Título Aquí (Times New Roman 16, negrita, centrado)

Título Aquí (Times New Roman 16, negrita, centrado)

XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
S10_016
Age and sedimentary environment of the Estratos de Puñihuil
(northwestern Chiloé, 41º56'S)
Encinas, A.1, Finger, K.2, Duhart, P.3, Quiroz, D.3
(1) Departamento de Ciencias de la Tierra, Facultad de Ciencias Químicas, Universidad
de Concepción, Casilla 160-C, Concepción, Chile.
(2) University of California Museum of Paleontology, 1101 Valley Life Sciences
Building, Berkeley, CA 94720-4780, United States.
(3) Oficina Técnica Puerto Varas, Servicio Nacional de Geología y Minería. La Paz 406
Puerto Varas, Chile.
[email protected]
Introduction
Marine strata that crop out on the Puñihuil Peninsula northwest of Chiloé Island were
first cited by [1], who observed them overlying volcanic rocks of the Complejo
Volcánico de Ancud. [2] assigned the marine succession to the Miocene, while [3]
reported that the marine strata are interbedded with lava flows, and therefore included
them in the Complejo Volcánico de Ancud. In order to verify the stratigraphy and obtain
new data pertinent to interpreting the geological history of the area, we carried out
sedimentological, ichnological and micropaleontological studies on the Puñihuil
Peninsula.
Results
Two sections were measured in the Estratos de Puñihuil at the homonymous peninsula.
The first one shows a discordant contact with the underlying lavas of the Complejo
Volcánico de Ancud [3] followed by a basal breccia made of centimeter- to meter-sized
volcanic clasts and a fining-upward succession of sandstone and siltstone with
Chondrites, molluscs, barnacles, foraminifera, and ostracodes. To the west, in the second
section, the marine unit is in faulted contact with the volcanics. It is a succession of
coarse-grained sandstone with scarce volcanic clasts up to 1 m followed by fine-grained
sandstones with Chondrites, echinoid spines, molluscs, foraminifera, and ostracodes. A
basal contact was not observed. Strata are slightly inclined to subhorizontal in both
sections.
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XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
The microfossils occur as a mixed-depth association that includes lower-bathyal (>2000
m depth) foraminifera and psychrospheric (cold deep-water) ostracodes. This assemblage
also includes a single species of planktic foraminifera, Globoturborotalita woodi Jenkins,
that ranges from the Oligocene to late Pliocene. This long range overlaps the Late
Miocene-early Pliocene age determined by [4] for strata of the Lacui Formation [3] that
crop out in northwestern Chiloé. In addition, the species of benthic foraminifera and
ostracodes in the Estratos de Puñihuil Formation also occur in the the Lacui Formation.
Discussion
Our field observation confirms that the Estratos de Puñihuil Formation unconformably
overlies the volcanic beds of the Complejo Volcánico de Ancud, which has important
implications regarding the geological history of this area. The marine succession is
transgressive, with a basal breccia overlain by a fining-upward succession containing
deep-water foraminifera and ostracodes. The lithofacies and benthic microfauna of the
Estratos de Puñihuil Formation are similar to those of the Estratos de Lacui Formation [3]
in Chiloé and the Santo Domingo Formation [5] in south-central Chile. Thus, our findings
suggest that these three units are coeval and reflect a regional tectonic event that resulted
in subsidence of the continental margin during the Miocene.
References
[1] García, F. (1968). Estratigrafía del Terciario de Chile Central. En, Cecioni, G., Ed,
Terciario de Chile Central, Editorial Andrés Bello, Santiago, pp. 25-57
[2] Tavera, J. (1982). Informe estudio sobre material paleontológico procedente de la
localidad de Chiloé: Informe inédito, Departamento de Geología, Universidad de Chile, 2
p.
[3] Valenzuela, E. 1982. Estratigrafía de la boca occidental del canal de Chacao, X
Región, Chile. III Congreso Geológico Chileno (Actas), p. A343-A376, Concepción,
Chile.
[4] Finger, K.L., Nielsen, S.N., DeVries, T.J., Encinas, A., Peterson, D.E. (2007)
Paleontologic evidence for sedimentary displacement in Neogene forearc basins of
central Chile. Palaios, vol. 22 (1), 3–16.
[5] Martínez-Pardo, R., Pino, M. 1979. Edad, paleoecología y sedimentología del
Mioceno marino de la Cuesta Santo Domingo, Provincia de Valdivia, X Región. II
Congreso Geológico Chileno (Actas), p. H 103-H 124, Arica, Chile.
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