Abstract

Subdivision of sedimentary rocks into genetically related sequences reflects the interaction between the extrinsic and intrinsic processes that affect a basin's accommodation potential as well as its environment. In nonmarine settings, sequence-bounding surfaces are often difficult to locate, and several causal mechanisms for sequences (e.g. tectonism, climate change, and autogenic variability) combine in complex and still poorly understood ways. Here we draw upon paleosols as sources of evidence for causes of sequence boundaries in the upper Miocene–lower Pliocene continental Iglesia wedgetop basin, San Juan Province, Argentina. Stable-isotope compositions (δ13C and δ18O), clay mineralogy, inorganic carbon weight percentages, and micromorphologic features of paleosols that span 9.0–4.6 Ma reveal environmental conditions concurrent with the development of ten continental stratigraphic sequences. Charaetcristics of compound pedofacies and regionally recognizable, abrupt vertical lithofacies changes suggest fluctuations in both accumulation rate and climate across sequence boundaries and between structural subbasins. We interpret that the Iglesia Basin strata accumulated under conditions that were generally semiarid with seasonal precipitation. Isotopic ratios and soil inorganic carbon weight percentages indicate increased aridity at 6.9 and 5.2 Ma, each a time of sedimentary hiatus. Notable enrichment of the δ13C signal at 6.9 Ma also suggests the onset of dominance of C4 plant biomass in northwestern Argentina. In combination with seismic data, we interpret that four of the ten sequence boundaries formed during intervals of marginal surface uplift concurrent with greater aridity, circumstances that led to erosion of proximal strata and starved the central basin of sediment input, resulting in hiatuses in sediment accumulation.

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