (wMississippian) Ambo Group (Newell et al., 1953).

An uncleaved succession, dominated by red beds, unconform-ably overlies the Marañón Complex, Contaya Formation and AmboGroup. In the study area, its upper section consists of a thicksequence of red and grey, medium to fine sandstone, with angularclasts, shale and volcanic and pyroclastic intercalations, and mud-crack textures. The lower sequence is predominantly representedby thick intervals of conglomerate and/or breccia, containing clastsof quartz, phyllite, and schist, and volcanic rocks. Within theconglomerate, clasts of intrusive rocks, limestone, quartzite, andgneiss are less abundant. The matrix is a red muddy arkosic sand-stone. We interpret this sedimentary succession to correlate withthe Mitu Group (McLaughlin, 1924). These rocks are exposed in theeastern part of the study area, where the thickness of the group ishighly variable, from approximately 1100 m near Margos to 800 mnear Cauri to zero at other localities. These large variations inthickness occur predominantly across faults, clearly demonstratedin the valley south of Cauri (Fig. 4). The lower facies indicatescontinental alluvial fan deposits (with eastward flow), whereasthe upper facies is associated with flood plain deposits. (Half-)grabens can control rapid fluctuations in their thicknesses, and

Fig. 3. Geological map of study area (revised from base maps of Cobbing and Sánchez, 1996a,b; Cobbing and Garayar, 1998) showing the three locations of logging spread across twoancient sub-basins of the WPT (deep basin and shelf) separated by the Chonta fault (CF). ‘Eye’ symbol shows view direction of cross-section CeC0 presented in Fig. 4.


Fig. 4. Schematic cross section along the valley south of Cauri with photos (view toward W) showing large thickness changes in the Permian Mitu red beds (Ps-m) across faults. Thefaults that do not influence the overlying Goyllarisquizga Group (Ki-g) were inactive during Early Cretaceous deposition. One fault also bounds the Contaya metasediments andCauri volcanic rocks (depicting a pre-rift geometry), indicating that it had been active earlier. More thickness changes across growth faults in the Mitu Group can be found along thisvalley south of Cauri, and indicate a syn-depositional extensional tectonic regime. PE-cm: Marañón Complex (part of metamorphic basement). (For interpretation of the referencesto color in this figure legend, the reader is referred to the web version of this article.)


consequently indicate an extensional/rifting tectonic regime duringthe Permo-Triassic deposition of the Mitu Group.

Concordantly overlying the Mitu Group is a brownish greymicritic limestone (fine facies) that is finely laminated, containsabundant quartz grains as well as fossils, and outcrops as large,thin, karstified lenses or patches. These fine facies alternate withthick horizons of slope breccia facies that comprise limestone clastsand a fine conglomeratic matrix. These rocks are exposed in theeastern part of the study area. Their thickness changes from ca.130 m near Margos to 60 m near Cauri to zero west of Jesús and atsome other localities. These thickness changes are related to pinch-outs and faults. The rocks represent supra-, inter- and subtidalprocesses, and we correlate them with the Chambará Formation(Harrison, 1943; Mégard, 1968) of the Upper Triassic e LowerJurassic Pucará Group (McLaughlin, 1924). The two youngerformations that complete this group, the dominant supercrustalsequence in the eastern Cordillera, are not exposed and most likelyare not represented in the study area.

Along the NW limb of theMargos Syncline (on the Jesús-Margosroad) is a thin (60e70 m) horizon of conglomeratic breccia withabundant milky quartz and rare limestone clasts, discordantlyoverlying the Chambará Formation. The breccia unit displays weakcross-bedding, and contains a white sandy-fine conglomeraticmatrix. We think that these rocks represent alluvial fan depositsthat are related to a brief period of erosion and tectonic quiescencein the Upper Jurassic. We first discovered this unit outside the study

area at Huacaybamba Puente Copuma along the Marañón Riverfrom whence the name Copuma breccia is derived. At this locality,the breccia overlies the Pucará Group discordantly and is discor-dantly overlain by the Goyllarisguizga sandstone.

3.3. Cretaceous sequences

An alternating sequence of thin-bedded, sub-greywacke sand-stone and carbonaceous shale associated with coal lenses occurs inthe cores of the Andean anticlines in the western part of the studyarea, but appears to be completely absent in the eastern part. Itsestimated thickness of >375 m is minimal because the base of thissequence is not exposed in the study area. We consider these rocksto represent swamp deposits of the basal Cretaceous Oyón Forma-tion (Wilson, 1963) that only appear in central Peru. The earliestCretaceous age of this formation is based on plant remains and itsstratigraphic location relative to other better-dated units.

