PALEOBIOCHEMISTRY OF FOSSIL REMAINS (JURASSIC CAÑADÓN ASFALTO FORMATION, CHUBUT, ARGENTINA)

D'ANGELO JOSÉ; MONFERRAN, MATEO DANIEL; VOLKHEIMER, WOLFGANG; CABALERI, NORA

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Simposio; V Simposio Argentino del Jurásico; 2013

Museo Paleontológico Egidio Feruglio

Jurassic remains of animals (?conchostracans?) and plants (gymnosperms) from Cañadón Lahuincó locality, middle Chubut River (near Cerro Cóndor village) are studied using scanning electron microscopy combined with energy-dispersive X-ray spectrometry (SEM-EDS) and Fourier Transform Infrared (FTIR) spectroscopy analyses. Provenance is the middle section of the Las Chacritas Member (Aalenian-Bathonian) of the Cañadón Asfalto Formation. SEM-EDS spectra of ?conchostracan? carapaces indicate the presence of chemical elements mainly related to silicate and phosphate minerals. Gymnosperms are preserved as coalified remains without cuticle (cuticle-free coalified layers). FTIR spectra reveal a molecular structure which is similar in composition to Type III kerogen, i.e. characterized mainly by hydrocarbon compounds with a dominant aromatic character. This is evidenced by small aliphatic C?H stretching peaks (2800-3000 cm−1 region) and distinct skeletal C=C stretching vibrations (in benzene rings, 1500-1700 cm−1 region). FTIR spectra of associated fusain remains show that only a few functional groups are left. This is in agreement with medium to high reflectance values of ~3.70 % measured in fusain remains, suggesting high temperatures of ~600º C. Alkaline medium prevalence is indicated by both phosphate mineral preservation in conchostracan carapaces and cuticle loss in cuticle-free coalified layers of gymnosperm leaves. Combined evidence points out to harsh post-diagenetic conditions related to volcanic-ash burial of this Jurassic flora and fauna. However, as demonstrated by a well preserved palynoflora in the same locality a few stratigraphic meters above the macrofossils here studied, different thermal alterations are evident due to varying degrees of heating by basaltic dikes.