Bull Volcanol DOI 10.1007/s00445-006-0053-x RESEARCH ARTICLE Wendy Pérez . Guillermo E. Alvarado . Phillip B. Gans The 322 ka Tiribí Tuff: stratigraphy, geochronology and mechanisms of deposition of the largest and most recent ignimbrite in the Valle Central, Costa Rica Received: 9 February 2004 / Accepted: 8 February 2006 # Springer-Verlag 2006 Abstract The Tiribí Tuff covered much of the Valle trachyandesites are predominant. Replicate new 40Ar/39Ar Central of Costa Rica, currently the most densely populated age determinations indicate that widespread exposures of area in the country (~2.4 million inhabitants). Underlying this tuff represent a single ignimbrite that was erupted the tuff, there is a related well-sorted pumice deposit, the 322±2 ka. The inferred source is the Barva Caldera, as Tibás Pumice Layer. Based on macroscopic characteristics interpreted from isopach and isopleth maps, contours of the of the rocks, we distinguish two main facies in the Tiribí ignimbrite top and geochemical correlation (~10 km in Tuff in correlation to the differences in welding, devi- diameter). The Tiribí Tuff caldera-forming eruption is in- trification, grain size, and abundance of pumice and terpreted as having evolved from a plinian eruption, during lithic fragments. The Valle Central facies consists of an which the widespread basal pumice fall was deposited, ignimbritic plateau of non-welded to welded deposits followed by fountaining pyroclastic flows. In the SW part within the Valle Central basin and the Orotina facies is a of the Valle Central, the ignimbrite flowed into a narrow gray to light-bluish gray, densely to partially welded rock, canyon, which might have acted as a pseudo-barrier, re- with yellowish and black pumice fragments cropping out flecting the flow back towards the source and thus mainly at the Grande de Tárcoles River Gorge and Orotina thickening the deposits that were filling the Valle Central − plain. This high-aspect ratio ignimbrite (1:920 or 1.1×10 3) depression. The variable welding patterns are interpreted to covered an area of at least 820 km2 with a long runout of be a result of the lithostatic load and the influence of the 80 km and a minimum volume outflow of 25 km3 (15 km3 content and size of lithic fragments. DRE). Geochemically, the tuff shows a wide range of compositions from basaltic-andesites to rhyolites, but Keywords Tiribí Tuff . Ignimbrite . Welding facies . Channelized . 40Ar/39Ar geochronology . Barva caldera . Costa Rica Editorial responsibility: C. Kilburn W. Pérez (*) SFB 574, University of Kiel and Leibniz Institute Introduction for Marine Science (IFM-GEOMAR), Wischhofstr. 1-3, 24148 Kiel, Germany The Valle Central (Central Valley) in central Costa Rica, a e-mail: [email protected] volcanic plateau underlain mainly by the Tiribí Tuff, constitutes the most densely populated area of the country, G. E. Alvarado hosting more than 2.4 million people. Until now, little was Área de Amenazas y Auscultación Sismo-volcánica, Instituto Costarricense de Electricidad. Apartado, known about stratigraphy, precise age, source area, trans- 10032-1000 San José, Costa Rica port and, depositional characteristics of this tuff. Since it forms terraces that have been displaced by W. Pérez . G. E. Alvarado active faults, the Tiribí Tuff is important for its implications Escuela Centroamericana de Geología, in the volcanic history of the region and its utility in Universidad de Costa Rica, San José, Apartado 35-2060 UCR, Costa Rica neotectonic studies (Borgia et al. 1990; Marshall et al. e-mail: [email protected] 2003). Also, due to its relatively low-permeability, the Tiribí Tuff protects the Valle Central aquifers, located in the P. B. Gans underlying Colima lavas (Fernández 1968; Echandi 1981), Department of Geological Sciences, University of California, from urban contamination. It also is an important source Santa Barbara, 93106-9560, USA of construction and road aggregate producing about 1.9 e-mail: [email protected] million tons of aggregates per year (Berrangé et al. 1990). Despite its economic and scientific importance, as well as analyses, we propose that the Barva caldera was the source the presence of well-exposed artificial and natural out- of this ignimbrite. crops, there have been virtually no studies on its eruptive and emplacement history, and a precise age has not been firmly established. Methods This work, which focuses on stratigraphic and geochro- nologic aspects of the Tiribí Tuff, complements a geo- The Tiribí Tuff, as well as the overlying and underlying chemical and petrological study made by Hannah et al. volcanic units of the Valle Central and Cordillera Central, (2002). The Tiribí Tuff comprises pyroclastic deposits that were examined in detail at 76 localities and 7 boreholes, in crop out in the lower SW flanks of Barva volcano, in deep an area of about 900 km2. Sizes of the five largest gorges into the Valle Central plateau, and distal exposures fragments (pumice and lithic), percentage of components along the Grande de Tárcoles River, including the Orotina (ash to