Paleobiochemistry of Jurassic gymnosperm compressions (Cañadón Asfalto Formation, Chubut, Argentina)

CAMÍ, G; D`ANGELO, J A; VOLKHEIMER, W; CABALERI, N

Corrientes

Simposio; XV Simposio Argentino de Paleobotánica y Palinología; 2012

Unviversidad Nacional del Nordeste - CONICET

Vegetative and reproductive compressions of Jurassic gymnosperms from Cañadón Lahuincó locality, middle Chubut River (Cerro Cóndor village) are studied by the first time using Fourier Transform Infrared (FTIR) spectroscopy analysis. Provenance is the middle section of the Las Chacritas Member (Aalenian-Bathonian) of the Cañadón Asfalto Formation. Gymnosperms are preserved as cuticle-free coalified layers, i.e. coalified remains without preserved cuticle. FTIR spectra show a relatively small number of peaks, reflecting a molecular structure which is characterized mainly by hydrocarbon compounds with a dominant aromatic character. Aliphatic and some oxygen-containing functional groups are present as well. The main peaks found in fossil spectra can be assigned to the following functional groups: hydroxyl O?H stretching (3100-3700 cm−1 region), aliphatic C?H stretching (2800-3000 cm−1 region), skeletal C=C stretching vibrations (in benzene rings, 1500-1700 cm−1 region), and alkyl C?H bending (1250-1500 cm−1 region). Comparisons with kerogen types indicate that most of the cuticle-free coalified layers are Type III kerogen and similar in composition to some vitrinites studied for comparison. Overall, the spectrochemical evidence suggests that cuticle-free coalified layers are characterized by medium to low contributions from aliphatic and oxygen-containing compounds and a high content of aromatic structures. This is indicative of harsh post-diagenetic conditions likely related to volcanic-ash burial of a Jurassic flora inhabiting a lacustrine and volcanic landscape characterized by intensive and repeated volcanism.