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Last changes 22. Mai 1999 by Steffen Kutterolf
Abstract from members of the Institute of Geology and Palaeontology, University of Stuttgart (IGPS).
Holger Appel, Gerhard Wörner, Guillermo Alvarado Chris Rundle & Siegfried Kussmaul
Abstract
New K-Ar age data and unpublished age information on igneous rocks from Costa Rica are summarized and discussed. Whole-rock K-Ar ages for older Jurassic mafic rocks from the Upper and Lower Nicoya Complex are strongly disturbed and record periods of thermal and chemical alteration with a cluster of "ages" around 60 Ma. Isolated "flakes" of intra plate basalt compositions record alteration ages from 65 Ma to 35 Ma, indicating an origin distinct from, and possibly younger than, the Nicoya Complex. Rocks from the older Talamanca and Aguacate series represent the early stages of arc formation on the evolving landbridge starting from around 35 Ma reaching ages as young as 2 Ma. The presently active mature arc in Central Costa Rica and the Guanacaste range is younger with ages from 1.5 Ma to the Present.
Addresses:
Appel, Wörner: Geochemisches Institut der Universität Göttingen, Goldschmidtstr. 1, D-37077 Göttingen, Germany
Alvarado: Dept. Geología, Instituto Costarricense de Electricidad, Apdo 10032, 1000 San José, Costa Rica and GEOMAR, Universität Kiel, Wischhofstr. 1-3, 2300 Kiel, Germany
Rundle: NERC Isotope Geosciences Laboratory, Keyworth, Nottingham NG12 5GG, United Kingdom
Kussmaul: Escuela Centroamericana de Geología, Universidad de Costa Rica, Apdo. 35, 2060 Ciudad Univérsitaria, Costa Rica
Resumen
La secuencia sedimentaria del Jurásico-Cretácico incluida dentro del Complejo Ofiolítico de Nicoya (NW de Costa Rica) es separada en dos unidades cronoestratigráficas: La unidad inferior de edad Jurásico tardío-Cretácico temprano (pre-Albiano), consiste de pedernales radiolaríticos ("ribbon cherts") asociados con nódulos de Manganeso y sobreyace un Complejo Básico (inferior), que caracteriza un corteza oceánica típica. La unidad superior de edad Albiano - Campaniano tardío basal, consiste de una secuencia de varios cientos de metros de espesor, conformada por: a) facies pelágicas de fangos radiolaríticos (radiolaritas), depositadas bajo una zona de alta productividad, relacionda a una zona "upwelling" ecuatorial; b) facies hemipelágicas de barros sapropélicos (lutitas negras), originadas por la expansión del nivel de oxígeno mínimo durante un evento anóxico ocurrido sobre una zona levantada del piso oceánico; c) facies de turbiditas volcaniclásticas (lutitas tobáceas, areniscas y brechas), y carbonatadas (calizas silíceas), que evidencian la redepositación de sedimentos desde relieves volcánicos cercanos. Los tres intervalos reconocidos con mayor desarrollo de lutitas negras, laminadas, ricas en materia orgánica son correlacionados con los eventos anóxicos oceánicos (OAE) descritos para el Cretácico: Aptiano-Albiano (OAE1), Cenomaniano/Turoniano (OAE2), y Santoniano-Campaniano temparano (OAE3). Estas sucesiones sobreyacen y están intercaladas con un Complejo Básico (superior), que consiste de flujos de lava y filones-manto de basaltos y doleritas, además de brechas de basaltos almohadillados. Este volcanismo submarino pudo ser localmente somero (hasta emergido), y se considera como la causa de los relieves submarinos que proveyeron los sedimentos turbidíticos a la secuencia. Se interpreta entonces, las secciones sedimentarias como una sedimentación pelágica/hemipelágica desarrollada sobre un piso oceánico levantado y posiblemente engrosado por el efecto de una amplia actividad volcánica submarina contemporánea.
