Andes Ecuatoriano.pdf

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A. A. GENERAL G E N E R A L DATA D A T A ON O N THE T H E SEGMENT SEGMENT 1. THE T H E SEGMENT SEGMENT STUDIED STUDIED

The Ecuadorian Ecuadorian Andes Andes comprise comprise the the southernmost southernmost segment The segment of of the the Northern which border border the the Guayana through Ecuador, Ecuador, Northern Andes Andes which Guayana Shield Shield through

ECUADORIAN ECUADO RIAN ANDES ANDES COLIN COLIN JOHN JOHN CAMPBELL CAMPBELL Hawksgrove, Lane.Gerrards GerrardsCross, Cross, Buckinghamshire Buckinghamshire Ha wksgrove, Hawkswood Hawkswood Lane,

CONTENTS CONTENTS A. General data on on the the segment segment C. Data on on individual individual structural structural zones zones

1. Western Western Andes: Andes: a. Cordillera Cordillera Occidental Occidental Amotape-Chanch~in Belt Belt b. Amotape-Chanchán Intermontane Depression: Depression: QuitoQuito2. Intermontane Cuenca Depression Cuenca Depression Central Andes: Andes: Cordillera Cordillera Real 3. Central 4. Eastern Eastern Andes: Andes: a. Napo Napo Uplift Uplift a.

the Tertiary Tertiary and and Quaternary Quaternary strata strata of ofthe theCoastal Coastal Depression. Depression. DeDe725 731 73I 731 731

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Cutucfi Range Range b. Cutucü c. Cordillera Cordillera de Condor C6ndor

D. References References

Colombia and and Venezuela. Venezuela. They They adjoin adjoin and and impinge impingeupon upon the the Peruvian Peruvian Andes, the segment of Andes, the northernmost segment of the the Southern Southern Andes Andes bordering bordering the the Brazilian Shield. They They are are flanked flanked to to the the west west by by a structural depresBrazilian Shield. depression that extends into Colombia and to the east by the Oriente sion that extends into Colombia and Oriente Basin, Basin, a Sub-Andean extending from from southern southern Colombia Colombia through through Sub-Andean depression depression extending Ecuador Ecuador into north-eastern north-eastern Peru. Peru. Segment: thisdescription descriptionrelates relatestotothe the 750 750km km long long section sectionof ofthe theAndes Andes Segment: this in Ecuador. The from 130 to 230 230km kmwide. wide.Its Itseastern easternmargin margin The orogen orogen is from 130 to is defined by thrusts thrusts and and abrupt flexures defined by flexures separating separating the Andean Andean footfoothills from from the the Sub-Andean Sub-Andean province province to to the the east. east. The The western western margin margin is is along aa partly partly faulted faulted flexure flexure where where Mesozoic Mesozoic rocks drawn along rocks dip dip beneath beneath

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extends into depressions bordering formation extends into the the structural structural depressions bordering the the Andes; Andes; in the east whereas to to the the west west important important east it it is is limited to mild foldings, foldings, whereas faulting is is present. present. faulting Zones: four principal principal zones zones are are recognized Western Andes Andes Zones: four recognized as follows. follows. Western 1)--with Mesozoic (zone 1)—with Mesozoic eugeosynclinal eugeosynclinalsediments sedimentsand andlate lateCretaceousCretaceousPalaeocene syn-orogenic granoPalaeocene syn-orogenicsediments, sediments,intruded intrudedby by diorite diorite and and granodiorite intrusions. nonintrusions. Intermontane Depression Depression (zone 2)—containing 2)--containing nonmarine Tertiary strata and with major Quaternary vulcanism. Central vulcanism. Central Andes (zone (zone 3)--ranges Andes 3)—ranges of ofmetamorphic metamorphicrocks rockscapped cappedby byQuaternary Quaternary volcanoes. Eastern Andes, volcanoes. Eastern Andes Andes (zone (zone 4)--the 4)—the frontal frontal ranges ranges of the the Andes, with folded the folded and and thrust thrust Mesozoic-Tertiary Mesozoic-Tertiary sequences, sequences, comprising comprising the uplifted western western flank of the Sub-Andean province. province. History: the History: the segment segmenthas hasbeen beenaffected affectedby byintermittent intermittent orogenic orogenic activity activity since the Pre-Cambrian. Probably Probably at no no time time has has itit been been tectonically tectonically since the Pre-Cambrian.

dormant, occurred inin the the midmiddormant, but the the main main phases phases of of deformation deformation occurred Palaeozoic, the Palaeozoic, the early early Mesozoic, Mesozoic, the thepre-Albian pre-Albian (or (or U. U. Aptian), Aptian), the the prepre-

Campanian, and the is Campanian, the pre-Eocene pre-Eocene and the pre-U. pre-U. Miocene. Miocene. Insufficient Insufficient is known the Andes Andes of of Ecuador Ecuador to to date all the known of the the movements movements accurately.

2. SHAPE OF THE SHAPE OF THE OROGEN O R O G E N IN PLAN PLAN 16 16The The orogen orogen

continues continues into into Colombia Colombia to to the the north north with with little littleinterrupinterrup-

tion. The The southern southern boundary boundary however however borders borders the Huancabamba tion. the Huancabamba Deflection in which forms Deflection in northern northern Peru, Peru, which forms aa major major break break between between the the Northern Andes the Southern Southern Northern Andes bordering bordering the the Guayana Guayana Shield Shield and and the

Andes bordering the Brazilian Andes bordering Brazilian Shield, Shield, which which in many many respects respects consticonstitotally different different orogenic orogenic belts. belts. The The Huancabamba Huancabamba Deflection Deflection tute totally itself corresponds Andes, the the itself correspondswith withaa palaeogeographic palaeogeographicgap gapinin the the Andes, Marafidn Portal, Portal, that thatappears appearstotobe beaawesterly westerly extension extension of the Marañón the Amazon Amazon which separates separates the the two two shields. shields. Graben, which

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ECUADORIAN ECUADORIAN ANDES ANDES 24 The segment has has a general trend of sigmoidal ,4 The of N. 20° 20 ° E., but is is slightly sigmoidal

in plan plan due due to to offsets offsets by by transcurrent faulting. faulting.

