Tectonic and Climatic Controls on Sequence Stratigraphy Revealed by Geometric and Paleosol Analyses in a Nonmarine Foreland Basin

Jordan, Teresa¹, Brian Ruskin² (1) Cornell University, Ithaca, NY (2) Shell International Exploration and Production Inc, Houston, TX

It has proven difficult to develop nonmarine sequence stratigraphic models in wholly nonmarine settings such as are common in foreland basins. Compared to their marine counterparts there are few tried-and-true “rules” for interpreting nonmarine stratigraphic sequences. We examine a nonmarine wedgetop foreland basin on multiple scales (seismic, field and geochemical measurements of paleosols) to discriminate the main forcings (tectonics and climate) on sequence genesis. Paleosols prove especially useful in defining changes in sedimentation and climate.

The Iglesia wedgetop foreland basin of the Argentine Central Andes (S 30-31°, W 69-70°) contains a lens shaped profile of syntectonic fill with 11 basin-scale seismic sequences. Ten dated sequences span 9 to 4.3 Ma (magnetostratigraphic and ash dating), a period of global cooling and regional glaciation. Rotation of sequence bounding reflections along the western margin is enhanced at 6.9, 6.6, and 5.2 Ma as well as during the interval 5.2 – 4.3 Ma. We interpret these as evidence for accelerated uplift of the Frontal Cordillera, which forced the western depositional margin basinward. At the same times, δ¹³C and δ¹⁸O for pedogenic carbonates indicate enhanced aridity in Iglesia Basin during formation of those specific sequence boundaries, as well as at a sequence boundary dated 8.7 Ma. Only one sequence boundary is associated with humid conditions (7.7 Ma), coinciding with a globally warm episode and interpreted low rates of local tectonism. Neither climate nor tectonism alone fully explains the sequence boundaries. The combination of these variables suggests that complex feedback cyclicity affected sequence formation.