Sedimentology (2007) 54, 1191–1222 doi: 10.1111/j.1365-3091.2007.00878.x Lacustrine sedimentation in active volcanic settings: the Late Quaternary depositional evolution of Lake Chungara´ (northern Chile) A. SA´ EZ*, B. L. VALERO-GARCE´ S ,A.MORENO ,R.BAOà, J. J. PUEYO*, P. GONZA´ LEZ- SAMPE´ RIZ ,S.GIRALT§,C.TABERNER§,C.HERRERA– and R. O. GIBERT* *Facultat de Geologia, Universitat de Barcelona, c/Martı´ Franques s/n, 08028 Barcelona, Spain (E-mail: [email protected]) Instituto Pirenaico de Ecologı´a, Consejo Superior de Investigaciones Cientı´ficas, Apdo 202, 50080 Zaragoza, Spain àFacultade de Ciencias, Universidade da Corun˜ a, Campus da Zapateira s/n, 15071 A Corun˜ a, Spain §Instituto de Ciencias de la Tierra ‘Jaume Almera’ – CSIC, c/Lluı´s Sole Sabaris s/n, 08028 Barcelona, Spain –Departamento de Ciencias Geolo´ gicas, Universidad Cato´ lica del Norte, Casilla 1280, Antofagasta, Chile ABSTRACT Lake Chungara´ (18°15¢S, 69°09¢W, 4520 m above sea-level) is the largest (22Æ5km2) and deepest (40 m) lacustrine ecosystem in the Chilean Altiplano and its location in an active volcanic setting, provides an opportunity to evaluate environmental (volcanic vs. climatic) controls on lacustrine sedimentation. The Late Quaternary depositional history of the lake is reconstructed by means of a multiproxy study of 15 Kullenberg cores and seismic data. The chronological framework is supported by 10 14C AMS dates and one 230Th/234U dates. Lake Chungara´ was formed prior to 12Æ8 cal kyr bp as a result of the partial collapse of the Parinacota volcano that impounded the Lauca river. The sedimentary architecture of the lacustrine succession has been controlled by (i) the strong inherited palaeo-relief and (ii) changes in the accommodation space, caused by lake-level fluctuations and tectonic subsidence. The first factor determined the location of the depocentre in the NW of the central plain. The second factor caused the area of deposition to extend towards the eastern and southern basin margins with accumulation of high-stand sediments on the elevated marginal platforms. Synsedimentary normal faulting also increased accommodation and increased the rate of sedimentation in the northern part of the basin. Six sedimentary units were identified and correlated in the basin mainly using tephra keybeds. Unit 1 (Late Pleistocene–Early Holocene) is made up of laminated diatomite with some carbonate-rich (calcite and aragonite) laminae. Unit 2 (Mid-Holocene–Recent) is composed of massive to bedded diatomite with abundant tephra (lapilli and ash) layers. Some carbonate-rich layers (calcite and aragonite) occur. Unit 3 consists of macrophyte-rich diatomite deposited in nearshore environments. Unit 4 is composed of littoral sediments dominated by alternating charophyte- rich and other aquatic macrophyte-rich facies. Littoral carbonate productivity peaked when suitable shallow platforms were available for charophyte colonization. Clastic deposits in the lake are restricted to lake margins (Units 5 and 6). Diatom productivity peaked during a lowstand period (Unit 1 and subunit 2a), and was probably favoured by photic conditions affecting larger areas of the lake bottom. Offshore carbonate precipitation reached its maximum during the Early to Mid-Holocene (ca 7Æ8 and 6Æ4 cal kyr bp). This may have been favoured by increases in lake solute concentrations resulting from evaporation Ó 2007 The Authors. Journal compilation Ó 2007 International Association of Sedimentologists 1191 1192 A. Sa´ ez et al. and calcium input because of the compositional changes in pyroclastic supply. Diatom and pollen data from offshore cores suggest a number of lake-level fluctuations: a Late Pleistocene deepening episode (ca 12Æ6 cal kyr BP), four shallowing episodes during the Early to Mid-Holocene (ca 10Æ5, 9Æ8, 7Æ8 and 6Æ7 cal kyr BP) and higher lake levels since the Mid-Holocene (ca 5Æ7 cal kyr BP) until the present. Explosive activity at Parinacota volcano was very limited between c. >12Æ8 and 7Æ8 cal kyr bp. Mafic-rich explosive eruptions from the Ajata satellite cones increased after ca 5Æ7 cal kyr bp until the present. Keywords Andean Altiplano, carbonate, diatomite, Holocene, lacustrine ecosystem, tephra. INTRODUCTION A seismic survey and some littoral cores obtained in 1993 facilitated a preliminary recon- Sedimentary successions of lakes in active vol- struction of the Late Quaternary evolution of the canic areas in the Andean Altiplano have provi- lake (Valero-Garce´s et al., 1999b, 2000, 2003). In ded detailed records of global changes November 2002, a coring expedition with the (environmental, climatic and cultural) during the Limnological Research Center (University of Min- Late Quaternary (Grosjean, 1994; Grosjean et al., nesota, USA) retrieved 15 Kullenberg cores up to 1997, 2001; Valero-Garce´s et al., 1999b). Many of 8 m long along several transects in the lake basin. the palaeoenvironmental and palaeohydrological Stratigraphical and sedimentological analyses of fluctuations have been attributed to climatic vari- the new