Considerando las características y el tiempo de formación de esta unidad, y a partir de la integración de otros datos locales y regionales, tales como: a) Datos geoquímicos publicados para los basaltos de la unidad Albo-Campaniano (toleítas de afinidad océanica), b) Los datos sedimentológicos, petrológicos y geofísicos de la región del Caribe y circumcaribe que evidencian la existencia de un piso oceánico engrosado pre-Campaniano tardío (bajo el horizonte sísmico B"), asociada a sedimentación pelágica con niveles de lutitas negras; y c) la gran similitud de las características de esta unidad, con las secciones estratigráficas de la misma edad reportadas por el DSDP para las "Plateaus" del Pacífico Oeste; se interpreta a la unidad Albo-Campaniano y el Complejo Básico superior, como una sección aflorante de una "Plateau" oceánica del Aptiano - Campaniano temprano, que tiene extensión en la región del Caribe y que se designa como "Paleo-Plateau" Caribe. A partir de lo anterior se establece el siguiente modelo general de evolución cretácica para este límite: 1) Generación de corteza oceánica (Complejo Básico inferior) durante el Jurásico-Cretácico temprano en un centro de dispersión del Pacífico (Farallón-Pacífico?) y cobertura de la misma por sedimentos oceánicos durante su movimiento hacia el Este. 2) Desarrollo durante el Aptiano - Campaniano temprano (Zona de Quietud Magnética del Cretácico y anomalía 33r) de un extenso volcanismo de origen intraplaca (Complejo Básico superior), genéticamente asociado al evento volcanotermal delPacífico. Este volcanismo originó el piso engrosado del Caribe, que con respecto al segmento del NW de Costa Rica, para el Aptiano se ubicó cerca de 30_ de longitut hacia el W-SW de su posición actual, en una latitud cercana al Ecuador. 3) Producto de cambios mayores en el movimiento de las Placas del Pacífico, durante el Campaniano temprano, se abre el margen convergente de América Central. 4) Inicio del volcanismo de arco de islas primitivo durante el Campaniano tardío - Maastrichtiano y posible extensión del mismo hasta el Paleógeno.
Address of author: Refinadora Costarricense de Petroleo S.A. (Recope), Apdo 4351-1000, San Jose, Costa Rica, America Central.
Guillermo Barboza & Héctor Zucchi
Resumen
En las cuencas "forearc" localizadas entre la Península de Nicoya y el promontorio de la Península de Burica, se reconocen tres ciclos de sedimentación ligados a tres fases tectónicas bien definidas. El ciclo "C" representa la fase inicial de apertura de las cuencas "forearc". El mismo está caracterizado por sedimentos de plano de cuenca, depositados entre el Campaniano tardío y el Paleoceno tardío Basal. El estilo estructural de esta fase tectónica es de desarrollo de pequeños grábenes y semigrábenes. El ciclo "B" se caracteriza por sedimentación turbidítica, depósitos contemporáneas a la formación de profundos grábenes y semigrábenes que lograron individualizar las cuencas del Golfo de Nicoya - Parrita - Quepos y Dominical. Este ciclo representa los sedimentos depositados durante la segunda fase tectónica, desde el Paleoceno tardío basal, hasta el Oligoceno tardío. El límite superior de este ciclo de está limitado por una discordancia regional, cuya edad se deduce Oligoceno tardío - Mioceno temprano. A partir del Mioceno temprano, esta discordancia marca el establecimiento de las plataformas, denominado ciclo sedimentario "A", que representa al menos tres secuencias agradantes, depositados durante un período de dominio transgresivo, desde el Mioceno temprano al Plio-Pleistoceno. Los límites entre secuencias en el área de estudio, están marcados por un efecto combinado de levantamiento tectónico y cambios eustáticos.
Addresses:
Barbosa: Refinadora Costarricense de Petróleo, s.A. Apdo. 4351-1000, San José, Costa Rica;
Zucchi: 211 de Setiembre 2568, Buenos Aires, Argentina
Giovanni Bottazzi, J.A. Fernandez & Guillermo Barboza
Resumen
La historia del relleno sedimentario, en la cuenca de Limón Sur, se inicia a partir del Campaniano tardío y refleja una somerización hasta su colmatación a partir del Plioceno. Se reconocen cinco secuencias sedimentarias, que en su conjunto sobrepasan los 7000 m de espesor. Dos fases compresivas se documentan bien en la cuenca, la del Eoceno medio-tardío y la del Mioceno tardío. La etapas tectono-sedimentarias reconocidas son: Cretacico tardío - Eoceno medio: Caracterizada por un régimen tectónico distensivo típico de la región tras arco y sedimentación pelágica carbonatada: Eoceno medio/tardío - Oligoceno tardío: Formación de altos estructurales a raiz de una etapa tectónica compresiva que caracteriza a la cuenca como tipo foreland, en dichos altos se instalan plataformas carbonatadas que aportan materialers al talud y a las áreas de centro de cuenca. Oligoceno tardío - Mioceno tardío: Se inicia una etapa de tranquilidad tectónica con inversiones estructurales que indican una subsidencia continua y general de la cuenca a partir del Mioceno temprano. En el Mioceno tardío se da la emersión de gran parte de la cuenca debida a sobrecorrimientos relacionados con el desarrollo del cinturón deformado de Panamá y parcialmente a la inicio del levantamiento del intrusivo de Talamanca, la sedimentación característica es deltaica. Plioceno - Reciente. Los esfurzos compresivos continuan actuando provocando fuertes levantamientos y desarrollo de depresiones o pequeñas cuencas intermontanas marino someras, donde se da una sedimentación de bahías y fan deltas. Finalmente la cuenca es colmatada por la progradación de abanicos aluviales.
