Reference
Basin, central western Colombia
SUTER, Fiore
Abstract
La première étape de cette recherche a été d'éclaircir la sédimentologie de la Formation Zarzal et ses relations avec le Fan volcanoclastique de Quindío-Risaralda. Un modèle de dépôt allant du (Plio)-Pléistocène au présent, incluant les deux formations, a été élaboré. Par ailleurs, la présence de grains de pollen d'Aulne dans des argiles a démontré un âge pléistocène pour les dépôts affleurant de la Formation Zarzal. La seconde phase de cette recherche a été d'améliorer les connaissances concernant la tectonique qui affecta ces bassins. La troisième phase de ce travail a été l'étude néotectonique des dépôts pléistocènes et holocènes.
SUTER, Fiore. Tectono-sedimentary study of the Interanden north Cauca Valley Basin, central western Colombia. Thèse de doctorat : Univ. Genève, 2008, no. Sc. 3981
URN : urn:nbn:ch:unige-22818
DOI : 10.13097/archive-ouverte/unige:2281
Available at:
http://archive-ouverte.unige.ch/unige:2281
Disclaimer: layout of this document may differ from the published version.
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UNIVERSIDAD DEL QUINDÍO FACULTAD DE INGENIERÍA
CEIFI Profesor A. Espinosa
T ECTONO - SEDIMENTARY STUDY OF THE I NTERANDEAN
N ORTH C AUCA V ALLEY B ASIN , C ENTRAL W ESTERN
C OLOMBIA
THESE
Présentée à la Faculté des Sciences de l’Université de Genève pour obtenir le grade de Docteur ès sciences, mention Sciences de la Terre
par
Fiore SUTER
de
Genève (Suisse)
Thèse No 3981
GENEVE 2008
Suter, F.: Tectono-sedimentary study of the interandean North Cauca Valley Basin, central western Colombia.
Terre & Environnement, vol. 78, viii + 145 pp. (2008)
ISBN 2-940153-77-9
Section des Sciences de la Terre, Université de Genève, 13 rue des Maraîchers, CH-1205 Genève, Suisse Téléphone ++41-22-702.61.11 - Fax ++41-22-320.57.32
http://www.unige.ch/sciences/terre/
Acknowledgements - Remerciements - Agradecimientos I
Abstract III
Resumen V
Résumé VII
Chapter I Introduction
1. Consequences of Armenia Earthquake on 25th January, 1999 2
2. Importance of studying the Quaternary: follow-up studies 2
3. Relationship between the Quindío-Risaralda Basin and the Cauca Depression – this study 3
4. Aims of this study 4
5. Organization of the manuscript 4
Chapter II Geological knowledge prior to this study
1. Paleostress evolution of Colombia from the Early Cretaceous to Present 8
2. Location of study area 10
3. Evolution and structure of study area 11
4. Rocks cropping out in the study area 12
5. Faults in and around the study area 17
6. Remaining contradictions in the interpretation of the study area Cenozoic rocks 17
7. The Cauca Depression – hypotheses about its formation 18
Chapter III Soft-sediment deformation
1. Introduction 22
2. Geology of Zarzal Formation 22
3. Overview of soft-sediment deformations and classifications 24
4. Soft-sediment deformations in the Zarzal Formation 27
5. Discussion 34
6. Conclusions 42
Chapter IV Sedimentary model
1. Introduction 46
2. Geological Framework 46
3. State of knowledge prior to this study 49
4. Depositional Record 50
5. Discussion 56
6. Conclusions 61
1. Introduction – Aims of study 100
2. Geodynamic and tectonic settings 100
3. Geology of study area 101
4. Methods 105
5. Results 105
6. Discussion 117
7. Conclusions 124
Chapter VII Synthesis
1. Major conclusions in the study area (cf. chapters 3, 4, 5 and 6) 128
2. Cinematic reconstruction of the North Cauca Valley Basin 129
3. Final conclusion 131
4. Perspectives 132
References 135
Acknowledgements - Remerciements Agradecimientos
Ce travail de thèse ne serait pas arrivé a terme sans la participation de beaucoup de per- sonnes qui, de près ou de loin, ont collaboré avec enthousiasme et qui, à un moment ou à un autre, ont permis que cette recherche fasse un pas en avant.
Mes remerciements s’adressent en premier lieu au Pr. Georges Gorin, pour son enthou- siasme et son amour de la Colombie, qui s’est lancé dans cette longue aventure en nous faisant confiance et en prenant à sa charge une lourde tâche administrative. Sans lui, ce projet n’existerait pas.
En collaboration avec Georges Gorin, le Pr. Armando Espinosa a été le « pied-à-terre » en Colombie. Connaissant les nécessités et les difficultés du terrain, il a su nous guider habi- lement afin d’orienter notre recherche et nous mettre en contact avec des personnes clés.
Un immense merci au Dr Mario Sartori. Ces remerciements vont au-delà du cadre de ce travail de thèse. Mario m’a appris à être rigoureux sans trop de sérieux. Il a su me mettre sur les bonnes pistes aux bons moments, constamment me faire voir de nouvelles pos- sibilités, m’apprendre à ne jamais concevoir une idée comme acquise. Son expérience a permis de faire de grands pas en avant pendant l’acquisition et le traitement des données, comme lors de la rédaction. Par ailleurs, l’expérience pédagogique partagée au cours des travaux pratiques de Géologie Structurale et notamment sur le terrain en Corse est d’une grande valeur.
La parte sedimentológica de este trabajo no se hubiera desarrollado de la misma manera sin el aporte del Pr. Carlos Guzmán. Le agradezco por habernos ayudado a desenredar los complicados vínculos entre sedimentos y sismos; así como por su ayuda en campo, su activa participación en el proceso de análisis de datos y en la redacción de este trabajo.
También le agradezco por habernos puesto en contacto con sus colegas de la Universidad de Caldas, por haber confiado en mí durante las salidas de campo con sus estudiantes y por habernos hospedado calurosamente en su casa en Manizales.
Ces remerciements s’adressent également au Pr. Wilfried Winkler qui a accepté de lire, corriger et évaluer ce manuscrit.
Cette thèse ne représente qu’une partie de l’étude du nord de la vallée du Cauca, qui a été entreprise et a évolué en étroite collaboration avec Ralph Neuwerth. Mes remerciements lui sont adressés pour avoir tout partagé autant sur le terrain, qu’au labo et au bureau. Ces remerciements vont bien au-delà de la collaboration géologique. Un grand merci pour tous les moments inoubliables et l’amitié sans égal.
Guzmán, Ralph Neuwerth, Lina Ospina, Fernando Guarín, Sandra Bedoya, Gabriel Paris, Jaques Metzger, Mapathe Ndiaye, Hugo Monsalve, Marisol Gómez, Juan Carlos Zorilla, Gloria Toro, Franck Audemard, José Cortés, Hans Diederix, Rossana Martini, Nathalia Rodríguez, Chadia Volery, Eliana Torres, Tatiana Gaona, Myriam López, les reviewers des articles présentés aux chapitres 3, 4 et 5, Ana Luisa Castillo, Luz Mary Toro Toro et ses étudiantes, ainsi que l’Université de Genève et particulièrement la Section des Scien- ces de la Terre, le Département de l’Instruction Publique de la Ville de Genève, le Fonds National Suisse de la Recherche Scientifique, la bourse Ernst et Lucie Schmidheiny, la Société Académique, l’Observatoire Sismologique du Quindío, la Corporation Régionale du Quindío, Ingeominas et l’Institut Géographique Agustín Codazzi.
Para la ayuda en campo: Ralph, Huber, Don Daniel, Germán, Olivier, Lalo, Manilo, Sol, JuanK y La Cifu.
Pour l’aide au labo: Ralph, Olivier, Luc, Roelant, Richard Spikings, François, Pierrot et Peter.