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8 Andean orogeny.ByByanalogy analogywith withColombia Colombiaititmmay beassumed assumed that that Andean orogeny. a y be acme in late Miocene Miocene the Andean orogeny in in Ecuador reached its acme time, although although the movements movements cannot time, cannot be be more more accurately accurately dated dated than than 'late 'late Tertiary'. Tertiary'. The The movements movements were were probably probably most most propro-

3. SHAPE OF OF TTHE 3. SURFACE S U R F A C E SHAPE H E SEGMENT S E G M E N T IN IN ELEVATION ELEVATION 3° 30 The The

average height of the highest highest 5% 5% of segment is average of the the segment is 4500 4500 m: m: the the 31 the 31 western margin margin has an average average height height of about about 300 300 m m whereas whereas the 32 82 eastern margin lies lies 1000 1000 m above above sea sea level. level.

zone 44 where nounced in zone where they they caused causedthrusting thrustingalong alongthe themountain mountain

front front and and the the development development of of related related asymmetrical asymmetrical folds. folds. The The orogeny also Cotoorogeny also probably probably led led to to movement movement on on the the postulated postulated Cotopaxi-Baños Faults as as well well as as posthumous posthumousmovement movement on on the the DoloresDolorespaxi-Bafios Faults system. This This faulting faulting caused caused lines lines of of weakness weakness resulting Guayaquil system. resulting

in the in zones zones 22 and and 33 which the outbreak outbreak of of volcanic volcanic activity activity in which 4. 4. GEOPHYSICAL G E O P H Y S I C A L DATA DATA Gravity, magnetometer and surveyshave havebeen beenunderunderand reflection reflection seismic seismic surveys taken in the basins in connection with oil exploraexplorabasins adjoining the Andes Andes in connection with

tion. gravity measurements measurementsofofBouguer Bouguerininthe the last last tion. After After the classic classic gravity century, studies have have been been conducted conducted in in the century, no recent recent geophysical geophysical studies the Andes themselves. themselves.

5. ACTIVITY 5. PRESENT-DAY PRESENT-DAY A CTIVITY Andes active: aa list Andes are are seismically seismically active: list of all all known known earthquakes earthquakes 1534 to 1958, together accompanying map showing epifrom 1534 to 1958, together with with an an accompanying map showing epicentres, has been published published by by the Comité Comit~ del del Aflo Afio GeofIsico Geoflsico Intercentres, has been Internacional del Ecuador, Ecuador, 1959. 1959.From Fromthis thisitit appears appearsthat that there there are are two two nacional del principal of seismic seismicactivity activityinin Ecuador: Ecuador: one, one, characterized characterized by by principal zones zones of 68 n8 The The

(h 70—290 = 70-290 km), km), follows follows the earthquakes of intermediate intermediate depth (h the DoloresDoloresGuayaquil Fault system; another, shallow earthquakes Guayaquil Fault system; another, characterizedby characterizedby shallow earthquakes offshorebeing beingrelated relatedtoto the the northern northern extremity extremity of of the the (h < kin), lies lies offshore < 70 km), Peruvian Peruvian Trench. Trench.

6. 6. TIME T I M E RELATIONS RELATIONS 85 85 The The oldest

undeformed undeformed rocks rocks in in the the Ecuadorian Ecuadorian Andes Andes are are lacustrine lacustrine and terrace in zone zone 22 which which have have not not been been terrace deposits deposits and volcanic volcanic ashes ashes in dated accurately accurately but are probably probably of of late late Quaternary age. age. dated 83 ss The The youngest deformed rocks rocks consist consist of ofpyroclastic pyroclasticrocks rocksand andterrace terrace material material that that have have not been been dated dated accurately accurately either either but but which which probprobably from the the Pliocene into the the early early Quaternary. Quaternary. ably extend extend from Pliocene into 91-5 Phases of of mobility: phases mobility:ititisisprobable probablethat that most, most, ifif not not all, all, of of the the phases of in the the of mobility mobility recognized recognized in the Colombian Colombian Andes Andes also also affected affected the Ecuadorian segment, segment, but the basis basis of present Ecuadorian but on the present information information only only the the following have been been identified: identified: following have

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Plio-Pleistocene uplift. 99 Plio-Pleistocene uplfi.GeomorphologicaI Geomorphologicalevidence evidence suggests suggests that that zones 1, and 33 have have been been subjected subjected to to recent recent uplift uplift that zones 1, 22 and that is

probably at the present probably continuing continuing at present time. time. The The movements movements have have not been been accurately accurately dated are tentatively tentatively designated designated as PlioPlionot dated but are Pleistocene.

late orogenic orogenic to to post-orogenic time. spanned late 7 Early movements. Unconformities reportedbelow belowthe the Early Miocene Miocene movements. Unconformities arearereported Miocene in zone zone 22 and and in the Coastal Miocene rocks rocks in Coastal Depression Depression to the west

of the but the age has has not It is the Andes, Andes, but the precise precise age not been been defined. defined. It therefore if these these movements uncertain therefore movements correspond correspondwith with the the protoprotoAndean movements movements of slightly earlier Andean of Colombia Colombia or or with with aa slightly earlier phase phase of mobility. 6 Laramide orogeny.These Thesemovements movementswere werepronounced pronouncedininzone zone 1, 1, Laramide orogeny. where they they were were accompanied accompanied by the where by igneous igneous activity, activity,and and in in the

coastal region, an important important unconunconcoastal region, where where they they gave gave rise rise toto an formity that separates separates beds beds of U. U. Eocene Eocene age age from from Palaeocene Palaeocene formity strata. ItIt isis probable Fault probable that that the the postulated postulated Dolores-Guayaquil Dolores-Guayaquil Fault

system was a y have system was active activeduring during the the orogeny, orogeny,but but itit mmay have had had earlier earlier origins. Although less movements also also affected affected zone zone 44 less severe, severe, these movements the adjoining adjoining Sub-Andean Sub-Andean province province where where they were were responand the responsible for for the the influx influx of of coarse coarseclastic clasticmaterial materialforming formingthe theTiyuyacfi Tiyuyacü Fm with some discordanceon onthe the underlying underlyingTena Tena Fro. Fm. Fm that rests rests with some discordance 5 Palaeocene movements. zone 2 animportant importantunconformity unconformityseparseparPalaeocene movements. In In zone 2 an ates from underlying underlying U. U. Cretaceous flysch ates a Palaeocene Palaeocene sequence sequence from Cretaceous flysch

deposits. In zone zone 44 and deposits. In and the the western western part part of of the the Sub-Andean Sub-Andean province an unconformity also separates separatesthe the Tena Tena Fm Fm from from the the province an unconformity also underlying Napo the top of of which which is eroded. eroded. However, underlying Napo Fm, Fm, the However, the the of this this interval is not yet fully understood, stratigraphy of understood, so so that that it is difficult movementsfrom fromthe the SubSubdifficult to to distinguish distinguish these these possible possible movements movements, discussed discussed below. hercynian movements, below.