Address of authors: Refinadora Costarricense de Petroleo S.A. (Recope), Apdo 4351-1000, San Jose, Costa Rica, America Central.
Claudio Calvo & Angela Bolz
Abstract
This paper presents evidence for an Albian to Campanian explosive calcalkaline island arc volcanism in Costa Rica. The evidence comes from the Loma Chumico Formation which is an island arc sedimentary succession within the Upper Nicoya Complex. It is composed of radiolarite, clayey radiolarite, siliceous limestone, radiolarian claystone, black shale, and tephra as well as epiclastic deposits. Three main facies of tephra deposits can be distinguished: pyroclastic flows, fallout deposits, and tuffaceous hemipelagic sediments. Tephra deposits principally consist of volcanogenic material but may contain minor amounts of non-volcanic components. Glass shards are composed by highly vesicular pumice fragments, fragments of broken bubble walls and shards with perlitic craks. Pumice shards include particles with tabular, subparallel vesicles as well as particles with subspherical vesicles. These features indicate that vitric shards were generated by vesiculation of silicic magma in a subaerial environment. Crystal fragments and phenocrysts comprise plagioclase, K-feldspar, quartz, pyroxene and minor amounts of hornblende and biotite. Co-magmatic lithic clasts are dominated by recrystallized trachyte/latite fragments and polycrystalline quartz grains. We interpret this fabric as the product of a low-grade metamorphic overprint. The explosive volcanic activity corresponds to a magmatic arc which probably was located in the area of the present Cordillera de Tilarán. The tephra deposits indicate a highly differentiated rhyodacitic magma composition. The Albian to Campanian sedimentary succession reveals that calcalkaline volcanic activity was continuous. This in turn leads to the conclusion that the Farallón Plate was equally continuously subducted beneath southern Central America since at least the Albian. In combination with Sick's paleomagnetic data, which prove that the Upper Nicoya Complex was formed in an equatorial position, the volcanic activity recorded by the Loma Chumico Formation points to an in-situ origin of the oceanic crust under the proto-Caribbean seaway.
Adress of authors: Institut für Geologie und Paläontologie, Universität Stuttgart, Herdweg 51, D-70174 Stuttgart
Thomas W. Donnelly
Abstract
The Nicoya Complex is a part of the Caribbean Cretaceous basaltic province. Available information suggests that voluminous basalts were erupted in the Venezuelan Basin and westward to the ocean adjacent to presentday Central America and Columbia in the middle Cretaceous. The entire province is one of the largest of the Phanerozoic Large Igneous Provinces (LIP) and is of approximately the same age (dominantly Cenomanian - Coniacian) as similar provinces in the western Pacific Ocean. Contrary to previous statements, there is no firm evidence that the "Lower Nicoya" contains igneous rocks older than those of the "Upper Nicoya". The lower contact of the intervening chert sequence is nowhere demonstrably a true stratigraphic contact. Doubtless some of the basaltic rocks of the "Lower" complex are older, but these have not yet been identified. There are three recognizable petrologic types within the basalt complex. All of these are represented elsewhere in the Caribbean. One is close to N-MORB, differing mainly in Nd isotopes. The second is more alkalic and characterized by higher LIL and an inclined REE pattern; it is characteristic of the latest magmas of this event. The third is Mg-rich and approaches the komatiitic rocks of Curaçao and Isla Gorgona. Part of the failure to recognize all of these types as basaltic results from the wide range of Mg numbers of the province. There is no good evidence that any of these rocks originated in an island arc setting. The major task facing us is to establish the age of inception of this event, in Costa Rica and elsewhere.
Address: Department of Geological Sciences, State University of New York, Binghamton NY 13902-6000
Rudolf Fischer & Teresita Aguilar
Abstract
The tectonic evolution of the Central American Isthmus controlled the development of marine faunal associations within the Pacific and the Caribbean Oceans and their predecessor, as well as of the terrestrial life within the area between the two oceans. Additionally, sea level changes and alterations of oceanographic factors influenced the biological evolution. The fossil record of Central America documents such evolutionary processes through faunal associations which seem to be specific for the colonization of island arc environments.
