Cenozoic sedimentary evolution of the Amazonian foreland basin system

Amazonia, Landscape and Species Evolution: A Look into the Past,1st edition. Edited by C. Hoorn and F.P. Wesselingh. © 2010 Blackwell Publishing

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Cenozoic sedimentary evolution of the Amazonian foreland basin systemMartin Roddaz1, Wilber Hermoza2, Andres Mora3, Patrice Baby1, Mauricio Parra4,Frédéric Christophoul1, Stéphane Brusset1 and Nicolas Espurt5

1Université de Toulouse, France2REPSOL-YPF, Madrid, Spain3ECOPETROL, Instituto Colombiano del Petroleo, Santander, Colombia4The University of Texas, Austin, USA5Université Paul Cézanne, Aix en Provence, France

Abstract

In this chapter we present a synthesis of the Cenozoic evolution of the Amazonian foreland basin system, based on a review of the estimated ages, lithology and sedimentary structures, palaeontological content, and inferred depositional environments of sedimentary units in the basin. In addition, we have calculated maximum sedi-mentation rates for the Cenozoic formations of the northern Peruvian foreland basin and integrated these with existing data on sedimentation rates, subsidence analysis, migration of depocentre and depositional environ-ments. Based on this information we propose a model for the Cenozoic evolution of the Amazonian foreland. The sedimentary architecture of this foreland basin indicates that Cenozoic evolution was marked by several periods, which were roughly synchronous and of similar effect, along the entire Amazonian foreland basin system. Tectonic loading of the Andes of Colombia, Ecuador, Peru and northern Bolivia, and development of the Amazonian foreland, was initiated during Late Cretaceous-Paleocene times and followed by an unloading stage during the Early-Middle Eocene period. The Middle-Late Eocene marine transgression and the increase in sedimentation rates, associated with westward migration of the depocentre, were all indicative of a renewed phase of tectonic loading of the Peruvian Western Cordillera and the Ecuadorian and Colombian Eastern Cordillera. Subsequent Oligo-Miocene increase in sedimentation rates and further migration of the depocen-tres towards the present-day sub-Andean zone, are all indicative for a thrust-induced uplift and loading of the Eastern Cordilleras of Peru, Bolivia and Colombia. This Oligo-Miocene loading stage maintained high subsi-dence rates that favoured the sedimentation of aggradational fl oodplain and coastal plain and tidally infl uenced deposits. Nevertheless, the processes that controlled the Early-Middle Miocene marine ingressions remain to be determined. Late Miocene ongoing thrust tectonic loading of the Eastern Cordillera, initial structuring of the sub-Andean zone and the onset of the main phase of Andean surface uplift induced fl exural subsidence in the foredeep depozones of the entire Amazonian foreland basin. This process also drove the Late Miocene marine transgressions that characterized the fi lled stage of the Ecuadorian, Peruvian and Bolivian Amazonian foreland basin system. Valley incisions and full relief development in the hinterland during the Late Miocene-Pliocene provided increased sediment supply and overfi lled the Amazonian foreland basin system. Finally, the fl at-slab subduction of the Nazca ridge induced Pliocene (~4 Ma) uplift of the Fitzcarrald Arch and subdivided the Amazonian foreland basin into the northern and southern Amazonian foreland basins.

Introduction

The sedimentary basins adjacent to the eastern side of the Central Andes form one of the best examples in the world of retroarc

foreland basin systems (Fig. 5.1; Jordan et al. 1983; Jordan 1995; Horton & DeCelles 1997). Foreland basins are a favoured area for studying the interplay at different scales of tectonics, climate and sedimentation (see, e.g., Beaumont 1981; Burbank 1992; Jordan 1995; DeCelles & Giles 1996; Horton 1999; Catuneanu 2004 amongst many others) as they record the denudation of the adjacent mountain belt and hence the interaction between erosion and mountain growth. The stratigraphic record of foreland basins is generally very complete (Jordan 1995), and

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62 M. Roddaz et al.

shortening, mountain building and the initiation of the Andean foreland basin started in Late Cretaceous-Paleocene times (Balkwill 1995; DeCelles & Horton 2003; Barragan et al. 2005; Martin-Gombojav & Winkler 2008 and references therein). South of 22°S, although debated (see Jordan et al. 2007 and references therein), the Andean foreland basin initiated in the Cretaceous-Early Tertiary (Arriagada et al. 2006) and migrated eastwards throughout the Paleogene (Carrapa & DeCelles 2008). In southern Bolivia, the Andean foreland basin defi nes a four-component fore-land basin system (wedge-top, foredeep, forebulge and backbulge

therefore they provide ample information on the geometry ofthrust sheets, ages of movement on particular thrust sheets, rheology of the lithosphere, drainage history, denudationhistory, slab subduction rate and dip history, and climate his-tory (Jordan et al. 2001). These processes all may have promoted isolated areas capable of fostering conditions for biodiversity development.

The Andes are the second largest mountain belt in the world, spanning more than 50° of latitude with a maximum width of ~800 km and peak elevations exceeding 6.7 km. Regional

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Fig. 5.1 Map delimiting the present-day location of the Amazonian foreland basin depozones. NAFB, North Amazonian foreland basin; SAFB, South Amazonian foreland basin. Modifi ed from Roddaz et al. 2005b.


The Amazonian foreland basin system 63

An important difference between the pro- and retro-foreland basins is that retro-foreland basins are subjected to sublithos-pheric loads mainly caused by the drag force generated by viscous mantle corner fl ow beneath the retro-foreland (e.g. dynamic sub-sidence: Mitrovica et al. 1989; Gurnis 1992; Catuneanu et al. 1997; Pysklywec & Mitrovica 2000; Catuneanu 2004). Combined with orogenic supracrustal loading (load of the mountain belt exerted on the continental lithosphere), these are the primary subsidence mechanisms that control accommodation and sedimentation pat-terns in retro-foreland basin (DeCelles & Giles 1996; Pysklywec & Mitrovica 1999; Catuneanu 2004). Local-scale mechanisms such as reactivation of weak structures (Bayona & Thomas 2003), three-dimensional (3D) confi guration of the orogenic load (Whiting & Thomas 1994) or variable strengths of the lithosphere (Cardozo & Jordan 2001) can also play an important role in subsidence within a foreland basin.

Orogenic loading leads to the partitioning of the foreland systems into four discrete depositional areas: the wedge-top, the foredeep, the forebulge and the backbulge depozones (DeCelles & Giles 1996) (Fig. 5.2). The wedge-top depozone is the mass of sediment that accumulates on top of the frontal part of the orogenic wedge, including ‘piggyback’ and ‘thrust top’ basins (DeCelles & Giles 1996). The foredeep depozone consists of the sediment deposited between the structural front of the thrust belt and the forebulge. The forebulge depozone is the broad region of potential fl exural uplift between the foredeep and the backbulge depozones. The backbulge depozone is the mass of sediment that accumulates in the shallow but broad zone of potential fl exural subsidence cratonward of the forebulge (DeCelles & Giles 1996).

Renewed thrusting in the orogenic belt (orogenic loading) results in foredeep and backbulge subsidence and forebulge uplift (DeCelles & Giles 1996; Catuneanu 2004 and references therein), and the reverse occurs as orogenic load is removed by erosion or

depozones, after DeCelles & Giles 1996) that propagated eastwards throughout the Tertiary since at least the Late Paleocene (DeCelles & Horton 2003). Two Miocene marine ingressions interrupted the continental foreland sedimentation. The fi rst transgression occurred between 15 and 13 Ma and was tectonically and eus-tatically controlled; a second one, predominantly tectonically controlled, younger than 10 Ma, and was generated by tectonic loading of the Eastern Cordillera fold-and-thrust belt (Hernández et al. 2005). At least one other marine ingression occurred in the Western Amazon basin during the Eocene (Christophoul et al. 2002a; Hermoza et al. 2005b; Santos et al. 2008) but the extent and wider signifi cance of this marine ingression have not been fully explored.

In spite of the large size of Amazonian foreland basins, their Tertiary evolution received comparatively little attention. Some of the diffi culties are the remote access, political instabilities, wide extension of sedimentary units into different countries (Bolivia, Peru, Ecuador and Colombia), diffi culties in accessing subsurface data, poor stratigraphic control and numerous local formation names. Oil exploration started as early as the 1920s in the Amazonian regions of Peru and Ecuador, with a production boom arriving in the 1970s. Due to this renewed interest, many informal reports contain valuable regional data. Some recent studies have proposed regional basin analysis in Peru (Hermoza 2004; Hermoza et al. 2005a, 2005b), in Ecuador (Christophoul et al. 2002a) and in Colombia (Bayona et al. 2007; Parra et al. 2009) but lack stratigraphic homogenization. As a result, political frontiers have prevented an earlier synthesis of the Tertiary evo-lution of the Amazonian foreland basin.

This chapter presents the fi rst compilation of the Cenozoic evo-lution of Amazonian foreland basin that attempts to correlate the Paleocene to Recent development over the entire Colombian-to-Bolivian Amazonian foreland basin. The aims of this chapter are: (i) to present an overview of the stratigraphy and depositional environment of the Paleogene to Neogene infi ll of Bolivian to Colombian Amazonian foreland basin; (ii) to identify the main periods of foreland basin development and to discuss the controls on accommodation; and (iii) to emphasize how the sedimentary processes were controlled by the growth of the Andes.

Basic concepts

This section is intended for readers who are not familiar with foreland basin and basin analysis. Here we simply give basic defi -nitions and concepts used in the chapter. For further details, the reader is referred to publications cited in this section.

A foreland basin generally is defi ned as an elongate region of potential sediment accommodation that forms between a linear contractional orogenic belt and the stable craton, mainly in response to fl exural subsidence that is driven by thrust-sheet loading in the orogen (Dickinson 1974; Beaumont 1981; Jordan 1995; DeCelles & Giles 1996). Foreland basins can develop on the subducting lithosphere in a forearc setting, when they are referred to as pro-foreland basins; or they can form on the overriding lithosphere (behind the orogenic belt), when they are termed retro-foreland basins (Catuneanu 2004 and references therein).

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Fig. 5.2 Orogenic loading and unloading stages and the associated depozones in a retro-foreland basin. (+, −) refer to increases and decreases in orogenic load, respectively; see text for explanations. Modifi ed from Catuneanu 2004.


64 M. Roddaz et al.

defi nition for continental foreland basins in which the underfi lled basin is characterized by longitudinal drainage because of the sub-sidence in the foredeep due to tectonic loading and the overfi lled basin is characterized by transverse drainage due to increasing erosion. Despite these differences in their defi nitions, both Jordan (1995) and Sinclair (1997) proposed an increase in tectonicshortening to explain the transition from underfi lled to over-fi lled. In contrast, other authors (Blair & Bilodeau 1988; Heller et al. 1988; Burbank 1992; Heller & Paola 1992; Christophoul et al. 2003) suggest that this transition is mainly due to erosional unloading (isostasic rebound) succeeding tectonic loading.

Geological setting and stratigraphy of Amazonian foreland basin deposits

The western Amazon drainage basin extends from southern Colombia to northern Bolivia. Since the Pliocene (Espurt et al. 2007; see also Chapter 6), the Amazonian foreland basin has been divided into two foreland basin systems (sensu DeCelles & Giles 1996): the North Amazonian foreland basin system and the South Amazonian foreland basin, separated by the Fitzcarrald Arch (see Fig. 5.1) (Roddaz et al. 2005b).

The North Amazonian foreland basin system comprises the Colombian, Ecuadorian and Northern Peruvian foreland basins. The basins in the sub-Andean zone, including the Huallaga Basin, defi ne the present-day wedge-top depozone and are separated from the foredeep depozone basins by the sub- Andean thrust front. The Oriente (Ecuador) and Marañón (Peru) delimit the foredeep depozone. We here include the Llanos foredeep (Colombia) as part of the Amazonian foreland until Late Miocene times (see Chapter 4); the Putumayo Basin (Colombia) and the Oriente basin (Ecuador) are considered as a single basin (Mora et al. 1998). Because the Oriente Basin is much bigger than the Putumayo Basin, we did not include the stratigraphy of the Putumayo Basin (Colombia); however, correlations were made where appropriate. To the east the foredeep depozones end with the high of the Iquitos forebulge. East of the Iquitos forebulge is the Pebas backbulge (Fig. 5.3; see also Fig. 5.1).

extension (i.e. orogenic unloading, Catuneanu, 2004). The orogenic unloading stage causes the wedge-top and foredeep depozones to uplift and erode, referred to as the foreslope topographic zone, and the main zone of low sediment accumulation (foresag depozone) is located cratonward (Catuneanu et al. 1998) (see Fig. 5.2).