Para la logística: Sol, Huber, Germán, Hugo Monsalve, Armando, Carlos Agudelo, Al- berto Núñez, Don Daniel, Lina, Deisy, Doña Martha, Lalo, La mamá de Don Daniel, la familia Montoya en La Victoria, la familia “2R” en Bogotá, la familia Ostios y la Cifu en Pereira, las familias Bedoya Cuervo, Guzmán y Gómez Cano en Manizales, Tatiana Gaona, la familia Guarín en Bogotá, la familia Isaza en Ginebra y Labopat en Bogotá, la familia Espinosa en Armenia, Carlos Monsalve.
Et les autres, ceux qui ont toujours été là: mes parents (les 6 !), mes grands-parents, mes frangins (les 3 !), Juliette, Ralph, Anouk, Joël, Katia, Masumi, Huguito, Mia, les collè- gues de volée et ceux du 3ème étage.
Así como los que estuvieron ahí sin quizás darse cuenta: Ana, Lucho, Marito, Álvaro Montoya, Wilson y la gente de la salsoteca Senegal en Armenia, Doris y Martha, Sergio, Ignacio Martínez, los profesores de la Universidad de Ginebra y los que fueron mis asis- tentes durante la carrera.
The 6.2 Mw Armenia earthquake of January 25th, 1999, underlined the lack of scientific and social pre- pardness in this prosperous region of Colombia. The area affected lies inbetween the Western and Central Cordilleras of the Colombian Andes, in a zone do- minated by the interaction between three major tec- tonic plates (Nazca, Caribbean and South American plates) and some secondary blocks such as the Cho- có-Panamá and North-Andes blocks. The necessity to improve the geological information in this zone for risk assessment was pointed out, particularly in the fields of structural geology, neotectonics, paleo- seismology and sedimentology. This Ph.D. thesis was subsequently undertaken in the framework of a greater Swiss National Science Foundation pro- ject concentrating on “Tectonics, neotectonics and sedimentation in an active fault zone: examples of Plio-Pleistocene deposits in the Northern Andes of Central Colombia”. The study presented here con- centrates on Pleistocene deposits outcropping on both sides of the Serrania de Santa Barbara (SSB), an active, Tertiary fold and thrust range separating the Cauca Depression to the west from the Quindío- Risaralda Basin to the east where the city of Arme- nia is located. Preliminary work had shown that the essentially fluvio-lacustrine deposits of the Zarzal Formation, well known in the Cauca Depression, are also present on the eastern side of the Serranía de Santa Barbara (SSB), where they interfinger with mass-flow deposits of the Quindío-Risaralda volca- niclastic Fan. These observations testify to the tec- tono-sedimentary relationships between the Quin- dío-Risaralda Basin and the Cauca Depression. The aims of this study are to better understand these re- lationships and the Late Cenozoic evolution of these two basins.
The first step of the research has been to unravel the Zarzal Formation sedimentology and its relationship with the Quindío-Risaralda volcniclastic Fan. A de-
positional model including both formations from (Plio)-Pleistocene to Present has been established.
Additionally, the presence of Alnus pollen grains in clays demonstrated a Pleistocene age for the out- cropping deposits of the Zarzal Formation. In these sediments, widespread soft-sediment deformations testify to the continuous seismicity which affected both basins during Pleistocene times. All the faults observed in these superficial sediments are exten- sive, which is surprising with respect to the present- day, WNW-ESE maximum horizontal stress known in the study area.
Consequently, the second phase of this research has been to improve the knowledge about the tectonics which affected these basins. A structural study of the SSB was carried out. Inversion methods to determine paleostress axis orientations were performed. They pointed out that this basement-involved thrust range underwent a compressive tectonic regime since its formation. The normal faults affecting the flat-lying, incised, superficial, sediments of the Zarzal Forma- tion and Quindío-Risaralda volcaniclastic Fan were attributed to lateral spreading during earthquakes.
Furthermore, similar orientations of discontinuities, at every scale and in each lithology, show that strain partitionning occurs between the still-active Creta- ceous Romeral Fault System and a younger set of shear fractures, apparently as a consequence of the Chocó-Panamá Block collision in Mio-Pliocene ti- mes.
The third phase of the study was the neotectonic stu- dy of the outcropping Pleistocene to Holocene rocks.
Indices of active tectonics such as the ratio of valley floor width over valley height, drainage inversions, alignment of alluvial fans and terraces evidence the present-day uplift of the SSB. Observations made on digital elevation models suggested the presence of an active pull-apart basin opening up in the southern
Abstract
ley Basin (i.e. The Quindío-Risaralda Basin and the Cauca Depression).
Based on these data, the Oligocene to Present ci- nematic evolution of both basins could be better understood. The folding of the SSB began in Oli- go-Miocene times, coeval with the Farallon Plate break-up into Nazca and Cocos Plate and the Wes- tern Cordillera uplift beginning. The Cauca Valley began to form. Probably during the onset of the Chocó-Panamá Block collision into the NW cor- ner of South-America, the Garrapatas-Ibagué active fault zone developed at a latitude of 4.5°N. Then, as the buoyancy of this collided oceanic island arc pre- vented its subduction, it bended northeastwards and pushed the Western Codillera eastwards. The Cauca Valley was closed north of the latitude of 5°N and began to fill in, leading to the formation of the Cauca Valley Basin between 3.5 and 5°N. The activity of thrusts at the outlet, which closed the valley, may be responsible for the present-day “drowning” of the alluvial plain topography.
This work has demonstrated the widespread neotec- tonic activity in this area, where many superficial soft sediments are susceptible to amplify seismic waves.
It gives clues for the location of trenches in order to perform plaeoseismological studies and define more accurately the recurrence interval of displacements along particular faults. It has to be taken into account for further seismic hazard and risk assessment stu- dies.
Resumen
El sismo de Armenia de magnitud 6.2 (Mw), del 25 de enero de 1999, evidenció la falta de preparación tanto a nivel social como científico en esta próspera región de Colombia, la cual se ubica entre las Cor- dilleras Occidental y Central de los Andes, en el centro del país. Allí convergen tres placas tectónicas mayores, i.e. Nazca, Caribe y Suramericana, y algu- nos bloques secundarios como el Bloque Chocó-Pa- namá y el Bloque Norandino. Dentro de la necesidad por mejorar el conocimiento geológico en esta área para la mitigación del riesgo sísmico se destacó la necesidad de adelantar esfuerzos en los campos de la geología estructural, la neotectónica, la paleosi- mología y la sedimentología. Esta tesis doctoral fue emprendida en el marco del macroproyecto « Tectó- nica, neotectónica y sedimentación en una zona de fallas activa: ejemplos de los depósitos Plio-Pleis- tócenos en el norte de los Andes de Colombia cen- tral » apoyado por el Fondo Nacional Suizo para la Investigación. El estudio presentado aquí se focaliza en los depósitos pleistocénicos aflorantes alrededor de la Serranía de Santa Barbara (SSB), un estrecho cinturón de pliegues y cabalgamientos activos que separa la Depresión del Cauca al oeste, de la cuenca de Quindío-Risaralda al este, en la cual se ubica la ciudad de Armenia. Trabajos preliminares mostraron que los depósitos de la Formación Zarzal, bien cono- cidos en la Depresión del Cauca como de origen flu- vio-lacustre, principalmente, se presentan también al este de la SSB, donde se encuentran interdigitados con depósitos de remoción en masa del Abanico vol- canoclástico de Quindío-Risaralda. Estas observa- ciones atestiguan las relaciones tectono-sedimenta- rias entre ambas cuencas. Las metas de este estudio han sido las de comprender mejor estas relaciones así como la evolución de ambas cuencas durante el Cenozoico tardío.
