Preservation modes of Triassic gymnosperm wood (Ischigualasto, San Juan, Argentina). First spectrochemical evidence of paleofires.

D`ANGELO, J A; COLOMBI, C; CAMÍ, G; ZODROW, E L

Corrientes

Simposio; XV Simposio Argentino de Paleobotánica y Palinología; 2012

Unviversidad Nacional del Nordeste - CONICET

Several specimens of gymnospermous wood from the Triassic of Ischigualasto (San Juan, Argentina) are studied using Fourier Transform Infrared (FTIR) spectroscopy analysis. Fossils come from Valle de la Luna Member of the Upper Triassic Ischigualasto Formation (310?450 meters from the base). The tree trunks were incorporated, together with large amounts of leaf compressions, into fluvial systems during the peaks of discharges for short transporting distances before being buried in the channel bars. Studied specimens (collection of Instituto y Museo de Ciencias Naturales, UNSJ) assigned to Rhexoxylon Bancroft emend Archangelsky and Brett 1961 (Corystospermales, Corystospermaceae), are usually associated with coalified corystosperm foliage. Fossil woods exhibit two preservation modes including (i) coalification to vitrinite and (ii) silicified vitrinite-semifusinite transition. FTIR spectra of coalified specimens are characterized by small peaks at 1711 cm-1 and prominent bands at 1565 cm-1 assigned to aromatic carboxyl groups and benzene rings, respectively. The small number of peaks in the FTIR spectra of coalified remains, and the virtual absence of aliphatic peaks could be indicative of harsh post-diagenetic conditions. FTIR spectra of silicified, vitrinite-semifusinite transition fossils show dominant silicate components. Smaller peaks (at 1685 and 1620 cm-1) are indicative of carbonyls in ketonic structures. The low absorbance in the region 3000?2800 cm-1 indicates lower contents of aliphatic compounds. The spectrochemical evidence suggests that these transition fossils represent coalified, partially charred stems, whereas the uncharred plant portions were silicified and three-dimensionally preserved. These results provide the first spectrochemical evidence of paleofires in the Triassic of Ischigualasto.