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Evidencias paleosismicas en depositos recientes de la Laguna de Urao, Andes de Merida, Venezuela.

PALEOSEISMIC EVIDENCES IN RECENT DEPOSITS OF URAO LAKE, ANDES OF MERIDA, VENEZUELA

EVIDENCIAS PALEOSISMICAS EM DEPOSITOS RECENTES DA LAGUNA DE URAO, ANDES DE MERIDA, VENEZUELA

INTRODUCTION

This paper describes the paleoseismic evidences in the Urao lake deposits. This lake is known by the alkaline carbonates (trona and gaylussite) which have been traditionally extracted by the locals to produce "Chimo" something like chewing tobacco. The Urao lake occurs in the Venezuelan or Merida Andes which is a mountain range with NE-SW trend related to the Late Miocene collisional event between the Maracaibo block and the Guyana Shield (Audemard and Audemard, 2002) (Fig. 1). The Bocono fault is a large (500 km long) dextral strike-slip which separates, in the Merida area, the Sierra de La Culata in the northern block and the Sierra Nevada in the southern block.

In the central sector of the Merida Andes, the Bocono fault bifurcates and produces a pull-apart basin known as La Gonzalez basin in which the Lagunillas pull apart basin is developed (Schubert, 1980; Alvarado, 2008).

The Lagunillas basin, and the Urao lake (1020 m, absl) locate in the lower block of two faults that bound the basin by the north and south. The northern basin margin has high relief (2200 m, absl) and the Jurassic siliciclastic rocks of La Quinta Fm. nourish the alluvial fans that fringe this margin (Fig.1).

The fault in the southern basin margin produces a 20 m high shutter ridge, with E-W trend developed on Quaternary alluvial deposits.

In the knick point that connects the alluvial fan deposits of the Lagunillas pull apart basin and the Jurassic basement there is probably another fault that in some parts is covered by alluvial fan deposits, and no rupture evidences have been detected, probably because is covered by houses.

The fault in the southern part of the basin, continues laterally outside of the watershed of the Urao lake, towards the west the Quinanoque trench was excavated by a Funvisis-ULA team (Alvarado, 2008). In these trench the same range of events than in the Pantaleta trench was found. The recurrence interval of the events found in this trench is about 400-450 years (Alvarado, 2008).

THE LAKE DEPOSITS

The Urao lake shows a transition from the alluvial fan deposits in the northern margin of the basin towards the fine clay deposits of the lake centre. The palustrine areas are characterized by floating vegetation that extends into the lake several tens of meters.

Six cores were extracted in the lake bottom sediment reaching from 50 cm to 212 cm depth (all core depths are referred to the lake bottom). All the sediment The age of the core E99 at 238 cm depth is 300 years BP [sup.14]C cal), what is the year 1 650 AD. So the sedimentation rate till this core was extracted is about 0.68 cm/year. If we assume that this sedimentation rate is constant during this period the event at 120 cm depth occurred in 1823 AD and the event recorded at 60 cm depth was produced in 1911 AD. But if the sedimentation rate is increased in 0.05 cm/year (0.73 cm/year), the event at 120 cm depth corresponds with the year 1811 AD and the event at 60 cm depth is equivalent to the year 1894 Ad. So we conclude that the event recorded at 120 cm depth represents the Merida earthquake of 1812 AD recorded with an VIII Intensity in the area (Soulas et al, 1987; Audemard, 1997, 1998), and the event at 60 cm depth corresponds with the Santa Cruz de Mora earthquake, also with VIII intensity in the area (Soulas et al, 1987; Audemard, 1997, 1998). Analyzed is constituted by chlorite, muscovite, quartz, calcite and analcime (Huerta, et al., 2012) (Fig. 2).

The sediment is mainly dark grey to white mud with some intercalations of sand. The lamination is only well preserved in some beds and there are at least to levels with disrupted lamination and sand dykes which are interpreted as paleoseismites.

The paleoseismites occur at two depths in the most of the cores studied, 40-60 cm (SGL-01; SGL-03; SGL-05; SGL-06) and at 100-120 cm (SGL-03; SGL-05; SGL-06) (Fig. 3). In E99 core extracted by a team from the University of Massachusetts Amherst (Mazzarino, 2000) dated a bed with a depth of238 cm by the [sup.14]C cal. as 300years BP.

DISCUSSION

The two levels with faulted beds, convoluted and disrupted lamination, sand injections and pillar structures are interpreted as two paleoseismic events recorded in the most of the lake as liquefaction processes. Similar examples have been described in lacustrine and other sedimentary environments (Sims, 1975, Audemard and Santis, 1991; Rodriguez-Pascua, et al., 2000, Alfaro, et al., 2010).