La primera etapa de este estudio fue la de compren- der la sedimentología de la Formación Zarzal y su
relación con el Abanico volcanoclástico de Quindío- Risaralda. Un modelo de depositación incluyendo ambas formaciones desde el (Plio)-Pleistoceno hasta el Presente fue establecido. Adicionalmente, la pre- sencia de granos de polen del género Alnus en los depósitos arcillosos de la Formación Zarzal demos- tró su edad Pleistocena. Deformaciones de sedimen- tos no consolidados son comunes en la Formación Zarzal y atestiguan una actividad sísmica continua a lo largo del Pleistoceno en ambas cuencas. Las fallas encontradas en estos sedimentos superficiales son extensionales, cosa que no se esperaría encon- trar teniendo en cuenta el esfuerzo máximo horizon- tal con orientación WNW-ESE conocido en la zona de estudio.
Por consiguiente, la segunda fase de esta investiga- ción fue la de mejorar el conocimiento relacionado a la tectónica que afectó estas cuencas. Un estudio estructural de la SSB fue llevado a cabo. Métodos de inversión para determinar las orientaciones de los ejes de paleoesfuerzo fueron aplicados. Estos mostra- ron que este estrecho cinturón de pliegues y cabalga- mientos, el cual comprende escamas de basamiento, fue afectado por un régimen tectónico compresivo desde su formación. Las fallas normales afectando los depósitos superficiales subhorizontales y erosio- nados de la Formación Zarzal y del Abanico volca- noclástico de Quindío-Risaralda fueron atribuidas a expansión lateral durante los sismos. Adicionalmen- te, orientaciones similares de las discontinuidades, a cada escala y en cada litología, mostraron que una partición de la deformación ocurre entre el Sistema de Fallas de Romeral de edad cretácica, el cual sigue siendo activo, y un conjunto más joven de fracturas de rumbo, aparentemente como consecuencia de la colisión del Bloque Chocó-Panamá en el Mio-Plió- ceno.
La tercera fase de esta investigación fue el estudio
les de elevación mostró la probable apertura reciente de una cuenca de tracción en el extremo sur del Aba- nico volcanoclástico de Quindío-Risaralda. Ésta se encuentra limitada por las extremidades de fallas de rumbo dextrales en-échelon alineadas en la zona de fallas Garrapatas-Ibagué. La geomorfología mostró que los sedimentos sub-horizontales de la Forma- ción Zarzal y del Abanico volcanoclástico de Quin- dío-Risaralda sufrieron una fase erosiva anterior al relleno holocénico de la cuenca del Valle del Cauca, i.e., la cuenca de Quindío-Risaralda y la Depresión del Cauca.
Con base en estos datos, la comprensión de la evo- lución cinemática de ambas cuencas del Oligóceno al Presente pudo avanzar. El plegamiento de la SSB comenzó alrededor de la transición Oligo-Miocena cuando la placa Farallón se divisó en las de Nazca y Cocos y la Cordillera Occidental empezó a levantar- se. Fue entonces cuando el Valle del Río Cauca em- pezó a formarse. Al colisionar contra el occidente de Colombia, el Bloque Chocó-Panamá primero hubiera generado la zona activa de fallas de Garrapatas-Iba- gué localizada a 4.5°N. La flotabilidad de este arco de islas oceánicas colisionado contra el continente impidió su subducción, al tiempo que se curvó hacía el noreste y empujó la Cordillera Occidental hacía el este. El Valle del Río Cauca se hubiera cerrado al norte de 5°N y empezado a rellenarse, llevando a la formación de la Cuenca del Valle del Cauca entre 3.5 y 5°N. La actividad de los cabalgamientos que cierran la cuenca en su extremo meridional podría ser responsable de su relleno y por consiguiente del presente « ahogamiento » de la paleotopografía en
la localización de futuras trincheras para la realiza- ción de estudios paleosismológicos para definir con precisión la recurrencia de fallas particulares. Este trabajo ha de ser tomado en cuenta para próximos estudios de amenaza sísmica y de mitigación del riesgo sísmico.
Résumé
la sédimentologie de la Formation Zarzal et ses re- lations avec le Fan volcanoclastique de Quindío-Ri- saralda. Un modèle de dépôt allant du (Plio)-Pléisto- cène au présent, incluant les deux formations, a été élaboré. Par ailleurs, la présence de grains de pollen d’Aulne dans des argiles a démontré un âge pléistocè- ne pour les dépôts affleurant de la Formation Zarzal.
Dans ces sédiments, des déformations de sédiments non-consolidés, largement répandues, témoignent de l’activité sismique continue qui affecta ces deux bas- sins au cours du Pleistocène. Toutes les failles obser- vées dans ces sédiments superficiels sont extensives, ce qui est énigmatique par rapport à la contrainte horizontale maximale connue actuellement dans la zone d’étude, d’orientation WNW-ESE.
Par conséquent, la seconde phase de cette recherche a été d’améliorer les connaissances concernant la tectonique qui affecta ces bassins. Une étude struc- turale de la SSB a été mise en œuvre. Les axes des paléocontraintes ont été calculés grâce aux métho- des d’inversions. Ceux-ci indiquent que cette étroite ceinture de plis et chevauchements, qui inclut des écailles de socle océanique, subit un régime tecto- nique compressif depuis sa formation. Les failles normales affectant les sédiments sub-horizontaux, superficiels et incisés de la Formation Zarzal et du Fan volcanoclastique de Quindío-Risaralda ont été attribuées à de l’expansion latérale durant les séis- mes. Les orientations similaires des discontinui- tés, à toutes échelles et dans toutes les lithologies, montrent qu’un partitionnement de la déformation à lieu entre le Système de failles de Romeral, d’âge crétacé et encore actif, et un ensemble de fractures cisaillantes plus jeunes, apparemment consécutives à la collision du Bloc Chocó-Panamá à la transition Mio-Pliocène.
La troisième phase de ce travail a été l’étude néo- tectonique des dépôts pléistocènes et holocènes. Les Le séisme de magnitude 6.2 Mw qui secoua la ville
d’Armenia le 25 janvier 1999 a souligné le manque de préparation, tant sur le plan scientifique que so- cial, de cette prospère région de Colombie centrale.
La zone affectée est localisée entre les cordillères Centrale et Occidentale, dans une zone située à la confluence de trois plaques tectoniques majeures (les plaques Nazca, Caraïbe et Sud-Américaine) où des blocs secondaires comme le bloc de Chocó-Panamá ou le bloc Norandin accommodent la déformation.
La nécessité d’améliorer l’information géologique en vue de diminuer le risque lié aux séismes a été soule- vée, en particulier dans les domaines de la géologie structurale, de la néotectonique, de la paleosimolo- gie et de la sédimentologie. Cette thèse de doctorat a été entreprise dans le cadre d’un projet plus vaste subventionné par le Fonds national suisse (FNS) de la recherche scientifique intitulé : « Tectonique, néo- tectonique et sédimentation dans une zone de failles active : exemples de dépôts Plio-Pléistocènes dans le nord des Andes de Colombie centrale ». L’étude pré- sentée ici traite des dépôts d’âge pléistocène affleu- rant au pied des deux flancs de la Serranía de Santa Barbara (SSB), une étroite ceinture de plis et che- vauchements qui sépare la Dépression du Cauca à l’ouest du Bassin de Quindío-Risaralda à l’est, où se trouve la ville d’Armenia. Des travaux préliminaires ont montré que les dépôts de la Formation Zarzal, essentiellement fluvio-lacustres et bien connus dans la Dépression du Cauca, affleurent également à l’est de la SSB, où ils sont interdigités avec les dépôts de mass-flows du Fan volcanoclastique de Quindío-Ri- saralda. Ces observations témoignent des relations tectono-sédimentaires existant entre la Dépression du Cauca et le Bassin de Quindío-Risaralda. Les buts de cette étude sont de mieux comprendre ces relations ainsi que l’évolution de ces deux bassins au cours du Cénozoïque supérieur.