4 Subhercynian movements. Importantearth earthmovements movementsoccurred occurred in in the the Subhercynian movements. Important Cretaceous, reaching Andean region generally generally in in the the late late Cretaceous, reaching aa climax climax in late synlate Santonian Santonian times. times. In Ecuador Ecuador they they led to the the onset onset of synorogenic sedimentation in zones and 2, which affected orogenic sedimentation zones 1 and which were were also also affected by intrusive igneous activity. activity. In In zone zone 44 the the Napo Napo Fm Fm was was uplifted uplifted intrusive igneous and eroded of the the Tena Tena Fro, Fm, and and contemeroded prior to to the the deposition deposition of contemporaneous uplift of the source areas caused caused the the deposition of sandsandsource areas deposition of stones (Vivian Fm) Fm) on on the stones (Vivian the margin margin of the the Guayana G u a y a n a Shield. Shield. 33 Mid-Cretaceous movements. Subsidence of zone andthe the SubSubMid-Cretaceous movements. Subsidence of zone 4 4 and Andean province widespread marine marine transgression transgression in in late late province caused aa widespread Aptian Aptian to to early early Albian Albian times times heralding heralding the the onset onset of ofmiogeosynmiogeosynclinal deposition deposition in in this this region. region. Contemporaneous Contemporaneous movements movements were were also for the the unconformity below the the Puyango Puyango Fm Fm in in also responsible responsible for unconformity below zones llb b and 2. 2. zones

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2 J"fevadan orogeny. analogy withColombia Colombiait itisispresumed presumedthat that the the Nevadan orogeny. ByBy analogy with Andes were affected by strong orogenic orogenic movements Ecuadorian Andes movements in in

late Jurassic times, prior prior to to the the Tithonian In the west Jurassic times, Tithonian stage. stage. In west they they

are are thought thought to to be beresponsible responsible for for the the onset onsetofofeugeosynclinal eugeosynclinal deposition (Piñon Fm). Fm). In In zones led to to deposition (Pifion zones lb, 22 and 33 they may m a y have have led the metamorphism age has has not not howmetamorphism of of pre-existing pre-existing rocks, rocks, whose whose age how-

ever yet been and in in zone ever yet been determined; determined; and zone 4 and and the the Sub-Andean Sub-Andean province, uplift a y bebe in province, uplift associated associatedwith withthis thisorogeny orogenymmay in part part responsible for the the hiatus hiatus between between the theJurassic JurassicChapiza ChapizaFm Fm and and responsible for the Cretaceous. Cretaceous. Hercynian orogeny. Again onon regional 11 Hercynian orogeny. Again regionalgrounds, grounds,it itisispresumed presumedthat that the Ecuadorian by important important movements in Ecuadorian Andes Andes were were affected affected by movements in early Triassic Such movements movements mmay have caused caused the the metametaTriassic times. times. Such a y have

morphism of pre-existing pre-existingrocks rocksininzones zonesl b, 1 b,2 2and and33 and and the the morphism of emplacement of the eastern eastern margin margin of of zone zone 3, emplacement of alkali alkali granites granites on on the but the concerned have have not not been been dated. dated. In In zone an unconunconthe rocks rocks concerned zone 44 an formity below the the Liassic Liassic Santiago SantiagoFm Fmmmay be related related to to this formity below a y be this phase of mobility. phase of mobility.

87—9 definethetheinitiation initiationofofmobility mobilityassociated associatedwith withthe the MesozoicMesozoic8~-9 ToTo define Tertiary would give undue emphasis Tertiary orogeny orogeny would would be be misleading as it would give undue emphasis

to to an arbitrary arbitrary limit limit in in aa region region that that has has been been subjected subjected to to almost almost continuous mobility Phanerozoic time. continuous mobility throughout throughout Phanerozoic 78 In zones zones lb, lb, 2, 2, 33 and and 4 'basement' consists of Pre-Cambrian Pre-Cambrian igneous igneous ~8 In consists of

and rocksforming formingpart part ofof the the continental crust of of and metamorphic metamorphic rocks continental crust northern South and the the region to the the west west of of the the South America. America. In zone zone 1l aa and region to

Andes Andes 'basement' 'basement' is is thought thought to to consist consist of of early early Mesozoic Mesozoic oceanic oceanic deposits that are in the being welded welded to to the the continent. deposits that the process process of being continent. R E V I E W OF OF OROGENIC O R O G E N I C DEVELOPMENT DEVELOPMENT 9. REVIEW 133 133

TRAN5cURRENT TRANSCURRENT FAULTING FAULTING IN INECUADOR ECUADOR

The transcurrent faulting in the tectonics of the the Andean Andean region region The role of transcurrent faulting in tectonics of is discussed discussedininthe theprevious previousarticle articleon on Colombia. Colombia.Although Although the the evidence evidence even more open to differing differing interpretations, in Ecuador is even more limited limited and and open interpretations, transcurrent faulting faulting is believed transcurrent believed to to have have also also played played aa significant significant part part in in this Two major major in the the structural structural evolution evolution of the the Andes Andes in this segment. segment. Two

transcurrent are tentatively transcurrent fault fault systems systems are tentatively recognized: recognized: the DoloresDoloresGuayaquil Fault Fault. Fault system system and the the Cotopaxi-Baños Cotopaxi-Bafios Fault. the northern northern part part of of the the Cauca Cauca Basin Basin of of Colombia Colombia aa major In the major fault fault known as the the Dolores Dolores (or (or Romeral) Romeral) Fault Fault forms forms the the eastern eastern margin margin of of known as the basin. faults within within the the basin basin gradually gradually basin. Towards Towards the south south parallel parallel faults take so that that by by the the time is reached reached the the take up up the the displacement displacement so time Ecuador Ecuador is fracture has has shifted shifted to to the the western western margin of the basin, which which dominant fracture is is here here designated designated as as zone zone 2. This This system system of of faults faults almost almost certainly certainly continues southwardstoto the the latitude latitude of of Guayaquil. Guayaquil. The The course of the the continues southwards course of faults is indicated by by linear linear belts belts of of volcanic volcanic centres, centres, although although the the faults is indicated surface traces themselves themselvesare are largely largely obscured obscured below belowaa cover cover of of lava lava surface traces and ash. ash. At At the the latitude latitude of of Guayaquil Guayaquil the the principal principal fracture fracture swings swings and