cores and integration with the seismic ability. However, given the active volcanism and profiles allowed the three-dimensional (3D) related tectonics in the region, the role of volcanic reconstruction of the lake sediment architecture. processes in lacustrine sedimentation requires The chronological framework was based on 14C evaluation. Volcanic activity may strongly influ- AMS and 230Th/234U methods. The interpreted ence lake deposition by several processes: depositional environments may serve as modern (i) changes in the vegetation of the lake catchments analogues of lacustrine sedimentation of Quater- caused by increased wildfires and variations in nary and pre-Quaternary volcanic settings else- soil conditions favourable to pioneer plants (Hab- were. Diatomaceous sediments occur in a variety erle et al., 2000); (ii) changes in bathymetry and of Quaternary and pre-Quaternary lake succes- morphology of the lake basin caused by faulting sions (Bellanca et al., 1989; Owen & Crossley, and the construction and erosion of volcanic 1992; Owen & Utha-aroon, 1999; Sa´ez et al., 1999; structures (Colman et al., 2002); (iii) variability Zheng & Lei, 1999), often in volcanic-influenced in the sediment supply to the lake and in the basins where volcanic silica and hot waters chemistry of the waters caused by the supply of provide the dissolved silica necessary for diatom new volcanic materials in the watershed and by growth. The variety of diatomite facies from Lake the direct input of pyroclastic material into the Chungara´ illustrates the large compositional lake (Telford et al., 2004); (iv) addition of hydro- range of diatomite in a single basin. Diatomite thermal fluids to the lake system changing the commonly occurs with other offshore and littoral chemistry of water and sediments (Valero-Garce´s facies such as alluvial, carbonate-rich (Gasse et al., 1999a); and (v) ecological impacts on the et al., 1987; Bao et al., 1999) and aquatic macro- aquatic ecosystems (Baker et al., 2003). phyte-rich facies. A multiproxy approach enabled Lake Chungara´ (18°15¢S, 69°09¢W, 4520 m a.s.l.), the roles of tectonics, volcanism and climate at the base of the active Parinacota volcano (6342 m change in lake evolution to be characterized. a.s.l.), is the deepest and highest lacustrine eco- system in the Chilean Altiplano. The sedimentary record, including diatomite-dominated sediments GEOLOGICAL SETTING and tephra layers, provides a unique opportunity to analyse the interplay of climate change (Gros- The Lauca Basin and the origin of Lake jean, 1994; Grosjean et al., 1997, 2001; Valero- Chungara´ Garce´s, 1999b) and the activity of the Parinacota volcano during the Holocene (Wo¨rner et al., 1988, Lake Chungara´ is located on the NE edge of the 2000; Clavero et al., 2002, 2004). Lauca Basin (Fig. 1), an intra-arc basin bounded Ó 2007 The Authors. Journal compilation Ó 2007 International Association of Sedimentologists, Sedimentology, 54, 1191–1222 Depositional evaluation in Chungara´ Lake 1193 A B Fig. 1. (A) Map of the Lauca Basin and the two sub-basins that were created by the collapse of Parinacota volcano: the Chungara´ and the Cotacotani sub-basins. Lake Chungara´ occupies the highest sub-basin. It is topographically closed and surrounded by >5500 m a.s.l. volcanoes (modified from Ko¨tt et al., 1995). (B) Geological map of the Lake Chungara´ area. by faults and volcanoes. The peaks of the Western et al., 1995; Gaupp et al., 1999). During the Late Andean Cordillera (up to 5000 m a.s.l.) form the Pleistocene, the Palaeo-Lauca River flowed north- western margin, and a north–south ridge, made wards from Guallatire to Cotacotani, between the up of Parinacota, Quisiquisini, Guallatire and Ajoya and Parinacota volcanoes, turned west- Puquintica volcanoes, forms the eastern margin wards at Parinacota, then flowed southwards (Fig. 1A). The Lauca Basin is filled with an Upper for about 100 km, and finally eastwards to Miocene to Pliocene, volcaniclastic alluvial and Bolivia. Lacustrine depositional environments lacustrine sedimentary succession, >120 m thick, also occurred during the Late Pleistocene in the that rests unconformably on an Upper Creta- northern areas close to the village of Parinacota ceous–Lower Miocene volcanic substrate (Ko¨tt (Fig. 1A). Ó 2007 The Authors. Journal compilation Ó 2007 International Association of Sedimentologists, Sedimentology, 54, 1191–1222 1194 A. Sa´ ez et al. Parinacota volcano (6342 m a.s.l.) is a large followed by a short, wet episode during the Late composite stratocone of Late Quaternary age. It is Holocene. One of the most significant changes in built on an earlier stratocone in the Lauca Basin the sequence is a transition from carbonate-rich, that underwent a single catastrophic sector col- laminated lacustrine sediments to peat sediments lapse event and produced a ca 6km3 debris- that occurred at ca 7030 ± 245 14C yr BP (Baied & avalanche deposit that covered more than Wheeler, 1993). 140 km2 (Francis & Wells, 1988; Wo¨rner et al., 1988, 2000; Clavero et al., 2002, 2004).