La première étape de cette recherche a été d’éclaircir
s’ouvrant dans la partie sud du Fan volcanoclastique de Quindío-Risaralda. Il est localisé entre les extré- mités de failles dextres en-échelon appartenant à la zone de failles Garrapatas-Ibagué. La géomorpho- logie montre que les sédiments sub-horizontaux de la Formation Zarzal et du Fan volcanoclastique de Quindío-Risaralda ont souffert une phase d’érosion avant le remplissage holocène du Bassin de la Vallée du Cauca (c.à.d. du basin de Quindío-Risaralda et de la dépression du Cauca).
Basée sur ces données, l’évolution cinématique de ces deux bassins de l’Oligocène au Présent a pu être mieux comprise. Le plissement de la SSB a com- mencé vers la transition Oligo-Miocène, avec la rup- ture de la plaque Farallon, devenant les plaques de Nazca et Cocos, et le soulèvement de la Cordillère Occidentale. La vallée du Cauca a commencé à exis- ter. La zone de failles de Garrapatas-Ibagué, située à 4.5°N, est probablement apparue au début de la col- lision du Bloc Chocó-Panamá contre la Cordillère Occidentale. Comme la flottabilité de cet arc d’îles bloque sa subduction, il est probable qu’il se ploie en direction du nord-est et pousse la Cordillère Occi- dentale vers l’est, générant la fermeture de la Vallée du Cauca au nord de 5°N. Au sud de 5°N, celle-ci se rempli de sédiments et forme le Bassin de la Vallée du Cauca entre 3.5 et 5°N. L’activité des chevauche- ments qui ferment ce bassin à son exutoire pourrait être à l’origine de l’« ennoiement » actuel de la topo- graphie dans cette plaine alluviale.
Cette étude montre qu’une intense activité néotecto-
de futures études d’aléa et de risque sismiques.
Fiore Suter
Introduction
towns or municipalities, killing around 2000 people, injuring more than 5000 people and damaging more than 45’000 buildings, for a total financial loss rea- ching 2 billion US$ (Bohórquez et al., 2001; Gallego et al., 2005; INGEOMINAS, 1999; Monsalve and Vargas, 2002; Pulido, 2003; Restrepo, 2000).
This earthquake occurred on a left-lateral, strike- slip fault with a normal component (Bohórquez et al., 2001; Monsalve and Vargas, 2002), at a depth ranging between 10 km (Bohórquez et al., 2001) and 18.6 km (Monsalve and Vargas, 2002). The afters- hocks occurred at depths ranging between 8 and 20 km (Bohórquez et al., 2001; Monsalve and Vargas, 2002).
Until 1996, when results of the risk mitigation pro- ject for the region around the cities of Pereira, Dos Quebradas and Santa Rosa de Cabal were published (Espinosa, 1996), all the earthquakes which had af- fected the Viejo Caldas Region (i.e., the departments of Caldas, Risaralda and Quindío) were attributed to the deep seismicity related to the subduction of the Nazca Plate below the South American Plate. This report pointed out the occurrence of 32 earthquakes in this region between 1911 and 1995, which had a strong attenuation from their epicentre towards the surroundings. It also underlined that the destructive, 1983 Popayán earthquake was a shallow one, and that care had to be taken.
Unfortunately, the results of this report were not taken seriously until the Armenia Earthquake. The dramatic consequences of the latter highlighted the
red as active when it has moved at least once du- ring the Holocene and as potentially active when it has moved during the Quaternary (Keller and Pinter, 2002).
When seismotectonics studies allow linking instru- mental and historical seismicity data with particular faults, some important information can be obtained about their activity. But the observation window remains very short in terms of recurrence intervals (Yeats and Prentice, 1996), particularly in Colombia, where instrumental seismicity dates back only to 30 years, and the historical record to 500 years (Fig.
1) (Diederix, 2001). Therefore, paleoseismological studies have to be carried out in order to span the Quaternary.
To perform successful paleoseismological studies, choosing the best site to make a trench is very im- portant. The site has to be on an active fault (if pos- sible having an instrumental seismic record), where continuous sedimentation has occurred and good datable material is available. In order to choose the best site, preliminary neotectonic studies have to be performed, combining both structural and Quater- nary geomorphological studies.
Consequently, refined mapping of Quaternary de- posits and faults is the first step towards improving fault activity records. At the time of the Armenia ear- thquake, the state of knowledge of the Quaternary in the Quindío Department was totally insufficient to reach such a goal.
Cordillera eastern foothills and the Risaralda River Valley north of la Virginia (Neuwerth, in prep.).
Two other M.Sc. theses were achieved: one analy- zed Quaternary deposits at the western foothills of the Central Cordillera north of Calarcá, in order to supply data on the Quaternary activity of the Silvia- Pijao Fault (Ospina, 2007); the other refined the geo- logy of the urban zone of Armenia for land manage- ment (Duque, 2005). Two other M.Sc. theses are in preparation: one attempts to refine the cartography of Quaternary deposits in the area where the Cauca River changes its fluvial regime (Pahud, in prep.);
the other analyses the Quaternary deposits on the Ar- menia Fault between Filandia and Armenia (García Londoño, in prep.). Finally, a Ph.D. thesis providing stratigraphic and structural advances related with the recent geological processes and fault activity asso- ciated with the Armenia Earthquake epicentre zone is in process (Duque, in prep.).
3. Relationship between the Quindío-Risaralda Basin and the Cauca Depression – this study During the mapping of Quaternary deposits in the area where the Roble River joins the La Vieja Ri- Consequently, Espinosa (2000) began a detailed
study of the Quindío-Risaralda volcaniclastic Fan and, in the framework of a Swiss National Science Foundation project, six M.Sc. theses and four Ph.D.
theses have been undertaken at the Geneva Univer- sity, in collaboration with the Colombian Quindío and Caldas Universities.
The first M.Sc. thesis (Guarin, 2002) allowed the sedimentological characterization of the different geomorphological units defined by (Espinosa, 2000).
This work developed into a Ph.D. thesis covering the proximal and intermediate parts of the Quindío-Ri- saralda Fan, attempting to link the sedimentological processes having led to this large volcaniclastic fan formation with fault activity (Guarín, 2004). At the same time, another M.Sc. thesis allowed the detailed mapping of a small area in the distal part of the fan, where the latter is in contact with the folded Oligo- Miocene rocks of the Serranía de Santa Barbara (SSB) (Suter, 2003). This research developed into two Ph.D. theses, the first one around the SSB (this work) and the second one on the more distal part of this huge volcaniclastic complex, towards the wes- tern flank of the Cauca Depression, on the Western
Fig. 1: Geologic observation window and seismology in Colombia, after Diederix (2001).
The manuscript ends with general conclusions and recommendations for further investigations in the area.
Chapters 3 and 4 describe the sedimentological part of this research, whereas chapters 5 and 6 concen- trate on the structural aspects.
Chapter 3 presents sedimentological and palaeoseis- mological data from the Zarzal Formation, which provide a new case study of soft sediment deforma- tions. These results were published in 2006 in Sedi- mentary Geology (Neuwerth et al., 2006) and pre- sented as such in this chapter.
Chapter 4 describes a sedimentary model for the de- position of the Zarzal Formation in relation with that of the Quindío-Risaralda Fan. These results were published in 2008 in Geologica Acta (Suter et al., 2008a) and presented as such in this chapter.
Chapter 5 presents field, aerial photograph and DEM structural data about faults, lineaments and fractures which yielded the orientation of maximum horizon- tal stress and the dominant type of tensor in the area.
Together with some neotectonic observations, these results are to be published in Tectonophysics (Suter et al., 2008b).