A thick sequence dominated by sandstone discordantly overliesthe limestone units of the Chambará Formation or the meta-morphic rocks of the Marañón Complex in the eastern part of thestudy area, and the shale/coal of the Oyón Formation in thewesternpart of the study area. The dominantly sandstone sequence corre-lates with the Lower Cretaceous Goyllarisquizga Group (Wilson,1963). The group shows a pronounced facies variation across thearea. In the eastern half of the study area, the units are entirelysandstone dominated and the group is undifferentiated. In the


western half, carbonate-rich units are intercalated into the sand-stone sequences, and the group comprises four lithologicallydistinct units: the Chimú, Santa, Carhuaz and Farrat Formations.From east to west, the thickness of the Goyllarisquizga Groupchanges from 600 to 1100 m in the Margos section, tow500e800 m in the Cauri section, and then to w2000 m in thewestern succession near Iscaycruz.

In the eastern half of the area, we have divided the Goyllar-isquizga Group into a lower unit that consists of white coarse to finequartzitic sandstone with large (5e10 m) sets of cross-beds, and anupper unit that is represented by medium to fine quartzitic sand-stone, light grey in colour, with distinctive medium to large (1e5 mscale) cross-bedding, and intercalated by thin red shale-like layerswithmud cracks and load casts. We interpret the facies of the lowerunit of this group as eolian to fluvio-eolian, indicative of desertenvironments. The upper unit comprises fluvial facies deposited inwide flood plains typical of littoral to distal deltaic systems.

In the western half of the area, the basal unit of the Goyllar-isquizga Group that discordantly overlies the Oyón Formation isa w600e900 m thick sandstone-dominated unit (Chimú Forma-tion). It can be subdivided into a basal sequence of w1 m thicklayers of medium to coarse grained sandstone, intercalated by coalhorizons and various coloured shale, overlain by>1m thick beds ofwhite orthoquartzite that form the middle and upper part of theunit. Cross-beds and wave ripples are prominent and indicate W,SW and S current directions. We interpret the Chimú Formation(Benavides-Cáceres, 1956) as a fluvio-deltaic (transitional) systemthat had an eastern source area, probably in the eolian sandstone ofthe Brasilian and Guianan cratons.

Dark grey, microgranular fossiliferous dolomitic limestone andmarl alternations concordantly overlie the orthoquartzite of theChimú Formation. They have a thickness ranging from w100 m to260 m across the western part of the study area and are absent inthe east. These rocks and their stratigraphic position are typicalof the Santa Formation (Benavides-Cáceres, 1956) in central Peru.Based on fossil content and sedimentary textures, we interpret itsdepositional environment as transitional, with brackish conditionsnear the shoreface, displaying oolitic tidal facies. This formationrepresents a high concentration of maximum transgressive sealevels in Valanginian times.

A w400e800 m thick sequence of fossiliferous (plant remains)shale and marl intercalated by fine-medium grained, reddish toviolet sandstone with flaser-bedding and cross-bedding of theCarhuaz Formation (Benavides-Cáceres, 1956) overlie the SantaFormation limestone concordantly, and only occur in the westernpart of the study area. Some fossils are characteristic of brackishconditions, such as coastal marsh environments that incur sporadicshallow marine transgressions as indicated by the occasionalgypsum and oolitic limestone, bivalve trigoniidae, gastropoda,bioclastic limestone, and dinosaur tracks (in the Huallanca region).Consequently, we interpret the Carhuaz Formation as beingdeposited in a tidal flat environment, which coincides with the endof the Valanginian transgression, represented by the SantaFormation.

The Carhuaz Formation is overlain concordantly by a sequenceof 0.1e1 m thick beds of orthoquartzitic sandstone that display redpatina, common cross-beds and ripple-marks, and are intercalatedby 1e10 cm thick layers of fine organic-rich marl. This unit maycorrelate with the Farrat Formation (Stappenbeck, 1929; Wilson,1963), and is the uppermost part of the Goyllarisquizga Group inthe western part of the study area. Rare, poorly preserved plantremains and ripple-marks in the sandy marl horizons indicatea fluvial-deltaic depositional environment.

