Triassic Macrostratigraphy of the Western United States
The Triassic is an extraordinary time interval that experienced a Phanerozoic low in sedimentary packages, a major shift from carbonate to siliciclastic deposition, biotic recovery, the origination of the modern fauna, the peak of Pangaean accretion (and its initial break-up), and the end-Triassic extinction. However, quantification of the sedimentary record is necessary to more fully interrogate the nature of Triassic deposition. To better understand the factors that influenced the formation and preservation of the sedimentary record, this study describes the spatiotemporal distribution of sedimentary packages (and their properties) in the western US; this region preserves the most complete Triassic record in North America. The results demonstrate a significant climatic control on Early Triassic sediment accumulation, which is up to 4.5 times greater than that seen in the more tectonically controlled accumulation of Late Triassic sediment. It is demonstrated that global perturbations in δ13Ccarb closely match variability in the Early Triassic sedimentation rates, volume, and lithology. It is also during this interval that the highest turnover rate of sedimentary packages occurs. In contrast, the Late Triassic has much lower rates of sediment accumulation but greater lithological heterogeneity, including several intervals of coarse-grained sedimentation linked to the initiation of Andean-type subduction along the Cordilleran margin. Further results show that the distribution of sedimentary package duration throughout the Triassic interval is nearly identical to that reported for the entire Phanerozoic. This suggests that tectonic processes that govern package longevity in the Triassic of the western US are similar to those that govern larger-scale patterns across the continent. Quantification of the spatiotemporal distribution of Triassic sediments has provided a framework for comparing the structure of the sedimentary record across this dynamic interval. It is anticipated that this study will be the starting point for further investigations of attributes integral to the Triassic system, including improved studies of mass balance, interrogations of the Middle Triassic problem, and analyses of the preservation and distribution of body and trace fossils essential for understanding biotic recovery and the origins of the modern fauna.
KeywordsMacrostratigraphy Triassic Modern fauna
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