Chapter 6 is a review of geomorphological and neo- tectonic data obtained in and around the SSB. To- gether with some new structural data, these data per- mit the elaboration of a relative uplift rate map of the SSB and surroundings and give some clues towards extend the study of Quaternary deposits westwards
in the western foothills of the SSB and across the Cauca River Valley up to the Western Cordillera eas- tern foothills.
4. Aims of this study
The Cauca River is meandering in a 900 m high, 200 km long flood plain from Cali up to La Virginia (Fig.
2). The latter town is located about 15 km north of the Serranía de Santa Barbara (SSB) northern end.
At this location, its fluvial regime changes abruptly:
the Cauca River enters and cuts its way through the Central Cordillera and changes its behaviour from meandering and depositional to turbulent and ero- sive (Fig. 2). The Cauca flood plain, or Cauca De- pression, corresponds to a 200 km-long, interandean basin, in which a high rate of sedimentation resulted in the drowning of the surrounding foothills.
This study aims at understanding the mechanisms which might have formed such an intramontane ba- sin.
The following methods were used to reach this goal:
bibliographical revision; aerial photograph interpre- tation; 30 and 90 meter-DEM observations; compa- rative facies sedimentology in numerous field sec- tions; palaeocurrent, fault plane, striae and fracture plane measurements, structural geology standard techniques; Quaternary geological mapping; drai- nage anomaly detection and morphometric tools.
5. Organization of the manuscript
future paleoseismological studies in order to better assess the seismic risk. This chapter will be slightly reduced and submitted as a manuscript to the Journal of Quaternary Science.
Because chapters 3 to 6 correspond to published and submitted papers, there is a repetition of some figu- res in these chapters.
Fig. 2: Location of study area with respect to the Cauca Depression and the Cauca River. Note the abrupt change in landscape morphology from La Virginia towards the north: the Cauca River flood plain ends up. DEM from USGS (2005).
Fiore Suter
Geological knowledge prior to this study
Restrepo and Toussaint, 1988) outlines the suture between accreted oceanic crust and its associated volcanic and sedimentary rocks to the west, and the
in some parts, the oceanic rocks seem to have ob- ducted upon rather than accreted onto the continen- tal margin (Kerr et al., 1998; Nivia, 1996). At that
Fig. 1: Geodynamics of NW South America: velocities and directions of motion for the different plates and blocks with respect to South America (Freymueller et al., 1993; Kellogg et al., 1985; Pennington, 1981; Trenkamp et al., 2002); tectonic data modified after Cortes and Angelier (2005), Gutscher et al. (1999), Taboada et al. (2000); DEM from USGS (2005). The study area is located to the north of the Cauca Depression, south of La Virginia, i.e., at the front of the Chocó-Panamá Block collision boundary and west of the Romeral suture.
Fig. 2: Factual diagram from Cretaceous to Present, linking the deposits and unconformities in the study area with continental or regional scale events. Causal relationships are indi- cated by arrows. The unconformity “AU2” could be older, i.e., it could have an Upper Miocene age and be directly related to the Chocó-Panamá Block collision. “AU1” is dealt with further in this study. Ages after the ICS International Stratigraphic Chart (2005).
times (Cortes et al., 2005) (Fig. 2).
Using stress inversion of fault slip data sets, Cor- tés et al. (2005) deduced three stress regimes from Late Cretaceous times until present day in the Eas- tern Cordillera. They related the first change in stress direction in the Late Paleocene-Early Eocene with the shift from divergence to convergence between the North and South American Plates. Their model predicts an E-W to WSW-ENE stress orientation be- fore the Late Paleocene-Early Eocene, followed by a NW-SE stress pattern. This change in stress direc- tion corresponds to what they call “the major Ceno- zoic unconformity in Colombia” (Fig. 2).
At 23-27 Ma, during the Late Oligocene-Early Mio- cene, the Farallon oceanic plate breaks up into the Nazca and Cocos plates (Hey, 1977; Lonsdale, 2005;
Pennington, 1981; Taboada et al., 2000) and the di- rection of convergence between the oceanic plate and the continental margin becomes W-E (Taboada et al., 2000). Although the kinematics of the wrench faults in the continental margin of south-western Colombia stayed right-lateral, the strain partitioning reduced and the normal strain increased. As a con- sequence of this event, the Cordilleras uplift rate in- creased and, consequently, the Cauca Valley began to develop (McCourt et al., 1984) (Fig. 2).
The second change in stress direction occurred in middle Miocene times (Cooper et al., 1995; Taboada et al., 2000), as a consequence of the Chocó-Panamá Block collision onto NW South America, which led to the closure of the seaway between the Atlantic
sive stage is responsible for the cinematic inversion of extensional faults, folding and rise of the Eastern Cordillera of Colombia, as well as major uplifts in the Western and Central Cordilleras. After Cortes et al. (2005), the Andean tectonic phase appears to be active in the Eastern Cordillera at present day.
South of 4-5°N, where the present-day kinematics of the Romeral Fault System shifts from right-lateral in the south to left-lateral in the north, the paleostress evolution was influenced rather by the Farallon and Nazca Plates than by the Caribbean Plate.
2. Location of study area
The study area is located between 4.4 - 4.8°N and 75.8 - 76.1°W (Figs. 3 and 4), in the Quindío, Valle and Risaralda Departments. It encompasses the Ser- ranía de Santa Barbara (SSB) and its foothills. The SSB is a small mountain range lying between the Central and Western Cordilleras of Colombia. This range forms the eastern boundary of the northern end of the Cauca River flood plain (referred in this work as Cauca Depression) some 15 km south of its outlet (Figs. 1, 3 and 4).
The area studied is located immediately west of the Romeral suture, at the front of the Chocó-Panamá Block collision boundary, in the transition zone where the RFS shifts its strike-slip kinematics (Ego et al., 1995; Paris et al., 2000; Taboada et al., 2000) (Figs. 1 and 3).
Fig. 3: 3D block diagram of the lithosphere from 4°N to 8°N. See figure 1 for location. The Chocó-Panamá Block and the Carib- bean slab appear in brownish, the Nazca slab in blue. This model illustrates the slab tearing (white arrows) proposed by Cortés &
Angelier (2005) to explain the extensional focal mechanisms of the Cauca deep seismicity nest. The Romeral suture and the ENE striking transverse faults appear in green and red respectively. E-W cross-section and fault locations modified after Taboada et al.
(2000); DEM after USGS (2005).
3. Evolution and structure of study area
In the northern part of the Cauca Depression, no sedimentary rocks were deposited between the Pa- leocene and Late Eocene. The first sedimentary se- quence to be deposited on the suture zone between oceanic and continental basement is the Oligocene Cartago Formation (Rios and Aranzazu, 1989) (Figs.
2 and 5).
The clastic Cartago Formation began to be folded in Late Oligocene-Early Miocene times (Keith et al., 1988; Rios and Aranzazu, 1989). The syn-kinema- tic La Paila Formation deposited uncomformably on this nascent fold belt (Figs. 2 and 5).
These two continental sedimentary formations, to- gether with some oceanic basement slices, form the northern end of the Tertiary fold and thrust belt of the Cauca Valley Basin (Alfonso et al., 1994). In this
Formation and Quindío-Risaralda volcaniclastic Fan deposited during (Plio-)Pleistocene times.
4. Rocks cropping out in the study area
Names of formations refer to the compiled geologi- cal map of figure 5.
