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Jurassic seas spread southward from the Arctic region four times into the western interior of North America. During Early Jurassic time a narrow, shallow arm of the sea extended southward as far as southeastern British Columbia. During the Middle and Late Jurassic epochs three successive seas extended about 2500 miles south of the Arctic region and attained a maximum width of at least 800 miles. Each invasion spread farther south and southeast than the preceding one, but none connected directly with the Mexican sea or with the sea in the Pacific coast area. The site of most continuous marine deposition and of thickest accumulation was near the western margin of the seaway. Extensive areas in the eastern and southern parts of the sea-way received gypsiferous red beds suggestive of brackish-water or lagoonal origin. Most of the bordering land masses were low, furnished few coarse sediments, and probably had arid climates except in the area west of Montana.

The Middle Jurassic sea, as indicated by its faunal assemblages and lithologic characteristics, was very shallow, warm, clear, generally well aerated, and its bottom in many places supported benthonic organisms.

The Early Callovian sea was extremely shallow throughout its eastern part, as indicated by an abundance of Lingulas and Ostreas and the presence of sandstones interpreted as littoral deposits. Lingulas were less common in the central and western parts of the sea, but an abundance of Grypkaea and other benthonic mollusks attests to the shallowness of the water. The fauna is similar to that of the Middle Jurassic except for the absence of corals and nerineid gastropods, which may indicate cooler waters during Callovian time.

The Oxfordian sea was probably somewhat deeper than the Callovian sea, and locally the waters may have been stagnant, as indicated by pyritized fossils. However, some of the sandy beds along its western margin were probably deposited in the littoral zone, as indicated by the presence of Ostrea and Mytilus. Its deposits are distinguished from the underlying Jurassic rocks by having abundant glauconite and numerous belemnites. These probably signify an abundance of organic matter and of minerals, such as phosphate, derived from the lands to the west. The presence of considerable driftwood in the Oxfordian sandstone in Montana shows that the bordering lands were covered with vegetation. The faunal assemblage is similar to that of the lower Callovian, although most of the species are different.

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