The recognition of a forebulge depozone is thus of particular importance for reconstructing loading/unloading cycles. As fore-bulges are positive structures, their past existence has been inferred from progressively cratonward migration of distal unconformities and onlaps of the overlying sediments (Crampton & Allen, 1995; White et al. 2002). Although they are generally associated with non-deposition and/or erosion, forebulges can occasionally pre-serve thin and condensed sedimentary sequences (Crampton & Allen 1995; DeCelles & Horton 2003; Catuneanu 2004; Roddaz et al. 2005a; Dávila et al. 2007), which suggests that they can pre-sent some accommodation even during tectonic loading. Several causes can explain sediment deposition in the forebulge depozone including overfi lling of the foreland basin (Crampton & Allen 1995; Dávila et al. 2007), temporal changes in the lithosphere rigidity (Garcia-Castellanos et al. 2002), overdensifi cation of the lower crust (Leech, 2001) and large-scale dynamic subsidence (Catuneanu 2004). The forebulges can thus be the loci of inter-ferences between long-wavelength dynamic subsidence and fl ex-ural uplift. It is diffi cult to discern the infl uence of these processes on accommodation, and this explains why the identifi cation and exact localization of past forebulges remains a diffi cult task.

Accommodation in a retro-foreland basin depends on the interplay of base-level changes and sediment supply and is mostly controlled by the interaction of tectonic and sublithospheric static and dynamic loading (e.g., Catuneanu et al. 1997; Catuneanu 2004). The degree to which the accommodation space is con-sumed by deposition is refl ected in the depositional setting of the sedimentary record of the foreland basin system. Three stages are conventionally defi ned: underfi lled, fi lled and overfi lled, domi-nated by deep marine, shallow marine and fl uvial environments, respectively (Sinclair & Allen 1992; Sinclair 1997). Foreland basins are generally viewed as evolving in a predictable way starting from an underfi lled stage to fi nish with an overfi lled confi guration (Crampton & Allen, 1995). Jordan (1995) provided a different

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Fig. 5.3 Schematic cross-section depicting the concept of foreland basin system sensu DeCelles & Giles (1996). Depozone is labelled dz. SAZ, sub-Andean Zone.



fi ne-to-medium-grained, cross-bedded sandstones and minor coal beds (Fig. 5.4). The Guaduas Formation progressively thins eastwards, beneath the fi rst regional unconformity of the Llanos Basin. The formation is only ~50 m thick in the eastern foothills of the Eastern Cordillera and is absent further to the east (Santos et al. 2008). The Guaduas Formation and the laterally equiva-lent units in the Magdalena Valley Basins (Guaduala, Seca and Lisama Formations) register the onset of protracted non- marine deposition along fl uvial plains. The Lower Hoyon Formation, which is restricted to the westernmost Middle Magdalena Valley Basin, consists of clast-supported, horizontally bedded conglom-erates that are interlayered with lenticular-shaped cross-bedded sandstones, interpreted as alluvial fan deposits (Gómez et al.2003). Gravel petrography suggests a sediment source situated in the west, the Central Cordillera, and the fi rst appearance of plu-tonic lithic fragments.

Late Paleocene to Early Eocene

Upper Paleocene-Lower Eocene units defi ne an eastward-tapering wedge onlapping onto the Upper Cretaceous-Early Paleogene units toward the Llanos Basin. From west to east, this sedimentary wedge includes (see Fig. 5.4):

Eastward thinning, west-southwesterly derived, clast- and 1matrix-supported conglomerates and interbedded reddish mudstones and cross-bedded sandstone lenses that com-prise the upper part of the Hoyón Formation in the Middle Magdalena Valley Basin (Gómez et al. 2003). These rocks coarsen upwards and represent alluvial fan sediments depos-ited in a proximal foredeep (Gómez et al. 2003).An up to ~1200 m-thick fi ning-upward sequence evolving 2from sandstone-dominated braided plain deposits of the Cacho Formation to the variegated mudstones and sandstones of the Bogotá Formation, deposited in fl uvial plains (Hoorn et al. 1987) within the foredeep depozone in the present axial Eastern Cordillera.A lateral equivalent, ~700 m-thick fi ning upward sequence 3including the distal alluvial and coastal plain deposits of the sandstone-dominated Barco and mudstone-dominated Los Cuervos Formations (e.g. Santos et al. 2008), which are inter-preted as the distal part of the Late Paleocene foredeep (Parra et al. 2009).

Eocene

Lower Eocene units are absent in the Middle Magdalena Valley Basin (see Fig. 5.4). Middle to Upper Eocene rocks correspond to the Almacigos Member of the San Juan de Río Seco Formation (see Fig. 5.4). East of the Middle Magdalena Valley Basin, this unit is ~900 m thick, and onlaps westwards onto the substratum of the basin, defi ning a regional unconformity (Gómez et al. 2003). The Almacigos Member is made up of fi ning-upward, cross-bedded, conglomeratic sandstones with beds roughly metre scale in thick-ness (Gómez et al. 2003).

To the east, in the axial Eastern Cordillera, Eocene units include the ~1000 m-thick Regadera and Usme Formations

The South Amazonian foreland basin system comprises the Southern Peruvian and Northern Bolivian foreland basins. The sub-Andean zone basins defi ne the present-day wedge-top depozone and are delimited from the foredeep depozone by the sub-Andean thrust front (see Figs 5.1 & 5.3). The Madre de Dios and Beni-Mamoré Basins defi ne the foredeep depozone. As the topographic expression of the forebulge is minor, it is diffi cult to separate the forebulge and the backbulge depozones. These two depozones are grouped and termed Beni forebulge-backbulge depozones (see Figs 5.1 & 5.3).

Cenozoic sedimentary evolution of the Colombian foreland basin system

Late Cretaceous to Early Paleocene depositional systems

Following the widespread Cretaceous, rift-related shallow marine deposition in Colombia, sea withdrawal and establishment of non-marine deposition up to ~1500 m thick is recorded in shallow-ing-upward Maastrichtian to Paleocene sedimentary units. This succession represents a doubly tapered geometry with a maximum thickness of ~1500 m along the western margin of the Eastern Cordillera, which disappears toward the Central Cordillera, to the west, and the Llanos Basin to the east (Cooper et al. 1995; Gómez et al. 2003, 2005). This doubly tapered geometry is locally irregular in the foredeep (Sarmiento 2002) and, although not properly doc-umented, minor intra-foredeep structures cannot be discounted.

Maastrichtian units

To the west, in the Middle Magdalena Basin (see Fig. 5.4), ~80 m of calcareous mudstones and interlayered thin, cross-laminated sand-stones comprise the Buscavida Formation (Gómez et al. 2003). This unit is overlain by up to ~80 m of multistoreyed cross-bedded con-glomerates, fi ne-grained sandstones, minor mudstones, and for-aminiferan- and mollusc-rich limestones that were interpreted as fan-delta deposits (Gómez & Pedraza 1994; Gómez et al. 2003). To the southwest, in the Upper Magdalena Valley Basin, lateral equiva-lent units (La Tabla and Monserrate Formations; Montes et al.2005; Ramon & Rosero 2006) are continental (braided river depos-its; Ramon & Rosero 2006). Palaeocurrent and provenance data for these units indicate a western source located in the present-day Central Cordillera (Gómez et al. 2003; Montes et al. 2005; Ramon & Rosero 2006). To the east, in the Eastern Cordillera and Llanos Basin, sediments consist of ~250 m-thick easterly derived, laterally continuous, fi ne-to-coarse-grained sandstones with lenticular and fl aser bedding. At the top of this sequence the Labor and Tierna Formations are characterized by sandstones with large-scale cross-stratifi cation, which were interpreted as tidal fl at deposits (Pérez & Salazar 1978).

Paleocene units

The Cretaceous units in the Eastern Cordillera are conform-ably overlain by the Guaduas Formation, which is constituted by up to 1400 m of variegated mudstones interlayered with


66 M. Roddaz et al.

sandstones and sandy pebble conglomerates interbedded with minor variegated mudstone that was deposited in braided fl u-vial systems. To the east, the Eocene units progressively onlap onto the Llanos Basin substratum, resulting in a regional uncon-formity with an eastward increase in the chronostratigraphic gap (Jaramillo 2007).

Thus, Eocene deposits in central Colombia display a double tapered shape along a transverse WNW–ESE section (see Fig. 5.3). Lower Eocene units are restricted to the central part of this wedge; they outcrop in the Eastern Cordillera but are absent in the Middle Magdalena Valley Basin and Llanos Basin. It is important to notice that the Late Eocene shaly units of the Upper Mirador Formation and Lower Carbonera C8 member indicate a change from fl uvial conditions to marine-infl uenced conditions (Pulham et al. 1997).

Oligocene

In the Eastern Cordillera, Oligocene units are restricted to the uppermost ~200 m of the Usme Formation (Hoorn et al. 1987;

(Hoorn et al. 1987). The Early to Middle Eocene Regadera Formation is 650–750 m thick and consists of tens of metre-thick packages of interlayered (i) cross-bedded conglomeratic sandstones, clast-supported pebble conglomerates, and thin variegated mudstones, and (ii) variegated mudstones with minor, lens-shaped, fi ne-grained sandstones. The Regadera Formation was interpreted as deposits formed in a braided alluvial environ-ment (Hoorn et al. 1987; Kammer 2003). The Late Eocene-Early Oligocene Usme Formation is ~300 m thick and is unconform-ably overlain by the Early Miocene Tilatá Formation. The lower 100 m of the Usme Formation consist of tens-of-metres-thick brownish and greyish mudstones; the upper 200 m are formed by medium-grained sandstones, variegated mudstones and minor coal interbeds. A deltaic depositional environment has been pro-posed for the Usme Formation (Hoorn et al. 1987).

In the eastern part of the Eastern Cordillera, Eocene rocks are ~400 m thick and correspond to the Mirador Formation and the lower part of the Carbonera Formation (C8 Member; Parra et al. 2008; see next section). The Lower Mirador Formation consists of ~250 m of multistoreyed, metre-thick, cross-bedded

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Fig. 5.4 Stratigraphic overview (time–distance diagram, or Wheeler diagram) of the Paleogene to Neogene Colombian fore-land basin. Modifi ed from Parra et al. (2009).



conglomerates interbedded with very thick beds of sandy mud-stones and mudstones where palaeosol development is common. The upward increase in grain size and the upward appearance of channelized conglomeratic levels in the top of the Lower Guayabo Formation suggest a change in depositional environment from meandering to braided rivers. The Upper Guayabo Formation consists of clast-supported cobble to pebble conglomerate that is organized in very thick beds and interbedded with isolated dark mudstones beds, of less than 2 m thick. The age of the lowermost dark mudstones level of the Upper Guayabo Formation is esti-mated as Late Miocene-Early Pliocene (Mora 2007). Thus, the upper parts of this unit are likely to have been deposited during the Pliocene (see Fig. 5.4).

Cenozoic sedimentary evolution of the Ecuadorian foreland basin system

Paleocene

The Tena Formation is poorly documented but known to con-sist of monotonous reddish shales interbedded with thin beds of fi ne sandstones that were deposited in a distal meandering fl ood-plain (Fig. 5.5). The age is based on Paleocene charophytes that were found in the upper part of the formation (Fauchet & Savoyat 1973). The sediments of the Tena Formation derived from the Eastern Cordillera (Ruiz et al. 2004, 2007; Martin-Gombojav & Winkler 2008), and the formation is thought to be an analogue of the Rumiyaco Formation in the Colombian Putumayo Basin (Mora et al. 1998).

Eocene

The Tiyuyacu Formation (Tschopp 1953; Baldock 1982) is divided into two members. The Lower Member has been dated by zircon fi ssion track analysis at 51 ± 5 Ma (Ruiz et al. 2004). In the north of the sub-Andean zone, the lower part of the Upper Member contains a tuffaceous layer dated at 46 ± 0.4 Ma (argon-argon [Ar/Ar] dating on biotite, Christophoul et al. 2002a). The Tiyuyacu Formation is fed by a higher grade of metamorphic rocks of the Eastern Cordillera than those of the Tena Formation (Martin-Gombojav & Winkler 2008). This unit can be correlated with the Pepino Formation in the Colombian Putumayo Basin (Mora et al. 1998). The Tiyuyacu Lower Member is variable in thickness (150–548 m) and the base is a regional erosional unconformity that can be identifi ed in both outcrops and seismic sections (Christophoul et al. 2002a) (see Fig. 5.5). This member consists of conglomerates and reddish shales. The con-glomerates are composed of 80–90% reddish chert clasts and 10–20% white quartz pebbles and metamorphic rock fragments (Marocco et al. 1996; Valdez Pardo 1997). These deposits range from conglomerates to mudstones and are organized in typical channel-fi lling, fi ning-upward, 10 m-thick sequences. Each base of a sequence corresponds to conglomerates and sandstones dis-playing trough cross- and planar stratifi cations and ripple cross-laminations deposited in gravel bars (Miall 1996). These deposits

Parra et al. 2009), which are unconformably overlain by Early Miocene fl uvial strata of the Tilatá Formation (see Fig. 5.4). Further to the east, Oligocene deposits of the Carbonera Formation show a progressively eastward thinning succession (Cooper et al. 1995; Gómez et al. 2005; Parra et al. 2009). In the eastern foothills of the Eastern Cordillera, this unit conformably overlies the Mirador Formation and has a maximum thickness of ~3400 m, which diminishes by a half ~100 km basinward (Bayona & Thomas 2003; Bayona et al. 2007).