Addresses:
Fischer: Institut für Geologie und Paläontologie, Callinstraße 30, D-30167 Hannover (Germany)
Aguilar: Escuela Centroamericana de Geología, Universidad de Costa Rica, Apdo 35, San Pedro Montes de Oca (SJ), América Central
Hans-Jürgen Gursky
Abstract
Sequences of radiolarian chert of ?Lower Jurassic to Late Cretaceous ages are minor, but significant intercalations in the Nicoya Ophiolite Complex exposed along the Pacific coast of Costa Rica and western Panama. The radiolarites overlie ridge-derived tholeiites (oceanic crust) and are overlain and intensively intruded by younger within-plate and island-arc basalts. Monotonous, rhythmically bedded, relatively pure, biosiliceous material in sections of some tens of meters in thickness is dominant. Much thicker and detritus-rich sections are regionally present. Bedding characteristics reflect deposition by low-velocity, low-density turbidity currents and/or bottom currents. Slumping and coarse-grained clastic intercalations indicate local rough topography. Detritus is mostly of intraoceanic origin. Generally high biosiliceous purity, scarcity of terrigenous input, relatively high hematite content, local presence of sedimentary manganese nodules, low-energy hydrodynamic conditions, and generally low vesicularity of associated basalts suggest an O2-rich, tranquil, deep-sea, basinal realm below the CCD. Local bituminous horizons reflect deposition under temporally stagnating conditions. The cherts were probably deposited at moderate to large distances from land areas in the equatorial Paleopacific Ocean and shifted by sea-floor spreading to their actual position. Mode of final emplacement, stratigraphic significance, and structural position within the ophiolite basement of Southern Central America are under discussion.
Address: Geologisch-Paläontologisches Institut, Technische Hochschule Darmstadt, Schnittspahnstr. 9, D-64287 Darmstadt
Henry Hooghiemstra
Abstract
Pollen records of two deep boreholes, Funza I (357m) and Funza II (586m) from the high plain of Bogotá (2550 m altitude, Colombia) document Late Pliocene through Latest Quaternary climatic change. Five zircon fission track dates from intercalated volcanic ash provide absolute time control. Both records correlate with high precision and represent the interval from 3.2 Ma through ca. 24 ka BP. A provisional graphical correlation of the Funza I arboreal pollen record with ODP Site 677 ∂18O record is shown for the last 1.2 Ma. The history of the montane forests and open (alpine) paramo vegetation is documented with a temporal resolution of ca. 6-5 ka; for the upper 1.1 Ma with a resolution of ca. 1.2 ka. The climate-stratigraphical sequence and biozonation of the Bogotá sediments, spanning the last ca. 5 Ma (Van der Hammen et al., 1973) is confirmed and additionally documented. Evidence of the upheaval of the Eastern Cordillera and closure of the Panamanian Isthmus is shown. The immigration of Alnus (at ca. 1 Ma) and Quercus (at ca. 0.33 Ma), originating from the northern hemisphere, caused significant changes in the composition of the Andean montane forests. The Pleistocene glaciations forced the Andean vegetation belts to an almost continuous altitudinal movement; the upper forest line (at present at ca. 3200 m) shifted between 1800 m (glacials) and ca. 3500 m (interglacials), corresponding to a maximum variation in temperature between ca. 5°ree; and 15°ree;C at 2550 m altitude.
Address: Department of Palynology and Paleo/Actuo-Ecology, University of Amsterdam, Kruislaan 318, 1098 SM Amsterdam, The Netherlands.
Abstract
The El Fraile Formation represents continental to shallow-marine siliciclastic environments, which were established during the Miocene in the forearc area of Southwestern Nicaragua. The simultaneous occurrence of shelfal, estuarine-deltaic, and alluvial systems point to a more or less equilibrated open shoreline system. Facies relations, stratal architecture, and stacking patterns of the depositional systems suggest an arrangement into two depositional sequences which result from cycles of relative sea-level change. The basal part of the sequences is made up by retrogradational parasequences, representing consecutive flooding events. This transgressive tendency culminates in an amber-bearing condensed layer formed during maximum transgressive onlap of marine beds. Most portions of the sedimentary suite record the sea-level rise relatied to a transgressive-regressive cycle. The record of sea-level fall, on the other hand, is restricted to a thin portion of the section revealing progradational parasequences.
Addresses:
Krawinkel, H: Institut für Geologie und Paläontologie der Universität, Herdweg 51, D-70174 Stuttgart
Kolb: Ingenieurbüro Asal + Partner, Barbarossastraße 30, 67655 Kaiserslautern
Justus Krawinkel & Hartmut Seyfried
Abstract
The present configuration of tectonic elements in the southern Central American landbridge is the product of crustal mobility driven by escape tectonics and coastwise transport of forearc units. Six different tectonic segments are described in detail. The main structural elements are trans-isthmic and trench-parallel strike-slip faults as well as fold and thrust belts. The tectonic history of each segment and their assemblage into the present configuration at the southwestern edge of the Caribbean Plate is reviewed in detail.