4.1. Pre-Cenozoic
Immediately west of the Romeral suture (i.e., the SSW-NNE striking structure, the Mesozoic ophioli-
tic basement and its Cenozoic sedimentary cover are involved in westvergent fault-propagation folds stac- ked in an imbricated thrust system showing a thick- skin structural style (Fig. 6). According to Alfonso et al. (1994), the thrusts root at a 10 km depth, where they join a common detachment. The term “Serranía de Santa Barbara” (SSB) refers to the northern termi- nation of this fold-and-thrust belt. The SSB separates the Cauca Depression to the west from the Quindío- Riseralda basin to the east (Fig. 6), where the Zarzal
Fig. 4: Location of study area with respect to the Cauca Depression and the Cauca River. Note the abrupt change in landscape morphology from La Virginia towards the north: the Cauca River flood plain ends up. DEM from USGS (2005).
4.2. Cenozoic 4.2.1. Introduction
The terms pre-, syn- and post-orogenic refer to the faulting and thrusting phase which affected the Car- tago and La Paila Fm, allowing the uplift of the SSB fold-and-thrust range. This tectonic phase started at Late Oligocene-Early Miocene times (Fig. 2) (Keith et al., 1988).
4.2.2. Pre-orogenic deposits
The Cartago Fm (Rios and Aranzazu, 1989; Schwinn, 1969), also referred as Cinta de Piedra Fm (Hubach and Alvarado, 1934; Keith et al., 1988; McCourt, 1984) or Cinta de Piedra Member of the Cauca Su- perior Fm (Van der Hammen, 1958) was first descri- bed by Hubach and Alvarado (1934).
The age of this formation ranges between Lower Oligocene for its intermediate member based on palynology (Rios and Aranzazu, 1989) and Middle Miocene (Schwinn, 1969). McCourt (1984) gives an Oligocene age based on stratigraphic relationships.
This formation is characterized by well-bedded, oxidized sandstones with intercalations of clays or conglomerates. The works of Keith et al. (1988) and Ríos and Aranzazu (1989) are the most extensive.
They describe a general coarsening-up tendency for the whole formation and define an upper conglome- ratic member. In terms of depositional setting, the Cartago Fm is interpreted as a fine-grained meander belt system which evolves into braided streams and possibly coarse-grained meandering channels with time. This depositional setting is associated with a humid alluvial fan system (Keith et al., 1988). Pa- leocurrents measured in the through cross-bedded sandstones of the intermediate “Piedras de Moler”
Member show a SSW direction of flow (Rios and Aranzazu, 1989).
Ríos and Aranzazu (1989) measured a thickness of 3109 m. for the whole Cartago Fm, the lower contact Cauca-Almaguer Fault in the study area, Fig. 5), the
first sequence of oceanic rocks corresponds to the Amaime Fm. It constitutes the basement on top of which the Cenozoic, continental sedimentary rocks of the study area were deposited. Further west, one distinguishes from east to west, the volcanic rocks of the Diabase Group and the metasediments and metavolcanics of the Dagua Group (McCourt et al., 1984). Both Groups are interfingering and partially equivalent (McCourt and Aspden, 1983), and form the core of the Western Cordillera.
The Amaime Formation (McCourt et al., 1984;
McCourt, 1984), also referred as Amaime Group (McCourt et al., 1984) or Amaime Complex (Mo- reno and Pardo, 2002) is part of the Diabase Group of Nelson (1962). It corresponds to a suite of basic volcanic rocks which are generally massive tholeii- tic basalts with important horizons of pillow lavas, locally associated with komatiitic basalts (A. Espi- nosa, in McCourt (1984) and McCourt et al. (1984)).
Together with the Ginebra, Cauca, and Los Azules ophiolitic complexes, the Amaime Formation is part of the Amaime Terrane, which is interpreted as an incomplete ophiolite (Aspden and McCourt, 1986).
After Pindell et al. (2005) the basalts of the Amaime Terraine originated in back-arc. The Amaime Ter- rane is intruded by Lower Cretaceous stocks (e.g.
the Buga Batholith (McCourt et al., 1984), the Saba- nalarga Stock, and the Támesis Stock (Moreno and Pardo, 2002)) and tectonically associated with Upper Cretaceous, basic and sedimentary rocks (Moreno and Pardo, 2002; Nivia et al., 2001). The contacts of the Amaime Fm are faulted; the Cauca-Almaguer Fault limits the Amaime Fm to the east (Aspden and McCourt, 1986; Moreno and Pardo, 2002) and, lo- cally, the Guabas-Pradera Fault corresponds to the western limit of the latter, although in general it is hidden beneath Tertiary and Quaternary deposits of the Cauca Valley (Aspden and McCourt, 1986). The Amaime Fm has a probable Jurassic-Lower Cre- taceous age (McCourt, 1984; Moreno and Pardo, 2002).
Fig. 5: Geology of study area. Simplified and compiled from sheets 223-El Cairo (Parra, 1983), 224-Pereira (Ca- ballero and Zapata, 1983), 242-Zarzal (Nivia et al., 1995), and 243-Armenia (McCourt et al., 1984). See figure 4 for location.
of which is unconformable and depositional (Keith et al., 1988) or faulted (McCourt, 1984) or locally faulted (Rios and Aranzazu, 1989) over the Creta-
ceous Amaime Fm and Buga Batholith.
The Cartago Fm is a time equivalent of the Vijes and
Guachinte/Ferreira Fms cropping out further south in the Cauca River Valley (Keith et al., 1988) and of the Amagá Fm (Alvárez, 1983; McCourt, 1984) cropping out further North, outside of the Cauca De- pression.
4.2.3. Syn-orogenic deposits
The La Paila Fm (Alvárez, 1983; González and Núñez, 1991; Keith et al., 1988; McCourt, 1984; Ni- via et al., 1992; Rios and Aranzazu, 1989) was first described by Van der Hammen, (1958), it is also re- ferred to as Buga Fm (Schwinn, 1969).
Its age is Lower Miocene based on structural and sedimentological relationships with porphyric intru- sions (McCourt, 1984), Middle Miocene (Nivia et al., 1992; Schwinn, 1969), Middle to Upper Miocene based on Ar40/Ar39 datings (12,7 Ma, (Suter, unpu- blished)), or Miocene based on palynology (Van der Hammen, 1958).
The La Paila Fm is characterized by a lower interval of dacitic or volcanic tuffs followed by a sequence of conglomerates and sandstones with some intercala- ted clays. Silicified wood fragments are common in these sediments (McCourt, 1984; Schwinn, 1969), which rework fragments of the underlying Cartago Fm (Schwinn, 1969) and contain tuffaceous mate- rial (Keith et al., 1988; Rios and Aranzazu, 1989).
The basal tuff is interpreted by Nivia et al. (1992) as some compacted ash fall levels, whereas Suter (2003) interpreted it as a rhyolithic lava flow. The upper member is interpreted as braided streams (Nivia et al., 1992) of a humid alluvial fan origina- ting in the slopes of the Central Cordillera (Keith et al., 1988; Rios and Aranzazu, 1989) and extending south-westwards (McCourt, 1984).
The volcanic input in these deposits testifies to the onset of the Miocene to Recent calcoalkaline volca- nic activity in the Central Cordillera (Fig. 2) (Mon- salve and Mora, 2005). The La Paila Fm is correlated with the Combia Group (Van der Hammen, 1958) or
Combia Formation (McCourt, 1984) of Antioquia and the Honda Fm in the Magdalena River Valley (McCourt, 1984; Van der Hammen, 1958).
McCourt (1984) makes a difference between the La Paila Fm and the La Pobreza Fm, because of the pre- sence in the latter of a very coarse, basal conglomera- te containing boulders of the regional porphyry suite (named “Albania type”). This assumption is in disa- greement with the observations of Keith et al. (1988) and Rios and Aranzazu (1989). Because both units unconformably overlie the Catago Fm and underlie the Zarzal Fm or modern alluvium and because they were deposited in the same depositional setting and contain similar lithologies, the latter authors grou- ped both units into the La Paila Fm and attributed it a Miocene age. For González and Núnez (1991), the La Paila Fm corresponds to the La Pobreza Fm of McCourt (1984). For Nivia et al. (1992), the La Po- breza Fm corresponds to the proximal and oriental part of the La Paila Fm.