In the proximity of the Eastern Cordillera, the Carbonera Formation mainly consists of two interlayered facies associations (Parra et al. 2008) (see Fig. 5.4):

Up to 10 m-thick, thickening and coarsening-upwards inter-1vals composed of tabular sandstone with minor dark-grey mudstone interbeds, common fl aser and lenticular lamination, laminae rich in organic matter and dewatering structures;up to 100 m-thick intervals of dark-grey to greenish mudstone 2with occasional bioturbation and coal interbeds.

Freshwater molluscs are occasionally present in either of the facies associations. Punctuated marine infl uence is registered by the presence of discrete, thin levels with foraminiferal lin-ings. According to the relative abundance of these lithofacies, the Carbonera Formation is divided into eight members (C8 to C1 members, from older to younger) of alternating sandstone-dominated and mudstone-dominated deposits, that are inter-preted as tidally infl uenced lacustrine and coastal plain deposits (Parra et al. 2009). Westward facies variations toward braided fl uvial deposits in the lower part of the Carbonera Formation (C7–C6 members) have been documented in the westernmost reaches of the eastern foothills region ((Parra et al. 2009). Finally, a coarsening-upwards interval of fl uvial deposits constitutes the ~uppermost 1 km of the unit (C1 member). Toward the east, the basal deposits of the Carbonera Formation become younger and overlie a progressively older substratum (Gómez et al. 2005; Santos et al. 2008).

Miocene to Present

The Middle Miocene León Formation consists of a package of ~500 m of thin, laminated dark-grey mudstones and conform-ably overlies the Carbonera Formation (see Fig. 5.4). Subsurface and well data in the Llanos Basin indicate lateral continuity, albeit minor facies changes occurred across the Llanos Basin. Towards the north and west of the basin these changes include the pres-ence of sandstone interbeds a few metres thick (Cooper et al.1995). Scarce interlayered, thin fossiliferous horizons show rela-tive high abundances of freshwater molluscs (Parra et al. 2006). Brackish-water palynological associations, dinofl agellate cysts and foraminiferal test linings (Bayona et al. 2007) suggest the punctuated infl uence of brackish waters within an otherwise continuous lacustrine environment. The León Formation is over-lain by the Upper Miocene Lower Guayabo Formation, which is composed of a succession of very thick beds of channelized, texturally immature sandstones, conglomeratic sandstones and


68 M. Roddaz et al.

Early Oligocene

The Early Oligocene Orteguaza Formation, formerly known as the Chalcana Formation (Tschopp 1953), conformably overlies the Upper Tiyuyacu Member (see Fig. 5.5) and has a variable thick-ness that ranges between 40 and 341 m. Palynological dating by Zambrano et al. (1999) suggest a Late Eocene to Early Oligocene age for the Orteguaza Formation (Table 5.1). The provenance of the Orteguaza Formation is similar to that of the Tiyuyacu Formation (Martin-Gombojav & Winkler 2008).

In the sub-Andean zone, the Orteguaza Formation consists of fi ne fl uvial deposits similar to the overlying Chalcana Formation. Eastwards within the basin, the Orteguaza Formation consists

grade upwards into coarse and then fi ne sandstones, indicating downstream accretion macroforms and lateral accretion macro-forms (Miall 1996). The top of the sequence consists of fi ne silt-stones and mudstones and palaeosols characterizing fl oodplain and overbank deposits. These elemental sequences are repeated, showing an overall fi ning upward trend.

The Tiyuyacu Upper Member is variable in thickness and ranges between 150 and 548 m. This member outcrops continuously in the sub-Andean zone and its facies associations are quite similar to the Lower Tiyuyacu Member (see Fig. 5.5). The main difference is that 90% of the conglomerates are composed of well- to very-well-sorted clasts of white vein quartz in a rare blue clay matrix (Christophoul et al. 2002a; Ruiz 2002).

Series

PLEISTOCENE

Alluvial fan

Erosion

Fan delta

?

?Growth strata

Fluvial

Distal floodplain deposits

Late

Middle

Middle

Middle

Early

Early

Early

Legend

Marine deposits Continental deposits

Trough cross-stratification

Palaeosol

Mudstones

Sandstones

Limestones

Clasts

Mud clasts

Erosional surface

Lenticular/wavy bedding

Channel

Early

Late

EO

CE

NE

PAL

EO

-C

EN

EO

LIG

OC

EN

EM

IOC

EN

E

NE

OG

EN

EPA

LE

OG

EN

E

Late

Late

PLIO-CENE

West

Chambira Fm

Arajuno Fm

Curaray Fm

Chalcana Fm

Orteguaza Fm

Upper Tiyuyacu Mb.

Lower Tiyuyacu Mb.

Tena Fm

Mera FmOriente Basin East

Fig. 5.5 Stratigraphic overview (Wheeler diagram) of the Paleogene-Neogene Ecuadorian foreland basin. Fm, formation; Mb, member.


Tab

le 5

.1

Ove

rvie

w o

f m

ain

feat

ures

of

the

diff

eren

t pa

rts

of t

he f

orel

and

basi

n sy

stem

. Bio

stra

tigra

phic

and

rad

iom

etric

dat

a ar

e in

dica

ted

sepa

rate

ly.

Fore

lan

d

bas

inFo

rmat

ion

sA

ge

Ch

arac

teri

stic

fo

ssils

Rad

iom

etri

c d

atin

gD

epo

siti

on

al

sett

ing

Pa

laeo

curr

ent

dir

ecti

on

sR

epre

sen

tati

ve

ou

tcro

ps

Co

lom

bia

Ba

rco

Early

Pal

eoce

neFl

uvia

l bra

ided

riv

ers

Gua

dual

era

Cre

ek

Lo

s C

uerv

osM

iddl

e-La

te

Pale

ocen

eBo

mba

caci

dite

s an

nae,

Fov

eotr

icol

pite

spe

rfor

atus

, Pro

xape

rtite

s op

ercu

latu

s,

Tetr

acol

poro

polle

nite

s m

acul

osus

(Jar

amill

o et

al.

2005

)

Floo

dpla

in d

epos

itsPi

ñale

rita

Cre

ek

M

irado

rEo

cene

Tetr

acol

porit

es m

acul

osus

, Spi

nozo

noco

lpite

sgr

andi

s (J

aram

illo

et a

l. 20

09)

Coa

stal

pla

in a

nd

estu

arin

e de

posi

tsPi

ñale

rita

Cre

ek

C

arbo

nera

C8

Early

Olig

ocen

eEc

hitr

ipor

ites

tria

ngul

iform

is o

rbic

ular

is,

Not

hofa

gidi

tes

huer

tasi

i (Pa

rra

et a

l. 20

09)

Coa

stal

pla

in a

nd

estu

arin

e de

posi

tsPi

ñale

rita

Cre

ek

C

arbo

nera

C7-

C6

Early

to

Late

O

ligoc

ene

Sand

ufou

ria s

eam

rogi

form

is, M

agna

stria

tites

gran

diou

sius

, Mau

ritiid

ites

fran

cisc

oi

min

utus

and

Ver

ruca

tosp

orite

s us

men

sis,

C

icat

ricos

ispo

rites

dor

ogen

sis

(Par

ra e

t al

. 20

09)

Coa

stal

pla

in a

nd

estu

arin

e de

posi

tsN

E–E

(Par

ra e

t al

. 20

09)

Gua

dual

era-

Gac

ener

a C

reek

s

C

arbo

nera

C5-

C1

Early

Mio

cene

Coa

stal

pla

in a

nd

estu

arin

e de

posi

tsH

umea

and

Gaz

aunt

a Ri

vers

Le

ónM

iddl

e M

ioce

neLa

cust

rine

Gaz

amum

o Ri

ver

G

uaya

boLa

te M

ioce

ne-

Plio

cene

Brai

ded

river

de

posi

ts a

nd

allu

vial

fan

s

E, N

E–SW

Tont

ogüe

Cre

ek

Ecu

ado

r

Tena

Fm

Pale

ocen

eC

haro

phyt

es (F

auch

er &

Sav

oyat

, 197

3)Fl

oodp

lain

dep

osits

Ti

yuya

cu L

ower

M

bLo

wer

to

Mid

dle

Eoce

neFl

uvia

l dep

osits

Ti

yuya

cu U

pper

M

bM

iddl

e to

Lat

e Eo

cene

Tuff

46

± 0

.4 M

a ag

e (A

r/A

r da

ting

on b

iotit

e;

Chr

isto

phou

l et

al. 2

002a

)

Brai

ded

river

de

posi

ts

(Con

tinue

d)(C

ontin

ued)


O

rteg

uaza

Fm

Late

Eoc

ene-

Early

O

ligoc

ene

Paly

nolo

gy (Z

ambr

ano

et a

l. 19

99):

Del

toid

ospo

ra s

p., S

ynco

lpite

s sp

., C

icat

ricos

ispo

rites

sp.

, Cic

atric

osis

porit

esdo

roge

nsis

, Ver

ruca

tosp

orite

s sp

., St

riatr

icol

pite

s ca

tatu

mbu

s, L

aevi

gato

spor

ites

sp.,

Retit

ricol

pite

s sp

., M

agna

perip

orite

ssp

inos

us, S

teph

anop

orite

s sp

., Sp

inoz

onoc

olpi

tes

echi

natu

s, M

agna

stria

tites

ho

war

di, V

erru

cato

spor

ites

usm

ensi

s,

Mon

ocol

pite

s sp

.

Mar

ine

depo

sits

C

halc

ana

Fm

Late

Olig

ocen

e to

Ea

rly M

ioce

neM

iosp

ore

(Lae

viga

tosp

orite

s sp

.) an

d fr

eshw

ater

alg

a (E

dwar

ds, 1

983)

Spor

omor

phs

(Cic

atric

osis

porit

escr

iatu

s, M

agna

stria

tites

how

ardi

and

Ve

rruc

atos

porit

es u

smen

sis

(Lor

ente

198

6)C

haro

phyt

es o

ogon

es (T

ecto

char

a cf

. uc

ayal

iens

is)

Mea

nder

ing

or

anas

tom

osed

riv

er

depo

sits

NW

–SE,

N–S

(Chr

isto

phou

l et

al.

2002

b)

Nap

o an

d A

guar

ico

Rive

rs

A

raju

no F

mM

iddl

e to

Lat

e M

ioce

neFo

ram

inife

ra B

athy

siph

onsp

., Ps

amm

osph

aera

sp.,

Troc

ham

min

a sp

. and

Va

lvul

ina?

(Bris

tow

& H

offs

tett

er, 1

977)

Echi

perip

orite

s an

d ra

re E

chitr

icol

porit

esm

aris

tella

e(V

illan

o-2

wel

l)Fr

eshw

ater

fer

n sp

ore

(Azo

lla)

Apa

tite

and

zirc

on fi

ssio

n tr

ack

datin

g (A

FT a

nd Z

FT)

~ 2

2 M

a (R

uiz

et a

l. 20

04)

Gra

vel-w

ande

ring

river

WN

W–E

SE t

o W

E an

d N

–S

(Chr

isto

phou

l et

al.

2002

b)

Ara

juno

and

Nap

o Ri

vers

C

ham

bira

Fm

Mid

dle

to L

ate

Mio

cene

Verr

ucat

otril

etes

eta

yoi (

Edw

ards

, 198

3)C

rass

oret

itrile

tes

vanr

aads

hoov

enii

(Mul

ler

et a

l. 19

87)

Gra

vel b

raid

ed r

iver

SW–N

E, E

–WPa

staz

a D

epre

ssio

n, N

apo

river

C

urar

ay F

mEa

rly t

o ea

rly L

ate

Mio

cene

Fora

min

ifera

: Am

mob

acul

ites

(2 s

pp.),

Si

gmoi

lina

sp.,

Poly

stom

ella

sp.

