The relationship between carbonate facies, volcanic rocks and plant remains in a late Palaeozoic lacustrine system (San Ignacio Fm, Frontal Cordillera, San Juan province, Argentina)

Abstract

The San Ignacio Fm, a late Palaeozoic foreland basin succession that crops out in the Frontal Cordillera (Argentinean Andes), contains lacustrine microbial carbonates and volcanic rocks. Modification by extensive pedogenic processes contributed to the massive aspect of the calcareous beds. Most of the volcanic deposits in the San Ignacio Fm consist of pyroclastic rocks and resedimented volcaniclastic deposits. Less frequent lava flows produced during effusive eruptions led to the generation of tabular layers of fine-grained, greenish or grey andesites, trachytes and dacites. Pyroclastic flow deposits correspond mainly to welded ignimbrites made up of former glassy pyroclasts devitrified to microcrystalline groundmass, scarce crystals of euhedral plagioclase, quartz and K-feldspar, opaque minerals, aggregates of fine-grained phyllosilicates and fiammes defining a bedding-parallel foliation generated by welding or diagenetic compaction.

Pedogenic and subaerial exposure microfabrics in a late Carboniferous-early Permian carbonate-volcanic lacustrine-palustrine system (San Ignacio Formation, Frontal Cordillera, Argentina)

“…The observed pedogenic microfabrics on the top of the alternating carbonate and volcanic deposits through this succession confirm the statement referred to the medium to high exposure index by Busquets et al (2013b). Furthermore, the pedogenic microfeatures recorded in these rocks can be significant for evaluating the prevailing paleoclimatic conditions during the subaerial exposure intervals.…”

“…The studied material comes from a previous work focused on a thorough facies and a multidisciplinary analysis of carbonates deposits, volcanic rocks and plant remains that led to a depositional model of the upper part of the San Ignacio Formation (Busquets et al, 2013b). Samples of the pedogenically modified deposits were collected from stratigraphic sections at the Quebradas de Ñipas (Ñipas-El Boliche), San Ignacio (sector 1, Fig.…”

“…According to Busquets et al (2013b) carbonate and volcanic deposits are organized in sequences that occur repeatedly (Fig. 3A).…”

“…Evidence of synsedimentary instability such as slumps and breakages of carbonate beds is also present. Commonly, the carbonate sequences rest on volcanic rocks, and also are sharply overlain by volcanic deposits consisting of pyroclastic rocks related to explosive eruptions, ignimbrites and ash-fall tuffs, epiclastic deposits of volcanic conglomerates, breccias and sandstones, and less frequent lava flows (Busquets et al, 2013b). The volcanism, hydrothermal fluids, abundance of organic nutrients provided by decaying flooded plants, as well as slow water circulation and water features, favoured the growth of microbes, microbial carbonate precipitation and silica permineralization of tree trunks and other plant remains (Busquets et al, 2013b;Césari et al, 2010Césari et al, , 2012.…”

“…These sequences have been deposited in a shallow lacustrine-palustrine system subject to fluctuations in water level giving rise to repeated events of flooding and desiccation. Fluctuations in water level are related to contemporary volcanic supply, tectonic subsidence and microbial growth, without ruling out the effects of an oscillating climate (Busquets et al, 2013b and references therein); climate or water level fluctuations are also indicated by the growth under recurrent wetter and drier conditions of the fossil forest (Césari et al, 2010(Césari et al, , 2012. The thrombolites were formed in relatively deeper conditions of the lake and the stromatolites grew on them in shallower water areas.…”