The Huallaga foreland basin evolution: Thrust propagation in a deltaic environment, northern Peruvian Andes

The Huallaga foreland basin evolution: Thrust propagation

in a deltaic environment, northern Peruvian Andes

Wilber Hermozaa,*, Stephane Brusseta,b, Patrice Babya,b, Willy Gilc, Martin Roddaza,

Nicole Guerrerob, Molando Bolanosd

aLMTG-UMR 5563, Universite Paul Sabatier Toulouse III, 38 rue des 36 Ponts 31400 Toulouse, FrancebIRD UR 104 LMTG, 38 rue des 36 Ponts 31400 Toulouse, France

cConsultor, La Mariscala N8115, San Isidro Lima, PerudPeruPetro S.A., Luis Aldana 320, San Borja, Peru

Received 1 June 2003; accepted 1 June 2004

Abstract

The sub-Andean Huallaga basin is part of the modern retroforeland basin system of Peru. It corresponds to a thrust-and-fold belt

superimposed on inverted and halokinetic structures and is characterized by Eocene–Pliocene, thick synorogenic series that have controlled

the burial history of petroleum systems. Sedimentological analysis and a sequentially restored cross-section based on seismic data and new

field studies show three sequences of synorogenic deposits. The Eocene (Lower Pozo member) developed in shoreface environments, when

the basin morphology corresponded to a foresag depozone linked to an orogenic unloading period. The Middle Eocene sequence (Upper Pozo

member) developed in shallow marine environments and recorded a change in Andean geodynamics and the retroforeland basin system. The

basin morphology corresponded to a foredeep depozone linked to an orogenic loading period. This configuration remained until the Middle

Miocene (Chambira Formation). The Middle Miocene–Pliocene sequence recorded the onset of the modern sub-Andean Huallaga basin that

became a wedge-top depozone. Thrust propagation occurred in a deltaic environment, which evolved progressively to an alluvial system

linked to the modern Amazon River.

q 2005 Published by Elsevier Ltd.

Keywords: Deltaic and estuarine deposits; Eocene; Foreland basin; Huallaga basin; Miocene; Peru; Petroleum systems; Sub-Andean

1. Introduction

The Huallaga sub-Andean and Amazonian basins of the

northern Peruvian Andes (Fig. 1) belong to the retroforeland

basin system linked to the Andean orogen. The Huallaga

basin is mainly structured by thrust systems such as duplex,

fault bend folds, and fault-propagation folds associated with

syntectonic sedimentation. Cenozoic foreland deposits are

exceptionally thick in this part of the sub-Andean zone

(about 8 km) and have never been approached using

descriptive sedimentary parameters and modern foreland

propagation concepts.

0895-9811/$ - see front matter q 2005 Published by Elsevier Ltd.

doi:10.1016/j.jsames.2004.06.005

* Corresponding author.

E-mail address: [email protected] (W. Hermoza).

The aim of this article is to present new data about the

Cenozoic sedimentary environments observed in the

Huallaga basin, interpret paleoenvironmental evolution

from a stratigraphic architecture point of view, and propose

a sequential restoration of the Huallaga portion of the

northwestern Amazonian foreland system.

2. Geological setting

The sub-Andean zone is an active fold-and-thrust belt on

the eastern edge of the Andean orogen that constitutes the

wedge-top depozone of the Andean retroforeland basin

system. In the sub-Andean zone, the Huallaga basin is

N160E elongated approximately 400 km long and 100 km

wide and located between 768–778W and 68–98S (Fig. 1). It

is bounded to the north by the Santiago basin. To the east,

Journal of South American Earth Sciences 19 (2005) 21–34

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Fig. 1. Structural map of the northern Peruvian Andes, showing the Western Cordillera, Eastern Cordillera, sub-Andean basins (Santiago, Huallaga, and

Ucayali), and Amazonian basin (Maranon).

W. Hermoza et al. / Journal of South American Earth Sciences 19 (2005) 21–3422

the Huallaga basin is restrained from the Maranon foredeep

basin by the NE-vergent Shanusi-Chazuta thrust, which

overthrusts the WNW–ESE-oriented Contaya arch (Fig. 1).

To the south, the Huallaga basin progressively terminates

along the backlimb of the Shira high.