south-westwards through a y be be clearly south-westwards through the the Western Western Andes, Andes, where where itit m may observed at the surface. surface. It then almost almost certainly certainly continues continues along observed at the It then along the the southern shore shore of the Gulf Gulf of of Guayaquil, Guayaquil, probably probably being being responsible responsible southern for the complex faulting in in the the Gulf Gulf of of Guayaquil Guayaquil and and on on the the coast coast of of complex faulting north-west Peru. It continues and apparently apparently causes causesaa dextral dextral north-west Peru. continues offshore offshore and offset at the the northern northern end end of of the the Peruvian Peruvian Trench. Trench. offset at Although the many Although the fault fault has has not not been been mapped mapped in in detail detail and and is in many areas obscured obscured below below Quaternary Quaternary sediments, sediments, itit isisinterpreted interpretedasasaamajor major fracture system system forming forming part zone dextral transcurrent fracture part of a great shear zone bordering the the South South American American continent. continent. Furthermore, Furthermore, as shown shown on bordering on crustal model model in Fig. 2, it is believed believed to the crustal in Fig. to mark mark the the western western limit limit of of Pre-Cambrian Pre-Cambrian continental continental crust. crust. second tectonic line--the a u l t - - m a y be The second line—the Cotopaxi-Bafios Cotopaxi-BaflosFFault—may betraced traced from the from the Pichincha Pichincha volcano volcano near near Quito, Quito, through through the the Cotopaxi Cotopaxi volcano, volcano, to the intersection of the the Rio RIo Santiago Santiagoand and the the Peruvian Peruvian frontier. frontier.The The intersection of Andes are number of offsets Andes are deflected deflectedon on this this line, line, and and aa number offsets and and abrupt abrupt changes m a y be observed, observed, including: offset of structural changes may including: the the offset ofthe the thrust thrust belt bordering of the the Dolores-Guayaquil Dolores-GuayaquilFault; Fault; and and bordering zone zone 3; the the offset offset of the southern from the the Cerros Cerros de de southern termination termination of of zone zone 4b, 4b, which which is is offset offset from Campanguiz, extensioninin Peru. Peru. Although Althoughitit has has not not been been mapped mapped Campanguiz, its its extension is suggested suggested that a y correspond on the ground itit is that this this line line m may correspond with with aa major major sinistral transcurrent fault. fault. The The alignment centres toto the the sinistral transcurrent alignment of volcanic volcanic centres south further suggests that it is made made up of a series offaults faultsrather rather than than suggests that series of single fracture. fracture. It m a y be be significant significant that this postulated postulated fault a single It may that this fault is is approximately parallel Santa Marta M a r t a Fault Fault of of Colombia, Colombia, also also approximately parallel with with the the Santa considered to sinistral transcurrent considered to be be aa sinistral transcurrent fault. fault. Both and the Faults are are Both the the Dolores-Guayaquil Dolores-Guayaquil and the Cotopaxi-Baños Cotopaxi-Bafios Faults almost certainly still still active, active, being almost being responsible responsible for for current current seismic seismicand and volcanic activity. If If the Dolores-Guayaquil Fault coincides coincideswith with the the volcanic activity. Dolores-Guayaquil Fault edge of of Pre-Cambrian Pre-Cambrian continental continental crust, crust, it is is reasonable reasonable to to suppose suppose edge has been been active active throughout throughout Phanerozoic Phanerozoic time. that itit has time. Although Although both both faults have have probably probably had had aa long long history history of of movement, movement, the the CotopaxiCotopaxifaults Bafios Fault appears to displace displace the Dolores-Guayaquil Fault, suggestsuggestBaflos Fault appears that its its main main phase phase of of movement movement m a y have ing that may have occurred occurred during during the the Andean orogeny, movement on on the the Doloresorogeny, whereas the main phase of movement DoloresGuayaquil Fault m a y have have occurred occurred during during Laramide Laramide or or earlier earlier Guayaquil Fault may orogenies. orogenies.

very difficult difficult to estimate estimate the amount amount of of displacement displacement on these these It isis very little known Fault appears appears to to offset the known faults. faults. The The Cotopaxi-Baflos Cotopaxi-Bafios Fault offset the thrust belt on the eastern margin of zone zone 33 by by about about 50 50 km. km. If If the the Gulf Gulf Guayaquil should displacement of of Guayaquil should reflect reflect the the Guayaquil Guayaquil Fault, Fault, displacement of the the coast line, line, which which itself itself is is presumably presumably geologically geologically controlled, coast controlled, would would point to to aa dextral dextral movement movement of of about 300 300 km. km.

Volcanic activity. The succession Volcanic activity. TheEcuadorian EcuadorianAndes Andesare arecapped capped by by a succession of volcanic volcanic peaks peaks rising rising above above the the snow snow line line to to altitudes in excess of altitudes in excess of 5000 m. They are presently presently in aa state state of of quiescence quiescence although although many many 5000 m. They violent eruptions some within within the the last last eruptions have occurred occurred in in historic historic times, times, some hundred years. whose craters craters and and cones cones years. In In addition addition to to these these volcanoes, volcanoes, whose remain intact, are are other other more more deeply deeply eroded eroded volcanic volcanic centres centres reprerepreremain

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The main activity occurs occursboth bothwithin withinand and on on the the main belt belt of of volcanic volcanic activity margins of zone 2, 2, and, as previously,isisthought thoughtto to be be conconas discussed discussed previously, trolled by by aa series series of major faults. Another Another line line of of volcanoes, including

senting earlier phases of of activity activity during the Pleistocene Pleistocene and Pliocene. Pliocene. According to Sauer (1957), the earlier volcanic rocks rocksare arepredominantly predominantly andesites, dacites and rhyolites, rhyolites, whereas the the later laterrocks rocks consist consist of basic andesites: decline in in orogenic orogenic activity. activity. andesites: a change that that coincides coincides with aa decline

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++++++++++++++++++ + -+ + ++++++++++ + Figure 2. the separate separate regions regions of Ecuador. Ecuador. Figure 2. Diagram Diagram of the the stratigraphical stratigraphical sequences in the

+ + + + Ii IGNEOUS & METAMORPHIC ROCKS OF GUAVANA SHIELD

VVVVVVI TUFFS & TUFFACEOUS

________

SEDIMENTS

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730

almost unique occurrence volcanic activity activity in in the the Eastern Eastern Andes, Andes, but but almost unique occurrence of volcanic may be due to faulting related to the Baños-Cotopaxi Fault with with which which Bafios-Cotopaxi Fault is roughly parallel. it is MaraIión Portal.The TheMarafi6n MarañónPortal, Portal, which which lies lies mainly mainly outside outside the the Mara~dn Portal. segment under Peru, deserves deserves mention, segment under consideration consideration in in northern northern Peru, mention, for foritit affects the southern southern part part of the the Ecuadorian Ecuadorian Andes and also also has has palaeopalaeoaffects the Andes and geographic significance.ItIt appears appears to to be be an extension of the the Amazon Amazon geographic significance. extension of Graben, a structural structural depression depression dividing Shield from Graben, dividing the the Guayana Guayana Shield from the the Brazilian Shield, that that was uplifted in in late late Tertiary Tertiary times times and and incorporincorporBrazilian Shield, was uplifted ated within topographic saddle saddle and and within the the Andes. Andes. It is is still still nevertheless nevertheless aa topographic separates the NE-trending Northern Northern Andes Andes from from the the SE-trending SE-trending separates the NE-trending

