The evolution of the Interior Mesozoic basin is presented in terms of an evolving Gulf of Mexico which had its origin with the rifting and breakup of Pangea, particularly with the separation of North and South America. This Mesozoic event was preceded by the formation of Pangea in the late Paleozoic when plate collision produced the Appalachian-Ouachita-Marathon orogeny. As a result of this orogenic episode of plate collision and accompanying crustal dislocation along three major transcurrent-fault systems, the Texas, Wichita, and Mississippi megashears, a proto-Atlantic was closed and a distributive pattern of pre-Mesozoic rocks was created that was to have a lasting effect on the shape of the Interior Mesozoic basin.

Rifting in the Early Triassic created an incipient Gulf of Mexico with associated peripheral grabens that defined the shape of Mesozoic sedimentation. Crustal thinning and attenuation accompanied the divergent rifting of Pangea and early sedimentation in rift grabens are represented by the Eagle Mills Formation. Deltaic prisms are postulated, coincident with the three megashears, and represent the positions of ancestral Rio Grande, Red, and Mississippi Rivers. They augment the continental redbeds of the grabens formed during early rifting and the succeeding marine-shelf sediments of a diverging plate margin and constitute exploratory objectives.

The thick evaporite deposition, represented by the Werner evaporite and Louann Salt, in a shallow basin on a subsiding-plate margin is the result of a unique combination of events. The updomed rift margin of the trailing plate formed a restricting barrier that allowed the continued influx of sea water into the attenuated and rifted part of the plate that was subsiding to form the Interior Mesozoic basin. The sea water, on encountering the

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highly saline waters of this subsiding basin, initiated rapid salt deposition by the brine-mixing method. Eastward rotation of Mexico into its present position deepened the Gulf of Mexico and peripheral rifting aided in continued submergence with normal marine deposition being established in Late Jurassic time.

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