Climate Change Control of Mio-Pliocene Nonmarine Stratigraphic Sequences: Iglesia Basin, Northwestern Argentina

Brian G. Ruskin and Teresa E. Jordan
Cornell University, Ithaca, NY

Subdivision of sedimentary rocks into genetically related sequences reflects interaction between extrinsic and intrinsic processes affecting a basin's accommodation potential. Models for stratigraphic sequence genesis in alluvial settings are at present oversimplified, appeal to eustatic comparisons and lack quantification of each mechanism's role. In net aggradational continental systems, erosional unconformities are commonly local features (i.e. channels), confounding field identification of sequences in the much broader interfluves. The Tertiary wedgetop foreland Iglesia Basin (S 30-31°, W 69-70°, NW Argentina) provides multiple datasets (basin-scale seismic network revealing numerous sequences, chronostratigraphic constraint, outcrop continuity, geochemical proxies) for improved understanding of nonmarine sequence formation. Extensive field mapping and reinterpretation of seismic boundaries/facies suggest that, in the field, lithofacies changes are more plentiful, recognizable and useful in defining sequence boundaries than are erosive contacts. Though accommodation space and sediment distributions are influenced by Andean uplift and thrust belt propagation, we postulate that seismically defined sequence boundaries are represented in outcrop by regionally continuous, climatically controlled lithofacies transitions. Climatic conditions from 9 - 4 Ma (encompassing ~10 seismic sequences) were quantified by combining paleosol profile descriptions, stable isotopic, X-ray diffraction, and micromorphologic analyses with facies characterization. Long-term increases in humidity and accumulation rates accompanied decreased pedogenesis, with superimposed higher-frequency inter- and intra-sequence climate variation. Coeval deposits in the adjacent foreland show similar climatic influences. Iglesia Basin sequences thus provide unique documentation of climate variability in entirely nonmarine strata, contributing to more realistic sequence-bounded systems tract characterizations.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005