, Rot

alia

sp.

(T

iput

ini w

ell)

Cyt

herid

ea c

f. k

ollm

ani,

Cyp

ridei

s af

f. h

owei

(t

oday

Vet

usto

cyth

erid

ea b

risto

wi)

in T

iput

ini

wel

lRe

titric

olpo

rites

gui

anen

sis,

Zon

ocos

tites

ra

mon

ae, L

aevi

gato

spor

ites

sp. a

nd

fora

min

ifera

n A

mm

onia

bec

carii

(Vin

ita w

ell)

Cro

codi

lian

and

turt

les

Tida

l dep

osits

Cur

aray

Riv

er, N

apo

Rive

r

M

era

FmPl

io-P

leis

toce

neLa

har

depo

sits

Tab

le 5

.1

Con

tinue

d.

Fore

lan

d

bas

inFo

rmat

ion

sA

ge

Ch

arac

teri

stic

fo

ssils

Rad

iom

etri

c d

atin

gD

epo

siti

on

al

sett

ing

Pa

laeo

curr

ent

dir

ecti

on

sR

epre

sen

tati

ve

ou

tcro

ps


No

rth

ern

Per

u

Ya

huar

ango

Fm

Pale

ocen

eFl

oodp

lain

and

la

cust

rine

depo

sits

Uca

yali

Basi

n (K

umm

el,

1946

)

Lo

wer

Poz

o M

b (P

ozo

sand

s)M

iddl

e Eo

cene

-Ea

rly O

ligoc

ene

Ost

raco

ds, f

oram

inife

rs, g

astr

opod

s an

d pa

lyno

mor

phs

(Sán

chez

& H

erre

ra 1

998;

Se

min

ario

& G

uiza

do 1

976;

Will

iam

s 19

49)

43.0

± 9

.9 M

a (C

arm

en

1508

wel

l) an

d 35

.1 ±

4.4

M

a (C

orrie

ntes

115

)A

FT d

atin

g on

tuf

fs

(Her

moz

a 20

04)

Estu

arin

e an

d sh

oref

ace

depo

sits

Shap

aja

area

(6.5

8555

°S,

76.3

0250

°W, H

ualla

ga

basi

n; H

erm

oza

et a

l. 20

05) M

arañ

ón f

ored

eep

(wel

ls)

U

pper

Poz

o M

b (P

ozo

shal

e)M

iddl

e Eo

cene

-Ea

rly O

ligoc

ene

Ost

raco

ds, f

oram

inife

rs a

nd p

olle

n of

Eo

cene

-Olig

ocen

e ag

e (S

ánch

ez &

Her

rera

19

98; S

emin

ario

& G

uiza

do 1

976;

Will

iam

s 19

49)

Shal

low

cla

stic

she

lf de

posi

tsJu

anju

i-Toc

ache

roa

d (P

eru,

7.2

3471

°S,

76.7

4628

°W; H

erm

oza

et a

l. 20

05M

arañ

ón f

ored

eep

(wel

ls)

C

ham

bira

Fm

Olig

ocen

e-M

iddl

e M

ioce

neC

haro

phyt

es (T

ecto

cara

sup

rapl

ana)

in S

anta

Lu

cia

2X w

ell (

Her

moz

a 20

04)

Tide

-infl u

ence

d fl u

vial

dep

osits

and

di

stal

fl oo

dpla

in

depo

sits

Tara

poto

–Bel

lavi

sta

road

(6

.709

05°S

, 76.

2879

0°W

)Be

llavi

sta

area

(7

.071

66°S

, 76.

5744

0°W

)M

arañ

ón f

ored

eep

(wel

ls)

(Her

moz

a et

al.

2005

)

Ip

urur

o Fm

Mid

dle

Mio

cene

-Pl

ioce

neD

elta

ic t

o co

ntin

enta

l de

posi

ts

SAZ

only,

Rio

Sis

a an

d Sa

poso

a ar

eaSa

canc

he a

rea

(7.0

7235

°S, 7

6.70

231°

W)

Juan

jui-T

ocac

he r

oad

(7.5

3722

°S, 7

6.68

028°

W)

(Her

moz

a et

al.

2005

)

Pe

bas

FmEa

rly M

ioce

ne-

early

Lat

e M

ioce

neSe

e C

hapt

er 1

8

Ju

anju

i Fm

Plio

-Ple

isto

cene

Fluv

ial t

o al

luvi

al

fan

depo

sits

Onl

y SA

Z (H

erm

oza

et a

l. 20

05)

M

arañ

ón F

mLa

te M

ioce

ne-

Plio

cene

Floo

dpla

in d

epos

itsM

arañ

ón f

ored

eep

(wel

ls) (

Her

moz

a 20

04;

Wes

selin

gh e

t al

. 200

6)

C

orrie

ntes

Fm

Plei

stoc

ene

Floo

dpla

in d

epos

its

Mar

añón

for

edee

p (w

ells

)

(Con

tinue

d)


Sou

ther

n P

eru

an

d n

ort

her

n B

oliv

ia H

erm

oza

(20

04)

H

uaya

bam

ba F

mPa

leoc

ene

Cha

roph

ytes

(Sph

aero

char

as a

nd T

ecto

char

a su

prap

lana

) (G

utié

rrez

197

5)C

haro

phyt

es (S

phae

roch

aras

sp.

, Por

ocha

ragi

ldem

eist

eri c

osta

ta, P

. gild

emei

ster

i sol

ensi

s,

Spha

eroc

hara

s hu

aroe

nsis

) in

Can

dam

o 78

–53-

1X w

ell (

Car

pent

er &

Ber

umen

199

9)C

haro

phyt

es (N

itello

psis

sup

rapl

ana)

in P

uqiri

Sy

nclin

e (In

amba

ri Ri

ver,

Mad

re d

e D

ios

Basi

n;

Her

moz

a 20

04)

Floo

dpla

in a

nd

lacu

strin

e de

posi

tsPu

quiri

syn

clin

e, In

amba

ri Ri

ver

(Mad

re d

e D

ios

Basi

n) (H

erm

oza

2004

)

Ba

la F

mO

ligoc

ene-

Mio

cene

Q

uend

eque

Fm

Late

Mio

cene

Cha

roph

ytes

(Tec

toch

ara

ucay

alie

nsis

co

rona

ta) F

oram

inife

ra (B

athy

siph

on) i

n C

anda

mo

78–5

3-1X

wel

l (C

arpe

nter

&

Beru

men

199

9)

Fluv

ial d

epos

its

loca

lly t

ide-

infl u

ence

d

Sout

hern

Per

uvia

n an

dno

rthe

rn B

oliv

ian

SAZ

C

harq

ui F

mLa

te M

ioce

ne8.

7 ±

0.9

Ma

and

7.96

±

0.5

8 M

a (A

r da

ting

on m

icas

) in

nort

hern

Bo

livia

n SA

Z (1

4.71

°S,

67.5

8°W

; Str

ub 2

006;

St

rub

et a

l. 20

05)

Brai

ded

and

mea

nder

ing

river

de

posi

ts

Sout

hern

Per

uvia

n an

d N

orth

ern

Boliv

ian

SAZ

Ip

urur

o/M

adre

de

Dio

s Fm

Mio

cene

Am

bros

ia s

p., M

ultim

argi

nite

sva

nder

ham

men

i, K

uylis

porit

es w

ater

bolk

i, C

aryo

phyl

lace

ae, P

rote

acea

e, C

orsi

nipo

lleni

tes

ocul

us n

octis

and

Com

posi

tae/

Poly

gonu

m(M

obil

Oil

Cor

p., 1

998)

9.01

± 0

.28

Ma

(Ar

datin

g on

fel

dspa

rs) (

Cam

pbel

l et

al.

2006

)

Tide

-dom

inat

ed

delta

s an

d es

tuar

ies

Mad

re d

e D

ios

fore

deep

M

asuk

o an

d Tu

tum

o Fm

Plio

-Ple

isto

cene

Fluv

ial t

o al

luvi

al

fan

depo

sits

Sout

hern

Per

uvia

n an

d no

rthe

rn B

oliv

ian

SAZ

M

adre

de

Dio

s Fm

Plio

cene

3.12

± 0

.02

(Ar

datin

g on

fel

dspa

rs a

nd b

iotit

es)

(Cam

pbel

l et

al. 2

006)

Brai

ded

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Fm, F

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sub

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.

Tab

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ion

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ge

Ch

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stic

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Chambira Formation

The Chambira Formation is of Middle to Late Miocene age, based on biostratigraphic markers (see Table 5.1) and stratigraphic rela-tions with the underlying Arajuno Formation. Previous interpreta-tions attributing a Pliocene age to this formation (Tschopp 1953) cannot be entirely ruled out but might only apply to the upper-most part of the formation. The Chambira Formation is composed of quartz pebble-bearing conglomerates included in a quartz-rich argillaceous matrix. The basal part contains trough cross-bedded and matrix-supported conglomerates. The upper part is composed of conglomerates with horizontal and trough cross-stratifi cations grading up to trough cross-bedded and ripple cross-laminated sandstones and massive siltstones (see Fig. 5.5). These assemblages characterize high-energy gravel-braided rivers with frequent mud-fl ows (Miall 1996). Palaeocurrents indicate main fl ow directions, ranging from SW–NE to E–W, i.e. transverse drainage.

Curaray Formation

The Curaray Formation is of Early to early Late Miocene age based on the biostratigraphic markers and correlations with other, radio-metrically dated formations. For instance, the Loyola Formation was dated at 13.9 ± 1.4 and 11.1 ± 1.0 Ma (Serravallian-Early Tortonian) and the Mangán Formation at 9.9 ± 1.2 to 9.5 ± 1.0 Ma (Tortonian: Hungerbuhler et al. 2002). The Middle to Late Miocene age con-fi rms that the Curaray Formation is the easterly lateral equivalent of the Arajuno and Chambira Formations (see Fig. 5.5). This forma-tion is made up of sandy to silty tidalites, containing crocodilians and marine turtles (Bristow & Hoffstetter 1977, but see Chapter 16for the putative marine character of Neogene South American turtles), which indicate deposition in a tidal environment.

Pliocene to Present

The Mera Formation unconformably overlies the Chambira and Arajuno Formations and is covered by volcaniclastic deposits (see Fig. 5.5). The age of the Mera Formation is poorly constrained and a Plio-Pleistocene age has been ascribed due to its stratigraphic position. The oldest dated deposits are 40,580 ± 1030 years BP in age (14C dating on charcoal; Bes de Berc et al. 2005).

Typically, the lower part of the formation is composed of well-sorted rounded clasts (mainly volcaniclastics with minor meta-morphic fragments from the Eastern Cordillera) that range from 1 cm to 20–50 cm, and are included in a volcanic sandy matrix. These sediments are arranged into at least three units, each 15 m thick, with poorly convex erosional bases, corresponding to wide and shallow channels. The middle part of the formation (20 m) is made up of unsorted angular andesitic clasts (60%), which are included in an ash-rich silty-to-sandy matrix. These beds are likely to represent lahars deposits. The upper surface of the lahars deposits is oxidized and hardened and corresponds to the Mera surface (Bes de Berc et al. 2005). The Mera Formation mainly crops out at the apex of the Pastaza Megafan (Räsänen et al. 1992) where it reaches its maximum thickness. The thickness decreases downstream along the Pastaza River.

of marine deposits composed of greenish shales and medium-to-coarse, locally glauconitic, sandstones (Christophoul et al.2002a). Sedimentary structures such as fl aser and wavy bedding, 2D ripple marks and trough cross-beddings indicate tide- infl uenced deposition (Christophoul et al. 2002a) within a clastic marine depositional environment. As the Orteguaza Formation overlies continental deposits of the Tiyuyacu Upper Member, its base corresponds to a transgressive surface. Well log sequential analyses suggest that the Orteguaza Formation is composed of two transgression–regression cycles (T–R cycles) (Christophoul et al. 2002a). At the end of the second T–R cycle, the reddish shales of the Chalcana Formation rapidly prograded throughout the Oriente Basin.

Late Oligocene to Miocene

The Late Oligocene-Miocene formations are formed of sediments issued from both the Eastern and Western Cordilleras (Martin-Gombojav & Winkler 2008, and references therein).

Chalcana Formation

The Chalcana Formation is Late Oligocene to Early Miocene in age (see Table 5.1) and mostly consists of reddish shales interca-lated with rare fi ne-grained and thin sandstone beds displaying trough cross-bedded stratifi cations and horizontal laminations (see Fig. 5.5). These lithofacies represent fl oodplain fi nes and crevasse channels (Miall 1996) that possibly characterized the inter-distributaries of a sandy, low-sinuosity, meandering or anastomosed river system, similar to the present-day Amazonian plain. Palaeocurrent measurements show two main directions, NW–SE and N–S, which may be interpreted as transverse river systems perpendicular to the Andean proto-cordillera, debouch-ing into river systems parallel to it. The thickness of the Chalcana Formation is variable (255–455 m).