The geological evolution of the Peruvian Andean

retroforeland basin is ascribed to the onset of the Nazca

subduction, which started in Late Cretaceous times

(Peruvian phase; Megard, 1984) along the western margin

of the South American continental lithosphere. In the

Ecuadorian Oriente basin, which is the northern

continuation of the Peruvian Maranon basin, compressional

deformation began in the Late Turonian (Baby et al., 1999;

Barragan, 1999). Consequently, two sedimentary succes-

sions can be distinguished (PeruPetro, 2002; Fig. 2): (1) the

pre-Andean series that consists of Paleozoic–Early Meso-

zoic deposits (McLaughlin, 1924; Kummel, 1946; Huff,

1949; Jenks, 1951; Rosenzweig, 1953; Zegarra and

Olaechea, 1970) and (2) the Andean series that corresponds

to Late Mesozoic marine to continental foreland successions

(Moran and Fyfe, 1933; Kummel, 1948; Rodrıguez and

Chalco, 1975; Pardo and Zuniga, 1976).

Fig. 2. Synthetic lithostratigraphic section of the sub-Andean zone showing

the two sedimentary successions: pre-Andean and Andean series.

W. Hermoza et al. / Journal of South American Earth Sciences 19 (2005) 21–34 23

The western part of the Huallaga basin is structured by

N160E-trending, thrust-related anticlines spaced 25 km

apart, whereas its eastern part consists of the broad Biabo

syncline developed at the hangingwall of the Shanusi-

Chazuta thrust system. The geometry of the deformation

results from thin-skinned thrust tectonics marked by NE-

verging thrust systems superimposed on inverted grabens

and halokinetic domes. Thrusts branched onto the regional

decollement formed by the Jurassic evaporites of the Pucara

Formation (Baby et al., 1995; Gil, 2001). Synorogenic

sedimentation is well preserved in the Huallaga basin, as

recorded by the 7000 m thick Eocene–Neogene deposits of

the Biabo syncline and a syntectonic series that displays

growth stratal patterns in smaller piggyback basins (Biabo,

Juanjui, and Huicungo synclines; Figs. 3 and 4).

3. Geometric and kinematic analysis of the Huallaga

fold-and-thrust belt

To study the tectonic evolution of the Huallaga basin,

surface data, regional mapping, and seismic reflection data

were integrated to construct a balanced cross-section

between the Contaya arch and the Eastern Cordillera

(Fig. 3). Surface data were obtained from 1:100,000

INGEMMET geologic maps, regional cross-sections from

Baby et al. (1995) and Gil (2001), and fieldwork carried out

in 2001 and 2002. Well and seismic reflection data

published by the Parsep project (PeruPetro, 2002) were

used to constrain the depth of the geometry of the thrust

systems. The updated cross-section was constructed and

balanced on the basis of the consistency of the bed lengths

and the restorability of the cross-section (Dahlstrom, 1969;

Woodward et al., 1985). The realization of this balanced

cross-section throughout the surveyed area (Fig. 4) led us to

improve the geometrical interpretation, calculate the

horizontal shortening rate, and propose a sequential

restoration from the Eocene to the Present.

The oblique WNW–ESE-oriented Contaya arch (Fig. 3),

which corresponds to a single, broad, extrusive structure

limited by opposite basement reverse faults, represents the

easternmost part of the cross-section. The greater part of the

Maranon structures, the Contaya arch results from inversion

tectonics and started to develop in Late Cretaceous times

(Baby et al., 1999; PeruPetro, 2002).

The seismic section of Fig. 5 shows that the Chazuta

thrust corresponds to a low-angle thrust fault deformed by

deep structures, which we interpret as inversions of grabens

that probably are Permo-Triassic in age (Mitu Formation).

According to Fig. 5, the displacement of the Chazuta thrust

sheet is approximately 47 km. To the south, the Ponasillo

anticline is directly associated with a W-verging basement

inverted fault, which deformed the backlimb of the Chazuta

thrust sheet. To the north, a similar basement fault truncates

the Chazuta thrust and emerges with the Pucara Formation

(Figs. 1 and 3). The kinematics of the Chazuta thrust can be

deciphered on the basis of apatite fission track analysis

results (Alvarez-Calderon, 1999), which show cooling

events that can be interpreted as thrust-related uplifts

between 10 and 15 Ma. In the Biabo syncline, Eocene–

Neogene thickness reaches 7000 m (Fig. 4), and the upper

part of the series exhibits a typical growth stratal pattern,

which progressively seals the E-verging Biabo blind thrust

to the west. The Biabo anticline is an elongated NNE–SSW

fault propagation fold (200 km long). The Eastern Cordil-

lera consists of a duplex of outcropping Paleozoic rocks and

feed slips, which is accommodated by the Pungoyacu and

Pachicillo thrusts. The total amount of shortening of the

Huallaga basin, calculated from the balanced cross-section,

is approximately 84 km (i.e. 40%) (Fig. 4).