The Pariahuanca Formation (Benavides-Cáceres, 1956) isexposed as a yellow-beige sequence containing thin layers (up to

0.5 m thick) of fine sandstone with a calcareous matrix. It displaysripple-marks, flaser bedding, and cross bedding, concordantlyoverlies the Goyllarisquizga Group, and appears as lenses in boththe eastern and western parts of the study area. The PariahuancaFormation generally thins eastward (w200 m near Iscaycruz tow60 m near Cauri), but shows a slight increase in thicknessbetween Cauri and Margos (w100 m). The facies and textures aretypical of foreshore environments, and farther east may evenchange into continental facies (perhaps across a fault) (Palacioset al., 1995).

A sequence composed of 0.5e1 m thick, light grey-bluish(sandy) limestone layers, local sandstone, and calcareous shale,containing sporadic fine algal laminations and lenses of breccia andchert, concordantly overlies the Pariahuanca Formation, andgradually thickens westward fromw100m atMargos tow300m atCauri. The sequence splits into two distinguishable units nearIscaycruz and in the western half of the study area. Its fossil contentis abundant, and represented by numerous bivalves, pelecypods,ostreidae and ammonites. Oolitic limestone with echinodermremains intercalates with the limestone layers, of which some havea strong hydrocarbon odour. Facies and rhythmic sequencesobserved in the lower part of the unit indicate an external platformenvironment, but toward the top, its textures and structures indi-cate a shallower, more energetic setting. This unit marks the changefrom a transitional environment with dominant continental influxinto deeper marine. It corresponds to the Crisnejas Formation(Benavides-Cáceres, 1956), and is equivalent to the thicker Chulecand Pariatambo formations combined in the western part of thestudy area.

The equivalent rocks, which concordantly overly the Pariah-uanca Formation in the western part of the study area, constitutea w200 m thick sequence of clear grey marl and limestone that isfossil-rich. We think that these rocks represent the Chulec Forma-tion (McLaughlin, 1924; Benavides-Cáceres, 1956). The lower two-thirds of this unit consists generally of thick limestone beds,whilst the upper part mainly comprises shaley and sandy marlintercalated by limestone. This sequence represents a depositionalenvironment associated with an open platform to the west, severaltransgressive episodes, ammonite-rich fauna, and a period ofmoderate subsidence.

Alternating beds of thin grey limestone with dark grey marly-shale (totalling w150e250 m in thickness) concordantly overliethe Chulec Formation limestone. Fossils are abundant from base totop, and chert nodules mark the top. We interpret this as the Par-iatambo Formation (McLaughlin,1924; Benavides-Cáceres,1956). Itsoverall lithology indicates anoxic conditions that provided bitu-minous facies. During this time, local “oceanic-upwelling” wasassociated with global eustatic sea-level changes and bituminousfacies were deposited in an anoxic environment.

Thick limestone sequences concordantly overlie the westernPariatambo and eastern Crisnejas Formations, and can be dividedinto three common subsequences: (1) the lower unit, comprisingthin to medium beds (0.5e1 m thick) of limestone intercalated byblack marl and chert, and locally, at the base of the sequence, bybasalt; (2) the middle unit, representing thick limestone beds,occasionally associated with syn-sedimentary slumping; and (3)the upper unit, consisting of thin tabular limestone beds alternatingwith blackmarl. A section (5e10m thick) of blackmarl, observed allover the region, characterises the middle-upper boundary. Anothercharacteristic of this formation includes the presence of severallayers with a strong hydrocarbon smell, bioturbation, oolites, andseveral ammonite species, including Llyelliceras ulrichi, Paren-gonoceras cf P.haasi, Manuaniceras Peruvianum multifidum, Morto-niceras sp. and Parengonoceras aff. pernodosum. We interpret theserocks as the Jumasha Formation (McLaughlin, 1924), which is


thought to have a variable thickness (e.g. Wilson, 1963). Ourthickness estimates of this limestone sequence show an apparenteast to west thickening from Margos (w1000 m) through Cauri(w1500 m) to Iscaycruz (>2000 m). The top of the sequence at thelatter two localities is not present, so the thickness is under-estimated. Based on the faunal diversity in the study area, thisformation developed in an “epicontinental” carbonate platform,where an open, shallow platform was associated with submarinebars of oolitic sand, distinct layers of ammonites and moderate tostrong bioturbation. Intercalations of hydrocarbon-rich rocks withno fossils demonstrate eustatic sea-level variations ranging up tomaximum transgressive sea-level stands. The observed ammonitespecies indicate a middle/late Albian to late Turonian age for theJumasha Formation, which is similar to previously publishedbiostratigraphic ages (Benavides-Cáceres, 1956; Von Hillebrandt,1970; Romani, 1982; Jaillard, 1986).