Arajuno Formation

The contact between the Chalcana Formation and the overlying Arajuno Formation (Tschopp 1953) is conformable and gradual (Campbell 1970). Based on spores and fossil assemblages (see Table 5.1), its base is ascribed to late Early Miocene Zone 27 (Late Aquitanian-Burdigalian), with an age ranging from ~22 to 16.2 Ma (Muller et al. 1987; Rull 2002). Fission track dating on volcanic zircons and apatites from the base of the formation yields Early Miocene ages of ~22 Ma (Ruiz et al. 2004); the age of the upper part is less constrained. The Arajuno Formation is essen-tially composed of fi ne-to-coarse-grained sandstones with trough cross-bedded stratifi cations and horizontal laminations and con-glomerates displaying horizontal bedding and trough cross-strat-ifi cations interbedded with minor siltstone beds that represent lateral accretion and downstream accretion deposits (Miall 1996). These associations of lithofacies can be interpreted as the distal part of gravel-wandering rivers. Palaeocurrent measurements in-dicate two drainage directions, WNW–ESE to W–E and N–S, as in the Chalcana Formation.


74 M. Roddaz et al.

tuffaceous sandstones of the base of the Pozo Formation gave two stratigraphic ages at 43.0 ± 9.9 Ma and 35.1 ± 4.4 Ma (Hermoza 2004). The Pozo Formation is divided into two members, a lower sandy member (Lower Pozo Member or Pozo Sand) and an upper muddy member (Upper Pozo Member or Pozo Shale), both outcropping continuously in the sub-Andean zone and in the Marañón foredeep (see Fig. 5.6).

Lower Pozo Member

In the sub-Andean zone the lower Pozo Member reaches up to 20 m in thickness. The base of the formation consists of unconsolidated conglomerates composed of Cretaceous sandstones, Paleozoic quartzites, and tuffaceous sandy clasts in a well-sorted sandy ma-trix, which are interpreted as lag pebble deposits (Hermoza et al. 2005b). Immediately following these lag pebble deposits is a succes-sion of 30 cm-thick fi ning-upward sandy sequences composed of coarse-to-medium-grained sandstones with tidal bundles, sigmoid

Cenozoic sedimentary evolution of the Peruvian and northern Bolivian foreland basin system

Paleocene

The Yahuarango Formation (northern Peru) and the Huayabamba Formation (southern Peru) are poorly dated and consist mainly of red siltstones and mudstones forming distal fl uvial deposits (Figs 5.6 & 5.7) (Gil 2001; Hermoza 2004). Paleocene deposits are not encountered in Bolivia.

Eocene

Northern Peru: Pozo Formation

Based on biostratigraphic markers, the Pozo Formation is Eocene-Oligocene in age (Williams 1949; Seminario & Guizado 1976; Sánchez & Herrera 1998). Apatite fi ssion track dating on

EO

CE

NE

PAL

EO

-C

EN

EO

LIG

OC

EN

EM

IOC

EN

E

NE

OG

EN

EPA

LE

OG

EN

E

PLIO-CENE

Juanjui FmCorrientes Fm

Marañón Fm

Pebas Fm

Upper Chambira Fm

Lower Chambira Fm

Upper Pozo (shale) Mb.

Lower Pozo (sand) Mb.

Yahuarango Fm

?

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? ?

Alluvial fan

Fan delta

Growth strata

Distal floodplain deposits

Series

Q

Late

Middle

Middle

Middle

Early

Early

Early

Legend

Marine deposits Continental deposits


Palaeosol

Mudstones

Sandstones

Limestones

Clasts

Mud clasts

Erosional surface


Channel

Early

Late

Late

Late

West (SAZ) Marañón foredeep East (lquitos Fb.)

Fig. 5.6 Stratigraphic overview (Wheeler diagram) of the Paleogene-Neogene northern Peruvian foreland basin. Fm, forma-tion; Mb, member; SAZ, sub-Andean zone.



Upper Pozo Member

In the sub-Andean zone, the Upper Pozo Member consists of a succession of reddish/greenish shales associated with sandstones and shallow marine limestones containing marine ostracods and foraminiferans (see Table 5.1) that disappear westwards. These sediments were deposited in a shallow clastic shelf environment (Hermoza et al. 2005b).

In the Marañón foredeep, the thickness of the Upper Pozo Member ranges from ~50 m to 156 m. It consists of a succession of intercalations of black-to-grey and green shales and siltstones with abundant glauconite and occasional pyrite. Limestone levels can also occur. These sediments are interpreted to representshallow marine deposits. In the western part, the Upper Pozo Member is interpreted as regressive system tracts at its base followed by transgressive system tracts, whereas in the eastern part it consists entirely of transgressive system tracts (Hermoza 2004).

laminations, planar foresets, and herringbone cross-stratifi cations (Hermoza et al. 2005b). These sandy sequences represent shoreface deposits (i.e. marine deposits; Hermoza et al. 2005b).

In the Marañón foredeep, the thickness of the Lower Pozo Member varies from 20 to 56 m. The Lower Pozo Member con-sists here of well-sorted sandstones intercalated with silts and grey shales. An 18 m-long core of the Lower Pozo Member (Carmen1508 well; Hermoza 2004) showed that the upper part of the Lower Pozo Member is composed of well-sorted medium-to-coarse-grained sandstones (8 m thick) with abundant sigmoid, planar and trough cross-bedded laminations, followed by fi ne sandstones and grey to black siltstones and muds displaying fl aser and lenticular bedding. Both sandstones and siltstones are strongly bioturbated. The Lower Pozo Member sediments were deposited within a tide-infl uenced deltaic and estuarine environment. Overall and in each part of the Marañón foredeep, the Lower Pozo Member defi nes a regressive system tract followed in the distal part by a transgressive system tract (Hermoza 2004).

Series

Alluvial fan

Tide-influenced

Growth strataLate

Middle

Middle

Middle

Marine deposits

Legend

Continental deposits


Palaeoso

Mudstones

Sandstones

Limestones

Clasts

Mud clasts

Erosional surface


Channel

Early

Early

Early

Early

Late

EO

CE

NE

PAL

EO

-C

EN

EO

LIG

OC

EN

EM

IOC

EN

E

NE

OG

EN

EPA

LE

OG

EN

E

Late

Late

QPLIO-CENE

Masuko Fm

Charqui Fm

Quendeque Fm

Lower Pozo (sand) Mb.

Huayabamba Fm

Bala Fm

?

? ? ?

?

?

?

?

?

?

?

?? ?

? ?

West (SAZ) Madre de Dios/Beni foredeep East

Madre de Dios Fm

Fig. 5.7 Stratigraphic overview (Wheeler diagram) of the Paleogene-Neogene southern Peruvian foreland basin. Fm, forma-tion; Mb, member; SAZ, sub-Andean zone.


76 M. Roddaz et al.

thickness. Hermoza et al. (2005b) divided the Ipururo Formation into three members (see Fig. 5.6).

The Lower Member consists of microconglomerates and mudstones followed upwards by medium-to-coarse-grained sandstones with oblique planar stratifi cations and low-angle cross-laminations. The Lower Member represents regressive system tracts composed of successive prograding deltaic lobes (Hermoza et al. 2005b). The Middle Member consists of marls and limestones associated with fi ne- and very fi ne-grained hummocky cross-stratifi ed calcarenites. These deposits represent westward transgressive storm deposits (Hermoza et al. 2005b). The Upper Member is composed of conglomerates of well-rounded volcanic and quartzite clasts with trough and planar cross-bedding stratifi cations intercalated with siltstones and mudstones. This sequence is succeeded by trough and planar cross-bedded and horizontal-bedded sandstones. The Upper Member represents fl uvial to braided river deposits (Hermoza et al. 2005b).

Pebas Formation

A detailed description of this formation can be found in Chapter 8. The basal part of this formation is not well dated. For Hermoza (2004) it is Middle-Late Miocene in age, whereas for Wesselingh et al. (2006) it is Early to early Late Miocene.

The Pebas Formation is continuously present in wells of the Marañón foredeep (see Fig. 5.6). Based on the study of three wells, Wesselingh et al. (2006) determined the thickness of the Pebas Formation depending on the presence of coaly intervals; the fi rst coaly interval encountered would be the base of the Pebas Formation whereas the last one would be the top. For these authors, the Pebas Formation is about 1000 m thick. Based on an extensive study of wells and seismic lines, Hermoza (2004) suggests that its thickness is fairly constant (400–500 m). For Hermoza (2004), the base of the Pebas Formation is marked by a lowering in sonic interval travel time, a lowering in gamma ray response and an in-crease in resistivity, which is interpreted as a transgressive surface. On seismic lines, the base of the Pebas Formation corresponds to a sharp refl ector, and channel structures are absent. The basal part consists of glauconite-rich sandstones, and siltstones and mud-stones with fi sh and ostracod remains. The upper part is made of blue mudstones typical of the Pebas Formation. Calcareous intervals are also present (Hermoza 2004). In the absence of bio-stratigraphic or radiometric dating, the exact thickness of the Pebas Formation remains unclear. However, the study of Hermoza (2004) is based on extensive studies of well and seismic lines cov-ering the entire northern Peruvian foreland basin and delimita-tions are based on more criteria; for this reason we favour here a maximum thickness of ~500 m for the Pebas Formation.

Southern Peru and northern Bolivia

The Ipururo Group (Valdivia 1974) comprises Late Oligocene to Miocene deposits of southern Peru and northern Bolivia. The Ipururo Group is divided into three formations: the Bala Formation, the Quendeque Formation and the overlying Charqui Formation (see Fig. 5.7).


The Pozo Sand Member is preserved at some places in the frontal thrust of the southern Peruvian sub-Andean zone and conglom-erates of this member were found in some wells, but in general Eocene deposits are absent (see Fig. 5.7). Eocene deposits are not encountered in Bolivia.

Late Oligocene to Miocene

Northern Peru

Chambira Formation

The age of the Chambira Formation (Kummel 1946), which is a different unit from the Chambira Formation from Ecuador dealt with above, is poorly constrained. For Marocco (1993), it is Late Oligocene-Middle Miocene, whereas for Seminario & Guizado (1976) it is Miocene. Charophytes (Tectocara supraplana) found in Santa Lucia 2X well suggest an Oligocene to Middle Miocene age (Hermoza 2004; see Table 5.1). The Chambira Formation outcrops almost continuously in the sub-Andean zone and in the Marañón foredeep. The Chambira Formation has been divided into two members (Lower Chambira Member and Upper Chambira Member), both of them representing a similar deposi-tional setting (see Fig. 5.6).

In the sub-Andean zone, the Lower Member consists of a suc-cession of sand bars with trough and planar cross-stratifi cations, mudstones and channels with sand-mud couplets. Several chan-nels exhibit coarse-to-medium-grained sigmoid beds, sandstone and planar foreset stratifi cations. Mudstones and sandy bars with trough and planar cross-bedding indicate deposition within a meandering fl uvial system. Sigmoid beds and sand-mud cou-plets suggest a tide-infl uenced system (Hermoza et al. 2005b). The Upper Member is thicker and is characterized by sequences of tidal sand bars, sigmoid bedded sandstones, and trough cross-bedded sandstones, with intercalations of reddish to brownish argillites and silts. In comparison with the Lower Member, the silt/sand ratio is higher, but the Upper Member was deposited in a similar tide-infl uenced fl uvial system (Hermoza et al. 2005b).

In the Marañón foredeep, the thickness of the formation varies from 580 to 1500 m and is formed by an alternation of red siltstones and mudstones with intercalations of fi ne sandstones. The Lower and Upper Members are thought to represent distal aggrading fl oodplain deposits in a meandering fl uvial system. Channel structures can be easily visualized on seismic lines and anhydrite occurrence is frequent (Hermoza 2004). Wesselingh et al. (2006) have added a small regressive basal subunit dividing the Chambira Formation into three subunits based on the study of three wells located in the Pastaza Megafan close to the Ecuadorian frontier.

Ipururo Formation

The Mio-Pliocene Ipururo Formation (Kummel 1946) is present only in the sub-Andean zone, is poorly dated and has an uncertain



trough and planar cross-stratifi cations. The Charqui Formation sediments correspond to braided river deposits. Moreover, the formation also exhibits frequent syntectonic growth strata indi-cating that deposition is strongly controlled by thrust tectonics (Hermoza 2004).