4. Cenozoic sedimentology and stratigraphy

of the Huallaga basin

Our field observations provide new data and enable us to

propose a new sedimentary interpretation of the Eocene–

Neogene series of the Huallaga basin on the basis of our

facies analysis. This interpretation consists of the charac-

terization of each formation in terms of depositional

Fig. 3. Geological map of the Huallaga basin. The dashed line indicates the location of the cross-section of Fig. 4. Localizations of the sedimentary logs in

Figs. 7–10 are indicated by black rhombi. Cities are represented by black circles.

Fig. 4. Balanced cross-section and restored counterpart. Note that the sedimentary logs have been projected onto the cross-section. See location in Fig. 3.

W. Hermoza et al. / Journal of South American Earth Sciences 19 (2005) 21–3424

Fig. 5. Seismic line 91-MPH-23 (PeruPetro, 2002), crossing from SW to NE in the Biabo syncline and Contaya arch. Seismic line shows the structural style of

the Huallaga basin. The decollement level is indicated by a sharp black line. Note the growth stratal pattern on the backlimb of the Chazuta thrust sheet.

W. Hermoza et al. / Journal of South American Earth Sciences 19 (2005) 21–34 25

environments that we integrate in a new, dynamic model of

the northwestern Amazonian foreland basin system.

4.1. Stratigraphic background

In the Huallaga basin, the stratigraphic succession of

Cenozoic strata traditionally has been divided into the

following five formations (Kummel, 1946, 1948; Williams,

1949; Seminario and Guizado, 1976; Fig. 6):

1.

(Fig. 6), which has been defined by Kummel (1948) and

dated by Gutierrez (1982) on the basis of its charophytes.

It contains reddish to grayish silts interbedded with

sandstones. In Pongo de Tiraco (Eastern Huallaga basin;

Fig. 3), this formation is approximately 500 m thick

(Caldas and Valdivia, 1985). In this locality, its base

consists of conglomeratic sandstones with limestone

clasts. East of Chazuta (Fig. 3), its thickness consider-

ably increases to 1000 m. The Yahuarango Formation

traditionally is considered to have been deposited in a

continental environment (floodplain and lacustrine;

Kummel, 1946, 1948; Williams, 1949; Sanchez and

Herrera, 1998; Dıaz et al., 1998).

2.

Fig. 6. Chronostratigraphic chart of the Huallaga basin.

The Eocene–Oligocene Pozo Formation (Fig. 6), which

was first described at the confluence of the Santiago

and Maranon Rivers by Kummel (1948) and Williams

(1949). The numerous fauna (ostracods, foraminifers,

charophytes, gasteropods, palynomorphs) show that the

Pozo Formation is Eocene–Oligocene in age (Williams,

1949; Seminario and Guizado, 1976; Valdivia, 1982 in

Sanchez and Herrera, 1998). This formation is made of

two sequences. The lower is formed by conglomeratic

sandstones, and the upper contains grayish coal-bearing

shales interbedded with limestones. In the Chazuta area

(Fig. 3), Sanchez et al. (1997) describe a sequence

beginning with medium to coarse, well-sorted grayish

Fig

the

Tid

W. Hermoza et al. / Journal of South American Earth Sciences 19 (2005) 21–3426

sandstones and topped by grayish siltstones, green

shales, and limestones. This formation is interpreted to

have been deposited in a marine environment.

