Cretaceous Paleogeography, Southwestern US

Early Cretaceous Paleogeography, Southwestern US (130 Ma). The Western Interior was the site of an encroaching epicontinental seaway from the north. To the west lay uplands and a thrust belt in Nevada and western Utah. Transtensional basins in southern Arizona and california were the sites of thick marine and continental deposition. The Cordilleran arc was now a classic continental (Andean-style) arc along all or most of its extent. The arc consisted of a fore arc-trench system, fore arc basin, and Andean arc. The fore arc-trench was the site of the famous Franciscan melange'. The Great Valley sequence was deposited in the fore arc basin and the Sierra Nevada batholith complex formed in the bowels of the arc

Middle Cretaceous Paleogeography, Southwestern US (90 Ma). The Western Interior seaway has expanded to one of the greatest eoicontinental seas of all times. It stretched from central Utah to the western Appalachians and from the Arctic to the Gulf of Mexico. The Sevier orogenic belt bordered the sea to the west. This great thrust belt resulted from compression of the Cordilleran arc, perhaps further mitigated by collision with the huge Wrangellia oceanic plateau. The nature of the collision, its exact location, and its magnatude are anything but clear. Pieces of Wrangellia and Wrangellia-like plateaus are presently strewn along the west coast of North America from Mexico to Alaska

Late Cretaceous Paleogeography, Southwestern US (75 Ma). The Western Interior seaway slowly retreated to the northeast. Vast alluvial plains marked its past locations. Some of the World's greatest dinosaur remains are found in these deposits. The Sevier orogeny was at its climax. Regional metamorphism affected western Arizona and eastern California. Paleozoic and Mesozoic sandstone, mudstone, and limestone were metamorphosed to quartzite, schist, and marble. The west coast was marked by strongly oblique collision between the oceanic and continental plates and right-lateral transform faults transported pieces of Wrangellia and other terrains northward along the edge of North America. The Franciscan forearc complex was a broad accretionary prism that was constantly being deformed by complex stresses into a melange'

Latest Cretaceous-Early Tertiary Paleogeography, Southwestern US (65 Ma). Huge changes marked western North America. The Western Interior seaway had withdrawn and in its place were the embryonic Laramide uplifts of the Rocky Mountains. Debate rages over the origin of the Rockies, but the most popular theory states that the subduction of the Farallon oceanic plate under western North America changed to a very shallow angle, perhaps less than 10 degrees. This drove subduction eastward into the Rocky Mountain region and caused the uplifts and plutons there while the Cordilleran arc in California was shut down. Other theories blame collision with Wrangellia and the resulting compression for the Laramide orogeny. Perhaps large fragments of Wrangellia were partly responsible for the shallow angle of subduction.

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