In the Bolivian sub-Andean zone, the Charqui Formation out-crops only in the Madidi syncline. Compared with its southern Peruvian sub-Andean zone equivalent, it exhibits higher propor-tions of sandstones and mudstones but similar syntectonic growth strata (Strub 2006). The Charqui Formation probably represents meandering and braided river deposits

Madre de Dios foredeep: Ipururo and Madre de Dios Formations

In the Madre de Dios foredeep, the Ipururo Group comprises the Ipururo Formation and Unit A and B of the Madre de Dios Formation sensu Campbell et al. (2001). Based on stratigraphic correlations, the Ipururo Formation is estimated to be Miocene in age (Hermoza 2004). The Madre de Dios Formation is Late Miocene (40Ar/39Ar dating on feldspars at 9.01 ± 0.28 Ma, Campbell et al. 2001). The Madre de Dios Formation can thus be considered as a lateral equivalent of the Charqui Formation.

The Ipururo Formation is continuously present in well and seismic sections all along the Madre de Dios foredeep; the Madre de Dios Formation outcrops continuously across the Madre de Dios foredeep (Roddaz et al. 2004; see also Fig. 5.7). Subsurface data indicate that the thickness of the Ipururo Formation ranges from 1100 to 1300 m and that the thickness of the Madre de Dios Formation is fairly constant (~400 m in the three studied wells; Hermoza 2004).

The upper part of the Ipururo Formation consists of subtidal channel sediments deposited in a tide-dominated delta (Roddaz 2004; Roddaz et al. 2004) and the Unit A and B of the Madre de Dios Formation were deposited in tide-dominated estuaries (Roddaz 2004; Roddaz et al. 2004; Hovikoski et al. 2005). For fur-ther details on tide-dominated estuaries see Chapter 9.

Neogene(?) to Present

Northern Peru

The sub-Andean zone (Juanjui Formation)

The Plio-Pleistocene Juanjui Formation is about 100 m thick and can be found at various locations in the sub-Andean zone (Díaz et al. 1998; Sánchez & Herrera 1998). In areas close to Tocache, it is named the Tocache Formation (Díaz et al. 1998). The forma-tion is composed of polygenic well-rounded conglomerates. The clasts are usually less than 15 cm long and consist of intrusive vol-canic schist, gneisses, quartzite, limestones and sandstones. The conglomerates show frequent trough and planar cross-bedded stratifi cations. Clast-supported and inverse-grading facies are also present. The conglomerates usually coarsen upwards (see Fig. 5.6). The Juanjui Formation developed in fl uvial to alluvial fan environments (Hermoza et al. 2005b).

Bala Formation

The Bala Formation is considered as the age-equivalent of the Petaca Formation (Sempere et al. 1990; see also Chapter 7) and hence is Oligocene-Miocene in age. This formation unconform-ably overlies the Jurassic Beu Formation. Its basal part is estimated at ~27 Ma based on lithostratigraphic correlations (Baby et al.1995). This formation has been poorly studied and no detailed sedimentological study exists. The formation is up to 200 m thick and is composed of fl uvial sandstones and conglomerates interca-lated with muddy palaeosol intervals. The clasts consist of cherts, quartzites and reworked sandstones of the Beu Formation and usually the matrix is sandy Fe-rich. Iron and siliceous nodules are frequent both in the sandstones and conglomerates. Lateritic palaeosols can occasionally occur at the basal part of the forma-tion. The sediments of the Bala Formation were deposited by a fl uvial system that developed on a very low topographic gra-dient. Abundant palaeosol horizons and low sediment thickness (< 200 m) indicate predominantly non-deposition. Lateritic pal-aeosols and Fe- and Si-rich nodules suggest intense meteorization compatible with a tropical climate (Strub 2006).

Quendeque Formation

Based on biostratigraphic markers (see Table 5.1), the base of the Quendeque Formation is Late Oligocene-Miocene in age. Stratigraphic correlation (Baby et al. 1995) further suggests a Late Miocene age for the basal part of the formation.

In the Peruvian sub-Andean zone, the Quendeque Formation is up to 1500 m thick and consists of sequences of 6–8 m-thick red quartz and feldspar-rich sandstone bars separated by 10–15 m-thick siltstones and mudstones. The sandstones are character-ized by trough and planar cross-stratifi cations and ripple cross-laminations. The siltstone and mudstone beds are massive. The Quendeque Formation deposits represent distal meandering and fl oodplain sediments (Hermoza 2004).

In the Bolivian sub-Andean zone, the Quendeque Formation is about 2 km thick in the external part of the sub-Andean zone (Madidi syncline) (Strub 2006). The Quendeque Formation deposits consist mainly of aggrading anastomosed fl uvial and fl oodplain deposits (Strub 2006). Tidally infl uenced point bar and estuarine/deltaic interdistributary bay facies coexisting with fl uvial facies described above can occasionally occur (Hovikoski et al. 2007)

Charqui Formation

Argon-40/argon-39 (40Ar/39Ar) dating on a tuffaceous level of the upper part of the Charqui Formation in the northern Bolivian sub-Andean zone gave ages of 8.7 ± 0.9 Ma (Strub et al. 2005; Strub 2006), 7.96 ± 0.58 Ma (micas) and 7.79 ± 0.03 Ma (feld-spars) (Hérail et al. 1994).

In the Peruvian sub-Andean zone, the Charqui Formation is up to 1750 m thick and consists of conglomerates, quartz and feldspar-rich sandstones and rare massive mudstones. The con-glomerates exhibit trough and planar cross-bedded stratifi ca-tions and horizontal stratifi cation whereas the sandstones have


78 M. Roddaz et al.

Plio-Pleistocene sedimentation in the Madre de Dios and Beni foredeeps

The upper part of Madre de Dios Formation (Campbell et al.2006) is part of the Madre de Dios foredeep sedimentary record, and is of Pliocene age based on absolute dating of a tuffaceous level intercalated in these deposits (40Ar/39Ar dating at 3.12 ± 0.02 Ma; Campbell et al. 2001). These deposits have variable thicknesses (10–30 m) and exhibit several facies that are characteristic for braided river and meandering river systems (Roddaz 2004). The braided-river deposits consist of gravels in a sandy matrix and with clasts (1–5 cm in length) of quartzites, intrusive rocks or white weathered sandstones. Trough and planar cross-bedding as well as horizontal laminations are present. Meandering river deposits are characterized by muds, silts and sands. The sands exhibit trough cross-stratifi cations and have a channel-shaped base, defi ning channel infi ll deposits. They are associated with muds with faint planar laminations interpreted as oxbow-lake depos-its. Red massive muds and silts are also frequent and represent fl oodplain deposits. Closely associated with these facies, well-developed palaeosols occur (Roddaz 2004). Macrofossil evidence of a pre-Holocene thorny bamboo similar to Guadua (Poaceae: Bambusoideae: Bambuseae: Guaduinae) has recently been found in these deposits (Olivier et al. 2009). Drainage systems of the Pliocene Madre de Dios deposits are similar to present-day drain-age systems of the Madre de Dios Basin such as the braided Inambari River and the meandering Madre de Dios River.

The modern sedimentation in the Beni foreland basin is dominated by episodic accumulation of fl oodplain deposits con-trolled by El Niño-Southern Oscillation (ENSO) cycles (Aalto et al. 2003; see also Chapter 14).

Sedimentation rates

Colombian foreland basin

Geohistory analysis in the eastern foothills area suggests limited subsidence during the Eocene and earliest Oligocene (Mirador Formation and C8 Member of the Carbonera Formation; Parra et al. 2009; see also Chapter 4). At ~31 Ma, subsidence rates increased and fl uvial-dominated deposition was restricted to the proximal eastern foothills region (Parra et al. 2009).

Ecuadorian foreland basin

Isopach maps and sedimentations rates for the Tiyuyacu, Orteguaza and Chalcana Formations are available in Christophoul et al. 2002a. The Tiyuyacu Lower Member is variable in thickness (150–548 m) and the depocentre is localized in the centre of the Oriente Basin. Calculated sedimentation rates range from 0.01 to 0.05 mm/year. The thicknesses of the Tiyuyacu Upper Member range from 59 to 319 m and its depocentre was located in the centre of the Oriente Basin. Calculated sedimentation rates are similar to those of the Lower Tiyuyacu Member and range from 0.01 to 0.05 mm/year. The Orteguaza Formation is variable in thickness (40 to 341 m) and its depocentre is localized in the

Marañón Formation

The Marañón Formation is poorly dated but considered to be of Pliocene age. Its thickness ranges from 220 to 600 m (Hermoza 2004). The base of the formation consists of well-sorted sand-stones, some of which are glauconite-rich with intercalations of red siltstones and mudstones. The middle part of the formation is composed of massive sandstone beds (up to 30 m thick) interca-lated with siltstones and mudstones. Pyrite, gypsum and anhy-drite are occasionally found. Some limestone levels are also found. The upper part of the formation consists mainly of red siltstone and mudstone with thin sandstone intercalations (see Fig. 5.6) with occasional occurrences of anhydrite (Hermoza 2004). The Marañon Formation probably represents meandering and fl ood-plain fl uvial deposits.

Corrientes Formation

The Corrientes Formation represents Pleistocene deposits and ranges in thickness from 400 to 850 m. The formation is composed of massive sandstone beds (10–30 m thick) intercalated with red siltstones and mudstones (see Fig. 5.6). Coaly intervals are locally present. The Corrientes Formation represents aggrading chan-nel infi ll and fl oodplain sediments (Hermoza 2004) deposited in a meandering fl uvial system probably similar to present-day Amazonian rivers.


The sub-Andean zone (Masuko and Tutumo Formations)

The Masuko Formation is estimated to be Plio-Pleistocene in age. This formation outcrops in the Peruvian sub-Andean zone (see Fig. 5.7) and has a variable thickness. The formation con-sists of gold-bearing conglomerates of economic interest that are presently being mined. This formation is separated from the underlying Charqui Formation by an erosional uncon-formity (Hermoza 2004). The conglomerates are composed of intrusive volcanic, schist, quartzitic, gneissic and sandstone clasts, 15–30 cm in diameter. Trough and planar bedded cross-stratifi cations are present. Massive clast-supported and inverse clast-supported facies also occur. This formation exhibits well-developed syntectonic growth strata indicating thrust-controlled deposition. These deposits correspond to prograding alluvial fan deposits controlled by the activity of sub-Andean zone thrusts (Hermoza 2004).

The Plio-Pleistocene Tutumo Formation (Davila et al. 1965) comprises the Pliocene Bolivian sub-Andean zone deposits and Quaternary aggradational terraces of the Beni River and its tributaries. The Tutumo Formation is variable in thickness (20–700 m). Unfortunately, precise sedimentological studies of this formation are not yet available. It consists mainly of conglomerates of Andean origin and trough cross-stratifi ed sands (Strub 2006). This formation is considered as the lat-eral equivalent of the Masuko Formation (Hermoza 2004) and hence probably represents braided river and alluvial fan deposits.



in several papers (e.g. Ungerer et al. 1990). To calculate sedimen-tation rates, the program uses the backstripping method. The necessary input consists of stratigraphic time–depth informa-tion (thicknesses, ages, lithology and depositional water depths). The decompaction model is based on the algorithm described by Perrier and Quiblier (1974).

To calculate sedimentation rates, we constructed fi ctitious wells where maximum thickness and lithology for each formation formed the input. Due to poor stratigraphic control regarding the Neogene formations, we proposed four different scenarios(Table 5.2). Scenario 1 ascribed a Late Oligocene age to the Chambira Formation and a Miocene age to the Pebas Formation. Scenario 2 proposed a Late Oligocene to Early Miocene age for the Chambira Formation and Middle to Late Miocene age for the Pebas Formation. Scenarios 3 and 4 are the same

centre of the Oriente Basin. Calculated sedimentation rates range from 0.009 to 0.07 mm/year. The thicknesses of the Chalcana Formation range from 255 to 455 m and its depocentre is located in the centre of the Oriente Basin. Calculated sedimentation rates (0.07–0.12 mm/year) increased when compared with underlying formations. Overall, sedimentation rates increased throughout the Cenozoic with the depocentre remaining at a constant place.

Northern Peruvian foreland basin

For the reconstruction of the burial history and basin subsidence rates, we have used the Genex 1D basin modelling software (IFP-BEICIP). The basic concepts of the Genex program can be found

Table 5.2 Calculated sedimentation rates for the northern Peruvian foreland basin. Maximum sedimentation rates calculated from GENEX 1D basin modelling software (BEICIP-IFP; see text for explanations). Maximum sedimentation rates are based on stratigraphic thickness, time as well as compaction.