Address: Institut für Geologie und Paläontologie der Universität Stuttgart, Herdweg 51, D-70174 Stuttgart, Germany
Siegfried Kussmaul, Jean Tournon & Guillermo Alvarado
Abstract
Lava flows and pyroclastic rocks of the Aguacaste Group (Oligocene - Miocene) are the product of the evolution of a primitive island arc (represented by low-K tholeiites) towards andesites to dacites. This transformation was accompanied by gabbroic to granitic intrusives, mainly of Miocene age. During Pliocene and Pleistocene time much of the northern and central part of Costa Rica was covered by large volumes of andesitic to rhyolithic ignimbrites. The stratovolcanoes of the Cordillera de Guanacaste and the Cordillera Central were built up towards the end of Pleistocene. Based on their chemical composition, the Plio-Pleistocene volcanic rocks can be divided into 4 geochemical groups: (a) dacitic to rhyolithic ignimbrites of Guanacaste overlain by a chain of stratovolcanoes, composed of medium-K andesites; (b) andesitic to dacitic ignimbrites of the Valle Central overlain by composite volcanoes, composed mainly of K-rich basaltic andesites; (c) low-K basalts to dacites of the Arenal-Chato volcanoes, situated between the two main volcanic chains; (d) alkaline basalts concentrated in the backarc area along N-S trending fracture zones and along the Costa Rican Transcurrent Fault System. The basalts and part of the basaltic andesites are the product of partial melting of the upper mantle. Much of the intra-edifice variation in lava composition can be modeled by fractional crystalization of olivine, clinopyroxene, orthopyroxene and plagioclase, with smaller amounts of titanomagnetite and rare apatite. Processes such as magma mixing, contamination with subducted oceanic crust and sediments, and the assimilation of subvolcanic rocks also affected the magma composition. The geochemical differences between the two volcanic chains seem to be the result of geotectonic factors and different magma sources.
Addresses:
Kussmaul: Escuela Centroamericana de Geología, Universidad de Costa Rica, Apdo. 35, 2060 Ciudad Universitaria, Costa Rica.
Tournon: Laboratoire de Petrographic, Université P. et M. Curie, 4 Place Jussieu, 75252 Paris, France.
Alvarado: Dept. Geología, Instituto Costarricense de Electricidad, Apdo. 10032, 1000 San José, Costa Rica.
Spencer G. Lucas & Guillermo E. Alvarado
Abstract
Prior to the late Cenozoic, Central America played a minor role in the dispersal of land vertebrates between North and South America. During the latest Cretaceous-Paleocene and the Eocene-Oligocene, when significant land vertebrate dispersal took place between the Americas, the Central American arc was a volcanic archipelago isolated by significant oceanic gaps from either North or South America or from both. Land-vertebrate dispersal, other than some possible island hopping, probably took place via the more extensive antillean arc to the east. Central America first emerged as a significant route for land-vertebrate dispersal during the Miocene when it was a dense, volcanic archipelago traversed by island hopping mammals. During the Pliocene-earliest Pleistocene (about 3.7-1.5 Ma), extensive interchange of vertebrates between North and South America took place via Central America. This "great American interchange" resulted from the completion of a Central American land bridge between the two continents prior to the late Pliocene and is reflected well, but incompletely, by the Plio-Pleistocene mammal faunas of Central America. The meagre record of Plio-Pleistocene mammals from Central America needs to be augmented in order to evaluate better the role of Central America as a filter to dispersal and as a center of evolution during the great American interchange.
Addresses:
Lucas: New Mexico Museum of Natural History, 1801 Mountain Road N.W., Albuquerque, NW 87104, USA.
Alvarado: Dept. Geología, Instituto Costarricense de Electricidad, Apdo. 10032, 1000 San José, Costa Rica.
Geochemical characteristics of basaltic rocks from the Central American ophiolites
Martin Meschede & Wolfgang Frisch
Abstract
The ophiolite complexes along the Pacific coast of Costa Rica and Panama contain basaltic rocks (dolerites, massive flows, pillow basalts) which chemically resemble mid-ocean ridge basalts, island-arc tholeiites, within-plate tholeiites, and alkali basalts. Mid-ocean ridge basalts, together with ultramafic (partly cumulate) rocks, gabbros, plagiogranites, and pelagic sedimentary rocks constitute the oceanic basement of presumed Jurassic age. A tendency towards high-field strength element depletion in many of the basalts of the oceanic basement may be interpreted in terms of contribution of a subduction component. Basalts displaying a within-plate tholeiite character correlate with similar basalts elsewhere in the Caribbean attributed to the Cretaceous sill event that thickened the oceanic crust of the Caribbean plate. A within-plate environment of formation is also indicated by alkaline basalts from Santa Elena and within-plate tholeiites from Quepos. The Quepos basalts differ from the other within-plate tholeiites in composition and age. This complex is interpreted as an accreted seamount.
Addresses: Institut für Geologie und Paläontologie, Universität Tübingen, Sigwartstr. 10, D-72076 Tübingen, Germany.
