Cretaceous marginal seas extending across America, Asia, Africa, and Australia connected oceanic areas in different climate zones. Water masses in such regions may have had the same density but very different salinities and temperatures, as is the case with the modern Gulf of Mexico and eastern Arctic Ocean. Cretaceous seaways permitted such different water masses to mix, resulting in a third water mass of greater density. Preliminary calculations indicate that this mechanism may have caused the Western Interior seaway of North America to become a source of intermediate water with a strength equal to the modern Weddell Sea, the source of Antarctic bottom water. Intermediate water production would be maximized at times of maximum transgression. The surface waters of the world ocean would be drawn into both ends of such a seaway and the plankton killed at the front, where mixing occurred, as environmental conditions changed drastically. This introduced large amounts of organic matter into the intermediate water, causing development of an intense oxygen minimum and deposition of large quantities of organic carbon in seas of intermediate depth to form extensive hydrocarbon source beds.