A thin (up to 120 m thick) unit concordantly overlies theJumasha Formation and comprises a progressively thinningsequence of grey to greenish blue limestone, marl and gypsum. Itsfossil content includes several species of echinoderm, bivalves,invertebrate remains (annelids), and rare pectinids. This representsthe Celendín Formation (Benavides-Cáceres, 1956). Although thisformation appeared absent in thewestern part of the study area, weobserved fish teeth and scales in some levels of the eastern part ofthe study area. The evaporites and annelids indicate a Sabkhadepositional environment that evolved into a slightly deeper waterenvironment westward, where ammonites occurred. The faunalcontent of the Celendín Formation, combined with a regionalcorrelation to equivalent units in northern Peru, indicate a Con-iacian-Santonian age, perhaps ranging up to the early Campanian.

3.4. Cretaceous-Palaeogene foreland sequence

The appearance of widespread red beds marks a transition intocompletely terrestrial sequences that are broadly coeval with theinitiation of Andean orogenesis. In the study area, the CasapalcaFormation (McLaughlin, 1924) is characterised by red, cross-beddedand graded sandstone, alternating with mica-poor red shale. At thebase of the formation, calcareous marl with colours ranging fromgreenish to reddish-purple indicates sporadic marine incursions ina tidal flat environment. Toward the top, sporadic flood plain andabundant alluvial fan deposits, including thick fluvial sequences ofbraided channels, dominate the sequence. The prominent crossbeds indicate NE current directions near Margos and WSW direc-tions near Lauricocha. To the east, the Casapalca Formation overliesthe Celendín Formation disconformably and is quite widespread. Inthe west, it is only preserved in the cores of synclines paralleled bythrust faults. Its map pattern indicates foreland depocentres east ofthe paralleling fault traces. Its thickness is unknown due to theabsence of a concordantly overlying sequence. Age estimates forthe unconformably overlying Calipuy volcanics (see below) indicatea minimum Palaeocene age for the red beds.

3.5. PalaeogeneeNeogene volcanic arc sequences

Discrete bodies of andesitic-rhyolitic volcanic rocks uncon-formably overly the folded Cretaceous-lower Palaeogene sequencesthroughout the area. These volcanic rocks not only preservea record of arc activity across this continental margin, but alsoprovide minimum age constraints on tectonic events within thebroader Andean Orogeny. The rocks unconformably overly thestrongly folded Cretaceous sequence, but some are themselvesbroadly folded, thus providing evidence for younger deformation.

Typically, conglomerates at the base of the volcanic sequenceare overlain by a thick series (>500 m) of broadly folded andesitic

pyroclastic, breccia and lava flows overlain slightly discordantly bya thick sequence of unfolded lithified ash-flow tuffs (ignimbrites).This sequence is typical of the Calipuy Group (Cobbing et al., 1981).Facies and thickness variations in the andesitic and pyroclasticflows (which are associated with minor discordances) indicatedifferent volcanic centres. We observed various dykes and sills inthe study area, of which some could represent subvolcanic conduitsof these volcanic rocks.

4. Discussion

4.1. Stratigraphic correlations across the study area

Fig. 5 shows the age, thickness and stratigraphic relationships oftheMesozoic formations in thewestern part of the study area (fromthe Iscaycruz section), and their correlation with the Palaeozoic toMesozoic formations in the eastern parts of the study area (Cauriand Margos sections). The Marañón Complex, Mitu Group andChambará Formation are thought to underlie (most of) the westernpart, whereas the other units appear to pinch out in the east. TheMarañón Complex is suggested to represent the basement of thecentral and northern Peruvian Andes (e.g. Mégard, 1987b), and istherefore presumed to form part of the substratum of the westernpart of the study area. The Mitu Group and Chambará Formationrocks are not exposed in the western part of the study area, but canbe observed in the area of Cauri and Margos.

The observed eastward disappearance of the basal CretaceousOyón shale unit, and the distinct facies changes and thinning of theGoyllarisquizga Group fromwest to east in the study area, confirmearlier suggestions that the Lower Cretaceous sedimentary cover ofthe WPT is characterised by east-west thickness variations andfacies changes (Wilson, 1963). However, the change observed hereoccurs across a reverse fault, the Chonta Fault (CF), well inside theMFTB and not along the western or eastern margins. This fault doesnot coincide with the proposed boundary position ofWilson (1963)that supposedly separates the WPT into a marginal trough in thewest and the slope to the Marañón High (back-arc basin in Fig. 1) inthe east, nor with the proposed boundary of Romero (2008) andCarlotto et al. (2009) that supposedly separates a structural high tothe east and basin to the west. The question then becomes whetherthe WPT has any other major boundaries that divide it into addi-tional sub-basins.