Southern orogenic belts. belts. Relics Relics Southern Andes, Andes, which which are in effect effect two two discrete discrete orogenic

of the early structural grain grain are are still still visible visibleinin the the Andes Andes of of of the early easterly easterly structural southern Ecuador, Ecuador, especially especially in rocks of b. southern in the older rocks of zone zone 1lb. The Marañón during the the CretaceCretaceMarafi6n Portal Portal was was evidently evidently in existence existence during ous if not earlier, of earlier, and and was was responsible responsible for a local local westerly westerly extension extension of the miogeosynclinal province, represented represented by by the the limestones limestonesand andsandsandmiogeosynclinal province, stones of the the Puyango Fm in an area fall stones of Puyango Fm area that that in in regional regional terms terms would would fall within the eugeosyncline. eugeosyncline. During within Duringthe the early early Tertiary Tertiary itit formed formed aa gap gap in in the ancestral Andes through which which flowed the rivers rivers draining draining the the SubSubAndes through flowed the Andean The Tertiary of the the Gulf Gulf of of Guayaquil Guayaquil Andean province. province. The Tertiary sediments sediments of and N.W. laid down down in in the the delta delta of of this this river river N.W. Peru Peru may may include include beds beds laid system. The gap furthermore furthermore allowed allowed periodic periodic marine incursions to system. The gap marine incursions to sweep eastwards into into the the Sub-Andean for sweep eastwards Sub-Andean province, province, being being responsible responsible for

the One the brackish brackish faunas faunas that that occur occur within within the the Tertiary Tertiary succession. succession. One implication of this this interpretation interpretation isis that that the Amazon implication of Amazon drainage drainage system system did not until after the Andes at not come come into existence existence until after the final uplift uplift of the Andes at the end of of the Tertiary. Tertiary. 134 134

OVERALL OF THE THE BELT OVERALL EVOLUTION EVOLUTION OF

Early Palaeozoic. Palaeozoic. Regionally early Palaeozoic Palaeozoic (i) Early Regionallyit itisisclear clearthat that an an early geosyncline followedthetheline lineofofthe thepresent presentEastern EasternAndes, Andes,and and inin geosyncline followed Ecuador through zone zone 4.4. ItIt was probably bordered bordered to to the the east Ecuador passed passed through was probably east shelf conditions by shelf conditionsflanking flankingthe theGuayana GuayanaShield. Shield.InInthe the Andean Andean region region itself the L. L. Palaeozoic Palaeozoic rocks rocks were were metamorphosed metamorphosed by mid-Palaeozoic mid-Palaeozoic earth movements, movements, and recognition is difficult. They are, however, however, earth and recognition is difficult. They are,

recognizable in zone 4b where where they they form form the the Pumbuiza Pumbuiza Fm. clearly recognizable Fm. (ii) Late Late Palaeozoic. Palaeozoic. Based (ii) Basedononinferences inferencesfrom fromColombia Colombiaititmay may be be assumed that geosynclinal conditionsexisted existedininzones zonesl1b,b,2 2and and33of of assumed that geosynclinal conditions Ecuador Ecuador during the the late late Palaeozoic. Palaeozoic. The The rocks rocks concerned concerned were were subsesubsequently and have quently metamorphosed metamorphosed and have not been been distinguished distinguished from from other other metamorphosed sequences sequences except metamorphosed except in in zone zone 1l b,b, where where low-grade low-grade metametamorphic rocks rocks are are correlated correlated with with the the Amotape Amotape Fm morphic FmofofPeru Peruthat that has has

may be be present present in the the Andean Andean region, region, but has has not not been been distinguished distinguished from sequences.InIn the the east, the shelf shelf from other other metamorphic metamorphic sequences. east, however, however, the environment gave way way to to continental continental conditions environment of the late Palaeozoic Palaeozoic gave conditions early Mesozoic, Mesozoic, and sequence of during the early and a sequence of red red beds, beds, comprising comprisingthe the Chapiza Fm Chapiza Fmwas wasdeposited depositedininzone zone44and and the the Sub-Andean Sub-Andean province province to to east. This This sequence sequence consists consists of of sandstones, sandstones, arkoses, arkoses, conglomerates, the east. conglomerates, shales and minor and was laid shales and minor intercalations intercalations of of evaporite, evaporite, and was evidently evidently laid down under arid arid conditions. conditions. Contemporaneous Contemporaneous igneous igneous activity down activity led led to to the emplacement granites in in the the Andean Andean region and the extrusion of emplacement of of granites region and extrusion of lavas, with associated pyroclastic rocks, rocks, to to the the east. east. associated pyroclastic (iv) new geosynclinal geosynclinal environment environment became became estab(iv) Late Late Mesozoic. Mesozoic. AA new established in the the Andean formed lished during during the late late Mesozoic Mesozoic in Andean region. region. Zone Zone 3 formed mainly submerged submerged welt separating separating a miogeosynclinal miogeosynclinal province aa mainly provinceinin the the east from province in in the the west. from a eugeosynclinal eugeosynclinal province west. In the the eugeosyncline eugeosyncline a great great thickness thickness ofofsubmarine submarine lava, lava,comprising comprising the thePiutón Pi~dn Fm, Fm, was was extruded on the the ocean ocean floor, floor, and was was followed followed by sequence of extruded by aa sequence of conconglomerates, limestone, limestone, cherts, cherts, siliceous siliceous argillites shales with glomerates, argillites and and shales with tuffaceous ceous intercalations intercalations making making up the the Callo Callo Fm. Fm. In In the the miogeosyncline miogeosyncline deposition opened an epicontinental epicontinental sequence sequence of of sandstones, sandstones, deposition opened with with an comprising the Hollln HollInFm, Fm,that that was was succeeded succeededby by the the Jfapo Fm, aa comprising the Napo Fm, sequence of euxinic shales shales and and limestones, limestones, passing passing eastwards eastwards into into sequence of euxinic glauconitic In the of the the welt welt (zone (zone 3) 3) there there was was aa glauconitic sandstones. sandstones. In the vicinity vicinity of certain certain degree degree of of interdigitation interdigitation between between the two two facies: facies: for example, example, Puyango Fm, westerly the Puyango Fm,found foundlocally locallyininzone zone l1b,b,appears appearstoto be be a westerly prolongation of of miogeosynclinal miogeosynclinal deposition. deposition. prolongation (v) Late Late Cretaceous—Early Cretaceous--Early Tertiary. move(v) Tertiary.Subhercynian Subhercynian orogenic orogenic movements conments in late Santonian Santonian times times brought to to aa close close the geosynclinal geosynclinal conditions They were during late late Cretaceous ditions of the Mesozoic. Mesozoic. They were followed followed during Cretaceous and Palaeocene Palaeocene times times by a transitional transitional phase phase preceding preceding the the establishestablishment ment of of the the Tertiary Tertiary cycle cycle in in the theEocene. Eocene. Syn-orogenic Syn-orogenic sedimentary sedimentary conditions characterizedthe the Andean Andean region, region, especially especiallyzones zonesl a,la, llb conditions characterized b and overlain by by and 2, 2, where where late late Cretaceous Cretaceous flysch flysch is is unconformably unconformably overlain