Whereas McCourt et al. (1984) mention that the Neogene sequence known as the La Paila Fm is pos- sibly up to 2000 m thick, Keith et al.(1988) and Rios and Aranzazu (1989) measured a thickness of 1400 m for the grouped La Paila and La Pobreza Fms.
Angularly unconformable with the underlying Cinta de Piedra Fm (González and Núñez, 1991; Keith et al., 1988; Rios and Aranzazu, 1989), the Amaime Fm basalts (Nivia et al., 1992), or the Diabase group and the large tonalite body east of Buga (Schwinn, 1969), the contact is mainly depositional but someti- mes tectonic (Keith et al., 1988; Rios and Aranzazu, 1989). South of their study area, Ríos and Aranzazu (1989) observed an inverse-type, faulted contact with the Amaime Fm.
4.2.4. Post-orogenic deposits
During Plio-Pleistocene times, the flat-lying Zarzal Fm and the Quindío-Risaralda volcaniclastic Fan were deposited. Both units contain a high percentage of volcanic material, and are derived from the vol-
Fig. 6: Published cross-sections of the study area showing the Serranía de Santa Barbara (SSB) fold-and-thrust belt: A) Alfonso et al. (1994), see figure 5 for locations of cities; B) Ingeominas (1999), see figure 4 for location of Armenia. Refer to these publications for details.
canic activity of the Cerro Bravo-Machín volcanic system. No or very few minor deformations affect these sediments: both formations are considered to be post-orogenic.
The Zarzal Fm is a flat-lying to locally slightly dip- ping sequence of diatomites, clays, tuffaceous sands or sandy tuffs (Cardona and Ortiz, 1994; Keith et al., 1988; McCourt, 1984; Nivia et al., 1992; Van der Hammen, 1958). Keith et al. (1988) noted the pre- sence of conglomerates as well. Nivia et al. (1992)
subdivided it into three intervals: a lower one cha- racterized by conglomeratic sandstones, sandstones and claystones; an intermediate one rich in claysto- nes; and an upper one dominated by diatomites.
Whereas Keith et al. (1988) interpreted this sequen- ce as representative of a shallow lacustrine setting bordered by small deltas and braided streams emit- ting from small alluvial fans, Cardona and Ortiz (1994) interpreted it as braided streams and flat zo- nes between channels and lakes.
These lithologies were first described by Boussin- gault (1903), but the name “Zarzal Formation” was given by Van der Hammen (1958), who attributed it a probable Pliocene age without confirmation by palynological data. Keith et al. (1988) considered both the Zarzal Fm and the Armenia Fm of McCourt (1984) as being Plio-Pleistocene in age. Cardona and Ortiz, (1994) observed the interfingering of this sequence with the Pereira Fm, also referred to as Ar- menia Fm (McCourt, 1984).
The Zarzal Fm is at least 20 m thick (Nivia et al., 1992) and overlies unconformably the La Paila Fm (Cardona and Ortiz, 1994; McCourt, 1984; Nivia et al., 1992; Van der Hammen, 1958), the Cartago Fm (Cardona, 1994), or the whole underlying Eocene- Miocene sequence (Keith et al., 1988).
Keith et al. (1988) and Nivia et al. (1992) noted the absence of important deformation in these sedi- ments. According to Rios and Aranzazu (1989), their horizontal to subhorizontal position testifies to a relative tectonic stability from Pliocene to Re- cent times. However, McCourt (1984) and Van der Hammen (1958) reported that it suffered minor dislocations which are probably related to very re- cent movements along buried faults in the pre-Ter- tiary basement (McCourt, 1984). Cardona and Ortiz (1994) and Pardo et al. (1994) also observed recent faulting. Cardona and Ortiz (1994) mentioned that to the north of Obando anticlines and synclines with an amplitude of 200 m deform the Zarzal Fm beds.
The Quindío-Risaralda volcaniclastic Fan (Guarín et al., 2004), also referred as Quindío Mud-flow (Mosquera, 1978), Quindío-Pereira Fan (Thouret, 1988), Armenia Formation (McCourt, 1984), Quin- dío Glacis (González and Núnez, 1991), Pereira For- mation (Cardona and Ortiz, 1994), or Quindío Fan (“Abanico del Quindío”) (Espinosa, 2000), is a stac- ked succession of volcaniclastic mass-flows (Espi- nosa, 2000; Guarín, 2004). Ash fall deposits are also observed (Cardona and Ortiz, 1994; Espinosa, 2000;
McCourt, 1984), as well as pyroclastic flow depo- sits (Cardona and Ortiz, 1994). The proximal part of this fan is dominated by debris-avalanche depo- sits with no or little matrix, the intermediate part by matrix-supported mass-flow deposits which may be interbedded with hyperconcentrated flow deposits, and the distal part by hyperconcentrated flow depo- sits and normal stream flow deposits (Guarín et al., 2004). In the intermediate and distal parts, (Cardona and Ortiz, 1994) observed braided-stream, low si- nuosity stream, floodplain, and lacustrine deposits in association with the debris-flow deposits. The latter authors pointed out the interfingering of the Pereira Fm with the Zarzal Fm.
5. Faults in and around the study area
An extensive bibliographical revision of fault data in and around the study area is presented in Chapter 5.
6. Remaining contradictions in the interpretation of the study area Cenozoic rocks
6.1. Contacts in geological maps
The western, lower contact of the Cartago Fm is sup- posed to be tectonic and outlined by the Quebrada- nueva fault. On the western side of the Serranía de Santa Barbara, the Cartago Fm is in contact either with the La Paila Fm (Caballero and Zapata, 1983;
McCourt et al., 1984; Rios and Aranzazu, 1989), or with the La Pobreza Fm, which is itself in strati- graphic contact with the La Paila Fm further west (Nivia et al., 1992; Nivia et al., 2001).
It seems relevant to point out that the 1:250’000 Geological Map of the Valle del Cauca Department (Nivia et al., 2001), the 1:100’000 Sheets 242-Zarzal (Nivia et al., 1992) and 243-Armenia (McCourt et al., 1984) (Fig. 6), and the 1:100’000 structural map of Rios and Aranzazu (1989) do not agree neither on the fault location nor on the contact location.
eastern one corresponding to the Quebradanueva Fault. The latter authors position the tectonic contact along the Holguín Fault. Consequently Nivia et al.
(2001) are in agreement with those authors for the location of the contact, but uses a different name for this fault.
Alfonso et al. (1994) and Rios and Aranzazu (1989) are the first ones to characterize the Holguín and Quebradanueva Faults as west-verging thrusts (Fig.
6A). Nivia et al. (2001) mention them as faults wi- thout any kinematics indication, whereas Guzmán et al. (1998) deduce an inverse, right-lateral kinematics for the Quebradanueva fault.
East of the Serranía de Santa Barbara, published data show the same type of contradictions. The eastern contact of the Cartago Fm is tectonic according to González and Núnez (1991), or partly tectonic ac- cording to McCourt et al. (1984) and Rios and Aran- zazu (1989), or stratigraphic according to Nivia et al.
(2001). The contact follows in part the La Holanda Fault (McCourt et al., 1984; Rios and Aranzazu, 1989), or the Potrerillos Fault located 2 km further east (González and Núñez, 1991). This contact se- parates the Cartago Fm from the La Paila Fm (Gon- zález and Núñez, 1991; Rios and Aranzazu, 1989;
Suter, 2003) or from the La Pobreza Fm (McCourt et al., 1984; Nivia et al., 2001).