The change in the Upper Cretaceous and uppermost LowerCretaceous units across the area is recognised by thickness varia-tions across faults and in facies changes within two units. Primarily,the thinner mixed facies of the Crisnejas Formation in the eastbecome the clearly distinguishable middle Albian Chulec limestoneand Pariatambo shale Formations in thewest, with the Chonta Faultmarking the boundary zone. The Pariahuanca Formation, however,is thinning to the east, and is suggested to show an eastward facieschange from transitional to continental (Palacios et al., 1995). Thischange could be across the Chonta Fault, but this has not beenclearly observed by us. Additionally, the lower Upper CretaceousJumasha Formation clearly shows a westward thickness increaseacross the Chonta Fault. Facies changes are less distinct, althoughfurther subdivision of this unit into subunits, as done in northernPeru (Fig. 2), could lead to better lateral facies distinction.

The Upper Cretaceous Celendín Formation does not extend intothe western area. This could be the result of erosion, but could alsobe related to the onset of the Andean Orogeny, which presumablyuplifted the tectonic block west of the Chonta Fault above sea level.This scenario could explain the absence of the Celendín marlhorizons in the western part. The facies changes in the CelendínFormation in the eastern part of the study area, from limestonethrough marl to gypsum, indicate desiccation of the eastern block

Fig. 5. Stratigraphic columns, showing the age, thickness (in metres) and stratigraphic relationships of Mesozoic formations in the western part of the study area (from the Iscaycruzsection), and their correlation with the Palaeozoic to Mesozoic formations in the eastern parts of the study area (Cauri and Margos sections). An overall SW-ward thickness increaseis apparent in the Cretaceous sedimentary rocks, and distinct facies changes occur between the Cauri and Iscaycruz columns. Note that the horizontal grain size scale does not applyto the limestone units. F, fine grained; M, medium grained; C, coarse grained; V, very coarse grained; P, pebble size; Cb, cobble size.


due to either very slow uplift or a drop in sea level. This differencein uplift rate between the eastern and western tectonic blocksexplains why Campanian to Neogene foreland red beds are foundonly in the eastern part of the study area.

The Casapalca Formation red beds also only persist as far as theChonta Fault. Beyond that boundary, the Jumasha is the youngestexposed unit of the Cretaceous sequence. Similarly to the CelendínFormation, it may have been present in the west but removed byerosion. Except of the Celendín Formation, the Casapalca Formationis the earliest unit that shows evidence of syn-deformationaldeposition, with the establishment of foreland-like depocentres.It is unlikely that such foreland systems would persist much fartherwest beyond the Chonta Fault. The incorporation of these red bedsinto the subsequent thin-skinned fold-thrust belt implies an east-ward stepping of the deformation front into the foreland system,masking the original westward margin of that depositional system.

4.2. Fault architecture and basement involvement

The Chonta Fault is a major boundary within the study area ofthe MFTB that not only separates the Cretaceous sedimentarywedge into thicker western and thinner eastern facies, but alsopenetrates the Palaeozoic units. This provides insight into itsarchitecture and suggests that the fault is basement-involved. Inthe context of this discussion, the term basement has two mean-ings. All rocks below the major unconformity at the base of theCretaceous sequences (base of Oyón Fm in the west and base of theGoyllarisguizga Group in the eastern part) are the stratigraphicbasement to the Cretaceous WPT (Mégard, 1984; Vicente, 1989).These basement rocks include the Lower Palaeozoic metamorphicrocks, and a relatively thin supercrustal sequence of Carboniferous-Jurassic sedimentary rocks. To the northwest, just outside the mainstudy area, rocks of the Mitu Group red beds have been observed,

Fig. 6. Block diagram showing the palaeogeography of central Peru during the Late Cretaceous (revised from Mégard, 1987b; Marocco, 1987; Jacay, 1992) and a schematic cross-section of the eastern basins of the WPT illustrating in more detail the basin structure. The latter shows the facies and thickness changes across the Chonta Fault, eastward tilting ofthe sub-basins, stratigraphic thinning on the shoulder of the half-graben, and increasing bed dips with depth. The absence of the Celendín and Casapalca Formations to the west ofthe Chonta Fault may be due to erosion or may be the effect of the build up of sufficient relief. The thickness and type of facies at depth in both sub-basins can vary, as well as thethickness of units to the east and west of the Chonta Fault, which is marked by the question marks. Arrows on either side of the schematic cross-section indicate two of theprominent facies changes. sb, stratigraphic basement.


folded together with the Cretaceous sequence. This suggests thateven though the basal Cretaceous Oyon shales were thought torepresent the main décollement level for the thin-skinned FTB (e.g.Wilson et al., 1967), a deeper basal detachment may be present inthe Palaeozoic sequences. Tectonically, the basement to the thin-skinned FTB represents the rocks beneath the basement detach-ment, and these are dominantly metamorphic rocks (althoughlocally may include some of the Carboniferous-Jurassic sedimen-tary rocks).

