Original paper

Los depósitos terciarios lacustres de Lonquimay, Chile

[Tertiary lacustrine sediments of Lonquimay, Chile]

Cisternas, Maria Eugenia; Diaz, Leonardo L.

Abstract

El detallado análisis de microfacies de las secuencias terciarias en la región de Lonquimay ha permitido precisar la evolución de esta depresión continental que forma parte de una extensa cadena desarrollada en el sector cordillerano de Chile Central-Sur. Cuatro ambientes de depositación lacustre, desde subaéreo a subacuático profundo se distinguen en una fase intermedia del desarrollo de esta secuencia continental. En las facies lacustres se encuentran niveles fuertemente bituminosos conocidos de antaño. En el presente trabajo se destaca además el hallazgo de hidrocarburos livianos en estas facies. La existencia de un volcanismo contemporáneo se registra a lo largo de toda la evolución de la cuenca. En la etapa lacustre se evidencia como fino material piroclástico presente en todas las facies. Una supuesta conección de esta cuenca con el mar abierto es descartada en base a la total ausencia de indicadores de facies lagunares o marinas.

Abstract

In the central-southern part of Chile deposited during the Tertiary volcanic and sedimentary rocks in intracontinental basins. Through the study of the depositional facies of one of these basins can be concluded that the facies are strictly continental, and that any connection with the open sea can be discarded. Two stratigraphical units have been recognized in the area: the Chilpaco unit and the Lonquimay Formation. The Chilpaco unit is a more than 1000 m thick sequence that outcrops on the northern part of the area. In the lower parts it is composed of conglomerates and coarse volcanoclastic sandstones with some coal intercalations. Rhyodacitic lavas and breccias with some black shales intercalations are more common in the upper part of the sequence. This unit separates the Lonquimay basin from another similar basin deposited to the north. The Lonquimay Formation was divided in four members: the member I represents the initial, pre-lacustrine facies, with sandstones and vitro-clastic tuffs, deposited discordantly over the lower jurassic limestones and volcanics. The upper pyroclastic levels were deposited subaerially as shown by the presence of pisolites and the absence of alteration of the glass particles. The lacustrine facies are referred as member II. On the northern part they interfinger with the Chilpaco volcanoclastics. Four depositional environments were recognized on this member: The subaerial one is characterized by clastic alluvial deposition and abundant tuffitic and vitroclastic components. The shoreline environment is characterized by tuffitic arenites, lithic and vitroclastic tuffs with fish scales and spines and stromatolitical limestones. Dryness cracks, retraction-desiccation-dissolution structures show brief over-water conditions, very normal in closed lacustrine complexes, since they depend on the seasonal influx and evaporation. The nearshore deposits show greater facies diversity. The sedimentary structures and the fossils point to a high energy area with persistent bottom currents. This environment is characterized by oosparitic and biosparitic facies. On the transition to the shoreline facies are more common the oncosparites. In the offshore environment the sediments deposited under the wave level, in reducing conditions favoured by a chemical and thermal stratification. Characteristic for this environment are the fine laminated bituminous carbonatic cherts. Both member III and IV represent the basin closing facies. They are characterized by volcanoclastic and volcanic rocks with lithic sandstones on the lower beds. Paleontological determinations let suppose a marsh or very shallow lake, surrounded by thick forests. The upper terminal facies are coarse lithic volcanic sandstones, tuffs and coarse breccias. Plio-quaternary basaltic flows appear discordantly over them. The study of other basins has been continued by the Departamento de Geociencias of the Concepción University, Chile and will continue trying to get a better comprehension of the tectonic and depositional phenomena during the last stages of the Andean development.

Kurzfassung

Eine ausführliche Mikrofazies-Analyse der tertiären Abfolgen im Gebiet Lonquimay zeigt die Entwicklung eines Beckens, das der Beckenkette der zentralen bis südlichen andinen Kordillere angehört. Durch die Faziesinterpretation der intermediären lakustrischen Etappe der Beckenentwicklung können vier Ablagerungstypen unterschieden werden, von subaerisch bis tiefaquatisch. Der Fund von leichten Kohlenwasserstoffen verstärkt das bergwirtschaftliche Interesse an dieser Fazies, deren Bitumenvorkommen seit langem bekannt sind. Beweise für einen synchronen Vulkanismus sind in der ganzen Beckenentwicklung zu finden. In der lakustrinen Fazies ist immer ein fein-pyroklastischer Bestandteil vorhanden. In Lonquimay liegen ausschließlich kontinentale Fazien vor. Die früher angenommene Verbindung mit dem Ozean ist, nachdem keinerlei Anzeichen einer lagunaren bzw. marinen Fazies gefunden wurden, abgelehnt. Sie wurden in zwei gleichalte lithostratigraphische Einheiten gegliedert: die Schichten von Chilpaco und die Lonquimay-Formation. Die erstgenannte Einheit setzt sich aus subaerisch gebildeten klastisch-vulkanischen Faziestypen zusammen; sie verzahnt sich teilweise mit der lakustrinen Fazies der Lonquimay-Formation.

Keywords

Tertiaryvolcanic rockssedimentary rocksChilevolcanoclastics