Influence of Sill Intrusions from the Organic Geochemistry of Upper Triassic Lacustrine and Fluviatile Sediments from Timber Peak (Antarctica)
The Timber Peak of North Victoria Land, Antarctica, exposes, embedded between two 150 m thick Early Jurassic mafic sills, 80 m of Late Triassic sediments of the Beacon Supergroup. The sediments comprise fluvial channel sandstones interbedded with lacustrine and palustrine deposits containing Dicroidium-dominated floras. The sandstones show a thin fused margin at the contact to the sills.
During the German Antarctic North Victoria Land Expedition IX (2005/2006) an approximately 3.5 m thick section of carbonaceous mudstones and coals, 12 m above the lower sill has been sampled for organic geochemical and petrographical analyses in order to better describe the depositional environment and to investigate whether the sill intrusions are responsible for thermal alteration processes of the sedimentary organic matter.
Hydrogen and oxygen indices of RockEval pyrolyses suggest that the sediments contain degraded terrestrial organic matter (Type IV). Carbon isotope ratios of aliphatic and aromatic fractions show typical values related to land plant dominated material. Varying abundances of C27- to C29-steranes reveal mixtures between land plant and aquatic organic matter, which in turn suggests that the sediments were likely deposited in swamp-like and/or lacustrine environments.
Sterane isomerisation parameters indicate that the sediments have reached maturities of the oil window, which is supported by vitrinite reflectance data. We observe a general increase of vitrinite reflectance (VR) values from the top to the lower parts of the analyzed section. The increase of approximately 0.4 %-VR over a depth interval of 3.5 m is unusual and cannot be explained through subsidence processes, but must be related to an abnormal heat flow related to the sill intrusions. The maturity data suggests that the underlying sill had a greater thermal influence on the sediments than the overlying one. This assumption seems plausible as the vertical distance between the sample positions and the lower sill is 12 m compared to 65 m towards the upper sill. The increase of reflectance values, however, is not uniform and the observed scattering of the data might be attributed to the influence of the variability of the thermal conductivity of different sediment types.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009