3.

which was defined as part of the Contamana group by

Kummel (1946; Fig. 6) and whose stratigraphic position

was given by Caldas and Valdivia (1985). This formation is

made up of red sandstones exposing trough cross-bedding

interbedded with reddish to grayish siltstones. In the

Huallaga basin, it outcrops in the Biabo syncline, in the

Caspisapa area, and south of Chazuta (Fig. 3). In these

areas, the Chambira Formation consists of reddish to

grayish silts interbedded with medium to coarse sandstones

and a few limestones. Various authors have indicated that

the Chambira Formation varies in thickness between 3000–

5000 m (Rodrıguez and Chalco, 1975) and 1000 m (Caldas

and Valdivia, 1985). The Chambira Formation has been

interpreted to have been deposited in a meandering fluvial

environment (Kummel, 1948; Williams, 1949; Sanchez

and Herrera, 1998; Dıaz et al., 1998).

4.

defined in the Cushabatay River by Kummel (1946), who

. 7. Measured sedimentologic sections (Logs 1 and 2; see location in Fig. 3) of the P

Shapaja area and Log 2 on the Juanjui–Tocache road. (Photo 1) Unconsolidated c

al bundles, sigmoidal bedding, planar foresets, and herringbone cross-stratificatio

described it as the upper part of the Contamana group

(Fig. 6). This formation consists of greyish to brownish

sandstones interbedded with reddish silts. It is divided

into two members: the lower member of coarse light to

brownish sandstones interbedded with reddish to grayish

silts and the upper member with vertical stacking of

decimetric fining-upward sandstone beds over polygenic

conglomerates. The formation is approximately 3500 m

thick. In the Rio Sisa and Saposoa area, it reaches its

maximum thickness of 5700 m (Vargas, 1965 in Sanchez

and Herrera, 1998). The Ipururo Formation is interpreted

to have been deposited in a fluvial environment

(Kummel, 1946, 1948; Williams, 1949; Rodrıguez and

Chalco, 1975; Sanchez and Herrera, 1998; Fig. 6).

5.

which is composed of polygenic conglomerates with a

sandy matrix. The clasts consist of intrusive, volcanic

gneisses, schists, and reworked sandstone pebbles

deposited in a fluvial to alluvial fan environment. This

formation is approximately 100 m thick. In the Tocache

area, it is named the Tocache Formation (Sanchez and

Herrera, 1998; Dıaz et al., 1998).

ozo Formation (Middle Eocene–Oligocene). Log 1 has been observed in

onglomerates of the lag pebbles of the Lower Pozo member. (Photo 2)

n of the shoreface part of the Lower Pozo member.

W. Hermoza et al. / Journal of South American Earth Sciences 19 (2005) 21–34 27

4.2. New sedimentological and facies analysis

In the Huallaga basin, the Cenozoic stratigraphic interval

has never been precisely observed or discussed because all

sedimentological studies have focused on the Jurassic and

Cretaceous, which provided the principal source rocks and

reservoirs. Nevertheless, the analysis of the Cenozoic

overburden rocks is crucial for understanding the petroleum

systems. The sedimentary architecture of the Cenozoic

series is based on facies recognition. Sedimentologic

successions have been studied in many localities. We

present the most characteristic sedimentologic logs to

illustrate our observations in Shapaja, Bellavista, Saposoa,

Sacanche, and Juanjui and along the Tarapoto–Juanjui–

Tocache and Tarapoto–Chazuta roads (Fig. 3). In the

seismic section of Fig. 5, the upper part of the Cenozoic

sequence displays progressive unconformities and thickness

variations that we correlate with field observations.

4.2.1. Pozo Formation

The Pozo Formation consists of the Lower and Upper

Pozo members. The sedimentary series observed in the

Shapaja area (6.585558S, 76.302508W; Log 1, Fig. 7)

illustrates the typical sedimentary succession of the Lower

Pozo member, which is represented by unconsolidated

conglomerates displaying well-rounded tuffaceous sand-

stones and medium to coarse, well-sorted sandstones

(Fig. 7). The pebbles are less than 5 cm in diameter and

Fig. 8. Measured sedimentologic section (Log 3; see location in Fig. 3) of the lowe

Bellavista road. (Photo 3) Sigmoidal laminations and tidal mud-sand couplet lam

lamination.

mainly composed of Cretaceous sandstones and Paleozoic

quartzites. This unconsolidated conglomerate is topped by a

set of sequences, each of which is composed of coarse- to

medium-grained sandstones displaying different sedimen-

tary structures. From the base to the top, these are as

follows: tidal bundles, sigmoidal laminations, planar

foresets, and herringbone cross-stratifications (Fig. 7).