The facies/thickness boundary described here (cf. Chonta Fault)could reflect a long-lived intrabasinal structure. Such a structuremay have extended into the metamorphic basement and reac-tivated as a basin structure through multiple basin-forming events.The same boundary also corresponds to a major change in defor-mation style across theMFTB (Mégard,1987b; Scherrenberg, 2008),further reinforcing the fundamental nature and likely basementinvolvement of the controlling structure. The Chonta Fault, thepresent-day expression of the structure, is a steeply dipping reversefault, which we interpret as an inverted normal fault.

The pattern of thickness variation across the Chonta Faultprovides a constraint on the likely geometry of the postulated basinfault. Units are consistently thickest west of the boundary. On theeastern side, units are thinnest close to the boundary (in the Caurisection) and thicken away from the boundary (Margos section).These observations are consistent with a half-graben geometry,with the graben floors tilted to the east (Fig. 6). In addition to itsPermian precursor, the Chonta Fault must have persisted asa growth fault throughout much of the history of the Cretaceousbasin.

4.3. Basin reconstruction of the central MFTB

Fig. 6 shows a schematic reconstruction of the main tectonicelements in central Peru during the Late Cretaceous with anemphasis on the back-arc basin of the WPT. The reconstructionshows distinct facies changes and thickness variations in the LowerCretaceous sedimentary cover of the MFTB, confirming the rela-tionship to a basement-involved fault that divides the FTB into two


parts (Mégard, 1987b). The stratigraphic relationships suggest thatthe growth fault was active from the early Permian until at least thelatest Cretaceous and probably up to the Neogene. Most impor-tantly, Fig. 6 illustrates schematically the distinct facies changesand thickness variations in the Permian to Neogene rocks. Theabrupt change occurs across a major reverse fault inside the MFTB(as suggested by Mégard, 1987b). This fault has experienceda complex kinematic history. It was active as a normal fault duringbasin formation, and was subsequently inverted during the devel-opment of the MFTB (Fig. 1c). In addition, local slickenlines on thefault indicate a (local) strike-slip component. The episodic activityof this fault is further expressed in the stratigraphic unconformities(Fig. 5), which we attribute to temporal changes in the geodynamicsetting, such as plate realignments, changes in convergence ratesand ridge subduction.

5. Conclusions

A major boundary exists within the MFTB, across which strati-graphic units show distinct facies and thickness changes. Thesechanges are observed within a large section, from the PermianMituGroup to at least the Upper Cretaceous Jumasha Formation. Themost pronounced variations are found in the lower units (Oyón Fm,Goyllarisquizga Group, Crisnejas-Chulec-Pariatambo Fm) and oneupper unit (Jumasha Fm) of the Cretaceous basin. The boundarycorresponds to a prominent reverse fault, the Chonta Fault, withinthe MFTB. The Chonta Fault is a long-lived intrabasinal structure,which extends into the (metamorphic) basement and reactivated asa basin structure through multiple basin-forming events. Atpresent, the Chonta Fault is a steeply dipping reverse fault repre-senting an inverted normal fault.

The architecture of the Chonta Fault depicts a half-grabengeometry, with the graben floors tilted to the east (Fig. 6). Thepattern of thickness variation across the fault is thickest west of theboundary, while eastward it is thinnest close to the boundary (inthe Cauri section) and thickens away from the boundary (Margossection). Besides its Permian precursor, the Chonta Fault persistedas a growth fault throughout much of the history of the Cretaceousbasin.

We conclude that during most of the Cretaceous, the WPT wasdivided into three sectors: an intra-arc trough, a deep basin anda platform/shelf. This basin architecture provides insight into theinherited structural framework of the Andean Orogeny in centralPeru.

Acknowledgements

AngloGold Ashanti Ltd Pty is thanked for their generous finan-cial and logistical support.

References

Benavides-Cáceres, V.E., 1956. Cretaceous system in northern Peru. Bulletin of theAmerican Museum of Natural History 108, 353e494.