Formations Period Depth (m) Sedimentation rates

End Start Top Bottom m/Ma mm/year

Scenario 1Corrientes 0 1.8 0 850 725.3 0.7253

Marañón 1.8 5.3 850 1450 306.0 0.306

Pebas 5.3 23 1450 1950 83.1 0.0831

Chambira 23 28.4 1950 3450 527.0 0.527

Upper Pozo 28.4 37.2 3450 3606 38.3 0.0383

Lower Pozo 37.2 48.6 3606 3662 14.9 0.0149

Scenario 2Corrientes 0 1.8 0 850 725.3 0.7253

Marañón 1.8 5.3 850 1450 306.0 0.306

Pebas 5.3 16 1450 1950 137.5 0.1357

Chambira 16 28.4 1950 3450 229.5 0.2295

Upper Pozo 28.4 37.2 3450 3606 38.3 0.0383

Lower Pozo 37.2 48.6 3606 3662 14.9 0.0149

Scenario 3Marañón 0 5.3 0 1450 431.4 0.4314

Pebas 5.3 23 1450 1950 83.1 0.0831

Chambira 23 28.4 1950 3450 527.0 0.527

Upper Pozo 28.4 37.2 3450 3606 38.3 0.0383

Lower Pozo 37.2 48.6 3606 3662 14.9 0.0149

Scenario 4Marañón 0 5.3 0 1450 431.4 0.4314

Pebas 5.3 16 1450 1950 137.5 0.1357

Chambira 16 28.4 1950 3450 229.5 0.2295

Upper Pozo 28.4 37.2 3450 3606 38.3 0.0383

Lower Pozo 37.2 48.6 3606 3662 14.9 0.0149


80 M. Roddaz et al.

Valley, Eastern Cordillera and Llanos Basins is much more con-strained and illustrate well the position of the tectonic load and the extent of the foreland depozones. In the Magdalena Valley Basin, several stratigraphic observations have been used to suggest Late Cretaceous initiation of uplift of the Central Cordillera (Gómez et al. 2003) and coeval associated foreland basin development. In addition, these authors calculated that about 7–13 km thickness of Central Cordilleran rocks were removed from the Campanian to the Eocene, due to kilometre-scale uplift of this range. Foredeep depos-ition occupied the Middle Magdalena Valley and the axial Eastern Cordillera. The accumulation of westerly derived coarse-grained deposits (Cimarrona and Hoyón Formations) occurred adjacent to the topographic front along the uplifting Central Cordillera. In contrast, the deposition of mudstone-dominated fl uvial plain and estuarine deposits associated with high subsidence rates occurred in the distal part of the foredeep, along the axial part of the Eastern Cordillera. Further east, either erosion or limited deposition in the Llanos Basin indicated forebulge conditions.

In Ecuador, the Tena Formation is the oldest formation to be derived from the Eastern Cordillera (Ruiz et al. 2004; Martin-Gombojav & Winkler 2008). Detrital zircon fi ssion track analysis of the sediments of the Tena Formation suggests rapid exhumation and uplift of the Eastern Cordillera (Ruiz et al. 2004) consistent with the 65–55 Ma period of elevated cooling rates and exhum-ation rates of the Eastern Cordillera (Spikings et al. 2001). The fast exhumation and topographic growth of the Eastern Cordillera is related to the Late Cretaceous-Paleocene initial collision of the Caribbean with the South American Plate (Vallejo et al. 2006) and marked in the Ecuadorian foreland basin the onset of tectonic load-ing and related fl exural subsidence. Unfortunately, no such studies exist for the northern Bolivian and Peruvian Eastern Cordillera. Additionally, no detailed sedimentological studies of the Eastern Cordillera of northern Bolivia, Peru and Ecuador have so far been undertaken, so that the existence of a Paleocene forebulge is diffi -cult to establish. Hence, we suggest that the distal fl uvial fl oodplain and continental deposits of the Huayabamba, Yahuarango and Tena Formations were deposited in backbulge or distal foredeep

as scenario 1 and 2 respectively with the exception that the Corrientes and Marañón have been grouped into one formation. Isopach maps for the Marañón Basin are available in Hermoza (2004) and can be provided upon request.

The lowest sedimentation rate was found for the lower Pozo Member (~0.01 mm/year), which has its depocentre located in the distal part of the present-day Marañón foredeep. The depocen-tre of the Upper Pozo Member migrated westwards, close to the present-day orogenic front (Hermoza 2004), and its sedimentation rate increased at ~0.04 mm/year (see Table 5.2). The depocentre of the Chambira Formation migrated towards the present-day sub-Andean zone (Hermoza 2004; Hermoza et al. 2005a) and its sedimentation rate increased at 0.23 mm/year or 0.53 mm/year, depending on the scenario chosen. The Pebas Formation has a constant thickness and its sedimentation rates decreases at 0.14 mm/year or 0.08 mm/year (see Table 5.2). The highest sedi-mentation rates are found, depending on the scenario chosen, for the Corrientes or Marañón Formations (~0.73 mm/year and ~0.43 mm/year), and the locus of the depocentre migrated in the present-day Marañón foredeep (Hermoza 2004).

Discussion

Late Cretaceous-Paleocene: initial tectonic loading and partitioning of the foreland basin

Although there is still debate, most recent studies suggest that the initiation of the Andean foreland basin started in Late Cretaceous-Paleocene times (Balkwill 1995; DeCelles & Horton 2003; Barragan et al. 2005; Martin-Gombojav & Winkler 2008 and references therein). These authors suggest that in southern Bolivia the Paleocene Santa Lucia Formation, outcropping in the Eastern Cordillera, was deposited in the Paleocene backbulge depozone of the Central Andean foreland basin (DeCelles & Horton 2003).

In Colombia, the distribution of facies and thickness of the Late Cretaceous to Paleocene foreland deposits of the Magdalena

– – – – – – – –

–

Fig. 5.8 Paleogene palaeogeographic maps; black lines with black triangles indicate the positions of the Andean thrust front ; light grey indicates areas of marginal marine and lacustrine wetlands. ECC, Eastern Cordillera of Colombia.



In northern Peru, the Lower Eocene unconformity of the Pozo sand Member constitutes a regional subaerial unconformity that marked an important change in geodynamic conditions. Low sedimentation rates (max. ~0.04 mm/year), cratonward migration of the Pozo sand depocentre and reciprocal architecture of the depositional system tracts (regressive system tracts in the prox-imal part of the basin and transgressive system tracts in the distal part) characterized an orogenic unloading stage probably due to the erosion of the Western Cordillera. The Middle to Late Eocene Pozo shale Member defi ned transgressive system tracts occur-ring throughout the basin. These transgressive system tracts, as well as increased sedimentation rates and westward (toward the Andes) migration of the depocentre, characterized a change to a tectonic loading stage where thrust-related loading of the Western Cordillera provoked fl exural subsidence and transgression in the foredeep depozone.

Limited deposition of the Lower Pozo sand Member in south-ern Peru and the absence of deposition of the Pozo shale Member in southern Peru as well as the absence of Eocene deposition in northern Bolivia suggest low accommodation space compatible with an Eocene unloading stage (see Fig. 5.8).

Based on this review, we suggest that the Colombian, Ecuadorian and northern Peruvian foreland basins were characterized by an Early-Middle Eocene unloading stage corresponding to the ero-sion of the Central Cordillera of Colombia, Eastern Cordillera of Ecuador and Western Cordillera of Peru. The confi guration of the southern Peruvian and northern Bolivian foreland basin remains unclear. The Eocene erosional surface and low sediment accu-mulation could either mark an erosional unloading stage or be produced in a distal backbulge setting, as proposed by DeCelles & Horton (2003) for the southern Bolivian foreland basins. The Middle(?)-Late Eocene period marked the onset of tectonic load-ing of the Western Cordillera of Peru and renewed tectonic load-ing of the Eastern Cordillera of Ecuador.

Oligocene-Middle Miocene: generalized loading stage

In Colombia, continued westward onlapping of fl uvial Oligocene-Miocene units in the Middle Magdalena Valley Basin refl ects ero-sional retreat of the Central Cordillera (Gómez et al. 2003). This Oligocene-Miocene erosional retreat of the Central Cordillera was contemporaneous with generalized low shortening and up-lift of the Eastern Cordillera (Gómez et al. 2003, 2005; Mora 2007; Parra et al. 2009). Generalized deformation of the Eastern Cordillera resulted in a Late Oligocene increase of tectonic subsid-ence in the eastern foothills of the Eastern Cordillera (Parra et al.2009; see also Chapter 4) and Llanos Basin (Bayona & Thomas 2003). This episode thus reveals a stage of eastward migration of the foreland basin system. The observed greater subsidence pat-terns roughly coincide with the time of deposition of the coastal plain; tidally infl uenced deposits of the Carbonera Formation and are prolonged throughout the Miocene (see Chapter 4; Fig. 5.9). Consequently, because of the Oligocene uplift of the Eastern Cordillera, there is no record of Middle Oligocene to Middle Miocene deposits. The mountain-building and exhumation pat-terns recorded in the hinterland (Parra et al. 2009) were rather similar during the Late Oligocene-Middle Miocene. Thus, the

position, these depozones being formed as a response to initial tec-tonic loading of a proto-Andean Cordillera (Fig. 5.8).

Eocene: erosional unloading

In the Colombian Llanos Basin, east of the Eastern Cordillera, the absence of Lower and Middle Eocene units (Santos et al. 2008) is ascribed to Eocene forebulge uplift (Parra et al. 2009). Thus the Colombian Eocene foredeep could have been much narrower than the Paleocene foredeep. At fi rst glance, this evidence may suggest thrust loading and cratonward progradation of the orogenic front of the Central Cordillera (e.g. Gómez et al. 2005). However, there is no other direct and unambiguous evidence of renewed thrust load-ing and eastward progradation of the Central Cordillera during the Eocene. Most of the structures of the Magdalena Valley below a conspicuous Eocene unconformity are older than the Eocene (Suarez et al. 2000) and could correspond to a Late Paleocene deformation event. Moreover, the westward onlapping sequences of the Magdalena Valley most likely suggest erosional retreat of the Central Cordillera. In addition, to the east, along the eastern foothills of the Eastern Cordillera, the deposits of the Mirador Formation registered a 56–31 Ma slow sediment accumulation under estuarine and coastal plain conditions (Parra et al. 2009). Thus, if the absence of Lower and Middle Eocene deposits in the Llanos Basin is due to post-Middle Eocene erosion, then a much wider Early-Middle Eocene basin could have been possible. In such case, low Early-Middle Eocene sedimentation rates to the east and absence of coeval newly created accommodation space to the west, adjacent to the Central Cordillera, would coincide with a confi g-uration typical of an erosional unloading stage (Catuneanu 2004). The exact signifi cance of the Magdalena Valley unconformity remains unclear. It could be formed by Late Paleocene deform-ation in the valley followed by Early Eocene erosional unloading in the Central Cordillera or by Eocene advance of the orogenic front towards the valley. None of the two hypotheses can be ruled out with the available data, but in line with observations in Peru, Bolivia and Ecuador, we suggest that it is more likely that Early-Middle Eocene times corresponded to a stage of tectonic quiescence and erosional unloading in the Colombian Central Cordillera.

In Ecuador, the Early-Middle Eocene period is marked by low sedimentation rates (max. ~0.05 mm/year), by the development of braided rivers fed by sediments from the Eastern Cordillera (Ruiz et al. 2004; Martin-Gombojav & Winkler 2008) and by lower ex-humation rates of the Eastern Cordillera (Spikings et al. 2001). Therefore, the erosional base of the Lower Tiyuyacu Member and its associated coarse sedimentation is interpreted to mark the onset of tectonic unloading due to isostatic readjustment of the Eastern Cordillera. This tectonic quiescence stage lasted until the Middle Eocene with the deposition of the Upper Tiyuyacu Member. The Late Eocene-Early Oligocene transgressive marine deposits of the Orteguaza Formation are characterized by an increase in sedimentation rates (~0.07 mm/year) and by the appearance of high-grade metamorphic minerals, coming from the Eastern Cordillera (Martin-Gombojav & Winkler 2008). Associated with an increase in exhumation rates in the Eastern Cordillera (Spikings et al. 2001), these suggest the end of the isostatic readjustment and renewed tectonic loading of the Eastern Cordillera.


82 M. Roddaz et al.

this sedimentological evidence indicates that sedimentation in the Peruvian and Bolivian parts of the Amazon foreland Basin was controlled by tectonic loading of the Eastern Cordillera.