Palaeocene conglomeratesand and coarse coarse clastic of mixed Palaeocene conglomerates clastic sediments sediments of mixed marine and sequence of of and continental continental Origin. origin. In the the west west was deposited deposited aa sequence post-orogenic cherts cherts comprising comprising the the Guayaquil Guayaquil Fm, post-orogenic Fm,whereas whereasininthe the east, east,

in zone the deposition of zone 4 and and the the adjacent adjacent Sub-Andean Sub-Andean province, province, the deposition of local was followed followedby by aa sequence sequence of of paralic paralic clays, clays, minor minor local sandstones sandstones was sandstones and limestones, limestones, comprising comprising the Tena Tena Fm. sandstones (vi) Eocene—Miocene. Eocene--Miocene. Laramide (vi) Laramideorogenic orogenicmovements movementsininlate late M. Eocene times Eocene times uplifted uplifted the the Andean Andean ranges ranges (principally (principally zones zones 11and and 3) 3) and set for the the present present structure structure of of the the country. country. These These set the framework framework for movements were movements were accompanied accompanied by by widespread widespread igneous igneous activity activity of of both both intrusive and extrusive character that was probably intrusive and extrusive character probably most most pronounced pronounced in zone zone 1. 1.

yielded Carboniferous Carboniferous fossils. geoyielded fossils.Shelf Shelf conditions conditionsseparated separated the the geo-

the coastal coastal region region to to the the west west of of the the Andes Andes was was deposited deposited aa In the

synclinal belt Shield, and limestones limestones representative synclinal belt from from the the Guayana Guayana Shield, representative of this this province province outcrop outcrop in in zone zone 4b 4b where where they they comprise comprise the the CarboniCarboni-

succession of clays with intercalations of shallower shallower succession of deep deep water water marine marine clays with intercalations water conglomerates, and local Turbidites and and conglomerates, sandstones sandstones and local limestones. limestones. Turbidites

ferous Macuma Macuma Fm. (iii) Early Early Mesozoic. Mesozoic. An Mesozoic geosynclinal geosynclinal development development (iii) An early early Mesozoic

olistostromes developed olistostromes developedlocally locallyininthe theSta. Sta.Elena Elena Peninsula. Peninsula.InIn the the intermontane depression depression forming forming zone sequence of intermontane zone 22 was was deposited deposited aa sequence of

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ECUADORIAN ANDES ANDES ECUADORIAN generally coarse-grained coarse-grained continental are, however, however, generally continental sediments, sediments,that that are, mainly mainly obscured obscured below below aa cover cover of of Pliocene Pliocene to Quaternary Quaternary volcanic volcanic rocks. and rocks. In zone zone 4 and the the adjacent adjacent Sub-Andean Sub-Andean province province fiuviatile fluviatile and

lacustrine silts and and minor minor sands lacustrine clays, clays, silts sands were were deposited. deposited. Conglomerates Conglomerates with some intercalations are are however howeverfound foundatat the the base base of of some tuffaceous tuffaceous intercalations the sequence in zone zone 44 adjacent adjacent to to the source areas. sequence in source areas. (vii) Andean orogeny theend endofof the the (vii) Pliocene—quaternary. Pliocene--Quaternary. TheThe Andean orogeny at atthe Miocene period gave gave the the country country its its present present structure structure and and caused caused the the Miocene period uplift of the Andean The orogeny alsoled led toto the the outbreak outbreak of of Andean ranges. ranges. The orogeny also volcanic which has has persisted persisted to to the the present present day: day: the volcanoes volcanic activity which volcanoes are located major faults faults that that cut and border located on on major border zone zone 2. Zone Zone 4 which which

had had hitherto hitherto been been part part of ofthe theSub-Andean Sub-Andean sedimentary sedimentary basin basin was was uplifted to form form the the Andean Andean foothills. foothills. Erosion of the the uplifted uplifted mountain mountain chains led to to the the deposition deposition of ofwidewideErosion of chains led spread tuffaceous tuffaceous terraces terraces on both both sides sides of the Andes. Andes.

C. DATA D A T A ON O N INDIVIDUAL I N D I V I D U A L STRUCTURAL STRUCTURAL ZONES ZONES The within the The Andes Andes within the segment segment under under consideration consideration may m a y be classified classified as follows follows (see as (see Fig. Fig. 1): 1): WESTERN ANDES ANDES WESTERN

Zone Zone la Cordillera Cordillera Occidental Occidental Zone Zone lb Amotape-Chanchán Amotape-Chanch~n Belt Belt INTERMONTANE INTERMONTANE DEPRESSION DEPRESSION

Zone 22 Quito-Cuenca Quito-Cuenca Depression Depression Zone

731 731

tion the zone and are thought tion of the zone and thought to to be be of of Eocene Eocene age. age. The The southerly southerly attenuation in part part due for CretaCretaattenuation of the the zone zone is is in due to to recent recent subsidence, subsidence, for ceous rocks identical identical toto those thoseinin the the mountains mountains lie lie atat shallow shallowdepth depthinin ceous rocks the eastern part of of the the Coastal Coastal Depression. Depression. Zone lb, ib, Amotape-Chanchdn Belt. Thisisisa asomewhat somewhatill-defined ill-defined strucstrucAmotape-Chanchdn Belt. This tural unit unit which which extends extends for for aa distance distance of of 250 250 km km from from the the DoloresDoloresGuayaquil Fault to Guayaquil Fault to the Peruvian Peruvian frontier frontier and and ranges ranges in in width width from from about 40 rocks in in 40 km km in the the north north to to 150 150 km in the the south. south. The oldest oldest rocks zone are gneisses gneisses and hey the zone and schists, schists,probably probablyof ofPre-Cambrian Pre-Cambrian age. age. TThey are overlain metamorphic rocks rocksthat that are are correlated correlated with with overlain by low-grade low-grade metamorphic the Amotape Amotape Fm Fm of ofPeru Peruwhich whichhas hasyielded yieldedCarboniferous Carboniferous fossils. fossils. Above thick sequence Mesozoic rocks rocks with with Above follows follows aa thick sequence of eugeosynclinal eugeosynclinal Mesozoic some limestones and and sandstones, the Puyango some intercalations intercalations of shelf shelf limestones sandstones, the Puyango Fm, belonging belonging to the the miogeosynclinal miogeosynclinal province Capping Fm, province toto the the east. east. Capping these are littoral littoral deposits and extensive pyrothese various various rock rock sequences sequences are deposits and extensive pyroclastic sheets. The The latter latter are mainly clastic sheets. mainly of of Recent Recent age, age, although although Eocene Eocene components may m a y also also be be locally locally preserved. components preserved. The zone large number number of intrusions intrusions of ofdioritic dioriticand and grangranzone includes includes aa large itic composition that, unlike composition that, unlike those those of zone zone 1l a,a, appear appear to to follow follow no