Each sequence is composed of approximately 30 cm thick,

well-laminated beds and fines upward (Log 1, Fig. 7).

Sigmoidal cross-stratified sandstones are interpreted as

shoreface deposits dominated by tidal influences,

as corroborated by the presence of herringbone cross-

stratifications, which require opposing current directions.

The facies association of the Lower Pozo member suggests a

shoreface depositional environment overlying a lag pebble.

Sedimentologic observations of Upper Pozo member

have been carried out along the Juanjui-Tocache road

(7.234718S, 76.746288W; Log 2, Fig. 7). The Upper Pozo

member facies consists of a succession of reddish/greenish

argillites associated with sandstones and shallow marine

limestones (Fig. 7). In the westernmost part of the Huallaga

basin, this succession is replaced by sandier siliciclastic

sequences without any lime. Marine argillaceous levels

contain ostracods, foraminifers, and pollens of Eocene–

Oligocene age (Williams, 1949; Seminario and Guizado,

1976; Valdivia, 1982 in Sanchez and Herrera, 1998). To

the north in the Santiago basin, these strata were dated

as Eocene (QMC, internal report). The depositional

r part of the Chambira Formation. Log 3 has been observed on the Tarapoto-

ination. (Photo 4) Planar foresets, sigmoidal lamination, and tidal rhythmic

W. Hermoza et al. / Journal of South American Earth Sciences 19 (2005) 21–3428

environment of the Upper Pozo member seems in

accordance with shallow clastic shelf models.

4.2.2. Chambira Formation

This sedimentary succession can be divided into a lower

part observed along the Tarapoto–Bellavista road

(6.709058S, 76.287908W; Log 3; Fig. 8) and an upper part

observed in the Bellavista area (7.071668S, 76.574408W;

Log 4; Fig. 9). The lower part of the Chambira Formation is

considered Oligocene–Miocene in age (Blasser, 1946 in

Dıaz et al., 1998; Gutierrez, 1982; Seminario and Guizado,

1976). It is composed of a repeating succession of sand bars

that display trough cross-stratifications and planar cross-

stratifications, flood plain argillites, and channels that

display sand-mud couplets (Log 3, Fig. 8). Several channels

exhibit coarse- to medium-grained sigmoidal beds, sand-

Fig. 9. Measured sedimentologic sections of the upper part of the Chambira Form

location in Fig. 3). (Photo 5) Typical succession of the Upper Chambira Formation

planar foresets, and tidal rhythmic horizontal laminations of the Lower Ipururo m

member. (Photo 8) Conglomerates of the Juanjui Formation overlying a sharp ero

stone, and planar foresets laminations. The upper part of the

Chambira Formation is characterized by sequences of tidal

sand bars, sigmoidal bedded sandstones, and trough cross-

bedded sandstones, with intercalations of reddish to

brownish argillites and silts. The upper part of the Chambira

Formation is marked by an increase in the silt: sand ratio

(Fig. 9). The facies association of the Chambira Formation

suggests a tidal-influenced fluvial depositional environment.

4.2.3. Ipururo Formation

During the Middle Miocene–Pliocene, the Ipururo

Formation was deposited. We distinguish three members

in the Ipururo Formation: the Lower, Middle, and Upper

Ipururo members.

The Lower Ipururo member is partially exposed in the

Sacanche area in the central part of the Huallaga basin

ation (Oligocene–Middle Miocene) and Lower Ipururo member (Log 4; see

where fluvial and tidal influences interfere. (Photo 6) Sigmoidal lamination,

ember. (Photo 7) Mammal remains in a sandstone bar of the Lower Ipururo

sive surface and removing at least the Middle and Upper Ipururo members.

Fig. 10. Measured sedimentologic sections (Logs 5 and 6; see location in Fig. 3) of the Upper Miocene–Pleistocene Ipururo Formation. (Photo 9) Decametric

sand bar of the deltaic system displaying large-scale, low-angle foresets. (Photo 10) Hummocky cross-stratification of the transgressive Middle Ipururo

member.

