Redefinition, Stratigraphy and Facies of the Lo Valdés formation (Upper Jurassic-Lower Cretaceous) in Central Chile

Abstract

Lo Valdés Formation was informally divided in three members (Spilitas, Arenáceo and Calcáreo). A redefinition of the Lo Valdés Formation is proposed here which reduces this lithostratigraphic unit to the "Arenáceo" and "Calcáreo" members; the "Spilitas" member is treated as a separate unit, termed the Baños Morales Formation. The new Baños Morales Formation (formerly "Spilitas" member of the Lo Valdés Formation) is 760 meters thick and consists predominantly of porphyry andesite and volcanic breccia, while intercalated sedimentary rocks are rare and restricted to four units, each a few meters thick. The type locality for the Baños Morales Formation is on the southern side of the El Volcán valley, in the high Andes southeast of Santiago (70°02`57``W and 33°49`41``S). Ammonites and other molluscs from the sedimentary rocks intercalated with the andesitic rock sequence indicate an earlymiddle Tithonian age for the Baños Morales Formation. The Lo Valdés Formation as re-defined here conformably overlies the Baños Morales Formation. Its type locality is located on the southern side of the El Volcán River (70°02`52``W and 39°49`50``S). The unit consists of siliciclastic and carbonate sedimentary rocks separated into three formal members (from base to top), the Escalador, Placa Roja and Cantera Members. The Lo Valdés Formation is late Tithonian to Hauterivian in age, based on abundant and relatively diverse ammonite assemblages.

The lithological composition and biotic content of the Lo Valdés Formation suggest shoreface, offshore transition and offshore environments. The carbonate content increases upsection. The Escalador Member represents shoreface facies, with transgressive shallow marine intervals and a storm-dominated shelf facies. Offshore transition facies are reflected in the Placa Roja Member by rhythmically-bedded siltstone, calcareous siltstone, wackestone and packstone. The presence of disseminated pyrite and high content of organic matter indicate reducing and low-energy environments.

An offshore (outer-ramp) environment is present in the Cantera Member towards the top of the section and is represented by an increase in silty wackestone and mudstone and a decrease in faunal richness and abundance. The re-definition of Lo Valdés Formation, and assignation of the new Baños Morales Formation show that two main events occurred in the Andean Basin during the Late Jurassic - Early Cretaceous transition. The Baños Morales Formation records a dominance of andesitic lithologies that indicates volcanic events with quiescence, the latter evidenced by decimetre-scale sedimentary layers. A prominent lithological change, from volcanic dominance to the exclusively marine succession of the Lo Valdés Formation, marked the second event during this time in the Andean Basin.