block/bomb size, juvenile and lithic fragments) and plain near the Pacific coast (Fig. 1). We use the term “Tiribí thickness were measured. In addition to the outcrops and Tuff” to denote only the youngest and most widespread of boreholes examined, we also used the well logs of 233 the Valle Central ignimbrites, which is part of the Tiribí water-supply boreholes from the database of the SENARA Formation of Fernández (1968) and Echandi (1981)orof (National Service of Waters, Irrigation and Draining) to what has been also referred to as “Glowing Avalanche determine thickness and locate the top of this unit. Isopach Deposits” by other authors (Williams 1952; Kussmaul and and ignimbrite top-height maps, as well as lithic and Sprechmann 1982). Several older ignimbrites have also juvenile-fragment isopleth maps, were made for the Tiribí been identified in the Valle Central region and are de- Tuff in order to assess its volume and source area. Bulk scribed elsewhere (Gans et al. 2003; Vogel et al. 2004). densities of whole-rock and juvenile fragments were mea- The main purpose of this investigation is to clarify the sured in the laboratory. Petrographic examination of 50 eruptive and transport history of this unit, which con- samples from a broad spectrum of positions in the tuff stitutes perhaps the most explosive pyroclastic event of were conducted to document lateral and vertical variations central Costa Rica. In addition, we present the results in the modal mineralogy and textural characteristics. of a detailed 40Ar/39Ar dating investigation of the tuff and Seven samples from widely separated sections of the tuff, units directly underlying and overlying it. We also describe representing the entire range of facies, were dated by the interaction of a widespread ignimbrite with a variable the 40Ar/39Ar method, using both step heating and total- forested area and determine the effect of a canyon system fusion experiments on high-purity plagioclase separates to during a rapid aggradation of an ignimbrite deposit. establish its precise age. Finally, based on geological and geomorphological map- ping, contours of the height of the top, isopleth and isopach maps, as well as geochemical and geochronological Fig. 1 Geologic map of the Valle Central and Cordillera Central, including volcanic structures and faults. The Tiribí Tuff is exposed mainly along the canyons of the Virilla and Grande de Tárcoles rivers, while in the southwestern slope of Barva volcano, it is shown only in the area where it has been detected in water-supply bore- holes. The inset shows the location of Ledbetter (1985) piston cores where the layer I6 was found (correlated with the Tiribí Tuff). MAT Middle America Trench, CC Cordillera Central, VC Valle Central, CG Cordillera de Guanacaste, CT Cordillera de Talamanca, Pl Platanar volcano, Por Porvenir volcano, Po Poás volcano, Co Congo volcano, Hu Hule Maar, Ba Barva volcano, BC Barva Caldera, CN Cacho Negro volcano, Iz Irazú volcano, Tu Turrialba volcano Geological setting La Caja Members corresponds with different degrees of welding, while Nuestro Amo Member is a non-related Since at least 80 Ma, Costa Rica, located on the Central debris flow deposit. Kussmaul and Sprechmann (1982) America isthmus, hosts a mature volcanic arc formed as the suggested a new formal name of “Formación Depósitos result of the subduction of the Cocos Plate beneath the de Avalancha Ardiente” (Glowing Avalanche Deposits Caribbean Plate (Alvarado et al. 1992; Tournon and Formation). Madrigal (1970) defined the Orotina Forma- Alvarado 1997). Subduction of the Cocos Plate is re- tion consisting of ignimbrites close to the Pacific coast. sponsible for active volcanism in Costa Rica, mainly in Marshall et al. (2003), based in geologic mapping, cor- northern and central Costa Rica (Cordillera de Guanacaste relation columns and new 40Ar/39Ar dating, established and Cordillera Central). The Cordillera Central has been that what they call “Snake Flow Member” of Orotina built up by several andesitic shield volcanoes and strato- Formation can be traced from Orotina plain to the Valle volcanoes, some of which have erupted during historical Central, where it merges with the Tiribí Tuff. Geological time (e.g., Poás, Irazú and Turrialba), and Holocene time mapping of the Tiribí Tuff was done by Madrigal (1970), (e.g., Hule and Barva; Fig. 1). Echandi (1981), Denyer and Arias (1991) and Tournon and The Valle Central is an elongated volcanic plateau, Alvarado (1997), among others. The first chemical analysis 50 km long by 20 km wide, limited to the north by the was made by Schaufelberger in 1935 (Dengo and Chaverri Cordillera Central, and to the south by a Tertiary volcano- 1951), and a detailed petrologic study was made by sedimentary range. Quaternary volcanic activity began Hannah et al. (2002). around 1.0 Ma in the Cordillera Central, whose Middle Pleistocene history was characterized by alternating periods of andesitic effusive volcanism (constructional) Characteristics of the Tiribí Tuff and short-lived silicic pyroclastic eruptions (Alvarado et al.