6.2. Existence of the La Pobreza Fm ?
So far, there are not enough stratigraphic data on the La Paila and La Pobreza Formations in order to con-
Various hypotheses have been proposed to explain the occurrence of such an intramountane basin at an altitude of 900m: some authors have interpreted it as a graben (Acosta, 1978; Bermúdez et al., 1985;
Droux and Delaloye, 1996; MacDonald et al., 1996;
McCourt, 1984; Padilla, 1982) or a pull-apart basin (Kellogg et al. (1983) in Alfonso et al. (1994)), whe- reas various studies have demonstrated the compres- sive character of the faults and folds observed within Cretaceous to Miocene structures (Alfonso et al., 1994; Keith et al., 1988; Rios and Aranzazu, 1989).
Recent studies have shown neotectonic compressive activity in some faults bounding this floodplain (Ló- pez et al., 2005; López and Moreno, 2005).
Ralph Neuwerth, Fiore Suter, Carlos Guzmán and Georges Gorin
Sedimentary Geology (2006)
Soft-sediment deformation in a tectonically ac- tive area: The Plio-Pleistocene Zarzal Forma-
tion in the Cauca Valley (Western Colombia)
neras), 1999). The Ruiz-Tolima volcanic system in the Central Cordillera is associated with these faults and formed the source of the fluvio-volcanic fans deposited to the west (Fig. 2).
The Zarzal Formation encountered in the Cauca Valley was deposited in a fluvio-lacustrine environment.
It contains numerous soft-sediment deformation structures, particular- ly in the Cartago area (Fig. 4). The aims of this paper are to describe the various types of deformations encountered and discuss their po- tential triggering mechanisms.
2. Geology of Zarzal Formation The Zarzal Formation has been so far poorly studied. Boussingault (1903) was the first to describe si- liceous deposits intercalated with tonic regime in the north-western part of Colombia,
particularly in the studied area located between the Western and Central Cordilleras, north of the city of Cali (Fig. 1).
The studied area covers parts of three Colombian departments (Fig. 2): Quindío (city of Armenia), Ri- saralda (city of Pereira) and Valle del Cauca (cities
mentary units interfinger in the western part of the Cartago, Pereira and Quindío Fans. This tectonically active zone is dissected by several major SSW–NNE trending fault lineaments such as the Romeral Fault System (Fig. 3). In particular, the rupture at shallow depth of the Armenia fault caused the dramatic earth- quake of Armenia in January 1999 (INGEOMINAS (Instituto nacional de investigaciones geológico-mi-
Fig. 1: Megatectonic framework and lo- cation of study area.
sand and sandy clay beds in the Cartago area, where they form low-relief hills. He considered these sedi- ments as the infill of a lake. The name Zarzal Forma- tion was attributed in 1955 to these deposits of dia- tomite, clay and volcanic sand (Keiser, Nelson and Van der Hammen, unpublished, in Van der Hammen, 1958; see also De Porta, 1974).
Cardona and Ortiz (1994) were the first to analyze in detail the depositional environment of these sedi- ments. They interpreted three types of facies: brai- ded-stream channel deposits, floodplain sediments and lake deposits. They also noted the geomorpholo- gical expression of this unit: low-relief hills dissec- ted by a well-marked drainage pattern and numerous surface fractures. They recognized traces of block tectonics younger than the Pliocene Andine orogeny and also described the interdigitation of the Zarzal Formation with the fluvio-volcanic fans of Cartago and Pereira to the east.
So far, no precise age dating has been achieved for
Fig. 2: Regional distribution of Plio-Pleistocene sediments in the Valle del Cauca, Risaralda and Quindío Departments. Location of Figs. 3 and 4.
the Zarzal Formation. According to Van der Ham- men (1958), it assumes a probable Pliocene age for the Zarzal Formation without any scientific eviden- ce. The Zarzal Formation disconformably overlies the Miocene La Paila Formation on the western flank of the Serranía de Santa Barbara (Fig. 2; McCourt, 1984; Nivia et al., 1995). In the Cauca Valley, the Zarzal Formation is disconformably overlain by gra- vels of alluvial fans fed by the surrounding reliefs, by grey palaeosols rich in volcanic materials and by recent alluvial sediments (Nivia et al., 1995). In fact, prior to the present research, the only certainty that exists is that the Zarzal Formation postdates the Pliocene Andine compression, i.e. it has a Late Plio- cene to Pleistocene age.
The dramatic earthquake of Armenia in January 1999 prompted the detailed geological study of the Plio-Pleistocene deposits in the Quindío Depart- ment, especially of the fluvio-volcanic fans and their relation with the Zarzal Formation in Quindío and the Cauca Valley (Figs. 2 and 3; Guarin, 2002;
fluvio-volcanic sediments of the Quindío Fan (Suter, 2003). Preliminary palynological data from clays of the Zarzal Formation show a significant presence of Alnus pollen. Because the first record of this tree in Colombia dates back to 1 my (Hooghiemstra and Cleef, 1995), a large part of the Zarzal Formation is probably of Pleistocene age. These data are awai- ting confirmation from ongoing radiometric dating in volcanic ashes.
Field work and aerial photographs have confirmed the evidence of recent tectonic activity, particularly the diverging drainage pattern of Zarzal outcrops in the Cauca Valley (Fig. 4). Most of the soft-sediment deformation structures described below are within the sections shown in Fig. 5.
3. Overview of soft-sediment deformations and classifications
Guarin et al., 2004; Gorin et al., 2003; Suter, 2003;
Suter et al., 2003). Detailed field studies confirm the observations of Cardona and Ortiz (1994). The Zarzal Formation consists of autochthonous lacus- trine sediments (diatomites) with a variable degree of interfingering with volcanic, fluvio-volcanic and fluviatile influxes derived from surrounding sources.
In the Cauca Valley, where numerous sections can be observed (Figs. 4 and 5), the degree of interfingering increases towards the east and ongoing studies show that the Cartago Fan (Fig. 2) probably supplied most of the allochthonous material. At the western edge of the Cartago Fan, volcanic mass flows are inter- calated with Zarzal fluvio-lacustrine sediments and can be observed westwards up to the Ansermanuevo area (Figs. 4 and 5). Moreover, thin intercalations of volcanic ash are encountered in the diatomites. In the La Vieja and Roble valleys, east of the Serrania of Santa Barbara (Fig. 2), lacustrine deposits of the Zarzal Formation can also be observed underlying
Fig. 3: Geological cross-section across the Valle del Cauca and Quindío Departments. See Fig. 2 for location.
Although different authors have proposed classifications, no univer- sally accepted scheme exists for various reasons. First of all, the me- rely descriptive classifications do not take into account the processes
Fig. 4: Detailed geological and location map of study area. See Fig. 2 for location.
which formed the structures. On the other hand, the genetic classi- fications are based on assumptions about the inferred processes and parameters that have acted during the deformation. In many cases, the relationships are not clear and their application to field work is diffi- cult. Although some classifications have been proposed in order to link morphology and genetic processes, they do not answer all questions.
Another difficulty is the large va- riety of descriptive terms. Although this paper does not aim at elabora- ting a new classification, a review of some proposed classifications is necessary, in order to clarify the ter- minology that will be used (Table 1). It refers mainly to Lowe (1975), Brenchley and Newall (1977), Mills (1983) and Owen (1987, 2003).
Lowe (1975) proposed a classifica- tion for water escapes structures. It is quite comprehensive but confu- sion may exist for some structures.
Nevertheless, Lowe’s classification has been used in recent papers (Ros- setti, 1999). Brenchley and Newall (1977) proposed a classification for contorted bedding, which has been updated by Mills (1983). Although the diagnostic features are relati- vely easy to establish, the direction of movement is difficult to infer.
Moreover, it does not include soft- sediment structures such as dykes and water escape structures. More recently, Owen (1987, 2003) pro- posed two attractive classifications encompassing respectively all soft- sediment deformation structures
Fig. 5: E–W and N–S trending correlations tion. See Fig. 4 for location of field sections.