Considering that this two levels with paleoseismites appear at the same depth range (40-60 and 100-120) along all the lake centre and that each one records the same seismic event, it is possible interpret that the sedimentation rate is constant all along the lake centre during the last centuries.

ACKNOWLEDGEMENTS

The first author acknowledges the travel grant that received from the University of Salamanca for a research stay in the University of Los Andes (Venezuela). And CDCHTA-ULA in the project FO-706-11-02-B.

REFERENCES

Alfaro, R, Gibert, L, Moretti, M., Garcia-Tortosa, F. J., Sanzde Galdeano, C., Galindo-Zaldivar, J., Lopez-Garrido, T. C. (2010). The significance of giant seismites in the Plio-Pleistocene Baza palaeo-lake (Spain). Terra Nova, 22 (3), 172-179.

Alvarado,M.(2008). Caracterizacion Neotectonica de la cuenca de traccion La Gonzalez, Merida, Venezuela. Tesis de Maestria, Universidad Central de Venezuela, 89 p. Audemard F.A. (1997) Holocene and historical earthquakes on the Bocono Fault System, southern Venezuelan Andes: Trench Confirmation. Journal of Geodynamics 24 (1-4), 155-167.

Audemard, F. (1998) Contribucion de la paleosismologia a la sismicidad historica: los terremotos de 1610 y de 1894 en los Andes venezolanos meridionales. Revista Geografica Venezolana, 39 (1-2), 87-105.

Audemard, F. E., Audemard, F. A.( 2002). Structure of the Merida Andes, Venezuela: relations with the South America-Caribbean geodynamic interaction. Tectonophysics, 345 (1-4), 299-327.

Huerta, P, Guerrero, O., Cuevas, R., Armenteros, I., F. A. Audemard, F. A., Paredes, J., (2012). Caracterizacion sedimentologica de la laguna de Urao (Andes de Merida, Venezuela). Geo-Temas, 8..

Mazzarino, M. (2000) Clay and evaporite-mineralogy in the lago de Urao basin: clues to environmental fluctuations in the Holocene of Venezuela. Master thesis, University of Massachusetts Amherst, 82 p.

Rodriguez-Pascua, M. A., Calvo, J. R, De Vicente, G., (Gomez-Gras, D.(2000) Soft-sediment deformation structures interpreted as seismites in lacustrine sediments of the Prebetic Zone, SE Spain, and their potential use as indicators of earthquake magnitudes during the Late Miocene. Sedimentary Geology, 135 (1-4), 117-135.

Schubert, C. (1980). Late Cenozoic pull-apart basins, Bocono Fault Zone, Venezuela Andes. Journal of Structural Geology 2, 463-468.

Sims, J. (1975). Determining earthquake recurrence intervals from deformational structures in young lacustrine sediments. Tectonophysics, 29, 41-152.

Soulas, J. P., Giraldo, C., Bonnot, D. (1987). Actividad cuaternaria y caracteristicas sismogenicas del sistema de fallas de Oca-Ancon y de las fallas de Lagarto, Urumaco, Rio Seco y Pedregal. Afinamiento de las caracteristicas sismogenicas de las fallas de Mene Grande y Valera. FUNVISIS para INTEVEP, S.A. 65 p + anexos. Inedito.

Pedro Huerta (1) Omar Guerrero (2) Romulo Cuevas (3) Gabriela Cantos (4) Emilio Sanchez (5)

(1) Geo, Dr. Departamento de Geologia, Escuela Politecnica Superior de Avila, Universidad de Salamanca. Espana. e-mail: phuerta@usal.es

(2) Geog, Geo, Dr. Universidad de Los Andes (ULA). e-mail: oguerre@ula.ve

(3) Ing Geo, ULA. e-mail: cuevasromulo@gmail.com

(4) Ing Geo, ULA. e-mail: gabrielacantos@ula.ve

(5) Ing Geo, ULA. e-mail: emilisanchez05@gmail.com

Recibido: 17-6-13; Aprobado: 19-7-13

Leyenda: Fig. 1: Situation of the Urao lake. A) Regional geological sketch. A star marks the situation of the Urao lake. B) Geomorphological map of the Urao lake watershed. The red line are faults, the drainage is in blue.

Leyenda: Fig. 2: Detailed log of the Core SLG-06.

Leyenda: Fig. 3: Fluid escape structures and fractures in the Urao lake cores. A) Core SLG-03, B) Core SLG-06 with fluid escape structures marked by organic matter oxidation.
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Title Annotation:Neotectonica
Author:Huerta, Pedro; Guerrero, Omar; Cuevas, Romulo; Cantos, Gabriela; Sanchez, Emilio
Publication:Geominas
Article Type:Report
Date:Aug 1, 2013
Words:1382
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