Paleoseismic record in the recent deposits of Urao Lake (Merida Andes, Venezuela)

Paleoseismic record in the recent deposits of Urao Lake (Merida Andes, Venezuela)

Huerta, Pedro (1), Audemard, Franck. (2), Guerrero, Omar (3), Cuevas, Rómulo (3) Alvarado, Miguel (3), Silva, Pablo, G (1).

 (1)  Dpto. Geología, Escuela Politécnica Superior de Ávila, Universidad de Salamanca. Ávila. SPAIN. phuerta@usal.es

 (2)  Fundación Venezolana de Investigaciones Simológicas, FUNVISIS, Final Prolongación Calle Mara, Quinta Funvisis, El Llanito, Caracas 1073, Venezuela. faudemard@funvisis.gob.ve

 (3)  Dpto. Geología General, Escuela de Ingeniería, Universidad de los Andes, Campus de La Hechicera, Mérida (Venezuela). oguerre@ula.ve

Abstract: The Urao lake locates in the Merida Andes where the Boconó fault is the main fault system, a large dextral strike-slip fault system, in the area. This lake occurs in a pull apart basin developed between two branches of the above mentioned fault system. In this paper we describe the paleoseismic record found in 5 cores extrated in the lake. The paleoseismic evidences are faulted laminae, convoluted lamination, sand injections, and pillar structures. The proximity of these structures with active faults belonging to the Boconó fault system suggest that the mentioned structures record seismic events in this area of the Venezuelan Andes. The paleoseismic structures appear in two levels, approximately at 40-60 cm and 100-120 cm from the lake bottom. The ¹⁴C cal. age of a bed situated at 238 cm is 300 years BP (Mazzarino, 2000). So the two levels with paleoseismic structures could correspond to two events later to 1650 AD. Assuming a sedimentation rate of 0.73 cm/year (0.68+ 0.05 cm/year), the bed at 120 cm depth corresponds with the event of 1812 AD (VIII intensity in the area), and the bed at 60 cm depth corresponds with the event of 1894 AD (VIII intensity in the area).

Key words: Urao lake, Lagunillas, Merida, Santa Cruz de Mora, earthquake. 

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 “Chimó” 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 Boconó 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 Boconó fault bifurcates and produces a pull-apart basin known as La González 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.