In the Rancho Delicias area south of Tijuana, Baja California, well preserved remnants of an ancient weathering profile are developed on mid-Cretaceous granodioritic rocks. The paleosol is buried beneath Middle Eocene marine sedimentary rocks of the Delicias Formation. It has a 16 m thick C horizon characterized by grus decomposed to varying degrees. The A horizon is a severely leached oxisol that exceeds 13 m thickness and consists of approximately [2]/[3] kaolinite and [1]/[3] residual quartz gains with minor iron oxide concretions near the top. Thick latosolic soil profiles dominated by kaolinite form today only in near equatorial areas which have around 25 degrees C average annual temperature and greater than 1,200 mm yearly rainfall. The Late Cretaceous to Early Eocene climate of the San Diego-Tijuana region was probably like that of the hot and humid areas found within 20 to 30 degrees of the modern equator, and very unlike the present temperate, arid climate.

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