Sequence stratigraphy and paleopedology of nonmarine foreland basins: Iglesia Basin, Argentina and Axhandle Basin, Utah

Abstract

Subdivision of strata into genetically related sequences reflects interaction between extrinsic and intrinsic processes affecting a basin's accommodation potential and environment. In nonmarine settings, sequence bounding surfaces are difficult to locate and several causal mechanisms (tectonism, climate change, and autogenic variability) combine in complex and still poorly-understood ways. However, integrating basin-scale analyses of subsurface and surface stratigraphy can promote interpretations about the magnitude of individual extrinsic forcings and suggest feedback mechanisms for sequence genesis. Extensive field measurements, seismic stratigraphy and facies analyses, and examination of fossil soils (paleosols) are utilized in the Miocene-Pliocene Iglesia wedgetop basin (Central Andean foreland, northwestern Argentina) and the Late Cretaceous-Early Eocene Axhandle wedgetop basin (Sevier foreland, central Utah, USA) to characterize extrinsic variability during nonmarine sequence deposition and refine general base level models.

Seismic stratigraphy of Iglesia Basin is reassessed and compared with correlative outcrops to evaluate tectonic and climatic control of sequence development. Intrabasinal deformation and surface uplift of the frontal Cordillera of the Andes are documented. In basin-central outcrops, vertical transitions in lithofacies and paleosol characteristics demarcate seismic sequence boundaries. Paleosol profile measurements, stable isotopic ([delta] 13 C and [delta] 18 O) time series, micromorphology and clay mineralogic analyses document paleoclimatic variability. Four sequence boundaries are associated with climate shifts away from semiaridity. Increased aridity occurred at 9 Ma, 6.9 Ma and 5.2 Ma, concurrent with sequence boundary formation.

At least three sequence boundaries formed during intervals of concurrent marginal surface uplift and climate shifts that worked in accord to starve the central basin of sediment.

Six stratigraphic sequences are defined between 76--49 Ma in Axhandle Basin, central Utah. Previous assessments of the basin's stratigraphy emphasized the role of tectonics. Herein, local paleoclimate variability is explored through analyses of paleosols and their relationship to the proposed stratigraphic sequences.

Previously proposed nonmarine systems tract models are compared with the Axhandle and Iglesia Basin sequences. Models characterized by lithofacies and sedimentary architecture are found to be incongruous with features observed and measured from the test basins' strata because of inadequate spatial, lithologic, aggradation rate and extrinsic mechanism definitions.