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Fluviatile clastics of the nonmarine, Early Cretaceous Gannett and Wayan groups were deposited on wet alluvial megafans and on intervening interfan piedmont slopes which declined eastward into more poorly drained lowlands from a western highland source area uplifted episodically by movements of the Paris overthrust. The deposits accumulated in the tectonic foredeep depressed by that thrust. The Gannett alluvial fan deposits (earliest Cretaceous) graded eastward into nonmarine lowland alluvial plain deposits of the Cloverly Group. Subsequent Wayan alluvial fan clastics (Early Cretaceous) graded into Bear River, Aspen, and lower Frontier mixed nonmarine, brackish, and marine facies. Lacustrine episodes of deposition intercalated Peterson and Draney limestones with Gannett f uvial clastics. Westward marine transgressions (Skull Creek, Mowry) intercalated mixed lacustrine and brackish facies (Smiths and Cokedale formations) into Wayan fluviatile clastics. Newly discovered fossil vertebrate and invertebrate materials (all fragmentary but identifiable) include: Gannett Group--large reptiles including turtles; Thomas Fork Formation--freshwater gastropods and unionid pelecypods, gastroliths(?), two types of turtles, large reptilian fragments (dinosaur?), and abundant dinosaur eggshell fragments; Wayan Formation--perennially aquatic snails, turtles, unidentifiable large reptiles, two types of crocodilians, an iguanodontid dinosaur (Tenontosaurus), an ankylosaurian dinosaur, a large ornithopod dinosaur, gastroliths(?), abundant and ubiquitous dinosaur eggshell frag ents (numerous types and sizes), and miscellaneous unidentifiable small vertebrate bone fragments. The dinosaurs are the first reported from Idaho and from these stratigraphic units.
Faunal, sedimentologic, and stratigraphic data indicate perennially wet, upland (proximal fan) lithotopes and biotopes on a deposurface of much lower gradient than characterizes semiarid or arid fans.
A census of analogous modern reptile reproductive behaviors supports the conclusion that the Wayan, and probably also the Gannett, alluvial fan environments were used as upland breeding grounds by dinosaurs and perhaps other reptiles. Comparison of these Early Cretaceous data with observations on the tectonic setting, sedimentology, and biology of the Quaternary indo-gangetic plain suggests many close analogies between the two sedimentary tectonic settings.
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