Walter Montero
Abstract
The neotectonics of Costa Rica and the related maximum horizontal compressive stress directions (MHCSD), are evaluated through structural, seismological and geomorphological data. In the Pacific forearc region, the MHCSD is trending 030°ree;E. This direction agrees with the relative plate velocity vector. The MHCSD trend changes along the inner arc. In the Guanacaste Quaternary Volcanic Cordillera, in northwestern Costa Rica, the MHCSD is not well defined according to the evaluated data. Weak coupling along the interplate boundary could explain why plate boundary forces are not well transmitted to the inner arc. In the central and southeastern Costa Rican inner arc area (Central Quaternary Volcanic Cordillera and Talamanca range), the MHCSD rotates between N-S and NE respectively. Stresses are related to the collision of the oceanic Cocos Ridge with southern Costa Rica. In this paper I propose that the Cocos Ridge behaves as a rigid indenter and the region of central and southern Costa Rica as a rigid-plastic material. Strike slip faults and volcanic lineaments of central and southern Costa Rica correspond with the slip lines and the MHCSD of the theoretical model, respectively. Overthrusts are nearly perpendicular to the MHCSD predicted. Finally, a discussion is presented about the tectonic role of the shear zone that occurs throughout central Costa Rica. The geometrical distribution of faults in this region (with predominant trends NE-SW to E-W) suggests that this shear zone represents the western border of the Panama microplate. Alternatively, it could be interpreted as a fault system transferring slip motion from the Pacific to the Caribbean side of the isthmus, as a consequence of the Cocos Ridge collisional event.
Address: Escuela Centroamericana de Geología, Universidad de Costa Rica, Red Sismológica Nacional (RSN, ICE-UCR), Centro de Investigaciones Geofísicas, Apartado 36, 2060 Ciudad Universitaria, Costa Rica, América Central
Luis Guillermo Obando, Giovanni Bottazzi & Fernando Alvarado
Resumen
En el área del Valle Central de Costa Rica, aflora una secuencia principalmente arenosa muy rica en e carbón (lignitos 0,3 a 1 m de espesor) y lutitas muy ricas en materia orgánica. La reinterpretación de las secuencias aflorantes y el aporte de nuevos datos, permitió definir las siguientes Facies sedimentarias: Facies de Plataforma silicoclástica, Facies de Bahía Abierta, Facies de Frente de Barra (Bar Front), Facies de Barras Arenosas Costeras (Playa y Tras playa). Estas facies en su conjunto caracterizan un "Strand Plain" generado por corrientes interoceánicas que se desarrollaban, en esa época, a través de un canal genéticamente relacionado, a una falla de desplazamiento de rumbo sinestral. Dicha falla segmentaba el territorio y generaba una cuenca "Pull-apart", que por su alta subsidencia acumulaba espesos depósitos de sedimentos someros.
Addresses:
Escuela Centroamericana de Geología, Universidad de Costa Rica, Apartado 35, 2060 Ciudad Universitaria, Costa Rica, América Central
Bottazzi, Alvarado: Refinadora Costarricense de Petroleo S.A. (Recope), Apdo 4351-1000, San Jose, Costa Rica, America Central.
Hartmut Seyfried, Hannelore Krawinkel & Teresita Aguilar
Abstract
The Punta Judas Formation originated within a coastal embayment on the Pacific side of the southern Central American isthmus. During the Middle Miocene, more than 1300 m of mainly shallow marine fine sandstones accumulated within this embayment. Two main types of stratigraphic bounding surfaces can be distinguished: a) erosional boundaries and non-erosional transitions between depositional systems (which are defined by sedimentological criteria) and b) fossiliferous bounding surfaces (which are defined by palaeoecological or taphonomical criteria; mainly shell concentrations). Erosional boundaries were produced by block tilting, tectonic reactivation of cliffs, or wave and tidal ravinement during sea level rises, and are interpreted as sequence boundaries and bases of parasequences sets, respectively. Non-erosional transitions delimit genetically closely related depositional systems such as, for instance, the delta-related systems. They are mostly developed as gradual or interfingering contacts, more seldom as sharp contacts (diastems) and considered as a product of eustatic changes of sea level or autocyclic shifts of facies belts. Sharp contacts are considered as bases of parasequence sets, diffuse contacts are interpreted as parasequences. Fossiliferous bounding surfaces are represented by autochthonous and allochthonous shell concentrations. Autochthonous shell concentrations are commonly developed as concretion horizons. They contain layers with epifaunal and semi-infaunal mollusc associations in growth position and layers with infaunal bivalve associations in growth position. They invariably signal a relative rise of sea level and are interpreted as bases of parasequences or parasequence sets. Allochthonous shell concentrations are represented by shell concentrations produced by longshore shelf currents, tempestitic shell concentrations, nearshore wave-swept shell concentrations, and shell concentrations produced by tidal currents. Their appearance in certain intervals of the section, and disappearance in others, is clearly a signal of shallowing or deepening seas. Only the shell concentrations produced by longshore shelf currents can be interpreted as bases of parasequences; the other allochthonous shell concentrations have no sequence stratigraphic significance. Ichnofabrics can be used as an additional tool for the identification and interpretation of bounding surfaces. Erosional boundaries are marked by well-known features such as Gastrochaenolites; sequence boundaries can be preceeded by Thalassinoides boxworks strongly engrossed by concretionary growth. At non-erosional boundaries trace fossil assemblages are most characteristic under transgressive control: After prolonged swamping with sand during relative sea level lowstands, trace fossils "explode" in diversity during transgressions. Trace-making organisms seem to have been busiest after the dumping of organic matter during the highstand; at this stage they commonly rework their habitat to the degree of total destratification. The degree of reworking by bioturbation seems to be a very good and easily recognizable indicator of the internal architecture of parasequences and parasequence sets.