Benavides-Cáceres, V.E., 1999. Orogenic evolution of the Peruvian Andes: theAndean Cycle. In: Skinner, B.J. (Ed.), Geology and Ore Deposits of theCentral Andes, vol. 7. Society of Economic Geologists Special Publication,pp. 61e107.

Cardona, A., Cordani, U.G., Ruiz, J., Valencia, V.A., Armstrong, R., Chew, D.,Nutman, A., Sánchez, A.W., 2009. U-Pb zircon geochronology and Nd isotopicsignatures of the pre-Mesozoic metamorphic basement of the eastern PeruvianAndes: growth and provenance of a Late Neoproterozoic to Carboniferousaccretionary orogen on the northwest margin of Gondwana. The Journal ofGeology 117, 285e305.

Carlotto, V., Quispe, J., Acosta, H., Rodríguez, R., Romero, D., Cerpa, L., Mamani, M.,Díaz-Martínez, E., Navarro, P., Jaimes, F., Velarde, T., Lu, S., Cueva, E., 2009.Dominios Geotéctonicos y Metalogenésis del Perú. Boletín Sociedad Geológicadel Perú 103, 1e89.

Chew, D., Schaltegger, U., Ko�sler, J., Whitehouse, M.J., Gutjahr, M., Spikings, R.A.,Mi�skovi�c, A., 2007. UePb geochronologic evidence for the evolution of theGondwanan margin of the north-central Andes. Geological Society of AmericaBulletin 119 (5e6), 697e711.

Cobbing, E.J., 1976. The geosynclinal pair at the continental margin of Peru. Tecto-nophysics 36 (1e3), 157e165.

Cobbing, E.J., 1978. The Andean geosyncline in Peru, and its distinction from Alpinegeosynclines. Journal of the Geological Society 135 (2), 207e218.

Cobbing, E.J., Garayar, J., 1998. Mapa geológico del cuadrángulo de Oyón. PerúInstituto Geológico Minero y Metalúrgico Hoja 22-j, scale 1:100,000.

Cobbing, E.J., Pitcher, W.S., Wilson, J.J., Baldock, J.W., Taylor, W.P., McCourt, W.,Snelling, N.J., 1981. The geology of the Western Cordillera of northern Peru.Overseas Memoir, Institute of Geological Sciences 5, 132.

Cobbing, E.J., Sánchez, A.W., 1996a. Mapa geológico del cuadrángulo de La Unión.Perú Instituto Geológico Minero y Metalúrgico Hoja 20-j, escala 1:100,000.

Cobbing, E.J., Sánchez, A.W., 1996b. Mapa geológico del cuadrángulo de Yana-huanca. Perú Instituto Geológico Minero y Metalúrgico Hoja 21-j, escala1:100,000.

Coward, M.P., 1994. Inversion tectonics. In: Hancock, P.L. (Ed.), Continental Defor-mation. Pergamon Press Ltd, pp. 289e304.

Dalmayrac, B., Laubacher, G., Marocco, R., 1980. Caractères généraux de l’évolutiongéologique des Andes péruviennes. In: Travaux et Documents de l’ORSTOM, vol.122, Paris, 501 pp.

Harrison, J.V., 1943. Geología de los Andes centrales en parte del departamento deJunín, Perú (estudio definitivo). Boletín de la Sociedad Geológica del Perú 16,1e52.

Jacay, J., 1992. Estratigrafía y sedimentología del Jurásico curso medio del valle deChicama y esbozo paleogeográfico del Jurásico Cretáceo del nor Perú (6� 300 8�

latitud sur). Tesis Ingeniero, Universidad Nacional Mayor de San Marcos, 180 p.Jaillard, E., 1986. La sédimentation Crétacée dans les Andes du Pérou central:

exemple de la Formation Jumasha (Albien moyen à Turonien supérieur)dans la région d’Oyón (département de Lima). Géodynamique 1 (2),97e108.

Janjou, D., Bourgois, J., Mégard, F., Sornay, J., 1981. Rapports paléogéographiqueset structuraux entre Cordillères occidentale et orientale des Andes nord-péruviennes: les écailles du Marañon (7�S, Départements de Cajamarca et deAmazonas, Pérou). Bulletin de la Société Géologique de France 23 (6),693e705 (7).

Marocco, R., 1987. Evolución tectonosedimentaria del Perú central y meridionaldurante el cretácico. Programme International de Correlation Geologique, 242“Cretácico de América Latina”.

McLaughlin, D.H., 1924. Geology and physiography of the Peruvian Cordillera,Departments of Junin and Lima. Geological Society of America Bulletin 35,591e632.

