El sistema lacustre de la Formación Mollar en el depocentro triásico de Santa Clara (provincia de Mendoza, Argentina)

SPALLETTI, L. & ZAVATTIERI, A.M.

ANDEAN GEOLOGY

Servicio Nacional de Geología y Minería

Lugar: Santiago; Año: 2009 vol. 36 p. 236 - 263

0718-7092

ABSTRACT.            The lacustrine system of the Mollar Formation in the Triassic Santa Clara Depocenter (Mendoza Province, Argentina). The Triassic Santa Clara depocenter is characterised by thick fine-grained deposits formed in lacustrine systems. One of them is the record of the Mollar Formation, that exceeds 345 m. The lower to middle section of this unit is dominated by a monotonous succession of black shales having a high content of amorphous organic matter produced by algae and highly degraded plant remains. These deposits were formed by suspension fallout in the anoxic offshore sector of a hydrologically closed lake. Thin and fine-grained turbiditic beds and stromatolitic carbonates are intercalated in the fine-grained succession. Small-scale coarsening-upward cycles (PACs) are attributed to episodes of lake expansion-retraction driven by climate change. A facies association composed of heterolithic package represents the deposits of the transition between nearshore and offshore settings. The upper section of the Mollar Formation essentially consists of bioturbated mudstones suggesting a greater oxygenation of the substrate. They are assigned to the offshore environment of a holomictic hydrologically open lake system. Sporadic hyperpycnal flows are documented by intercalations of sandy turbidites. Bioturbated sandstone beds with primary structures indicative of both normal and storm wave action and unidirectional flows, represent the marginal deposits of the lacustrine system. In addition to the high-frequency cycles, three larger scale asymmetrical sequences are identified in the Mollar Formation. These sequences suggest episodes of major expansion-contraction of the lake system due to the interaction between climatic and tectonic factors. The overall shallowing up stacking pattern of the Mollar Formation and the gradual transition to fluvial deposits of the overlying Montaña Formation reflect a steady reduction of accommodation space in the Santa Clara depocenter during the accumulation of the studied succession.