particular structural trend. trend. The The intrusions are probably probably mainly mainly ofof particular structural intrusions are Eocene age, age, although although some some were were emplaced emplaced during during Subhercynian Subhercynian Eocene orogenic movements inin the the late late Cretaceous. Cretaceous. (A (A radiometric radiometric age age deterdeterorogenic movements mination +30 m.y. m.y. has has been from aa granomination of 111 111 4-30 been made made on samples samples from granodiorite 25 km diorite 2"5 km north north of of Macará.) Macar~.) The zone zone is cut cut by SW-trending SW-trending faults The faults that that probably probably belong belong to to the the Guayaquil In the south Guayaquil Fault Fault system. system. In south of of the the zone, zone, however, however, another another system of system of faulting faulting with with aa strong strong easterly easterly component component isis recognized recognizedand and thought to be be related related to to the the MaraIión Marafidn Portal, Portal, described described previously. previously. is thought

CENTRAL CENTRAL ANDES ANDES

Zone Zone 33 Cordillera Cordillera Real Real EASTERN EASTERN ANDES ANDES

Zone 4a Napo Uplift Uplift Zone 4a Napo Zone 4b Cutucü Zone 4b Cutucfl Range Range Zone 4c Cordillera Zone 4c Cordillera de de Condor C6ndor

ZONE Z O N E 1. 1. WESTERN W E S T E R N ANDES ANDES The Western Cordillera Western Andes Andes are divided divided into into two two discrete zones—the zones--the Cordillera

Occidental Occidental and and the the Amotape-Chanchán Amotape-Chanch~n Belt—by Belt--by the the GuayaquilGuayaquilDolores Fault. Dolores Fault. Zone la, la, CordilleraOccidental. Occidental. Thisis is arcuatebelt belt400 400km kmlong longand and This ananarcuate between and 80 km wide wide and and isis aa direct direct continuation continuation of of the the CordilCordilbetween 20 20 and 80 km lera Occidental The east Occidental of Colombia. Colombia. The east margin margin of the zone zone coincides coincides with the Guayaquil-Dolores Fault and and the west west margin, margin, which which isispartly partly Guayaquil-Dolores Fault obscured by terrace material, is is probably probably cut cut by obscured by terrace material, by thrusts. thrusts. The The zone zone is built mainly submarine mainly of of strongly strongly deformed deformed eugeosynclinal eugeosynclinal Mesozoic Mesozoic submarine lavas overlaininin the the east east by by late late Cretaceous lavas and and oceanic oceanic sediments, sediments, overlain Cretaceous and Palaeocene sediments.ItIt isis invaded invaded by by aa sequence sequence of of Palaeocene syn-orogenic syn-orogenic sediments. granodiorite intrusions, intrusions, which follow follow the diorite and granodiorite the arcuate arcuate configuraconfigura-

ZONE Z O N E 2. 2. INTERMONTANE I N T E R M O N T A N E DEPRESSION DEPRESSION This is the Quito-Cuenca Depression—a direct extension extension of of the the Cauca Cauca Depression--a direct Basin of Colombia. Colombia. ItIt extends extends 450 450 km km from from the the Colombian Colombian frontier frontier to to Basin of

a point 50 km km point near near Cuenca, Cuenca, has has an an average average width width of of approximately approximately 50 and with a sequence of nonmarine nonmarineTertiary Tertiary strata strata that that are, are, and is is filled filled with sequence of

however, in many below aa cover however, in m a n y areas areas obscured obscured below cover of of Quaternary Quaternary volcanic The zone faulted by by the two volcanic rocks. rocks. The zone is strongly strongly faulted two major systems systems discussed previously, which which have have formed formed lines lines of weakness weakness controlling controlling discussed previously, the volcanic activity. activity. In In detail, detail, a number the location location of of volcanic number of ofcross-highs cross-highs divide divide the zone zone into into sub-basins sub-basins with topographic topographic expression. expression. series of A series of striking striking volcanic volcanic cones cones(Cotopaxi, (Cotopaxi,5287 5287m; m;Tungurahua, Tungurahua, 5016 m; Pichincha 4794 as more more deeply deeply eroded eroded older older centres, centres, 4794 m) as well well as including Chimborazo Chimborazo (6269 including (6269 m), m), the the highest highest peak peak in in the the Ecuadorian Ecuadorian Andes, lie Andes, dominate the the landscape landscape of of zone zone 2. 2. Between Between these these eminences eminences lie depressions filled depressions filled with with ash ash deposits, deposits, pumice pumice beds, beds, and and Plio-Pleistocene Plio-Pleistocene gravels and lacustrine lacustrine strata. strata. Furthermore, the four different different gravels and the remains remains of of four phases of Pleistocene phases Pleistoceneglaciation glaciationcan can be be found. found. During During these these the the permaperma-

nent m lower lower than than its nent snow snow level level lay lay approximately approximately 1500 1500 m its present present position of between and 4800 between 4600 4600 and 4800 m above above sea sea level. level.

