Most of this world's oil was generated in the late Mesozoic, and most of these deposits consist of middle Cretaceous oil. A study of Deep Sea Drilling Project results combined with marine geophysical data has yielded a large-scale, plate-tectonic history for this period. The most significant events in this history are (1) the opening of the north-central Atlantic between North America and Africa about 180 m.y. ago; and (2) the fragmentation of Gondwanaland 130 m.y. ago, a worldwide phenomenon when Africa separated from South America, Australia-Antarctica separated from greater India, and spreading patterns in the Pacific were greatly altered in response to the large-scale continental breakup. The middle Cretaceous is not characterized by continental breakup, but has unusu l, and probably highly significant, characteristics. An apparent increase in worldwide spreading rates occurred from 110 to 80 m.y.B.P., coincident with a period that lacked magnetic-field reversals. Subduction rates increased, sea levels rose, and batholiths formed behind subduction zones as results of these spreading-rate increases. Mid-plate volcanism created most of the seamounts and guyots present today in the western Pacific, as well as voluminous sills that are chemically similar to midocean ridge tholeiite. These events probably controlled the generation and subsequent preservation of late Mesozoic oil deposits in a yet unknown way.

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