W. Hermoza et al. / Journal of South American Earth Sciences 19 (2005) 21–34 29

(7.072358S, 76.702318W; Log 5, Fig. 10). The sedimentary

succession and facies association is composed of reddish

argillites and cross-stratified sandstones, followed by

microconglomerates and medium to coarse sandstones

that display oblique planar stratifications and low-angle

cross-laminations (Hermoza, 2001; Fig. 10). In this

sequence, we have collected some mammal remains

(scapula of sloth identified by J. Flynn). The vertical

organization shows a deltaic environment topped by

fluvial-influenced deposits. Such a stacking pattern prob-

ably is related to an increase in sediment supply. To the

south in the Bellavista area (7.072778S, 76.573058W; Log

4, Fig. 9), this sequence is laterally replaced by coarser

sandstone lenses spread into reddish/greenish argillites that

contain bone remains (Fig. 9). The lenses exhibit tidal

couplets and trough cross-bedding that can be interpreted

as a point bar system.

The Middle Ipururo member is exposed in the western part

of the Huallaga basin at the Juanjui-Tocache road

(7.537228S, 76.680288W; Log 6, Fig. 10). It is composed

of grayish to blackish marls and limestones associated with

fine- and very fine-grained hummocky cross-stratified

calcarenites and reworked continental fauna (Fig. 10). This

facies association can be interpreted as a storm-induced

deposit.

The Upper Ipururo member is mainly exposed in the

central and western parts of the basin, where it

unconformably overlies the Middle member or directly

overlies the Lower member. The lower part of the Upper

Ipururo member is characterized by a succession of

conglomerates of well-rounded volcanic and quartzitic

pebbles with trough cross-bedding (Gt facies of Miall,

1996) and planar cross-beds (Gp facies of Miall, 1996),

intercalated with siltstones and argillites (Fsm facies of

Miall, 1996). It is succeeded by trough cross-bedded

(St facies), planar cross-bedded, and horizontal bedded

sandstones (Sp and Sh facies of Miall, 1996; Fig. 10). The

facies association suggests a depositional fluvial environ-

ment of channel infill deposit.

4.2.4. Juanjui Formation

The Juanjui (or Tocache) Formation is composed of

polygenic well-rounded conglomerates. The pebbles’ com-

position is mainly intrusive, volcanic schist, gneisses,

quartzite, limestones, and sandstones, and the pebbles are

less than 15 cm in diameter. This conglomerate facies

exhibits trough cross-bedding (Gt facies of Miall, 1996),

planar cross-bedding (Gp facies), and clast-supported and

inverse-grading facies (Gcm and Gci facies of Miall, 1996).

Facies association suggests development in fluvial to alluvial

fan environments. Analyses of the clast imbrications show

transport to the north to northwest. The Juanjui Formation

thus developed in fully continental environments. It is

characterized by coarsening-upward conglomerates.

W. Hermoza et al. / Journal of South American Earth Sciences 19 (2005) 21–3430

4.3. Regional sequence architecture

Our sedimentological and stratigraphic reappraisal of the

Huallaga basin leads us to propose an Eocene–Pliocene

stratigraphic architecture based on stacking pattern ten-

dencies: transgressive or landward, vertical or aggrada-

tional, and regressive or seaward (Van Wagoner et al., 1988;

Embry, 1995; Catuneanu, 2002).

The Lower Pozo member is interpreted as a shoreface

deposit that overlies a regional unconformity underlain by

lag pebbles. We interpret this unconformity as formed in

Fig. 11. Synthetic stratigraphic section and sequence stratigraphy of the

Huallaga basin.

a subaerial environment. Therefore, this member has

recorded a base level rise, but its stacking pattern remains

vertical. This vertical stacking is considered to represent the

proximal aggradation that occurs in the incipient stage of

normal regression. Therefore, we consider the Lower Pozo

member a regressive systems tract (RST; Fig. 11).

The Upper Pozo member exhibits an important change of

facies characterized by a brutal decrease in grain size and the

occurrence of shallow marine limestone. Therefore, we

interpret this retrogradational stacking pattern as the

transgressive phase, which occurs when the base level rise

outpaces the sediment supply. Therefore, we consider

the Upper Pozo member a transgressive systems tract

(TST; Fig. 11).

The Chambira Formation exhibits a vertical stacking

pattern. The facies variations between fluvial and estuarine

environments are considered autocyclic. The top of

this formation exhibits dominant facies of floodplain

reddish argillites and fluvial channelized sandstone. These

aggrading–prograding stacking patterns characterize a

normal regression. Therefore, we interpret the Chambira

Formation as an RST (Fig. 11). This latest fluvial succession

ends with an abrupt decrease in grain size. This top part is

the best candidate to represent a continental TST.