Addresses:
Seyfried, Krawinkel: Institut für Geologie und Paläontologie, Universität Stuttgart, Herdweg 51, 70174 Stuttgart, Germany
Aguilar: Escuela Centroamericana de Geología, Universidad de Costa Rica, Apartado 35, 2060 Ciudad Universitaria, Costa Rica, América Central
Peter Sprechmann, Allan Astorga, Claudio Calvo & Alberto Fernandez
Abstract
Costa Rican sedimentary rocks deposited since the Campanian are subdivided into supergroups, groups, subgroups, and formations. The Curime, Boruca and Cerere supergroups are unconformity-bounded units. The preexisting number of formations is considerably reduced. The remaining rock units described in this paper are defined by their depositional environment, using differential sedimentologic criteria. The use of sequence stratigraphic terminolgy is discussed.
Project coordinated by the authors listed below. Further members of the working group are listed in the legend to Fig. 1.
Addresses:
Sprechmann, Fernandes: Rufino Bauzá 2460, 11300 Montevideo, Uruguay
Astorga: Recope, Apartado 4351, 1000 San José, Costa Rica
Calvo: Institut für Geologie, Universität Stuttgart, Herdweg 51, 70174 Stuttgart, Germany
Jean Tournon
Abstract
The Santa Elena peninsula, northern Pacific coast of Costa Rica, is formed by three structural units: a relative autochthonous unit, an allochthonous peridotitic unit and a sedimentary cover (Campano/Maastrichtian to Paleogene in age). The autochthonous unit crops out as tectonic windows in the Center and the South of the peninsula. It consists of strongly folded layered radiolarites and cherts, Lias-lower Dogger to Cenomanian in age. Various sequences of dykes and pillowed basaltic flows display alkaline compositions. The allochthonous unit is a 35 km long peridotitic massif. Most peridotites are diopside bearing harzburgites and lherzolithes and correspond to relatively poorly depleted mantle rocks. Plagioclase peridotites and scarce dunites also occur. Pyroxenitic layers parallel the foliation of the enclosing peridotites. The mantle sequence is cut by dykes composed of ultramafic cumulates, pegmatitic gabbros and very abundant dolerites. A layered sequence made of cumulates, gabbros and scarce plagiogranites is overthrust by the peridotites. The allochthonous unit does not present the stratigraphy of a classical ophiolitic complex and the various sequences of mafic rocks are not cogenetic. Similar mantle peridotites occur in the Río San Juan area. The possible occurence of a 150 km long East-West peridotitic suture is discussed. Such a structure could be the result of the Senonian closure of an "oceanic Basin" by convergence between the Southern Central American block and the Chortis block.
Address: Département de Pétrologie, Université Pierre et Marie Curie,4 place Jussieu, 75252 Paris cedex 05, France.
Roland von Huene & Ernst R. Flüh
Abstract
The history of marine geophysical studies-including scientific bathymetry - shows the steady development of tectonic knowledge about the convergent margin of Costa Rica. Major features of the ocean floor were known 30 years ago and the proposed plate tectonic history of the ocean crust has not changed much during a decade. However, in the last few years it was realized that the character of the subducting plate greatly influences continental margin structure. The consequences of subducting Cocos Ridge is the broad uplift of the margin opposite Osa Peninsula; the consequences of subducting the adjacent seamounts is a rugged continental margin topography. These margin morphologies contrast with the simple slope topography where relatively featureless ocean crust is subducted. Here its structure consists of a frontal accretionary complex, a non-reflective margin wedge, and a cover of slope deposits. This basic margin structure was affected by subducting plate relief to produce morphological and structural differences. An unanswered question is how far seaward the igneous complex on the Nicoya Peninsula extends and whether a body of highly consolidated accreted sediment not known on land could be hidden in the margin wedge. Seismic velocities somewhat higher than those generally observed across Neogene accretionary wedges is revealed by new wide angle seismic data. This seismic velocity structure is like that off Guatemala where ophiolitic rock has been sampled across the margin. The Costa Rican margin wedge has reacted to convergent margin tectonism as a semi-rigid body since middle Miocene time, subduction of sediment is efficient, and accretion occurs very slowly.