Mégard, F., 1968. Geología del Cuadrángulo de Huancayo. Boletín del Servicio deGeología y Minería 18, 123.

Mégard, F., 1984. The Andean orogenic period and its major structures incentral and northern Peru. Journal of the Geological Society 141 (5),893e900.

Mégard, F., 1987a. Cordilleran Andes and marginal Andes; a review of Andeangeology north of the Arica Elbow (18� S). In: Monger, J.W.H., Francheteau, J.(Eds.), Circum-Pacific Orogenic Belts and Evolution of the Pacific Ocean Basin.Geodynamics Series, pp. 71e95.

Mégard, F., 1987b. Structure and evolution of the Peruvian Andes. In: Schaer, J.P.,Rodgers, J. (Eds.), The Anatomy of Mountain Ranges. Princeton University Press,pp. 179e210.

Myers, J.S., 1975. Vertical crustal movements of the Andes in Peru. Nature 254(5502), 672e674.

Newell, N.D., Chronic, B.J., Roberts, T.G., 1953. Upper Paleozoic of Peru. GeologicalSociety of America, Memoir 58, 111.

Newell, N.D., Tafur, I.A., 1943. Ordovícico fosilífero en la selva oriental del Perú.Boletín de la Sociedad Geológica del Perú 14e15, 5e16.

Palacios, O., Mendoza, M.J., Chacón Abad, N., Sánchez, A.W., León, L.W., Canchaya, S.,Aranda, A., 1995. Geología del Perú. In: Serie A: Carta Geológica Nacional, vol 55.Perú Instituto Geológico Minero y Metalúrgico, p. 177.

Rodríguez, R., 2008. El Sistema de Fallas Chonta y sus Implicancias Metalogenéticasentre 12�150 S y 13�300 S (Huancavelica e Perú). Tesis Ingeniero, UniversidadPolitecnica Madrid, 102 p.

Romani, M., 1982. Gèologie de la région minière Uchucchacua; Hacienda Otuto,Pérou, Ph.D. thèse, Université Joseph Fourier Grenoble I, 176 p.

Romero, D., 2008. The Cordillera Blanca fault system as structural control of theJurassic-Cretaceous basin in central-northern Peru. In: 7th InternationalSymposium on Andean Geodynamics, Nice, Extended Abstracts,pp. 465e468.

Scherrenberg, A.F., 2008. Structural framework of mineralisation, Marañón Fold-Thrust Belt, Peru. Ph.D. thesis, The University of Queensland, 232 p.

Scherrenberg, A.F., Jacay, J.P., 2006. Nuevas unidades litoestratigráficas Paleozoicasreconocidos en la regíon de Huánuco del Perú central; Implicancias tectónicas ymetalogenéticas para la faja plegada y corrida del Marañón. In: Caillaux, V.C.(Ed.), XIII Congreso Peruano de Geología. Sociedad Geológica del Perú, Lima,pp. 320e323.

Stappenbeck, R., 1929. Geologie des Chicamatales in Nordperu und seiner Anthra-cit-Lagerstatten. Neues Jahrbuch für Geologie und Paläontologie Abhandlung 16(4), 305e355.

Steinmann, G., 1929. Geologie von Peru. Karl Winter Universitat, Heidelberg, 248 p.


Vicente, J.C., 1989. Early Late Cretaceous overthrusting in the Western Cordillera ofsouthern Peru. In: Ericksen, G.E., Canas Pinochet, M.T., Reinemund, J.A. (Eds.),Geology of the Andes and its Relation to Hydrocarbon and Mineral Resources.Circum-Pacific Council for Energy and Mineral Resources Earth Sciences Series,vol. 11, pp. 91e117.

Von Hillebrandt, A., 1970. Die Kreide in der Zentralkordillere ostlich von Lima Peru,Sudamerika. Geologische Rundschau 59 (3), 1180e1203.

Wilson, J.J., 1963. Cretaceous stratigraphy of central Andes of Peru. AmericanAssociation of Petroleum Geologists Bulletin 47 (1), 1e34.

Wilson, J.J., Reyes, L., 1964. Geología del cuadrángulo de Pataz: hoja 16-H. Serie A:Carta Geológica Nacional, Boletín 9, 91.

Wilson, J.J., Reyes, L., Garayar, J., 1967. Geología de los cuadrángulos de Mollebamba,Tayabamba, Pomabamba y Huarí, Lima, Peru. Servicio de Geología y Minería delPerú, Boletín 16, 95.