The Lower Ipururo member is characterized by a

progradational stacking pattern of deltaic lobes (Hermoza,

2001). The spatial facies distribution suggests that the

progradation is to the northeast. This member is interpreted

as an RST (Fig. 11). The Middle Ipururo member overlies

the Lower member with a transgressive surface and consists

of westward transgressive storm deposits. This Middle

member is typically a TST. The Upper Ipururo member is a

fully continental system in which local base level changes

control sedimentation (Fig. 11). The tectonic control on the

depositional area is evident; the westernmost part of the

wedge-top depozone is infilled by coarse conglomerates

lying above a sharp erosional surface. The alluvial fan

environment is restricted to piggyback synclines, though the

erosion surface may correspond to a bypass surface.

5. Tectonosedimentary evolution of the Huallaga basin

Several authors (Fauchet and Savoyat, 1973; Megard,

1984; Aspden and Litherland, 1992; Baby et al., 1999;

Barragan, 1999; Christophoul et al., 2002) have proposed

that the onset of the Andean foreland basin system occurred

during Late Cretaceous times. In the Maranon basin, this

onset occurred during the period of the sedimentation of the

Upper Chonta Formation. Since the Eocene, we propose a

foreland system interpretation that is based on the sedimen-

tary expression of orogenic loading and unloading stages

(DeCelles and Giles, 1996; Catuneanu et al., 1997, 2000).

Our sedimentological and structural analyses of the

Huallaga basin deposits enable us to distinguish three stages

controlled by orogenic processes. These stages are

Fig. 12. Tectonostratigraphic diagram of the Huallaga basin. Foreland basin system dynamics consist of three stages: (1) Early Eocene, with large wavelength

tectonics controlled by orogenic unloading. The Huallaga basin corresponds to a foresag basin; (2) Middle Eocene–Miocene, with large wavelength tectonics

controlled by orogenic loading. The Huallaga basin corresponds to a foredeep basin; and (3) Late Miocene–Pliocene, with short wavelength tectonics

controlled by thrust-related structures. The Huallaga basin corresponds to a wedge-top depozone.

W. Hermoza et al. / Journal of South American Earth Sciences 19 (2005) 21–34 31

characterized by typical sedimentary record and basin

morphology (Fig. 12). The first stage (Eocene) is charac-

terized by orogenic unloading and large wavelength

tectonics, the second stage (Middle Eocene–Miocene) is

Fig. 13. Sequential restoration illustrating the three stages of the geodynamics of

4440 m of Early Eocene–Late Miocene strata, and AFTA 2 indicates a max

(Alvarez-Calderon, 1999).

characterized by orogenic loading and large wavelength

tectonics, and the third stage (Middle Miocene–Pleistocene)

is characterized by short wavelength tectonics with

synsedimentary thrust-related folds.

the Huallaga basin. AFTA 1 indicates a maximum burial corresponding to

imum burial of 3250 m corresponding to Early Eocene–present strata

W. Hermoza et al. / Journal of South American Earth Sciences 19 (2005) 21–3432

5.1. Lower Eocene (orogenic unloading,

large wavelength tectonics)

The Lower Eocene unconformity constitutes a regional

subaerial unconformity that marked an important change in

geodynamic conditions. It is capped by lag pebble deposits,

which indicates a reworking of the series ranking in the

Paleozoic–Cretaceous and suggests a deep erosion of at

least the Eastern Cordillera. During the Lower Eocene,

erosion processes dominated thrust tectonic activity. The

stratigraphic architecture of a subaerial unconformity

overlapped by an RST typically characterizes an unloading

period (Fig. 12). Therefore, in the structural context of the

Huallaga foreland system, the subaerial unconformity and

the lag pebble deposits of the Lower Pozo member are the

best candidates for an eastward-dipping foreslope surface

(Catuneanu et al., 1997, 2000), and the sag geometry of the

basin reconstructed by balancing techniques displays

characteristics of a foresag basin (Fig. 13).

5.2. Middle Eocene–Miocene (orogenic loading, large

wavelength tectonics)

The TST of the Upper Pozo member occurred in a basin.

