HR: 0830h
AN: T31C-0852    [PDF]
TI: Testing the Origins of Nonmarine Stratigraphic Sequences, Iglesia Basin, Northwest Argentina
AU: * Ruskin, B G
EM: bgr9@cornell.edu
AF: Department of Earth and Atmospheric Sciences Cornell University, Snee Hall Cornell University, Ithaca, NY 14853 United States
AU: Jordan, T
EM: tej1@cornell.edu
AF: Department of Earth and Atmospheric Sciences Cornell University, Snee Hall Cornell University, Ithaca, NY 14853 United States
AB: The Iglesia Basin is an entirely nonmarine Andean foreland basin consisting of approximately 3.5 km of Tertiary strata unconformably overlying Paleozoic basement. Best described as a wedge-top basin, Iglesia Basin is located in San Juan Province, Argentina at S 30-31\deg between the Frontal Cordillera and Precordillera fold-thrust belt. Interpretations of seismic reflection profiles and field reconnaissance have suggested basin-wide stratigraphic sequences. Additionally, radiometric and magnetostratigraphic data constrain sequence deposition between approximately 17 and 4 Ma. However, a fundamental question remains unanswered: temporal variability of which control caused development of unconformity-bound nonmarine sequences? Prior to this work, hypotheses about the factors at play, notably tectonism and climate change, remained untested, and fieldwork provided only localized information about the nature of the sequences. The present study examines basin lithofacies more broadly and will independently constrain discharge history (a proxy for climate) and intrabasinal tectonics. Thus far, fuller knowledge of the sedimentation patterns, structural expression, and volcanic history of Iglesia Basin is supplied by information from new outcrop localities near the northern paleomargin and basin center, and from reinterpretation of previously studied localities. A substantial volcanic component to the history of the oldest sequences is inferred from age relationships and continuity of deposits in proximity to the Cerro Negro intrabasinal andesitic center. Reassessment of field-assigned sequence boundaries in terms of continuity and expression, both along-strike and across intrabasinal faults, suggests that lithofacies shifts are more prevalent than erosive surfaces. Radiometric dating of additional tuffaceous units in the Tertiary sequences will allow more conclusive correlation among discontinuous outcrops. Floodplain assemblages of Aridosols and Inceptisols indicate notable sedimentary hiatuses, allow pedofacies definition, and provide material for temporally constrained $^{13}$C and $^{18}$O analysis as a proxy for climate fluctuation. Preliminary interpretation of paleosol micromorphology and isotopic time series suggests minimal diagenesis, as well as an evolution towards moisture-stressed conditions and depletion of heavy isotopes, possibly caused by the orographic effect of the rising Andes. Isotopic signal variability can be compared to sequence chronostratigraphy to test the hypothesis that sequence formation results from stream discharge variations.
DE: 1815 Erosion and sedimentation
DE: 3675 Sedimentary petrology
DE: 9360 South America
SC: Tectonophysics [T]
MN: 2003 Fall Meeting