Union Oil Co. Calif., United States

Holes 535 and 540 were drilled in a basinal terrane for maximum penetration of the Cretaceous-Tertiary sequence. Rhythmic alternations of light bioturbated and dark laminated carbonaceous limestone represent the Early Cretaceous. Late Cretaceous rocks are almost totally missing. Holes 536, 537, and 538A were drilled on high-standing fault blocks. Hole 537 recovered phyllite that records (super 40) / (super 39) Ar plateau ages of about 500 m.y. In Hole 538A, basement consists of mylonitic gneiss and amphibolite, intruded by diabase dikes. (super 40) Ar/ (super 39) Ar dates of the regional metamorphic rocks suggest a 500-m.y. age; dike intrusions between 190 and 160 m.y. ago. Among the most significant results are: (1) recovery of "transitional" crust with early Paleozoic (Pan-African) metamorphic rocks, (2) recovery of Early Cretaceous deep-water limestones with immature petroleum source beds, (3) recovery of mid-Cretaceous platform talus resembling the reservoirs in the Poza Rica and some of the Reforma fields, and (4) discovery of a Late Cretaceous hiatus of 30 m.y. that corresponds to the "mid-Cretaceous unconformity" recognized widely on seismic records in the Gulf of Mexico.--Modified journal abstract.