Carbon and oxygen isotope ratios vary with time, and researchers can use those to map subtle changes that occurred in the paleoenvironment. The basic concept in stratigraphy, called the law of superposition, states: in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence.Ĭhemostratigraphy studies the changes in the relative proportions of trace elements and isotopes within and between lithologic units. Key concepts in stratigraphy involve understanding how certain geometric relationships between rock layers arise and what these geometries imply about their original depositional environment. These variations provide a lithostratigraphy or lithologic stratigraphy of the rock unit. This variation can occur vertically as layering (bedding), or laterally, and reflects changes in environments of deposition (known as facies change). Variation in rock units, most obviously displayed as visible layering, is due to physical contrasts in rock type ( lithology). 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.Chalk layers in Cyprus, showing sedimentary layering Previously proposed nonmarine systems tract models are compared with the Axhandle and Iglesia Basin sequences. Herein, local paleoclimate variability is explored through analyses of paleosols and their relationship to the proposed stratigraphic sequences. Previous assessments of the basin's stratigraphy emphasized the role of tectonics. Six stratigraphic sequences are defined between 76-49 Ma in Axhandle Basin, central Utah. Increased aridity occurred at 9 Ma, 6.9 Ma and 5.2 Ma, concurrent with sequence boundary formation.Īt 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. Four sequence boundaries are associated with climate shifts away from semiaridity. Paleosol profile measurements, stable isotopic ( 13 C and 18 O) time series, micromorphology and clay mineralogic analyses document paleoclimatic variability. In basin-central outcrops, vertical transitions in lithofacies and paleosol characteristics demarcate seismic sequence boundaries. Intrabasinal deformation and surface uplift of the frontal Cordillera of the Andes are documented. Seismic stratigraphy of Iglesia Basin is reassessed and compared with correlative outcrops to evaluate tectonic and climatic control of sequence development. 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. 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. 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. Subdivision of strata into genetically related sequences reflects interaction between extrinsic and intrinsic processes affecting a basin's accommodation potential and environment.
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