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ECUADORIAN E C U A D O R I A N ANDES ANDES

ZONE Z O N E 3. 3. CENTRAL C E N T R A L ANDES ANDES The of the the Cordillera Cordillera Central Central of of The Cordillera Cordillera Real Real is a direct direct extension extension of Colombia through Ecuador 650 km km into into Colombia and and extends extends through Ecuador for a distance distance of 650 northern about 45 45 km km and and northern Peru. Peru. It It has has aa relatively relatively uniform uniform width width of of about is built of high-grade uaternary is built of high-grade metamorphic metamorphic rocks rocks capped capped locally locallyby byQQuaternary volcanoes, including volcanoes, including Sangay Sangay (5290 (5290m) m)and and Altar Altar (5319 (5319m). m). The The metametamorphic complexes complexes have may morphic havenot not been been investigated investigatedinin detail, detail, but but may

include rocks rocks of include of widely widely different different ages, ages,the the oldest oldestof ofwhich whichare areprobably probably

Pre-Cambrian. The southern southern extremity extremityofofthe the zone zone isis invaded invaded by by Pre-Cambrian. The intrusions similar to to those those found foundininzone zone 1lb. An important important belt of highhighintrusions similar b. An angled the eastern eastern margin margin of ofthe thezone. zone.InIn the the north north itit angled thrusting thrusting marks marks the attains aa width described under under zone 4a. width of of as as much much as as 20 20 km which which is described zone 4a. The zone zone is is also also cut and apparently apparently deflected deflected by the the Cotopaxi-Baflos Cotopaxi-Bafios Fault, described described previously. previously. Fault,

ZONE Z O N E 4. EASTERN E A S T E R N ANDES ANDES From a historical historical and and stratigraphic stratigraphic standpoint standpoint the the Andean Andean foothills foothills From form of the form the the west west flank flank of the Oriente Oriente Basin, Basin, aa part of of the the Sub-Andean Sub-Andean province that was uplifted uplifted during end-Tertiary Andean Andean orogeny. orogeny. province that was during the the end-Tertiary From view on on the the other otherhand, hand, they they are are the the frontal frontal From a structural structural point of view

Cutting obliquely across the the Napo Napo Uplift Uplift and and the the thrust thrust zone zone isis aa line line obliquely across of three three volcanic volcanic centres: of centres: Sumaco Sumaco (3900 (3900m), in), Pan Pan de de Azficar Azücar and and Reventador m), the the latter latter being still active. active. The The alignment alignment of of Reventador (3458 (3458 m), being still these suggeststhat thatthey they mmay overlie aa basement these volcanoes volcanoes suggests a y overlie basement fault, fault,

similar similar to the the Cotopaxi-Baños Cotopaxi-Bafios Fault, discussed discussed previously. previously. Zones lb and It: de Cdndor. Condor.The TheCutucfi Cutucü Zones 4b 4c:Cutucü Cutuc~tRange Range and and Cordillera Cordillera de Range structural uplift uplift marking marking the the northern northern termination termination Range is a complex complex structural of the the virtually del COndor that runs runs into into Peru. Peru. virtually unexplored unexplored Cordillera Cordillera del C6ndor that Palaeozoic phyllites core of uplift are are L. Palaeozoic phyllites (Pumbuiza (PumbuizaFro) Fm) inin the the core of the the uplift overlain by Carboniferous limestones M a c u m a Fm) overlain by Carboniferous limestones((Macuma Fm) inin turn turn succeeded succeeded by aa full full Mesozoic Mesozoic and and Tertiary Tertiary succession. succession. O n the the east east of of the the uplift, uplift, the the very very large large Macuma M a c u m a and and Cangaime Cangaime On Anticlines are flanked flanked by by a zone Anticlines are zone of of reverse reverse faulting faulting separating separating the the uplift the Sub-Andean basin to to the the east. east. In In the is the the more uplift from from the Sub-Andean basin the west west is more strongly deformedand andfaulted faultedcore coreofofthe the uplift, uplift, which whichisisinin turn turn strongly deformed bordered Fault. Still Still farther farther west bordered by the the postulated postulated Cotopaxi-Baflos Cotopaxi-Bafios Fault. west lies structure isis virtually virtually ununlies a broad broad zone zone of of Mesozoic Mesozoic rocks rocks whose whose structure

known. At the southern end end of of this this tract, tract, to to the the east of Loja, Loja, isisanother another known. At the southern east of large granite granite body large body intruded intruded into into the the thrust thrust belt belt bordering bordering zone zone3.3.InIn all probability activity as as the the probability it it belongs belongs to to the the same same phase phase of of igneous igneous activity granite granite in the the thrust thrust belt belt west west of the the Napo Napo Uplift. Uplift.

ranges the Andes. They with the the CordilCordilranges of the Andes. T h e y exhibit exhibit certain certain similarities similarities with lera Oriental as Oriental of of Colombia Colombia and and in in aa regional regional sense sense may m a y be be classified classified as belonging to the the Eastern They fall into into three the belonging to Eastern Andes. Andes. T h e y fall three sub-zones: sub-zones: the Napo in the north and and the and the the Cordillera de Napo Uplift Uplift in the north the Cutucü Cutucfi Range Range and Cordillera de

D. D. REFERENCES REFERENCES

Condor in the are separated fromeach eachother otherby by aa Cdndor in the south, south, which which are separated from depression, known structural depression, knownasasthe thePuyo PuyoRe-entrant. Re-entrant.The Thestratigraphy stratigraphy this zone zone has described in by Tschopp Tschopp (1953). (1953). of this has been been described in detail detail by Zone 4a: J'tfapo Upljfl.This Thisisisa abroad broad domal domal feature, feature, 170 km long Zone 4a: Napo Uplift. 170 km long and 35 35 km km wide, wide, which, which, for for convenience convenience of of description description here, and here, will will be be taken taken to to include include a 20 20 km km wide wide thrust thrust belt belt separating separating the uplift uplift from from zone 3. Jurassic Jurassic rocks rocks (Chapiza (Chapiza Fm) Fm) are are exposed exposed in deep deep gorges gorges cut cut zone through part of by a Cretaceous through the the axial axial part of the the uplift uplift and and are overlain overlain by Cretaceous

succession, made up of sandstones sandstones of of the theHollln HollIn FFm below and and shales shales and and succession, made m below limestones of the theNapo NapoFFm above.Lower LowerTertiary Tertiarystrata strata outcrop outcrop on on the the limestones of m above. margins of the uplift. uplift. margins The eastern eastern flank flank of the uplift uplift is cut by by aa zone zone of of reverse reverse faulting, faulting, extending into the basin as much much as as 40 40 km. km. The The faults faults dip dip extending outwards outwards into basin for for as towards the mountains mountains and and in many border broad broad asymmetrical towards the m a n y cases cases border asymmetrical anticlines. Transverse faults faults atat the the northern northern end end of of the the uplift uplift are are interinteranticlines. Transverse preted preted as dextral tear faults. faults. The Napo Napo Uplift Uplift is is separated separated from from zone zone 3

by by an an important important thrust thrust zone zone in in which which are are exposed exposed slivers slivers of of many many different rock rock types, types, including low-grade metamorphic different including low-grade metamorphic rocks rocks believed believed of L. Palaeozoic Palaeozoic age; an alkali alkali granite granite tentatively tentatively considered considered to to be of age; an to be of of Triassic Triassic age; age; and and strongly strongly deformed deformed Cretaceous Cretaceous and be and Jurassic Jurassic strata. strata.

J. CAMPBELL,C. C.J. CAMPBELL,

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