Lower and Middle Jurassic rocks of the Gulf region have been identified only in southern Mexico and northern Central America. They consist of several thousand feet of dark marine beds and of varicolored continental beds that contain much carbonaceous shale and some coal. They were deposited in a geosyncline north of a rising land mass occupying the site of Honduras and southernmost Guatemala, Chiapas, and Oaxaca. Marine waters entered the region of Veracruz early in Lower Jurassic time, spread west and southwest as a narrow embayment that reached the region of northeastern Guerrero by late Lower Jurassic time, and spread more and more widely during Middle Jurassic time. The climate must have been hot and humid, at least seasonally.

Upper Jurassic rocks of the Gulf region are dominantly marine, occur in northern and southern Mexico, the southern United States, and the northern Antilles, and attain thicknesses of more than 5,000 feet at many places. The Callovian stage is known only from southern Mexico, where it is represented by over 2,000 feet of dark-colored marine beds that indicate a widespread sea and a fairly humid climate. The Divesian stage is represented in northern Central America, southern Mexico, and the southern United States by 600 to 1,000 feet of rock salt associated with a few hundred feet of anhydrite and redbeds that grade shoreward into thick redbeds without salt. Thick redbeds were formed in northern Mexico apparently at the same time. Probably the entire Gulf of Mexico was a salt-depositing basin for several million years. The Argovian stage is represented in many parts of Mexico and the southern United States by dark-colored limestone, but locally by considerable sandstone and conglomerate, and some shale, and ranges in thickness from a few hundred to more than 2,000 feet. Sites of salt deposition during Divesian time gradually became sites of lime deposition during Argovian time, but elsewhere, as in northern Mexico, the Argovian sea transgressed across redbeds and older rocks. The Argovian stage is represented in Cuba by partly metamorphosed sedimentary rocks that contain considerable argillaceous material. The climate, as during the Divesian, must have been very arid. During the Kimmeridgian occurred the most intense orogeny in North America since the Paleozoic. Early i the lower Kimmeridgian marine waters retreated basinward from 50 to 100 miles, giving rise to extensive lagoons in which accumulated thick masses of anhydrite and redbeds. Then followed uplift of the positive areas into highlands, or mountains, and considerable erosion of the marginal areas of the Jurassic rocks. Orogeny was expressed by block faulting (Palisade disturbance) in the Atlantic Coast region, and by strong folding and metamorphism in Cuba. Late in the lower Kimmeridgian marine waters transgressed widely in the northern parts of the Gulf of Mexico and the Mexican sea, but Cuba apparently remained a land area until the upper Portlandian. The late Upper Jurassic deposits, formed after the orogeny of the early Kimmeridgian, range in thickness from a few hundred to a few thousand feet and consist mainly of bituminous and carbonaceous shale and limestone, but nearshore deposits contain much conglomerate and sandstone, mostly Kimmeridgian and lower Portlandian in age. The northern part of the Mexican sea, unlike the Gulf of Mexico, was bordered by lagoons in which some gypsiferous and coaly deposits accumulated. The climate of the Kimmeridgian, Portlandian, and Tithonian stages must have been more moist than that of the Argovian and Divesian stages. Marine waters retreated slightly from the northern part of the Gulf region at the end of the Jurassic, but uplift was much less than during the early Kimmeridgian. Continued erosion of the highlands formed during the Palisade disturbance eventually produced the Fall Zone peneplain by early Upper Cretaceous time.

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Fig. 1. Index map of southern Arkansas and northern Louisiana, showing locations of fields producing from Jurassic formations.

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Fig. 1. Continued. See caption on page 1408.

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