The Miocene Pebas Formation in the Marañón foredeep shows two transgressive-regressive cycles that could probably be cor-related with tide-controlled sedimentation of the Chambira Formation of the sub-Andean zone and with the Ecuadorian Curaray and Colombian León formations. However, due to poor stratigraphic control it is diffi cult to unravel which processes con-trolled these marine ingressions (fl exural subsidence, eustasy or a combination of both).

Late Miocene to present-day: loading and transition from fi lled to overfi lled

In the Colombian foredeep, there is a well-documented transi-tion from Middle Miocene tidal-infl uenced lacustrine deposits of the León Formation, to fl uvial environments of the Guayabo Formation. The transition from a meandering-to-braided river depositional environment of the Late Miocene Lower Guayabo Formation to alluvial fan deposits of the Late Miocene-Pliocene Upper Guayabo Formation documents an increase in grain size and a passage from a distal to a proximal fl uvial depositional environment. Mora (2008), document that the main facial and granulometric change between the Lower and Upper Guayabo Formation roughly coincides with a dramatic Mio-Pliocene acceleration in denudation rates in the Eastern Cordillera, which reached a critical elevation. However, part of such an acceleration could be due to a progressively increasing size of the catchment areas in the hinterland because of widespread incision. Therefore it could be expected that the process of increasing exhumation is not a point in time but a time-range event. Provided that the calculated subsidence rates are roughly constant in the Middle-Late Miocene in the Colombian foredeep (see Chapter 4), then increasing denudation rates in the source areas may result in an overfi lled foredeep.

Middle Miocene Leòn tidal-infl uenced lacustrine transgressive deposits cannot be solely explained by the onset of Andean-scale mountain-building processes (Bayona et al. 2007).

The Oligo-Miocene infi ll of the Ecuadorian Amazonian foreland basin comprises thick non-marine deposits (Chalcana and Arajuno Formations) passing eastward to shallow marine to lacustrine depos-its (Curaray Formation). Increasing sedimentation rates contem-poraneous with Oligocene exhumation of the Western and Eastern Cordillera (Spikings et al. 2001, 2005; Martin-Gombojav & Winkler 2008) are indicative of ongoing tectonic loading of the proto-Andes. Upward coarsening of the series, westward/upward passage from meandering to braided streams, and reduction of the fl oodplain/channel infi ll ratio indicate an increase in slope and in erosion rate during Early(?)-Middle Miocene (Burgos 2006). The convergence of the palaeocurrent directions along with the channel instability shows that these deposits formed a distributary system with a fanlike arrange-ment. The sedimentary evolution thus records the evolution through time of a shallow-dipping alluvial fan grading into a large-scale fan delta to a piedmont fan prograding eastwards (see Fig. 5.9). Despite the absence of a visible transition, it should be postulated that a delta marked the transition from the piedmont deposits to the deposits of the Curaray Formation. Eastern onlaps of the Curaray Formation indicative of uplift of the basement and eastern progradation of the Arajuno and Chambira Formations suggest ongoing tectonic loading throughout Early-Middle Miocene times.

In northern Peru, the depocentre of the Chambira Formation was located in the present-day sub-Andean zone and in this zone the Chambira Formation is tide-infl uenced suggesting marine ingression throughout the foredeep parallel to the Oligo-Miocene palaeo-thrust front (see Fig. 5.9). In the distal part, the Oligo-Miocene period is marked by increasing sedimentation rates (see Table 5.2) and aggrading fl oodplain deposits. Similar features are found in the southern Peruvian and northern Bolivian parts with locally tide-infl uenced sedimentation and signifi cant thicknesses of the deposits of the Quendeque/Ipururo Formations in the sub-Andean zone and distal aggrading sedimentation (Ipururo Formation). Together with other tectonic evidence (see Chapter 4),

Fig. 5.9 Oligocene and Middle Miocene palaeogeographic maps; black lines with black triangles indicate the positions of the Andean thrust front. ECC, Eastern Cordillera of Colombia.



2004). To reinforce this interpretation, it is worth noting that in the Latest Miocene, a shift in palaeofl ow directions from parallel to the orogen to perpendicular to the orogen has been documented in Ecuador (Christophoul et al. 2002b; Burgos 2006). This change may also be symptomatic of a transition from fi lled to overfi lled stages. The overfi lled stage could also be refl ected in the present-day sedimentation from Bolivia to Colombia as most of the sub-Andean rivers run perpendicular to the deformation front, like in overfi lled systems (Jordan 1995).

Transition from fi lled to overfi lled is caused by a decrease in accommodation space, which in turn depends on the interplay between sediment supply and base level changes (see Catuneanu 2004 and references therein). In the case of the Amazonian fore-land basin, this transition is marked by an increase in sedimen-tation rates in northern Peru (see Table 5.2) and by eastward cratonic migration of the depocentres in Peru and southern Bolivia. A global eustasy sea-level fall can be ruled out as the Late

Other parts of the Amazonian foreland sedimentation were widely controlled by deltaic and estuarine sedimentation dur-ing the Late Miocene (Fig. 5.10) including the wedge-top depo-zone (Hermoza et al. 2005b), the foredeep depozone (Hermoza 2004; Roddaz 2004; Roddaz et al. 2004; Hovikoski et al. 2005; Burgos 2006), the forebulge depozone (Roddaz et al. 2005a, 2006; Rebata-Hernani et al. 2006a, 2006b) and the backbulge depozone (Gingras et al. 2002; Roddaz et al. 2005a, 2006). The Late Miocene Amazonian foreland basin system may therefore be interpreted as a fi lled foreland basin system (Catuneanu 2004). The Latest Miocene to Pliocene sedimentation in the Amazonian foreland basin system is characterized by continental deposits (see Fig. 5.10), including prograding alluvial fan and braided river deposits in the wedge-top depozone, aggrading meandering rivers, and fl oodplain and lacus-trine deposits in the foredeep and forebulge and backbulge depo-zones (Hermoza 2004; Roddaz 2004). The Amazonian foreland basin system may therefore be interpreted as overfi lled (Catuneanu

Fig. 5.10 Late Miocene, Pliocene and present-day maps. NAFB, North Amazonian foreland basin; SAFB, South Amazoni-an foreland basin; MA, Mérida Andes; ECC, Eastern Cordillera of Colombia; Iq Fb, Iquitos forebulge; PC fb, Puerto Cavinas forebulge; Ca, Contaya Arch; Fa, Fitzcarrald Arch; SAZ, sub-Andean Zone. Black lines with black triangles indicate the positions of the Andean thrust front.


84 M. Roddaz et al.

of the Eastern Cordillera (Spikings et al. 2001) and increases in sedimentation rates of the Orteguaza Formation. At the same time, in the Northern Andes signifi cant tectonic loads were located in the Eastern Cordillera of Colombia. During approximately the same time, the southern Peruvian and northern Bolivian parts of the foreland basin were still in an erosional unloading stage.

The Oligocene-Miocene period was marked by a general load-ing stage along the entire Amazonian foreland. Increasing sedi-mentation rates in Ecuador and development of a large-scale alluvial fan were largely controlled by thrust-induced uplift and loading of the Western and Eastern Cordillera. Increasing sedi-mentation rates and migration of the depocentres, which occu-pied the present-day sub-Andean zone, indicate a thrust-induced uplift and loading of the Eastern Cordilleras of Peru, Bolivia and Colombia. In Colombia, Oligo-Miocene loading of the Eastern Cordillera maintained high subsidence rates, refl ected by coastal plain and tidally infl uenced deposits. Consequently, the Middle Miocene León tidal-infl uenced lacustrine transgressive deposits cannot be solely explained by the onset of Andean-scale moun-tain-building processes (Bayona et al. 2007). Similarly, the proc-esses that controlled Early-Middle Miocene marine ingressions in other parts of the foreland remain to be determined

Ongoing thrust-tectonic loading of the Eastern Cordillera and sub-Andean zone and the onset of the main phase of Andean sur-face uplift induced enhanced fl exural subsidence in the foredeep depozones of the entire Amazonian foreland basin from Colombia to Bolivia and drove Late Miocene marine transgressions that characterized the fi lled stage of the Ecuadorian, Peruvian and Bolivian Amazonian foreland basin. Valley incisions and full relief development in the hinterland during the Late Miocene-Pliocene provided increased sediment supply and caused the Amazonian foreland basin to be overfi lled. During that period, the Ecuadorian, Peruvian and Bolivian Amazonian foreland basin formed a unique Amazonian foreland basin system partitioned into the four discrete depozones.

The fl at-slab subduction of the Nazca ridge induced Pliocene (~4 Ma) uplift of the Fitzcarrald Arch (see Chapter 6) and divided the Amazonian foreland basin into the North and South Amazonian foreland basin systems.

This fi rst compilation of the foreland sedimentary and basin evolution from Colombia to Bolivia shows that, like subduction processes adjacent to the Andes, foreland basin processes are roughly synchronous and similar along the entire Amazonian foreland. This reinforces the point that subduction and foreland basin development have a close causal linkage to each other in the Andes.

Acknowledgments

We thank the Instituto Colombiano del Petroleo (ICP), the Institut de Recherche pour le Développement (IRD) and PeruPetro for material and fi nancial support. We also acknowledge fi nancial grants from INSU-CNRS DyETI and ECLIPSE II programmes. This study has been supported by BQR grant « Mise en place d’équipements pour la thermochronologie basse température en Sciences de la Terre » from the Université de Toulouse. We apolo-gize in advance to those whose work we may have unintentionally

Miocene tidal sedimentation occurred when the sea level was lower or equal to its present level (Haq et al. 1987). Hence, Latest Miocene emersion of the Amazonian foreland basin and associ-ated continental sedimentation cannot be due to eustatic sea level fall. Rather, the presence of growth strata in Late Miocene tidal wedge-top deposits associated with Late Miocene forebulge up-lift and structuring and uplift of the Eastern Cordillera and the sub-Andean zone (see Chapter 4) indicate that the Late Miocene ingression was driven by fl exural subsidence as a result of renewed thrust tectonic loading. The absence of Late Miocene tidal depos-its in Colombia could be explained by the Late Miocene uplift of the Mérida Andes (Colletta et al. 1997; Audemard & Audemard 2002) that would have closed the connection with the Caribbean Sea. If correct, this suggests a southern connection for the Late Miocene Amazonian marine ingression.

Increasing sediment supply from the Andean highland is the more plausible mechanism to explain the Neogene transition from fi lled to overfi lled as suggested by the exhumation data from the Colombian Eastern Cordillera. We propose a two-step re-sponse of the Amazonian foreland basin system to the Neogene uplift and relief acquisition triggered by tectonic loading. A Late Miocene (~9 Ma) tidal transgression (fi lled stage) is roughly con-temporaneous with the initiation of the inferred surface uplift and consequent increased load of the Andean wedge, as a result of increasing tectonic shortening of both the sub-Andean zone and the Eastern Cordillera (see Chapter 4 and Fig. 5.10). Later, wide-spread incision of the newly created high relief was probably due to the transition from topographic pre-steady state to steady state. As a consequence exhumation rates increased and more sediment was supplied to the Amazonian foreland basin, achieving overfi ll-ing of the Amazonian foreland basin at ~6 Ma.

The Pliocene (~4 Ma) uplift of the Fitzcarrald Arch as a result of the fl at-slab subduction of the Nazca Ridge (Espurt et al. 2007) is then responsible for the partitioning of the Amazonian foreland basin into the North Amazonian foreland basin system and the South Amazonian foreland basin system (see Fig. 5.10).

Conclusions

During the Cenozoic, the development of the Amazonian foreland basin as recorded by its sedimentary architecture was strongly controlled by Andean tectonics and related subduction proc-esses. Initial tectonic loading of the Andes of Ecuador, Peru and northern Bolivia occurred in the Late Cretaceous-Paleocene and favoured distal fl oodplain sedimentation in the Amazonian fore-land. Similar processes occurred in the Colombian Andes with the onset of a Late Cretaceous-Paleocene foreland basin coupled with the Central Cordillera loading.

Tectonic quiescence and an orogenic unloading stage prevailed during the Eocene. The Early-Middle Eocene period was marked by an unloading stage affecting most of the Amazon foreland basin. During the Middle(?)-Late Eocene, increasing sedimen-tation rates and migration of the depocentre westwards within the northern Peruvian Amazonian foreland basin indicate a tec-tonic loading stage, probably due to thrust-related uplift of the Western Cordillera. In Ecuador, Middle-Late Eocene renewed tectonic loading is also documented by high exhumation rates



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misrepresented or failed to quote. We thank Carina Hoorn and the editors for inviting us to contribute to this book. Comments by Frank Wesselingh were appreciated. This chapter benefi ts from constructive reviews by German Bayona, Victor Ramos and Wilfried Winkler.

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