Such a retrogradational package records an abrupt base level

rise, classically interpreted in foreland basins as a renewal of

loading by an active thrust wedge. During this period, thrust

tectonics dominated erosion. This pre-steady-state period

was followed by increasing sediment supply, which

recorded a renewal of erosion in the active thrust wedge.

In the foredeep, this turn back to the steady state was

recorded by the deposition of the aggradational Chambira

Formation, whose vertical stacking pattern indicates

equilibrium between accommodation and sediment supply

within a foredeep basin (Fig. 13).

Fig. 14. Three-dimensional paleogeographic sketch

5.3. Late Miocene–Pliocene (short wavelength tectonics,

fold/thrust structures)

The stratigraphic succession of the highest part of the

Chambira Formation and the lowest part of the Lower

Ipururo member displays a fining-upward trend that we

interpret as a continental TST. This continental TST

indicates a pre-steady-state period controlled by an

increase in thrust activity. In the Chazuta thrust unit,

these transgressive strata were deformed by thrust-related

structures, which were responsible for the uplift-related

first cooling event recorded by apatite fission track (10–

15 Ma, Alvarez-Calderon, 1999) (Fig. 13). Since then, the

Huallaga basin sedimentation has been controlled by

thrust emplacement. In the innermost part of the thrust

wedge, the wedge-top depozone was characterized by

fluvial to deltaic sedimentation developed in a piggyback

basin, whereas the foredeep Maranon depozone, east of

the Chazuta tipline, must be marked by marine

sedimentation (Fig. 14). Marine sedimentation is well

known in the Iquitos area of the Maranon basin (Pebas

Formation; Gabb, 1869; Seminario and Guizado, 1976;

Hoorn, 1993; Rasanen et al., 1998), which recorded the

orogenic loading (forebulge onset) of Late Miocene times

(Roddaz et al., 2004). Until 5 Ma, the Huallaga basin

corresponded to a near sea-level depozone, and then the

thrust wedge grew vertically and became subaerial.

Piggyback basins became fully continental and trapped

coarse sediments while the fine sediments were

transported eastward into the Amazon lowland by alluvial

systems (Fig. 13). The geometry of the basin recon-

structed through balancing techniques displays character-

istics of a thrust-top basin, which constitutes the

wedge-top depozone of the northwestern Amazonian

foreland basin system (Fig. 13).

of the Huallaga basin in the Late Miocene.

W. Hermoza et al. / Journal of South American Earth Sciences 19 (2005) 21–34 33

6. Summary and conclusion

The analysis of new structural and sedimentological data

leads us to propose an evolutionary scheme for the Huallaga

basin that agrees with foreland system dynamics (DeCelles

and Giles, 1996; Catuneanu et al., 1999). In this scheme,

tectonics is the predominant control over sedimentation.

This control acted at two different wavelengths: a large

wavelength due to the loading-unloading cycle and a short

wavelength due to thrust-related structures. In the Huallaga

basin, large and short wavelength tectonics succeeded in the

Eocene–Middle Miocene and Late Miocene–Pliocene,

respectively. The onset of large wavelength tectonics due

to the unloading stage is evidenced by the subaerial

unconformity foreslope surface and the RST of the Lower

Pozo member. The Upper Pozo member, the Chambira

Formation, and their bounding surface recorded large

wavelength tectonics due to a loading stage. The onset of

short wavelength tectonics due to the emplacement of

thrust-related structures is recorded by the prograding

deltaic lobes of the Lower Ipururo member. Sedimentary

structures indicate that this major deltaic feature prograded

to the NNE. The deltaic depositional environment is

restricted to the Late Miocene piggyback basin, which

may constitute an important zone for hydrocarbon gener-

ation contemporaneous with the development of structural

traps. Consequently, the timing of thrust emplacement

should be studied to improve petroleum exploration. Until

the present, thrust tectonics were going on, and a coarse

alluvial fan system occurred in the Huallaga wedge-top

depozone of the northwestern Amazonian foreland system.

Acknowledgements

This research was supported by IRD, INSU grant

99PNSE59 (Tectonique, erosion et sedimentation dans

le bassin de l’Amazone: du Mio-Pliocene a l’Actuel), and

INSU grant (Erosion des Andes). PeruPetro is acknowl-

edged for its technical support. The manuscript largely

benefited from constructive reviews by Th. Nalpas and

J. Verges.

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