Addresses: GEOMAR Forschungszentrum für Marine Geowissenschaften der Christian-Albrechts-Universität zu Kiel, 24148 Kiel
Jutta Winsemann
Abstract
Data from southern Central America indicate a Caribbean provenance of the anomalously thickened Caribbean crust. Rocks from the early Caribbean ocean floor are recorded from northern Costa Rica and are present as imbricated structural units, which are interpreted to be the remains of a late Jurassic to Cenomanian accretionary prism. The existence of such an accretionary prism in northern Costa Rica can best be explained by a north-to northeasterly subduction of Caribbean crust underneath the southern Chortís block. A contemporaneous south to southeasterly dipping subduction system at the north-western margin of South America is indicated by the occurrence of early Cretaceous metamorphites and primitive island arc rocks in Panama and the Dutch and Venezuelan Antilles. At the end of the early Cretaceous the Caribbean region was affected by an episode of within-plate volcanism, which created a plateau-like structure and was presumably linked to a slowed-down spreading between the North and South American plates. As a result, subduction reversal occurred at the northern Caribbean margin and a sinistral stike-slip system developed at the former convergent margin of the Chortís-Antilles arc, represented today by the Hess-Escarpment. After the north Caribbean subduction reversal, the Central American subduction system, from which the later Central American island arc emerged, probably formed as early as Turonian. During the Campanian the divergent movements between the North and South American plates stopped. The subsequent formation of a compressive regime (roughly running east-west) caused a major structural change in the Caribbean region and led to arc-continent collisions at the northern and southern Caribbean margins. The Central American island arc was uplifted and southward thrusts took place along the Hess-Escarpment in northern Costa Rica, where fragments of the old subduction complex of the Chortís-Antilles arc were thrusted onto younger structural units. The contemporaneous strong uplift, which can be observed all over southern Central America, can most probably be related to the collision of the Central American island arc with South America, which resulted in the accretion of the Western Cordillera of Colombia. As a response, subduction processes ceased in the southernmost Central American island arc segments and a transform margin formed during the Paleocene. After the accretion of the Western Cordillera was completed, a new subduction system evolved at the beginning of the Middle Eocene and was associated with the onset of volcaniclastic sedimenation in southern Costa Rica and Panama. The subsequent collsion of the Central American island arc with South America during the Miocene resulted in the accretion of the Serrania de Baudó and the formation of the Panama Deformed Belt.
Address: Geologisch-Paläontologisches Institut und Museum, Universität Hamburg, Bundesstrasse 55, D-20146 Hamburg, Germany
Jutta Winsemann
Abstract
Trench slope deposits of the early Central American island arc are exposed along the pacific coast of the Nicoya Peninsula and were deposited on the seaward margin of an outer structural high, which emerged during the Campanian. Two different basin-fill systems can be recognized in the southwestern Nicoya Peninsula. 1) The Sámara basin in the northwestern part of the study area, where a coarse-grained channel-lobe complex developed mainly during the Paleocene. The lobe deposits consist of massive or graded sandstones, which are organized into thickening- and coarsening-upward cycles. Channels have been filled primarily by conglomeratic debris-flows, high-density turbidity currents and slump masses. Associated are poorly developed overbank deposits.The initial position of the feeding canyon was most probably structurally controlled and the arc-derived sediment load completely bypassed the inner forearc basin. The rapid rise of the sea-level towards the end of the Late Paleocene caused the deactivation of this canyon system. Subsequently only fine-grained siliceous limestones and mudstones were deposited in the Sámara basin. 2) In contrast, the sedimentary sucession of the Cabo Blanco basin in the southwestern part of the Nicoya Peninsula is dominated by slump masses, thin-bedded turbidites, hemipelagic mudstones, black organic-rich shales and short-lived shallow channels. Deposits display a slope parallel arrangement of coarse to fine-grained facies and thick turbidite sand units are restricted to a small feeding canyon. As in the inner forearc basins, second order sequence boundaries can be correlated basin-wide and coarse volcaniclastic material reached the trench slope only during episodes of relative lowstands of sea-level. During transgressions and highstands of sea-level the structural high became a favoured place of carbonate production.The resedimentation of these calcareous muds led, during periods of relative sea-level highstands to a carbonate-dominated sedimentation and thus the formation of hybrid carbonate-clastic sedimentation cycles, which can likewise be observed along the landward margin of the structural high. Owing to lower sedimentation rates, eustatic signals are generally less masked than in the inner forearc basins and tectonic effects can be more easily recognized at the base of clast composition. The greatest variable in the sediment distribution was the size and regional distribution of submarine canyons. Depending on the location of these feeding systems, complete channel-lobe sequences developed or locally derived mass-flow deposits and thin-bedded turbidites prevailed.
Address: Geologisch-Paläontologisches Institut und Museum, Universität Hamburg, Bundesstrasse 55, D-20146 Hamburg
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Last changes 22. Mai 1999 by Steffen Kutterolf